Pharmacological effects of zotepine and other antipsychotics on the central 5-HT2 receptors

The effects of the atypical neuroleptic zotepine in comparison with clozapine, risperidone and haloperidol were examined in tests related to the central 5-HT2 activity. Zotepine, as well as clozapine and risperidone, were very potent inhibitors of the 5-HTP-induced head twitches (the behavior mediated by 5-HT2 receptors) in mice, with rank order of potency risperidone > zotepine > clozapine; haloperidol used in high doses only slightly reduced the effect of 5-HTP. Zotepine, clozapine, risperidone and haloperidol antagonized the stimulatory effects of mCPP on the hind limb flexor reflex in spinal rats. As the mCPP-induced stimulation is considered to be mediated by 5-HT2 receptors, the above results provide further evidence for the 5-HT2 antagonistic activity of the drugs studied. Te rank order of potency in that test was the same as in the 5-HTP-induced head twitches (risperidone > zotepine > clozapine). The effect of haloperidol was somewhat unspecific, since its active doses were very high. In the model of hind limb flexor reflex of spinal rats, the anti-alpha 2-adrenergic effect was also tested. Clonidine-induced stimulation of the hind limb flexor reflex (an alpha 2-adrenergic effect) was antagonized by zotepine, clozapine, risperidone and haloperidol. It may be concluded that zotepine, clozapine and risperidone show an alpha 2-adrenergic antagonistic activity (order of potency: zotepine = risperidone > clozapine). The effect of haloperidol may be considered unspecific, since it is observed at high (cataleptic) doses only. In conclusion, it appears that zotepine, as well as clozapine and risperidone, are strong 5-HT2 antagonists, while haloperidol is not.