GABA(B) receptor-mediated increase of neurosteroids by gamma-hydroxybutyric acid

Among the pharmacological actions of gamma-hydroxybutyric acid (GHB), some may involve GABA(A) receptor-mediated mechanisms. GHB, however, fails to directly interact with sites for agonists and modulators on the GABA(A) receptor complex. We hypothesized that, in vivo, GHB may interfere with GABA(A) receptor function by altering the brain concentrations of the neurosteroids 3 alpha-hydroxy-5 alpha-pregnan-20-one (allopregnanolone, AP) and 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one (allotetrahydrodeoxycorticosterone, THDOC), positive allosteric modulators of GABA-gated chloride currents. In male Wistar rats, GHB dose-dependently (75-1000 mg/kg, i.p.) increased AP, THDOC and their precursors pregnenolone and progesterone in brain cortex and hippocampus. The increases of AP (4-5 fold) and THDOC (3-4 fold) elicited by 300 mg/kg GHB peaked between 30 and 90 min and abated by 180 min. The selective GABA(B) receptor antagonist SCH 50911 (50 mg/kg, i.p.) prevented the action of GHB, while the GABA(B) receptor agonist baclofen (5-10 mg/kg) mimicked it. NCS-382 (50 mg/kg, i.p.), the purported selective antagonist of the GHB receptor, failed to antagonize GHB, but at 300 mg/kg increased brain cortical neurosteroids to the same extent as 300 mg/kg GHB; coadministration of GHB and NCS-382, however, failed to yield an additive effect. These results strongly suggest that GHB, via a GABA(B) receptor-mediated mechanism, increases the brain concentrations of neurosteroids, whose properties as amplifiers of the GABA-gated chloride conductances may play a role in the GABA(A) receptor-mediated pharmacological actions of GHB.     

Olanzapine and clozapine increase the GABAergic neuroactive steroid allopregnanolone in rodents.

The neuroactive steroid allopregnanolone is a potent gamma-aminobutyric acid type A (GABA(A)) receptor modulator with anxiolytic and anticonvulsant effects. Olanzapine and clozapine also have anxiolytic-like effects in behavioral models. We therefore postulated that olanzapine and clozapine would elevate allopregnanolone levels, but risperidone and haloperidol would have minimal effects. Male rats received intraperitoneal olanzapine (2.5-10.0 mg/kg), clozapine (5.0-20.0 mg/kg), risperidone (0.1-1.0 mg/kg), haloperidol (0.1-1.0 mg/kg), or vehicle. Cerebral cortical allopregnanolone and peripheral progesterone and corticosterone levels were determined. Adrenalectomized animals were also examined. Both olanzapine and clozapine increased cerebral cortical allopregnanolone levels, but neither risperidone nor haloperidol had significant effects. Olanzapine and clozapine also increased serum progesterone and corticosterone levels. Adrenalectomy prevented olanzapine- and clozapine-induced elevations in allopregnanolone. Allopregnanolone induction may contribute to olanzapine and clozapine anxiolytic, antidepressant, and mood-stabilizing actions. Alterations in this neuroactive steroid may result in the modulation of GABAergic and dopaminergic neurotransmission, potentially contributing to antipsychotic efficacy.     

Characterization of steroid interactions with gamma-aminobutyric acid receptor-gated chloride ion channels: evidence for multiple steroid recognition sites.

The potentiation of gamma-aminobutyric acid (GABA) receptor-mediated 36Cl- uptake by various steroids has been characterized in rat cerebral cortical synaptoneurosomes. Several of these steroids, including 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha-OH-DHP) and 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one (THDOC), increase the potency of muscimol to stimulate 36Cl- uptake in a concentration-dependent and stereospecific manner. Concentration-response curves for 3 alpha-OH-DHP, THDOC, 3 alpha-hydroxy-pregn-4-en-20-one, and pentobarbital enhancement of muscimol-stimulated 36Cl- uptake are biphasic, with Hill coefficients significantly less than 1.0. Computer-modeling (ALLFIT analysis) of these curves suggests that these steroids and pentobarbital interact with multiple binding sites on GABAA receptor(s). In contrast, the concentration-response curve for THDOC 21-mesylate is monophasic, with a smaller maximal response, and yields a Hill coefficients of 1.0. In addition to modulating GABA receptor-mediated 36Cl- uptake, THDOC enhanced the ability of the benzodiazepine clonazepam to potentiate muscimol-stimulated 36Cl- uptake. The central benzodiazepine antagonist Ro15-1788 failed to inhibit THDOC-induced potentiation of muscimol-stimulated 36Cl- uptake, although it has been previously reported to inhibit some of the behavioral actions of THDOC. In contrast to the A ring-reduced metabolites and analogues of progesterone and deoxycorticosterone, glucocorticoids had no effect on muscimol-stimulated 36Cl- uptake in cerebral cortical synaptoneurosomes at concentrations between 20 nM and 5 microM.     

