8-OH-DPAT, a 5-HT1A agonist and ritanserin, a 5-HT2A/C antagonist, reverse haloperidol-induced catalepsy in rats independently of striatal dopamine release

The alpha 2-adrenoceptor antagonist idazoxan reverses catalepsy induced by haloperidol in rats independent of striatal dopamine release: role of serotonergic mechanisms

The alpha(2)-adrenoceptor antagonist idazoxan may improve motor symptoms in Parkinson's disease and experimental Parkinsonism. We studied the effect of idazoxan on haloperidol-induced catalepsy in rats, an animal model of the drug-induced extrapyramidal side effects in man. Catalepsy was induced by a subcutaneous (s.c.) injection of haloperidol (1 mg/kg) and measured by the bar test for a maximum of 5 min. At 3 h after haloperidol, rats were given 0.16-5.0 mg/kg s.c. idazoxan, and descent latency was measured 1 h later. Idazoxan potently reversed haloperidol-induced catalepsy with an ED(50) of 0.25 mg/kg. This effect was mimicked by the selective alpha(2)-adrenoceptor antagonist RS-15385-197 (0.3 and 1 mg/kg orally). We assessed how dopaminergic mechanisms were involved in the anticataleptic effect of idazoxan by studying its effect on dopamine (DA) release in the striatum, with the microdialysis technique in conscious rats. Idazoxan (0.3 and 2.5 mg/kg) had no effect on extracellular DA and did not modify the rise of extracellular DA induced by haloperidol, indicating that changes of striatal DA release were not involved in the reversal of catalepsy. The anticataleptic effect of 2.5 mg/kg idazoxan (haloperidol+vehicle 288+/-8 s, haloperidol+idazoxan 47+/-22 s) was attenuated in rats given an intraventricular injection of 150 microg of the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (haloperidol+vehicle 275+/-25 s, haloperidol+idazoxan 137+/-28 s). The 5-HT(1A) receptor antagonist WAY100 635 (0.1 mg/kg s.c.) did not affect the anticataleptic effect of idazoxan. The results suggest that idazoxan reversed haloperidol-induced catalepsy by a mechanism involving blockade of alpha(2)-adrenoceptors and, at least in part, 5-HT neurons.

Serotonin1A-receptor agonism attenuates the cocaine-induced increase in serotonin levels in the hippocampus and nucleus accumbens but potentiates hyperlocomotion: an in vivo microdialysis study

The hippocampus and the nucleus accumbens (Nac) are important structures for the modulation of spontaneous locomotor activity. Both structures receive a serotonergic (5-HT) innervation. We have previously reported that the 5-HT(1A)-receptor antagonist WAY 100635 blocked cocaine-induced hyperactivity, while potentiating cocaine-induced 5-HT increases in the hippocampus and the Nac. In order to further investigate the relationship between extracellular 5-HT concentration and cocaine-induced behaviour, we used in vivo microdialysis to measure the effects of the 5-HT(1A)-receptor agonist 8-OH-DPAT on cocaine-induced changes in the extracellular 5-HT concentration in the hippocampus and the Nac and on behavioural activity. Following a pilot pretest in which we determined the lowest effective dose of 8-OH-DPAT for potentiating cocaine-induced hyperlocomotion, four groups of rats were given one of the following drug treatments: 8-OH-DPAT (0.2 mg/kg) and cocaine (10 mg/kg), saline and cocaine (10 mg/kg), 8-OH-DPAT (0.2 mg/kg) and saline, or saline and saline. The injections were administered i.p. and spaced 30 min apart. We found that the 5-HT(1A)-receptor agonist 8-OH-DPAT attenuated the cocaine-induced increases in 5-HT in the hippocampus and the Nac, but potentiated cocaine-induced hyperlocomotion. 5-HT metabolite measurements revealed a complex role for the 5-HT(1A)-receptor in the broad spectrum of cocaine's neurochemical effects. Altogether, these observations support an important role of the 5-HT(1A)-receptor in the hippocampus and Nac in the modulation of cocaine stimulant effects.

Conditional involvement of striatal serotonin3 receptors in the control of in vivo dopamine outflow in the rat striatum

Serotonin3 (5-HT3) receptors can affect motor control through an interaction with the nigrostriatal dopamine (DA) neurons, but the neurochemical basis for this interaction remains controversial. In this study, using in vivo microdialysis, we assessed the hypothesis that 5-HT3 receptor-dependent control of striatal DA release is conditioned by the degree of DA and/or 5-HT neuron activity and the means of DA release (impulse-dependent vs. impulse-independent). The different DA-releasing effects of morphine (1 and 10 mg/kg), haloperidol (0.01 mg/kg), amphetamine (1 and 2.5 mg/kg), and cocaine (10 and 20 mg/kg) were studied in the striatum of freely moving rats administered selective 5-HT3 antagonists ondansetron (0.1 mg/kg) or MDL 72222 (0.03 mg/kg). Neither of the 5-HT3 antagonists modified basal DA release by itself. Pretreatment with ondansetron or MDL 72222 reduced the increase in striatal DA release induced by 10 mg/kg morphine but not by 1 mg/kg morphine, haloperidol, amphetamine or cocaine. The effect of 10 mg/kg morphine was also prevented by intrastriatal ondansetron (1 microm) administration. Reverse dialysis with ondansetron also reduced the increase in DA release induced by the combination of haloperidol and the 5-HT reuptake inhibitor citalopram (1 mg/kg). Considering the different DA and 5-HT-releasing properties of the drugs used, our results demonstrate that striatal 5-HT3 receptors control selectively the depolarization-dependent exocytosis of DA only when central DA and 5-HT tones are increased concomitantly.

