Chronic, but not acute, dosing of antipsychotic drugs alters neurotensin binding in rat brain regions.

The present study compared high affinity neurotensin (NT) binding in rat brain following acute or chronic treatment with the classical antipsychotic, haloperidol, and the newer antipsychotic drugs, clozapine and zotepine. Drugs were given orally, as an acute treatment (1 dose) or chronically (21 day dosing) and binding to the NT high affinity receptor was examined in three brain regions; striatum, nucleus accumbens/olfactory tubercle and frontal cortex. Acute dosing with either vehicle, haloperidol, clozapine or zotepine produced no significant changes in NT binding from controls (naïve rats). Chronic (21 day) dosing resulted in an increase in the K:(D:) and B(max) of high affinity receptors in the striatum following haloperidol, but not clozapine, zotepine or vehicles. In contrast, the newer antipsychotics, clozapine and zotepine but not haloperidol or vehicles, significantly altered NT binding in the nucleus accumbens/olfactory tubercle by decreasing the K:(D:) and B(max). Further differentiation between the two newer antipsychotic drugs occurred in the frontal cortex. Clozapine had no significant effect on NT binding, whereas zotepine significantly reduced the K:(D:) of the high affinity receptor with no alteration in B(max). The antipsychotic drugs tested did not interact directly with the NT high affinity receptor. Therefore, they must be acting indirectly via an alternative receptor mechanism to alter NT high affinity binding. In accordance with previously reported NT/dopamine receptor interactions, this would suggest cross-talk between these systems. Overall, these data demonstrate that chronic, but not acute, administration of antipsychotic drugs alters NT binding in the rat brain. In addition, anatomical differences in NT binding arise according to the antipsychotic drug under test. This may be predictive of drug side-effect profile, antipsychotic efficacy or atypicality.

A highly sensitive and selective radioimmunoassay for the measurement of neurotensin.

A highly selective and sensitive radioimmunoassay (RIA) for the detection of endogenous neurotensin (NT) has been developed. We have raised a C-terminally-directed antibody (CAb) that specifically binds 'biologically active' NT (NT and NT(8-13)) and that does not significantly cross-react with inactive NT metabolites or other bioactive peptides in the CNS. By reducing the volume of the assay to a low volume-RIA (30 microl), such that in vivo measurements can be made, we have increased the sensitivity (<0.3 fmol per tube), with inter- and intra-assay variations of 11.2 and 5.8%, respectively. Comparisons with similar methods of detecting NT have demonstrated that this RIA has a higher sensitivity than previously used RIA's and ELISA's. The data presented suggests that this sensitive RIA is a reliable method ideal for the detection of small quantities of biologically active NT.

Further evidence that behavioral tests and neuropeptide mRNA and tissue level alterations can differentiate between typical and atypical antipsychotic drugs.

This study was designed to compare some behavioral and biochemical effects of chronic treatment with a range of antipsychotic drugs. Gene expression of enkephalin, chromogranin A, chromogranin B, and secretogranin II and their respective peptide products were studied with in situ hybridization and radioimmunoassays after daily oral administration of haloperidol, clozapine, risperidone, or zotepine for 21 days. In behavioral tests, significant catalepsy was induced by haloperidol only. All four antipsychotic drugs increased hind paw retraction time but only haloperidol also increased forelimb retraction time. In the caudate putamen, haloperidol increased both enkephalin mRNA expression and enkephalin tissue levels. Neither of these parameters was altered by the other three drugs. In the prefrontal cortex, antipsychotic drugs generated a distinct pattern of gene expression in two regards. First, the dopamine D(2) receptor antagonist, haloperidol, did not significantly alter synaptic protein levels or their encoding mRNAs. Secondly, there was a differential change in tissue levels and mRNA expression since secretogranin II was not affected by any tested antipsychotic drug. This study shows that different types of antipsychotic drug induce distinct behavioural effects as well as differential changes in the biosynthesis of synaptic proteins and their encoding mRNAs. The data reinforce the notion that haloperidol can be classed as a typical antipsychotic drug whilst clozapine, zotepine, and risperidone reflect their atypical classification.

