E. Chleide

Nitric-oxide - a New Messenger in the Brain

The important role played by nitric oxide (NO) in the central nervous system has largely been emphasized in the recent literature. It can originate at least from four different sources: the endothelium of cerebral vessels, the immunostimulated microglia and astrocytes, the nonadrenergic noncholinergic nerve, and the glutamate neuron. NO has been implicated in a large number of pathologies (such as neurotoxicity in Alzheimers disease and Huntingtons disease, cerebral ischemia, stroke, and anxiety) and also in normal physiological functions (such as memory and learning, regulation of the cerebrovascular system, modulation of the wakefulness, mediation of nociception, olfaction, food intake and drinking, regulation of noradrenaline, and dopamine release). The aim of this paper is to review and to integrate the most recent advances in our understanding of the roles of NO in the brain.

Anxiolytic potential of sulpiride, clozapine and derivatives in the open-field test

Recently acquired data question the sharp dichotomy between anxiolytics and neuroleptics, since disinhibitory effects have been measured in the rat with very low doses of haloperidol and higher doses of atypical neuroleptics in FI and DRL schedules, but also in the open-field test. That the DA transmission in certain brain regions is involved in some aspects of anxiety has recently been suggested. The present study confirms this hypothesis particularly with high doses of sulpiride (80 mg/kg) and clozapine (24 mg/kg) when tested in the open-field test. Moreover, the results show how a slight chemical modification of clozapine can give a direction to pharmacological activity with one derivative still resembling clozapine and the second one resembling haloperidol. As neuroleptics do not seem to influence the synthesis and utilization of GABA, the higher entry score observed with them would seem to depend above all on DA antagonism in the mesolimbic system.

Clozapine: An atypical neuroleptic

Since it was synthesized in 1960, much has been written about clozapine. Although a number of its properties are those of a neuroleptic, it displays marked differences from classical antipsychotics to the extent that it is currently listed as an atypical neuroleptic. A classical neuroleptic has been defined in man according to its antipsychotic properties, accompanied by extrapyramidal effects, and in animals according to its cataleptic properties, its ability to antagonize apomorphine and amphetamine stereotypies and to suppress the conditioned avoidance response. Moreover, the classical neuroleptic exerted depressive and anhedonic effects in most conditioning schedules. With clozapine, most of these properties are no longer strictly in force to the point that they call in question the validity of the tests carried out to detect the potential of neuroleptics. This article attempts to compare the characteristics of clozapine with those of classical neuroleptics from a toxicological, neuropharmacological, psychopharmacological and clinical point of view.

Effects of specific dopaminergic agonists and antagonists in the open-field test

It has been found that dopaminergic transmission could be involved in some aspects of anxiety. The present study aims to explore this hypothesis further, using specific DA1 (SKF 38393) and DA2 (bromocriptine) agonists or DA1 (SCH 23390), and DA2 (zetidoline) antagonists in the open-field test. The results confirm previous studies indicating that DA1 and DA2 agonists predominantly increase locomotor activity, while DA1 and DA2 antagonists predominantly decrease it. However, at low doses, the four drugs increase the peripheral ambulation score significantly and, with the exception of zetidoline, also increase the central ambulation score. The observations made with zetidoline confirm the hypothesis that a specific presynaptic DA2 antagonism could be determinant for the disinhibitory effects of low doses of neuroleptics. A collateral action on 5HT transmission is also suggested to explain an hypothetic anxiolytic action of DA agonists and SCH 23390 at lower doses.

Comparative study of typical neuroleptics, clozapine and newly synthesized clozapine-analogues: correlations between neurochemistry and behaviour.

While neuroleptic therapy with classical compounds has frequently been associated with extrapyramidal side effects, clozapine has revealed an interesting antipsychotic profile without producing any clearcut motor side effects. However, some adverse reactions remained that stimulated the search for improved antipsychotic agents. The aim of this study was to characterize the behavioural and neurochemical profiles of typical neuroleptics (chlorpromazine, haloperidol), clozapine, and four newly synthesized clozapine-analogues. Affinity for dopaminergic (D1, D2), serotonergic (5-HT2) and cholinergic (muscarinic) receptors were measured and the ratios of these different binding affinities were determined and correlated with the behavioural effects of the drugs in a complex temporal regulation task in the dog. The four clozapine-analogues showed most of the behavioural characteristics previously described for neuroleptics and their neurochemical profile, particularly their 5-HT2/D2 pKi ratio, was compatible with an atypical antipsychotic effect. Among these drugs, JL5 and JL13 showed a high degree of similarity with clozapine. Like clozapine, they did not induce catalepsy and stereotypy/hyperkinesia. Moreover, other motor effects were also reduced (ataxia, akinesia, dystony). and tremor and sialorrhea were completely absent with these two molecules.

