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IRL790 ([2-(3-fluoro-5-methanesulfonylphenoxy)ethyl](propyl)amine, mesdopetam) is a novel compound in development for the clinical management of motor and psychiatric disabilities in Parkinson disease. The discovery of IRL790 was made applying a systems pharmacology approach based on in vivo response profiling. The chemical design idea was to develop a new type of DA D3/D2 receptor type antagonist built on agonist rather than antagonist structural motifs. We hypothesized that such a dopamine antagonist with physicochemical properties similar to agonists would exert antidyskinetic and antipsychotic effects in states of dysregulated dopaminergic signaling while having little negative impact on physiologic dopamine transmission and, hence, minimal liability for side effects related to dopamine-dependent functions. At the level of in vivo pharmacology, IRL790 displays balancing effects on aberrant motor phenotypes, reducing l-DOPA-induced dyskinesias in the rodent 6-hydroxydopamine lesion model and reducing psychostimulant-induced locomotor hyperactivity elicited by pretreatment with either d-amphetamine or dizocilpine, without negatively impacting normal motor performance. Thus, IRL790 has the ability to normalize the behavioral phenotype in hyperdopaminergic as well as hypoglutamatergic states. Neurochemical and immediate early gene (IEG) response profiles suggest modulation of DA neurotransmission, with some features, such as increased DA metabolites and extracellular DA, shared by atypical antipsychotics and others, such as increased frontal cortex IEGs, unique to IRL790. IRL790 also increases extracellular levels of acetylcholine in the prefrontal cortex and ventral hippocampus. At the receptor level, IRL790 appears to act as a preferential DA D3 receptor antagonist. Computational docking studies support preferential affinity at D3 receptors with an agonist-like binding mode. SIGNIFICANCE STATEMENT: This paper reports preclinical pharmacology along with molecular modeling results on IRL790, a novel compound in clinical development for the treatment of motor and psychiatric complications in advanced Parkinson disease. IRL790 is active in models of perturbed dopaminergic and glutamatergic signaling, including rodent 6-hydroxydopamine l-DOPA-induced dyskinesias and psychostimulant-induced hyperactivity, in a dose range that does not impair normal behavior. This effect profile is attributed to interactions at dopamine D2/D3 receptors, with a 6- to 8-fold preference for the D3 subtype.

The individual weight loss response to obesity treatment is diverse. Here we test the hypothesis that the weight loss response to the CB1 receptor antagonist rimonabant is influenced by endogenous levels of receptor agonists. We show that baseline anandamide levels and body weight independently contribute to predict the treatment response to rimonabant in rodents, demonstrating that addition of biomarkers related to mode of action is relevant for a personalized health care approach to obesity treatment.

Some clinical reports suggest that tricyclic antidepressants which block both noradrenaline and serotonin (5-HT) reuptake (SNRIs) are more effective than selective 5-HT reuptake inhibitors (SSRIs) in treating severe depression. Moreover, one neurochemical study reported larger increases in extracellular 5-HT in rat frontal cortex in response to the tricyclic antidepressant imipramine compared to the SSRI fluoxetine. However, imipramine, which blocks both 5-HT and noradrenaline reuptake, also binds with relatively high affinity to receptors for noradrenaline, histamine and acetylcholine. Thus, to test the hypothesis that compounds that inhibit both 5-HT and noradrenaline reuptake produce larger increases in 5-HT efflux, we compared the effects of acute systemic administration of several SNRIs and SSRIs. Extracellular 5-HT was measured using microdialysis probes implanted in the diencephalon and frontal cortex of unanesthetized rats. We tested the SSRIs paroxetine (0.3-10 mg/kg), citalopram (10-20 mg/kg) and fluoxetine (10 mg/kg), the nonselective tricyclic antidepressant imipramine (20 mg/kg) and the more selective SNRIs duloxetine (3-30 mg/kg) and venlafaxine (30-50 mg/kg). During the lights-off period, paroxetine and duloxetine increased 5-HT in the diencephalon approximately 300 and approximately 200%, respectively. During the lights-on period, paroxetine and duloxetine each increased 5-HT approximately 400% in the diencephalon. In the frontal cortex, both paroxetine and duloxetine increased 5-HT approximately 200%. Citalopram and venlafaxine each increased 5-HT in the diencephalon approximately 300%. Fluoxetine and imipramine increased 5-HT in the diencephalon by approximately 125 and approximately 80%, respectively. Thus, these results do not support the hypothesis that compared to SSRIs, compounds which inhibit both 5-HT and noradrenaline reuptake have a larger acute effect on extracellular 5-HT.

Patients with chronic hepatic encephalopathy (HE) may present affective symptoms and antidepressant drug treatment in this condition is not uncommon. The present microdialysis study investigated treatment with the chronic antidepressant venlafaxine (VEN) in experimental HE with regard to tentative changes in pharmacokinetic and/or pharmacodynamic parameters. Three weeks after portacaval shunt (PCS) or sham operation in rats, VEN (10 mg/kg daily) was administered by implanted osmotic minipumps. VEN treatment for 14 days resulted in higher concentrations of VEN in PCS rats than in sham controls in serum and brain compartments, and the VEN levels in serum and brain were strongly inter-correlated. The serum N-desmethylvenlafaxine concentration did not differ between the groups, but correlated with the serum VEN levels. The other VEN metabolites were below the quantification limits. VEN treatment for 9-12 days significantly stimulated locomotion and rearing in the open field in sham controls, but failed to do so in the PCS rats. The concentrations of noradrenaline, dopamine, 5-HT, and 5-HIAA in neocortical dialysates were higher in PCS than in sham rats after 14 days of VEN treatment, but the elevations reached statistical significance only in the case of dopamine and 5-HIAA. In summary, there were significant pharmacokinetic and pharmacodynamic alterations in rats with experimental HE as compared to controls. The described experimental HE model may be useful for continued pharmacokinetic/pharmacodynamic interaction studies to unravel the pathophysiological consequences of HE on the CNS.

Rats were administered venlafaxine (10 mg/kg per day) for 14 days by using subcutaneously implanted osmotic minipumps. The present study assessed the distribution of VEN in different compartments, whether the VEN concentration in the compartments correlated, the effect of VEN on dialysate monoamine levels and on the spontaneous open-field behavior, and possible relations between the pharmacokinetic and pharmacodynamic parameters. The venlafaxine level in serum after sustained treatment was about 25% of the concentration in brain parenchyma and much higher than in brain dialysate. There was a clear correlation between venlafaxine concentrations in blood and brain compartments. The sustained venlafaxine challenge resulted in higher neocortical concentration of serotonin and noradrenaline, lower 5-hydroxyindole-3-acetic acid levels and increased locomotor activity in the central part of the test arena as compared to controls. No correlations were found between the venlafaxine concentration and brain monoamine parameters or the open-field behaviors. We conclude that, although species differences in pharmacokinetic properties for venlafaxine between rat and man exist, the pharmacokinetic correlations found after sustained treatment add information to the in vivo nature of the drug. Also, more studies like the present need to be performed to find the pharmacokinetic/pharmacodynamic interrelations for drugs like VEN.

THE NMDA receptor antagonist phencyclidine (PCP) has low micromolar affinity for the 5-HT reuptake site, but it is uncertain whether PCP blocks 5-HT reuptake when given systemically to rats in behaviourally stimulating doses. We here report for the first time that systemically administered PCP (5 mg/kg, s.c.) increases extracellular 5-HT levels in the rat medial prefrontal cortex (to 322%) and dorsal hippocampus (to 233%). Increases were found also when citalopram (1 microM) was included in the perfusion medium (to 184 and 180%, respectively). Extracellular 5-HIAA concentrations increased during both conditions, and extracellular GABA decreased in the dorsal hippocampus. It is concluded that systemic PCP treatment elevates extracellular 5-HT levels, probably through mechanisms other than a blockade of 5-HT reuptake.

