Inhibition of Milk-Ejection Reflex in Lactating Rats by Systemic Administration and Intracerebral Implantation of Atropine

Abstract

The present study was undertaken to determine the site(s) of action of the suckling stimulus in the hypothalamus and involvement of the cholinergic link in the milk-ejection reflex, by using systemic administration and intracerebral implantation of atropine in lactating rats. Atropine administered subcutaneously blocked the milk-ejection reflex almost completely at dose levels of 700 or 350 mg/kg body weight. Implants of atropine in the paraventricular nucleus blocked the milk-ejection reflex in all 5 animals used. The mammary glands of 4 out of 5 animals were filled with milk for 3 days after the operation. The blockade of the milk-ejection reflex in the rats implanted with atropine in the paraventricular nucleus was overcome by the injection of oxytocin. These results suggest that the paraventricular nucleus is concerned with the regulation of the milk-ejection reflex and that the neural relay might be of a cholinergic nature. Implants of atropine in the median eminence and the arcuate nucleus caused blockade of the milk-ejection reflex in 5 out of 6 animals. However, since milk accumulation following the operation was not found in 4 animals of this group, the cholinergic link in the median eminence may also affect milk secretion in the rat.