Serotonin denervation enhances responsiveness of presynaptic dopamine efflux to acute clozapine in nucleus accumbens but not in caudate-putamen

Differential effects of intra-midbrain raphe and systemic 8-OH-DPAT on VTA self-stimulation thresholds in rats

RATIONALE

Intra-median raphe nucleus (MRN) administration of the 5-HT(1A) receptor agonist 8-OH-DPAT decreases lateral hypothalamic self-stimulation thresholds and is reported to have biphasic effects following systemic administration. These experiments attempted to extend the previous findings to mesolimbic pathway self-stimulation at ventral tegmental area (VTA) electrodes.

OBJECTIVES

This study was conducted to provide comparative data for systemic and intra-dorsal raphe nucleus (DRN) and intra-MRN effects of 8-OH-DPAT on VTA self-stimulation.

METHODS

Male Sprague-Dawley rats with VTA electrodes were trained to respond for electrical stimulation. Systemic and intra-midbrain raphe 8-OH-DPAT effects on rate-frequency thresholds were measured. Systemic administration of WAY 100635 was used to confirm 5-HT(1A) receptor mediation of 8-OH-DPAT effects.

RESULTS

8-OH-DPAT (0.003-0.3 mg kg(-1) SC) increased rate-frequency thresholds and decreased maximal response rates. WAY 100635 alone (0.0125-0.1 mg kg(-1) SC) did not alter these measures. Intra-DRN and intra-MRN 8-OH-DPAT (5.0 microg) decreased rate-frequency thresholds without altering maximal response rates. Intra-DRN 8-OH-DPAT (0.1-5.0 microg) induced a slight decrease and intra-MRN 8-OH-DPAT a slight increase in locomotor activity. WAY 100635 (0.1 mg kg(-1)) blocked effects of 8-OH-DPAT on VTA self-stimulation.

CONCLUSION

These results confirm threshold-decreasing effects of intra-MRN 8-OH-DPAT and extend this to the DRN and to VTA thresholds. Monophasic dose dependent increases in VTA thresholds following systemic 8-OH-DPAT are not equivalent to reports for hypothalamic self-stimulation. Differences between studies may be attributable to stimulation site and/or differences in threshold measurement procedures. Effects of WAY 100635 in this study indicate 5-HT(1A) receptor mediation of these 8-OH-DPAT effects.

The latent inhibition model of schizophrenia: further validation using the atypical neuroleptic, clozapine

Latent inhibition (LI) refers to retarded conditioning to a stimulus that has been repeatedly presented without reinforcement. LI is impaired in schizophrenia patients and in rats treated with amphetamine. Neuroleptic drugs produce two effects in this test paradigm: antagonism of amphetamine-induced disruption of LI, and enhancement of LI when administered on their own. The present experiments tested the effects of the atypical neuroleptic, clozapine, on LI. The experiments used a conditioned emotional response procedure in rats licking for water, consisting of three stages: preexposure, in which the to-be-conditioned stimulus (tone) was repeatedly presented without reinforcement; conditioning, in which the preexposed stimulus was paired with reinforcement (foot shock); and test, in which LI was indexed by animals' degree of suppression of licking during tone presentation. In experiments 1 and 2, the effects of 5.0 and 10.0 mg/kg clozapine on LI were assessed following 20 or 10 tone preexposures, respectively. Experiments 3 and 4 used 40 preexposures and investigated antagonism of amphetamine-induced disruption of LI by 5.0 and 10.0 mg/kg clozapine, respectively. The results demonstrated that clozapine possesses a neuroleptic profile in the LI model, namely, it facilitates the development of LI and antagonizes amphetamine-induced disruption of LI.

Effects of chronic neuroleptic treatment on dopamine release: insights from studies using 3-methoxytyramine

Antipsychotic medications appear to exert their therapeutic effects by blocking D2 receptors. While D2 blockade occurs rapidly, reduction in psychotic symptoms is often delayed. This time discrepancy has been attributed to the relatively slow development of depolarization inactivation (DI) of dopaminergic neurons. The reduced firing rates associated with DI has been hypothesized to reduce dopamine release and thus psychotic symptoms. Studies assessing changes in dopamine release during chronic neuroleptic treatment, using microdialysis and voltammetry, have been inconsistent. This may be due to methodological differences between studies, the invasive nature of these procedures, or other confounds. To investigate the effects of DI on dopamine release, 3-MT accumulation, an index of dopamine release that does not involve disruption of brain tissue, was measured during acute and chronic neuroleptic treatment. These results are compared with those using other techniques. 3-MT levels remained elevated after chronic treatment, suggesting that DI does not markedly reduce release. Regulation of dopamine release during DI was examined using two techniques known to block dopamine neuronal impulse flow. 3-MT levels were markedly reduced by both, implying that DI does not alter the portion of dopamine release mediated by neuronal impulse flow. Overall, studies to date suggest that the delayed therapeutic effects of neuroleptics are not due to reductions in impulse dependent dopamine release. Recent studies using a neurodevelopmental animal model of schizophrenia have pointed to altered pre- and post-synaptic indices of dopamine neurotransmission. The results suggest that neuroleptics may exert their therapeutic effects, in part, by limiting the fluctuations in dopamine release, and raise new issues for future research.

