Organization of LHRH cells: differential apposition of neurotensin, substance P and catecholamine axons

A wealth of evidence suggests that catecholamines (particularly norepinephrine) influence gonadotropin secretion via a direct interaction with the LHRH neurons. Neuropeptides such as neurotensin (NT) and substance P (SP) are likewise implicated in the control of LHRH secretion, based on pharmacological and preliminary anatomical studies. Since sub-populations of LHRH neurons project to areas of the brain other than the median eminence, a detailed analysis of the topography of axonal interactions of catecholamines (CA), substance P and neurotensin with LHRH cells was conducted in adult male mice using dual immunocytochemical techniques. An analysis of the patterns of apparent contact of NT or SP axons on LHRH cells as determined by close apposition of immunoreactive axons to LHRH cells when viewed under a light microscope at high magnification revealed that the density of NT or SP axons was not a reliable index of the degree of contact; in many locations, NT and SP had similar densities yet a greater portion of the LHRH cells appeared contacted by SP than NT. NT axons were in close contact with up to one-third of the LHRH cells. Analysis of the location of these "contacted" cells did not reveal a discrete subnucleus controlled by NT. Rather, the NT-contacted cells were scattered throughout the LHRH cell field. Interactions of LHRH cells with SP axons were likewise uniform throughout most of the LHRH cell field, with the exception of the most anterior portion of the field. In the anterior septum, few SP axons appeared to contact LHRH cells. Elsewhere, most of the LHRH cells were in contact with SP axons. For the CAs, the fiber density in the regions of the LHRH cells was uniformly moderate, yet the pattern of cells contacted showed variation across the LHRH cell field, with most of the "contacted" cells located near the OVLT and medial preoptic area. These data suggest that LHRH cells may be differentially regulated by NT, SP and the CAs.