Effects of antipsychotic drugs on 5-HT2 receptors in the medial prefrontal cortex: microiontophoretic studies

Cellular rules underlying psychedelic control of prefrontal pyramidal neurons

Classical psychedelic drugs are thought to increase excitability of pyramidal cells in prefrontal cortex via activation of serotonin 2A receptors (5-HT2ARs). Here, we instead find that multiple classes of psychedelics dose-dependently suppress intrinsic excitability of pyramidal neurons, and that extracellular delivery of psychedelics decreases excitability significantly more than intracellular delivery. A previously unknown mechanism underlies this psychedelic drug action: enhancement of ubiquitously expressed potassium "M-current" channels that is independent of 5-HT2R activation. Using machine-learning-based data assimilation models, we show that M-current activation interacts with previously described mechanisms to dramatically reduce intrinsic excitability and shorten working memory timespan. Thus, psychedelic drugs suppress intrinsic excitability by modulating ion channels that are expressed throughout the brain, potentially triggering homeostatic adjustments that can contribute to widespread therapeutic benefits.

Serotonergic Regulation of Prefrontal Cortical Circuitries Involved in Cognitive Processing: A Review of Individual 5-HT Receptor Mechanisms and Concerted Effects of 5-HT Receptors Exemplified by the Multimodal Antidepressant Vortioxetine

It has been known for several decades that serotonergic neurotransmission is a key regulator of cognitive function, mood, and sleep. Yet with the relatively recent discoveries of novel serotonin (5-HT) receptor subtypes, as well as an expanding knowledge of their expression level in certain brain regions and localization on certain cell types, their involvement in cognitive processes is still emerging. Of particular interest are cognitive processes impacted in neuropsychiatric and neurodegenerative disorders. The prefrontal cortex (PFC) is critical to normal cognitive processes, including attention, impulsivity, planning, decision-making, working memory, and learning or recall of learned memories. Furthermore, serotonergic dysregulation within the PFC is implicated in many neuropsychiatric disorders associated with prominent symptoms of cognitive dysfunction. Thus, it is important to better understand the overall makeup of serotonergic receptors in the PFC and on which cell types these receptors mediate their actions. In this Review, we focus on 5-HT receptor expression patterns within the PFC and how they influence cognitive behavior and neurotransmission. We further discuss the net effects of vortioxetine, an antidepressant acting through multiple serotonergic targets given the recent findings that vortioxetine improves cognition by modulating multiple neurotransmitter systems.

Functional and structural brain changes associated with methamphetamine abuse

Methamphetamine (MA) is a potent psychostimulant drug whose abuse has become a global epidemic in recent years. Firstly, this review article briefly discusses the epidemiology and clinical pharmacology of methamphetamine dependence. Secondly, the article reviews relevant animal literature modeling methamphetamine dependence and discusses possible mechanisms of methamphetamine-induced neurotoxicity. Thirdly, it provides a critical review of functional and structural neuroimaging studies in human MA abusers; including positron emission tomography (PET) and functional and structural magnetic resonance imaging (MRI). The effect of abstinence from methamphetamine, both short- and long-term within the context of these studies is also reviewed.

Differential effects of spinal 5-HT1A receptor activation and 5-HT2A/2C receptor desensitization by chronic haloperidol

The effects of 7- and 21-day haloperidol treatment on the spinal serotonergic system were examined in vivo in acutely spinalized adult rats. Intravenous administration of a selective 5-HT(2A/2C) receptor agonist, (+/-)-2,5-Dimethoxy-4-iodoamphetamine hydrochloride (0.1 mg/kg) significantly increased the excitability of spinal motoneurones as reflected by increased monosynaptic mass reflex amplitude. This was significantly reduced in rats treated with haloperidol (1 mg/kg/day, i.p.) for 7 and 21 days. Administration of a 5-HT(1A/7) receptor agonist, (+/-)-8-Hydroxy dipropylaminotetraline hydrobromide (0.1 mg/kg, i.v.) significantly inhibited the monosynaptic mass reflex. This inhibition was greatly prolonged in haloperidol treated animals. These results demonstrate that the effects of haloperidol on the activation and desensitization of 5-HT(1A) and 5-HT(2A/2C) receptors respectively, may be mediated via intracellular mechanisms shared by these receptors with dopamine D(2) receptors in the mammalian spinal cord. The above serotonergic mechanisms may be partly responsible for haloperidol-induced extrapyramidal motor dysfunction.