Mechanisms of action of atypical antipsychotic drugs: a critical analysis

Various criteria used to define atypical antipsychotic drugs include: 1) decrease, or absence, of the capacity to cause acute extrapyramidal motor side effects (acute EPSE) and tardive dyskinesia (TD); 2) increased therapeutic efficacy reflected by improvement in positive, negative, or cognitive symptoms; 3) and a decrease, or absence, of the capacity to increase prolactin levels. The pharmacologic basis of atypical antipsychotic drug activity has been the target of intensive study since the significance of clozapine was first appreciated. Three notions have been utilized conceptually to explain the distinction between atypical versus typical antipsychotic drugs: 1) dose-response separation between particular pharmacologic functions; 2) anatomic specificity of particular pharmacologic activities; 3) neurotransmitter receptor interactions and pharmacodynamics. These conceptual bases are not mutually exclusive, and the demonstration of limbic versus extrapyramidal motor functional selectivity is apparent within each arbitrary theoretical base. This review discusses salient distinctions predominantly between prototypic atypical and typical antipsychotic drugs such as clozapine and haloperidol, respectively. In addition, areas of common function between atypical and typical antipsychotic drug action may also be crucial to our identification of pathophysiological foci of the different dimensions of schizophrenia, including positive symptoms, negative symptoms, and neurocognitive deficits.

Regional differences in the effect of haloperidol and atypical neuroleptics on interstitial levels of DOPAC in the rat forebrain: an in vivo microdialysis study

The effect of 'typical' and 'atypical' neuroleptics on interstitial levels of the dopamine metabolite 3,4- dihydroxyphenylacetic acid ([DOPAC]e) in the dorsolateral striatum (DLSt), the nucleus accumbens (NAc) and the medial prefrontal cortex (PFC) was investigated in awake rats by use of the microdialysis technique. All neuroleptics increased [DOPAC]e in the DLSt, NAc and in PFC. However, the 'atypical' neuroleptics clozapine, risperidone, sertindole and NNC 22-0031 showed an apparent cortical selectivity by preferentially elevating [DOPAC]e in the PFC compared with the DLSt and NAc, a feature which was not observed with the 'typical' neuroleptic haloperidol. Our data suggest that 'atypical' neuroleptics can be differentiated from the 'typical' neuroleptic, haloperidol, with respect to their ability to increase [DOPAC]e in PFC relative to DLSt and NAc.

Extrapyramidal symptoms with selective serotonin reuptake inhibitors

BACKGROUND

Several case reports in the literature suggest that selective serotonin reuptake inhibitors can produce extrapyramidal symptoms.

METHODS

Computerised literature searches were used to identify reports on extrapyramidal symptoms and serotonin reuptake inhibitors. Subsequently, manual searches were made for articles in which there was any indication of the mechanisms responsible for these extrapyramidal symptoms.

RESULTS

Only a few reports could be identified in which serotonin reuptake inhibitors were implicated in extrapyramidal symptoms in some patients.

CONCLUSIONS

Evidence is discussed from preclinical and clinical studies suggesting the interaction between serotoninergic and dopaminergic neurotransmitter system, as a possible mechanism for production of extrapyramidal symptoms.

Clozapine's functional mesolimbic selectivity is not duplicated by the addition of anticholinergic action to haloperidol: a brain stimulation study in the rat

This study examined whether the anticholinergic potency of the clinically superior antipsychotic drug clozapine contributes to clozapine's anatomically-selective functional inhibition of the mesolimbic dopamine (DA) system, using an electrical brain-stimulation reward (BSR) paradigm in rats that has been previously shown to be highly sensitive to clozapine's mesolimbic functional selectivity. Rats were chronically administered saline, clozapine, haloperidol, or haloperidol plus the anticholinergic compound trihexyphenidyl, and threshold sensitivity of the mesolimbic and nigrostriatal DA systems was assessed using the BSR paradigm, to infer degree of functional DA blockade produced by the chronic drug regimens. Chronic saline produced no change in either DA system. Congruent with previous findings, chronic clozapine powerfully inhibited the mesolimbic DA system but spared the nigrostriatal DA system. Also congruent with previous findings, chronic haloperidol powerfully inhibited both DA systems. Compared to chronic haloperidol alone, chronic haloperidol plus chronic trihexyphenidyl exerted diminished anti-DA action in both the mesolimbic and nigrostriatal DA systems. These results suggest that clozapine's anticholinergic potency is not an adequate explanation for its functional mesolimbic selectivity.

Adenosine A2a receptors in the nucleus accumbens mediate locomotor depression

The effects on locomotor activity (LA) of selective agonists for adenosine receptor subtypes were examined in mice following bilateral injections into the nucleus accumbens (ACB). The ACB is not only richly innervated by dopaminergic (DA) terminals but also exhibits the highest densities of adenosine A2a receptors in the brain. CGS 21680 (2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamidoadenosi ne), a potent and highly selective adenosine A2a receptor agonist, elicited pronounced, dose-related reductions in LA (ID50 dosage = 0.0031 nmol/mouse). NECA (5'-N-ethylcarboxamidoadenosine), a mixed adenosine receptor agonist which exhibits high selectivity and potency at striatal A2a receptors, similarly elicited dose-related reductions in LA (ID50 dosage = 0.0023 nmol/mouse). In contrast, intra-ACB injections of CPA (N6-cyclopentyladenosine), a highly selective agonist for adenosine A1 receptors, did not exert any significant effects on LA, even at 2.0 nmol/mouse, a dosage at which both CGS 21680 and NECA depressed LA by almost 90% compared to vehicle controls. Further, the pronounced locomotor depression elicited by intra-ACB injections of both CGS 21680 and NECA, at approximately the ID65 dosage, was significantly antagonized by IP pretreatment with DMPX, (3,7-dimethyl-1-propargylxanthine), an adenosine receptor antagonist with selectivity for A2 receptors in the striatum, at a dosage (0.15 micromol/mouse) [corrected] which alone had no significant effect on LA. These observations provide support for the notion that adenosine may selectively modulate DA-mediated mesolimbic behavioral circuits via agonist actions at a population of A2a receptors densely concentrated in the ventral striatum.