Quetiapine attenuates levodopa-induced motor complications in rodent and primate parkinsonian models

The contribution of serotoninergic mechanisms to motor dysfunction in Parkinson's disease (PD) has yet to be fully elucidated. Recent clinical observations increasingly suggest that drugs able to block serotonin 5HT2A/C receptors can benefit patients with certain extrapyramidal movement disorders. To further explore the roles of these and other neurotransmitter receptors in the pathogenesis of parkinsonian signs and levodopa-induced dyskinesias; we evaluated the effects of quetiapine, an atypical antipsychotic with 5HT2A/C and D2/3 antagonistic activity, on motor behavior in 6-hydroxydopamine-lesioned rats and MPTP-lesioned nonhuman primates. In hemiparkinsonian rats, quetiapine (5 mg/kg, po) reversed the shortened motor response to levodopa challenge produced by 3 weeks of twice-daily levodopa treatment (P < 0.01). Quetiapine (5 mg/kg po) also normalized the shortened response to the acute injection of either a dopamine D1 receptor agonist (SKF 38392) or a D2 agonist (quinpirole) in rats that had received chronic levodopa treatment. Quetiapine had no effect on parkinsonian dysfunction when given alone or with levodopa to parkinsonian rats and monkeys. Quetiapine (4 mg/kg, po) did, however, substantially reduce levodopa-induced dyskinesias when coadministered with levodopa (P < 0.05). These results suggest that quetiapine could confer therapeutic benefits to patients with levodopa-induced motor complications. Moreover, our findings may indicate that 5HT2A/C receptor-mediated mechanisms, alone or in combination with other mechanisms, contribute to the pathogenesis of the altered motor responses associated with the treatment of PD.

The selective serotonin(1A)-receptor antagonist WAY 100635 blocks behavioral stimulating effects of cocaine but not ventral striatal dopamine increase

An increase in the extracellular dopamine (DA) concentration is generally accepted as an important neurochemical mediator of the behavioral effects of cocaine. Cocaine induced increases in serotonergic (5-HT) activity also appears to be involved in these effects. Here we describe the effects of the 5-HT(1A)-receptor antagonist WAY 100635 on the behavioral and neurochemical effects of cocaine. In-vivo microdialysis was used in behaving rats to measure extracellular concentration of DA in the nucleus accumbens (Nac). Four groups of animals received one of the following drug combinations: WAY 100635 (0.4 mg/kg) and cocaine (10 mg/kg), saline and cocaine (10 mg/kg), WAY 100635 (0.4 mg/kg) and saline, or saline and saline. The injections were administered i.p. and spaced 20 min apart. The pretreatment with WAY 100635 significantly attenuated the locomotor stimulant effects of cocaine without altering the DA overflow in the Nac. WAY 100635 itself did not modify locomotion or the extracellular DA concentration in the Nac. These results indicate that (1) the 5-HT(1A)-receptor is an important component in the mediation of cocaine locomotor stimulant effects, and (2) an increase in the extracellular DA concentration in the Nac might be a necessary but is not a sufficient condition for the locomotor stimulant effects of cocaine.

Differential effects of cyclooxygenase inhibitors on haloperidol-induced catalepsy

Prostaglandins (PGs) might play a role as putative transmitters or as modulators in the central nervous system (CNS). In the present study, haloperidol (1 mg/kg i.p.)-treated mice showed significant cataleptic behaviour. Naproxen, a nonselective cyclooxygenase (COX) inhibitor, dose-dependently (5 and 10 mg/kg) antagonized the haloperidol-induced catalepsy in mice. Nimesulide, a preferential COX inhibitor, also dose-dependently reduced the cataleptic score in haloperidol treated animals at the first and second hour but not at the third and fourth hour of haloperidol treatment. Rofecoxib, a selective COX-2 inhibitor, did not show any effect on haloperidol-induced catalepsy. The major findings of the present study suggest that PGs might play a significant role in haloperidol-induced catalepsy. The findings of the present study further suggested that COX-1 derived rather than COX-2 derived PGs might play a potential role in haloperidol-induced catalepsy. In conclusions COX inhibitors can be screened as potential drug candidates for the treatment of neuroleptic-induced extrapyramidal side effects (EPS).

Central 5-HT(4) receptors and dopamine-dependent motor behaviors: searching for a functional role

In this study, we evaluated the role of central 5-HT(4) receptors in the control of motor behaviors related to change of nigrostriatal dopamine (DA) transmission, namely, stereotyped behavior and catalepsy in rats. Indeed, given that 5-HT(4) receptors indirectly modulate nigrostriatal DA neuron activity, we hypothesized that these receptors would regulate nigrostriatal DA transmission in the basal ganglia, and consequently, associated motor responses. Stereotypy was induced either by an acute administration of apomorphine (0.3 and 1.5 mg/kg sc), or by a single morphine administration (15 mg/kg sc) in chronically morphine-treated (15 mg/kg sc, twice daily for 10 days) rats. Catalepsy was induced by the typical neuroleptic haloperidol (HAL; 1 mg/kg sc). The selective 5-HT(4) antagonist, GR 125487 (1 mg/kg ip), modified neither apomorphine- nor morphine-induced stereotypy. HAL-induced catalepsy, while reduced by the systemic administration of the 5-HT(1A) agonist 8-OH-DPAT (0.1 mg/kg sc), was insensitive to GR 125487, systemically (1, 3, 10 mg/kg ip) or locally (20 and 40 nmol/20 microl) administered into the third ventricle. Also, HAL-induced catalepsy was not affected by the selective 5-HT(4) antagonist GR 113808 (3 mg/kg ip). The obtained results indicate that 5-HT(4) receptor antagonism does not modulate motor behaviors related to change of striatal DA transmission.

