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Previous work has suggested that activation of mGlu5 receptor augments NMDA receptor function and thereby may constitute a rational approach addressing glutamate hypofunction in schizophrenia and a target for novel antipsychotic drug development. Here, we report the in vitro activity, in vivo efficacy and safety profile of 5PAM523 (4-Fluorophenyl){(2R,5S)-5-[5-(5-fluoropyridin-2-yl)-1,2,4-oxadiazol-3-yl]-2-methylpiperidin-1-yl}methanone), a structurally novel positive allosteric modulator selective of mGlu5. In cells expressing human mGlu5 receptor, 5PAM523 potentiated threshold responses to glutamate in fluorometric calcium assays, but does not have any intrinsic agonist activity. 5PAM523 acts as an allosteric modulator as suggested by the binding studies showing that 5PAM523 did not displace the binding of the orthosteric ligand quisqualic acid, but did partially compete with the negative allosteric modulator, MPyEP. In vivo, 5PAM523 reversed amphetamine-induced locomotor activity in rats. Therefore, both the in vitro and in vivo data demonstrate that 5PAM523 acts as a selective mGlu5 PAM and exhibits anti-psychotic like activity. To study the potential for adverse effects and particularly neurotoxicity, brain histopathological exams were performed in rats treated for 4 days with 5PAM523 or vehicle. The brain exam revealed moderate to severe neuronal necrosis in the rats treated with the doses of 30 and 50 mg/kg, particularly in the auditory cortex and hippocampus. To investigate whether this neurotoxicity is mechanism specific to 5PAM523, similar safety studies were carried out with three other structurally distinct selective mGlu5 PAMs. Results revealed a comparable pattern of neuronal cell death. Finally, 5PAM523 was tested in mGlu5 knock-out (KO) and wild type (WT) mice. mGlu5 WT mice treated with 5PAM523 for 4 days at 100 mg/kg presented significant neuronal death in the auditory cortex and hippocampus. Conversely, mGlu5 KO mice did not show any neuronal loss by histopathology, suggesting that enhancement of mGlu5 function is responsible for the toxicity of 5PAM523. This study reveals for the first time that augmentation of mGlu5 function with selective allosteric modulators results in neurotoxicity.

Inhibition of phosphodiesterase 9 (PDE9) has been reported to enhance rodent cognitive function and may represent a potential novel approach to improving cognitive dysfunction in Alzheimer’s disease. PF-04447943, (6-[(3 S ,4 S )-4-methyl-1-(pyrimidin-2-ylmethyl)pyrrolidin-3-yl]-1-(tetrahydro-2 H -pyran-4-yl)-1,5-dihydro-4 H -pyrazolo[3,4- d ]pyrimidin-4-one), a recently described PDE9 inhibitor, was found to have high affinity ( Ki of 2.8, 4.5 and 18 nM) for human, rhesus and rat recombinant PDE9 respectively and high selectivity for PDE9 versus PDEs1–8 and 10–11. PF-04447943 significantly increased neurite outgrowth and synapse formation (as indicated by increased synapsin 1 expression) in cultured hippocampal neurons at low (30–100 nM) but not high (300–1000 nM) concentrations. PF-04447943 significantly facilitated hippocampal slice LTP evoked by a weak tetanic stimulus at a concentration of 100 nM but failed to affect response to the weak tetanus at either 30 or 300 nM, or the LTP produced by a theta burst stimulus. Systemic administration of PF-04447943 (1–30 mg/kg p.o.) dose-dependently increased cGMP in the cerebrospinal fluid 30 min after administration indicating target engagement in the CNS of rats. PF-04447943 (1–3 mg/kg p.o.) significantly improved cognitive performance in three rodent cognition assays (mouse Y maze spatial recognition memory model of natural forgetting, mouse social recognition memory model of natural forgetting and rat novel object recognition with a scopolamine deficit). When administered at a dose of 3 mg/kg p.o., which improved performance in novel object recognition, PF-04447943 significantly increased phosphorylated but not total GluR1 expression in rat hippocampal membranes. Collectively these data indicate that PF-04447943 is a potent, selective brain penetrant PDE9 inhibitor that increased indicators of hippocampal synaptic plasticity and improved cognitive function in a variety of cognition models in both rats and mice. Results with PF-04447943 are consistent with previously published findings using a structurally diverse PDE9 inhibitor, BAY73-6199, and further support the suggestion that PDE9 inhibition may represent a novel approach to the palliative remediation of cognitive dysfunction.

