LHRF, LHRH, GnRH: what controls the secretion of this hormone?

Abstract

In the present study, the possible direct effects of opioids on the release of LHRH have been studied utilizing the GT1-1 cell model. It has been shown that only opioid agonists which bind to delta receptors bring about a significant inhibition of the release of LHRH, when this is stimulated by forskolin- or PGE2. The effects of delta opioid agonists are dose-dependent, and are reversed by the delta specific antagonist naltrindole. If one assumes that the GT1-1 cells represent a good model for the study of LHRH-secreting neurons, the obvious conclusion from these data is that endogenous opioid peptides may influence LHRH secretion also by acting directly on LHRH-secreting neurons. In another study it has been shown that GT1-1 cells possess high affinity-low capacity binding sites for estradiol and androgens. Moreover, a two-fold induction of androgen-binding sites has been observed after treatment of GT1-1 cells with estradiol, indicating that estrogen receptors are functional. The present observations suggest that gonadal steroids might influence LHRH release acting directly on the cells which manufacture and release the hypothalamic hormone. In order to evaluate the possible existence of a humoral communication between glial cells and LHRH-secreting neurons, two different approaches have been used: (a) GT1-1 cells were co-incubated with purified cultures of type 1 astrocytes; (b) GT1-1 cells were exposed to the conditioned medium (CM, untreated or submitted to different experimental procedures) where type 1 astrocytes were grown for 24 h. In both sets of experiments LHRH was measured by RIA in the incubation media of GT1-1 cells. The data show that short periods of either co-culture or exposure to previously frozen or heated CM significantly increase the release of LHRH from the GT1-1 cells. The stimulatory effect on LHRH release appears to be specific for type 1 astrocytes. Surprisingly, fresh CM proved to be inactive; this suggested that the factor(s) involved needed to be 'activated'. Since it is known that TGF beta may be liberated from its supporting proteins by freezing or heating, the effects of TGF beta and of a TGF beta-neutralizing antibody were analyzed. The stimulatory effect exerted by the frozen CM was completely abolished by the antibody, while TGF beta 1 proved able to significantly increase LHRH release from GT1-1 cells. The present data unequivocally show that type 1 astrocytes in culture release some principle(s), probably TGF beta, able to stimulate LHRH release from the LHRH-producing GT1-1 cell line. To the authors' knowledge, the present data provide the first clear-cut demonstration that the glia may directly intervene in the control of LHRH release via the secretion of humoral factors.