Differentiation of haloperidol and clozapine using a complex operant schedule in the dog

This study aimed to differentiate chronically administered typical (haloperidol) and atypical (clozapine) neuroleptics in the dog using a complex temporal regulation schedule combining operant, voluntary, and involuntary motor parameters. Although clozapine and haloperidol showed some characteristics of neuroleptics, justifying their adherence to the same class of compounds, differences have also been highlighted and compared to the clinical observations. Haloperidol induced catalepsy, tremor, dystony, hyperkinesia, and stereotypy. Subjects produced anticipated responses before any stimulus. Incomplete and delayed responses were also produced. An interpretation in terms of akathisia and anhedonia has been suggested. Clozapine induced tremor, exploration, dystony, and hypersalivation. Subjects produced disinhibitory responses to the negative stimulus and incomplete responses but these latter were submitted to tolerance. The simultaneous presence of tranquilizing and disinhibitory effects has been reported on the clinical potential of clozapine both in cases of positive and negative schizophrenic symptomatologies.

Effect of chronic hypoxic treatment on the retention of fixed-interval responding

The present experiments aimed to study the effects of hypoxic treatment (10 min on 3.5% oxygen) on the stabilized performance of rats in a fixed-interval schedule (FI 60 s). The hypoxic treatment was given once a day, for 3 days, immediately after the FI session. The results showed that the operant performance was disturbed from the very first posthypoxia FI session. There was a sharp drop in response rate, and changes in the temporal distribution of responses occurred. The data obtained greatly limit the importance of nonspecific factors in explaining these changes in performance. The results are discussed with reference to the retrieval hypothesis. In particular, it is suggested that the decreases of performance may be due to an effect of hypoxia on the retrieval mechanisms associated with long-term memory.

Stimulant effect of the β-carboline FG 7142 in the open-field test

Abstract The anxiogenic activity of N-methyl-β-carboline-3-carboxamide (FG 7142) is sometimes difficult to observe in rats. As the open field has recently been applied successfully to test the anxiogenic potential of n-butyl-β-carboline-3-carboxylate (β-CCB) in mice, a comparable experiment was performed with FG 7142 (1, 5, 10, 30 mg/kg i.p.) in rats. In contrast to the inhibitory effects measured with β-CCB, FG 7142 significantly increased the ambulation and rearing scores and induced aggressivity in some animals. A differential sensitivity of mice and rats to β-carbolines, predominant analeptic properties of FG 7142, and differences in the types of anxiety induced are proposed to account for this discrepancy.

Comparative study of pirlindole, a selective RIMA, and its two enantiomers using biochemical and behavioural techniques.

The interaction with monoamine oxidase A (MAO-A) and B has been shown to be sensitive to the absolute configuration of molecules. Therefore, the aim of this study was to compare the effects of the racemic pirlindole (a selective and reversible MAO-A inhibitor) and its two enantiomers using biochemical techniques (in vitro and ex vivo determination of rat brain MAO-A and MAO-B activity) and behavioural models (forced swimming test and reserpine-induced hypothermia and palpebral ptosis test). In vitro, the MAO-A IC50 of (±)-pirlindole, R-(—)-pirlindole and S-(+)-pirlindole were 0.24, 0.43 and 0.18 (j,M, respectively. Ex vivo, their ID50 were 24.4, 37.8 and 18.7 mg/kg i.p. The differences between the three compounds were not significant, with a ratio between the two enantiomers [R-(—)/S-(+)] of 2.2 in vitro and 2.0 ex vivo. MAO-B was only slightly inhibited. In the forced swimming test and the reserpine-induced hypothermia and ptosis model, the three compounds had an antidepressant profile. In the forced swimming test, the minimal effective dose ratio between the R-(-) and the S-(+) was again around 2.0. The behavioural observations were thus clearly in accordance with the biochemical data.

Amineptine improves the performance of dogs in a complex temporal regulation schedule

Amineptine is a tricyclic antidepressant with activating properties, that stimulates spontaneous locomotor activity in rodents and elevates mood in humans. It mainly inhibits dopamine uptake and weakly increased dopamine release. Formulating the hypothesis that this drug would decrease waiting capacity, we decided to test amineptine in a Differential Reinforcement of Response Duration schedule (DRRD 9 s Limited Hold 1.5 s) in the dog. The drug was administered orally at 2.5, 5.0, 7.5, 10 and 20 mg/kg, 1 h before the experimental session. Between 2.5 and 10 mg/kg, amineptine improved the performance by increasing the response (nonsignificantly) and reinforcement (significantly) rates and by increasing the peak of correct responses (significantly). The inverse effect was measured for the reinforcement rate (nonsignificantly) and for the peak of correct responses (significantly) at the dose of 20 mg/kg. These results were compared to those obtained with other classes of drugs, like neuroleptics, barbiturates or anxiolytics, that disturbed the performance, and particularly with low doses of neuroleptics, which also increase the dopamine release. The positive effects of amineptine on performance (2.5-10 mg/kg) were related to its inhibitory effect on dopamine uptake and discussed in terms of improved vigilance and attention, increase of waiting capacity, improved anticipation, and cognitive enhancement.