Introduction of selected amine containing side chains into the 3-position of N',2-diphenylquinoline-4-carbohydrazide based NK3 antagonists abolishes unwanted hPXR activation. Introduction of a fluorine at the 8-position is necessary to minimize unwanted hI(Kr) affinity and a piperazine N-tert-butyl group is necessary for metabolic stability. The lead compound (8m) occupies receptors within the CNS following oral dosing (Occ(90) 7 mg/kg po; plasma Occ(90) 0.4 microM) and has good selectivity and excellent PK properties.

d -Amino acid oxidase (DAO) degrades d -serine, a co-agonist at the NMDA receptor (NMDAR). Hypofunction of the NMDAR has been suggested to contribute to the pathophysiology of schizophrenia. Intriguingly, DAO has been recently identified as a risk factor for schizophrenia through genetic association studies. A naturally occurring mouse strain (ddY/DAO−) has been identified which lacks DAO activity. We have characterized this strain both behaviorally and biochemically to evaluate DAO as a target for schizophrenia. We have confirmed that this strain lacks DAO activity and shown for the first time it has increased occupancy of the NMDAR glycine site due to elevated extracellular d -serine levels and has enhanced NMDAR function in vivo. Furthermore, the ddY/DAO− strain displays behaviors which suggest that it will be a useful tool for evaluation of the clinical benefit of DAO inhibition in schizophrenia.

Compared to other rat strains, the Wistar-Kyoto rats show increased amount of REM sleep, one of the characteristic sleep changes observed in depressed patients. The aims of this study were firstly to validate a simple sleep stage discriminator and then compare the effect of antidepressants on suppression of rapid eye movement (REM) sleep in Wistar-Kyoto rats and an outbred rat strain (Sprague-Dawley). Rats were implanted with telemetry transmitters with electroencephalogram/electromyogram electrodes. Following recovery, the animals were orally dosed at light onset with either desipramine (20 mg/kg), fluoxetine (10 mg/kg), citalopram (10 or 40 mg/kg) or vehicle in a cross-over design. Every 12-s epoch was automatically scored as WAKE, NREM or REM sleep. Results confirm that Wistar-Kyoto rats show increased amount of REM sleep and decreased REM latency compared with Sprague-Dawley rats. All antidepressants significantly suppressed REM sleep in Sprague-Dawley rats, but only the high dose of citalopram suppressed REM sleep in Wistar-Kyoto rats. These findings suggest that the enhanced REM activity in Wistar-Kyoto rats is less sensitive to the effect of antidepressants and therefore does not provide any additional predictive validity for assessing antidepressant efficacy.

Incorporation of fluorine at the 4-position of an existing series of sulfonyl piperidine 5-HT2A antagonists gave compounds with increased selectivity over the IKr potassium channel. This work led to the identification of 3b, a compound that gave no increase in QTc in the anesthetized dog up to plasma levels as high as 148 μM. Furthermore, 3b has been shown to increase slow-wave sleep bout duration and to decrease the number of awakenings in rats, indicating the potential utility of 5-HT2A antagonists in the treatment of insomnia.

(2S)-2-{[(3,5-Diflurophenyl)acetyl]amino}-N-[(3S)-1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-1,4-benzodiazepin-3-yl]propanamide (compound E) is a gamma-secretase inhibitor capable of reducing amyloid beta-peptide (1-40) and amyloid beta-peptide (1-42) levels. In this study we investigated the effect of in vivo administration of compound E on guinea-pig plasma, CSF and cortical amyloid beta-peptide (1-40) concentration. Using repeated sampling of CSF, compound E (30 mg/kg p.o.) was shown to cause a time-dependent decrease in CSF amyloid beta-peptide (1-40) levels, which was maximal at 3 h (70% inhibition), compared to baseline controls. After 3 h administration, compound E (3, 10 and 30 mg/kg p.o.), reduced plasma, CSF and DEA-extracted cortical amyloid beta-peptide (1-40) levels by 95, 97 and 99%; 26, 48 and 78%; 32, 33, and 47%, respectively, compared to vehicle control values. In the same animals, compound E (3, 10 and 30 mg/kg p.o.) inhibited cortical gamma-secretase activity, determined ex vivo using the recombinant substrate C100Flag, by 40, 71 and 79% of controls, respectively. These data demonstrate the value of determining not only the extent by which systemic administration of a gamma-secretase inhibitor reduces amyloid beta-peptide, but also the inhibition of brain gamma-secretase activity, as a more direct estimate of enzyme occupancy.

