Cyproheptadine resembles clozapine in vivo following both acute and chronic administration in rats.

Cyproheptadine is a cheap, widely available anti-allergy drug with a broad receptor binding profile which resembles that of clozapine. In rats discriminating clozapine from vehicle cyproheptadine mimicked clozapine very closely. Acutely it induced full generalization in the absence of response suppression, as observed with clozapine. Chronic administration of clozapine and cyproheptadine induced tolerance and cross-tolerance respectively to the clozapine stimulus. This was characterized by circa 3.5-fold parallel shifts to the right in the clozapine generalization curves. Such tolerance and cross-tolerance was spontaneously reversible, suggesting that it was pharmacodynamic, and that clozapine and cyproheptadine induce similar neuroadaptations when administered chronically. Administration of chlordiazepoxide at a very high dose induced no cross-tolerance to the clozapine stimulus showing the pharmacological specificity of tolerance. The clozapine stimulus is a compound cue involving actions at various receptors, and various clozapine-like antipsychotic (APD) drugs generalize fully to it. These data demonstrate that in vivo cyproheptadine resembles clozapine both acutely and chronically. Our findings, in conjunction with other actions of cyproheptadine -- induction of weight gain, alleviation of clozapine withdrawal, anxiolytic actions, alleviation of 'typical' APD-induced motoric side effects, and some preliminary clinical findings -- suggest that further study of cyproheptadine in conjunction with a 'typical' APD for the possible treatment of schizophrenia is merited at both pre-clinical and clinical levels.