Perospirone, a novel atypical antipsychotic drug, potentiates fluoxetine-induced increases in dopamine levels via multireceptor actions in the rat medial prefrontal cortex

Perospirone is a novel atypical antipsychotic with a unique combination of 5-HT1A receptor agonism as well as 5-HT2A and D2 receptor antagonism. We investigated the effect of perospirone in combination with fluoxetine on dopamine release in the rat medial prefrontal cortex using microdialysis. Perospirone and fluoxetine increased dopamine release to 270 and 210% of the baseline value, respectively. A combination of perospirone and fluoxetine markedly increased dopamine release to 800% of the baseline value. Pretreatment with a selective 5-HT1A receptor antagonist, WAY 100635, suppressed the increase in dopamine levels induced by the administration of perospirone and fluoxetine to 330% of the baseline value. These findings suggest that perospirone potentiates fluoxetine-induced dopamine increases in part via the action of the 5-HT1A receptor and may augment the effect of fluoxetine in treatment-resistant depression.

Antipsychotics and single-cell activity in the rat superior colliculus

Schizophrenic patients have problems with saccadic eye movements that can be characterized as a loss of control over the saccadic system. Preliminary clinical results suggest that antipsychotics can either disrupt or improve saccadic performance. The brain mechanism through which antipsychotics might affect saccades is the subject of study. The superior colliculus (SC) is crucially involved in the generation of saccades. Previous experimental studies showed that the substantia nigra reticulata (SNr), a structure with profound inhibitory influence on the SC, is differentially affected by classical and atypical antipsychotics. In this study, we investigated the potential effects of atypical antipsychotics (clozapine, olanzapine, and risperidone) and a classical antipsychotic (haloperidol) on the firing rate of SC cells in the rat. In anesthetized rats, we performed extracellular recordings on spontaneous active neurons in the intermediate and deep layers of the SC. After subcutaneous injection of an antipsychotic drug, changes in firing rate were compared with responses upon saline injection. Olanzapine (1.0 mg/kg), risperidone (0.3 and 1.0 mg/kg), and haloperidol (0.5 mg/kg) did not significantly alter cell activity, but clozapine (10.0 mg/kg) induced a short-lasting but significant decrease. Except for clozapine, the effects of antipsychotics on the SC were nonsignificant and therefore independent of the effects in the SNr. Our results support the notion that clozapine is different from the other atypical antipsychotics.

Decreased pallidal GABA following reverse microdialysis with clozapine, but not haloperidol

Changes in striatopallidal GABA are believed to play a significant role in the motor side effects produced by antipsychotic drugs (APDs). In the current study, we measured extracellular GABA in the globus pallidus (GP) of rats. GABA release was partially impulse- and Ca2+-dependent, as evidenced by decreased efflux following tetrodotoxin (TTX) or removal of Ca2+. In addition, GABA release was significantly increased by high K+ (100 mM KCl) stimulation. Reverse dialysis of the atypical APD, clozapine (1-100 microM), produced a concentration dependent decrease in extracellular GABA. In contrast, the typical APD, haloperidol (1-100 microM), had no significant effect on GABA levels. These results suggest that clozapine has direct actions within the GP, while the effects of haloperidol are most likely mediated through its effects in the striatum. The clozapine-induced decrease in pallidal GABA may account for its low motor side effect liability.

Dizocilpine-induced neuropathological changes in rat retrosplenial cortex are reversed by subsequent clozapine treatment

In this study, we examined the effect of post-treatment with clozapine on the neuropathological changes in the rat retrosplenial cortex induced by the administration of non-competitive NMDA receptor antagonist dizocilpine ((+)-MK-801). The maximal increase in vacuolized neurons, which are representative of neuropathology, was observed 4 hours after a single injection of dizocilpine (0.5 mg/kg s.c.), with a complete reversal of the neuropathology after 16-24 hours. The administration of clozapine (10 mg/kg, i.p.,) 4 hours after the administration of dizocilpine significantly decreased the number of vacuolized neurons in the retrosplenial cortex 6, 8 or 10 hours after administration of dizocilpine, compared to vehicle-treated animals. Furthermore, the administration of clozapine (5, 10 or 20 mg/kg i.p.) 4 hours after the administration of dizocilpine produced a significant decrease in the number of vacuolized neurons in the retrosplenial cortex in a dose-dependent manner when measure 6 hours post-dizocilpine. These results show that neuropathological changes in the rat retrosplenial cortex produced by dizocilpine can be attenuated by post-treatment with clozapine.

