[Hypothalamic inhibiting factor of luteinizing hormone secretion: relation to the fragment 1-5 of the LH releasing hormone].

In this paper we review the mechanisms underlying the control of gonadotrophin (Gn) release and present evidences of the existence of a luteinizing hormone release-inhibitory factor. We have extracted and partially characterized this factor from rat hypothalamus and bovine median eminence. Our data indicate that the factor is a peptide that has a common antigenic determinant with GnRH, but is of smaller molecular weight than GnRH (750 daltons approximately). These facts strongly suggest that it may be a fragment of the GnRH molecule. The synthetic fragment GnRH (1-5) has similar biologic effects and molecular weight to those of the inhibitory factor obtained from the median eminence and hypothalamus. GnRH (1-5) has been shown to be produced in vitro by cleavage of GnRH by an hypothalamic endopeptidase. Based on this evidences, we suggest that the inhibitory peptide found by us is formed in vivo by degradation of GnRH. Moreover, we suggest that this factor may play a role on the regulation of LH release induced by GnRH.