Clozapine decreases serotonin extracellular levels in the nucleus accumbens by a dopamine receptor-independent mechanism

Acute administration of a dopamine D2/D3 receptor agonist alters behavioral and neural parameters in adult zebrafish

The dopaminergic neurotransmitter system is implicated in several brain functions and behavioral processes. Alterations in it are associated with the pathogenesis of several human neurological disorders. Pharmacological agents that interact with the dopaminergic system allow the investigation of dopamine-mediated cellular and molecular responses and may elucidate the biological bases of such disorders. Zebrafish, a translationally relevant biomedical research organism, has been successfully employed in prior psychopharmacology studies. Here, we evaluated the effects of quinpirole (dopamine D2/D3 receptor agonist) in adult zebrafish on behavioral parameters, brain-derived neurotrophic factor (BDNF) and neurotransmitter levels. Zebrafish received intraperitoneal injections of 0.5, 1.0, or 2.0 mg/kg quinpirole or saline (control group) twice with an inter-injection interval of 48 h. All tests were performed 24 h after the second injection. After this acute quinpirole administration, zebrafish exhibited decreased locomotor activity, increased anxiety-like behaviors and memory impairment. However, quinpirole did not affect social and aggressive behavior. Quinpirole-treated fish exhibited stereotypic swimming, characterized by repetitive behavior followed by immobile episodes. Moreover, quinpirole treatment also decreased the number of BDNF-immunoreactive cells in the zebrafish brain. Analysis of neurotransmitter levels demonstrated a significant increase in glutamate and a decrease in serotonin, while no alterations were observed in dopamine. These findings demonstrate that dopaminergic signaling altered by quinpirole administration results in significant behavioral and neuroplastic changes in the central nervous system of zebrafish. Thus, we conclude that the use of quinpirole administration in adult zebrafish may be an appropriate tool for the analysis of mechanisms underlying neurological disorders related to the dopaminergic system.

Asenapine elevates cortical dopamine, noradrenaline and serotonin release. Evidence for activation of cortical and subcortical dopamine systems by different mechanisms

RATIONALE

Asenapine, a psychopharmacologic agent developed for schizophrenia and bipolar disorder, has higher affinity for 5-HT(2A/C,6,7) and alpha(2) adrenergic receptors than for D(2) receptors. Asenapine exhibits potent antipsychotic-like effects without inducing catalepsy, increases cortical and subcortical dopamine release, and facilitates cortical glutamatergic transmission in rats. In this study, we further analyzed the effects of asenapine on dopaminergic, noradrenergic, and serotonergic systems in the rat brain.

MATERIALS AND METHODS

We studied the effects of asenapine on (1) dopaminergic neurons in the ventral tegmental area (VTA) and noradrenergic neurons in the locus coeruleus using in vivo single cell recording, (2) release of dopamine and noradrenaline (medial prefrontal cortex), serotonin (frontal cortex), and dopamine (nucleus accumbens), using in vivo microdialysis.

RESULTS

Systemic asenapine increased dopaminergic (0.001-0.2 mg/kg, i.v.) and noradrenergic (0.025-0.05 mg/kg i.v.) neuronal firing, and asenapine (0.1-0.2 mg/kg, s.c) increased cortical noradrenaline and serotonin output. Local asenapine administration increased all three monoamines in the cortex but did not affect accumbal dopamine output. Intra-VTA tetrodotoxin perfusion blocked asenapine-induced accumbal but not cortical dopamine outflow.

CONCLUSION

Asenapine at doses associated with antipsychotic activity enhanced cortical monoamine efflux. Whereas the asenapine-induced dopamine increase in nucleus accumbens is dependent on activation of dopaminergic neurons in the VTA, the increase of cortical dopamine outflow involves largely a local action at nerve terminals. Our data provide further insight on the pharmacologic characteristics of asenapine that may have bearing on its clinical efficacy in the treatment of schizophrenia and bipolar disorder.

Presynaptic receptors for dopamine, histamine, and serotonin

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinson's disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinson's disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.