Clozapine blocks dopamine, 5-HT2 and 5-HT3 responses in the medial prefrontal cortex: an in vivo microiontophoretic study

Clozapine is an atypical antipsychotic drug active on both positive and negative symptoms of schizophrenia which has a unique serotonergic and dopaminergic profile. Given the putative role of the medial prefrontal cortex (mPFC) in negative symptoms of schizophrenia, the aim of this study was to assess the effects of clozapine on the dopamine- and serotonin-responsive neurons in that particular brain structure. D1 and D2 agonists (SKF 38393 and quinpirole) as well as 5-HT2 and 5-HT3 agonists (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, DOI, and phenylbiguanide) were applied by microiontophoresis alone and concurrently with clozapine while recording extracellularly mPFC neurons. Dopamine ejections inhibited firing activity while D1 and D2 agonists were ineffective. Clozapine did not change basal firing by itself, but was able to suppress the inhibition produced by dopamine and by the 5-HT2/5-HT3 receptor agonists. It is concluded that clozapine at the mPFC level exerts a complex modulatory activity on dopamine receptors, that is directly at the dopaminergic receptors and through 5-HT receptors on the same neurons.

Modification of haloperidol-induced pattern of c-fos expression by serotonin agonists

Acute challenge with clozapine and haloperidol produce different anatomical patterns of c-fos expression in the forebrain. The pharmacological profile of atypical antipsychotics suggests that serotonin might contribute to the unique therapeutic benefits of these drugs. In order to test this possibility, we examined the abilities of 5-HT1A and 5-HT2A/2c agonists to modify the pattern of c-fos expression induced by haloperidol and clozapine. Various groups of rats were pretreated with either saline, DOI, 8-OH-DPAT, and 8-OH-DPAT + DOI 30 min prior to haloperidol or clozapine administration. Rats were killed 90 min after antipsychotic administration. In saline-pretreated rats, haloperidol produced intense Fos-LI in all four striatal quadrants while the effect of clozapine was restricted to the medial part of the striatum. Prior administration of 8-OH-DPAT significantly reduced haloperidol-induced Fos-LI in all four striatal quadrants while DOI and 8-OHDPAT + DOI significantly reduced Fos-LI only in dorso- and ventrolateral quadrants. In the nucleus accumbens, haloperidol induced intense Fos-LI in the core and the shell regions whereas clozapine induced c-fos expression only in the shell. Pretreatment with 8-OHDPAT in haloperidol treated rats reduced Fos-LI in the core region yielding to a c-fos pattern similar to that induced by clozapine. In the prefrontal cortex of saline-pretreated rats, haloperidol produced a moderate c-fos expression compared with the intense expression produced by clozapine. Pretreatment with serotonin agonists before haloperidol brought the number of FOS-positive neurons to the same level as in clozapine treated rats. These results show the ability of 5-HT agonists to transform the typical pattern of c-fos expression induced by haloperidol into a pattern resembling that of clozapine.

Acute and chronic administration of clozapine produces greater proconvulsant actions than haloperidol on focal hippocampal seizures in freely moving rats

In this study, we assessed the effects of the acute (a single injection) and repeated (once daily injections for 21 days) administration of the atypical antipsychotic drug clozapine (1.5, 5, or 15 mg/kg i.p.) and the typical antipsychotic drug haloperidol (0.15, 0.5, and 1.5 mg/kg, i.p.) on hippocampal partial seizures generated by low-frequency electrical stimulation in male Wistar rats. The seizure threshold and severity were determined by measuring the pulse number threshold (PNT) and the primary afterdischarge duration (ADD), respectively. A single injection of either 5 or 15 mg/kg of clozapine significantly decreased the PNT and significantly increased the primary ADD, indicating a proconvulsant action. The repeated administration of clozapine (1.5, 5, or 15 mg/kg, i.p.) produced dose-dependent, proconvulsant effects by significantly decreasing the PNT and by significantly increasing the primary ADD. In contrast to clozapine, the acute administration of haloperidol did not significantly alter the PNT or the primary ADD. The repeated administration of haloperidol (0.5 and 1.5 mg/kg, i.p.), unlike clozapine, significantly decreased the primary ADD, but did not alter the PNT. Overall, clozapine produces a greater proconvulsant action than haloperidol in an animal model of hippocampal seizures.

Atypical antipsychotics block the excitatory effects of serotonin in septohippocampal neurons in the rat