Clozapine decreases rat brain dehydroepiandrosterone and dehydroepiandrosterone sulfate levels.

We examined the influence of the atypical antipsychotic agent clozapine compared to haloperidol, on levels of dehydroepiandrosterone and dehydroepiandrosterone sulfate ester (both gamma-aminobutyric acid(A) (GABA(A)) receptor allosteric antagonists) in the rat cortex. i.p. injections of clozapine (5 and 15 mg/kg), but not haloperidol (1 mg/kg), for 8 days decreased rat brain cortical dehydroepiandrosterone and dehydroepiandrosterone sulfate levels. These findings support the role of neurosteroids and possibly GABA(A) receptor modulation in the mechanism of action of clozapine.     

Atypical, but not typical, antipsychotic drugs increase cortical acetylcholine release without an effect in the nucleus accumbens or striatum.

The role of acetylcholine (ACh) in the action of antipsychotic drugs (APDs) was studied by microdialysis, without AChesterase inhibition, to facilitate the interpretation of any observed drug effects. The atypical APDs, clozapine (2.5-20 mg/kg), olanzapine (10 mg/kg), risperidone (1 mg/kg), and ziprasidone (3 mg/kg) significantly increased ACh release in rat medial prefrontal cortex (mPFC), whereas the typical APDs, haloperidol (0.1-1 mg/kg), S(-)-sulpiride (10-25 mg/g), and thioridazine (5-20 mg/kg) did not. None of seven APDs increased ACh release in the nucleus accumbens or striatum at the doses effective in the mPFC. Thus, atypical and typical APDs may differ in the ability to increase cortical ACh release, a possible factor contributing to cognitive improvement in schizophrenia. After perfusion with neostigmine, an AChesterase inhibitor, clozapine, but not haloperidol, increased ACh release in all three aforementioned brain regions with an enhanced effect in the mPFC, indicating the importance of studying ACh release in the absence of AChesterase inhibition. Clozapine, and perhaps other atypical APDs, alone or in combination with an AChesterase inhibitor, may improve cognition in schizophrenia, and perhaps other cognitive disorders, e.g., early Alzheimer's disease, by enhancing cortical cholinergic transmission.     

Chronic haloperidol and clozapine produce different patterns of effects on phosphodiesterase-1B, -4B, and -10A expression in rat striatum

Phosphodiesterase-10A (PDE10A), -1B (PDE1B), -4B (PDE4B), and -4A (PDE4A) are important regulators of signal transduction in striatum due to their catalysis of cyclic AMP and cyclic GMP. The typical antipsychotic drug haloperidol and the atypical antipsychotic drug clozapine are thought to regulate cyclic nucleotide signaling in striatum. Since this brain region is essential in mediation of both therapeutic and extrapyramidal side effects, it was of interest to determine whether chronic treatment for 21 days with haloperidol (1 mg/kg) or clozapine (20 mg/kg) affected PDE expression in rat striatum. This was accomplished using SDS-PAGE/immunoblotting and real-time RT-PCR. Both antipsychotic drugs increased PDE10A and did not change PDE4A. By contrast, PDE1B was increased by haloperidol treatment, but not clozapine treatment, while PDE4B was increased by clozapine, but not haloperidol. In all cases, changes in protein expression were accompanied by corresponding changes in mRNA, and only were observed with chronic treatment. Up-regulation of PDEs may represent compensatory responses to haloperidol and clozapine treatments, due to altered cyclic nucleotide signaling, and that different patterns of effects produced by these drugs may be associated with their mechanisms of action.     

