Thioridazine, unlike most other effective antipsychotic drugs, appears to be only a weak dopamine antagonist in various regions of the brain. We decided to test, indirectly, thioridazine's effects on another brain dopaminergic system, the tuberoinfundibular tract, which regulates prolactin secretion by stimulating hypothalamic secretion of prolactin-inhibiting factor. Chlorpromazine and several other phenothiazines have been shown to stimulate prolactin secretion.
Five healthy men ingested 50 mg of chlorpromazine concentrate on one occasion, and 50 mg of thioridazine concentrate on another. Both drugs noticeably stimulated prolactin secretion within two hours. It is concluded that thioridazine is a potent dopamine antagonist in the tuberoinfundibular system, and it is suggested that this system's regulation of prolactin secretion may provide a useful method for studying antipsychotic drug effects in man.