Adrenal vasoactive intestinal peptide participates in neonatal corticosteroid production in the rat

Adrenocortical sensitivity to ACTH in neonatal rats: correlation of corticosterone responses and adrenal cAMP content

A coordinated hypothalamic-pituitary-adrenal axis response is important for the survival of newborns during stress. We have previously shown that prior to postnatal day (PD) 5, neonatal rats exposed to hypoxia (one of the most common stressors effecting premature neonates) exhibit a large corticosterone response with a minimal increase in immunoassayable plasma ACTH and without a detectable increase in adrenal cAMP content (the critical second messenger). To explore the phenomenon of ACTH-stimulated steroidogenesis in the neonate, we investigated the adrenal response to exogenous ACTH in the normoxic neonatal rat. Rat pups at PD2 and PD8 were injected intraperitoneally with porcine ACTH at low, moderate, or high doses (1, 4, or 20 μg/kg body wt). Trunk blood and whole adrenal glands were collected at baseline (before injection) and 15, 30, or 60 min after the injection. ACTH stimulated corticosterone release in PD2 and PD8 pups. In PD2 pups, plasma corticosterone at baseline and during the response to ACTH injection was greater than values measured in PD8 pups, despite lower adrenal cAMP content in PD2 pups. Specifically, the low and moderate physiological ACTH doses produced a large corticosterone response in PD2 pups without a change in adrenal cAMP content. At extremely high, pharmacological levels of plasma ACTH in PD2 pups (exceeding 3,000 pg/ml), an increase in adrenal cAMP was measured. We conclude that physiological increases in plasma ACTH may stimulate adrenal steroidogenesis in PD2 pups through a non-cAMP-mediated pathway.

Adrenocortical control in the neonatal rat: ACTH- and cAMP-independent corticosterone production during hypoxia

We have previously demonstrated that the neonatal corticosterone response to acute hypoxia shifts from ACTH independence to ACTH dependence between postnatal days two (PD2) and eight (PD8). Cyclic AMP (cAMP) is the obligatory intracellular second messenger of ACTH action, and we hypothesized that corticosterone production in neonatal rats shifts from a cAMP-independent mechanism to cAMP-dependent mechanism between PD2 and PD8. Plasma ACTH and corticosterone and adrenal cAMP and cGMP responses to acute severe hypoxia (8% O₂ for 5, 10, 20, 30, and 180 min) were measured in neonatal rats at PD2, PD8, and PD15. Plasma ACTH and corticosterone were measured by radioimmunoassay, and adrenal cAMP and cGMP were measured by ELISA. Plasma corticosterone-binding globulin (CBG) was measured in normoxic pups by ELISA. The largest corticosterone response was observed in PD2 pups, despite only a small increase in plasma ACTH that was not sustained. The PD2 ACTH-independent increase in corticosterone occurred with no change in adrenal cAMP or cGMP content. Plasma CBG concentration was lowest in PD2 pups. Large corticosterone responses were measured during the first 30 min of hypoxia. Differences in corticosterone responses between PD2 and PD8 pups cannot be attributed to changes in plasma protein binding capacity, and the PD2 corticosterone response is consistent with a nongenomic mechanism of action. We conclude that the sustained corticosterone response to hypoxia in PD2 pups occurs with small and transient ACTH responses and independently of increases in adrenal cAMP or cGMP.

Immune-neuroendocrine integration at the adrenal gland: cytokine control of the adrenomedullary transcriptome

The bovine chromaffin cell represents an ideal model for the study of cell signaling to gene expression by first messengers. An abundance of GPCR, ionotropic, and growth factor receptors are expressed on these cells, and they can be obtained and studied as an abundant highly enriched cell population; importantly, this is true of no other postmitotic neuroendocrine or neuronal cell type. Chromaffin cells have now been shown to bear receptors for cytokines whose expression in the circulation is highly elevated in inflammation, including tumor necrosis factor, interferon, interleukin-1, and interleukin-6. The use of bovine-specific microarrays, and various biochemical measurements in this highly homogenous cell preparation reveals unique cohorts of distinct genes regulated by cytokines in chromaffin cells, via signaling pathways that are in some cases uniquely neuroendocrine. The transcriptomic signatures of cytokine signaling in chromaffin cells suggest that the adrenal medulla may integrate neuronal, hormonal, and immune signaling during inflammation, through induction of paracrine factors that signal to both adrenal cortex and sensory afferents of the adrenal gland, and autocrine factors, which determine the duration and type of paracrine secretory signaling that occurs in either acute or chronic inflammatory conditions.

