Lesion and electrophysiological studies on the hypothalamic afferent pathway of the milk ejection reflex in the rat

The afferent pathway of the milk ejection reflex in the hypothalamus was investigated with lesion and electrophysiological methods in anesthetized lactating rats. Destruction of the central region of the mid-hypothalamus (n = 12) blocked milk ejections induced by suckling, while that of the lateral region (n = 7) had no effect. In an electrophysiological study, extracellular recordings of neurons antidromically activated by electrical stimulation of the supraoptic nucleus were obtained from the ipsilateral hypothalamus caudal to the paraventricular nucleus (n = 84). Thirty-nine neurons were examined to see whether their firing activities changed during the milk ejection reflex. A group of 13 neurons were found to show changes in their activities prior to the reflex milk ejection; the neurons displayed a brief high-frequency burst of spikes before each milk ejection in the same manner as oxytocin neurons, and none of them antidromically responded to electrical stimulation of the neurohypophysis. The bursting neurons were recorded from the dorsomedial hypothalamic nucleus (n = 6), the region just lateral to that nucleus (n = 3) and the posterior hypothalamus (n = 4). The locations were included in a region whose destruction blocked the milk ejection reflex. These results indicate that the afferent pathway of the milk ejection reflex in the rat runs through the medial portion of the hypothalamus posterior to the paraventricular nucleus and that this region contains neurons which relay the input to the oxytocin neurons projecting in the neurohypophysis.