We recently reported that serotonin excites a subpopulation of GABAergic neurons in the rat medial septum/diagonal band of Broca complex via multiple serotonin receptors, including the serotonin2A subtype. Since a subpopulation of medial septum/diagonal band GABAergic neurons projects to the hippocampus, in the present study we tested the effect of serotonin on antidromically-activated septohippocampal neurons using extracellular recordings. Bath-applied serotonin had an excitatory effect in a majority of septohippocampal neurons; serotonin-excited septohippocampal neurons had a mean conduction velocity -1.63 +/- 0.07 m/s (n=101). Pharmacologically, MDL 100,907, a selective serotonin2A antagonist blocked the excitatory effect of serotonin in 78% of septohippocampal neurons tested, with a mean pA2 of 8.51 +/- 0.12 (n=22). Additionally, the atypical antipsychotics risperidone and clozapine but not the typical antipsychotic haloperidol, blocked the excitatory effects of serotonin at clinically relevant concentrations. The pA2 values of 8.84 +/- 0.11, 6.57 +/- 0.13 and 5.94 +/- 0.27 for risperidone, clozapine and haloperidol, respectively, obtained in the present study, give a rank order of potency risperidone (1.6 nM) clozapine (269 nM) haloperidol (1.1 microM) which corresponds to that reported in binding studies. Additionally, in whole-cell patch-clamp recordings, risperidone (10 nM) blocked serotonin-induced increase in GABAergic synaptic currents. In conclusion, serotonin excites septohippocampal neurons primarily via the serotonin2A receptor and atypical antipsychotics block this excitation at clinically relevant concentrations.

Effects of risperidone, clozapine and haloperidol on extracellular recordings of substantia nigra reticulata neurons of the rat brain

Risperidone has proven to be effective as an antipsychotic drug and has fewer extrapyramidal side-effects than classic neuroleptics. In addition to its dopamine D2 receptor antagonistic properties, this antipsychotic agent is a potent 5-HT2 receptor antagonist. The atypical antipsychotic, clozapine, also possesses both dopamine D2 and 5-HT2 receptor affinity next to affinities for other receptors. To gain an insight in the consequences for basal ganglia activity of treatment with these atypical neuroleptics vs. typical neuroleptics, the effects of cumulative doses of risperidone, clozapine and haloperidol on the firing rate of substantia nigra reticulata neurons were studied. Extracellular recordings were performed in chloralhydrate-anaesthetized male Wistar rats. Both risperidone (50-3200 micrograms/kg i.v.) and clozapine (100-6400 micrograms/kg i.v.) dose dependently decreased substantia nigra reticulata activity maximally to 70% of the basal activity. With both treatments, a dose of 800 micrograms/kg was significantly effective. In contrast, haloperidol (12.5-800 micrograms/kg i.v.) gradually induced a slight increase in substantia nigra reticulata activity, which was identical to the substantia nigra reticulata activity after saline treatment. Therefore, these results indicate that typical and atypical neuroleptics affect differentially the output of the basal ganglia in the substantia nigra reticulata. To evaluate the involvement of 5-HT2 receptors in the effect of risperidone, the 5-HT2 receptor agonist, quipazine (0.5 mg/kg i.p.), was administered 15 min preceding risperidone treatment. A 4-fold higher dose of risperidone was needed to significantly affect the substantia nigra reticulata firing rate. Thus, the 5-HT2 component of the effect of risperidone is, at least partly, responsible for the difference in effect on substantia nigra reticulata neurons in comparison to haloperidol.

Perspectives on the role of serotonergic mechanisms in the pharmacology of schizophrenia

In recent years, a number of research findings has renewed interest in the possible role of serotonin (5-HT) in the pharmacology of schizophrenia. Atypical antipsychotics that potently block 5-HT receptors have been shown to be at least as effective as classical antipsychotics as well as producing fewer extrapyramidal side-effects. In addition, molecular biological studies have suggested that allelic variations of 5-HT receptor genes may affect both susceptibility to schizophrenia and clinical response to atypical antipsychotics. Building on these findings, this article proposes that 5-HT receptors are critical sites of antipsychotic action, and examines the implications of this to the treatment and pathophysiology of schizophrenia. Possible pharmacological mechanisms underlying the clinical efficacy of 5-HT blocking antipsychotics are discussed, and the potential of functional neuroimaging techniques to further elucidate these mechanisms is emphasized.

The effects of clozapine and haloperidol on serotonin-1A, -2A and -2C receptor gene expression and serotonin metabolism in the rat forebrain

The therapeutic and side-effect profiles of clozapine differ from those of typical antipsychotic drugs such as haloperidol. Effects on the serotonin system, especially serotonin-2 receptors, may contribute to clozapine's atypicality. We injected rats for 14 days with clozapine (25 mg/kg/day) or haloperidol (2 mg/kg/day), and measured three aspects of the serotonin system in forebrain regions: abundance of serotonin-2A, -2C and -1A receptor messenger RNAs by in situ hybridization histochemistry; serotonin-2A and -1A binding sites using receptor autoradiography, and levels of serotonin and 5-hydroxyindoleacetic acid with high-performance liquid chromatography. Clozapine administration decreased serotonin-2A receptor messenger RNA and the density of [3H]ketanserin binding in cingulate and frontal cortex, but not in piriform cortex. Serotonin-1A receptor expression and serotonin-2C receptor messenger RNA were unchanged in all areas. The treatment markedly decreased serotonin and 5-hydroxyindoleacetic acid concentrations in striatum with similar trends in cortex and hippocampus. Haloperidol administration did not affect the expression of the three serotonin receptors, but was associated with a modest reduction of striatal and hippocampal 5-hydroxyindoleacetic acid. The selective reduction of serotonin-2A receptors confirms earlier findings and supports the view that this receptor may have relevance for the actions of clozapine. The fact that the encoding messenger RNA is decreased shows that the the effect is mediated at the level of gene expression. In contrast, the unchanged serotonin-2C receptor messenger RNA level indicates that the reported loss of serotonin-2C receptors after clozapine treatment is due to translational or post-translational events. The relationship between the reduction in serotonin-2A receptor expression and the altered serotonin metabolism remains unclear.

