This paper gives a brief survey of the structural organization of the GnRH neuronal system and of the chemically identified axons impinging directly on GnRH neurons. There are great species differences in the localization of the GnRH nerve cells. In the human and monkey brain, they are concentrated in the medial basal hypothalamus. GnRH neurons project not only to the median eminence, but terminate also on other nerve cells including GnRH elements. Under physiological circumstances the GnRH neurons are influenced by nervous structures in- and outside the medial basal hypothalamus and by ovarian and other hormones. Recent, primarily immunocytochemical studies indicate that catecholaminergic, serotoninergic, GABA-ergic, opioid peptidergic, immunoreactive substance-P and corticotropin releasing factor-immunoreactive axons synapse on GnRH neurons. These findings provide morphological basis for the view that neurons operating with these compounds can act directly on GnRH release. Direct connections of the GnRH neurons demonstrated so far does not exclude the possibility that these neurotransmitters or neuromodulators influence GnRH release also at other levels of the CNS.
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