Ensaculin (KA-672 HCl): a multitransmitter approach to dementia treatment

Ensaculin, a novel benzopyranone substituted with a piperazine moiety, showed memory-enhancing effects in paradigms of passive and conditioned avoidance in both normal and artificially amnesic rodents. It exhibited neuroprotective activities in an NMDA toxicity model and neurotrophic effects in primary cultured rat brain cells. The compound could be characterized as a weak NMDA receptor-operated channel blocker. In receptor-binding studies, ensaculin was found to have high affinities to serotonergic 5-HT(1A) and 5-HT(7) receptors, adrenergic alpha(1), and dopaminergic D(2) and D(3) receptors. Due to its unique pharmacodynamic profile, ensaculin may have potential as an antidementia agent acting on various transmitter systems.

Distribution of serotonin 5-HT(2A) receptors in afferents of the rat striatum

Treatment with conventional antipsychotic drugs (APDs) is accompanied by extrapyramidal side effects (EPS), which are thought to be due to striatal dopamine D(2) receptor blockade. In contrast, treatment with atypical APDs is marked by a low incidence or absence of EPS. The reduced motor side effect liability of atypical APDs has been attributed to a high serotonin 5-HT(2A) receptor affinity coupled with a relatively low D(2) affinity. Despite the high density of 5-HT(2A) binding sites in the striatum, there are few detectable 5-HT(2A) mRNA-expressing neurons in the striatum. This suggests that most striatal 5-HT(2A) receptors are heteroceptors located on afferent axons. A combined retrograde tracer-immunohistochemistry method was used to determine the sites of origin of striatal 5-HT(2A)-like immunoreactive axons. 5-HT(2A)-like immunoreactive neurons in both the cortex and globus pallidus were retrogradely labeled from the striatum; very few nigrostriatal or thalamostriatal neurons expressed 5-HT(2A)-like immunoreactivity. Within the striatum, parvalbumin-containing interneurons displayed 5-HT(2A) immunolabeling; these neurons are the targets of cortical and pallidal projections. Our data indicate that cortico- and pallido-striatal neurons are the major source of 5-HT(2A) receptor binding in the striatum, and suggest that cortico- and pallido-striatal neurons are strategically positioned to reduce the motor side effects that accompany striatal D(2) receptor blockade or are seen in parkinsonism.

Efficacy of mirtazapine add on therapy to haloperidol in the treatment of the negative symptoms of schizophrenia: a double-blind randomized placebo-controlled study

The negative symptoms of schizophrenia remain a major clinical challenge. Mirtazapine is an antidepressant with antagonist properties at 5-HT2A, 5-HT3 and alpha 2 receptors as well as indirect 5-HT1a agonist effects. Many of these pharmacological actions have clinical or preclinical evidence of efficacy in schizophrenia. This study was a 6-week randomized placebo-controlled trial of mirtzepine or placebo add on to haloperidol 5 mg in the treatment of 30 patients with DSM-IV schizophrenia. The primary finding of the trial was a 42% reduction in Positive and Negative Syndrome Scale (PANSS) negative symptom scores in the mirtazapine group compared to placebo at the end of 6 weeks (mirtazapine 13.9, SD 1.56; placebo 23.9, SD 1.56; P = 0.000, F = 20.31, d.f. = 1). The PANNS total scores, Clinical Global Impression severity and improvement scales in addition showed superiority of mirtazapine over placebo. There was no difference between the groups on the Hamilton depression scale at endpoint, suggesting that the improvement in negative symptoms was not an artifact of mood improvement. These results suggest a potential role for mirtazapine in the negative symptoms of schizophrenia.

Antagonism at 5-HT(2A) receptors potentiates the effect of haloperidol in a conditioned avoidance response task in rats

High affinity for serotonin-2A (5-HT(2A)) over dopamine (DA) D(2) receptors is a leading hypothesis for clozapine's favorable therapeutic profile. Recent preclinical studies also indicate that a sufficient antipsychotic effect might be obtained by a combined high 5-HT(2A)/low D(2) receptor blockade. Thus, addition of a 5-HT(2A) receptor antagonist to an ineffective dose of a D(2) receptor antagonist produces a robust antipsychotic-like effect in the conditioned avoidance response (CAR) test. Electrophysiological and biochemical studies also show that 5-HT(2A) receptor antagonists can confer an atypical (clozapine-like) profile on a D(2) receptor antagonist. Improved therapeutic efficacy by adjunctive 5-HT(2A) receptor antagonist treatment to a traditional D(2) receptor blocking regimen has been suggested. However, the ability of 5-HT(2A) receptor blockade to protect against, or ameliorate, parkinsonian symptoms still remains unclear. Using the CAR and the catalepsy (CAT) tests as indices for antipsychotic activity and extrapyramidal side effect (EPS) liability, respectively, the effects of the selective 5-HT(2A) receptor antagonist MDL 100,907 in combination with the DA D(2) receptor antagonists haloperidol or raclopride were studied in rats. Haloperidol (0.025 or 0.1 mg/kg sc, -30 min) produced a dose-dependent suppression of CAR. Pretreatment with MDL 100,907 (0.5, 1.0, or 1.5 mg/kg sc; -60 min) enhanced and prolonged the haloperidol-induced suppression of CAR without escape failures. MDL 100,907 (1 mg/kg sc, -60 min) had no effect on CAT when coadministered with ineffective doses of raclopride. Raclopride (1 mg/kg sc, -30 min) alone produced a submaximal cataleptic response that was significantly enhanced by pretreatment with MDL 100,907. The present results confirm and extend previous results by showing that 5-HT(2A) receptor blockade can enhance the antipsychotic-like effects of a very low dose of a commonly used traditional antipsychotic. 5-HT(2A) receptor blockade does not, however, prevent EPS (CAT). The therapeutic advantage of this combination might, therefore, operate within a fairly narrow window.