Serotonin-2 and dopamine-1 binding components of clozapine in frontal cortex and striatum in the human brain visualized by positron emission tomography

The specific binding of N-methyl-11C-clozapine in the human brain was studied in five healthy volunteers with positron emission tomography (PET). Four of the volunteers were reexamined after treatment with the dopamine D1 and D2 receptor antagonist flupenthixol, and all five volunteers were reexamined after pretreatment with the serotonin2 receptor antagonist ritanserin. The examinations after flupenthixol and ritanserin treatment were performed on different occasions. In the flupenthixol part of the study, two of the subjects were given an oral dose of 1 mg flupenthixol 2-3 h before the posttreatment study with PET. The other two subjects received 0.5 mg orally three times during the 24 h preceding the posttreatment PET study, with the last dose being administered < or = 4 h before the scan. All five ritanserin-treated subjects followed the same dosing regimen. During the 5 days preceding the posttreatment PET study, they were given a 10-mg tablet of ritanserin in the evening. The last dose was administered 2-1/2 hours before the study. Both flupenthixol and ritanserin pretreatment were associated with decreased binding of N-methyl-11C-clozapine in dorsolateral and medial frontal cortical regions. These results support previous findings that clozapine has affinity for both dopamine D1 and serotonin 5-HT2 receptors in the human frontal cortex. No consistent change of binding was observed in striatal regions following flupenthixol or ritanserin pretreatment. The clinical aspects of this feature are discussed, both with respect to efficacy and side effects.

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.

Risperidone: regional effects in vivo on release and metabolism of dopamine and serotonin in the rat brain

The antipsychotic drug risperidone shows high affinity for both central serotonin (5-HT)2A and dopamine (DA)-D2 receptors in vivo. By employing microdialysis in freely moving rats, the effects of acute risperidone administration on regional brain DA and 5-HT release and metabolism were compared with the corresponding effects of the atypical antipsychotic drug clozapine as well as amperozide, the selective DA-D2 receptor antagonist raclopride and the selective 5-HT2A/5-HT2C receptor antagonist ritanserin. Risperidone (0.2 or 2.0 mg/kg, SC) was found to increase DA release and metabolism to about the same extent in three major projection areas of the mesotelencephalic dopaminergic system, i.e. the nucleus accumbens (NAC), the medial prefrontal cortex (MPC) and the lateral striatum (STR). In contrast, clozapine and amperozide (both 10.0 mg/kg, SC), as well as raclopride (2.0 mg/kg, SC), were all found differentially to affect DA release and metabolism in the three projections areas. Specifically, clozapine and amperozide enhanced DA release in the MPC to a greater extent than in the NAC or the STR, whereas raclopride instead preferentially increased DA release in the NAC and the STR but not in the MPC. Ritanserin (3.0 mg/kg, SC) did not exert any major effects on DA metabolism in the three areas studied. In contrast to the regionally rather homogenous activation of brain DA systems caused by risperidone, the drug was found to enhance brain 5-HT metabolism preferentially in the MPC, as indicated by the elevated extracellular concentration of 5-hydroxyindoleacetic acid (5-HIAA) in this region. A similar elevation of the 5-HIAA level in the MPC was observed after amperozide and, to some extent, after clozapine and ritanserin administration. The risperidone-induced (2.0 mg/kg, SC) elevation of 5-HIAA concentrations in the frontal cortex was found to be paralleled by an increased 5-HT release in this brain area. Consequently, our findings demonstrate a pharmacological profile of risperidone, as reflected in brain DA metabolism, in between that of clozapine and the DA-D2 antagonists. The preferential activation of 5-HT release and metabolism in frontal cortical areas might be of particular relevance for the ameliorating effect of risperidone on negative symptoms in schizophrenia, especially when associated with depression.