A comparison of the acute effects of zotepine and other antipsychotics on rat cortical dopamine release, in vivo.

The acute effects of systemic administration of the antipsychotic drug, zotepine, on extracellular dopamine (DA) in the frontal cortex of freely-moving rats were studied using in vivo microdialysis and compared with the actions of clozapine, olanzapine and haloperidol. Treatment with zotepine (1.0 mg/kg, i.p.) resulted in a prolonged elevation of cortical DA levels for up to 180 min post-drug. A maximal rise of +333% was observed at 120 min post-zotepine treatment. Clozapine (10.0 mg/kg, i.p.) also evoked a rise in extracellular DA which was similar in duration (200 min) to that resulting from treatment with zotepine. A maximal rise of +223% was observed at 100 min post-clozapine treatment. Olanzapine (1.0 mg/kg, i.p.) resulted in an immediate increase in DA levels which was maximal 40 min post-treatment (+280%) with levels returning to pre-injection values by 100 min after dosing. In contrast, haloperidol (0.1 mg/kg, i.p.) had no measurable influence on cortical DA levels. Local perfusion with the NA uptake inhibitor, nisoxetine (10 microM), resulted in an increase in cortical DA levels which was maximal at 100 min post-onset of perfusion (+257% above baseline). Administration of zotepine (1.0 mg/kg, i.p.) during nisoxetine perfusion elevated DA levels to a maximum of +301% above baseline, 60 min post-zotepine. These results show that acute administration of each of three drugs with an atypical antipsychotic profile causes an elevation of cortical DA in freely-moving rats at doses relevant to those derived from animal models which predict antipsychotic activity. As a dysfunction in cortical DA is thought to be involved in both the negative symptoms of schizophrenia and cognitive deficits in schizophrenic patients, it is possible that zotepine's ability to elevate cortical DA levels may underlie its effectiveness in successfully treating these components of schizophrenia. Furthermore, the ability of zotepine to elevate cortical DA is more likely to derive from its inhibition of the NA transporter rather than DA receptor blockade in this region.

N-Substituted (2,3-dihydro-1,4-benzodioxin-2-yl)methylamine derivatives as D(2) antagonists/5-HT(1A) partial agonists with potential as atypical antipsychotic agents.

A series of N-substituted 1-(2,3-dihydro-1, 4-benzodioxin-2-yl)methylamine derivatives with D(2) antagonist/5-HT(1A) partial agonist activity has been prepared as potential atypical antipsychotic agents. Optimization of in vitro receptor binding activity and in vivo activity in rodent models of psychosis has led to compound 24, which showed good affinities for human D(2), D(3), and 5-HT(1A) receptors but significantly less affinity for human alpha(1) adrenoceptors and rat H(1) and muscarinic receptors. In rodents, 24 showed functional D(2)-like antagonism and 5-HT(1A) partial agonism. After oral dosing, 24 showed good activity in rodent antipsychotic tests and very little potential to cause extrapyramidal side effects (EPS), as measured by its ability to induce catalepsy in rats only at very high doses. In the light of this promising profile of activity, 24 has been selected for clinical investigation as a novel antipsychotic agent with a predicted low propensity to cause EPS.

Elevation of extracellular cortical noradrenaline may contribute to the antidepressant activity of zotepine: an in vivo microdialysis study in freely moving rats.

