Intercellular interactions in the anterior pituitary

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Intercellular communication in the anterior pituitary

In addition to hypothalamic and feedback inputs, the secretory cells of the anterior pituitary are influenced by the activity of factors secreted within the gland. The list of putative intrapituitary factors has been expanding steadily over the past decade, although until recently much of the work was limited to descriptions of potential interactions. This took the form of evidence of production within the pituitary of factors already known to influence activity of secretory cells, or further descriptions of actions on pituitary cells by such factors when added exogenously. A new phase of discovery has been entered, with extensive efforts being made to delineate the control of the synthesis and secretion of the pituitary factors within the gland, regulation of the receptors and response mechanisms for the factors in pituitary cells, and measurements of the endogenous actions of the factors through the use of specific immunoneutralization, receptor blockade, tissue from transgenic animals, and other means. Taken together, these findings are producing blueprints of the intrapituitary interactions that influence each of the individual types of secretory cells, leading toward an understanding of the physiological significance of the interactions. The purpose of this article is to review the recent literature on many of the factors acting as intrapituitary signals and to present such finding in the context of the physiology of the secretory cells.

Corticotropin-releasing hormone is produced by rat corticotropes and modulates ACTH secretion in a paracrine/autocrine fashion

Anterior pituitary hormone secretion is mainly regulated by hypothalamic releasing factors, which reach the pituitary via portal vessels. It has been demonstrated recently that these peptides can also be produced by the pituitary itself, thus possibly modulating hormone secretion in a paracrine/autocrine fashion. The object of this study was to seek evidence for the synthesis and secretion of corticotropin-releasing hormone (CRH) within the anterior pituitary and to ascertain its biological relevance. Messenger RNA from adult rat anterior pituitary fragments and cell cultures was reverse transcribed and subjected to PCR amplification using primers specific to the rat CRH gene. As in the hypothalamus, a single 232-bp band was obtained. The correspondence of the amplified fragment to the sequence of the CRH gene was confirmed by Southern blotting and restriction enzyme digestion. Combined in situ reverse transcription-PCR amplification/immunocytochemistry demonstrated the presence of CRH mRNA in corticotropes. Medium from anterior pituitary primary cultures contained approximately 7 pg/microg protein of CRH immunoreactivity which presented the same chromatographic profile on HPLC as the mature CRH peptide. Incubation of anterior pituitary cells with an antibody directed against CRH markedly reduced basal ACTH secretion compared with serum-treated control wells (0.89+/-0.11 vs. 1.74+/-0.14 ng/200,000 cells in control wells after 1 h, P < 0.05; 1.17+/-0.10 vs. 2.16+/-0. 39 ng/200,000 cells after 2 h, P < 0.05; 1.45+/-0.12 vs. 3.12+/-0.61 ng/200,000 cells after 3 h, P < 0.05). Further, the ACTH response to potassium and to forskolin was markedly blunted by the CRH antiserum as well as by the CRH antagonist, alpha-helical CRH(9-41). In conclusion, this study demonstrates the presence of CRH mRNA in normal rat corticotropes and the secretion of the mature peptide by the anterior pituitary, pointing to the production of CRH at the site of its target cells. In addition, intrapituitary CRH contributes in a paracrine/autocrine fashion to ACTH secretion.

Cholinergic regulation of the pituitary: autoexcitatory control by acetylcholine of melanotrope cell activity in Xenopus laevis

In the studies on Xenopus laevis presented here, the role of acetylcholine (ACh) in the regulation of pituitary activity was examined. The results are discussed in connection with other results on cholinergic regulation of pituitary functioning in vertebrates. It is demonstrated that dissociated melanotropes can synthesize ACh in vitro. In addition, immunocytochemical studies show the presence of the ACh-synthesizing enzyme choline acetyltransferase (ChAT) in the cytoplasm of melanotrope cells. ACh concentration-dependently raises the intracellular free calcium concentration as well as the release of proopiomelanocortin (POMC)-derived peptides. Selective muscarinic receptor antagonists showed that the actions of ACh are mediated through an M1-subtype muscarinic receptor. Immunofluorescence cytochemistry visualized muscarinic receptors on the surface of melanotrope cells. It is concluded that ACh stimulates the release of POMC-derived peptides from the Xenopus melanotrope cell in an autocrine, excitatory way. Finally, the possible mechanism through which ACh is released from melanotrope cells and the physiological significance of ACh in the pituitary gland are discussed.

Perivascular localization of nitric oxide synthase in the rat adenohypophysis: potential implications for function and cell-cell interaction

The possible localization of nitric oxide (NO) synthase (NOS) in proximity to the microvasculature was examined in the rat adenohypophysis using immunohistochemistry and nicotinamide adenine dinucleotide phosphate diaphorase histochemistry. A population of NOS-positive cells was localized in very close contact with the sinusoidal capillaries. The pattern of this perivascular localization was either unicellular, bicellular or multicellular. These observations suggest that, at least, some actions of NO in the adenohypophysis can be accounted for by a local regulation of the glandular microvasculature.

Growth hormone. A paracrine growth factor?

A number of tissues, including the brain, pituitary, immune system, placenta, mammary gland, and testis, may be self-contained units of GH regulation, production, and action. The production of GH and GH-releasing factors outside the hypothalamo-pituitary axis complements, rather than replaces, the traditional endocrine interactions between GH-releasing factors, GH, and its target tissues.

Anterior pituitary cells: getting to know their neighbors

We review in this article recent findings on the effects of cell proximity (nearness to each other) on anterior pituitary cell function in vitro. Important cellular processes, including protein synthesis, basal hormone release, response to hypothalamic regulatory factors and signal transduction can change as a function of the distance between adjacent cells. These findings, together with a growing body of evidence on intrapituitary intercellular interactions, form a basis for proposing a model of anterior pituitary cell regulation. In this scheme extrapituitary, intracellular and intercellular signaling pathways all contribute and interact to control secretory processes in cells.