Clozapine and the novel putative, antipsychotic S 16924 ((1-(benzodioxane-5-yl)-3-[3-(4-fluorophenacyl)pyrrolidine]-1-o xapropane HCl) share significant affinity for alpha1-adrenoceptors and 5-HT1A autoreceptors in vitro and display an 'atypical' behavioural profile in in vivo models used for detecting potential neuroleptic effects. In the present study, in vivo microdialysis was used to examine the effect of clozapine and S 16924 on 5-HT overflow in the rat ventral hippocampus, and to assess the relative role of putative alpha1-adrenoceptor antagonist and 5-HT1A autoreceptor agonist properties of the drugs in this regard. S 16924 (0.1-3 mg/kg, s.c.) reduced dialysate 5-HT in a dose- and time-dependent fashion by maximally approximately 70% from baseline 40-60 min after injection. Clozapine (0.1-10 mg/kg, s.c.) reduced 5-HT overflow in the same manner, with a maximum effect of approximately 60% from baseline, obtained after 60-80 min. The 5-HT decrease elicited by S 16924 (1.0 mg/kg, s.c.) was significantly, though only partially, antagonized by pretreatment with the selective 5-HT1A receptor antagonist WAY 100635 (0.3 mg/kg, s.c.). The selective alpha1-adrenoceptor agonist cirazoline (0.02 mg/kg, i.p.) alone did not significantly attenuate the effect of S 16924 (1.0 mg/kg, s.c.) on 5-HT overflow. Combined treatment with both WAY 100635 and cirazoline, however, totally reversed the 5-HT-suppressing effect of S 16924 (1.0 mg/kg, s.c.). By comparison, when given separately, neither WAY 100635 (0.3 mg/kg, s.c.) nor cirazoline (0.02 mg/kg, i.p.) antagonized the clozapine (0.3 mg/kg, s.c.)-induced decrease of 5-HT in ventral hippocampus dialysates. In the presence of both WAY 100635 and cirazoline, the response to this dose of clozapine was however significantly, though modestly, attenuated. In contrast, the WAY 100635/cirazoline combination failed to antagonise the 5-HT decrease resulting from a higher dose (3.0 mg/kg, s.c.) of clozapine. We conclude that both alpha1-adrenoceptor antagonist and 5-HT1A receptor agonist properties of clozapine and S 16924 contribute to the 5-HT release-reducing action of these drugs. Whereas these factors apparently explain the effect of S 16924 fully, additional mechanism(s) appear to be involved in the case of clozapine. With regard to the interplay between alpha1-adrenoceptor and 5-HT1A (auto)receptor mechanisms in the control of 5-HT release in the rat forebrain, the present data suggest that an excitation mediated by the former is outweighed by the simultaneous activation of the latter-inhibitory-receptors.

Portal-systemic encephalopathy (PSE) is associated with an increased brain tissue turnover of serotonin (5-HT). Despite increased 5-HT metabolism, brain 5-HT release in rats with a portacaval shunt (PCS) seems to be unaltered. Although this may indicate that the overall 5-HT output is unaltered in PSE, it is also possible that the 5-HT release pattern might be altered in some way. In the present study, the potassium-evoked frontal neocortical release of 5-HT was studied in experimental chronic PSE. KCI (60 mM) produced marked increases in the 5-HT output compared with basal values both in PCS and sham rats. Simultaneously, the KCI challenge resulted in significant elevations in the 5-HT release of PCS compared with sham. In Ca2+-free medium, the difference between PCS and sham rats in the KCl-evoked release of 5-HT was abolished. In the presence of TTX (1 mM), both groups displayed increased extracellular 5-HT levels. Again, a difference with higher amplitude of the 5-HT release in PCS compared with sham was evident. It is concluded that in experimental chronic PSE an augmented neocortical 5-HT release compared with the normal in vivo situation is available. The possible mechanism(s) responsible for the difference in neocortical 5-HT output between PCS and sham-operated rats in response to the KCl-challenge is discussed.

Portal-systemic encephalopathy (PSE) is associated with increased brain turnover of serotonin (5-HT) in vivo but the brain 5-HT output seems to be unaltered. Recent results suggest, however, that an augmented neocortical 5-HT release in experimental chronic PSE may prevail under certain conditions. In the present study, neocortical extracellular 5-HT and 5-hydroxyindoleacetic-3-acid (5-HIAA) levels were measured in portacaval shunted (PCS) rats and sham-operated controls following local administration of p-chloroamphetamine (pCA) and d-fenfluramine (dFEN), two specific 5-HT releasing agents. The basal neocortical extracellular 5-HT concentrations were unaltered and the 5-HIAA levels were elevated in experimental PSE, supporting an unchanged brain 5-HT output despite elevated brain 5-HT metabolism. Perfusion with pCA or dFEN (5 microM; one 20-min pulse) produced marked increases in brain 5-HT release both in PCS and sham-operated rats compared with corresponding basal values. While no difference in the 5-HT response to dFEN administration was seen between sham (5-HT levels increased by 330%) and PCS (500%) rats, a clear difference (p

Ammonia has been shown to cause release of neurotransmitters such as serotonin (5-hydroxytryptamine; 5-HT) from synaptosomal preparations in vitro. In the present study, frontal neocortical extracellular levels of 5-HT and its major metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA), were determined in vivo by the use of microdialysis in portacaval shunted (PCS) rats, an experimental model of chronic hepatic encephalopathy (HE), prior to and after an acute coma-inducing administration of ammonium acetate (NH4Ac; 5.2 mmol/kg, i.p.). PCS rats displayed elevated (P0.01) 5-HIAA but unaltered 5-HT extracellular levels compared with controls, supporting the contention of an increased neocortical 5-HT metabolism but unaltered neuronal 5-HT output in chronic HE. However, a transient elevation of extracellular 5-HT levels was observed when PCS-NH4Ac rats were in coma. Increased brain ammonia may thus augment neuronal 5-HT release in chronic HE, which in turn could be a causative for precipitation of more severe stages of HE.

Portal-systemic encephalopathy (PSE) is associated with increased brain turnover of serotonin (5-hydroxytryptamine; 5-HT). Despite this metabolic increase, neuronal release of 5-HT is unaltered in neocortex of portacaval shunted (PCS) rats. In the present study, frontal neocortical extracellular 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) levels were determined in PCS rats and sham-operated controls prior to, as well as, after acute challenge with L-tryptophan (L-TRP; a bolus dose of 280 mg/kg i.p. followed by 5 consecutive hourly doses of 50 mg/kg). Neither basal 5-HT nor 5-HIAA extracellular levels were significantly altered in PCS rats compared to controls. L-TRP administration resulted in unaltered extracellular 5-HT but elevated 5-HIAA levels in PCS and sham rats. These findings do not suggest that changes in brain neuronal 5-HT release play any major functional role in the pathogenesis of chronic PSE. The present data also emphasize the importance of distinguishing between brain 5-HT metabolism and brain 5-HT release.

Recent open clinical studies suggest that pindolol and buspirone may enhance the efficacy and/or shorten the latency to antidepressant action of selective serotonin reuptake inhibitors (SSRI) in unipolar major depressive disorder. The present investigation addressed the possibility that these agents share the ability to enhance the extracellular 5-hydroxytryptamine (5-HT)-elevating response to the SSRI citalopram. For the purpose, in vivo microdialysis in the rat ventral hippocampus was employed. (-)-Pindolol (8 mg/kg s.c.) augmented the citalopram (5 mg/kg s.c.)-induced rise of extracellular 5-HT levels, whereas buspirone (5 mg/kg s.c.) failed to do so. This effect of (-)-pindolol probably reflects its ability to block 5-HT1A autoreceptors, thereby abating the citalopram-induced indirect activation of these sites (secondary to the inhibition of 5-HT reuptake and elevation of extracellular 5-HT in the midbrain raphe). The lack of effect of buspirone in this model indicates that the clinically observed antidepressant augmentation action of buspirone is not mediated indirectly, via enhanced extracellular levels of 5-HT.

Twelve N,N-dipropyl-substituted derivatives of trans-2-arylcyclopropylamine have been prepared and assayed for their ability to displace [(3)H]-8-OH-DPAT from rat brain 5-HT(1A) receptors. The new derivatives include phenyl (7a), bromo- (7b) and fluorophenyl (7c-e), 2-methoxy-5-fluorophenyl (7h), and 2-hydroxy-5-fluorophenyl (7l) as well as trifluoromethylphenyl (7f) and 2,3-dichlorophenyl (7g) analogues. In the present series of compounds, electron-withdrawing substituents in the phenyl ring appear to decrease the affinity for 5-HT(1A) receptors. In contrast, electron-rich aryl groups, such as 2- or 3-thienyl (7j and 7k, respectively), provide compounds with high affinity. The additional bulk produced by the aromatic moiety in the 2-benzothienyl derivative 7i appears to be detrimental to 5-HT(1A) receptor affinity. The racemic mixtures of the interesting 7j and 7l were resolved into the enantiomers; 7j and 7l exhibited a high enantiomeric 5-HT(1A) receptor affinity ratio (75-fold and 100-fold, respectively). The enantiomers of 7j and 7l were evaluated in vivo by use of biochemical and behavioral tests in rats. Compound (1R,2R)-7j behaved as a partial agonist whereas (1R,2S)-7l appeared as an efficacious 5-HT(1A) receptor agonist, stimulating both autoreceptors and postsynaptic receptors.

