Projections from the arcuate/ventromedial region of the hypothalamus to the preoptic area and bed nucleus of stria terminalis in the brain of the ewe; lack of direct input to gonadotropin-releasing hormone neurons.

This study aimed to determine whether cells in the region of the arcuate and ventromedial hypothalamic nuclei (ARC/VMH) project to the gonadotropin-releasing hormone (GnRH) cells in the preoptic area (POA) and diagonal band of Broca (dbB) of the female sheep brain. An anterograde tracer, biotinylated dextran amine (BDA), was injected (70 nl) into the ARC/VMH (n=7) and the brains were perfused 3 weeks later. BDA terminals were mainly found in the dbB, POA and bed nucleus of stria terminalis (BNST). In order to determine the extent of input to GnRH neurons, we performed immunocytochemistry on the same sections with a GnRH antibody and examined close association of GnRH-immunoreactive (GnRH-IR) neurons (cell bodies and proximal dendrites) with BDA terminals. Of 223 GnRH-IR neurons that were examined, only three (1.3%) had BDA terminals in close proximity. Neither was close proximity observed between BDA terminals and GnRH-IR fibres. Injection of BDA into the BNST (n=6) showed terminals in POA, but only one of 273 GnRH-IR cells examined had BDA terminals in close proximity and no GnRH-IR fibres had BDA terminals in close proximity. Our results suggest that (1) although there are projections from the VMH/ARC to the dbB, POA and BNST, an interneuron or chain of interneurons is required for input to the GnRH neurones; (2) any input to GnRH neurons from the BNST involves at least one interneuron. The identity of these interneurons remains to be determined. Thus, input to the GnRH neurons from the estrogen receptor-rich area of ARC/VMH and from the BNST is not direct.