Effect of cyproheptadine and atropine on the diurnal prolactin responses to insulin-induced hypoglycemia in normal men.

There is a diurnal variation in the PRL response to insulin-induced hypoglycemia in normal men, with a substantially larger response in the evening. This study investigated the possible serotonergic and cholinergic influences on these diurnal PRL responses. Morning and evening PRL responses to hypoglycemia were compared in the same five normal young men without drug pretreatment, after cyprohepatadine, and after atropine sulfate. Cyproheptadine had no effect on the basal PRL concentration or the PRL response to hypoglycemia in either the morning or evening. Atropine had no effect on basal PRL concentrations in the morning or evening or on the morning PRL response to hypoglycemia. However, the evening PRL response was significantly inhibited by atropine (P less than 0.02), with an abolition of the normal diurnal difference in response (P less than 0.001) to insulin-induced hypoglycemia. These data suggest the involvement of a cholinergic mechanism in regulation of diurnal PRL responses to hypoglycemia.