Nicotine-induced changes in cerebrocortical neuroactive steroids and plasma corticosterone concentrations in the rat

Nicotine, one of the most widely used psychotropic substances, is able to induce both anxiolytic and anxiogenic effects. The effect of this drug on the brain and plasma concentrations of neuroactive steroids was examined in the rat. Anxiolytic doses of nicotine (0.03-0.3 mg/kg) had no significant effect, whereas administration of anxiogenic doses (0.5 to 2 mg/kg) produced a dose- and time-dependent increase in the cerebrocortical concentrations of pregnenolone, progesterone, and allopregnanolone, with the greatest observed effects (+180%, +223%, and +124%, respectively) apparent at the dose of 2 mg/kg. In contrast, nicotine (1-2 mg/kg) decrease by 31% and 38%, respectively, the concentration of 3alpha,21-dihydroxy-5alpha-pregnan-20-one (allotetrahydrodeoxycorticosterone, or THDOC) in the cerebral cortex. Nicotine also increased the plasma concentrations of pregnenolone and progesterone, whereas failed to affect significantly those of allopregnanolone or THDOC. Nicotine induced a dose- and time-dependent increase in the plasma concentration of corticosterone, indicating that this drug activates the hypothalamic-pituitary-adrenal (HPA) axis. These results suggest that the changes in emotional behavior elicited by nicotine, similar to those induced by stressful stimuli or other anxiogenic drugs, are associated with an increase in neuroactive steroids content of the brain.     

Effects of antipsychotic drugs on cholecystokinin and preprotachykinin (substance P) mRNA expression in the rat hippocampal formation.

To assess the involvement of substance P (SP) and cholecystokinin (CCK) in the effects of antipsychotic drugs, preprotachykinin-A (PPT-A) and CCK mRNA expression was studied in the hippocampal formation using in situ hybridisation following 21 daily i.p. injections with the typical antipsychotic drug haloperidol (1 mg/kg) and the atypical drug clozapine (15 mg/kg). PPT-A mRNA levels were increased in the hippocampal CA3 subregion and in the entorhinal cortex after haloperidol, whereas a decrease was observed in the CA1 after clozapine. CCK mRNA levels increased in the CA1, the entorhinal cortex and in hilus, following both haloperidol and clozapine. It is suggested that earlier findings of increased SP levels in the hippocampal formation of schizophrenics may be a consequence of haloperidol treatment and that reduced hippocampal CCK and CCK mRNA levels found earlier in schizophrenics do not result from antipsychotic drug treatment. These results are consonant to the hypothesis that increased cortical CCK transmission may be beneficial in the treatment of psychosis.     

Apomorphine-induced disruption of prepulse inhibition that can be normalised by systemic haloperidol is insensitive to clozapine pretreatment

Rationale Prepulse inhibition (PPI) of startle refers to the phenomenon in which a weak prepulse attenuates the startle response to a succeeding intense stimulus. PPI can be disrupted by systemic apomorphine in animals, and reduced PPI has been consistently reported in schizophrenia patients. The ability of the atypical antipsychotic clozapine to reverse apomorphine-induced PPI deficit has been demonstrated in the rat, but has not yet been tested in the mouse. The present study was designed to fill this gap.Objective and results We investigated the efficacy of clozapine in reversing apomorphine-induced (2.0 or 2.5 mg/kg, SC) PPI deficit in C57BL6 mice. Clozapine failed to restore PPI disruption in apomorphine-treated mice in two independent laboratories across two dose ranges (1–3 mg/kg, IP, or 3–30 mg/kg, PO), whereas the typical antipsychotic haloperidol (1 mg/kg,IP) completely normalised PPI performance.Conclusions Unlike the rat, apomorphine-induced PPI disruption in mice might be instrumental in distinguishing between typical and atypical antipsychotic drugs. This also lends further support to the suggestion that the neuropharmacology of PPI is not identical in the two rodent species.     

Behavioral effects of sertindole,risperidone, clozapine and haloperidol inCebus monkeys

Extrapyramidal side effects (EPS) are major limitations to neuroleptic treatment of psychoses. To evaluate further the behavioral characteristics of the novel antipsychotic agents, a wide range of single intramuscular doses of sertindole (0.1–2.5 mg/kg IM), risperidone (0.01–0.25 mg/kg IM), clozapine (1.0–25.0 mg/kg IM), and haloperidol (0.01–0.25 mg/kg IM) were blindly evaluated at weekly intervals inCebus monkeys previously sensitized to neuroleptics. All drugs except clozapine produced dystonia and parkinsonian symptoms, but haloperidol and risperidone were 50–100 times more potent than sertindole in producing EPS. Sertindole, risperidone and haloperidol had no significant sedative effects, whereas clozapine produced dose related sedation. Risperidone, clozapine and haloperidol but not sertindole decreased locomotor activity. Sertindole, risperidone and clozapine had a calming effect at doses below the EPS threshold, unlike haloperidol. Sertindole has many behavioral effects in nonhuman primates that are similar to those seen with the new antipsychotics, risperidone and clozapine, which suggests a favorable antipsychotic benefit/risk ratio in the clinic, especially regarding EPS.     