Evidence that 5-HT(2) Antagonism Elicits a 5-HT(3)-Mediated Increase in Dopamine Transmission

Amperozide, a novel atypical antipsychotic drug with few extrapyramidal side effects, is a strong serotonin(2) (5-HT(2)) antagonist but has low affinity for dopamine receptors in vitro. The effect of amperozide on the dopaminergic synapse was studied with an in vivo microdialysis technique using anesthetized male Sprague-Dawley rats. Following implantation of dialysis probes into the striatum and nucleus accumbens (NuAc), amperozide was intravenously infused as six consecutive incremental doses (0.5, 0.5, 1.0, 2.0, 4.0 and 8.0 mg/kg) at intervals of 15 min. From the beginning of drug infusion, perfusates were collected in fractions every 30 min throughout a total period of 120 min. The samples were then immediately analyzed by high-performance liquid chromatography with electrochemical detection. Amperozide induced a dose-related elevation of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindolacetic acid (5-HIAA) levels in both areas. p-Chlorophenylalanine (pCPA) pretreatment abolished the production of 5-HIAA in both areas and attenuated the amperozide-induced rise of DOPAC but not of dopamine. After pretreatment with an intravenous 5-HT(3) antagonist, MDL 72222, the amperozide-induced changes in dopamine, DOPAC and 5-HIAA in both areas were lower than in the saline control group. Preliminary data showed that after pCPA pretreatment, incremental concentrations of the 5-HT(3) agonist 1-(m-chlorophenyl)-biguanide perfused via the probe also produced significant elevation of dopamine and DOPAC levels in these two areas. Taken together, these results suggest that amperozide may directly block 5-HT(2) receptors in the striatum and NuAc, thereby enhancing 5-HT transmission. The enhanced 5-HT transmission may activate postsynaptic 5-HT(3) receptors located on the dopaminergic terminals, leading to changes in dopamine transmission in these two areas. Copyright 1995 S. Karger AG, Basel

Some general anesthetics reduce serotonergic neocortical activation and enhance the action of serotonergic antagonists

In urethane-anesthetized rats, neocortical LVFA induced by 100 Hz electrical stimulation of the median raphe area or by tail pinching was completely eliminated by a combination of scopolamine (5 mg/kg, IP) and p-chlorophenylalanine (500 mg/kg/day x 3, IP), providing evidence that LVFA is dependent on cholinergic-muscarinic and serotonergic inputs to the neocortex in urethane-anesthetized as well as in freely moving rats. The serotonergic receptor antagonists ketanserin and methiothepin (1-10 mg/kg, IP) also produced a dose-dependent blockade of LVFA in urethane-anesthetized rats, and eliminated virtually all LVFA when combined with scopolamine. A combination of diethyl ether anesthesia and scopolamine completely eliminated all neocortical LVFA without additional antiserotonergic treatment, and a combination of chloral hydrate anesthesia and scopolamine similarly blocked all LVFA in about 50% of the rats tested. In the remaining chloral hydrate-anesthetized rats, the residual LVFA could be eliminated by the serotonergic antagonist ritanserin (10 mg/kg, IP). As shown previously, in nonanesthetized rats treated with scopolamine, LVFA can be maintained by a serotonergic input to the neocortex. The present data suggest that some general anesthetics reduce or completely abolish this serotonergic LVFA. Further, the serotonergic antagonists used here exert much stronger antiserotonergic effects in rats anesthetized with urethane or chloral hydrate than in freely moving rats. Therefore, studies of serotonergic transmission or antagonist action, especially in the neocortex, in anesthetized rats may not be applicable to the waking state.

Interactions of clozapine with the stimulus effects of DOM and LSD

Two groups of rats were trained with the 5-HT2 agonists 2,5-dimethoxy-4-methylamphetamine (DOM) or lysergic acid diethylamide (LSD) in a two-lever discrimination task. Tests of generalization and antagonism were then carried out with clozapine. DOM did not generalize to clozapine. Partial antagonism of DOM was observed with 0.3, 1, and 2 mg/kg clozapine and statistically significant full antagonism with 3 mg/kg. LSD did not fully generalize to clozapine. Partial antagonism of LSD was observed with 3 and 4 mg/kg clozapine. Because clozapine is known to block muscarinic as well as 5-HT2 receptors, atropine was studied in DOM-trained rats. DOM partially generalized to 3 mg/kg atropine. Partial attenuation of DOM stimulus effects was observed with 3 mg/kg atropine, and no attenuation with 5 mg/kg. A combination of 2 mg/kg clozapine and 3 mg/kg atropine vs. DOM produced response suppression in five of seven rats. The atropine test results do not exclude the possibility of an antimuscarinic component in the observed attenuation of DOM and LSD stimulus effects by clozapine.