Neurochemical and electrophysiological evidence that 5-HT4 receptors exert a state-dependent facilitatory control in vivo on nigrostriatal, but not mesoaccumbal, dopaminergic function

In this study we investigated, using in vivo microdialysis and single unit recordings, the role of serotonin4 (5-HT4) receptors in the control of nigrostriatal and mesoaccumbal dopaminergic (DA) pathway activity. In freely moving rats, the 5-HT4 antagonist GR 125487 (1 mg/kg, i.p.), without effect on its own, significantly reduced the enhancement of striatal DA outflow induced by 0.01 (-35%) and 0.1 (-66%), but not 1 mg/kg, s.c. haloperidol (HAL). Intrastriatal infusion of GR 125487 (1 microM) had no influence on basal DA outflow, but attenuated (-49%) the effect of 0.01 mg/kg HAL. Systemic administration of GR 125487 modified neither basal nor 0.01 mg/kg HAL-stimulated accumbal DA outflow. In halothane-anaesthetized rats, 1 or 10 mg/kg GR 125487, without effect by itself, failed to modify the changes in accumbal and striatal DA outflow elicited by electrical stimulation (300 microA, 1 ms, 20 Hz, 15 min) of the dorsal raphe nucleus. Finally, GR 125487 (444 microg/kg, i.v.), whilst not affecting basal firing of DA neurons within either the substantia nigra or the ventral tegmental area, reduced HAL-stimulated (1--300 microg/kg, i.v.) impulse flow of nigrostriatal DA neurons only. These results indicate that 5-HT4 receptors exert a facilitatory control on both striatal DA release and nigral DA neuron impulse flow only when nigrostriatal DA transmission is under activated conditions. Furthermore, they indicate that the striatum constitutes a major site for the expression of the control exerted by 5-HT4 receptors on DA release. In contrast, 5-HT4 receptors have no influence on mesoaccumbal DA activity in either basal or activated conditions.

The 5-HT1A receptor in schizophrenia: a promising target for novel atypical neuroleptics?

Increasing attention is being directed towards the role of the serotonergic system in the neurochemistry of schizophrenia and antipsychotic drug treatment. This review considers the 5-HT1A receptor in this context. In patients with schizophrenia, the majority of post-mortem studies have reported increases in 5-HT1A receptor density in the prefrontal cortex in the approximate range 15-80%. Although the pathophysiological significance of this finding is unclear, given the location of a major proportion of these receptors on pyramidal cells, it may reflect an abnormal glutamatergic network. In terms of drug treatment, 5-HT1A agonists clearly display anticataleptic activity in rats. In addition, 5-HT1A agonists consistently increase dopamine release in the prefrontal cortex in rodents, which is an effect that might be predicted to improve negative symptoms. 5-HT1A agonists augment classical neuroleptics in some rat models of antipsychotic action and may be capable of modulating the glutamatergic network therapeutically. Despite the encouraging preclinical data, there is a paucity of clinical studies of 5-HT1A agonist augmentation of neuroleptics in the treatment of schizophrenia. However, the clinical relevance may be clarified by the atypical antipsychotic drugs clozapine, quetiapine and ziprasidone which combine D2 receptor antagonism and 5-HT1A agonism. In conclusion, given the increased prefrontal 5-HT1A receptor density in the illness, and the anticataleptic activity of 5-HT1A agonists combined with their ability to evoke prefrontal dopamine release, there is now a sufficient rationale to examine thoroughly the role of the 5-HT1A receptor in schizophrenia and antipsychotic drug treatment.

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.

5-HT(1A) receptor activation contributes to ziprasidone-induced dopamine release in the rat prefrontal cortex

BACKGROUND

Ziprasidone (Zeldox) is a novel antipsychotic with a unique combination of antagonist activities at monoaminergic receptors and transporters and potent agonist activity at serotonin 5-HT(1A) receptors. 5-HT(1A) receptor agonism may be an important feature in ziprasidone's clinical actions because 5-HT(1A) agonists increase cortical dopamine release, which may underlie efficacy against negative symptoms and reduce dopamine D(2) antagonist-induced extrapyramidal side effects. This study investigated the in vivo 5-HT(1A) agonist activity of ziprasidone by measuring the contribution of 5-HT(1A) receptor activation to the ziprasidone-induced cortical dopamine release in rats.