Evaluation of dopamine D-2 receptor occupancy by clozapine, risperidone, and haloperidol in vivo in the rodent and nonhuman primate brain using 18F-fallypride.

We have used the high-affinity dopamine D-2 receptor radioligand, 18F-fallypride for evaluating receptor occupancy by the antipsychotic drugs, clozapine, risperidone, and haloperidol in rodents and nonhuman primates. In rodents, clozapine (0.1 mg/kg to 100 mg/kg) competed with 18F-fallypride at all the doses administered. At doses over 40 mg/kg, clozapine was able to displace all the administered 18F-fallypride. A pseudobiphasic profile of receptor occupancy by clozapine was observed. This behavior was compared with such other neuroleptics as risperidone and haloperidol that exhibited over 90% receptor occupancy at doses over 0.1 mg/kg and did not exhibit a biphasic nature. Dopamine D-2 receptor occupancy in the monkeys was studied using positron emission tomography (PET) after acute subcutaneous doses of the various drugs. At therapeutically relevant doses, clozapine, risperidone, and haloperidol were able to compete significantly with the binding of 18F-fallypride in all brain regions in rhesus monkeys, and our analyses indicate that these drugs (clozapine, risperidone, and haloperidol) do not discriminate between the striatal (caudate and putamen) and the extrastriatal (thalamus and cortical regions) dopamine receptors. The following extent of D-2 receptor occupancies were measured in the monkey brain using PET: clozapine approximately 70% (dose of 9.7 mg/kg), risperidone approximately 75% (0.05 mg/kg), and haloperidol approximately 90% (0.05 mg/kg).     

A comparison of drug effects in latent inhibition and the forced swim test differentiates between the typical antipsychotic haloperidol,the atypical antipsychotics clozapine and olanzapine,and the antidepressants imipramine and paroxetine

Current animal models of antipsychotic activity that have the capacity to dissociate between typical and atypical antipsychotic drugs (APDs) have two drawbacks: they require previous administration of a psychotomimetic drug, and they achieve the dissociation by demonstrating effectiveness of atypical but not typical APDs, thus losing specificity and selectivity for APDs. The present experiments were designed to solve these problems by using two non-pharmacological tests: latent inhibition (LI), in which potentiation of the deleterious effects of non-reinforced stimulus pre-exposure on its subsequent conditioning served as a behavioral index for a common action of typical and atypical APDs (antipsychotic), and the forced swim test (FST), in which reduction of immobility served as a behavioral index for a dissimilar action of these drugs (antidepressant). The typical APD haloperidol (0.1 mg/kg), the atypical APDs clozapine (2.5 mg/kg) and olanzapine (0.6 mg/kg), and the antidepressants imipramine (10 mg/kg) and paroxetine (7.0 mg/kg), produced distinct patterns of action in the two tests: haloperidol potentiated LI and increased immobility in the FST, clozapine and olanzapine potentiated LI and decreased immobility in the FST, and imipramine and paroxetine decreased immobility in the FST and did not potentiate LI. Thus, the comparison of drug effects in LI and FST enabled a discrimination between typical and atypical APDs without losing selectivity for APDs.     

Investigation of the effects of lamotrigine and clozapine in improving reversal-learning impairments induced by acute phencyclidine and <Emphasis Type="SmallCaps">d</Emphasis>-amphetamine in the rat

