SUMMARY
Rabbits in pentobarbitone anaesthesia were implanted bilaterally with a pair of monopolar electrodes in the brainstem. Approximately 10 days after the operation, pseudopregnancy was induced by i.v. injection of human chorionic gonadotrophin, and 1 week later the rabbits received electrical stimulation with square-wave pulses through the implanted electrodes for two periods of 30 min. daily for 11 days. At autopsy on the following day the mammary glands were inspected for occurrence of lactogenesis and sites of electrode tips in the brain were determined histologically. Lactogenesis, indicating release of prolactin, occurred when electrical stimulation had been applied to sites in the lateral mesencephalic tegmentum and further forward in a region medio-ventral to the medial geniculate body. Passing rostrally, the pathway moved medially and then forwards in association with the extreme rostral central grey matter, and was traced as far forward as the posterior hypothalamus where sites were found close to, but not involving, the mammillo-thalamic tracts.
When compared with our previous studies on the afferent path of the milk-ejection reflex in this species, the ascending path for release of both oxytocin and prolactin appears to be the same in the mesencephalon. However, whereas the ascending path for oxytocin release bifurcates on each side into dorsal and ventral paths which reunite in the posterior hypothalamus, that for prolactin release appears to follow only the dorsal path, since stimulation of the subthalamus, through which the ventral path passes, was ineffective. It is proposed that the pathway traced in the present study represents the mesencephalic and posterior diencephalic route by which impulses initiated by the suckling stimulus attain the hypothalamus to evoke release of prolactin from the adenohypophysis.
Unit responses evoked in the amygdala and striatum by electrical stimulation of the medial geniculate body
