Role of striatal serotonin2A and serotonin2C receptor subtypes in the control of in vivo dopamine outflow in the rat striatum

Acute increases in monoamine release in the rat prefrontal cortex by the mGlu2/3 agonist LY379268 are similar in profile to risperidone, not locally mediated, and can be elicited in the presence of uptake blockade

Our recent work (Cartmell et al., Journal of Neurochemistry, 75 (2000) 1147-1154) demonstrated that systemic injection of the potent, selective mGlu2/3 receptor agonist, LY379268, acutely increased extracellular levels of dopamine, its metabolites DOPAC and HVA, and the 5-HT metabolite, 5-HIAA, in rat medial prefrontal cortex (mPFC). Here, we compared the acute effects of LY379268 with those of clozapine and risperidone (atypical antipsychotics) on extracellular levels of both dopamine and 5-HT in the mPFC of freely-moving rats. Uptake blockers were included to minimize metabolism of monoamines near the probe area. One hour after injection, LY379268 (10 mg/kg s.c.), clozapine (10 mg/kg s.c.) or risperidone (1 mg/kg s.c.) maximally increased dopamine by 224, 257 and 234% of basal levels. These effects were followed by maximal increases in DOPAC and HVA levels 2 to 3.5 hours after administration. LY379268, at 3 and 10 mg/kg s.c., and risperidone (1 mg/kg s.c.) also increased dialysate 5-HT to 169, 179 and 140% of basal levels and 5-HIAA to 144, 154 and 121% of basal levels, respectively. These neurochemical changes in the mPFC could not be mimicked when LY379268 (3 or 30 microM) was administered locally via the microdialysis probe. These data demonstrate that increases in extracellular monoamines in the rat prefrontal cortex evoked acutely by the mGlu2/3 agonist, LY379268, are similar in profile to risperidone, not locally mediated, and can be elicited in the presence of uptake blockade.

Differences in nicotine-induced dopamine release and nicotine pharmacokinetics between Lewis and Fischer 344 rats

Studies have shown a greater preference for the self-administration of drugs such as nicotine and cocaine in the Lewis rat strain than in the Fischer 344 strain. We examined some factors that could contribute to such a difference. The baseline level of extracellular dopamine in nucleus accumbens shell was about 3-times higher in Fischer rats than in Lewis rats (3.18 +/- 0.26 vs. 1.09 +/- 0.14 pg/ sample). Nicotine (50-100 microg/kg)-induced release of dopamine, expressed in absolute terms, was similar in the two strains. Dopamine release expressed in relative terms (as percent of baseline), however, was significantly greater in Lewis rats than in Fischer rats at 30 min after the first nicotine injection. We suggest that the relative increase is of more influence than the absolute level for determining preference; a lower physiological extracellular dopamine level thus represent a risk factor for increased preference. Amphetamine-induced dopamine release expressed in relative terms was not greater in the Lewis strain. In the initial time period of the microdialysis experiments, a sharper peak in nicotine-induced accumbal dopamine release in Lewis and a less but more sustained release in Fischer rats was observed. This release pattern paralleled the faster clearance of nicotine from blood of Lewis compared to Fischer rats. In tissue slices the electrically induced dopamine release was highest in the nucleus accumbens and lowest in the ventral tegmentum. A significant effect of nicotine was lowering the electrically induced release of dopamine in frontal cortex slices from Fischer brain and increasing this dopamine release in the ventral tegmentum of Lewis brain slices indicating that the ventral tegmentum, an area controlling dopamine release in the accumbens, is more responsive to nicotine in the Lewis rat. Nicotine levels tended to be more sustained in Fischer rats in different brain regions, although the difference in nicotine levels between the strains was not significant at any time period. Several factors contribute to nicotine preference, including the endogenous dopamine level, and the sensitivity of ventral tegmentum neurons to nicotine-induced dopamine release. Strain differences in pharmacokinetics of nicotine may also play a role.

Brain mediation of Anolis social dominance displays. II. Differential forebrain serotonin turnover, and effects of specific 5-HT receptor agonists

