Comparison of the effects of various typical and atypical antipsychotic drugs on the suppressant action of 2-methylserotonin on medial prefrontal cortical cells in the rat

Sodium- and magnesium-valproate in vivo modulate glutamatergic and GABAergic synapses in the medial prefrontal cortex

RATIONALE

Valproic acid (VPA) is a psychoactive drug currently used for the treatment of epilepsy. Recently it has been introduced in psychiatry for the treatment of bipolar disorders, aggression, impulsivity, and resistant schizophrenia, although the mechanism by which VPA acts on these psychiatric diseases remains still unknown.

OBJECTIVES

The aim of this study was to analyze the distinct effects of sodium-(Na-) and magnesium-valproate (Mg-VPA) in pyramidal neurons of the medial prefrontal cortex (mPFC) and their interactions with gamma-aminobutyric acid (GABA) and excitatory amino acid responses.

MATERIALS AND METHODS

In vivo electrophysiology and microiontophoresis techniques were used to attend these goals.

RESULTS

Both VPA salts decreased spontaneous neuronal firing activity in greater than 60% of recorded pyramidal neurons as well as potentiated GABA inhibitions. When injected at equal concentrations and currents, Mg-VPA blocked the excitatory responses induced by N-methyl-D-aspartate (NMDA) more frequently than Na-VPA. Both VPA salts equally blocked the excitatory responses of quisqualate and kainate.

CONCLUSIONS

These data suggest that VPA salts significantly modulate the activity of excitatory amino acid at mPFC pyramidal neurons and this mechanism should explain the therapeutic effects of valproate in psychiatric diseases involving NMDA, AMPA, and kainate receptors at the mPFC level.

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

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

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

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

Mechanisms of action of atypical antipsychotic drugs: a critical analysis

Various criteria used to define atypical antipsychotic drugs include: 1) decrease, or absence, of the capacity to cause acute extrapyramidal motor side effects (acute EPSE) and tardive dyskinesia (TD); 2) increased therapeutic efficacy reflected by improvement in positive, negative, or cognitive symptoms; 3) and a decrease, or absence, of the capacity to increase prolactin levels. The pharmacologic basis of atypical antipsychotic drug activity has been the target of intensive study since the significance of clozapine was first appreciated. Three notions have been utilized conceptually to explain the distinction between atypical versus typical antipsychotic drugs: 1) dose-response separation between particular pharmacologic functions; 2) anatomic specificity of particular pharmacologic activities; 3) neurotransmitter receptor interactions and pharmacodynamics. These conceptual bases are not mutually exclusive, and the demonstration of limbic versus extrapyramidal motor functional selectivity is apparent within each arbitrary theoretical base. This review discusses salient distinctions predominantly between prototypic atypical and typical antipsychotic drugs such as clozapine and haloperidol, respectively. In addition, areas of common function between atypical and typical antipsychotic drug action may also be crucial to our identification of pathophysiological foci of the different dimensions of schizophrenia, including positive symptoms, negative symptoms, and neurocognitive deficits.

Modulation of the A10 dopamine system: electrophysiological studies of the role of 5-HT3-like receptors

The distribution of 5-HT3-like receptors is primarily in the mesocorticolimbic structures. We have previously demonstrated that the atypical antipsychotic drug clozapine and a structurally related compound RMI 81,582 differ from other typical antipsychotic drugs in that they are effective 5-HT3 receptor antagonists. Our experimental results suggest that 5-HT3-like receptors play a permissive role in regulating or gating the inhibitory action of dopamine in the mesocorticolimbic areas. We hypothesize that the ability of clozapine to antagonize both 5-HT3-like and dopamine receptors may account for its preferential interaction with the mesocorticolimbic dopamine system and the higher efficacy in treating the schizophrenic symptoms.

Time course of dopamine1,2 and serotonin2 receptor binding of antipsychotics in vivo

An in vivo receptor binding technique was applied to evaluate the affinities of clozapine (20 mg/kg), RMI-81582 (20 mg/kg), and haloperidol (1 mg/kg) for dopamine D1, D2 and serotonin 5-HT2 receptors in rat brain with [3H]-SCH23390, [3H]-YM-09151-2, and [3H]-ketanserin as selective ligands. The time course study of receptor occupancy at 25 to 250 min after intraperitoneal administration of the drugs showed higher 5-HT2 and lower D2 receptor occupancies of clozapine and RMI-81582 than those of haloperidol both in the striatum and frontal cortex. The 5-HT2/D2 ratios of receptor occupancy for clozapine and RMI-81582 were about 6 to 8 times higher than that for haloperidol. Stable occupancies of D1 receptors were observed only with RMI-81582 and clozapine, the former demonstrating the higher occupancy. These findings are in agreement with the previous findings obtained under in vitro conditions and may account for some part of the properties of atypical antipsychotic drugs.

