Receptors
Studies on receptor localisation have indicated expression of CRH-receptor on corticotrophs (Smets et al., 1991). Variable levels of messenger RNA (mRNA) for the receptor for thyrotrophin-releasing hormone (TRH) have been detected in normal glands (Kaji et al., 1995) but it has not yet been localised to specific cells. The GH-receptor/binding protein has been demonstrated in somatotrophs, lactotrophs, and gonadotrophs by immunocytochemistry (Mertani et al., 1995) and PRLreceptor by radioligand binding (Ciccarelli et al., 1995) and in situ hybridisation (Jin et al., 1997), suggesting that these effect paracrine roles. Oestrogen receptor may be present in all cell types (Stefaneanu et al., 1994) but appears more highly expressed in gonadotrophs and lactotrophs (Friend et al., 1994;Stefaneanu et al., 1994;Zafar et al., 1995). Immunocytochemical studies have suggested that a number of variants of the thyroid hormone receptor may be expressed (Yen et al., 1992), and vitamin D receptor expression has been detected by molecular and immunocytochemical analysis (Perez-Fernandez et al., 1996).
Novel Peptides
There is increasing evidence for the synthesis and secretion of a wide range of non-classical peptides by the pituitary. Much of the work has been done in other species, particularly the rat (O'Halloran et al., 1991). However, studies on the human gland also indicate such complexity, although species differences may occur in the cell-specific expression of particular peptides. Hypothalamic peptides have been identified, including growth hormone-releasing hormone (GHRH) and somatostatin (Benlot et al., 1991;Joubert [Bression] et al., 1989; Levy et al., 1993), TRH (Le Dafniet et al., 1990) and gonadotrophin-releasing hormone (GnRH) (Miller et al., 1996). These may act locally in the fine tuning of GH, TSH, and gonadotrophin release. Activin and inhibin are dimers consisting of two of the three inhibin subunits (β£,β€A, and β€B), the activins being homo/heterodimers of the β€ subunits (Ling et al., 1986;