Neuropeptide Y (NPY)- and vasoactive intestinal peptide (VIP)-induced aldosterone secretion by rat capsule/glomerular zone could be mediated by catecholamines via beta 1 adrenergic receptors

Functional Zonation of the Adult Mammalian Adrenal Cortex

The standard model of adrenocortical zonation holds that the three main zones, glomerulosa, fasciculata, and reticularis each have a distinct function, producing mineralocorticoids (in fact just aldosterone), glucocorticoids, and androgens respectively. Moreover, each zone has its specific mechanism of regulation, though ACTH has actions throughout. Finally, the cells of the cortex originate from a stem cell population in the outer cortex or capsule, and migrate centripetally, changing their phenotype as they progress through the zones. Recent progress in understanding the development of the gland and the distribution of steroidogenic enzymes, trophic hormone receptors, and other factors suggests that this model needs refinement. Firstly, proliferation can take place throughout the gland, and although the stem cells are certainly located in the periphery, zonal replenishment can take place within zones. Perhaps more importantly, neither the distribution of enzymes nor receptors suggest that the individual zones are necessarily autonomous in their production of steroid. This is particularly true of the glomerulosa, which does not seem to have the full suite of enzymes required for aldosterone biosynthesis. Nor, in the rat anyway, does it express MC2R to account for the response of aldosterone to ACTH. It is known that in development, recruitment of stem cells is stimulated by signals from within the glomerulosa. Furthermore, throughout the cortex local regulatory factors, including cytokines, catecholamines and the tissue renin-angiotensin system, modify and refine the effects of the systemic trophic factors. In these and other ways it more and more appears that the functions of the gland should be viewed as an integrated whole, greater than the sum of its component parts.

Sodium depletion increases sympathetic neurite outgrowth and expression of a novel TMEM35 gene-derived protein (TUF1) in the rat adrenal zona glomerulosa

The adrenal zona glomerulosa (ZG) secretes aldosterone to regulate sodium balance. Chronic sodium restriction increases aldosterone accompanied by ZG expansion. The ZG is innervated by sympathetic, vasoactive intestinal polypeptide (VIP) and neuropeptide tyrosine (NPY), and sensory, calcitonin gene-related peptide, nerves. It is unclear whether innervation is affected by ZG growth. Therefore, we measured neurite outgrowth in the ZG of adult male rats after dietary sodium manipulation. In response to 1 wk sodium restriction, VIP and NPY fibers elongated in parallel with expansion of the ZG, shown by aldosterone synthase (AS) expression, but calcitonin gene-related peptide fibers were not affected. Sodium repletion resulted in parallel regression in VIP and NPY fiber length and AS expression. These results show that sympathetic, but not sensory, innervation is coordinated with ZG growth. Mediators underlying changes in innervation are unknown; therefore, we characterized a novel gene TMEM35 [termed the unknown factor-1 (TUF1) due to its unknown function] that shows extensive overlap with AS in ZG. After sodium restriction, TUF1 expanded in parallel with the ZG. TUF1 bound the low-affinity neurotrophin receptor, p75NTR, which was expressed in NPY fibers and showed a response similar to TUF1 after sodium manipulation. TUF1- p75NTR binding was competitively displaced by nerve growth factor but not by TUF1 lacking the p75NTR binding motif. Moreover, TUF1 mRNA in rat ZG cells increased after angiotensin II exposure in vitro. Collectively, these findings suggest that TMEM35/TUF1 is a candidate for modulating neurite outgrowth in the ZG after sodium depletion.

NPY receptors in human cancer: a review of current knowledge

Many peptide hormone receptors are over-expressed in human cancer, permitting an in vivo targeting of tumors for diagnostic and therapeutic purposes. NPY receptors are novel and promising candidates in this field. Using in vitro receptor autoradiography, Y1 and Y2 receptors have been found to be expressed in breast carcinomas, adrenal gland and related tumors, renal cell carcinomas, and ovarian cancers in both tumor cells and tumor-associated blood vessels. Pathophysiologically, tumoral NPY receptors may be activated by endogenous NPY released from intratumoral nerve fibers or tumor cells themselves, and mediate NPY effects on tumor cell proliferation and tumoral blood supply. Clinically, tumoral NPY receptors may be targeted with NPY analogs coupled with adequate radionuclides or cytotoxic agents for a scintigraphic tumor imaging and/or tumor therapy.