The inhibition of serotonin (5-HT) release produced by antidepressants varying in relative selectivity for blocking uptake of 5-HT and noradrenaline (NA) was compared. Release was measured by microdialysis in anesthetized rats with nerve terminal 5-HT uptake inhibited by local infusion of citalopram (1 μM) through a dialysis probe in hippocampus. With 5-HT uptake first blocked in hippocampus, systemic injection of uptake inhibitors produced decreases in dialysate 5-HT, presumably due to autoreceptor stimulation in the raphe. The largest decreases (about 60–70%) in 5-HT were produced by the selective 5-HT uptake inhibitors sertraline, paroxetine and citalopram. Nonselective blockers caused less suppression of release. Thus, the maximum decrease in 5-HT was 35% after clomipramine, a less selective 5-HT uptake inhibitor, and ⩽ 30% after the nonselective 5-HT/NA uptake blockers imipramine and amitriptyline, 5-HT was not decreased after maprotiline, a selective NA uptake blocker. Pretreatment with (+)WAY 100135 to block 5-HT 1A autoreceptors, abolished the inhibition of 5-HT release produced by systemic sertraline, clomipramine and imipramine. One explanation for the difference between selective and nonselective inhibitors with respect to central 5-HT release, is the excitatory effect of ( α 1 ) adrenergic receptor stimulation on 5-HT neuronal discharge. However, pretreatment with α -methyl- p -tyrosine to deplete NA, did not influence the inhibition of 5-HT release produced by imipramine.

The clinical efficacy of antidepressants that block serotonin (5-hydroxytryptamine, 5-HT) reuptake may be restrained by indirect activation of autoreceptors. In vivo microdialysis in rat hippocampus was used to assess the release-inhibitory properties of the 5-HT reuptake inhibitors citalopram and paroxetine. When reuptake was first blocked by infusing citalopram into the hippocampus, systemic administration of citalopram or paroxetine resulted in a 50–70% decrease in hippocampal 5-HT overflow. This presumably reflected the inhibition of 5-HT release subsequent to reuptake blockade in the raphe nuclei and, in turn, activation of somatodendritic autoreceptors. In support, pretreatment with (±)-pindolol or(+)-WAY100135 ((+)-N-tert-butyl-3-(4- (2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide dihydrochloride), to block 5-HT1A autoreceptors, abolished the decrease in 5-HT produced by systemic injection of the uptake blockers.

The two diphenylbutylpiperazinepyridinyl derivatives, FG5865 and FG5893, have a unique receptor binding profile in that they show very high and essentially equipotent affinities for both 5-HT 1A and 5-HT 2 receptors. The present report describes the acute effects of FG5865 and FG5893 on presynaptic 5-hydroxytryptamine (5-HT) neuronal function in the rat CNS, using established ex vivo and in vivo neurochemical techniques. Post-mortem measurements of tissue levels of 5-HT, its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), and of the formation of 5-hydroxytryptophan (5-HTP; after inhibition of aromatic amino acid decarboxylase by NSD 1015) showed that FG5865 (0.1–20 mg/kg, s.c.) and FG5893 (0.1–20 mg/kg, s.c.) dose dependently decreased the synthesis and the metabolism/turnover of 5-HT - this to an extent comparable to the reference 5-HT 1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin. Reserpine (5 mg/kg, s.c.) pretreatment did not prevent the FG5893-induced decrease of 5-HT synthesis rate. In contrast, about 25–50 times higher doses of FG5865 were required to produce a comparable decrease in brain 5-HT synthesis in reserpinized vs. non-pretreated rats. In in vivo microdialysis experiments, both FG5865 (0.1–3.0 mg/kg, s.c) and FG5893 (0.3–1.0 mg/kg, s.c.) caused a marked and dose-dependent decrease of 5-HT release in the ventral hippocampus. Pretreatment with the 5-HT 1A receptor antagonist, (±)-pindolol (8 mg/kg, s.c.), abolished the FG5865 (0.3 mg/kg, s.c.)-induced reduction of 5-HT release, and (−)-pindolol (8 mg/kg, s.c.) similarly reversed the FG5893 (0.3 mg/kg, s.c.)-induced decrease. Local infusion of FG5865 into the ventral hippocampus (10 μM, 20-min pulse) resulted in a rapid and transient elevation of the 5-HT output, an effect that was independent of extracellular Ca 2+ , FG5893, on the other hand, did not affect the 5-HT release upon local administration. The results demonstrate that FG5865 and FG5893 potently affect a range of neurochemical indices of rat brain 5-HT neuronal activity in vivo, in a way consistent with indirect (FG5865) and direct (FG5865 and FG5893) stimulation of the 5-HT 1A autoreceptors in the raphe nuclei.

The present study addressed the possibility that disinhibition of serotonin (5-HT) autoreceptor-mediated negative feedback might potentiate the elevation of nerve terminal 5-HT output induced by selective 5-HT reuptake blockade. To this end, rats were given citalopram and the 5-HT autoreceptor-blocking agents (S)-UH-301 (5-HT1A) and (-)-penbutolol (5-HT1A/1B), and the effect on extracellular 5-HT in the ventral hippocampus was monitored by means of in vivo microdialysis. Citalopram (5 mg/kg, s.c.) approximately doubled the 5-HT output, a response that was markedly augmented by (S)-UH-301 (3 mg/kg, s.c.) and (-)-penbutolol (8 mg/kg, s.c.) and by combined treatment with (S)-UH-301 (3 mg/kg, s.c.) plus (-)-penbutolol (1 microM; via the dialysis perfusion medium), but not by (-)-penbutolol (1 microM) alone. These findings provide evidence that 5-HT, in particular 5-HT1A, autoreceptor-mediated negative feedback mechanisms are pivotal in determining the nerve terminal 5-HT output level after 5-HT reuptake inhibition. These findings have important implications for the interplay between different processes controlling 5-HT transmission in vivo and might possibly offer a lead toward novel, therapeutically exploitable principles.

Brain 5-HT1A and 5-HT1B receptors are important targets for drug-induced modulation of 5-HT function in vivo. However, very few compounds are available that are effective antagonists at 5-HT1 receptors, thus hampering the progress of fundamental as well as clinical research in this area. The present study assessed the usefulness of the beta-adrenolytic agent (-)-penbutolol (and its (+)-counterpart) as a 5-HT1A receptor-blocking agent. The compound was found to counteract, in a stereospecific fashion, not only the behavioural and hypothermic but also the in vivo 5-HT synthesis/turnover-reducing effects of the specific 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). These findings indicate that (-)-penbutolol is an antagonist at both postsynaptic receptors and somatodendritic autoreceptors of the 5-HT1A subtype. Thus, (-)-penbutolol represents a useful addition to the array of pharmacological tools available for the study of central 5-HT1 receptor-mediated functions.

Injectable local drug delivery formulations—so-called microspheres —have recently been developed, in which drugs are microencapsulated within biocompatible and biodegradable copolymer excipients like poly[DL-lactide-co-glycolide]. In view of its potential therapeutical usefulness, we have studied the microsphere methodology as a means to substitute for experimentally induced subnormal levels of endogenous dopamine (DA). Administration of 6-hydroxydopamine (6-OH-DA) unilaterally in the medial forebrain bundle of rats results in an up-regulation of postsynaptic receptors in the denervated striatum, functionally manifested as contralateral rotational behavior after apomorphine. DA microspheres were implanted in the denervated striatum. The majority of the rats displayed an attenuation of the contralateral rotational behavior induced by apomorphine up to 8 wk postimplantation. Immunocytochemical observations unexpectedly demonstrated growth of DA and tyrosine hydroxylase immunoreactive fibers in the denervated striatum. Interestingly, there was an apparent correlation between functional recovery and the degree of growth of DA fibers. The present results suggest that implantation of DA microspheres may promote DA fiber growth and extended recovery of surviving DA neurons, and, therefore, could be of therapeutic usefulness in Parkinson’s disease.