Presynaptic modulation of 5-HT release in the rat septal region

5-HT released from serotonergic axon terminals in the septal nuclei modulates the activity of septal output neurons (e.g. septohippocampal cholinergic neurons) bearing somatodendritic 5-HT receptors. Therefore, we studied the mechanisms involved in the presynaptic modulation of 5-HT release in the lateral (LS) and medial septum (MS), and the diagonal band of Broca (DB). HPLC analysis showed that tissue concentrations of noradrenaline, dopamine and 5-HT were highest in DB (DB>MS>LS). Slices prepared from LS, MS and DB regions were preincubated with [(3)H]5-HT, superfused in the presence of 6-nitro-2-(1-piperazinyl)-quinoline (6-nitroquipazine) and electrically stimulated up to three times (first electrical stimulation period (S(1)), S(2), S(3); 360 pulses, 3 Hz, 2 ms, 26-28 mA). In all septal regions the Ca(2+)-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [(3)H] was inhibited by the 5-HT(1B) agonist CP-93,129 and the alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the mu- and kappa-opioid receptor agonists (d-Ala(2), N-Me-Phe(4), glycinol(5))-enkephalin (DAMGO) and [trans-(1S,2S(-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide hydro-chloride] (U-50,488H), respectively, acted inhibitory (although less potently), whereas the delta-opioid receptor agonist (d-Pen(2), d-Pen(5))-enkephalin (DPDPE), the dopamine D(2) receptor agonist quinpirole and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were all ineffective; the GABA(B) receptor agonist baclofen had weak effects. All inhibitory effects of the agonists were antagonized by the corresponding antagonists (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR-55,562), idazoxan, naloxone, nor-binaltorphimine), which also significantly enhanced the evoked release of 5-HT at S(1). It is concluded that 5-HT release in septal nuclei of the rat is modulated by presynaptic 5-HT(1B) autoreceptors, as well as by alpha(2)-, mu- and kappa-opioid heteroreceptors. All of these receptors seem to be under a tonic inhibitory influence of the corresponding endogenous agonists and show qualitatively comparable modulatory properties along the dorso-ventral distribution of the 5-HT terminals.

Heteromeric nicotinic acetylcholine-dopamine autoreceptor complexes modulate striatal dopamine release

In the striatum, dopamine and acetylcholine (ACh) modulate dopamine release by acting, respectively, on dopamine D(2) autoreceptors and nicotinic ACh (nACh) heteroreceptors localized on dopaminergic nerve terminals. The possibility that functional interactions exist between striatal D(2) autoreceptors and nACh receptors was studied with in vivo microdialysis in freely moving rats. Local perfusion of nicotine in the ventral striatum (shell of the nucleus accumbens) produced a marked increase in the extracellular levels of dopamine, which was completely counteracted by co-perfusion with either the non-alpha(7) nACh receptor antagonist dihydro-beta-erythroidine or the D(2-3) receptor agonist quinpirole. Local perfusion of the D(2-3) receptor antagonist raclopride produced an increase in the extracellular levels of dopamine, which was partially, but significantly, counteracted by coperfusion with dihydro-beta-erythroidine. These findings demonstrate a potent crosstalk between G protein-coupled receptors and ligand-gated ion channels in dopaminergic nerve terminals, with the D(2) autoreceptor modulating the efficacy of non-alpha(7) nACh receptor-mediated modulation of dopamine release. We further demonstrate physical interactions between beta(2) subunits of non-alpha(7) nicotinic acetylcholine receptors and D(2) autoreceptors in co-immunoprecipitation experiments with membrane preparations from co-transfected mammalian cells and rat striatum. These results reveal that striatal non-alpha(7) nicotinic acetylcholine receptors form part of heteromeric dopamine autoreceptor complexes that modulate dopamine release.

Microdialysis as a tool in local pharmacodynamics

In many cases the clinical outcome of therapy needs to be determined by the drug concentration in the tissue compartment in which the pharmacological effect occurs rather than in the plasma. Microdialysis is an in vivo technique that allows direct measurement of unbound tissue concentrations and permits monitoring of the biochemical and physiological effects of drugs throughout the body. Microdialysis was first used in pharmacodynamic research to study neurotransmission, and this remains its most common application in the field. In this review, we give an overview of the principles, techniques, and applications of microdialysis in pharmacodynamic studies of local physiological events, including measurement of endogenous substances such as acetylcholine, catecholamines, serotonin, amino acids, peptides, glucose, lactate, glycerol, and hormones. Microdialysis coupled with systemic drug administration also permits the more intensive examination of the pharmacotherapeutic effect of drugs on extracellular levels of endogenous substances in peripheral compartments and blood. Selected examples of the physiological effects and mechanisms of action of drugs are also discussed, as are the advantages and limitations of this method. It is concluded that microdialysis is a reliable technique for the measurement of local events, which makes it an attractive tool for local pharmacodynamic research.

Clozapine and haloperidol differentially alter the constitutive activity of central serotonin2C receptors in vivo

BACKGROUND

Central serotonin2C (5-HT2C) receptors are known to play a role in the mechanism of action of the antipsychotic drugs (APDs) clozapine and haloperidol. However, evidence for the involvement of the constitutive activity of 5-HT2C receptors in the dopamine (DA)ergic effects of APDs is lacking in vivo.

METHODS

Using in vivo microdialysis in halothane-anesthetized rats, we assessed the ability of selective 5-HT2C compounds to modulate the release of DA induced by haloperidol and clozapine in the nucleus accumbens and striatum.

RESULTS

Both APDs induced a dose-dependent increase in accumbal and striatal DA extracellular levels. The effect of .01 mg/kg haloperidol was potentiated by the 5-HT2C inverse agonist SB 206553 (5 mg/kg) but unaltered by the 5-HT2C antagonists SB 243213 and SB 242084 (1 mg/kg). Conversely, the effect of 1 mg/kg clozapine, a dose able to reverse the decrease in DA outflow induced by the 5-HT2C agonist Ro 60-0175 (3 mg/kg), was unaffected by SB 206553 but blocked by SB 243213 (1 mg/kg) and SB 242084 (.3 and 1 mg/kg).

