LHRH Priming in Gonadotrophs: A Model System for the Analysis of Neuroendocrine Mechanisms at the Cellular Level

Abstract

The priming effect of Luteinizing Hormone Releasing Hormone (LHRH), ie. the capacity of LHRH to increase the responsiveness of pituitary gonadotrophs to itself, is an important component of the mechanism by which changes in output of gonadotrophins during the oestrous cycle, in particular the preovulatory luteinizing hormone (LH) surge are controlled (Fink, 1979). LHRH priming can be elicited both in vivo (rat: Aiyer et al., 1974a; Fink et al.,1976; mouse: Fink et al., 1982) and in vitro (Pickering and Fink, 1976a; Waring and Turgeon, 1980). It is specific to LHRH and not elicited by secretagogues such as 40 mM K⁺ (Pickering and Fink, 1976b) and is not dependent on the initial release of LH (which normally occurs during the same 1h period as LHRH priming), since it occurs when LH release is blocked by removing calcium from the incubation medium (Pickering and Fink 1979). The priming effect of LHRH requires the synthesis of new protein (Pickering and Fink, 1979; Curtis et al., 1985) which is not LH (de Koning et al., 1976; Pickering and Fink, 1976a, 1979), and is dependent on the functional integrity of microfilaments (Pickering and Fink, 1979; Lewis et al., 1985). However, the subcellular mechanisms whereby the readily releasable pool of gonadotrophin is increased are still poorly understood. Previous ultrastructural analyses of LHRH stimulated gonadotrophs in vivo (Rommler et al., 1978) and in vitro (Tixier-Vidal et al., 1975; Childs, 1985) have reported changes in the position of granules but this has not been systematically or quantitatively analysed with respect to the priming effect of LHRH. The purpose of this report is to describe a series of quantitative ultrastructural analyses of gonadotrophs in different experimental and physiological states related to the augmented responsiveness of gonadotrophs produced by LHRH priming. The experiments have used normal mice, hypogonadal (hpg) mutants which do not produce detectable hypothalamic LHRH (Cattanach et al., 1977) and testicular feminised (tfm) mutants which do not respond to androgenic negative feedback (Naik et al., 1984). In addition, rats at different stages of the oestrous cycle have been studied to correlate changes in the morphology of gonadotrophs with changes in pituitary responsivness to LHRH. Our results indicate that augmentation of gonadotroph responsiveness to LHRH, such as that which occurs during the priming effect of LHRH involves a reversible migration of secretory granules to the surface of the gonadotrophs and a change in granule dimension indicative of increased intra-granular processing, associated with changes in the length and orientation of cytoskeletal microfilaments.