Rationale Phencyclidine (PCP), a glutamate/N-methyl-d-aspartate (NMDA) receptor antagonist, has been shown to induce a range of symptoms similar to those of patients with schizophrenia, while d-amphetamine induces predominantly positive symptoms. Previous studies in our laboratory have shown that PCP can selectively impair the performance of an operant reversal-learning task in the rat. Furthermore, we found that the novel antipsychotic ziprasidone, but not the classical antipsychotic haloperidol, could prevent the PCP-induced deficit.Objectives The aim of the present study was to validate the model further using the atypical antipsychotic clozapine and then to investigate the effects of lamotrigine, a broad-spectrum anticonvulsant that is known to reduce glutamate release in vitro and is able to prevent ketamine-induced psychotic symptoms in healthy human volunteers. A further aim was to compare effects of PCP and d-amphetamine in the test and investigate the effects of the typical antipsychotic haloperidol against the latter.Methods Female hooded-Lister rats were food deprived and trained to respond for food in a reversal-learning paradigm.Results PCP at 1.5 mg/kg and 2.0 mg/kg and d-amphetamine at 0.5 mg/kg significantly and selectively impaired performance in the reversal phase of the task. The cognitive deficit induced by 1.5 mg/kg PCP was attenuated by prior administration of lamotrigine (20 mg/kg and 30 mg/kg) or clozapine (5 mg/kg), but not haloperidol (0.05 mg/kg). In direct contrast, haloperidol (0.05 mg/kg), but not lamotrigine (25 mg/kg) or clozapine (5 mg/kg), prevented a similar cognitive impairment produced by d-amphetamine (0.5 mg/kg).Conclusions Our findings provide further data to support the use of PCP-induced disruption of reversal learning in rodents to investigate novel antipsychotic drugs. The results also provide evidence for different mechanisms of PCP and d-amphetamine-induced disruption of performance in the test, and their different sensitivities to typical and atypical antipsychotic drugs.     

Clozapine improves deficient inhibitory auditory processing in DBA/2 mice,via a nicotinic cholinergic mechanism

Abstract Rationale. Insufficient inhibitory processing of the P50 auditory evoked potential (AEP) is observed in most schizophrenia patients and is not improved by typical antipsychotic drugs, such as haloperidol. This inhibitory processing deficit is associated with a subnormal level of hippocampal α7 nicotinic receptors (nAChRs), and drugs that activate these receptors normalize the deficit. The atypical antipsychotic clozapine also normalizes this deficit in schizophrenia patients, but by an unknown mechanism. Objective. Similar to schizophrenia patients, DBA/2 mice spontaneously exhibit a deficit in inhibitory processing of the P20-N40 AEP, which is a rodent analogue of the human P50 AEP. The present study determined whether clozapine improved this deficit in DBA/2 mice, and by what mechanism. Method. Using a conditioning-testing paradigm with paired auditory stimuli to assess inhibitory P20-N40 AEP processing in DBA/2 mice, the effects of clozapine (0.1, 1, 3.33, or 10 mg/kg, i.p.) and haloperidol (1 mg/kg, i.p.) were assessed. The effect of clozapine (1 mg/kg) was assessed alone and after pre-administration of either α-bungarotoxin, an α7 nAChR antagonist, or dihydro-β-erythroidine, an α4β2 nAChR antagonist. Results. In a dose-dependent manner, clozapine improved the deficient inhibitory processing of the P20-N40 AEP normally exhibited by DBA/2 mice. Like α7 agonists, 1 mg/kg clozapine selectively increased the inhibition of the P20-N40 response to the second of paired auditory stimuli. The normalizing effect of 1 mg/kg clozapine was blocked by α-bungarotoxin, but not by dihydro-β-erythroidine. Haloperidol did not improve DBA/2's deficient P20-N40 AEP processing. Conclusions. Clozapine improved the deficient inhibitory processing of the P20-N40 AEP in DBA/2 mice, apparently through stimulation of α7 nicotinic receptors. This effect was not shared by the typical antipsychotic haloperidol. Electronic Publication     

The latent inhibition model dissociates between clozapine, haloperidol, and ritanserin.

Latent inhibition (LI), i.e., retarded conditioning to a stimulus following its nonreinforced preexposure, is impaired in some subsets of schizophrenia patients and in amphetamine-treated rats. Typical and atypical antipsychotic drugs (APD's) potentiate LI, but to date the model has not dissociated between them. This study demonstrates such a dissociation using haloperidol (0.1 mg/kg), clozapine (5 mg/kg), and ritanserin (0.6 mg/kg) administered in preexposure and/or conditioning. Under conditions which did not yield LI in vehicle controls (40 preexposures and five conditioning trials), both haloperidol and clozapine, but not ritanserin, led to LI when administered in conditioning. Under conditions which led to LI in vehicle controls (40 preexposures and two conditioning trials), clozapine and ritanserin, but not haloperidol, abolished LI when administered in preexposure. It is suggested that LI potentiation via conditioning detects the "typical" action of APD's whereas LI disruption via preexposure detects the "atypical" action of APD's.     

