Electrochemical stimulation of the median eminence evokes FSH but not LH release after LHRH antagonist treatment in vivo and in vitro.

Experimental data suggest that a follicle stimulating hormone-releasing factor (FSH-RF) distinct from luteinizing hormone-releasing hormone (LHRH) exists. In the present study, we investigated, in short-term ovariectomized (OVX) rats, whether FSH-RF(s) can be released from nerve terminals by electrochemical stimulation (ECS) of the median eminence. To prevent the effect of LHRH liberated by ECS, 100 microg of a potent LHRH antagonist (MI-1544) was administered to one group of OVX rats 60 min before ECS. Two groups of OVX rats were used as controls. One group was treated with the solvent of the LHRH antagonist 60 min before the ECS; the other group received sham-ECS only. In-vitro experiments using a hypothalamus-pituitary coperifusion system were also performed to investigate the direct effect of ECS of the median eminence on LH and FSH release from pituitary cells. ECS in vivo induced 4.6-fold (P<0.01) and 10.2-fold (P<0.01) elevation of serum LH concentration, measured by RIA at 10 min and 60 min after ECS, respectively. Serum FSH concentrations increased 1.35-fold at 10 min (P<0.01) and 1.50-fold at 60 min (P<0.01) after ECS, compared with sham-stimulated controls. Administration of LHRH antagonist attenuated the ECS-induced release of LH by 44% at 10 min and prevented it entirely at 60 min after ECS. However, the ECS-induced release of FSH was not modified by the antagonist at 10 min and was diminished by only 17% at 60 min after ECS, compared with solvent-treated and stimulated controls. Immunohistological examination of the hypothalami showed that LHRH-immunoreactivity was depleted in the region of ECS. In the study in vitro, substances released from the fragments of mediobasal hypothalami bearing ECS in the median eminence induced significant release of both LH and FSH, and the induced release of LH, but not FSH, was prevented by the LHRH antagonist. The present study suggests that FSH-releasing factor(s) different from LHRH can be released from the median eminence and that a significant portion of FSH secretion is independent of the control of LHRH.