D2 and 5-HT2 modulation of psychostimulant-induced facilitation of brain stimulation reward

Classical as well as novel antipsychotic drugs increase self-stimulation threshold in the rat--similar mechanism of action?

Antipsychotic drugs given acutely increase the threshold for intracranial self-stimulation elicited from the ventral tegmental area. As all the antipsychotic drugs share the dopamine D2-receptor antagonism it is reasonable to believe that this is the cause for suppression of intracranial self-stimulation behaviour. The objective of this investigation was to examine the effect of classical (haloperidol) as well as novel antipsychotic drugs (clozapine, olanzapine and sertindole) on intracranial self-stimulation behaviour. Furthermore, the effects of different specific receptor antagonists on intracranial self-stimulation behaviour were examined. Our results showed that both the classical (haloperidol) and the three novel antipsychotic drugs increase the threshold for intracranial self-stimulation. The results obtained with the receptor specific antagonists showed that dopamine D2, alpha1-adrenoceptor and serotonin 5-HT2A receptor antagonisms inhibit intracranial self-stimulation behaviour and that muscarinic receptor antagonism is without effect. Even though all the tested antipsychotic drugs inhibited intracranial self-stimulation behaviour, there seems to be a difference in their ratio between doses that inhibits intracranial self-stimulation behaviour and those that produce antipsychotic effect in a preclinical model (amphetamine hyperactivity). Sertindole was the only antipsychotic drug able to produce antipsychotic effect without significant inhibition of intracranial self-stimulation behaviour at a narrow dose interval. The remaining antipsychotic drugs all inhibited intracranial self-stimulation behaviour at equal or lower doses than those producing antipsychotic effect.

A comparison of the effects of risperidone, raclopride, and ritanserin on intravenous self-administration of d-amphetamine

These experiments were conducted to examine the effects of simultaneous blockade of dopamine D2 and 5-hydroxytryptamine2 (5-HT) receptor function on responding for intravenous infusions of d-amphetamine. Rats were trained to self-administer d-amphetamine intravenously according to a progressive ratio schedule of reinforcement, in which response requirements increased for successive infusions until responding extinguished. In the first experiment it was shown that increases in the unit infusion dose of d-amphetamine resulted in an increase in the number of amphetamine infusions earned. Thus, the strength of responding for d-amphetamine was linked to the dose of drug received. The mixed D2 and 5-HT2 receptor antagonist risperidone (0.1, 0.2, and 0.4 mg/kg) reduced responding for d-amphetamine (60 microg/kg infusion). The selective D2 antagonist raclopride (0.1, 0.2, and 0.4 mg/kg) also reduced responding for d-amphetamine. In contrast, the selective 5-HT2 antagonist ritanserin (0.63, 1.25, and 2.5 mg/kg) failed to alter amphetamine self-administration. Combined injections of raclopride (0.05 or 0.1 mg/kg) and ritanserin (2.5 mg/kg) were no more effective than injections of raclopride alone in reducing responding for d-amphetamine. Overall, these results suggest that 5-HT2 receptor blockade plays a negligible role in the rewarding effects of d-amphetamine measured by intravenous self-administration, and does not contribute to the suppressant effects of risperidone on this behavior.

Mixed D2/5-HT2 antagonism differentially affects apomorphine- and amphetamine-induced stereotyped behavior

Evidence supports the hypothesis that psychostimulant stereotypy is mediated through postsynaptic dopamine receptors. Given the recent findings of behavioral, neurochemical and electrophysiological studies showing 5-HT2 modulation of dopamine systems, a series of experiments were undertaken to assess the ability of D2 and 5-HT2 antagonists to reverse apomorphine and amphetamine stereotypy in the rat. Haloperidol reduced stereotyped behavior induced by d-amphetamine (50% reduction with 0.162 mg/kg) and apomorphine (50% reduction with 0.112 mg/kg) MDL 28,133A, a mixed D2/5-HT2 antagonist, also reduced stereotypy in the apomorphine group (50% reduction with 3.89 mg/kg) but was much less effective in antagonizing the effects of d-amphetamine (not even a 25% reduction with 9.0 mg/kg). MDL 100,907, a selective 5-HT2 antagonist, was ineffective at reducing stereotyped behavior induced by either stimulant. Thus, 5-HT2 modulation of dopaminergic activity was not demonstrated in the case of psychostimulant stereotypy. Furthermore, 5-HT2 antagonism did not induce stereotypy, as has been proposed in some models. These findings provide further support for the hypothesis that antipsychotic medications with high affinity for 5-HT2 receptors do not interfere with the regulation of the nigrostriatal dopaminergic system and, therefore, would be less likely to produce extrapyramidal side effects.

Mixed D2/5-HT2A antagonism of amphetamine-induced facilitation of brain stimulation reward

Recent experiments have demonstrated that 5-HT2A antagonists can modify electrophysiological, neurochemical, and behavioral responses to psychostimulants. These findings led to an interest in using 5-HT2A antagonists to block the effects of psychostimulants on brain reward mechanisms. The present experiments assessed the ability of mixed D2/5-HT2A antagonists to reverse amphetamine-induced facilitation of self-stimulation. The D2/5-HT2A antagonists MDL 28,133A and risperidone attenuated the effects of cocaine and amphetamine, but only at antagonist doses that elevated baseline self-stimulation thresholds. A comparison of the effects of the mixed antagonists to those of haloperidol and eticlopride revealed that all four antagonists produced similar anti-stimulant effects when the influence of the drugs on baseline responding was considered. The D2 activity of the antagonists appears to account for their ability to reduce the effects of psychostimulants on self-stimulation. 5-HT2A antagonism makes a negligible contribution to the anti-amphetamine effects.