It has recently been suggested that central 5-HT1A autoreceptors are already desensitised after single-dose 5-HT1A agonist treatment. In turn, this would lead to an attenuated feedback suppression of transmitter release from 5-HT neurones, and thus to enhanced 5-HT synaptic transmission. In the present study in vivo brain microdialysis techniques were used in an attempt to test this hypothesis. The results show that single-dose pretreatment with the reference 5-HT1A receptor agonist 8-hydroxy-2-(din-propylamino)tetralin, 8-OH-DPAT, (i) did not significantly alter the baseline output of 5-HT in the rat ventral hippocampus 24 h later, and (ii) did not alter the release-reducing response to 5-HT1A agonist (8-OH-DPAT, ipsapirone or BMY 7378) challenge under the same conditions. These observations indicate that the functional responsiveness of the 5-HT release-controlling 5-HT1A autoreceptors is maintained after bolus 8-OH-DPAT pretreatment. When related to the acute 8-OH-DPAT-induced reduction in raphe 5-HT1A radioligand binding density recently reported by others, the present results are consistent with a large functional overcapacity of this 5-HT1A receptor population. The mechanism by which 5-HT1A receptor-mediated hypothermia and hyperphagia are rapidly attenuated by a previous large single dose of a 5-HT1A receptor agonist remains to be explained.

Biodegradable controlled-release microsphere systems made with the biocompatible biodegradable polyester excipient poly [DL lactide-co-glycolide] constitute an exciting new technology for drug delivery to the central nervous system (CNS). The present study describes functional observations indicating that implantation of dopamine (DA) microspheres encapsulated within two different polymer excipients into denervated- striatal tissue assures a prolonged release of the transmitterin vivo. Moreover, in this regard, the results show that there were clear cut temporal differences in the effect of the two DA microsphere formulations compared in this study, probably reflecting variations in the actual composition (i.e., lactide to glycolide ratio) of the two copolymer excipients examined. This technology has considerable potential for basic research with possible clinical application.

The effects of MDL 73005EF (8-[2-(2,3-dihydro-1,4-benzodioxin-2-yl)methylamino]-8- azaspiro[4,5]decan-7,9-dione methyl sulphonate), a novel selective 5-HT1A receptor ligand with putative anxiolytic properties, were explored using models of central pre- and postsynaptic 5-HT1A receptor function in the male rat. MDL 73005EF dose dependently decreased the hippocampal 5-HT output measured by in vivo microdialysis in chloral hydrate-anaesthetised rats and this response was antagonised by the 5-HT1A/B receptor antagonist, pindolol. Local administration of MDL 73005EF had no effect on the hippocampal 5-HT output. MDL 73005EF failed to alter basal plasma adrenocorticotropin (ACTH) levels but, in common with pindolol, attenuated the ACTH response to the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). In contrast to 8-OH-DPAT, MDL 73005EF significantly increased plasma prolactin but apparently not through a 5-HT receptor-mediated mechanism. The results indicate that MDL 73005EF possesses mixed 5-HT1A receptor agonist/antagonist properties, acting as an agonist at presynaptic 5-HT1A receptors controlling 5-HT release and as an antagonist at postsynaptic 5-HT1A receptors mediating ACTH release.

5-HT (10 and 40 micrograms) and 8-OH-DPAT (1 and 5 micrograms) were locally applied into the dorsal or median raphe nuclei in awake, unrestrained, rats. All animals were also treated with the 5-HTP and DOPA decarboxylase inhibitor NSD-1015, 100 mg kg-1 SC, 30 min before decapitation. 5-HT or 8-OH-DPAT were administered 5 min before NSD-1015. The regional brain in vivo rate of tyrosine and tryptophan hydroxylase activity was estimated by measuring the accumulation of DOPA and 5-HTP. The following brain regions were sampled: neocortex, hippocampus, dorso-lateral neostriatum, ventro-medial neostriatum, nucleus accumbens, olfactory tubercle, globus pallidus, septum and the amygdala. Compared to normal controls, there were small and inconsistent effects on forebrain 5-HTP accumulation by saline injections into the dorsal or the median raphe (an increase in 3 out of 36 experiments), whereas strong effects by the injection procedure were noted on forebrain DOPA accumulation (an increase in 15 out of 36 experiments). Injections of 5-HT (same effect by 10 or 40 micrograms) into the dorsal raphe, produced a decrease in 5-HTP accumulation in all forebrain areas except for the hippocampus and the septum, whereas no effects were seen in any area after median raphe injections. In contrast, 8-OH-DPAT preferentially produced a decrease in forebrain 5-HTP accumulation after median raphe injections and less, but statistically significant effects by dorsal raphe injections. The 8-OH-DPAT injection into the median raphe primarily affected limbic forebrain areas (hippocampus, nucleus accumbens, ventro-medial neostriatum, amygdala and the septum). This dissociation of the effects of 5-HT and 8-OH-DPAT on forebrain 5-HT synthesis after local application into the dorsal or the median raphe strongly supports the contention of heterogeniety in the brain 5-HT receptor population in terms of receptor subtypes and/or receptor regulation.

Patients with chronic hepatic encephalopathy display monoaminergic perturbations together with affective symptoms. Thus, these patients belong to a group with a probability of receiving antidepressant drug treatment. The liver impairment may result in pharmacokinetic alterations of the antidepressant drug, which in turn may affect the already perturbed monoaminergic function. Venlafaxine (VEN) was administered as a single subcutaneous challenge to portacaval shunted (experimental hepatic encephalopathy model) rats (5 mg/kg) and sham-operated rats (5 and 10 mg/kg). The aims were to investigate whether a reduced dose in portacaval shunted rats resulted in higher concentrations of VEN and serotonin as compared to control rats, which had been demonstrated earlier when the rats received the same dose (10 mg/kg). A 50% reduction of the dose of VEN administered to portacaval shunted rats resulted in elevated levels of VEN in serum, brain parenchyma, and brain dialysate about 300 minutes after the injection. The VEN challenge increased the serotonin and noradrenaline concentrations in dialysate in portacaval shunted rats and both sham groups, but the three VEN groups did not differ in any major way in serotonin and noradrenaline output. Therefore, when the dose of VEN administered to experimental hepatic encephalopathy was reduced 50% as compared to control rats, mainly pharmacokinetic, and possibly also monoaminergic, alterations were observed.

In the present study, effects of citalopram (CIT) on brain 5-hydroxytryptamine (5-HT) release in experimental chronic hepatic encephalopathy (HE) were investigated. Neocortical administration of CIT (1.0 microM) increased the brain 5-HT output to a similar extent in portacaval shunted (PCS) rats and sham-operated controls, indicating that a previous described mismatch between increased 5-HT turnover and unchanged release in PCS rats is not explained by an accelerated brain 5-HT reuptake. Subsequent systemic administration of CIT (5 mg/kg subcutaneously) resulted in a more pronounced attenuation of the brain 5-HT release in PCS rats than in sham-operated controls, possibly indicating a higher susceptibility to indirect mid-brain 5-HT1A autoreceptor activation in experimental portal-systemic encephalopathy (PSE). A KCl (60 mM) challenge in the presence of locally administered CIT (1 microM) induced a more marked neocortical 5-HT response in PCS rats than in sham-operated controls, confirming previous results of a higher than normal amount of 5-HT available for depolarization-induced release in PCS rats. Although the pharmacodynamic parameters in this study was investigated for CIT, the likelihood of a parallel pharmacokinetic alteration existing for this drug in the PCS condition also was indicated. It is thus suggested that otherwise generally safe central nervous system 5-HT-active drugs may represent a potential hazard in patients with liver failure with or without PSE.

In vivo microdialysis in rat ventral hippocampus was used (i) to verify the importance of 5-HT1A autoreceptors in the raphe as targets for drugs that enhance the citalopram-induced elevation of forebrain 5-hydroxytryptamine (5-HT), and (ii) to further examine the specificity of (−)-penbutolol in this regard. The selective 5-HT1A receptor antagonist WAY100635 (s.c., or intra-raphe) or the mixed 5-HT 1A 1B β- adrenoceptor antagonist (−)-penbutolol (s.c.), potentiated the citalopram-induced 5-HT rise, whereas local ‘reverse’ dialysis of WAY100635 into the ventral hippocampus did not. Furthermore, the (−)-penbutolol-induced augmentation proved stereoselective and not mediated by β-adrenoceptors (no effect of s.c. (+)-penbutolol, or β1- and β2-adrenoceptor blockers (betaxolol, ICI118.551)). These data provide direct evidence that increased stimulation of 5-HT1A autoreceptors in the midbrain raphe impedes the effect of citalopram on forebrain extracellular 5-HT, whereas neither postsynaptic 5-HT1A receptors nor β-adrenoceptors appear to be involved.