Nicotine-antipsychotic drug interactions and attentional performance in female rats

Schizophrenia is marked by pronounced cognitive impairments in addition to the hallmark psychotic symptoms like hallucinations. Antipsychotic drugs can effectively reduce these hallucinations; however, the drugs have not resolved the cognitive impairment. Interestingly, nicotine, a drug commonly self-administered by people with schizophrenia, has been shown to significantly improve cognitive function of people with schizophrenia. The current study was conducted to determine the effect of typical (haloperidol) and atypical (clozapine and risperidone) antipsychotic drug treatment on sustained attention in rats performing a visual signal detection task. In addition, the interaction of haloperidol with chronic nicotine administration was assessed. Female Sprague-Dawley rats were injected subcutaneously with clozapine (0, 0.6, 1.25 and 2.5 mg/kg), risperidone (0, 0.025, 0.05 and 0.1 mg/kg) or haloperidol (0, 0.01, 0.02 and 0.04 mg/kg). In the second part of the study, the interaction of acute haloperidol (0, 0.005, 0.01 and 0.02 mg/kg) and chronic nicotine (5 mg/kg/day, for 4 weeks via osmotic minipump) was characterized. Clozapine, risperidone and haloperidol all caused dose-related impairments in percent hit performance. There was a significant linear dose-related impairment in percent hit caused by risperidone. All the doses of clozapine caused a significant impairment in percent hit at the higher luminance intensities in the visual signal detection task. The 0.01 and 0.02 mg/kg haloperidol doses caused significant decreases in percent hit. The 0.04 mg/kg haloperidol dose impaired performance of the task to the point that reliable choice accuracy measurements could not be made. Chronic nicotine infusion significantly diminished the impairing effects of haloperidol on performance during weeks 1-2. In summary, both typical and atypical antipsychotic drugs significantly impaired sustained attention in rats. Haloperidol was more detrimental than clozapine and risperidone. Chronic nicotine diminished the adverse effects of haloperidol on performance. This study establishes a paradigm to reliably determine the attentional impairment caused by antipsychotic drugs.     

Haloperidol, raclopride, and eticlopride induce microcatalepsy during operant performance in rats, but clozapine and SCH 23390 do not

 The purpose of this work was (1) to assess the ability of selected antipsychotic and comparison drugs to induce arrest of movement phenomena during operant responding and (2) to evaluate the capacity of muscarinic anitcholinergics to block such effects. The effects of haloperidol (0.02–0.12 mg/kg, IP, 45 min), raclopride (0.05–0.80 mg/kg, IP, 30 min) eticlopride (0.02–0.16 mg/kg, IP, 45 min), clozapine (1.0–8.0 mg/kg, IP, 60 min) and SCH 23390 (0.01–0.16 mg/kg, IP, 30 min) were administered to rats for 4 weeks in a between-groups dosing design. Operant responses in 15 min and the maximum duration of the rat’s muzzle entry into the reinforcement dipper well (the measure of arrest of movement that reflected microcatalepsy) were the quantitative measures of behavior. The D₂ antagonists dose-relatedly decreased operant responding and increased maximum muzzle duration, effects that were significantly reversed by the anticholinergic scopolamine (0.1 mg/kg) or atropine (6.0 mg/kg). Although the atypical antipsychotic drug clozapine and the selective D₁ antagonist SCH 23390 both significantly reduced operant responding, these drugs did not produce microcatalepsy. The results suggested that microcatalepsy expressed in the context of ongoing operant behavior may model low-dose extrapyramidal side effects. Received: 12 January 1998 / Final version: 27 March 1998     

The involvement of sigma and phencyclidine receptors in the action of antipsychotic drugs.