The present study examined the potential role of tachykinin NK1 receptors in modulating immobilisation stress-induced increase of dopamine metabolism in rat medial prefrontal cortex. In agreement with previous studies, 20 min immobilisation stress significantly increased medial prefrontal cortex dopamine metabolism as reflected by the concentration of the dopamine metabolite dihydroxyphenylacetic acid (DOPAC). Pretreatment with the high affinity, selective, tachykinin NK1 receptor antagonist (3(S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)-phenylmethyl amino)-2(S)-phenylpiperidine) ((S)-GR205171, 10 mg/kg, s.c.), a dose that in ex vivo binding studies extensively occupied rat brain tachykinin NK1 receptors for approximately 60 min, significantly attenuated the stress-induced increase of mesocortical DOPAC concentration without affecting cortical DOPAC levels per se. In contrast, pretreatment of animals with the less active enantiomer (R)-GR205171 (10 mg/kg, s.c.), which demonstrated negligible tachykinin NK1 receptor occupancy ex vivo, failed to affect either basal or stress-induced DOPAC concentration in medial prefrontal cortex. Furthermore, pretreatment of animals with the benzodiazepine/GABAA receptor antagonist, flumazenil (15 mg/kg, i.p.), did not affect the ability of (S)-GR205171 to attenuate the increase of medial prefrontal cortex DOPAC concentration by acute stress. Results demonstrate that the selective tachykinin NK1 receptor antagonist, (S)-GR205171, attenuated the stress-induced activation of mesocortical dopamine neurones by a mechanism independent of the benzodiazepine modulatory site of the GABAA receptor.

It is well established that the NMDA receptor antagonists block hippocampal long-term potentiation and impair acquisition in the Morris watermaze task, although the role of individual NMDA receptor subtypes is largely unknown. In the present study, we compared the effects of (±)-CP-101,606, an antagonist selective for NMDA receptor NR1/NR2B subunit-containing receptors and the nonselective NMDA receptor antagonist MK-801, on acquisition in the Morris watermaze. Male hooded Lister rats were given 4 trials/day to find a fixed hidden platform submerged beneath the opaque water of the Morris watermaze. Twenty-four hours after the last acquisition trial, a ‘probe trial’ was conducted to assess the rat's spatial memory for the location of the hidden platform. Those rats treated with MK-801 (0.1 mg/kg, i.p.) 60 min prior to the acquisition and probe trials took significantly longer to find the hidden platform during training and spent significantly less time searching the platform's location during the probe trial than vehicle-treated rats. In contrast, 60-min pretreatment with (±)-CP-101,606 (60 mg/kg, p.o.), a dose that fully occupied hippocampal NR1/NR2B subunit-containing receptors, as determined by ex vivo NMDA receptor-specific [3H]ifenprodil binding immediately following watermaze experiments, had no effect on acquisition or the probe trial. These results suggest that antagonists selective for NR1/NR2B subunit-containing receptors may not impair spatial memory in rats in the Morris watermaze.

