Dissociation of ACTH-Secretory Mechanisms in Rat Pituitary Cells: Evidence that Basal and Vasopressin-Stimulated Secretion Act via a Mechanism Distinct from that of Corticotrophin-Releasing Factor

In vivo correlation between c-Fos expression and corticotroph stimulation by adrenocorticotrophic hormone secretagogues in rat anterior pituitary gland

In the anterior pituitary gland, c-Fos expression is evoked by various stimuli. However, whether c-Fos expression is directly related to the stimulation of anterior pituitary cells by hypothalamic secretagogues is unclear. To confirm whether the reception of hormone-releasing stimuli evokes c-Fos expression in anterior pituitary cells, we have examined c-Fos expression of anterior pituitary glands in rats administered with synthetic corticotrophin-releasing hormone (CRH) intravenously or subjected to restraint stress. Single intravenous administration of CRH increases the number of c-Fos-expressing cells, and this number does not change even if the dose is increased. Double-immunostaining has revealed that most of the c-Fos-expressing cells contain adrenocorticotrophic hormone (ACTH); corticotrophs that do not express c-Fos in response to CRH have also been found. However, restraint stress evokes c-Fos expression in most of the corticotrophs and in a partial population of lactotrophs. These results suggest that c-Fos expression increases in corticotrophs stimulated by ACTH secretagogues, including CRH. Furthermore, we have found restricted numbers of corticotrophs expressing c-Fos in response to CRH. Although the mechanism underlying the different responses to CRH is not apparent, c-Fos is probably a useful immunohistochemical marker for corticotrophs stimulated by ACTH secretagogues.

Differential effects of the early and late intrauterine environment on corticotrophic cell development

The developing embryo and fetus respond to a range of intrauterine stressors, but the effect of chronic intrauterine stress on the programmed development of pituitary corticotrophs has not been investigated. We have used a pregnant sheep model in which the embryonic environment at conception has been surgically perturbed by uterine carunclectomy. This procedure results in the development of fetuses that either are placentally restricted and chronically hypoxemic or that demonstrate compensatory placental growth and maintain normoxemia throughout late gestation. We found that uterine carunclectomy resulted in the emergence of a population of non-corticotrophin-releasing hormone (non-CRH) target cells that secreted high amounts of adrenocorticotrophic hormone (ACTH) in the fetal pituitary. This change in corticotroph development was independent of late-gestation hypoxemia. However, chronic hypoxemia during late gestation (in either carunclectomized or non-carunclectomized uterine environments) resulted in a reduction in the proportion of ACTH stored in CRH-target. Thus, the early and late intrauterine environments differentially program the development of specific corticotrophic cell types in the fetal pituitary. These patterns of altered corticotroph development are important given the central roles of the hypothalamo-pituitary-adrenal axis in the fetal adaptive response to intrauterine stress and in the early programming of adult disease.

Corticotrophs and peptides

Corticotrophs were long thought to be a static, homogeneous population of cells that respond positively to hypothalamic stimulation, are inhibited by glucocorticoid feedback and secrete a single biologically active peptide, ACTH(1-39). Our current understanding is that this is an oversimplification and corticotrophs are a dynamic and more complex group of cells. The biosynthetic precursors of ACTH and other cleavage products of proopiomelanocortin (POMC) have been found to be secreted by anterior pituitary cells, to circulate and to have biological activity. POMC and the biosynthetic intermediate, pro-ACTH, exert activity antagonistic to ACTH(1-39) on glucocorticoid secretion by adrenal cells, and other derivatives of POMC are mitogenic to adrenocortical cells. In terms of responses to hypothalamic and peripheral factors, corticotrophs are functionally heterogeneous. This is reflected in the sensitivity of individual subtypes of corticotrophs to CRH, vasopressin and glucocorticoids. There is a functional plasticity amongst the various types of corticotrophs. During gestation, in fetal sheep, changes occur in the overall ACTH-secretory responses to CRH relative to vasopressin, the proportions of total corticotrophs that respond to the respective peptides and the average secretory response of individual cells. Corticotrophs also respond to locally produced pituitary factors. Local actions of leukaemia inhibitory factor are demonstrated by the effects of immunoneutralization of the peptide in pituitary cells. Urocortin and preproTRH(178-199) are locally produced peptides with potent stimulatory and inhibitory actions on corticotrophs, respectively. The specific roles of these peptides are under investigation.

Interactions between neurohypophysial hormones and the ACTH-adrenocortical axis

Figure 5 summarizes the aspects of the interaction of the HPA and magnocellular systems discussed above. Hormones classically considered confined to the magnocellular-neurohypophysial system are found in the parvocellular-long portal system and are known to be paramount in the hypophysiotropic control of ACTH release. It is now clear that hormones from the posterior pituitary can influence the secretion of ACTH via the short portal circulation and, possibly, by recirculation. There is some evidence that circulating AVP may affect adrenal sensitivity to ACTH. Corticosteroids, in addition to inhibiting parvocellular CRH and ACTH release, may inhibit the release of AVP from the neurohypophysis. The converse is also true in that patients with adrenal insufficiency may have an SIAD-like scenario. CRH may be synthesized in, and secreted from, magnocellular OT neurons and may be involved in the control of neurohypophysial function.

Chloroquine Decreases Adrenocorticotrophin-Secretory Response to Corticotrophin-Releasing Factor but not to Vasopressin in Rat Pituitary Cells: Further Evidence for Differentially Responsive Subpopulations

Abstract The relationships between adrenocorticotrophin (ACTH) content, basal ACTH secretion and the ACTH-secretory response to corticotrophin-releasing factor (CRF) or vasopressin in cultured rat anterior pituitary cells were examined in studies using chloroquine, an agent that diverts nascent ACTH from a storage pathway to direct release from cells. Chloroquine (200 mUM from t=minus 2 h) significantly elevated the basal ACTH secretory rate (by 0.7 +/- 0.2 ng over 30 min incubation, 1.1 +/- 0.3 ng over 60 min and 2.3 +/- 0.6 ng over 120 min). It also decreased the ACTH-secretory response to CRF (2.9+/-0.5 vs 4.6 +/- 0.5 ng/well over 30 min; 4.3 +/- 0.7 vs 8.0 +/- 1.4 ng/well over 60 min; 12.4+/-1.7 vs 20.2 +/- 3.6 ng/well over 120 min). In marked contrast, the net responses to vasopressin were unaltered (0.9 +/- 0.3 vs 0.9+/-0.2 ng/well over 30 min; 2.2 +/- 0.9 vs 2.1 +/-0.3 ng/well over 60 min; 3.8+/-0.9 vs 3.3 +/- 1.0 ng/well over 120 min). These data further support the association of the ACTH-secretory response to CRF with stored cellular ACTH, and the minimal dependence of the response to vasopressin on stored ACTH.