METHODS

Effects on dopamine release were measured by microdialysis in prefrontal cortex and striatum. The role of 5-HT(1A) receptor activation was estimated by assessing the sensitivity of the response to pretreatment with the 5-HT(1A) antagonist, WAY-100635. For comparison, the D(2)/5-HT(2A) antagonists clozapine and olanzapine, the D(2) antagonist haloperidol, the 5-HT(2A) antagonist MDL 100,907 and the 5-HT(1A) agonist 8-OHDPAT were included.

RESULTS

Low doses (<3.2 mg/kg) of ziprasidone, clozapine, and olanzapine increased dopamine release to approximately the same extent in prefrontal cortex as in striatum, but higher doses (> or =3.2 mg/kg) resulted in an increasingly preferential effect on cortical dopamine release. The 5-HT(1A) agonist 8-OHDPAT produced a robust increase in cortical dopamine (DA) release without affecting striatal DA release. In contrast, the D(2) antagonist haloperidol selectively increased striatal DA release, whereas the 5-HT(2A) antagonist MDL 100,907 had no effect on cortical or striatal DA release. Prior administration of WAY-100635 completely blocked the cortical DA increase produced by 8-OHDPAT and significantly attenuated the ziprasidone- and clozapine-induced cortical DA increase. WAY-100635 pretreatment had no effect on the olanzapine-induced DA increase.

CONCLUSIONS

The preferential increase in DA release in rat prefrontal cortex produced by ziprasidone is mediated by 5-HT(1A) receptor activation. This result extends and confirms other in vitro and in vivo data suggesting that ziprasidone, like clozapine, acts as a 5-HT(1A) receptor agonist in vivo, which may contribute to its activity as an antipsychotic with efficacy against negative symptoms and a low extrapyramidal side effect liability.

8-OH DPAT can restore the locomotor stimulant effects of cocaine blocked by haloperidol

In the first experiment, separate groups of rats (n = 7) were treated with either saline, cocaine (10 mg/kg), haloperidol (0.1 mg/kg), or cocaine (10 mg/kg) plus haloperidol (0.1 mg/kg). Locomotor behavior was measured in an open-field environment, and cocaine induced a reliable locomotor stimulant effect compared to saline-treated animals. Haloperidol produced a progressive decline in locomotion over the 5 test days. Haloperidol also blocked cocaine stimulant effects compared to cocaine-treated animals. In the second experiment, five groups (n = 7) of animals were treated either with saline, cocaine (10 mg/kg), 8-OH DPAT (0.2 mg/kg), 8-OH DPAT (0.2 mg/kg) plus haloperidol (0.1 mg/kg), or 8-OH DPAT (0.2 mg/kg) plus haloperidol 0.1 mg/kg plus cocaine (10 mg/kg). Over the course of 5 days of treatment, cocaine induced a locomotor stimulant effect. Saline and 8-OH DPAT animals did not differ in terms of locomotion. The 0.1 mg/kg haloperidol plus 0.2 mg/kg 8-OH DPAT treatment decreased locomotion compared to the saline group, but the group given 0.2 mg/kg 8-OH DPAT plus 0.1 mg/kg haloperidol plus cocaine (10 mg/kg) exhibited a locomotor stimulant effect equivalent to the cocaine group. In a third experiment, it was found that the 0.2 mg/kg 8-OH DPAT treatment did not enhance the locomotor stimulant effect of cocaine. Thus, the 8-OH DPAT treatment was able to restore a cocaine locomotor stimulant effect in animals treated with haloperidol without directly enhancing the locomotor stimulant effects of cocaine. In Experiments 2 and 3, entries into the central zone of the open field were measured. Cocaine reliably increased central zone entries. The 8-OH DPAT treatment, however, selectively blocked this behavioral effect of cocaine suggesting a qualitative influence of 5-HT(1A) receptors upon cocaine, independent of locomotion activation by cocaine. Ex vivo measurements of dopamine and 5-hydroxytryptamine metabolism in limbic tissue were consistent with the established effects of cocaine, haloperidol, and 8-OH DPAT upon dopamine and 5-hydroxytryptamine neurotransmission. In addition, measurement of cocaine brain concentration indicated that neither haloperidol or 8-OH DPAT affected cocaine concentration in brain.

Differential effects of acute administration of haloperidol and clozapine on ethanol-induced ascorbic acid release in rat striatum

Antipsychotic drugs were initially considered to act predominantly through their antagonism at dopamine D(2)-like receptors. However, reports have demonstrated that the typical neuroleptic drug haloperidol and the atypical neuroleptic drug clozapine showed differential actions in clinical, behavioral and biochemical studies. Since ascorbic acid has a potential usefulness in psychological therapeutics, the present study investigates the actions of these two drugs on ethanol-induced ascorbic acid release in the striatum in order to help explain the different mechanisms of these drugs. The results showed that clozapine, at the doses of 15 and 30 mg/kg, i.p., had no effect on basal ascorbic acid release. However, a synergistic tendency at a dose of 15 mg/kg and a significant synergism at a dose of 30 mg/kg were observed on ascorbic acid release when clozapine was used with ethanol. In contrast, haloperidol, at the doses of 0.5, 1.0 and 2.0 mg/kg, i.p., administered alone did not affect the basal release of striatal ascorbic acid, and when used together with ethanol had neither a potentiating nor an antagonizing effect on ethanol-induced ascorbic acid release. Chlorpromazine, a nonselective dopamine receptor antagonist, at the dose of 5 mg/kg, i.p., affected neither the basal nor the ethanol-induced ascorbic acid release. Ritanserin, a 5-HT(2) receptor antagonist, at the dose of 1 mg/kg, s.c., significantly antagonized ethanol-induced ascorbic acid release. These results demonstrate that clozapine dose-dependently potentiates the stimulatory effect of ethanol on striatal ascorbic acid release and this effect of clozapine may not be related to its dopamine D(2) receptor antagonism.