An alpha(1) adrenoceptor (alpha(1)-AdR) assay using [(3)H]prazosin binding in mouse brain is described which allows in vivo determination of central alpha(1)-AdR occupancy for ligands with alpha(1)-AdR affinity. Binding of [3H]prazosin in rat and mouse brain membranes in vitro was used to characterise the pharmacological profile of alpha(1)-AdRs in order to determine any potential species variations. Saturation and displacement studies yielded comparable affinity and pharmacological profile for [(3)H]prazosin binding in mouse and rat brain homogenates. These studies confirmed the absence of species variation for ligands in central alpha(1)-AdR pharmacology which is in good agreement with previous studies in rat brain. Subsequently, in vivo binding of [(3)H]prazosin in mouse whole brain was used to measure the occupancy of a number of AdR ligands. Timecourse studies revealed that a [3H]prazosin (5 mu Ci/mouse) pretreatment time of at least 20 min following intravenous (i.v.) administration was required for optimal specific binding. Ligands were administered systemically 40 min prior to i.v. administration of radiolabel. The alpha(1)-adrenoceptor ligands prazosin (ED(50)=0.15 mg/kg i.p.), benoxathian (0.52 mg/kg i.p.) and phentolamine (51 mg/kg i.p.) were all able to block in vivo [(3)H]prazosin binding from mouse brain. In addition, receptor occupancy values for a number of compounds including haloperidol (ED(50)=0.83 mg/kg s.c.), clozapine (2.2 mg/kg s.c.) and MDL-100907 [R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol], (10 mg/kg s.c.)], which possess high to moderate affinity at alpha(1)-adrenoceptors, were also determined. These results suggest that in the mouse, [(3)H]prazosin binding can be used to measure in vivo receptor occupancy of ligands with affinity at central alpha(1)-adrenoceptors.

Administration of the non-competitive NMDA receptor antagonists phencyclidine (PCP) (0.6-5 mg/kg s.c.) and MK-801 (0.1-0.8 mg/kg s.c. ) dose-dependently increased locomotor activity in the rat. Pre-treatment of rats with SB 221284 (0.1-1 mg/kg, i.p.) a 5-HT(2C/2B) receptor antagonist or SB 242084 (1 mg/kg, i.p.) a selective 5-HT(2C) receptor antagonist, doses shown to block mCPP induced hypolocomotion, significantly enhanced the hyperactivity induced by PCP or MK-801. Neither compound altered locomotor activity when administered alone. Furthermore, systemic administration of PCP (5 mg/kg s.c.) increased nucleus accumbens dopamine efflux in the rat to a maximum of approximately 220% of basal, 40-60 min after administration. Pre-treatment with the 5-HT(2C/2B) receptor antagonist SB 221284 (1 mg/kg, i.p.) and the 5-HT(2C) receptor antagonist SB 242084 (1 mg/kg i.p.) failed to affect nucleus accumbens dopamine efflux per se but significantly enhanced the magnitude and duration of the increase induced by PCP. However, the time course of the neurochemical and behavioural effects were qualitatively and quantitatively different, suggesting the potential involvement of other neurotransmitter pathways. Nevertheless, the present results provide behavioural and neurochemical evidence which demonstrate that, in the absence of effects per se, blockade of 5-HT(2C) receptors enhanced the activation of mesolimbic dopamine neuronal function by the non-competitive NMDA receptor antagonists PCP and MK-801.

Both pre-clinical and clinical studies suggest that additional treatment with 5-HT(1A) receptor antagonists may accelerate the antidepressant efficacy/onset of selective serotonin re-uptake inhibitors (SSRIs). Given that chronic SSRI treatment has been shown to desensitise 5-HT(2C) receptor mediated responses, we have used the rat social interaction test to determine if combined treatment with WAY 100,635, a selective 5-HT(1A) receptor antagonist, will accelerate this effect. In pairs of unfamiliar rats, acute administration of the 5-HT(2C) receptor agonist m-chlorophenylpiperazine (mCPP) or fluoxetine decreased the time spent in social interaction, responses which were reversed by the 5-HT(2C/2B) receptor antagonists SB 200646A and SB 221284. Similar reductions in social interaction were observed in rats treated with fluoxetine (10 mg/kg, i.p. daily) for 4, 7 and 14 days but was no longer apparent after 28 days of treatment. In contrast, only 7 days of combined treatment with WAY 100,635 (1 mg/kg/s.c./day) and fluoxetine were needed to reverse this response. The decrease in social interaction induced by an acute challenge of mCPP (1 mg/kg, i. p.) was also reduced after 6 days co-treatment with WAY 100,635 and fluoxetine. Thus, WAY 100,635 accelerates SSRI-induced desensitisation of 5-HT(2C) receptors, suggesting that this response might contribute towards the therapeutic effects of SSRIs in man.

