Study for the afferent hypothalamic pathway for the milk-ejection reflex in the rat

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We have previously reported that the dorsomedial hypothalamic nucleus (DMH) is the final relay site for the afferent pathway of the milk-ejection reflex in the rat (1992, Takano et al). The present experiments were undertaken to examine whether the DMH projects to supraoptic nuclei (SON) in both sides of the hypothalamus and contributes to the synchronized activation of oxytocin (OT) cells at milk-ejection reflex. In urethane anesthetized lactating rats, extracellular recordings of single OT cells in the SON were made. Twenty-six of 52 OT cells showed orthodromic excitation (OD+) or inhibition followed by the excitation (OD-+) in response to electrical stimulation of both ipsi-and contralateral DMH to the recording side. Eleven cells showed OD+ or OD-+ in responses to electrical stimulation of only ipsi-lateral DMH to the recording side. Continuous electrical stimulation of both ipsi- and contralateral DMH to the recording side at 10-50 Hz for 30-60 s increased firing rate in 7 of 12 O … More T cells tested. Fourty DMH cells were identified as projecting to the ipsilateral SON and 4 of them were also identified as projecting to the contralateral SON.Four cell were identified as projecting only to the contralateral SON.Of these, 25 were tested for theri firing pattern during milk ejection reflex. One cell, which had a projection to the contralateral SON, showed abrupt increase in firing rate (bursting activity similar to oxytocin cells) preceding milk ejection. This cell also showed an increase of basal firing rate in response to suckling stimulus. Two cells which had bilateral projectin and one cell which had a ipsilateral projectin showed reduction in firing rate preceding milk ejection. One cell which projected contralateral SON showed a reduction of firing rate in response to suckling stimulus. The results suggest that some of DMH neurones project to OT cells in both sides of hypothalamus and that these projections may contribute to the synchronized activation of OT cells during milk-ejection reflex. Less