In vitro Electrophysiological and Endocrinological Studies of the Developing Tubero-hypophyseal Dopamine System

Abstract

Some medio-basal hypothalamic dopamine neurons in areas such as the arcuate nucleus and periventricular nucleus send their projections to the pituitary pars interrredia gland. Most of these axons terminate freely in the gland, as opposed to making direct synaptic contacts with the individual endocrine cells. Dopamine is known to inhibit the spontaneous secretion of several pro-opiomelanocortin-derived peptides (beta-endorphin, ACTH, alpha-MSH, LPH) released from the sarre cell type in the pars intermedia (Randle et al., 1983). In previous in vivo studies (Davis et al., 1984) we have looked at how this dopamine system develops during the pre- and postnatal periods in rats. Dopamine “D2” receptors are present in late gestation and the pars intermedia cells become responsive to exogenous dopamine application at least as early as two days after birth. In addition, morphological and biochemical evidence showed that dopamine-containing nerve fibers begin infiltrating the neurointermediate lobe in the first few postnatal days. By measuring the serum melanophore stimulating hormone (MSH) profile, it appears that some type of inhibitory control becomes effective during the second postnatal week. A direct functional correlation between the dopamine projections and pars intermedia secretory activity is difficult to elucidate from in vivo experiments. However, since the entire tubero-hypophyseal dopamine system is quite small (2 mm at its greatest dimension), it is possible to investigate this neuroendocrine axis in vitro.