Morphologic evidence that neurokinin B modulates gonadotropin‐releasing hormone secretion via neurokinin 3 receptors in the rat median eminence

Abstract

Recent studies suggest that arcuate neurokinin B (NKB) neurons play a role in the regulation of gonadotropin secretion, but there is little information on the relationship between these neurons and the hypothalamic reproductive axis. In the present study, dual-label fluorescent immunohistochemistry was used to visualize the relationship between gonadotropin-releasing hormone (GnRH) neurons and either proNKB or NK₃ receptor (NK₃R) immunoreactivity. Immunocytochemistry was also combined with i.p. injections of the fluorescent retrograde tracer aminostilbamidine to determine whether arcuate neuroendocrine neurons expressed either proNKB or NK₃R. A dense interweaving and close apposition of GnRH and proNKB-immunoreactive (ir) fibers was observed within the rat median eminence, where GnRH axons expressed NK₃R immunoreactivity. These data provide morphological evidence that NKB neurons could influence GnRH secretion via interaction with NK₃R in the rat median eminence. Colocalization of GnRH and NK₃R was also identified in fiber tracts converging within the organum vasculosum of the lamina terminalis. In contrast, only a small number (16%) of GnRH-ir somata exhibited NK₃R staining. ProNKB and NK₃R-ir somata were identified within the arcuate nucleus, but none of these neurons were labeled by aminostilbamidine. Thus, we found no evidence that arcuate NKB neurons project to the primary capillary plexus of the portal system. Arcuate neuroendocrine neurons, however, were surrounded and closely apposed by proNKB-ir puncta and fibers. These data suggest that NKB neurons could indirectly influence anterior pituitary function by inputs to arcuate neuroendocrine neurons, but through a receptor other than NK₃R. Our results provide an anatomic framework for putative interactions between NKB neurons and the hypothalamic reproductive axis. J. Comp. Neurol. 489:372–386, 2005.