Autocrine regulation of prolactin secretion by endothelins: a permissive role for estradiol

Cocaine- and amphetamine-regulated transcript is localized in pituitary lactotropes and is regulated during lactation

Cocaine- and amphetamine-regulated transcript (CART) is a highly expressed peptide implicated in the regulation of feeding, reward and reinforcement, and stress-related behaviors. CART has been localized to discrete cell populations in the brain, gut, adrenal gland, and pancreas. In contrast, CART-producing cell types in the pituitary gland remain ill defined. In the present study, double-label immunohistochemistry, employing a high-affinity antiserum we generated against CART-(62-102), was used to identify CART-producing cells in the pituitary gland. In the anterior pituitary, the majority of CART immunoreactivity (-ir) was localized in lactotropes; minor populations of CART-ir cells were identified as somatotropes and corticotropes. In the posterior pituitary, CART-ir extensively colocalized with oxytocin-containing fibers; in contrast, only a few vasopressin fibers contained CART-ir. As expected, CART colocalized with oxytocin in magnocellular neurons of the supraoptic nucleus. The effects of bromocriptine, a potent dopamine receptor agonist, were examined to determine whether CART mRNA expression and protein release are regulated in a similar fashion as prolactin. Similar to prolactin, CART mRNA expression and protein release were significantly decreased after bromocriptine treatment of dispersed rat anterior pituitary cells in culture. To explore the putative physiological role of pituitary CART, we compared levels of CART mRNA expression in lactating and nonlactating female rats. CART mRNA levels were significantly increased in the anterior pituitary and supraoptic nucleus of lactating rats. Furthermore, levels of CART in the systemic circulation were significantly elevated at the onset of lactation, peaked on d 10 of lactation and returned to baseline values 10 d after pups were weaned. The current study describes the cellular localization and regulation of CART expression and protein release from the rat pituitary gland. These findings suggest a putative role for CART in lactation.

Prolactin-releasing peptides

Physiologic control of prolactin (PRL) secretion is largely dependent upon levels of dopamine accessing the adenohypophysis via the hypophysial portal vessels. However, it is clear that other factors of hypothalamic origin can modulate hormone secretion in the absence or presence of dopamine. Several neuropeptides have been identified as PRL releasing factors (PRFs) but none of these peptides appears to be a major determinant of PRL secretion in vivo. There remain uncharacterized activities in hypothalamic extracts that can alter secretion and production of the hormone. In addition, there exist a wide variety of substances (neurotransmitters, neuromodulators, neuropeptides) that can act within the hypothalamus to modify the neuroendocrine regulation of PRL secretion. These factors may not be considered true PRFs because their actions are not exerted directly at the level of the lactotroph; however, they can act in brain to stimulate PRL release in vivo and therefore might be considered PRL releasing peptides (PRPs).

Paracrine control of lactotrope proliferation and differentiation

Lactotropes in the pituitary gland might be useful models of how a cell type develops, differentiates, proliferates and regresses under the control of paracrine and autocrine signals. Lactotrope development during embryonic life is determined by a well-defined sequence of temporal and positional actions of locally produced members of the bone morphogenetic protein, hedgehog and fibroblast growth factor families. Transforming growth factor alpha (TGF-alpha), TGF-beta and galanin mediate the action of estrogen on the postnatal expansion of the lactotrope cell population and expression of the gene encoding prolactin in an autocrine/paracrine manner. Moreover, the classic hormone precursor pro-opiomelanocortin generates differentially glycosylated isoforms of its N-terminal fragment as paracrine controllers, which each induce distinct aspects of lactotrope differentiation and growth.

Autocrine regulation of prolactin secretion by endothelins throughout the estrous cycle

We have previously found that the ovarian steroid background determines the efficiency of the endothelin-mediated autocrine feedback regulation of prolactin (PRL) secretion. In this study, we investigated the role of endogenous endothelins in regulating PRL secretion during the estrous cycle. Adult female rats representing different stages of the 4-d cycle were sacrificed by decapitation, and the anterior pituitary cells were enzymatically dispersed using collagenase and hyaluronidase. PRL secretion of individual lactotrophs was measured in a PRL-specific reverse hemolytic plaque assay, and the influence of endogenous endothelins on PRL secretion was assessed by applying the selective ET(A) receptor antagonist peptide, BQ123. Blocking the endothelin-mediated autocrine feedback resulted in an increase in PRL secretion when cells were obtained at proestrus, estrus, and diestrus-1, whereas PRL secretion was decreased at diestrus-2 by ET(A) receptor blockade. These observations suggest that endogenous endothelins are predominantly inhibitory during proestrus, estrus, and diestrus-1, whereas at diestrus-2 their influence on PRL secretion is stimulatory. Whereas the bell-shaped concentration-response curves with BQ123 at proestrus and diestrus-1 may indicate a transition state in which endogenous endothelins can be both stimulatory and inhibitory, at estrus the influence of endogenous endothelins is unequivocally inhibitory in nature. We propose that intensification of the endogenous endothelin- mediated negative feedback at estrus may play a role in restraining PRL secretion following the estradiol- induced proestrous PRL surge.