Morphological evidence for a direct neuroendocrine GABAergic control of the anterior pituitary in teleosts.

The anterior pituitary of teleosts is unique among vertebrates in receiving a direct innervation which represents the morphological support of the neuroendocrine control of pars distalis functions. The participation of GABAergic fibers in this innervation was studied by means of immunocytochemistry at the light and electron microscopic levels, using antibodies against GABA. Immunoreactive fibers, characterized by the presence of small clear and dense cored vesicles, were detected in all parts of the gland. Immunopositive terminals were found in close, sometimes synaptic-like, contact with most glandular cell types in the anterior lobe. The data strongly suggest that in teleosts, as in mammals, GABA is involved in the neuroendocrine control of anterior pituitary functions.