Low-dose clozapine pretreatment partially prevents haloperidol-induced deficits in conditioned active avoidance

The effectiveness of neuroleptics in disrupting conditioned active avoidance has led to the widespread use of this test as an index of antipsychotic efficacy, whereas the tendency for these drugs to induce catalepsy is believed to reflect their propensity to cause extrapyramidal motor side-effects. Although the typical neuroleptic haloperidol produces catalepsy as well as profound deficits in conditioned active avoidance, the atypical neuroleptic clozapine does not induce catalepsy and is less effective than haloperidol in disrupting active avoidance. Furthermore, clozapine pretreatment prevents haloperidol-induced catalepsy. We investigated whether clozapine pretreatment might also reduce the disruptive effects of haloperidol on two-way active avoidance. We assessed the avoidance acquisition of the following drug treatment groups in which all animals received two injections prior to testing: vehicle + vehicle, vehicle + haloperidol (0.1 mg/kg, i.p.), clozapine (2.5, 5.0 or 10 mg/kg, i.p.) + haloperidol (0.1 mg/kg, i.p.), or clozapine (2.5, 5.0 or 10 mg/kg, i.p.) + vehicle. Haloperidol-pretreated animals showed markedly impaired active avoidance, deficits which were improved by 2.5 and 5 mg/kg but not by 10 mg/kg clozapine pretreatment. These data suggest that the disruptive effects of haloperidol on conditioned active avoidance partially mirror its capacity to induce catalepsy and extrapyramidal motor symptoms. Furthermore, this study indicates that clozapine may be effective in reducing motor side-effects caused by typical neuroleptics.

Combined 5-HT2/D2 receptor blockade inhibits the firing rate of SNR neurons in the rat brain

1. The aim of the present study was to evaluate the contribution of serotonin (5-HT) and dopamine (DA) receptor antagonism to the distinct inhibitory effects of the atypical antipsychotics clozapine and risperidone on SNR neurons, we have shown previously. 2. Utilizing extracellular recordings in the SNR in chloral hydrate anaesthetized rats, raclopride, a selective DA D2/D3 receptor antagonist and LY 53857, a 5-HT2A:2c receptor antagonist were studied separately and in combination for their effects on the firing rate of the SNR neurons. 3. Both raclopride and LY 53857 induced a slight but significant increase in the firing rate of the SNR neurons in a limited dose range. 4. Upon pretreatment with a single dose of raclopride, LY 53857 induced a dose-dependent inhibitory effect on the firing rate of the SNR neurons. 5. Concurrent 5-HT2 and moderate DA D2 receptor antagonism can mimic the in vivo effects of the atypical antipsychotics clozapine and risperidone on the firing rate of SNR neurons.

The effect of serotonergic agents on haloperidol-induced striatal dopamine release in vivo: opposite role of 5-HT(2A) and 5-HT(2C) receptor subtypes and significance of the haloperidol dose used

This study investigated, using microdialysis in freely-moving rats, the role of serotonin (5-HT) and 5-HT(2) receptor subtypes in the enhancement of striatal dopamine (DA) release induced by various doses of haloperidol. The subcutaneous injection of 0.01, 0.1 or 1 mg/kg haloperidol dose-dependently increased DA outflow (160, 219 and 230% of baseline, respectively). The effect of 0.01 mg/kg haloperidol was, respectively, potentiated by the 5-HT uptake inhibitor citalopram (1 mg/kg, s.c.; +35%) and reduced by the 5-HT(1A) receptor agonist 8-OH-DPAT (0.025 mg/kg, s.c.; -32%). Also, it was reduced by the 5-HT(2A) antagonist SR 46349B (0.5 mg/kg, s.c. ; -40%) or by the 5-HT(2A/2B/2C) antagonist ritanserin (1.25 mg/kg, i.p.; -34%), and potentiated by the 5-HT(2B/2C) antagonist SB 206553 (5 mg/kg, i.p; +78%). Further, only this latter compound significantly modified basal dopamine release by itself (+26%). Dopamine released by 0.1 mg/kg haloperidol was enhanced (+100%) by citalopram, decreased (-61%) by SR 4634B, but unaltered by SB 206553. Finally, none of the compounds used were able to modify the enhancement of dopamine release induced by 1 mg/kg haloperidol. These results show that central 5-HT(2A) and 5-HT(2C) receptors exert an opposite (respectively excitatory and inhibitory) influence on DA release. Moreover, they suggest that the 5-HT(2A)-dependent modulation depends on the degree of central DA receptor blockade.

