Cholinergic control of the nocturnal prolactin surge in the pseudopregnant rat

The possible involvement of cholinergic mechanisms in the control of rhythmic secretion of prolactin has been examined in the pseudopregnant rat. Baseline data were obtained in decapitated animals in which the diuranl surge was observed in the 1430--2030 h range and the nocturnal surge during the 2330-0530 h interval. Atrial cannulation permitted a faithful reproduction of the prolactin pattern seen in decapitated rats if at least 3 days elapsed between the cannulation operation and bleeding, while cardiac puncture, under light ether anesthesia, appeared to suppress the diurnal surge. This latter observation appeared to depend on the time of sampling in relation to the onset of the dark phase of the daily lighting cycle. Atropine (35 or 70 mg/100 g BW) was found to inhibit the nocturnal surge in animals bled by either the cardiac puncture or cannulation techniques --an effect which was reversed by pretreatment with eserine (50 or 100 mug/100 g BW). Nicotine (0.75 mg/100 g BW) was found to delay, but not completely to inhibit, the nocturnal surge. When evaluated in the light of other available information, these observations suggest that a complex cholinergic mechanism is involved in control of the nocturnal rhythm of prolactin in pseudopregnancy and that the two surges, diurnal and nocturnal, are differentially controlled.