Endogenous Ligands of PACAP/VIP Receptors in the Autocrine–Paracrine Regulation of the Adrenal Gland

Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are the main endogenous ligands of a class of G protein-coupled receptors (Rs). Three subtypes of PACAP/VIP Rs have been identified and named PAC(1)-Rs, VPAC(1)-Rs, and VPAC(2)-Rs. The PAC(1)-R almost exclusively binds PACAP, while the other two subtypes bind with about equal efficiency VIP and PACAP. VIP, PACAP, and their receptors are widely distributed in the body tissues, including the adrenal gland. VIP and PACAP are synthesized in adrenomedullary chromaffin cells, and are released in the adrenal cortex and medulla by VIPergic and PACAPergic nerve fibers. PAC(1)-Rs are almost exclusively present in the adrenal medulla, while VPAC(1)-Rs and VPAC(2)-Rs are expressed in both the adrenal cortex and medulla. Evidence indicates that VIP and PACAP, acting via VPAC(1)-Rs and VPAC(2)-Rs coupled to adenylate cyclase (AC)- and phospholipase C (PLC)-dependent cascades, stimulate aldosterone secretion from zona glomerulosa (ZG) cells. There is also proof that they can also enhance aldosterone secretion indirectly, by eliciting the release from medullary chromaffin cells of catecholamines and adrenocorticotropic hormone (ACTH), which in turn may act on the cortical cells in a paracrine manner. The involvement of VIP and PACAP in the regulation of glucocorticoid secretion from inner adrenocortical cells is doubtful and surely of minor relevance. VIP and PACAP stimulate the synthesis and release of adrenomedullary catecholamines, and all three subtypes of PACAP/VIP Rs mediate this effect, PAC(1)-Rs being coupled to AC, VPAC(1)-Rs to both AC and PLC, and VPAC(2)-Rs only to PLC. A privotal role in the catecholamine secretagogue action of VIP and PACAP is played by Ca(2+). VIP and PACAP may also modulate the growth of the adrenal cortex and medulla. The concentrations attained by VIP and PACAP in the blood rule out the possibility that they act as true circulating hormones. Conversely, their adrenal content is consistent with a local autocrine-paracrine mechanism of action.

Cholecystokinin and Adrenal‐Cortex Secretion

Cholecystokinin, or CCK, is a 33-amino acid peptide, originally considered a gut hormone, that acts via two subtypes of receptors, named CCK1-R and CCK2-R. CCK, along with its receptors, has been subsequently localized in the central nervous system, where it exerts, among other fuctions, antiorexinogenic actions. In this survey, we describe findings indicating that CCK, similar to other peptides modulating food intake (e.g., neuropeptide Y, leptin, and orexins), is also able to regulate the function of the hypothalamo-pituitary-adrenal axis, acting on both its central and peripheral branches. CCK stimulates aldosterone secretion via specific receptors (CCK1-Rs and CCK2-Rs in rats, and CCK2-Rs in humans) located in zona glomerulosa cells and coupled to the adenylate cyclase-dependent signaling cascade; and enhances glucocorticoid secretion from zona fasciculata-reticularis cells via an indirect mechanism mainly involving the CCK2-R-mediated stimulation of corticotropin-releasing hormone-dependent ACTH release.

Neuropeptide Y modulates pituitary-adrenal axis activity in the lizard, Podarcis sicula

The role of neuropeptide Y (NPY) in the modulation of the pituitary-adrenal axis activity in a lizard, Podarcis sicula, was investigated by in vivo NPY administration. The effects were evaluated by examination of the morphological and morphometrical features of the tissues as well as the plasma levels of ACTH, corticosterone, aldosterone, norepinephrine, and epinephrine. Intraperitoneally administered NPY (27 nmol /100g body wt) raised ACTH plasma levels (from 5.23+/-0.06 pg/ml in carrier injected specimens to 6.83+/-0.01 pg/ml, 24 h after the injection). In the steroidogenic cells a strong decrease of lipid amount was found; corticosterone plasma level increased from 6.28+/-0.02 ng/ml in carrier injected lizards to 7.96+/-0.01 ng/ml 24 h after the injection); aldosterone levels were raised from 1.88+/-0.02 ng/ml in carrier injected specimens to 6.38+/-0.05 ng/ml 24 h after the experimental treatment. In the chromaffin tissue, an increase in the number of epinephrine cells and a decrease in the number of norepinephrine cells were observed, decreasing the numeric norepinephrine/epinephrine (NE/E) cell ratio, from 1.4/1 of control specimens to 0.5/1 24 h after NPY administration. Moreover, norepinephrine plasma level were elevated from 922+/-4.30 pg/ml in carrier injected specimens to 3075+/-11.30 pg/ml 24 h after NPY administration; epinephrine plasma level increased from 502+/-2.40 pg/ml in carrier injected specimens to 2759+/-8.70 pg/ml 24 h after the experimental treatment. Consistent with these findings, morphological observations showed many chromaffin cells weakly stained and with a reduced content of secretory granules. These results suggest that, in P. sicula, NPY may play a role in the modulation of the pituitary-adrenal axis activity. Previous studies localized NPY in the epinephrine cells of P. sicula adrenal gland; taken together, these results suggest that this peptide might participate in the regulation of adrenal gland activity, enhancing corticosteroid and catecholamine secretion in a paracrine/autocrine manner. The mechanism of action of NPY is discussed.