Saturable radioligand binding of [3H]L-689,560 to the glycine site of the N-methyl-D-aspartate (NMDA) glutamate receptor complex was employed to determine the density of this receptor in putamen, caudate and nucleus accumbens taken post mortem from schizophrenic patients and matched controls. Receptor density was found to be significantly increased in putamen of schizophrenics (P=0.012), although no significant change was found in the other two areas studied. Further experiments following 21 days administration of haloperidol to rats provided no evidence that the increase was related to prior drug treatment.

Central administration of galanin dose-dependently (minimum effective dose, M.E.D. = 1 nmol) blocked the hypothermia induced by the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.5 mg/kg s.c.), in mice. This inhibitory effect was reversed by pretreatment with the galanin receptor antagonist galantide (0.3 nmol) and also by pretreatment with the ATP-sensitive potassium channel blockers glibenclamide (10 nmol) and gliquidone (10 nmol). The hypothermic response to 8-OH-DPAT was also blocked by the 5-HT1A receptor antagonist (N-(2,4(2-methoxyphenyl)-1-piperazinyl)ethyl-N-(2-pyridinyl)cyclohexane, (WAY 100,635, M.E.D. = 0.01 mg/kg s.c.), and the centrally acting muscarinic receptor antagonist scopolamine (M.E.D. = 10 mg/kg i.p.) but not the peripheral muscarinic receptor antagonist N-methylscopolamine. 8-OH-DPAT (0.5 mg/kg s.c.) also decreased cortical and hypothalamic 5-HT (5-hydroxytryptamine, serotonin) metabolism, an effect which was not blocked by pretreatment with galanin (0.3-3 nmol intracerebroventricular, i.c.v.). Neither did galanin (0.03-3 nmol/5 microliters i.c.v.) affect basal 5-HT metabolism in these brain regions. Furthermore, pretreatment in vitro of mouse cortical membranes with galanin (10 or 1000 nM) had no effect on 5-HT1A receptor affinity, Bmax or pharmacology determined using [3H]8-OH-DPAT. These results suggest that the inhibition of 8-OH-DPAT induced hypothermia by galanin is probably not mediated by an interaction with 5-HT1A receptors but more likely by blocking the indirect activation by 8-OH-DPAT of central cholinergic pathways involved in temperature regulation.

Corticotropin-releasing factor (CRF) receptors in IMR-32 human neuroblastoma cells were characterized after differentiation with 2.5 microM 5'-bromo-2'-deoxyuridine for 10 days. Scatchard analysis of [125I-Tyr0]ovine CRF binding revealed a high affinity binding site with a dissociation constant of 0.59 nM and a maximum binding capacity of 142 fmol/mg, the affinity of which was decreased by guanosine 5'-o-(3-thiotriphosphate). This binding was displaced in the following order of potency: human/rat CRF > ovine CRF > urotensin I > sauvagine > bovine CRF > [D-Phe12, Nle21,38, C alpha-MeLeu37]human/rat CRF-(12-41) > alpha-helical CRF-(9-41), indicative of the CRF1 receptor subtype. Functional coupling of this receptor was confirmed by CRF-induced increases in cyclic AMP, which were antagonised by alpha-helical CRF-(9-41) and [D-Phe12,Nle21,38,C alpha-MeLeu37] human/rat CRF-(12-41).

Intraperitoneal administration of the non-competitive NMDA receptor antagonists (5 R ,10 S )-(+)-5-methyl-10,11-dihydro-5 H -dibenzo[ a,d ]cyclohepten-5,10-imine (MK-801, 0.25 and 0.5 mg/kg) and 1-(1-phenylcyclohexyl)piperidine (PCP, 5 and 10 mg/kg) increased the extracellular concentration of acetylcholine in rat hippocampus but not striatum. In contrast, R -(+)-3-amino-1-hydroxypyrrolid-2-one (R(+)-HA-966, 30 and 60 mg/kg), an antagonist at the glycine modulatory site of the NMDA receptor, did not affect acetylcholine efflux in either region. (±)-3-[2-Carboxypiperazin-4-yl]-propyl-1-phosphonic acid ((±)CPP, 10 mg/kg) and cis -4-(phosphonomethyl)piperidine-2-carboxylic acid (CGS19755, 5 mg/kg), competitive antagonists at the glutamate agonist site of the NMDA receptor, marginally increased hippocampal acetylcholine efflux. Pretreatment with R(+)-HA-966 (60 mg/kg) or (±)CPP (10 mg/kg) attenuated the increase of hippocampal acetylcholine efflux by MK-801 (0.5 mg/kg). However, prior administration of CGS19755 (5 mg/kg) prolonged the MK-801-induced increase of hippocampal acetycholine efflux. Results demonstrate differential effects on hippocampal and striatal acetylcholine efflux of antagonists at different sites on the NMDA receptor complex and are discussed in relation to previously described effects of these drugs on mesolimbic dopamine function.

