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Locally administered angiotensin IV causes a dose-dependent increase of the dopamine levels in the striatum of the rat. The aminopeptidases insulin-regulated aminopeptidase (IRAP) and/or aminopeptidase N (AP-N) are proposed to be involved in this effect since both enzymes are inhibited by angiotensin IV. In agreement with this hypothesis we demonstrate that by using the AP-N selective inhibitor 7B, about 60% of the aminopeptidase activity in striatal membranes could be attributed to AP-N (pK(i)=9.20). Higher concentrations of 7B are capable of inhibiting IRAP as well (pK(i)=7.26). Interestingly, in vivo, inhibition of IRAP or AP-N activity does not appear to be involved in the angiotensin IV-mediated effect in the striatum since 7B itself is not capable to induce dopamine release such as observed with angiotensin IV. However, 7B at a concentration selective for inhibition of AP-N (100 nM) potentiates the angiotensin IV-mediated increase of dopamine, suggesting that inhibition of AP-N lengthens the half-life of angiotensin IV. On the other hand, inhibition of both AP-N and IRAP by perfusion of 500 nM 7B completely abolishes the effect of angiotensin IV. We therefore hypothesize that the effect of angiotensin IV on dopamine release in the striatum is mediated via activation of IRAP and/or AP-N, possibly acting as receptors for angiotensin IV.

We investigated the possible neuroprotective effect of the dopamine (DA) receptor agonist R-apomorphine (R-APO) within the striatal 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease. In one group of rats, R-APO administration (10 mg/kg/day, s.c.) started 15 min before 6-OHDA-injection. In a second group, R-APO administration started 24 h after lesion induction. Both groups received R-APO chronically for 11 days. Testing was carried out 2 weeks post-lesioning. R-APO treatment, whether started before or after the lesion induction, significantly reduced both the amphetamine-induced ipsiversive rotation and the size of the lesion at the level of the substantia nigra. Moreover, the dopamine cell shape and size resembled that observed in intact animals. R-APO treatment had no effect on the number of cells in the substantia nigra of intact rats, but significantly increased the number of cells in the ventral tegmental area (VTA), suggesting selective neurotrophic properties of R-APO in this region. R-APO treatment significantly attenuated the 6-OHDA-induced striatal DA depletion and DOPAC/DA ratios were normalized. Finally, an acute injection of 10 mg/kg R-APO was unable to scavenge 6-OHDA or MPP(+)-induced hydroxyl radicals as determined with the in vivo salicylate trapping technique. These data provide further evidence of the neurorescuing properties of R-APO. At least at the dose used in this study, this effect possibly occurs via mechanisms other than scavenging of hydroxyl radicals. In intact rats, we also show neurotrophic effects of the R-APO treatment. These seem to be limited to the VTA.

The present microdialysis study has examined whether exercise-elicited increases in brain tryptophan availability (and in turn 5-HT synthesis) alter 5-HT release in the hippocampus of food-deprived rats. To this end, we compared the respective effects of acute exercise, administration of tryptophan, and the combination of both treatments, upon extracellular 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels. All rats were trained to run on a treadmill before implantation of the microdialysis probe and 24 h of food deprivation. Acute exercise (12 m/min for 1 h) increased in a time-dependent manner extracellular 5-HT levels (maximal increase: 47%), these levels returning to their baseline levels within the first hour of the recovery period. Besides, exercise-induced increases in extracellular 5-HIAA levels did not reach significance. Acute administration of a tryptophan dose (50 mg/kg i.p.) that increased extracellular 5-HIAA (but not 5-HT) levels in fed rats, increased within 60 min extracellular 5-HT levels (maximal increase: 55%) in food-deprived rats. Whereas 5-HT levels returned toward their baseline levels within the 160 min that followed tryptophan administration, extracellular 5-HIAA levels rose throughout the experiment (maximal increase: 75%). Lastly, treatment with tryptophan (60 min beforehand) before acute exercise led to marked increases in extracellular 5-HT and 5-HIAA levels (maximal increases: 100% and 83%, respectively) throughout the 240 min that followed tryptophan administration. This study indicates that exercise stimulates 5-HT release in the hippocampus of fasted rats, and that a pretreatment with tryptophan (at a dose increasing extracellular 5-HT levels) amplifies exercise-induced 5-HT release.

Microdialysis was used to study the effect of M1 and M2 selective agonists and antagonists on striatal dopamine release and metabolism. Microdialysis probes were implanted, under anesthesia, in the left and the right striatum of the normal rats and in the normal and denervated striatum of the nigral 6-hydroxydopamine-lesioned rats. Dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined by liquid chromatography and electrochemical detection. The different drugs were infused through the dialysis probe during 40 min. Pirenzepine (5 microM), a selective M1 antagonist, produced a significant decrease in DA release in the normal and the 6-hydroxydopamine-lesioned rats, with no significant difference between both groups. Methoctramine, a selective M2 antagonist, produced a dose-dependent increase in DA release between 20 and 200 microM in the normal rats, with no significant effect on DOPAC and HVA. Infusing 75 microM methoctramine produced a significant increase in DA release with a more pronounced effect in the intact animals compared to the 6-hydroxydopamine-lesioned animals. The non-selective agonist carbachol produced a decrease in dopamine release after infusion of 50 microM (M2 effect) and an increase in dopamine release after infusion of 50 mM (M1 effect) in the normal rats. Infusing 50 microM carbachol in the denervated striatum, produced a slight increase in DA release. Our data suggest that presynaptic M1-muscarinic receptors enhance and M2-muscarinic receptors inhibit DA release in the striatum of the rat; and that 3 weeks after 6-hydroxydopamine lesioning there may be a normalisation of the number of M1-receptors with a loss of M2-receptors at the denervated side.