Dependence of fetal hairs and sebaceous glands on fetal adrenal cortex and possible control from adrenal medulla

Human fetal adrenal development is characterized by rapid growth, high steroidogenic activity, and a distinct morphology, including a unique cortical compartment known as the fetal zone. For most of gestation, the predominant fetal zone accounts for 80-90% of the cortical volume and is the primary site of growth and steroidogenesis, producing 100-200 mg/day of the androgenic steroid, dehydroepiandrosterone sulfate (DHEA-S). The physiological role of this zone during intrauterine life is not well understood. While the glands appear to be capable of DHEA-S synthesis early in gestation (8-10 weeks), we noticed that this event precedes the differentiation of hairs and sebaceous glands. Hairs begin to develop between 9 and 12 weeks and sebaceous glands between 13 and 15 weeks of gestation. Sebaceous glands form an oily secretion - sebum that mixes with desquamated epidermal cells to form vernix caseosa. Vernix caseosa protects the developing skin from constant exposure to amniotic fluid, and hairs helps to hold the vernix caseosa on the skin. We suggest therefore that the human fetal adrenal cortex produces DHEA-S beginning at around 8-10 weeks of gestation in sufficient quantities to influence the growth of hairs and sebaceous glands. Soon after birth, the fetal zone atrophies, and adrenal androgen production decreases to minimal levels. As a consequence, in concordance with the rapid decrease in adrenal androgen levels and in consistent with our hypothesis, fetal hairs are shed and sebaceous glands shrink to small structures. The mechanism that regulates fetal adrenal androgen production is a key unanswered problem in human adrenal biology. Since there exists a close relationship between epinephrine and DHEA-S levels during adrenarche which shows modulatory interactions between adrenal androgen production and adrenomedullary function, we suggest again that adrenomedullary function might play a role in the control of fetal adrenal androgen secretion.

Adrenocortical zonation and ACTH

The clear morphological distinction between the cells of the different adrenocortical zones has attracted speculation and experiment to interpret their functions and the ways in which they are regulated. Considerable data have been produced in recent years that has benefited a fuller understanding of the processes of steroidogenesis and of cell proliferation at the molecular level. This now enables the reexamination of earlier concepts. It is evident that there is considerable species variation, and this article, dealing mainly with the rat, reaches conclusions that do not necessarily apply to other mammals. In the rat adrenal, however, the evidence suggests that the greatest differences between the functions of the zones are between the glomerulosa and the fasciculata. Here the sometimes all-or-nothing demarcation in their complement of components associated with steroidogenesis or with cell proliferation suggests a stark division of labor. In this model the fasciculata is the main engine of steroid hormone output and the glomerulosa is the site of cell proliferation, recruitment, and differentiation. Regulating these functions are angiotensin II and other paracrine components that modulate and maintain the glomerulosa, and ACTH, that maintains the fasciculata, and recruits new fasciculata cells by transformation of proliferating glomerulosa cells. Grafted onto this mostly vegetative function of the glomerulosa is CYP11B2, limited to just a fraction of the outer glomerulosa in rats on a normal laboratory diet and generating aldosterone (and 18-hydroxycorticosterone) from precursors whose origin is not, from the evidence summarized here, very clear, but may include the fasciculata, directly or indirectly. The biosynthesis of aldosterone in the rat certainly requires reinterpretation.

Adrenal neuropeptides: regulation and interaction with ACTH and other adrenal regulators

It is now well accepted that both the cortex and medulla of the mammalian adrenal gland receive a rich innervation. Many different transmitter substances have been identified in nerves supplying both cortex and medulla and, as well as catecholamines, a wide range of neuropeptides has been found in the adrenal gland. There have been several studies on the affects of age, sodium intake, stress, ACTH, and splanchnic nerve activity on the regulation of adrenal neuropeptide content. There is evidence that the abundance of each of these peptides is actively regulated. Although there have been many studies addressing the individual actions of various neurotransmitters on steroid secretion, adrenal blood flow, and adrenal growth, few have attempted to determine the nature of any interaction between neurotransmitters and the classical adrenal stimulants. There are, however, some significant interactions, particularly in the regulation of zona glomerulosa function. This review necessarily focuses on vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), as these are the most abundant transmitter peptides in the adrenal gland and the majority of studies have investigated their regulation and actions. However, substance P, calcitonin gene-related peptide (CGRP), neurotensin, and the enkephalins are included where appropriate. Finally, it has been suggested that certain neurotransmitters, particularly VIP, may interact with classical hormone receptors in the adrenal, notably the ACTH receptor. This review attempts to evaluate our current state of knowledge in each of these areas.

