Diminished role of LHRH in the control of gonadotroph morphology and function in the long-term castrated male rat.

It was found in previous studies that the neurotransmitter control of the secretion of LHRH and LH differs between long-term castrated and ovariectomized rats. One interpretation of these data was that there was a reduced 'positive drive' in the male, and the question was raised 'how do the gonadotrophs of long-term castrated rats maintain a high level of LH secretion?'. In the present series of experiments, evidence for a reduced dependence of the gonadotrophs upon LHRH stimulation is provided. Although sensitivity to native LHRH was not completely lost in long-term castrated rats, two potent LHRH antagonists (D-pyroglu1,D-Phe2,D-Trp3,6)-LHRH and (N-acetyl-3,4-dehydro-Pro,p-fluoro-D-Phe2,D-Trp3,6)-LHRH, were found to inhibit LH secretion in short-term castrated and long-term ovariectomized rats, but not in long-term castrated rats. Neither blockade of axonal transport with colchicine nor immunoneutralization of LHRH with an antiserum against LHRH (both administered 48 h before blood sampling) produced reductions in serum concentrations of LH in long-term castrated rats, although these treatments significantly suppressed LH levels in short-term castrated animals. Chronic (6-day) infusions of the second LHRH antagonist (up to 450 micrograms/day) neither reduced LH secretion nor altered the morphology of the 'castration cells' in the pituitaries of long-term castrated rats. Chronic treatment with testosterone (15 days), however, reversed these parameters to some extent, and when the testosterone treatment was coupled with chronic infusions of the LHRH antagonist, significantly lower serum levels of LH and reductions in the size of the castration cells were observed. These data thus indicate that castration cells may function autonomously, without the need for LHRH, and that testosterone in some way restores the dependency on LHRH and/or the responsiveness to LHRH of these cells.