GABA AND ANTERIOR PITUITARY FUNCTION: ANATOMICAL DATA*

Anterior pituitary function is under hypothalamic control through hypophysiotropic hormones released into the hypothalamo-hypophyseal portal blood (Green & Harris, 1947). Several of these hormones have been isolated and identified (Guillemin, 1976; Schally et al., 1978). They have a widespread distribution throughout the central nervous system (Elde & HOkfelt, 1979), and they show more physiological effects than their primary hypophysiotropic property which permitted their isolation (Guillemin, 1978; Moss, 1979). Conversely, the capacity to influence endocrine functions by pharmacological manipulation of known central neurotransmitters, as well as localisation of the latter in appropriate anatomical sites within the hypothalamus and hypophysis, suggested the possible role of various neurotransmitters originally isolated in the peripheral or central nervous system in the control of hypophyseal function. For example, extensive investigations on the biogenic amines since the early visualisation of catecholamines in the hypothalamic median eminence (Carlsson, et aL, 1962; Fuxe, 1964) have led to the identification of dopamine as a prolactin (release) inhibiting factor (MacLeod et al., 1976; Weiner & Ganong, 1978; Lichtensteiger, 1979; Leong e ta l . , 1983). In this context, characterized by the progressive identification of various neuroregulatory messengers in neuroendocrine processes (MtiIler et al., 1977; Krtilich, 1979; Meites & Sonntag, 1981), a possible involvement of gamma-aminobutyric acid (GABA), a recognized inhibitory neurotransmitter in the central nervous system (Curtis & Johnston, 1974; Krnjevic, 1974; Roberts et al., 1976), has emerged (Racagni et al., 1982; Elias et al., 1982; McCann et al., 1982; Tappaz et al., 1982; Mtiller et al., 1983). In the following review, the histochemical findings which provide the anatomical basis for GABAergic control of anterior pituitary function will be considered. They have been obtained through two approaches: autoradiography of GABA high-affinity uptake sites (Makara et al., 1975, 1976; Tappaz et al., 1980) and immunocytochemical visualization of the GABA-biosynthetic enzyme glutamate decarboxylase (GAD), a reliable marker for GABAergic systems (Roberts, 1979; Ribak et al., 1981), first as incidental observations (HOkfelt et al., 1978; Fuxe et al., 1979; Perez de la Mora, 1981; Tappaz et al., 1981) and then in more comprehensive investigations at the light(Vincent et al., 1982) and electronmicroscopic level (Oertel et al., 1982a; Tappaz et al., 1982, 1983). The physiological significance of these findings will be discussed. The illustrations are taken from our work