CONCLUSIONS

These results show that clozapine and haloperidol differentially alter the constitutive activity of 5-HT2C receptors and suggest that clozapine behaves as a 5-HT2C inverse agonist in vivo.

Clozapine attenuates the locomotor sensitisation and the prepulse inhibition deficit induced by a repeated oral administration of Catha edulis extract and cathinone in rats

Locomotor sensitisation and deficits in prepulse inhibition (PPI) induced by psychostimulants are two paradigms that have been widely studied as animal behavioural models of psychosis. Clozapine is one of the atypical antipsychotic agents which has been widely employed to reverse the aforementioned behavioural changes in these usual models. In this particular study, locomotor sensitisation and prepulse inhibition deficit were induced under the same context by intermittent oral administration of S-(-)-cathinone or Catha edulis extract in rats. The rats were then challenged by administration of the atypical antipsychotic drug, clozapine and were finally challenged with psychostimulants after 2-week of withdrawal. Locomotor activity and PPI were assessed and later analyses of the neurotransmitter levels were made. The results of this experiment show that repeated oral administration of cathinone or C. edulis extract enhanced locomotor and exploratory activity and lead to a gradual deficit in prepulse inhibition. This locomotor sensitisation and PPI deficit could be reversed by administration of clozapine. A challenge with psychostimulant on day 40 (i.e., after 2-week of withdrawal) resulted in a response similar to the initial exposure (day 1). Neurotransmitter level analyses showed a significant increase in the level of dopamine in the prefrontal cortex (p < 0.05). There was also a significant decrease in the level of 5-hydroxytryptamine (5-HT) in the nucleus accumbens (p < 0.05) and its metabolite, 5-hydroxyindole acetic acid (5-HIAA) in the prefrontal cortex (p < 0.01). In the remaining regions (anterior and posterior striatum), there were no significant changes. In conclusion, this is the first study to demonstrate that repeated administration of C. edulis extract, or commercial cathinone, induces prepulse inhibition deficit and clozapine reverses both C. edulis or cathinone-induced sensitised locomotion and prepulse inhibition deficit.

Synergistic dopamine increase in the rat prefrontal cortex with the combination of quetiapine and fluvoxamine

RATIONALE

The combination of atypical antipsychotic drugs in addition to serotonin reuptake inhibitors has recently proven to be beneficial in a number of neuropsychiatric disorders, such as major depression, schizophrenia, and obsessive-compulsive disorder.

OBJECTIVES

To investigate the effects of an atypical antipsychotic drug in combination with a serotonin reuptake inhibitor on extracellular serotonin [5-HT]ex, and dopamine levels [DA]ex in different brain areas.

METHODS

The effects of quetiapine (10 mg/kg) with fluvoxamine (10 mg/kg) on [5-HT]ex and [DA]ex were compared in the rat dorsal striatum, prefrontal cortex, nucleus accumbens (core and shell), and thalamus by means of microdialysis coupled to HPLC with electrochemical detection.

RESULTS

Quetiapine had no significant effect on [DA]ex and [5-HT]ex levels in the prefrontal cortex and thalamus, but increased [DA]ex and [5-HT]ex levels in the dorsal striatum. In the accumbens, quetiapine increased [DA]ex levels and decreased [5-HT]ex levels. Fluvoxamine increased [5-HT]ex levels in all brain areas, and also increased [DA]ex levels in the striatum. The combination of quetiapine with fluvoxamine increased [DA]ex and [5-HT]ex levels in all brain areas compared with baseline. Although neither quetiapine nor fluvoxamine in monotherapy affected [DA]ex levels in the prefrontal cortex and thalamus, the combination produced a significant increase of [DA]ex levels in these two brain areas.

CONCLUSIONS

The combination of quetiapine with fluvoxamine causes a synergistic dopamine increase in the prefrontal cortex and the thalamus.

In vivo effects of aripiprazole on cortical and striatal dopaminergic and serotonergic function

In vivo microdialysis was used to monitor the effects of oral aripiprazole and olanzapine on basal extracellular concentrations of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindole acetic acid (5-HIAA) in the medial prefrontal cortex and striatum of conscious, freely moving rats. Acute aripiprazole administration did not affect dopamine output, but produced moderate increases in DOPAC and HVA concentrations, in medial prefrontal cortex or striatum of drug-naïve rats. Similarly, aripiprazole did not affect dopamine output but produced moderate elevations in DOPAC and HVA concentrations in the striatum of chronic aripiprazole-pretreated rats. Olanzapine produced comparatively larger elevations in dopamine, DOPAC, and HVA in both regions, which, in the striatum, were diminished after chronic olanzapine exposure. Aripiprazole reduced extracellular 5-HIAA concentrations in the medial prefrontal cortex and striatum of drug-nai;ve rats, but not in chronic aripiprazole-pretreated rats. Together, these data provide in vivo evidence of aripiprazole-induced changes in forebrain dopaminergic and serotonergic function that may reflect its partial agonist activity at presynaptic dopamine D(2) and 5-HT(1A) receptors and antagonist activity at 5-HT(2A) receptors.