Survey on the pharmacodynamics of the new antipsychotic risperidone

This review reports on the pharmacodynamics of the new antipsychotic risperidone. The primary action of risperidone is serotonin 5-HT2 receptor blockade as shown by displacement of radioligand binding (Ki: 0.16 nM), activity on isolated tissues (EC50: 0.5 nM), and antagonism of peripherally (ED50: 0.0011 mg/kg) and centrally (ED50: 0.014 mg/kg) acting 5-HT2 receptor agonists in rats. Risperidone is at least as potent as the specific 5-HT2 receptor antagonist ritanserin in these tests. Risperidone is also a potent dopamine D2 receptor antagonist as indicated by displacement of radioligand binding (Ki: 1.4 nM), activity in isolated striatal slices (IC50: 0.89 nM), and antagonism of peripherally (ED50: 0.0057 mg/kg in dogs) and centrally acting D2 receptor agonists (ED50: 0.056-0.15 mg/kg in rats). Risperidone shows all effects common to D2 antagonists, including enhancement of prolactin release. However, some central effects such as catalepsy and blockade of motor activity occur at high doses only. Risperidone is 4-10 times less potent than haloperidol as a central D2 antagonist in rats and it differs from haloperidol by the following characteristics: predominant 5-HT2 antagonism; LSD antagonism; effects on sleep; smooth dose-response curves for D2 antagonism; synergism of combined 5-HT2/D2 antagonism; pronounced effects on amphetamine-induced oxygen consumption; increased social interaction; and pronounced effects on dopamine (DA) turnover. Risperidone displays similar activity at pre- and postsynaptic D2 receptors and at D2 receptors from various rat brain regions. The binding affinity for D4 and D3 receptors is 5 and 9 times weaker, respectively, than for D2 receptors; interaction with D1 receptors occurs only at very high concentrations. The pharmacological profile of risperidone includes interaction with histamine H1 and alpha-adrenergic receptors but the compound is devoid of significant interaction with cholinergic and a variety of other types of receptors. Risperidone has excellent oral activity, a rapid onset, and a 24-h duration of action. Its major metabolite, 9-hydroxyrisperidone, closely mimics risperidone in pharmacodynamics. Risperidone can be characterized as a potent D2 antagonist with predominant 5HT2 antagonistic activity and optimal pharmacokinetic properties.

The effect of acute and chronic LY 277359, a selective 5-HT3 receptor antagonist, on the number of spontaneously active midbrain dopamine neurons

In this study, we have examined the effect of acute and chronic administration of LY 277359, a putative 5-HT3 receptor antagonist, on the number of spontaneously active dopamine cells in the substantia nigra pars compacta (SNC or A9) and ventral tegmental area (VTA or A10). This was accomplished using the standard extracellular single unit recording techniques. The acute administration of LY 277359 (0.1 or 1.0 mg/kg i.p.) produced a significant increase in the number of spontaneously active A10, but not A9, dopamine cells compared to saline controls. The acute administration of 10 mg/kg of LY 277359 did not significantly alter the number of spontaneously active dopamine cells in either area. In contrast to its acute effects, the administration of 0.1 mg/kg per day of LY 277359 for 21 days decreased the number of spontaneously active A9 and A10 dopamine cells. However, the i.v. administration of (+/-)-apomorphine (50 micrograms/kg) did not reverse LY 277359's action, suggesting that the chronic LY 277359-induced reduction of dopamine cells was not the result of depolarization block. To test whether chronic administration of LY 277359 at a high dose would induce depolarization block of dopamine cells, rats were treated with 1.0 or 10 mg/kg LY 277359. Interestingly, the chronic administration of 1.0 mg/kg LY 277359 increased the number of A10, but not A9 dopamine cells. In contrast, chronic treatment with 10 mg/kg selectively decreased the number of spontaneously active A10 dopamine cells.(ABSTRACT TRUNCATED AT 250 WORDS)