beta-endorphin-, adrenocorticotrophic hormone- and neuropeptide Y-containing projection fibers from the arcuate hypothalamic nucleus make synaptic contacts on to nucleus preopticus medianus neurons projecting to the paraventricular hypothalamic nucleus in the rat

The nucleus preopticus medianus is known to be situated in a key site in pa thways regulating the paraventricular hypothalamic nucleus. To investigate the innervation pattern to nucleus preopticus medianus neurons by afferent fibers containing beta-endorphin, adrenocorticotrophic hormone and neuropep tide Y, a retrograde tracing method was combined with immunohistochemistry for these peptides in the rat. In the first experiment with injection of a retrograde tracer in the nucleus preopticus medianus, retrogradely labeled neurons were found in many regions throughout the brain. Among these, the a rcuate hypothalamic nucleus contained a number of retrogradely labeled neur ons showing immunoreactivity to the neuropeptides examined. About 20%, 20% and 40% of retrogradely labeled arcuate hypothalamic nucleus neurons showed beta-endorphin, adrenocorticotrophic hormone and neuropeptide Y immunoreac tivity, respectively. About 18% and 57% of retrogradely labeled neurons in the nucleus tractus solitarius and ventrolateral medulla, respectively, wer e immunoreactive to neuropeptide Y. There were many more neuropeptide Y-imm unoreactive projections to the nucleus preopticus medianus from the arcuate hypothalamic nucleus than those from the medulla. None of the retrogradely labeled neurons in the medulla showed immunoreactivity to beta-endorphin o r adrenocorticotrophic hormone. In the second experiment with injection of a retrograde tracer in the paraventricular hypothalamic nucleus, electron m icroscopic observation revealed that retrogradely labeled neurons in the nu cleus preopticus medianus were in synaptic contact with beta-endorphin-, ad renocorticotrophic hormone- and neuropeptide Y-immunoreactive axon terminal s. The present finding indicates that nucleus preopticus medianus neurons proj ecting to the paraventricular hypothalamic nucleus are innervated by beta-e ndorphin-, adrenocorticotrophic hormone- and neuropeptide Y-containing arcu ate hypothalamic nucleus neurons in addition to being innervated by neurope ptide Y-containing catecholaminergic medullary neurons which have been repo rted in our previous study. (C) 2000 IBRO. Published by Elsevier Science Lt d. All rights reserved.