The effect of serotonin(1A) receptor agonism on antipsychotic drug-induced dopamine release in rat striatum and nucleus accumbens

Serotonin (5-HT)(1A) receptor agonism may be of interest in regard to both the antipsychotic action and extrapyramidal symptoms (EPS) of antipsychotic drugs (APD) based, in part, on the effect of 5-HT(1A) receptor stimulation on the release of dopamine (DA) in the nucleus accumbens (NAC) and striatum (STR), respectively. We investigated the effect of R(+)-8-hydroxy-2-(di-n-propylamino)-tetralin (R(+)-8-OH-DPAT) and n-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-n-(2-pyridinyl)cyclohe xanecarboxamide trihydrochloride (WAY100635), a selective 5-HT(1A) receptor agonist and antagonist, respectively, on basal and APD-induced DA release. In both STR and NAC, R(+)-8-OH-DPAT (0.2 mg/kg) decreased basal DA release; R(+)-8-OH-DPAT (0.05 mg/kg) inhibited DA release produced by the 5-HT(2A)/D(2) receptor antagonists clozapine (20 mg/kg), low dose risperidone (0.01 and 0. 03 mg/kg) and amperozide (10 mg/kg), but not that produced by high dose risperidone (0.1 and 1.0 mg/kg) or haloperidol (0.01-1.0 mg/kg), potent D(2) receptor antagonists. This R(+)-8-OH-DPAT-induced inhibition of the effects of clozapine, risperidone and amperozide was antagonized by WAY100635 (0.05 mg/kg). WAY100635 (0.1-0.5 mg/kg) alone increased DA release in the STR but not NAC. The selective 5-HT(2A) receptor antagonist M100907 (1 mg/kg) did not alter the effect of R(+)-8-OH-DPAT or WAY100635 alone on basal DA release in either region. These results suggest that 5-HT(1A) receptor stimulation inhibits basal and some APD-induced DA release in the STR and NAC, and that this effect is unlikely to be mediated by an interaction with 5-HT(2A) receptors. The significance of these results for EPS and antipsychotic action is discussed.

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

This study investigated, using in vivo microdialysis in the striatum of freely moving rats, the role of striatal serotonin2A (5-HT2A) and 5-HT2C receptor subtypes in the modulation of dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) outflow, both in basal conditions and under activation induced by subcutaneous administration of 0.01 mg/kg haloperidol. The different 5-HT2 agents used were applied intrastriatally at a 1 microM concentration through the microdialysis probe. Basal DA efflux was enhanced (27%) by the 5-HT2A/2B/2C agonist 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI) and reduced (-30%) by the 5-HT2B/2C antagonist SB 206553. It was unaffected by infusion of the 5-HT2A antagonist SR 46349B. The effect of DOI was abolished by SB 206553 but not modified by SR 46349B. Haloperidol-stimulated DA efflux (65-70%) was reduced by both SR 46349B (-32%) and the 5-HT2A/2B/2C antagonist ritanserin (-30%) but not affected by SB 206553. Conversely, the effect of haloperidol was potentiated (22%) when DOI was coperfused with SB 206553. Also, haloperidol-stimulated DOPAC outflow (40-45%) was reduced (-20%) by SR 46349B and potentiated (25%) by the combination of SB 206553 with DOI. These results indicate that striatal 5-HT2A receptors, probably through activation of DA synthesis, positively modulate DA outflow only under activated conditions. In contrast, striatal 5-HT2C receptors exert a facilitatory control on basal DA efflux, which appears to be both tonic and phasic.

Serotonin modulation of catalepsy induced by N(G)-nitro-L-arginine in mice

N(G)-(Nitro-L-arginine (L-NOARG), an inhibitor of nitric oxide synthase, induces catalepsy in mice. The objective of the present work was to investigate if serotonergic drugs are able to modulate this effect. Results showed that the cataleptogenic effect of L-NOARG (40 mg/kg) in male albino-Swiss mice was enhanced by pre-treatment with (+)-N-tert-butyl-3-(4-[2-methoxyphenyl]piperazin-1-yl)-2-phenylpro panamide ((+)-WAY-100135, 5 or 10 mg/kg), a 5-HT1A-selective receptor antagonist, and by ketanserin (5 or 10 mg/kg), a 5-HT2A receptor and alpha1-adrenoceptor antagonist. Prazosin (3 or 5 mg/kg), an alpha1-adrenoceptor antagonist, and endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3yl)-2,3-dihydro-3,3-dimet hyl-indole-1-carboxamide HCl (BRL-46470A, 0.05 or 0.5 mg/kg), a 5-HT3 receptor antagonist, did not interfere with L-NOARG-induced catalepsy. Ritanserin (3 or 10 mg/kg), a 5-HT2A and 5-HT2C receptor antagonist, tended to enhance the effect of L-NOARG. These results confirm that interference with the formation of nitric oxide induces catalepsy in mice, and suggest that this effect is modulated by 5-HT1A and 5-HT2A receptors.

Synthesis and Pharmacology of the enantiomers of the potential atypical antipsychotic agents 5-OMe-BPAT and 5-OMe-(2,6-di-OMe)-BPAT

