Hypothalamic Sites of Catecholamine Inhibition of Luteinizing Hormone in the Anestrous Ewe1

Norepinepherine (NE) and dopamine (DA) actively Inhibit the release of LH In anestrous ewes. This can be detected as an Increase In UI pulse frequency following l.v. injection of NE and DA antagonists.The objectiveof thisstudy was to determine the sites of these Inhibitory actions in the ovine hypothalamus by using local administrations of the NE antagonist, phenoxybenzamine (PBZ), or the DA antagonist, pimozide (PIM), Into specific hypothalamic areas. Each neurotransmltter antagonist was administered via a chronically Implanted steel guide tube into either the preoptic area (POA), retrochiasmatic area (RCh), or the median eminence region (ME). Blood samples were taken every 15 mIssfor 2 h before and 4 h during Implantation of these drugs and were analyzed for LH and prolactin by RL4..Control (no treatment) samples were obtained similarly from the same animals on another day. Placement of PBZ Into the POA significantly increased UI pulse frequency and mean UI concentrations over control values whereas PIM did not In contrast, NM significantly increased UI pulse frequency and mean UI concentrations when placed in the ME or in the RCh, but PBZ did not No effects of PIM on prolactin concentrations were detected. These results suggest that an NE neural system operates in the POA and that a DA system acts in the medial basal hypothalamus (RCh or ME) to suppressGnRH pulse frequency in the ovary-Intact anestrous ewe.