The regional distribution of [3H]idazoxan and [3H]rauwolscine was studied autoradiographically in human brain. [3H]Idazoxan binds with high affinity to alpha 2 adrenoceptors as well as to non-adrenergic sites (NAIBS). [3H]Rauwolscine, besides binding to alpha 2 adrenoceptors, also binds to 5-HT1A receptors. Both radioligands labelled the same population of alpha 2 adrenoceptors, defined as the epinephrine-displaceable binding component. The highest densities of alpha 2 adrenoceptors occur in the leptomeninges, cerebral cortex and claustrum; lower densities were visualised in the basal ganglia, thalamus, pons, substantia nigra, cerebellum and medulla oblongata; no alpha 2 adrenoceptors were detected in amygdala and nucleus ruber. NAIBS were present in all the examined brain areas, with the highest densities found in the basal ganglia and substantia nigra. The finding that certain brain regions, such as the amygdala, contained NAIBS but no detectable alpha 2 adrenoceptors, suggests that the binding sites are independent from each other. The regional distribution of 5-HT1A receptors labelled by [3H]rauwolscine is in agreement with previous studies using [3H]8-OH-DPAT.

Norepinephrine, epinephrine, dopamine, serotonin and their major metabolites were measured by high-performance liquid chromatography with electrochemical detection in 49 regions of the human brain. The regional distribution of the different monoamines in the subcortical areas was similar to previous reports. We report here the distribution pattern of the 4 monoamines observed in the cerebral cortex. Regional differences in concentration were observed for norepinephrine, epinephrine and serotonin, with high concentrations in the frontal and parietal regions. However, no regional difference in dopamine concentrations was detected. The possible role of norepinephrine and serotonin as conventional transmitters, and of dopamine and epinephrine as neurotransmission modulators is discussed.

Dopaminergic pathways to the cerebral cortex may be involved in cognitive function. We examined the effect of aging on the D1 dopamine receptors, and their high-agonist affinity (RH) sites, in postmortem human frontal cortex (n = 32; age range, 19-88 years). With aging, there was a significant decrease in the densities of the D1 dopamine receptors, and their RH sites, in human frontal cortex. The age-related reduction of cortical dopaminergic neurotransmission might contribute to the decline in cognitive abilities of elderly persons.

High-affinity binding sites for the D1 dopamine receptor antagonist [3H]SCH 23390 were identified in membranes from human putamen, frontal cortex and calf retina. In frontal cortex binding not only occurred to D1 but also to 5-HT2 receptors. In retina and putamen no binding to 5-HT2 receptors was detected. All further binding experiments in frontal cortex were carried out in the presence of 20 nM mianserin to prevent binding to 5-HT2 receptors. In the 3 tissues, antagonist competition curves were monophasic, whereas competition curves with the agonist dopamine revealed the presence of two binding sites, one having high affinity (RH) (32% in retina, 28% in putamen, and 15% in frontal cortex) and the other having low affinity (RL). In retina, the addition of 100 microM GTP caused a full conversion of RH into RL. In contrast, in frontal cortex, RH sites were not altered by 400 microM GTP or 100 microM 5'-guanylyl-imidodi-phosphate (Gpp(NH)p). In putamen, both guanine nucleotides provoked only a partial conversion of RH into RL. Dopamine (100 microM) produced a 220% and 56% increase in cAMP production in respectively retina and putamen homogenates, while no increase was observed in frontal cortex homogenate. These data may suggest that not all D1-receptors are coupled to the adenylate cyclase system.

Human brain contains two subtypes of D1 dopamine receptors, which both exist under high- (RH) and low-agonist affinity (RL) sites, but can be distinguished on the basis of the ability of GTP to convert RH into RL. The amygdala contains exclusively GTP-sensitive (GS) D1 receptors, frontal cortex exclusively GTP-insensitive (GI) D1 receptors, and putamen both GS and GI receptors. In contrast with controls, we were unable to detect RH sites in amygdala from patients with Huntington disease (HD). The amount of RH sites in normal and HD frontal cortex were similar. In putamen, the GTP-induced partial conversion of RH into RL, observed in controls, was absent in HD. The results suggest that coupling of GS-D1 receptors with the guanine nucleotide binding protein Gs may be deficient in HD.

In human brain, regulation of agonist binding to the D2 dopamine receptors by guanine nucleotides is different between several regions. In membranes of the anterior pituitary lobe, agonist binding is fully sensitive to GTP or Gpp(NH)p, whereas it is resistant in membranes of globus pallidus. Both guanine nucleotide-sensitive (G-S) and -insensitive (G-I) receptors are found in membranes of the striatum. The G-S and G-I type receptors display similar affinities for antipsychotic drugs of different classes, suggesting that they only differ in their effector-coupling system. We investigated the D2 receptors in striatal membranes of postmortem human brains in which corticostriatal pathways were unilaterally destroyed by an infarction. Compared to the intact side, D2 receptor densities in striata from the lesioned side were reduced by 46-65%, whereas the densities of the muscarinic cholinergic receptors, dopamine uptake sites, and D1 dopamine receptors were unchanged. In the deafferented striata all G-I, but also a substantial number of G-S receptors were lost, suggesting that both receptor subtypes are present on corticostriatal nerve terminals.