A series of C11-substituted (R)-aporphines and C11-oxygenated (R)-noraporphines has been synthesized and evaluated for central serotonergic and dopaminergic effects in vitro and in vivo. The various C11-substituents were introduced using efficient nickel- and palladium-catalyzed reactions of the corresponding triflate (R)-11-[[(trifluoromethyl)sulfonyl]oxy]aporphine (6). Several compounds display high affinity to serotonin 5-HT1A receptors in spite of major differences in steric bulk and electronic properties of the various C11-substituents. A change of the N-methyl group of the nonselective 3 to H [23, (R)-11-hydroxynoraporphine] or propyl [2, (R)-11-hydroxy-N-propylnoraporphine] increases the selectivity for 5-HT1A receptors (100-fold) and dopamine D2A receptors (3-fold), respectively. Compounds 3 and 23 have similar affinities to 5-HT1A receptors, whereas the propyl substituent of 2 not only enhances the selectivity for D2A receptors but also increases the D2A affinity. Modeling of ligand-receptor binding site interactions yielded an interaction site model for the 5-HT1A receptor that describes a gradual change in binding mode for C11-hydroxy, -methoxy-, and -phenyl-substituted derivatives. Hydrogen bonding is hereby gradually replaced by van der Waals interactions involving a relatively large lipophilic pocket. The derived D2A receptor model can accommodate both the N-propyl substituent of 2 and the C11-ethyl substituent of 11 [(R)-11-ethylaporphine].

Derivatives of the selective serotonin 5-HT1A receptor agonist (R)-11-hydroxy-10-methylaporphine (2) having various substituents in the C10-position or at the nitrogen have been synthesized from natural morphine or 6-O-acetylcodeine, respectively. The C10-substituents were introduced using efficient Stille or Suzuki cross-coupling reactions. The compounds were evaluated for their affinities to 5-HT1A and dopamine (DA) D1 and D2A receptors in vitro. All compounds tested displayed low (micromolar) affinities to D1 and D2A receptors. In addition, changes in steric bulk and/or electronic properties of the C10-substituent as compared to a C10-methyl group, as well as substitution of the N-methyl group for a hydrogen or a larger N-alkyl group, produced a marked decrease in the affinities to 5-HT1A receptors. Selected compounds that displayed moderate to high affinities to 5-HT1A receptors were evaluated for their ability to stimulate 5-HT1A receptors in vivo. The evaluated compounds behaved as agonists at 5-HT1A receptors, except for the N-propyl analogue of 2, (R)-11-hydroxy-10-methyl-N-propylnoraporphine (23), which displayed weak DA receptor agonism at the doses tested. Hence, the substitution pattern of 2 (a C10-methyl, a C11-hydroxy, and an N-methyl group) appears to be optimal for potent interaction of 10,11-disubstituted (R)-aporphines with 5-HT1A receptors. Modeling of ligand-5-HT1A receptor interactions was performed in an attempt to rationalize the observed affinity data. The binding site model suggests the presence of a "methyl pocket" in the 5-HT1A receptor binding ste. The C11-methoxy-substituted aporphines appear to have a different binding mode compared to 2, implying a different accessibility of these compounds to the "methyl pocket".

Rats were administered the selective serotonin (5-HT) uptake blocker citalopram or saline for 14 days to determine if prolonged treatment would lead to changes in extracellular 5-HT or autoreceptor sensitivity. One day after drug withdrawal, dialysis probes were implanted in the frontal cortex and dorsal hippocampus. Dialysis experiments were carried out using chloral hydrate anesthetized rats. The experimental protocol comprised the administration of three consecutive drug challenges: (1) After stable baseline levels were obtained, citalopram was infused through the dialysis probes to locally block uptake in the forebrain. (2) Subsequently, a 5-HT1B receptor agonist (RU24969 or CP93,129) was infused through the probe to test for changes in terminal autoreceptor sensitivity. (3) Last, citalopram was administered systemically to test the effect of indirect activation of somatodendritic autoreceptors. Under these conditions, with uptake already blocked locally in the forebrain, systemic citalopram produces a decrease in extracellular 5-HT, an effect that can be inhibited by pretreatment with antagonists of 5-HT1A receptors. The results indicate that during local infusion of citalopram extracellular 5-HT was significantly higher in the dorsal hippocampus of the chronic citalopram as compared to saline treatment group. This difference persisted throughout the full time course of the experiment. However, the decreases in 5-HT levels produced by local infusion of a 5-HT1B receptor agonist or after systemic citalopram administration were not significantly different between the chronic citalopram and saline treated groups. There were no significant differences between chronic citalopram and saline treated animals in frontal cortex. These results suggest that prolonged inhibition of 5-HT uptake may produce a selective change in the regulation of release from median raphe 5-HT neurons, but this change could not be clearly linked to a change in nerve terminal or somatodendritic autoreceptor sensitivity.

The present study utilized in vivo microdialysis to investigate the importance of 5-HT1A autoreceptors and alpha 1-adrenoceptors in the decreased 5-HT release obtained following administration of the mixed 5-HT1A autoreceptor partial agonist/alpha 1-adrenoceptor antagonist BMY7378, the selective 5-HT1A receptor agonist 8-OH-DPAT and the alpha 1-adrenoceptor antagonist prazosin. BMY7378 (0.25 mg/kg, s.c.), 8-OH-DPAT (0.025 mg/kg, s.c.) and prazosin (0.1-1.0 mg/kg, s.c.) all suppressed ventral hippocampal 5-HT efflux. The BMY7378- and 8-OH-DPAT-induced inhibition of 5-HT release were reversed by a 40 min pre-treatment with either (+/-)pindolol (8 mg/kg, s.c.) or WAY-100635 (0.3 mg/kg, s.c.), to block 5-HT1A autoreceptors. Neitehr of these antagonists altered the prazosin-induced (0.3 mg/kg, s.c.) 5-HT disease.(i) confirm that both an alpha 1-adrenoceptor antagonist (prazosin) and 5-HT1A autoreceptor stimulants (BMY7378 and 8-OH-DPAT) may reduce cerebral 5-HT release; (ii) support that the BMY7378-induced decrease in 5-HT release results from 5-HT1A autoreceptor agonism, rather than alpha 1-adrenoceptor blockade; and (iii) argue against "physiological" antagonism (i.e. via blockade of beta-adrenoceptors, 5-HT1B receptors or some other mechanism) as an explanation for the reversal by pindolol of 5-HT1A autoreceptor agonist-induced suppression of 5-HT release. These data support the usefulness of pindolol, as well as the more specific compound WAY-100635, to block 5-HT1A autoreceptors.

The effect of local injection of pertussis toxin (PTX) into the ventral tegmental area (VTA) on acoustic startle in rats was investigated. The PTX treatment caused only minor effects of its own on the acoustic startle response (ASR) or prepulse inhibition (PPI) of acoustic startle. However, systemic treatment with the indirect DA receptor agonist, amphetamine (2 mg/kg, SC) caused a significant increase in ASR magnitude and a significant disruption of PPI in PTX-treated rats while no such effects were observed in sham-treated rats. Treatment with the direct DA receptor agonist, apomorphine (2 mg/kg, SC), caused a significant disruption of PPI, an effect that was observed in both PTX- and sham-treated rats. Treatment with the 5-HT1A receptor agonist, 8-OH-DPAT (0.5 mg/kg, SC), did not affect PPI in either group but caused a marked increase in ASR magnitude in sham-treated rats. Interestingly, this effect was blocked in PTX-treated rats. The present results suggest that local injection of PTX into the VTA causes an increased sensitivity to the behavioural effects of psychostimulants on acoustic startle and may also suggest that intact midbrain 5-HT1A receptors are essential for the effect of 5-HT1A agonists on acoustic startle.

