The roles of extrahypothalamic and intrahypothalamic impulses to the medial basal hypothalamus in the control of the pituitary-gonadal-axis.

Adult female New Zealand white rabbits with chronically implanted electrodes were used. Potentials (EVPs) were consistently evoked in the medial basal hypothalamus (MBH) including the median eminence and the arcuate nucleus by stimulation of the dorsal. hippocampus (HPC), medial amygdala (AMYG) and preoptic area (POA). These EVPs were variously influenced under endogenous or exogenous hormonal condition such as during estrus, after copulation and after hCG administration. The MBH-EVP by HPC stimulation was depressed during estrus and was much more depressed after copulation than during estrus except the phasic facilitation at ten hours after copulation. The MBH-EVP by AMYG stimulation was significantly facilitated after copulation, nevertheless it was not so affected during estrus. The MBH-EVP by POA stimulation showed a reciprocal changes between during estrus and after copulation, that is, the facilitation in the former and the depression in the latter. Namely, the excitability of the HPC-MBH neuronal system may be inhibited, that of the AMYG-MBH neuronal system may be facilitated and that of the POAMBH neuronal system may be facilitated and successively inhibited from the beginning of estrus to the accomplishment of the copulatory ovulation. Finally, it was discussed that hCG directly might affect and depress the EVP representative of the neuronal pathways mentioned above.