Serotonergic dysfunction in schizophrenia assessed by the loudness dependence measure of primary auditory cortex evoked activity

Increased serotonergic activity is discussed as an important pathogenetic factor in schizophrenia. Further support for this hypothesis is difficult to obtain due to the lack of valid indicators of the brain's serotonin system. A great deal of evidence discovered through human and animal studies suggests that a weak loudness dependence of auditory evoked potentials (LDAEP) indicates high serotonergic activity and vice versa. The LDAEP is a measure of auditory cortex activity, reflecting increase or decrease of auditory evoked potential amplitudes with increasing tone loudness, which is probably modulated by the serotonergic innervation there. This is true only for the LDAEP of the primary auditory cortex, since this region is more highly innervated by serotonergic fibers than the secondary auditory cortex. The LDAEP (N1/P2 component) of 25 inpatients with schizophrenia free of medication and 25 healthy controls matched by age and gender, were recorded. Using dipole source analysis, the LDAEP of primary (tangential dipole) and this of secondary auditory cortex (radial dipole) was separately analyzed. Following a 4-week treatment with the 5-HT(2) antagonists clozapine or olanzapine, patients were once again studied. The LDAEP of the primary, but not of the secondary auditory cortex, was significantly weaker in the patients with schizophrenia than in healthy volunteers, indicating enhanced serotonergic neurotransmission. After treatment with the 5-HT(2) antagonists, the LDAEP (of the right hemisphere) tended to be increased, indicating normalization of serotonergic function in the patients with schizophrenia. These results suggest that the loudness dependence of primary auditory cortex evoked activity is well suitable to assess serotonergic dysfunction in schizophrenia.

Alpha-adrenoceptor modulation hypothesis of antipsychotic atypicality

Although all currently used antipsychotic drugs act as dopamine (DA) D2 receptor antagonists, clozapine, the prototype for atypical antipsychotics, shows superior efficacy, especially regarding negative and cognitive symptoms, in spite of a significantly reduced central D2 receptor occupancy compared with typical (conventional) antipsychotic drugs. Clozapine, as well as several other atypicals, displays significant affinities also for several other neurotransmitter receptors, including other dopaminergic receptors, alpha-adrenergic receptors and different serotonergic and cholinergic receptors, which in several ways may contribute to the clinical effectiveness of the drugs. Preclinical and clinical results suggest a dysregulated mesocorticolimbic DA system in schizophrenia, with an impaired prefrontal DA projection, which may relate to negative and cognitive symptoms, concomitant with an overactive or overreactive striatal DA projection, with bearing on psychotic (positive) symptomatology. Available data suggest that blockage of alpha1-adrenoceptors by antipsychotics may contribute to suppress positive symptoms, especially in acute schizophrenia, whereas alpha2-adrenoceptor blockage, a prominent effect of clozapine and, to some extent, risperidone but not other antipsychotics, may rather be involved in relief of negative and cognitive symptoms. Whereas alpha1-adrenoceptor blockage may act by suppressing, at the presynaptic level, striatal hyperdopaminergia, alpha2-adrenoceptor blockage may act by augmenting and improving prefrontal dopaminergic functioning. Thus, the prominent alpha1- and alpha2-adrenoceptor blocking effects of clozapine may generally serve to stabilize dysregulated central dopaminergic systems in schizophrenia, allowing for improved efficacy in spite of a reduced central D2 receptor occupancy compared with typical antipsychotic drugs.

Feeding-induced decrease in extracellular glutamate level in the rat nucleus accumbens: dependence on glutamate uptake

In vivo microdialysis combined with high-performance liquid chromatography and electrochemical detection was used to monitor extracellular glutamate levels in the medial nucleus accumbens of Sprague-Dawley rats during their feeding behaviour. Consumption of a palatable new diet or a diet to which rats were previously exposed caused a decrease in extracellular level of glutamate in the nucleus accumbens during and after feeding. The presentation of an inedible object (a piece of rubber) instead of the expected food caused a marked increase in extracellular glutamate levels. In contrast, if the piece of rubber was presented to rats that did not expect food delivery, the extracellular level of glutamate remained unchanged during the rubber presentation. The feeding-induced decrease in the extracellular glutamate level did not depend on food deprivation and was completely prevented by intraaccumbal infusions through the dialysis probe of 10 mM D,L-threo-beta-hydroxyaspartate (a glutamate uptake inhibitor). Intraaccumbal infusions of 10 microM S-(-)-raclopride L-tartrate (a D2/D3 dopamine receptor antagonist) or 1 microM tetrodotoxin (a voltage-dependent Na(+) channel blocker) also completely reversed the decrease in extracellular glutamate level in response to food intake. The D1/D5 dopamine receptor antagonist SCH-23390 (10 microM) administered into the nucleus accumbens had no significant effect on the feeding-induced decrease in extracellular glutamate level. From the data obtained we suggest that the decrease in the extracellular level of glutamate in the medial nucleus accumbens in response to feeding appears to arise from a temporal increase in glutamate uptake that is probably operated by dopamine inputs to the nucleus accumbens via D2/D3 receptors. Our findings also suggest that the dissociation between the expected biological value of a presented object and the reality might be an important determinant for regulation of glutamate release in this brain area during feeding behaviour.