To gain further insight into the operation of 5-HT autoreceptor-mediated feedback control of 5-HT biosynthesis in serotonergic nerve terminal areas, the effect of the 5-HT1B and the 5-HT1A receptor agonists, TFMPP and 8-OH-DPAT, respectively, were investigated in the rat central nervous system (CNS) using in vivo and in vitro neurochemical approaches. TFMPP suppressed 5-HT synthesis (5-HTP accumulation after decarboxylase inhibition) both in vivo and in vitro. In vivo, the 5-HT synthesis-suppressing effect of the drug (3.0 mg/kg, s.c.) proved resistant to either acute hemitransection or reserpine (5 mg/kg, i.p.; 90 min before) pretreatment. In vitro, in cortical, hippocampal and striatal slice preparations, TFMPP (0.1-10 microM) decreased 5-HT synthesis under basal and stimulated (30 mM K+) conditions, an effect which was unaltered by prior in vivo reserpine-induced 5-HT depletion but was attenuated in the presence of 5-HT1B receptor antagonists such as methiothepin, cyanopindolol or propranolol. The 8-OH-DPAT (0.1 mg/kg, s.c.)-induced decrease of 5-HT synthesis in vivo was abolished by hemitransection but resistant to acute reserpine pretreatment; 8-OH-DPAT (10 microM) did not decrease 5-HT synthesis in vitro. In conclusion, the present study confirms the importance of 5-HT autoreceptors in the feedback control of nerve terminal 5-HT biosynthesis. Specifically, our data indicate: (1) that the reduction of rat brain 5-HT synthesis after TFMPP is mediated by 5-HT1B autoreceptors located on the serotonergic axon terminals, and (2) that the effect is directly mediated and occurs independently of 5-HT neuronal firing and intact monoamine stores.

Intracerebral microdialysis was applied to monitor the neocortical extracellular levels of the aromatic amino acids phenylalanine, tyrosine, and tryptophan, the neurotransmitters dopamine (DA), noradrenaline (NA), and serotonin (5-HT), and the metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindole-3-acetic acid (5-HIAA) in rats with various forms of experimental hepatic encephalopathy (HE). The extracellular aromatic amino acid levels were clearly increased in acute, subacute, and chronic HE. No changes compared with controls in the neocortical DA release could be detected in the three experimental HE rat models investigated. The NA release showed a significant increase only in the subacute HE group. These data suggest that HE may not be associated with any major reduction of neocortical DA or NA release as previously suggested. In acute and subacute HE, decreased extracellular DOPAC but elevated 5-HIAA concentrations were seen. In chronic HE, elevations of both DOPAC and 5-HIAA were observed. Neocortical 5-HT release did not change in subacute and chronic HE, whereas it decreased in acute HE compared with control values. Significant increase in extracellular concentrations of 5-HIAA and of the 5-HIAA/5-HT ratio in the present study are in agreement with previously reported increases in 5-HT turnover in experimental HE. However, a substantially increased 5-HT turnover in experimental HE does not appear to be related to an increase in neuronal neocortical 5-HT release.

(R)-11-Hydroxyaporphine (2) and (R)-11-hydroxy-10-methylaporphine (3) were synthesized from natural morphine by using new, short, and efficient synthetic sequences. The dopaminergic and serotonergic effects of 2 and 3 were evaluated by use of in vitro and in vivo test systems. The results indicate that 3 is a potent, selective, and efficacious 5-HT1A receptor agonist. In contrast, 2 is a partial 5-HT1A receptor agonist of low potency which has affinity also for central D1 and D2A receptors. The differences in pharmacological profiles were rationalized by modeling of ligand-receptor interactions using homology-based receptor models of the 5-HT1A and D2A receptor binding site. The selective and pronounced serotonergic effects of 3 appear to be due to the C10-methyl group, which is accommodated by a lipophilic pocket in the 5-HT1A receptor. In contrast, the C10-methyl group of 3 is not accommodated by the binding site model of the D2A receptor.

Biodegradable controlled-release microsphere systems made with the biocompatible biodegradable polyester excipient poly(DL-lactide-co-glycolide) constitute an exciting new technology for drug delivery to the central nervous system (CNS). Implantable controlled-release microspheres containing dopamine (DA) or norepinephrine (NE) provide a novel means to compare DA- or NE -induced restitution of function in unilateral 6-hydroxydopamine lesioned rats. A suspension of 3 microL of DA- or NE-containing microspheres or empty microspheres was implanted in 2 sites of the DA denervated striatum of rats previously unilaterally lesioned with 6-hydroxydopamine. Contralateral-rotational behavior induced by apomorphine was used as an index of lesion success and, following implantation of the microspheres, also as an index of functional recovery. Interestingly, both DA- and NE-microsphere-implanted rats displayed a 30-50% reduction in the number of apomorphine-induced rotations up to 8 wk postimplantation. Rats implanted with empty microspheres did not demonstrate significant changes in contralateral rotational behavior. Behavioral studies following implantation of a mixture of DA and NE microspheres revealed an 80% decrease in the number of apomorphine induced rotations up to 4 wk. On conclusion of the studies, immunocytochemical examination revealed growth of DA and tyrosine hydroxylase immunoreactive fibers in the striatum of DA and NE microsphere-implanted rats. Functional behavior appeared to correlate with the degree of fiber growth. Preliminary electron microscopic studies showed signs of axonal sprouting in the vicinity of the implanted microspheres. No growth was noted in rats implanted with empty microspheres. This report reviews the abilities of both microencapsulated NE and DA to assure functional recovery and to promote DA fiber (re)growth in parkinsonian rats. This novel means to deliver these substances to the central nervous system could be of therapeutic usefulness in Parkinson's disease.

Electrophysiological studies suggest that 5-HT autoreceptor desensitization may be responsible for the delayed clinical efficacy of some antidepressant drugs, such as selective 5-HT reuptake inhibitors (SSRI) and certain MAO inhibitors (MAOI). In the present study we have used in vivo microdialysis to test this hypothesis. Rats were treated for 2 weeks with the antidepressant SSRI citalopram (5 mg/kg, s.c., b.i.d.). After 24 hr withdrawal, dialysis probes were implanted in the dorsal hippocampus (DH) and the frontal cortex (FCx). The rats then received as acute challenge, a 5-HT1A autoreceptor-active dose of the reference 5-HT1A agonist 8-OH-DPAT (0.025 mg/kg s.c.). The 8-OH-DPAT-induced changes in dialysate 5-HT from the DH and the FCx were monitored and taken as an index of autoreceptor sensitivity. Chronic citalopram and control animals responded similarly to 8-OH-DPAT with a drop of 5-HT of about 50-65%; no significant difference between the chronic citalopram and control groups were obtained, either in the DH or in the FCx. These data suggest that cell body 5-HT1A autoreceptors do not desensitize in response to repeated administration with antidepressant SSRI drugs such as citalopram.

1. Electrophysiological measurements of 5-HT neuronal activity report that repeated administration of 5-HT1A receptor agonists leads to desensitization of the 5-HT1A autoreceptor but this has not yet been detected in measurements of brain 5-HT synthesis or metabolism. Here we have determined the effect of repeated administration of 5-HT1A receptor agonists on brain 5-HT release using microdialysis. 2. Acute administration of the 5-HT1A receptor agonists buspirone (0.1–5 mg/kg s.c.) and ipsapirone (0.03–3 mg/kg s.c.) caused a dose-dependent decrease in 5-HT output in ventral hippocampus of the chloral hydrate anaesthetized rat. 3. The 5-HT response to buspirone (0.1 and 0.5 mg/kg s.c.) and ipsapirone (0.3 mg/kg s.c.) was significantly inhibited by pre-treatment with the 5-HT1/β-adrenoceptor antagonist pindolol (8–16 mg/kg s.c.). The 5-HT response to buspirone (0.1 mg/kg s.c.) and ipsapirone (0.3 mg/kg s.c.) was not blocked by pretreatment with a combination of the β1 and β2-adrenoceptor antagonists metoprolol and ICI 118,551 (4 mg/kg s.c.). 4. The effect of an acute challenge of buspirone (0.5 mg/kg s.c.) on 5-HT output in ventral hippocampus was not attenuated in rats treated twice daily for 14 days with 0.5 or 5 mg/kg s.c. buspirone compared to saline-injected controls. Similarly, the decrease in 5-HT induced by an acute challenge of ipsapirone (0.5 mg/kg s.c.) was not attenuated in rats treated twice daily for 14 days with 5 mg/kg s.c. ipsapirone. 5. In further experiments it was shown that the decrease in 5-HT induced in both ventral hippocampus and striatum by an acute challenge of the selective 5-HT1A receptor agonist 8-OH-DPAT (0.025 mg/kg s.c.), was not attenuated in rats treated twice daily for 14 days with 1 mg/kg s.c. 8-OH-DPAT. 6. Basal levels of 5-HT in hippocampal and striatal microdialysates of animals treated repeatedly with the 5-HT1A receptor agonists were not consistently altered relative to treatment controls. 7. In agreement with earlier studies measuring regional brain 5-HT synthesis and metabolism, the present microdialysis measurements of 5-HT release indicate that the inhibitory effect of 5-HT1A receptor agonists on presynaptic 5-HT function is maintained in rats treated repeatedly with the same drugs.