The antipsychotic, zotepine, as well as possessing affinity for dopamine D1- and D2-1ike receptors, has high affinity for the noradrenaline (NA) transporter and inhibits [3H]NA uptake by rat frontal cortex synaptosomes, in vitro. The present studies investigated the effects of zotepine on extracellular NA in the frontal cortex of freely moving rats using in vivo microdialysis. Removal of calcium from the perfusate reduced extracellular NA by 70.5% and prevented the 50 mM KCl-stimulated increase in NA levels. Zotepine (0.5-1.5 mg kg(-1) i.p.), evoked biphasic, dose-dependent rises in extracellular NA with maximal increases observed at 60 min (+ 171.0%) and 240 min (+ 211.5%) post-treatment. The increases in NA levels were sustained for up to 100 min post-dosing. Clozapine (10.0 mg kg(-1) i.p.), resulted in a smaller, transient increase in NA levels (+ 72.0%) which lasted for 20 min post-treatment. Neither ziprasidone (3.0 mg kg(-1) i.p.) nor olanzapine (1.0 mg kg(-1) i.p.) influenced extracellular NA. Systemic treatment with the antidepressant desipramine (0.3 mg kg(-1) i.p.) resulted in a prolonged elevation of NA levels over 240 min (maximal increase of + 354.3%), whilst local infusion of nisoxetine (1-100 microM) through the dialysis probe increased NA levels in a concentration-dependent manner (up to 587.8% of control values). These data suggest that the inhibition of NA uptake by zotepine and its subsequent prolonged elevation of extracellular cortical NA may underlie the reported antidepressant properties of zotepine in schizophrenic patients.

Zotepine: preclinical tests predict antipsychotic efficacy and an atypical profile.

Zotepine, an atypical antipsychotic structurally similar to clozapine, is in Phase III clinical trials in the United States and Europe for the treatment and management of acute and chronic schizophrenia. Zotepine's pharmacological profile has been compared with those of the atypical antipsychotic clozapine, and the typical neuroleptics haloperidol and chlorpromazine in preclinical tests that predict antipsychotic efficacy and extrapyramidal symptoms (EPS). Because zotepine causes potent, long-lasting inhibition of dopaminergic behavioral responses in animals, it may have an efficacious prolonged antipsychotic action in humans. In contrast, it induces little catalepsy, indicating that it should cause minimal motor side effects, such as EPS. Like clozapine but unlike the typical neuroleptics, zotepine's affinities for cloned human D1 and D2 receptors are very similar. Since a stimulation imbalance favoring D1 over D2 receptors has been suggested to induce dyskinesias, zotepine's reduced EPS profile in humans may derive, in part, from balanced inhibition of these receptors.

Reserpinization severs the cooperative but not the oppositional interaction between D1 and D2 receptors.

Dopamine D1 and D2 receptors in rat brain interact synergistically to produce stereotypy and antagonistically to induce atypical jerking. This study showed that repeated reserpine treatment (5 daily injections of 1 mg/kg, s.c.), which depleted dopamine stores by 98%, rendered D1 and D2 receptors independent with respect to the production of stereotypy, i.e. either D1- or D2-selective agonists given alone induced stereotypy. In contrast, the atypical jerking induced by the D2 agonist, quinpirole, was still inhibited by the D1 agonist, SKF 82958, indicating that reserpine treatment did not render these two receptors independent with respect to initiation of this behavior.

BRL 20596, a novel anilide with central dopamine antagonist activity.

BRL 20596 (N-(4-amino-5-chloro-2-methoxyphenyl)-1-phenylmethyl-4-piperidine-carbox amide) is a novel anilide related to clebopride (a gastric prokinetic benzamide) in which the sole change is reversal of the amide bond. Previous studies have shown conformational and electronic differences between these molecules which result in the anilide losing its gastric prokinetic activity, whilst retaining its central nervous system activity. Pharmacological and biochemical properties of BRL 20596 are compared here in animals with chlorpromazine, clebopride, haloperidol and sulpiride. BRL 20596 potently inhibited a number of behaviours, such as conditioned avoidance, amphetamine-induced stereotypy and turning, and apomorphine-induced climbing. Homovanillic acid (HVA) levels in the striatum and nucleus accumbens were raised at similar dose levels to those which inhibited these behaviours, whilst sedative activity was only exhibited at much higher dose levels. Haemodynamic changes were only observed with high IV doses of BRL 20596. Much lower doses of sulpiride were needed to raise prolactin levels than to raise HVA levels. This was not the case with BRL 20596 and the other drugs, where the doses needed for the two effects were similar. The results suggest that BRL 20596 is a central dopamine antagonist, with low sedative and haemodynamic activity.