Serotonin (5-HT) functions are associated with social dominance status in diverse species, but to date the brain regions wherein 5-HT exerts such effects are uncertain. Here, we indexed 5-HT turnover in male Anolis carolinensis as the ratio of 5-HT to its metabolite, 5-hydroxy-indol-acetic acid, and also as the accumulation of the in vivo tracer 14C-alpha-methyl-tryptophan (14C-AMT). After patching one eye, displaying dominant animals increased both measures of 5-HT turnover in the forebrain hemisphere receiving display-evocative visual stimuli, compared to control, contralateral brain, whereas both 5-HT turnover indices were decreased when animals displayed submissively. In contrast, various non-displaying controls showed forebrain symmetry on both measures. Drugs that stimulate 5-HT(2C) receptors in mammals, and have 5-HT(2C)-like binding in A. carolinensis, evoked some elements of dominant display behaviors in non-dominant anole males and also activated dorsolateral basal ganglia as seen in non-medicated dominants when they display [Baxter et al., 2001]. Thus, acute changes in forebrain 5-HT output from baseline equilibrium, acting at 5-HT(2C)-like receptors, might effect some elements of the dominant vs. submissive male anoles' territorial displays. A mechanistic model of how this might occur is offered. Given similarities in 5-HT systems, forebrain functions, and territorial display routines, similar mechanisms might have similar functions in other amniotes, including primates.

Decreased striatal dopamine efflux after intrastriatal application of benzazepine-class D1 agonists is not mediated via dopamine receptors

Previous pharmacological studies have reported that striatal dopamine efflux is negatively modulated not only by presynaptic D2 dopamine autoreceptors but also by striatal D1 dopamine receptors. The present experiments employed in vivo microdialysis to further examine the ability of widely used benzazepine-class D1 agonists to modulate striatal dopamine efflux. In the present study, both the partial D1 agonist (+/-)-SKF 38393 (10 microM) and the full D1 agonist (+/-)-SKF 82958 (10 and 100 microM) significantly reduced striatal dopamine efflux during intrastriatal application. Intrastriatal application of the less active enantiomer, S(-)-SKF 38393 (10 microM) did not decrease striatal dopamine suggesting a selective receptor-mediated mode of action of (+/-)-SKF 38393. Additional experiments were conducted with the full D1 agonist (+/-)-SKF 82958 in order to characterize the receptor(s) mediating the observed decrease in dopamine efflux. Neither local application of the D1 antagonist R(+)-SCH 23390 (100 microM) nor local application of the selective D2 antagonist raclopride (5 microM) blocked the ability of (+/-)-SKF 82958 (10 microM) to decrease striatal dopamine efflux. However, intrastriatal application of the less selective D2 antagonist haloperidol (1 microM) did prevent the decrease in striatal dopamine efflux observed during intrastriatal (+/-)-SKF 82958 application. The present data suggest that the ability of intrastriatally applied benzazepine-class D1 agonists to decrease striatal dopamine efflux is receptor-mediated, but this action apparently is not mediated at D1 or D2 receptors. There is therefore no indication for an intrastriatal population of D1 receptors capable of modulating dopamine efflux.

M100,907, a selective 5-HT(2A) antagonist, attenuates dopamine release in the rat medial prefrontal cortex

Previous research has suggested that serotonin 5-HT(2A) receptors modulate the functioning of the mesocortical dopamine (DA) pathway. However, the specific role of 5-HT(2A) receptors localized within the medial prefrontal cortex (mPFC) is not known. The present study employed in vivo microdialysis to examine the role of this receptor in the modulation of basal and K(+)-stimulated (Ca(2+)-dependent) DA release. The selective 5-HT(2A) antagonist M100,907 was infused directly into the mPFC of conscious rats. This resulted in a concentration-dependent blockade of K(+)-stimulated DA release. Intracortical application of M100,907 also blocked increases in DA release produced by the systemic administration of the 5-HT(2A/2C) agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). These findings demonstrate that local 5-HT(2A) antagonism has an inhibitory effect on stimulated, Ca(2+)-dependent DA release. They suggest that cortical 5-HT(2A) receptors potentiate the phasic release of mesocortical DA.

An evaluation of the role of 5-HT(2) receptor antagonism during subchronic antipsychotic drug administration in rats

A widely postulated mechanism of action for the atypical profile of many novel antipsychotic drugs (APDs) is their relatively high affinity for 5-HT(2) receptors. The present study investigated motor function and striatal dopamine (DA) efflux and metabolism in rats given 21 daily injections of drugs that differed in 5-HT(2) affinity. These drugs included: risperidone (high 5-HT(2A/2C)/high D(2)), clozapine (high 5-HT(2A/2C)/low D(2)), haloperidol (low 5-HT(2A/2C)/high D(2)), haloperidol+ritanserin (selective 5-HT(2A/2C)), or vehicle. Rats injected with haloperidol (0.5 mg/kg) or haloperidol+ritanserin (0.5 mg/kg and 1.0 mg/kg, respectively) showed extreme catalepsy on day 1, but significantly decreased catalepsy when tested again on days 7 and 21. Acute or subchronic risperidone (0.05 or 0.5 mg/kg), clozapine (20 mg/kg), or vehicle did not induce significant catalepsy. Microdialysis performed 24 h after the last injection demonstrated that rats treated with risperidone, clozapine, or vehicle showed similar increases in DA efflux and metabolism following an acute injection of a selective DA D(2/3) antagonist (raclopride, 0.5 mg/kg). DA efflux showed an attenuated response to raclopride in the haloperidol alone group; this effect was less apparent in the haloperidol+ritanserin group. However, both of these groups showed a similar tolerance effect to the raclopride-induced increase in DA metabolites. These results suggest that the profile seen after subchronic risperidone more closely resembles clozapine than haloperidol. While ritanserin reduced the tolerance-like effects of haloperidol on striatal DA efflux, the overall results demonstrate that potent 5-HT(2) blockade alone may not entirely account for the distinctive profile of novel APDs.