Metabotropic glutamate mGlu5 receptor-mediated modulation of the ventral striopallidal GABA pathway in rats. Interactions with adenosine A(2A) and dopamine D(2) receptors

Interactions between subtypes of dopamine, glutamate and adenosine receptors seem to play an important integrative role in the function of striatal gamma-aminobutyric acid (GABA)ergic efferent neurons. Recent behavioral and biochemical studies suggest the existence of specific interactions between adenosine A2A receptors (A(2A)R), dopamine D2 receptors (D2R) and the group I metabotropic mGlu5 receptors (mGlu5R) in the dorsal striatum. The dual-probe approach in vivo microdialysis technique in freely moving rats was used to study the role of mGlu5R/A2AR/D2R interactions in the modulation of the ventral striopallidal GABA pathway. Perfusion of a selective mGlu5R agonist (CHPG) in the nucleus accumbens facilitated GABA release in the ipsilateral ventral pallidum. This effect was strongly potentiated by co-perfusion with the A2AR agonist CGS 21680. Co-perfusion with the D2R agonist quinpirole counteracted the increase in pallidal GABA levels induced by CGS 21680 and by CGS 21680 plus CHPG. These results demonstrate that mGlu5R/A2AR/D2R interactions play an important modulatory role in the function of the ventral striopallidal GABA pathway, which might have implications for the treatment of schizophrenia and drug addiction.

MS-377, a novel selective sigma(1) receptor ligand, reverses phencyclidine-induced release of dopamine and serotonin in rat brain

A novel selective sigma(1) receptor ligand, (R)-(+)-1-(4-chlorophenyl)-3-[4-(2-methoxyethyl)piperazin-1-yl]methyl-2-pyrrolidinone L-tartrate (MS-377), inhibits phencyclidine (1-(1-phenylcyclohexyl)piperidine; PCP)-induced behaviors in animal models. In this study, we measured extracellular dopamine and serotonin levels in the rat brain after treatment with MS-377 alone, using in vivo microdialysis. We also examined the effects of MS-377 on extracellular dopamine and serotonin levels in the rat medial prefrontal cortex after treatment with PCP. MS-377 itself had no significant effects on dopamine release in the striatum (10 mg/kg, p.o.) nor on dopamine or serotonin release in the medial prefrontal cortex (1 and 10 mg/kg, p.o.). PCP (3 mg/kg, i.p.) markedly increased dopamine and serotonin release in the medial prefrontal cortex. MS-377 (1 mg/kg, p.o.), when administered 60 min prior to PCP, significantly attenuated this effect of PCP. These results suggest that the inhibitory effects of MS-377 on PCP-induced behaviors are partly mediated by inhibition of the increase in dopamine and serotonin release in the rat medial prefrontal cortex caused by PCP.

Atypical neuroleptic drugs downregulate dopamine sensitivity in rat cortical and striatal astrocytes

Psychotic symptoms in different neuropsychiatric disorders are treated by neuroleptic drugs. Neuroleptics are known to block dopamine (DA) neurotransmission, however, cell types mediating their actions have not been determined. Recently, astrocytes have been demonstrated to express D1- and D2-DA receptors, whose activation leads to transient increases in intracellular calcium concentration. We show here that DA-sensitivity of cortical and striatal rat astroglial cultures, as monitored by calcium imaging, is reduced by a 12-h exposure to the atypical antipsychotic agents Clozapine (>1 nmol/liter), Olanzapine (>100 nmol/liter), and Risperidone (>1 nmol/liter), but not by classical neuroleptics Haloperidol and Sulpiride. These effects could not be reverted by the receptor-specific antagonists SCH23390, Sulpiride, L745 870, Ergotamine, and Propranolol. In addition, RT-PCR and Western blot analyses concerning the effects of Clozapine, Olanzapine, and Risperidone on DA receptor expression in cortical and striatal astroglial cells revealed no alterations in mRNAs and immunoreactive protein of D1- and D2-DA receptor subtypes. These results provide the first evidence that atypical but not classical neuroleptic drugs reduce astroglial DA-sensitivity, a mechanism that may be important for a better understanding of differences in effects and side effects between atypical and classical neuroleptic drugs.

