Effect of atropine on the diurnal PRL responses to TRH in normal subjects

Prolactin (PRL) responses to TRH were investigated at 0900 and 1800 hr in six young healthy men under basal conditions and after atropine administration. TRH induced significantly higher PRL release during the evening administration compared to the morning. Atropine had no effect on basal PRL secretion. Unlike the reported influences of anticholinergic drugs on the diurnal PRL responses to other stimuli, atropine also had no effect on TRH-stimulated PRL release either in the morning or in the evening. These observations in part may be accounted for by the direct PRL-releasing effect of TRH (without neurotransmitter involvement).