The effects of serotoninergic drugs on dopaminergic neurotransmission in the substantia nigra, the striatum and the limbic forebrain of rat have been investigated. The accumulation of 3-methoxytyramine (3-MT) following inhibition of monoamine oxidase with pargyline was used as an indirect measure of dopamine (DA) activity in vivo. The effects of the following serotoninergic drugs were tested: the 5-HT1A receptor agonist 8-OH-DPAT, the 5-HT1B receptor agonist trifluoromethyl-phenylpiperazine (TFMPP), CGS 12066B and RU 24969, the 5-HT1A/1B antagonist (±)pindolol, the 5-HT2/1C receptor antagonist ritanserin, the 5-HT2/1C receptor agonist DL-1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI), the 5-HT3 receptor antagonist BRL 43 694, the unselective 5-HT1 receptor antagonist methiothepin, and carbidopa+L-5-hydroxytryptophan (L5-HTP) to achieve a general, unselective stimulation of multiple 5-HT receptors. In the substantia nigra, carbidopa + 5-HTP treatment increased the 3-MT accumulation by 26% and decreased the DA concentration to 67% of controls, tentatively suggesting a 5-HTP-induced displacement of nigral DA. A minor, non dose-related reduction in nigral 3-MT was seen after the 5-HT1A receptor agonist 8-OH-DPAT. None of the other serotonin receptor acting drugs induced any pronounced effect on the nigral 3-MT accumulation. Taken together, the findings provide little support for the idea that one single 5-HT1 receptor subtype serves a modulatory function on DA activity in the substantia nigra. In the striatum and the limbic forebrain, trifluoromethylphenylpiperazine dose-dependently increased the 3-MT accumulation to maximally 200%–220% of controls. In the limbic forebrain also the highest dose of RU 24 969 (15 mg/kg) increased the 3-MT accumulation (78%), whereas in the striatum the lowest does of the drug (1.5 mg/kg) decreased it by 30%. The trifluoromethylphenylpiperazine-induced stimulation of 3-MT accumulation was not blocked by ritanserin. In the limbic forebrain, also DL-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane and carbidopa+Lr5-HTP treatment increased the 3-MT concentrations to 120% and 150% of controls, respectively. Paradoxically, methiothepin also induced an increase of the 3-MT accumulation in these brain regions, probably due to its DA receptor antagonism. None of the other serotoninergic drugs induced any pronounced effects on the 3-MT accumulation in these brain parts. The results may overall support the hypothesis that 5-HT1 does modulate the DA activity in the striatum and limbic forebrain, tentatively via 5-HT1B receptors in the striatum and 5-HT1B and 5-HT2 or 5-HT1C receptors in the limbic forebrain. It may be speculated therefore, that clinical application of 5-HT1 receptor-modulating drugs to influence central dopaminergic activity might be of therapeutical benefit, for example, in motor disorders like Parkinson's disease.

The putative existence of a functional alpha 2-adrenoceptor-mediated modulation of 5-HT release in vivo from serotonergic neuronal terminals in the ventral hippocampus was investigated using intracerebral microdialysis in chloral hydrate-anaesthetised rats. The alpha 2-adrenoceptor agonist clonidine (0.01-0.3 mg/kg, SC) dose-dependently decreased the 5-HT output. The response to clonidine was antagonized by systemic or local administration of the alpha 2-adrenoceptor antagonist idazoxan (0.1 mg/kg, SC, or 10 mumol/l, via the dialysis perfusion medium). Similarly, the 5-HT release-suppressing response to the thiazole alpha 2-adrenoceptor agonist jingsongling (0.1 mg/kg, SC) was blocked by idazoxan (0.1 mg/kg, SC). The mixed beta-adrenoceptor/5-HT1 receptor antagonist pindolol (8.0 mg/kg, SC) did not affect the clonidine-induced reduction of 5-HT release. Tyrosine hydroxylase inhibition by means of alpha-methyl-para-tyrosine (alpha-MT; 250 mg/kg, IP) caused a drastic reduction (greater than 80%) in dialysate 3,4-dihydroxyphenyl acetic acid (DOPAC) output but did not affect the 5-HT output per se over 3 h post-injection. Nor did the alpha-MT pretreatment prevent, but instead significantly enhanced, the 5-HT release-suppressing effect of clonidine. The results demonstrate that the release of 5-HT from serotonergic nerve terminals in rat ventral hippocampus in vivo is modulated by alpha 2-adrenoceptors, probably both by heteroreceptors on the axon terminals of the serotonergic neurones and by other alpha 2-adrenoceptor sites situated pre- and/or postsynaptic to the noradrenergic terminals. Our results also suggest that while functionally operative, these sites may receive little physiological tone, at least in chloral hydrate-anaesthetised rats.

Notwithstanding recent advances in the understanding of central serotonin (5-HT) function, important basic aspects of the control of brain 5-HT neuronal transmission remain incompletely elucidated. The present experiments addressed the putative mechanism(s) by which acute reserpine treatment stimulates cerebral 5-HT synthesis; also, such studies might shed further light on the relation between impulse flow and transmitter synthesis in central 5-HT neurons. Reserpine (5 mg/kg, i.p., 90 min before death) markedly elevated 5-HT synthesis [5-hydroxytryptophan (5-HTP) accumulation after decarboxylase inhibition by means of NSD 1015] in the limbic, striatal, and cortical rat brain parts. Hemitransection of ascending neuronal connections between the brainstem and the forebrain, performed immediately before reserpine injection, did not affect the 5-HT synthesis per se but completely prevented the drug response on the lesioned side. Similarly, systemic administration of the selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (0.1 mg/kg, s.c., 60 min before death) blocked the reserpine-induced elevation of brain 5-HT synthesis. These findings indicate that the reserpine-induced rise in axon terminal 5-HTP accumulation is dependent on intact (5-HT) neuronal impulse flow, which may or may not involve a transient impairment of somatodendritic 5-HT release and, in turn, autoreceptor tone.

The effects of heterosexual interactions on the in vivo rate of regional brain monoamine synthesis were examined in the male rat. To this end, the animals were administered an inhibitor of cerebral aromatic L-amino acid decarboxylase, NSD-1015 (100 mg.kg-1 i.p.), and regional brain DOPA and 5-HTP accumulation, over a 15-35 min period of sexual interaction, was compared with the DOPA or 5-HTP accumulation in time-matched home cage controls. Using the DOPA and 5-HTP accumulation as an estimate for the rate of tyrosine and tryptophan hydroxylase activity, respectively, the present results demonstrate: (1) an increased demand on catecholamine synthesis in the neocortex, the amygdala and in the septal area; and (2) an increased dopamine and serotonin synthesis in the ventral striatum (excluding the olfactory tubercle), and in the dorsal striatum.

We have compared the ability of the new putatively specific 5-HT1B receptor agonist CP-93,129 (3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b] pyrid-5-one) and the structurally related mixed 5-HT1A/5-HT1B receptor ligand RU 24969, to influence 5-HT release in brain in vivo, using microdialysis techniques in chloral hydrate-anaesthetised rats. CP-93,129 (3 or 10 microM, via the dialysis perfusion medium) caused a concentration-dependent and methiothepin (10 microM)-sensitive suppression of ventral hippocampal 5-HT output. The effect of RU 24969 on 5-HT output was dependent on whether or not the 5-HT reuptake blocker citalopram was present in the perfusion medium. Thus, RU 24969 (0.1 microM) induced a decrease, or an increase followed by a decrease (1 microM), in 5-HT output in the absence of citalopram, but monotonically decreased (1 microM) 5-HT release when citalopram (1 microM) was present. CP-93,129 decreased dialysate 5-HT in either condition. Our findings are consistent with the characterisation of CP-93,129 as a 5-HT1B receptor agonist, and may thus represent in vivo support for 5-HT1B autoreceptor-mediated feedback control of 5-HT release in the rat brain. The 5-HT1B selectivity of CP-93,129, and its lack of 5-HT reuptake blocking properties, suggests that the compound compares favourably with other purported 5-HT1B receptor agonists.