The effects of antipsychotic drugs on serotonergic activity in the rat hippocampus

The serotonergic activity in hippocampus was investigated following acute and chronic treatment with the antipsychotic drugs haloperidol and risperidone. Acute administration of risperidone, the serotonin(2) (5-HT(2)) receptor antagonist ketanserin, and the dopamine (DA)-D(2) receptor antagonist raclopride increased the 5-hydroxyindoleacetic acid/serotonin (5-HIAA/5-HT) ratio. In contrast, acute administration of haloperidol did not affect this ratio. Chronic administration of risperidone maintained the increased 5-HIAA/5-HT ratio; a challenge dose of risperidone after the chronic treatment and the subsequent washout period also maintained the increased ratio. Chronic administration of haloperidol as well as a challenge dose of haloperidol following chronic treatment did not affect the serotonergic activity in hippocampus. Administration of ketanserin or raclopride after chronic treatment and the washout period induced an additional increase in the 5-HIAA/5-HT ratio in risperidone-treated rats. Moreover, a challenge dose of ketanserin, but not raclopride, increased the 5-HIAA/5-HT ratio in haloperidol-treated rats. The present results indicate that acute and chronic treatment of haloperidol or risperidone modified serotonergic activity in the hippocampus in a different way. Moreover, the augmentation of serotonergic activity induced by risperidone did not seem to be solely related to dopaminergic or serotonergic properties and may be of particular relevance for the amelioration of schizophrenia symptoms.

Effects of WAY 100635 on antipsychotic-induced catalepsy in 5-HT depleted animals: a role for tonic activation of 5-HT(1A) receptors

We recently observed that the 5-hydroxytryptamine (5-HT)(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-cycloh exanecarboxamide (WAY 100635) enhanced antipsychotic-induced catalepsy, which we hypothesized to be due to a blockade of tonic 5-HT(1A) receptor activation. Here, we examined this hypothesis by studying the effects of WAY 100635 in animals that were depleted of 5-HT by repeated treatment with the 5-HT synthesis inhibitor p-chlorophenylalanine methyl ester. Depletion of 5-HT abolished the enhancement by WAY 100635 of catalepsy induced by low doses of the antipsychotics nemonapride and raclopride, in agreement with the hypothesis that WAY 100635 enhances catalepsy by blocking tonic 5-HT(1A) receptor activation. Given the predictive validity of catalepsy, these findings indicate that 5-HT(1A) receptor blockade may enhance the extrapyramidal side-effects of antipsychotics in humans.

The dopamine D2 receptor antagonist sulpiride causes long-lasting serotonin release

The effects of the dopamine D2 receptor antagonist sulpiride on extracellular levels of serotonin (5-hydroxytryptamine, 5-HT) and the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) were examined by using in vivo voltammetry. Sulpiride (1 or 3 mM, 2 microl over 24 min) was administered to freely moving rats via a cannula implanted in the striatum and 5-hydroxyindole levels were measured by using a carbon fiber voltammetry electrode implanted in the ipsilateral striatum. Six to 8 h after injection, 5-hydroxyindole levels increased 3-fold, peaked 1 to 2 days post-injection, and returned to normal levels within 2 to 4 days. These effects were suppressed by pretreatment with p-chlorophenylalanine. Two days after sulpiride injection, high-performance liquid chromatography of striatal homogenates revealed that although the 5-HT concentration was unchanged, the 5-HIAA concentration was increased significantly. These results suggest that the long-lasting elevation of 5-hydroxyindole concentrations was primarily due to increased 5-HT release.

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

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.

Effect of antipsychotic drugs on extracellular serotonin levels in rat medial prefrontal cortex and nucleus accumbens

Amperozide, clozapine, olanzapine and risperidone are more potent serotonin (5-hydroxytryptamine, 5-HT)2A receptor antagonists than dopamine D2-like receptor antagonists. Haloperidol and S(-)-sulpiride are potent or selective dopamine D2-like receptor antagonists and lack 5-HT2A receptor antagonist properties. We studied the effect of these five proven antipsychotic drugs and one putative (amperozide) antipsychotic drug on extracellular 5-HT levels in the medial prefrontal cortex and the nucleus accumbens of awake, freely-moving rats, using in vivo microdialysis with dual probe implantation. Risperidone (1 mg/kg) and clozapine (20 mg/kg) significantly increased extracellular 5-HT levels in the medial prefrontal cortex and the nucleus accumbens, respectively. Amperozide (2 and 10 mg/kg) significantly increased extracellular 5-HT levels in both regions. Olanzapine (1 and 10 mg/kg), S(-)-sulpiride (10 and 25 mg/kg), haloperidol (0.1 and 1 mg/kg) and the selective 5-HT2A receptor antagonist MDL-100,907 (1 mg/kg) had no significant effect on extracellular 5-HT levels in either region. Thus, the ability to increase extracellular 5-HT levels in the medial prefrontal cortex and the nucleus accumbens by these antipsychotic drugs is not directly related to their affinity for 5-HT2A receptors since olanzapine and MDL-100,907 had no significant effect on extracellular 5-HT levels. A variety of mechanisms other than those involving 5-HT2A receptors, e.g., reuptake inhibition (amperozide) and blockade of alpha2-adrenoceptors (clozapine), may contribute to the ability to increase extracellular 5-HT levels in the brain. The increase in extracellular 5-HT levels in the medial prefrontal cortex or nucleus accumbens following amperozide, clozapine, or risperidone administration may not be related to the effect on psychotic symptoms but could be related to effects on other types of psychopathology such as depression, negative symptoms, or cognition.

