Opioid binding in the rostral hypothalamus is reduced following lesion of the ventral noradrenergic tract in female rats

Experiments were undertaken to establish whether opioid receptors exert a direct presynaptic influence on noradrenergic (NA) terminals in the preoptic/ anterior hypothalamus (PO/AH) of the female rat. Thus, opioid binding studies were performed in rats with lesions of the ventral NA tract (VNAT; the main NA projection to the hypothalamus) to assess whether a loss of NA terminals may also result in a decrease in opioid binding in the PO/AH. In the first experiment, unilateral electrolytic lesions of the VNAT caused a significant reduction in both the NA content and specific [3H]-diprenorphine binding to membrane homogenates in the ipsilateral PO/AH. In the second experiment bilateral 6-hydroxydopamine (6-OHDA)-induced lesions of the VNAT caused a significant reduction in NA levels in the PO/AH as well as significant decreases in the density of [3H]-diprenorphine binding to tissue sections of the PO/AH when compared to control animals. These results strongly suggest that the NA input to the PO/AH is regulated by endogenous opioid peptides, and provide an anatomical substrate to explain opioid-NA interactions in the control of gonadotrophin releasing hormone (GnRH) and gonadotrophin secretion.