An atypical antipsychotic drug clozapine and a selective sigma antagonist BMY 14802 were significantly less effective in the behavioural experiments (against apomorphine, d-amphetamine and MK-801), as well in the radioligand binding studies against 3H-spiperone (dopamine2-receptors) and 3H-haloperidol (sigma receptors) in the rat brain, as compared to a typical antipsychotic compound haloperidol. Contrary to haloperidol and BMY 14802, clozapine was a relatively selective antagonist of MK-801-induced motor excitation in the mouse. A nearly 3-fold lower dose of clozapine was needed to block the effect of MK-801 (6.4 mumol/kg) as compared to the action of amphetamine (17 mumol/kg). Haloperidol and clozapine, but not BMY 14802, antagonized apomorphine-induced aggressiveness in the rat. After long-term treatment (for 15 days) with BMY 14802 (10 mg/kg daily), haloperidol (0.5 mg/kg daily) and clozapine (10 mg/kg daily) the motor depressant effect of apomorphine (0.15 mg/kg) was reversed. Chronic haloperidol treatment, but not administration of BMY 14802 and clozapine, increased the number of dopamine2-receptors in the rat brain. BMY 14802 caused upregulation of sigma receptors in frontal cortex, whereas haloperidol induced the opposite change in cerebellum. Repeated treatment with clozapine significantly augmented the motor stimulating effect of MK-801 in rats. Simultaneously with a behavioural change the density of 3H-TCP binding sites in the rat forebrain was elevated after long-term treatment with clozapine, probably indicating the involvement of PCP binding sites at NMDA channel in the action of clozapine.     

Effect of antipsychotics on succinate dehydrogenase and cytochrome oxidase activities in rat brain

Typical and atypical antipsychotic drugs have been shown to have different clinical, biochemical, and behavioral profiles. It is well described that impairment of metabolism, especially in the mitochondria, leads to oxidative stress and neuronal death and has been implicated in the pathogenesis of a number of diseases in the brain. Considering that some effects of chronic use of antipsychotic drugs are still not well known and that succinate dehydrogenase (SDH) and cytochrome oxidase (COX) are crucial enzymes of mitochondria, in this work, we evaluated the activities of these enzymes in rat brain after haloperidol, clozapine, olanzapine, or aripiprazole chronic administration. Adult male Wistar rats received daily injections of haloperidol (1.5 mg/kg), clozapine (25 mg/kg), olanzapine (2.5, 5, or 10 mg/kg), or aripiprazole (2, 10 or 20 mg/kg) for 28 days. We verified that COX was not altered by any drug tested. Moreover, our results demonstrated that the atypical antipsychotic olanzapine inhibited SDH in the cerebellum and aripiprazole increased the enzyme in the prefrontal cortex. We also observed that haloperidol inhibited SDH in the striatum and hippocampus, whereas clozapine inhibited the enzyme only in the striatum. These results showed that antipsychotic drugs altered SDH activity but not COX. In this context, haloperidol, olanzapine, and clozapine may impair energy metabolism in some brain areas.     

Behavioural and electroencephalographic interactions between haloperidol and PCP/sigma ligands in the rat

Phencyclidine (PCP) and sigma ligands produce a typical excitatory behaviour in rats, characterized by circling and head- and body-weaving. Excitatory amino acid antagonists such as 2-amino 5-phosphonovaleric acid (AP5) or 3-(±)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) also produce a PCP-like excitatory behaviour in rats. In the present paper, the interactions between PCP/sigma drugs or excitatory amino acid receptor antagonists and haloperidol have been investigated in rats. In addition, the influence of two other butyrophenones having a different affinity for the sigma/haloperidol receptors, such as spiperone and 3-(4-(3(4-fluorobenzoyl)-propyl-piperazino-1-ylisoquinolino (HR 375), has been tested on the behavioural and EEG effects of PCP/sigma drugs and excitatory amino acid antagonists. PCP (2.5–5 mg/kg IP), (+) or (–) SKF 10,047 (1–15 mg/kg IP), (+) or (–) cyclazocine (2–8 mg/kg IP) and AP5 (0.5 µmol ICV) dose-dependently and significantly (P<0.01) antagonized the haloperidol-induced catalepsy in the horizontal bar and podium tests in rats. On the other hand, either haloperidol (1 mg/kg IP) or spiperone (1 mg/kg IP) reduced the head-weaving induced by (+) SKF 10,047, PCP, or AP5. On the contrary, HR 375 (6 mg/kg IP) was ineffective in blocking the excitatory effects of these drugs. In addition, either haloperidol (1 mg/kg IP) or spiperone (1 mg/kg IP), but not HR 375 (6 mg/kg IP) reduced the amplitude increase of the fast (20–30 Hz) frequency/low (30–50 µV) voltage background cortical activity elicited by PCP or (+) SKF 10,047. The results demonstrate an interaction between dopamine and excitatory amino acid receptors. At the same time, the data reveal the scarce relevance of the high affinity sigma/haloperidol receptors in the interference between PCP/sigma drugs and butyrophenones.