Dopaminergic regulation of serotonin release in the substantia nigra of the freely moving rat using microdialysis

The functional regulation by dopamine (DA) receptors of serotonin (5-HT) release from the rat substantia nigra (SN) was investigated using in vivo microdialysis. A D1- and D2-receptor-mediated inhibition of nigral 5-HT release was demonstrated in this study. Continuous administration of the D1-receptor agonist CY 208243 (10 microM) through the probe did not alter extracellular DA nor 5-HT from the SN, whereas intranigral administration of the D1-receptor antagonist SCH-23390 HCl (10 microM) significantly increased both DA (to 214%) and 5-HT release (to 168%) from the SN. Co-perfusion of the D1-receptor agonist and antagonist did not change nigral DA nor 5-HT release compared to perfusion of the antagonist alone. The continuous intranigral perfusion of the D2-receptor agonist, (-)-quinpirole HCl (1 microM) significantly decreased both DA ad 5-HT release to 71% and 78%, respectively. These decreases were abolished when the D2-receptor antagonist S(-)-sulpiride (10 microM) and the D2-receptor agonist (-)-quinpirole HCl (1 microM) were co-perfused. In contrast, the intranigral perfusion of the DA precursor, L-DOPA (5 microM; 1 h), significantly increased nigral and striatal 5-HT release to 202% and 155%, respectively. This enhanced nigral 5-HT release might not be receptor-mediated. The results of the present study suggest a D1 and D2 regulation of nigral 5-HT release, either directly mediated by DA receptors on nigral 5-HT terminals or indirectly by nigral GABA, Glu or Asp. Alternatively, the observed DA-5HT-interaction in the SN might not reflect a local interaction but might involve an interaction at the level of the serotonin cell body region, the dorsal raphe nuclei (DRN).

5-HT4 receptor involvement in the serotonin-enhanced dopamine efflux from the substantia nigra of the freely moving rat: a microdialysis study

The functional regulation by serotonin (5-HT) receptors of the 5-HT-enhanced dopamine (DA) release from the rat substantia nigra (SN) was investigated using in vivo microdialysis. Exogenously administered or extracellularly enhanced 5-HT (by means of intranigral citalopram perfusion) (both 1 microM for 1 h) significantly increased nigral DA efflux to 165% and 145%, respectively. Intranigral administration of pindolol (10 microM, 3 h), a 5-HT1A/1B receptor antagonist which is clinically used in order to block 5-HT1A/1B autoreceptors, did not affect DA levels but significantly increased nigral 5-HT levels to 135%. Co-perfusion of this antagonist with 5-HT (1 microM, 1 h) did not abolish the 5-HT-induced DA release from the SN as DA was increased to 166%. Local application of the 5-HT1A/1B receptor agonist, CP 93129 (1 microM, 1 h), increased DA release from the SN to 4770% whereas 5-HT release was significantly decreased to 75%. Co-perfusion of the 5-HT1A/1B receptor antagonist, pindolol, with this agonist only partly abolished the CP 93129-induced DA release whereas the CP 93129-induced decrease in nigral 5-HT release was completely abolished. Administration of the 5-HT2A/2C receptor antagonist, ketanserin (50 microM, 3 h), significantly increased DA to 143% and 5-HT release to 363%. Co-perfusion of this antagonist with 5-HT still caused an increase in nigral DA release to 214%. Intranigral perfusion of the 5-HT4 receptor antagonist, RS 39604 (10 microM, 3 h), did not affect DA levels but significantly decreased nigral 5-HT levels to 74%. Co-perfusion of this antagonist with 5-HT was able to prevent the 5-HT-enhanced DA efflux from the SN. From this study it can be concluded that the 5-HT-enhanced (and possibly the citalopram-induced) nigral DA release is 5-HT4 receptor mediated.