Systemic administration of amperozide, a new atypical antipsychotic drug, preferentially increases dopamine release in the rat medial prefrontal cortex

The putative atypical antipsychotic drug amperozide (APZ) shows high affinity for serotonin 5-HT2 receptors but only low affinity for dopamine (DA) D2 receptors. By employing microdialysis, we examined the effects of APZ on extracellular concentrations of DA in the nucleus accumbens (NAC), the dorsolateral striatum (STR) and the medial prefrontal cortex (MPC) of awake rats. A 5.0 mg/kg (SC) dose of APZ failed to affect DA concentrations in the NAC, while it increased DA outflow in the STR (by 46%) and the MPC (by 207%). A higher dose of APZ (10 mg/kg, SC) enhanced dialysate DA from the NAC and the STR by 30%, and from the MPC by 326%. Similarly, clozapine (2.5 and 10 mg/kg, SC) produced a greater release of DA in the MPC (+ 127 and + 279%) than in the NAC (+ 52 and + 98%). The selective 5-HT2 receptor antagonist ritanserin (1.5 and 3.0 mg/kg, SC) also produced a slightly higher increase of DA output in the MPC (+ 25 and + 47%) compared with the NAC (+ 19 and + 21%). In contrast, the selective D2 receptor antagonist raclopride (0.5 and 2.0 mg/kg, SC) increased DA release in the NAC (+ 65 and + 119%) to a greater extent than in the MPC (+ 45 and + 67%). These data suggest that the 5-HT2 receptor antagonistic properties of APZ and clozapine may contribute to their preferential effects on DA transmission in the MPC. Infusion of low doses (1, 10 microM, 40 min) of APZ through the probe in the DA terminal areas did not affect significantly DA outflow, while infusion of high doses (100, 1000 microM, 40 min) resulted in a more pronounced elevation of DA levels in the NAC (up to 961%) and the STR (up to 950%) than in the MPC (up to 316%). These findings indicate that the selective action of systemically administered APZ on DA in the MPC is most likely mediated at a level other than the terminal region. Taken together, the present results provide support for the notion that 5-HT2 receptor antagonism may be of considerable significance for the action of atypical antipsychotic drugs on mesolimbocortical dopaminergic neurotransmission.

The role of serotonin receptor subtypes in the behavioural effects of neuroleptic drugs. A paw test study in rats

The present study was designed to evaluate the roles of serotonin 5-HT1A and 5-HT2 receptors in the effects of neuroleptic drugs in the paw test. This behavioural test has been shown to model both the antipsychotic efficacy as well as the extrapyramidal side-effect liability of neuroleptic drugs. Whereas the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) reduced the effects of the classical neuroleptic haloperidol, it increased the effects of the atypical neuroleptic clozapine. The 5-HT2 receptor antagonist ketanserin as well as the 5-HT1C/5-HT2 receptor antagonist ritanserin, on the other hand reduced the effects of haloperidol, whereas the 5-HT1C/5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) reduced the effects of clozapine. The most important finding, however, was that the behavioural effects of different (putative) neuroleptics (fluphenazine, SCH-39166, remoxipride, prothipendyl, thioridazine and risperidone) were differentially influenced by both 8-OHDPAT and DOI, suggesting that there are important differences between the neuronal mechanisms underlying the behavioural effects of these neuroleptic drugs, even within the subclasses of classical and atypical neuroleptics.

Dopamine D2 and serotonin S2 receptors in susceptibility to methamphetamine psychosis detected by positron emission tomography

Positron emission tomography (PET) was used to assess the role of dopamine D2 receptors in the striatum and serotonin S2 receptors in the frontal cortex in the susceptibility to methamphetamine-induced psychosis. Subjects were six men who had previously experienced methamphetamine psychosis (methamphetamine subjects) and 10 age- and sex-matched control subjects. The radiotracer used was 11C-N-methylspiperone. Although binding availability, assessed by dynamic analysis, in the two regions did not differ between the two groups, the ratio of binding availability in the striatum to that in the frontal cortex significantly decreased in the methamphetamine subjects as compared with the control subjects. These findings suggest that an imbalance in the activity of these two receptors may be related to the susceptibility to methamphetamine psychosis.