A series of 1,2,4-triacylpiperidines are shown to be potent antagonists of the NK-1 (Substance P) receptor with significant affinity for the L-type calcium channel as well. The latter property can be diminished by suitable substitution on the terminal nitrogen while maintaining good NK-1 receptor binding.

The effect of nitrogen substitution on the binding affinity of the piperidine-ether substance P antagonist L-733,060 for the hNK 1 receptor and L-type Ca 2+ channel is discussed.

Central administration of galanin in the mouse dose-dependently blocked the hypothermia induced by the muscarinic receptor agonist, 2-ethyl 8-methyl-2,8-diazospiro[4,5]decan-1,3-dion hydrobromide, RS86 (minimum effective dose, MED = 3 nmol) and the acetylcholinesterase inhibitor tetrahydroaminoacridine, (MED = 3 nmol). This inhibitory effect was reversed over the dose range (0.1, 0.3, 1, 3 nmol) by the galanin receptor antagonist galantide (MED = 0.3 nmol). Furthermore, the ATP-sensitive K + channel blockers glibenclamide (MED = 1nmol) and gliquidone (10 nmol) both prevented the inhibitory effects of galanin on RS86 induced hypothermia. Glibenclamide (10 nmol) also reversed the inhibitory effects of galanin on tetrahydroaminoacridine induced hypothermia. Preincubation of rat cortical membranes with galanin (10 nM, 1000 nM) in vitro had no effect on binding affinity, receptor number or pharmacology of the rat cortical muscarinic receptor. In contrast to the high affinity of glibenclamide, galanin only weakly displaced [ 3 H]glibenclamide binding in mouse whole brain homogenates (36% at 10 μM). These studies suggest that the inhibitory effect of galanin on cholinergically mediated hypothermia induced by RS86 and tetrahydroaminoacridine may be exerted via an action at ATP-sensitive K + channels but is unlikely to be acting directly at the site labelled by [ 3 H]glibenclamide.

The concurrent release of endogenous ACh and GABA from the retina (in the presence of physostigmine) was measured using either an eye-cup preparation in rabbits anaesthetized with urethane or isolated rabbit retinas. There was a spontaneous resting release of ACh and GABA from the dark adapted retina of ca 5 and 160 pmol min-1 respectively. Stimulation of the initially dark adapted retina in vivo with flickering light (0.1-20 Hz) increased the release of ACh by up to 5 times the spontaneous resting release but did not cause a detectable increase in GABA release. The maximum light-evoked release of ACh was about 24 pmol min-1/retina and occurred at a frequency of 10 Hz. However, the maximum release of ACh per flash occurred at 0.1 Hz at which frequency the average ACh release per flash from one amacrine cell was ca 2.35 x 10(-18) mol. Exposure of the retina to the potent inhibitors of GABA uptake, SKF89976A and SKF100330A markedly reduced the resting release of ACh and abolished the light-evoked release of ACh but did not enable a light-evoked release of GABA to be detected. Bicuculline blocked the inhibitory actions of both SKF89976A and SKF100330A on ACh release but the combination of bicuculline and uptake inhibitor did not result in a light-evoked release of GABA. In contrast, KCl (20 mM) applied locally to the retina in vivo resulted in the release of both ACh and GABA (61 and 2.6-fold respectively). KCl (20 mM) also evoked large increases in ACh and GABA release from isolated rabbit retinas in room light (13.5 and 3.4-fold respectively). The K-evoked release of ACh and GABA from the rabbit retina both in vivo and in vitro was calcium dependent. These experiments are the first in which endogenous ACh and GABA release from the retina have been simultaneously measured and suggest that the release mechanisms for these transmitters are fundamentally similar.