Endogenous serotonin enhances the release of dopamine in the striatum only when nigro-striatal dopaminergic transmission is activated

In this study, we use in vivo microdialysis to investigate the influence of endogenous serotonin (5-HT) on striatal dopamine (DA) and 5-hydroxyidoleacetic acid (5-HIAA) efflux in both basal and activated conditions. The selective serotonin reuptake inhibitors citalopram and fluoxetine were used to mobilize endogenous 5-HT. In halothane-anaesthetized rats, citalopram (5 mg/kg, i.p.), administered either alone or in combination with the 5-HT(1A) receptor antagonist WAY 100635 (0.1 mg/kg, s.c.), while reducing striatal 5-HIAA outflow (-25 and -15%, respectively), had no effect on basal DA output. When locally applied into the striatum, citalopram had no effect at 1 microM concentration, but enhanced DA release after its perfusion at 25 and 100 mircroM concentrations (+27% and +67%, respectively). However, the injection of the neurotoxin 5,7-dihydroxytryptamine into the dorsal raphe nucleus, which markedly depleted 5-HT in the striatum, failed to modify the effect of 25 microM citalopram. In freely-moving rats, the intrastriatal infusion of citalopram or fluoxetine (1 microM each), had no effect on its own, but significantly enhanced the increase in DA outflow induced by the subcutaneous administration of 0.01 mg/kg haloperidol (+31% and +30% for citalopram and fluoxetine, respectively). These findings indicate that, in the striatum, endogenous 5-HT has no influence on DA release under basal conditions, but positively modulates DA outflow when nigro-striatal DA transmission is activated.

Identification and role of serotonin 5-HT1A and 5-HT1B receptors in primary cultures of rat embryonic rostral raphe nucleus neurons

Autoregulatory mechanisms affecting serotonin [5-hydroxytryptamine (5-HT)] release and synthesis during the early period of development were investigated in dissociated cell cultures raised from embryonic rostral rat rhombencephalon. The presence of 5-HT1A and 5-HT1B receptors in serotoninergic neurons was assessed using binding assays. The involvement of 5-HT1A and 5-HT1B receptors in the control of the synthesis and release of [3H]5-HT was studied using biochemical approaches with several serotoninergic receptor ligands. A mean decrease of 30% in [3H]5-HT synthesis and release was observed in the presence of 5-HT (10(-8) M), the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5HT1B/1A agonist 5-methoxy-3-(1,2,5,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969), the 5-HT1B agonist 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP-93,129), and the 5-HT(1D/1B) agonist sumatriptan. Inhibition of 5-HT synthesis and release induced by 8-OH-DPAT was blocked by chiral N-tert-butyl-3-[1-[1-(2-methoxy)phenyl]piperazinyl]-1-phenylpropionam ide dihydrochloride quaternary-hydrate (WAY 100135) (10(7) M) or methyl 4-[4-[4-(1,1,3-trioxo-2H-1,2-benzoisothiazol-2-yl)butyl]-1-p iperazinyl]-1Hindole-2-carboxylate (SDZ 216-525) (10(-7)M), and that of CP-93,129 was blocked by methiothepin (10(-7) M). Paradoxically, extracellular levels of [3H]5-HT increased in the presence of 8-OH-DPAT and RU 24969 at 10(-6) M. 5-HT uptake experiments showed that these two agonists interacted with the 5-HT transporter. 5-HT1 binding sites (620 fmol/mg of protein) and 5-HT1A (482 fmol/mg of protein) and 5-HT1B (127 fmol/mg of protein) receptors were detected in 12-day in vitro cell cultures. Experiments carried out with tetrodotoxin suggested that 5-HT1A receptors are located on nerve cell bodies, whereas 5-HT1B receptors are located on the nerve terminals. We concluded that autoregulatory mechanisms involving 5-HT1A and 5-HT1B autoreceptors are functionally mature in cells from rostral raphe nuclei during the early period of development.