Development of the ACTH and corticosterone response to acute hypoxia in the neonatal rat

Acute episodes of severe hypoxia are among the most common stressors in neonates. An understanding of the development of the physiological response to acute hypoxia will help improve clinical interventions. The present study measured ACTH and corticosterone responses to acute, severe hypoxia (8% inspired O(2) for 4 h) in neonatal rats at postnatal days (PD) 2, 5, and 8. Expression of specific hypothalamic, anterior pituitary, and adrenocortical mRNAs was assessed by real-time PCR, and expression of specific proteins in isolated adrenal mitochondria from adrenal zona fascisulata/reticularis was assessed by immunoblot analyses. Oxygen saturation, heart rate, and body temperature were also measured. Exposure to 8% O(2) for as little as 1 h elicited an increase in plasma corticosterone in all age groups studied, with PD2 pups showing the greatest response ( approximately 3 times greater than PD8 pups). Interestingly, the ACTH response to hypoxia was absent in PD2 pups, while plasma ACTH nearly tripled in PD8 pups. Analysis of adrenal mRNA expression revealed a hypoxia-induced increase in Ldlr mRNA at PD2, while both Ldlr and Star mRNA were increased at PD8. Acute hypoxia decreased arterial O(2) saturation (SPo(2)) to approximately 80% and also decreased body temperature by 5-6 degrees C. The hypoxic thermal response may contribute to the ACTH and corticosterone response to decreases in oxygen. The present data describe a developmentally regulated, differential corticosterone response to acute hypoxia, shifting from ACTH independence in early life (PD2) to ACTH dependence less than 1 wk later (PD8).

Vasoactive intestinal peptide is critical for circadian regulation of glucocorticoids

BACKGROUND/AIMS

Circadian control of behavior and physiology is a central characteristic of all living organisms. The master clock in mammals resides in the hypothalamus, where the suprachiasmatic nucleus (SCN) synchronizes daily rhythms. A variety of recent evidence indicates that the neuropeptide vasoactive intestinal peptide (VIP) is critical for normal functioning of the SCN. The aim of our study was to examine the possible role of VIP in driving circadian rhythms in the hypothalamic-pituitary-adrenal axis.

METHODS

Circulating ACTH and corticosterone concentrations were determined by round-the-clock sampling under diurnal and circadian conditions. The responsive aspects of the hypothalamic-pituitary-adrenal axis were tested by application of acute stress by footshock and light.

RESULTS

We demonstrate that the circadian rhythms in ACTH and corticosterone are lost in VIP-deficient mice. The ability of light to induce a corticosterone response was also compromised in the mutant mice, as was photic induction of Per1 in the adrenal glands. In contrast, the acute stress response was apparently unaltered by the loss of VIP.

CONCLUSION

Thus, our data demonstrate that VIP is essential for the circadian regulation of an otherwise intact hypothalamic-pituitary-adrenal axis.

Expression and fine mapping of murine vasoactive intestinal peptide receptor 1

Vasoactive intestinal peptide (VIP) plays multiple roles in the nervous, endocrine, and immune systems as a neurotransmitter, a hormone, and a cytokine. VIP is widely distributed in neurons of the central and peripheral nervous systems (CNS/PNS), and recently has been found to be an important neuroprotective agent. VIP actions are mediated through specific G protein-coupled receptors. We have cloned the cDNA of VIP receptor subtype 1 (VIPR1 or VPAC1) and have demonstrated the quantitative expression profile in mice. Fluorometric real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that VPAC1 is expressed in all tissues examined. Expression was highest in the small intestine and colon followed by the liver and brain. The high level of VPAC1 expression in forebrain and cerebellum suggests that VPAC1 may mediate the neuroprotective effect of VIP. We have refined the chromosomal localization of the mouse, rat, and human VPAC1 genes. This fine mapping of the VPAC1 gene extends the respective regions of synteny between the distal region of mouse chromosome 9, rat chromosome 8q32, and human chromosome 3p21.33-p21.31. Thus, VPAC, constitutes a functional-positional candidate for the tumor-suppressor function mapped to human 3p22-p21 where loss-of-heterozygosity is observed in small-cell lung carcinoma (SCLC) cell lines and primary tumors. Availability of the cDNA sequences for mouse VPAC1 will facilitate the generation of VPAC1 null mutant animals. Such studies will ultimately enhance our understanding of the role of VIP in the nervous system.

Expression and fine mapping of murine vasoactive intestinal peptide receptor 1

Vasoactive intestinal peptide (VIP) plays multiple roles in the nervous, endocrine, and immune systems as a neurotransmitter, a hormone, and a cytokine. VIP is widely distributed in neurons of the central and peripheral nervous systems (CNS/PNS), and recently has been found to be an important neuroprotective agent. VIP actions are mediated through specific G protein-coupled receptors. We have cloned the cDNA of VIP receptor subtype 1 (VIPR1 or VPAC1) and have demonstrated the quantitative expression profile in mice. Fluorometric real-time reverse transcription-polymerase chain reaction (RT-PCR) analysis demonstrated that VPAC1 is expressed in all tissues examined. Expression was highest in the small intestine and colon followed by the liver and brain. The high level of VPAC1 expression in forebrain and cerebellum suggests that VPAC1 may mediate the neuroprotective effect of VIP. We have refined the chromosomal localization of the mouse, rat, and human VPAC1 genes. This fine mapping of the VPAC1 gene extends the respective regions of synteny between the distal region of mouse chromosome 9, rat chromosome 8q32, and human chromosome 3p21.33-p21.31. Thus, VPAC, constitutes a functional-positional candidate for the tumor-suppressor function mapped to human 3p22-p21 where loss-of-heterozygosity is observed in small-cell lung carcinoma (SCLC) cell lines and primary tumors. Availability of the cDNA sequences for mouse VPAC1 will facilitate the generation of VPAC1 null mutant animals. Such studies will ultimately enhance our understanding of the role of VIP in the nervous system.