Recent electrophysiological studies, measurements of 5-HT synthesis and in vivo voltammetry recordings of 5-HT metabolism have suggested that serotoninergic neurones in the median raphe (MR) are less sensitive to 5-HT1A autoreceptor stimulation relative to those in the dorsal raphe (DR). To further study the putative differences in regulation between ascending 5-HT projections from the raphe nuclei we have used microdialysis to measure the release of 5-HT in ventral hippocampus, globus pallidus, dorsal hippocampus, frontal cortex, nucleus accumbens and medial septum, following systemic administration of the specific 5-HT1A agonist 8-OH-DPAT. The results show that the baseline output of 5-HT was similar in each of the areas studied. While 8-OH-DPAT decreased dialysate levels of 5-HT in all areas, the inhibition of 5-HT release seen in globus pallidus was significantly less marked compared to that observed in the other five regions. The results indicate that 5-HT1A autoreceptor-mediated control of 5-HT release is functional in all of the brain areas studied, including those receiving a preferential 5-HT innervation from the DR and MR. We find little evidence in support of the idea that brain 5-HT neuronal projections are heterogenous with respect to 5-HT1A autoreceptor regulation of 5-HT release; the globus pallidus, however representing a possible exception to this.

The somatodendritic 5-HT1A autoreceptor regulating 5-HT neuronal activity is currently poorly defined pharmacologically because there are no specific antagonists, but also because potent and stereoselective agonists are scarce. Moreover, there have been few, if any, attempts to specifically investigate structure-activity relationships for agonists acting at this site. Employing brain microdialysis techniques, we have examined the effects of the enantiomers of cis-8-hydroxy-1-methyl-2-(di-n-propylamino)tetralin (ALK-3; 0.01-0.3 mg/kg s.c.), its trans-1-methyl analogue (ALK-4; 0.3 mg/kg s.c.) and the pure enantiomers of the parent compound - 8-OH-DPAT (0.3 mg/kg s.c.) — in an attempt to address stereochemical agonist structure-activity requirements of 5-HT release-controlling 5-HT1A autoreceptors in brain. The cis-1-methylated 8-OH-DPAT analogue (+)ALK-3 was comparable to the parent compound in reducing the 5-HT output from rat ventral hippocampus. In comparison, both (−)ALK-3 and the racemic rans-diastereomer to ALK-3, ALK-4, were inactive, while the two stereoisomers of 8-OH-DPAT strongly reduced 5-HT release. Pretreatment with (−)pindolol (8 mg/kg s.c.), which has high affinity for 5-HT1A radioligand binding sites, blocked the reduction of hippocampal 5-HT release induced by a submaximally effective dose of (+)ALK-3. The direct intrahippocampal administration of (+)ALK3 (10 μM) via the perfusion medium did not affect 5-HT output. In summary, the data indicate that (+)ALK-3, like 8-OH-DPAT, is a very potent 5-HT receptor agonist which inhibits terminal 5-HT release in rat hippocampus, probably via activation of somatodendritic 5-HT1A autoreceptors. However, unlike 8-OH-DPAT, (+)ALK-3 is highly stereoselective and may therefore represent a useful probe in the further characterization of 5-HT1A receptor-mediated mechanisms and function. The present study defines some of the stereochemical requirements for 5-HT1A receptor interaction, emphasizing the importance of the receptor region complementary to the C1 and C2 carbons of the 8-OH-DPAT molecule. These findings contribute to the establishment of structure-activity relationships for the cell body 5-HT1A autoreceptors and might be of value in resolving structural features that determine agonist/antagonist activity at central 5-HT1A receptors. Finally, in conjunction with our recent finding that (+)ALK-3 is a partial agonist at postsynaptic 5HT1A receptors, the present study extends previous observations suggesting that pre- and postsynaptic 5-HT1A receptor populations differ in their characteristics.

The action of the dopamine autoreceptor agonist 3-PPP was examined in several putative models of antipsychotic action. Whereas this agent was capable of reducing d-amphetamine-induced locomotor stimulation at low doses, it was strikingly less potent in modifying conditioned avoidance responding and d-amphetamine-induced stereotypy. These findings are discussed in relation to a preferential action of 3-PPP on limbic dopamine mechanisms, and to the drug's potential as an antipsychotic agent devoid of extrapyramidal side-effects.

The effect of the (+)- and (-)-enantiomers of 3-PPP [conventional and atypical dopamine (DA)-receptor active agent, respectively] were investigated in a commonly used animal model of anxiety: the Vogel licking-conflict test. Low doses (≤0.5 mg/kg SC) of both 3-PPP enantiomers resulted in anti-conflict (=anxiolytic-like) actions in this test. (-)-3-PPP proved to be almost as potent as apomorphine in releasing the punished responding (minimum effective doses; (-)-3-PPP: 0.016, and apomorphine: 0.006 mg/kg SC), whereas (+)-3-PPP was about 10 times less effective than apomorphine. In the higher dose range (≥1.0 mg/kg), both 3-PPP enantiomers instead induced an apparent “pro”-conflict effect; i.e. decreased responding to a level significantly below baseline, thus resulting in a biphasic dose-response curve. Simple alterations in the animals' motivation to drink, in shock threshold or in motor capabilities did not seem to be major explanatory factors either for the anti- or for the “pro”-conflict effects. With regard to the latter, the possibility is discussed of an interaction between the experimental test situation and non conflict-related effects of the drugs, thus interfering with the punished drinking. The findings are interpreted within the concept that low doses of the 3-PPP enantiomers, in particular (-)-3-PPP, may attenuate anxiety-elicited increases in the neurotransmission in certain meso-cortical/limbic DA pathways, i.e. consistent with the previously shown preferentially “limbic” net antidopaminergic profile of action of (-)-3-PPP. The results support an active role for DA in conditions associated with anxiety and reinforce the view that novel putative anxiolytics might be found among selective DA-modulating agents like, e.g. the atypical DA receptor agonist (-)-3-PPP.

The weak neuroleptic agent buspirone, reported to possess ‘anxioselective’ and antidepressant actions in man, has pronounced effects on brain monoamine neurotransmission. Thus it is demonstrated that, apart from its dopamine antagonistic action, buspirone acts as a stimulant and a blocker of central 5-HT and α-adrenergic receptors, respectively. The relevance of these findings for the compound's clinical activity are discussed.

Eight monophenolic cis- and trans-4-n-propyl-1,2,3,4,4a,5,6,10b-octahydrobenzo[f]quinolines have been synthesized and tested for central dopamine- and serotonin-receptor stimulating activity, using biochemical and behavioral tests in rats. The trans-7-, -8-, and -9-hydroxy isomers all elicited central pre- and postsynaptic dopaminergic receptor stimulation, while the trans-10-hydroxy isomer was devoid of dopaminergic activity but instead showed central serotoninergic activity. In all four isomeric pairs, the trans isomers were consistently much more potent than their corresponding cis analogues. The apparent presynaptic selectivity of the dopaminergic cis isomer cis-9-hydroxy-4-n-propyl-1,2,3,4,4a,5,6,10b-octahydrobenzo[f]quinoline could not be confirmed due to the toxic properties of this compound. Central dopamine receptors (autoreceptors and postsynaptic receptors) can accept dopaminergic compounds with one of possibly two N-substituents being larger than n-propyl, if this substituent is properly oriented in relation to the rest of the molecule.

The two enantiomers of the putative centrally acting dopamine (DA) autoreceptor agonist 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP (Hjorth et al. 1981), were pharmacologically evaluated. An extensive series of biochemical and behavioural experiments unexpectedly revealed that both (+)- and (-)-3-PPP showed clear, but differential, effects on the DA receptors. Thus, (+)-3-PPP is a DA agonist with autoreceptor as well as postsynaptic receptor stimulatory properties. In contrast, although (-)-3-PPP similarly activates DA autoreceptors it acts concomitantly as an antagonist at postsynaptic DA receptors. Moreover, both behavioural and biochemical data on motor activity and DA synthesis and turnover suggest a preferential limbic action for the (-)-enantiomer. These results are discussed in terms of the dual antidopaminergic action of (-)-3-PPP coupled with anatomical differences in the feedback organisation in central (viz, limbic vs striatal) DA systems. It is suggested that compounds like (-)-3-PPP may be of potential clinical utility in the treatment of psychotic disorders, whilst lacking the seriously incapacitating motor dysfunctions produced by current neuroleptic therapy.