The effect of acute heat exposure on rat pituitary corticotroph activation: the role of vasopressin.

Abstract

Abstract:The increased ambient temperature affects the function of hypothalamic-pituitary-adrenal (HPA) axis. Since the correlation among vasopressin (VP), adrenocorticotropic hormone (ACTH) and corticosterone (CORT) responses to various stressors have been long recognized, the aim of this study was to reveal the aforementioned hormones production and morphology of the pituitary gland after exposure to acute heat. Rats were exposed to high ambient temperature (38°C) for 20 or 60 minutes. The circulating hormones were determined by an ELISA test or chemiluminescence's method. The results obtained show the elevation in ACTH and CORT secretion depending on the duration of heat exposure. The VP concentration increased only after prolonged exposure to heat (60 min). The pituitary morphology was examined by routine and fluorescent immunohistochemistry as well as electron microscopy. Observed changes in the anterior and posterior pituitary well corresponded to circulating hormones, regarding the volume density of ACTH-immunopositive cells, percentage of ACTH immunopositive area v. total area and number of VP-immunopositive containing varicose fibers per total area. Acute heat exposure also induced changes in shapes of ACTH-immunopositive cells. Cells appeared stellate with numerous slender cytoplasmic processes and degranulated, which is the most obvious after 20 min. In addition, immunopositivity of endothelial and anterior pituitary cells for VP suggests its influence on ACTH secretion.

The effect of vasopressin 1b receptors (V1bRs) blockade on the HPA axis activity in rats exposed to acute heat stress

“…Given that Nelivaptan blocks the action of AVP through activation of V1b receptors, mostly present in the pituitary and brain, it is reasonable to assume that the obtained increase in blood AVP concentration might result from the lack of its physiological action. In contrast to previously obtained results regarding the ability of circulating AVP to act on V1bR (Engelmann et al, 2004;Plotsky, 1991), our present results, and those published before (Jasnic et al, 2010), indicate that AVP, secreted into the general circulation, reaches the V1bR in concentrations sufficient for physiological action (Fig.7).…”

“…Previous results have revealed that thermal stressors, such as low or high ambient temperature, classified as physical stressors (Kvetnansky et al, 2009), elicit an abundance of neuroendocrine responses including activation of the hypothalamo-pituitary-adrenal (HPA) axis and arginine vasopressin (AVP) release (Aguilera, 1994;Featherby and Lawrence, 2004;Jasnic et al, 2010;Koko et al, 2006;Perčinić-Popovska et al, 2011). It was shown that several brain nuclei are involved in the integration and triggering of this response, with hypothalamic centers playing the pivotal role (Baffi and Palkovits, 2000;Dayas et al, 2001;Herman et al, 2005).…”

The Effect of Moderate Heat on Rat Pituitary ACTH Cells: Histomorphometric, Immunofluorescent and Hormonal Study

“…Actually, it was found that heat exposure provoked the rise of the plasma VP content and the pituitary VP receptor level, suggesting that VP could also be an important factor for ACTH cell activity regulation during prolonged thermal stress [4]. Furthermore, Jasnic et al [8] asserted that decreased density of ACTH cells upon heat exposure is not necessarily connected with their number changes, but it is rather a result of cells' degranulation, which is regularly followed with secretion of pre-synthesized hormone. Fillipa and Mohamed [31] also reported total or partial degranulation of ACTH cells after secreting the ACTH hormone.…”

“…Most of the studies that are investigating the effect of short-term exposure of rats to elevated ambient temperature (one hour at 38 o C) on the activity of the HPA axis [5,[7][8][9], have found increased blood ACTH and CORT concentrations. On the other hand, there is very few and ambiguous data about the effect of prolonged exposure to moderately high ambient temperature on the activity of the HPA axis, showing absence or increased values of blood ACTH and CORT concentrations after 4-5 days exposure to [35][36][37][38] o C [10,11].…”

Distinct vasopressin content in the hypothalamic supraoptic and paraventricular nucleus of rats exposed to low and high ambient temperature

“…Judging by the circulating ACTH content, our previous results showed not only that both cold and heat stress activate the HPA axis (Djordjevic et al, 2003;Jasnic et al, 2012), but also that AVP plays an important role in potentiating thermal stressor induced secretion of this pituitary hormone (Jasnic et al, 2013(Jasnic et al, , 2010. Driven by these findings, we decided to measure the AVP levels in the hypothalamus of rats exposed either to cold or heat (Fig.…”