5-HT1A receptor agonist properties of the antipsychotic, nemonapride: comparison with bromerguride and clozapine

5-HT1A receptor agonists are thought to enhance the antipsychotic-like effects of dopamine D2 receptor antagonists while reducing their potential to produce extrapyramidal side effects. Thus, 5-HT1A receptor agonist properties of mixed 5-HT1A receptor agonists/D2 receptor antagonists might be of clinical importance. The antipsychotics, clozapine and nemonapride, and the putative antipsychotic, bromerguride, have intermediate to high affinity for 5-HT1A receptors. The present study examined the 5-HT1A receptor agonist activity of nemonapride and bromerguride, in comparison with clozapine, which has partial 5-HT1A receptor agonist properties in vitro. Here, 5-HT1A receptor activation was examined in vitro, by measuring forskolin-stimulated cAMP accumulation in HeLa cells expressing human 5-HT1A receptors, and in vivo, by using microdialysis to measure the extracellular concentration of hippocampal 5-hydroxytryptamine (5-HT) in rats. Nemonapride markedly decreased both forskolin-stimulated cAMP accumulation and the extracellular concentration of 5-HT; both effects were antagonized by the 5-HT1A receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide (WAY100635). In contrast, clozapine only partially decreased forskolin-stimulated cAMP accumulation and extracellular 5-HT, and only its effects on cAMP accumulation were attenuated by WAY100635. Bromerguride decreased neither forskolin-stimulated cAMP accumulation nor extracellular 5-HT; instead, it antagonized the decrease of cAMP accumulation produced by 5-HT and the decrease of extracellular 5-HT produced by the 5-HT1A agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The selective D2 receptor antagonist, raclopride, affected neither forskolin-stimulated cAMP in vitro nor extracellular 5-HT in vivo. Thus, in contrast with clozapine and bromerguride, only the novel antipsychotic, nemonapride, exhibited marked 5-HT1A receptor agonist properties both in vitro and in vivo; conceivably, these properties may play a role in its preclinical and clinical effects.

Effect of chronic antipsychotic drug treatment on preprosomatostatin and preprotachykinin A mRNA levels in the medial prefrontal cortex, the nucleus accumbens and the caudate putamen of the rat

In situ hybridization histochemistry was used to study the expression of preprosomatostatin (PPSOM) and preprotachykinin A (PPT-A) mRNA in the medial prefrontal cortex (mPFC), the nucleus accumbens (NAC) and the caudate putamen (CP) of the rat after chronic (21 days) treatment with the classical antipsychotic drug haloperidol (1 mg/kg i.p.), the atypical antipsychotic drugs clozapine (15 mg/kg i.p.) and amperozide (5 mg/kg i.p.), and the selective dopamine (DA)-D2/D3 receptor antagonist raclopride (2 mg/kg i.p.). Whereas amperozide markedly elevated the numerical density of PPSOM mRNA expressing neurons in the mPFC (52%), the other drugs did not significantly affect PPSOM mRNA levels in any of the brain regions studied. Amperozide also altered PPT-A mRNA expression in the mPFC, i.e. a decrease (22%) was found. Of the other drugs tested only haloperidol significantly decreased PPT-A mRNA levels in the NAC shell (14%), in the dorso-lateral CP (19%) and in the medial CP (15%). In view of the differences between amperozide and the other drugs studied, as regards both pre-clinical and clinical characteristics, we suggest that the specific effects of amperozide on PPSOM and PPT-A mRNA in the mPFC may be related to its 5-HT releasing action in the frontal cortex, an effect possibly caused by its alpha2-adrenoceptor blocking activity. This effect, in turn, may be related to an antidepressant-like action that this compound exhibits in animal studies. The decrease in PPT-A mRNA levels seen after the haloperidol treatment is probably due to its potent DA-D2 receptor antagonism and may be related to side-effects, rather than therapeutic effects of this drug.

Kindling with clozapine: behavioral and molecular consequences

Clozapine is an 'atypical' neuroleptic that improves symptoms of many patients with schizophrenia whose illness is resistant to treatment with other neuroleptics. Unlike the 'typical neuroleptics (chlorpromazine, haloperidol), clozapine does not induce extrapyramidal symptoms such as Parkinsonism and tardive dyskinesia in humans or catalepsy in the rat. However, clozapine frequently causes epileptiform EEG changes and causes seizures in 3-5% of patients treated with this drug in therapeutic doses. Clozapine also induces dose dependent myoclonus in the partially restrained rat. In the experiments reported here, partially restrained rats were administered repeated alternate day or weekly low, fixed doses of clozapine (1 mg/kg). This dose initially caused no behavioral change. Following the third and subsequent administrations, the same dose elicited an increasing number of myoclonic seizure-like jerks reaching 140/h following the 15th injection in rats receiving the same low dose of clozapine on alternate days and 160/h following the 9th injection in animals that received the same dose once weekly. These effects are consistent with kindling, i.e. a progressive increase of brain excitability following repeated administration of a fixed subconvulsive dose of an excitatory agent. Clozapine kindled animals exhibited a significantly different pattern of early gene expression in ventral tegmental area, origin of the mesolimbic-mesocortical dopamine system and in the anterior thalamic nuclei, compared with saline treated controls subjected to exactly the same recording conditions. The evidence of central nervous system excitation with clozapine may be important to the unique therapeutic effect of this atypical antipsychotic in the treatment of symptoms, especially the deficit symptoms, of schizophrenia.

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.