Effects of serotonergic agents on apomorphine-induced locomotor activity

The interactions of serotonin 5-HT1A, 5-HT1C/2 and 5-HT3 receptor subtypes with apomorphine-induced locomotor activity (AILA) were investigated in Sprague-Dawley rats. The 5-HT3 antagonists ondansetron and ICS 205-930 had no significant effects on AILA. The 5-HT1A agonist 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) produced an increase in locomotor activity that was independent of DA neurotransmission. The locomotor activity induced by co-administration of apomorphine (APO; 0.25 mg/kg) and 8-OH-DPAT (0.25-1.0 mg/kg) was not significantly higher than those induced by APO alone during the peak period of APO stimulation of locomotor activity, nor were they higher than activity induced by 8-OH-DPAT alone during the same time intervals. The 5-HT1 antagonist (1)-propranolol had a depressant effect on AILA, but only at high doses. Coadministration of (1)-propranolol (5 mg/kg) and 8-OH-DPAT (1.0 mg/kg) elevated spontaneous locomotor activity for the first 10 min of the session when compared to 8-OH-DPAT (1.0 mg/kg) alone. The 5-HT2 antagonist ketanserin along with moderate and high doses of mesulergine depressed AILA, effects which may be mediated by the 5-HT2 antagonist properties of these drugs, by nonspecific sedation or by direct effects of these compounds on DA D2 receptors. In contrast to the high-dose mesulergine depression of AILA, a low dose (0.1 mg/kg) of mesulergine elevated AILA, an effect which was blocked by the 5-HT1C/2 agonist 1-(2,-5-dimethoxy-4-iodophenyl) -2-aminopropane (DOI; 1 mg/kg). Neither of these compounds at the doses tested had significant effects on spontaneous locomotor activity.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Clozapine alters mesolimbic dopamine (DA) function but spares nigrostriatal DA function in laboratory animals, but the underlying mechanism is unknown. In the present study, acute intraperitoneal injection of clozapine (5-40 mg/kg) increased extracellular DA levels in nucleus accumbens (Acb) and caudate-putamen (CPu) of awake, freely moving rats as measured by in vivo brain microdialysis, without anatomic selectivity. However, in serotonin (5HT)-denervated rats acute clozapine preferentially enhanced DA levels in Acb as compared to CPu. Since (i) up-regulation of 5HT receptors on DA neurons may result from 5HT denervation, (ii) clozapine has potent anti-5HT action, and (iii) 5HT receptors are more dense in Acb than CPu, these data appear to add additional weight to previous suggestions that a serotonergic mechanism may partly underlie clozapine's mesolimbic selectivity.

Neuroleptics increase C-FOS expression in the forebrain: Contrasting effects of haloperidol and clozapine

The mechanisms by which the atypical neuroleptic clozapine produces its therapeutic effects in the treatment of schizophrenia without causing the extrapyramidal side effects that are characteristic of most antipsychotic drugs remain unclear. Recently, a single injection of the typical antipsychotic haloperidol has been shown to increase c-fos expression in the striatum [Dragunow et al. (1990) Neuroscience 37, 287-294]. C-fos is a proto-oncogene that encodes a 55,000 mol. wt phosphoprotein, Fos, which is thought to assist in the regulation of "target genes" containing an AP-1 binding site. Because a wide variety of physiological and pharmacological stimuli increase c-fos expression, it has been proposed that Fos immunohistochemistry might be useful in mapping functional pathways in the central nervous system. The present experiments examined some potential neuroanatomical differences in the actions of clozapine and haloperidol by comparing their effects on c-fos expression in the medial prefrontal cortex, nucleus accumbens, striatum and lateral septum. The effects of the selective dopamine receptor antagonists SCH 23390 (D1) and raclopride (D2) were also examined. Haloperidol (0.5, 1 mg/kg) and raclopride (1, 2 mg/kg) produced large increases in the number of Fos-containing neurons in the striatum and nucleus accumbens. SCH 23390 (0.5, 1 mg/kg) reduced the number of Fos-positive neurons in the nucleus accumbens and striatum, and had no effect in the other regions. Neither haloperidol nor raclopride increased the number of Fos-positive neurons in the medial prefrontal cortex. Haloperidol, but not raclopride, produced a modest increase in c-fos expression in the lateral septal nucleus. Clozapine (10, 20 mg/kg) was without effect in the striatum; however, it significantly increased the number of Fos-positive neurons in the nucleus accumbens, medial prefrontal cortex and lateral septal nucleus. Destruction of mesotelencephalic dopaminergic neurons with 6-hydroxydopamine abolished the increase in Fos expression in the nucleus accumbens and striatum produced by haloperidol and raclopride, and also blocked the clozapine-induced increase in the nucleus accumbens. However, the inductive effects of clozapine and haloperidol on c-fos expression in the lateral septal nucleus and of clozapine in the medial prefrontal cortex were not affected by the 6-hydroxydopamine lesions. These results suggest that clozapine's unique therapeutic profile may be related to its failure to induce Fos in the striatum as well as its idiosyncratic actions in the lateral septum and medial prefrontal cortex. The effects of clozapine in these latter regions do not appear to be mediated by dopaminergic mechanisms.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.