A series of 3-(4-piperidinyl)- and 3-(8-aza-bicyclo[3.2.l]oct-3-yl)-2-phenyl-1 H -indoles have been prepared and evaluated as ligands for the h5-HT 2A receptor. 3-(8-Phenethyl-8-aza-bicyclo[3.2.l]oct-3-yI)-2-phenyl-1 H -indole is a high-affinity (1.2 nM), selective (>800 fold over h5-HT 2C and hD 2 receptors) antagonist at the h5-HT 2A receptor with oral bioavailability in rats.

Hippocampal and striatal extracellular acetylcholine levels were monitored in conscious rats using the technique of intracerebral dialysis. Scopolamine (1 mg/kg, i.p.) resulted in a marked increase of acetylcholine release in both regions whereas the thyrotropin-releasing hormone (TRH) analogue MK-771 (2.5, 5 and 10 mg/kg, i.p.) only increased acetylcholine release in the hippocampus. The relevance of the present findings in relation to the known cognitive enhancing effects of TRH analogues is discussed.

The glutamate hypothesis of schizophrenia suggests that NMethyl-D-Aspartic acid (NMDA) receptor function is dysfunctional in schizophrenia and may be responsible for the positive, negative and cognitive symptoms found in the disorder. One approach to modulate glutamate function is via activation of mGlu5 receptors which enhances NMDA receptor function. mGlu5 positive allosteric modulators enhance the potency of glutamate for the orthosteric site in the mGlu5 receptor subtype and are active in animal models of positive and negative symptoms. Moreover these molecules increase the cortical and hippocampal expression of proteins related to synaptic plasticity and reverse cognitive deficits induced by NMDA receptor antagonists. This overall preclinical profile suggests that mGlu5 positive allosteric modulators may be beneficial across all symptom domains in schizophrenia.

The pharmacological and neurochemical bases of hyperphagia induced by the serotonin agonist 8-OH-DPAT were examined. In addition, the possible therapeutic potential of 8-OH-DPAT and related drugs in the treatment of anorexic pathology was assessed in an animal model of anorexia (as induced by acute immobilization stress). In normal rats 8-OH-DPAT elicited feeding after peripheral injection and after intracerebral application to the brainstem raphe nuclei. Feeding elicited by peripheral injection of the drug was attenuated by pretreatment with the serotonin synthesis inhibitor para-chlorophenylalanine. Following a hyperphagic dose of 8-OH-DPAT, brain serotonin metabolism was reduced, particularly in midbrain and pons-medulla. Our interpretation of these data is that 8-OH-DPAT elicits feeding via an agonist action on serotonin autoreceptors in the raphe nuclei. These receptors are probably of the 5-HT 1A subtype as 8-OH-DPAT has a high affinity for this receptor and other putative 5-HT 1A gonist (i.e. buspirone, TVX Q 7821) also elicit feeding. In contrast, putative 5-HT 1B agonists (i.e. RU-24969 and quipazine) decrease feeding and cause anorexia. 8-OH-DPAT and other 5-HT 1A agonists attenuated the anorexia and body weight loss caused by immobilization stress. Therefore, it seems possible that 5-HT 1A agonists may be clinically useful in the treatment of anorexia.

Exposure to footshock (1 mA) for 30 sec induced a marked analgesia that was enhanced by pretreatment with the 5HT synthesis inhibitor, p-chlorophenylalanine, and attenuated by the 5HT releasing drugs p-chloroamphetamine and fenfluramine, by the 5HT re-uptake inhibitor, fluoxetine and by the 5HT agonists, 5-methoxy-N,N-dimethyltryptamine and MK212. However, agonists, quipazine and trifluoromethylphenylpiperazine, with greated reported affinities for 5HT binding sites on rat brain membranes than MK212 were without effect as were the antagonists metergoline, methysergide, cyproheptadine, mianserine and methiothepin. The specific opioid antagonist naloxone was also without effect. The results in general indicate that analgesia induced by brief footshock (1 mA, 30 sec) is inversely related to 5HT availability but thereis little evidence of involvement of known 5HT receptors.