The optically pure enantiomers of the potential atypical antipsychotic agents 5-methoxy-2-[N-(2-benzamidoethyl)-N-n-propylamino]tetralin (5-OMe-BPAT, 5) and 5-methoxy-2-{N-[2-(2,6-dimethoxy)benzamidoethyl]-N-n-propylamino}t etralin [5-OMe-(2,6-di-OMe)-BPAT, 6] were synthesized and evaluated for their in vitro binding affinities at alpha1-, alpha2-, and beta-adrenergic, muscarinic, dopamine D1, D2A, and D3, and serotonin 5-HT1A and 5-HT2 receptors. In addition, their intrinsic efficacies at serotonin 5-HT1A receptors were established in vitro. (S)- and (R)-5 had high affinities for dopamine D2A, D3, and serotonin 5-HT1A receptors, moderate affinities for alpha1-adrenergic and serotonin 5-HT2 receptors, and no affinity (Ki > 1000 nM) for the other receptor subtypes. (S)- and (R)-6 had lower affinities for the dopamine D2A and the serotonin 5-HT1A receptor, compared to (S)- and (R)-5, and hence showed some selectivity for the dopamine D3 receptor. The interactions with the receptors were stereospecific, since the serotonin 5-HT1A receptor preferred the (S)-enantiomers, while the dopamine D2A and D3 receptors preferred the (R)-enantiomers of 5 and 6. The intrinsic efficacies at the serotonin 5-HT1A receptor were established by measuring their ability to inhibit VIP-induced cAMP production in GH4ZD10 cells expressing serotonin 5-HT1A receptors. Both enantiomers of 5 behaved as full serotonin 5-HT1A receptor agonists in this assay, while both enantiomers of 6 behaved as weak partial agonists. The potential antipsychotic properties of (S)- and (R)-5 were evaluated by establishing their ability to inhibit d-amphetamine-induced locomotor activity in rats, while their propensity to induce extrapyramidal side-effects (EPS) in man was evaluated by determining their ability to induce catalepsy in rats. Whereas (R)-5 was capable of blocking d-amphetamine-induced locomotor activity, indicative of dopamine D2 receptor antagonism, (S)-5 even enhanced the effect of d-amphetamine, suggesting that this compound has dopamine D2 receptor-stimulating properties. Since both enantiomers also were devoid of cataleptogenic activity, they are interesting candidates for further exploring the dopamine D2/serotonin 5-HT1A hypothesis of atypical antipsychotic drug action.

Selective blockade of serotonin-2C/2B receptors enhances mesolimbic and mesostriatal dopaminergic function: a combined in vivo electrophysiological and microdialysis study

Electrophysiological techniques and in vivo microdialysis were used to investigate the relative contribution of central serotonin-2C/2B and serotonin-2A receptor subtypes in the control of mesolimbic and nigrostriatal dopaminergic function. Thus, extracellular single-unit recordings were performed from neurochemically identified dopamine neurons in the ventral tegmental area and the substantia nigra pars compacta, as well as simultaneous monitoring of accumbal and striatal basal dopamine release in anesthetized rats following the administration of serotonin-2C/2B (SB 206553), serotonin-2A (SR 46349B) or serotonin-2A/2B/2C (ritanserin) antagonists. Administration of SB 206553 (40-160 microg/kg, i.v.) caused a dose-dependent increase in the basal firing rate of ventral tegmental area and nigral dopamine neurons, reaching its maximum (45.2 and 28.5%, respectively) following 160 microg/kg. Moreover, burst activity was significantly enhanced by SB 206553 in the ventral tegmental area only. In contrast, injection of SR 46349B (40-160 microg/kg, i.v.), and ritanserin (40-160 microg/kg, i.v.) did not cause any significant change in the basal activity of these neurons. Basal dopamine release was significantly enhanced in both the nucleus accumbens (42%) and the striatum (33%) following the intraperitoneal administration of 5 mg/kg SB 206553. In contrast, SR 46349B (0.5 mg/kg, s.c.) and ritanserin (0.63 mg/kg, i.p.) failed to affect basal dopamine output in both regions. Taken together, these data indicate that the central serotonergic system exerts a tonic inhibitory control of mesolimbic and nigrostriatal dopaminergic pathway activity and that the serotonin-2C/2B receptor subtypes are involved in this effect. Moreover, these findings might open new possibilities for the employment of serotonin-2C/2B receptor antagonists in the treatment of neuropsychiatric disorders related to a hypofunction of central dopaminergic neurons.

The role of 5-HT1A autoreceptors and alpha1-adrenoceptors in the modulation of 5-HT release--III. Clozapine and the novel putative antipsychotic S 16924

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.

Cataleptogenic effect of subtype selective 5-HT receptor antagonists in the rat

5-HT receptor antagonists with selectivity for 5-HT1A WAY-100635 (N-[2-[-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide), 5-HT1B GR 127935 (N-[methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'(5-methyl-1,2, 4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide x HCl), 5-HT2C SB 200646A (N-(1-methyl-5-indolyl)-N'-(3-pyridyl)urea x HCl) and 5-HT2A (ketanserin, fananserin and MDL 100,151 ((+/-)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipe ridinemethanol) receptors were tested for cataleptogenic responses in rats. WAY-100635 (0.1-3 mg/kg, s.c.), ketanserin (0.1-3 mg/kg, s.c.), MDL 100,151 (0.3-3 mg/kg, s.c.) and fananserin (RP 62203; 3 mg/kg, s.c.) induced a significant catalepsy. GR 127935 (1 mg/kg, s.c.), SB 200646A (without effect per se at 10 mg/kg, s.c.) and MDL 100,151 (0.3 mg/kg, s.c.) did not inhibit the cataleptic response to the dopamine D2 receptor antagonist, loxapine (0.3 mg/kg, s.c.). Catalepsy induced by MDL 100,151 (3 mg/kg) was blocked by co-treatment with clozapine, but not by SB 200646A (both at 10 mg/kg, s.c.). Although clozapine displays significant affinity to 5-HT1A, 5-HT1B, 5-HT2A and 5-HT2C receptors, the present results suggest that blockade of these receptors is not responsible for clozapine's anticataleptic activity.