Direct neural regulation of the teleost adenohypophysis

The teleostean adenohypophysis has direct innervation by neurosecretory fibers; in effect, the median eminence is located in the adenohypophysis, and neurohormones are released directly in the proximity of the endocrine cells. Immunocytochemical studies demonstrate that neurohormonal fibers and terminals are specifically localized in the pituitary in association with the target endocrine cells. Because of this special anatomical arrangement, teleosts are a unique experimental model for determining the origin(s) in the brain of peptides and monoamines involved in regulation of the endocrine cells of the pituitary. To illustrate, the terminals of a specific group of dopaminergic neurons in the preoptic region of goldfish have been shown to be associated with gonadotropes, and likely serve as the source of dopamine for inhibiting gonadotrope activity. Functional studies on the regulation of activity of the adenohypophysis and release of neurohormones can be integrated. To illustrate, norepinephrine stimulates whereas dopamine inhibits gonadotropin release by acting directly on the gonadotropes. Catecholamines also modulate the release of gonadotropin-releasing hormone (GnRH); norepinephrine and epinephrine stimulate GnRH neurons centrally, whereas dopamine inhibits GnRH neurons centrally as well as at the level of the terminals in the pituitary. In contrast to the actions on gonadotropes, norepinephrine and epinephrine inhibit whereas dopamine stimulates growth hormone release. The overall role of catecholamines in the regulation of release of the neuropeptides involved in regulating the activity of somatotropes remains to be investigated. As demonstrated by these studies, teleosts are very useful models for neuroendocrine studies.