Expression and function of vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide, and their receptors in the human adrenal gland

VIP and pituitary adenylate cyclase-activating polypeptide (PACAP) are two regulatory peptides that possess remarkable amino acid sequence homology and act through common receptors, named PAC(1), VPAC(1), and VPAC(2). PAC(1) receptor is selective for PACAP, whereas VPAC(1) and VPAC(2) receptors bind both VIP and PACAP. We have investigated the expression and function of VIP, PACAP, and their receptors in the zona glomerulosa (ZG), zonae fasciculata and reticularis, and adrenal medulla (AM) of the human adrenal cortex. RT-PCR and RIA detected VIP and PACAP expression exclusively in AM cells. RT-PCR demonstrated the presence of PAC(1) mRNA only in AM and of VPAC(1) and VPAC(2) mRNAs in both ZG and AM cells. VIP and PACAP concentration-dependently increased aldosterone and catecholamine secretion from cultured ZG and AM cells. The catecholamine response to both peptides was higher than the aldosterone response, and the secretagogue action of PACAP was more intense than that of VIP. The aldosterone response of cultured ZG cells to VIP or PACAP was unaffected by the PAC(1) receptor antagonist PACAP-(6-38) (PAC(1)-A), but was significantly decreased by the VPAC(1) receptor antagonist [Ac-His(1),D-Phe(2),Lys(15),Arg(16)]VIP-(3-7),GH-releasing factor-(8-27)-NH(2) (VPAC(1)-A). The catecholamine response of cultured AM cells to VIP was lowered by VPAC(1)-A and unaffected by PAC(1)-A; conversely, the catecholamine response to PACAP was reduced by both PAC(1)-A and VPAC(1)-A. Simultaneous exposure to both antagonists did not abolish the catecholamine response to PACAP. Collectively, our findings allow us to conclude that in human adrenals 1) VIP and PACAP biosynthesis exclusively occurs in AM cells; 2) ZG cells are provided with functional VPAC(1) and VPAC(2) receptors, whose activation by VIP or PACAP elicits a moderate aldosterone response; 3) AM cells possess PAC(1), VPAC(1), and VPAC(2) receptors, whose activation evokes a marked catecholamine response; and 4) the catecholamine response to PACAP is more intense than that to VIP, because it is mediated by all subtypes of VIP/PACAP receptors.

Glucocorticoids impair fetal beta-cell development in rats

In rats, poor fetal growth due to maternal food restriction during pregnancy is associated with decreased beta-cell mass at birth and glucose intolerance in adulthood. Overexposure to glucocorticoids in utero can induce intrauterine growth retardation in humans and animals and subsequent glucose intolerance in rodents. The aims of this study were to investigate whether glucocorticoid overexposure mediates the effect of undernutrition on beta-cell mass and to study their potential role in normally nourished rats. Undernutrition significantly increased maternal and fetal corticosterone levels. Twenty-one-day-old fetuses with undernutrition showed growth retardation and decreased pancreatic insulin content; adrenalectomy and subcutaneous corticosterone implants in their dams prevented the maternal corticosterone increase and restored fetal beta-cell mass. In fetuses with normal nutrition, fetal corticosterone levels were negatively correlated to fetal weight and insulin content; fetal beta-cell mass increased from 355 +/- 48 microg in sham to 516 +/- 160 microg after maternal adrenalectomy; inhibition of steroid production by metyrapone induced a further increase to 757 +/- 125 microg. Our data support the new concept of a negative role of glucocorticoids in fetal beta-cell development.

Neuropeptide Y and the adrenal gland: a review

This paper sets out to review several aspects of NPY and adrenal function, starting with the localisation of NPY in the adrenal, then describing the regulation of NPY release and considering whether the adrenal is a significant source of circulating NPY. The review then describes the regulation of adrenal content of peptide, and finally covers the actions of NPY on the adrenal gland, and the receptor subtypes thought to mediate these effects. The regulation and actions of NPY are discussed with reference to both the adrenal cortex and the medulla.

Ectopic and abnormal hormone receptors in adrenal Cushing's syndrome

The mechanism by which cortisol is produced in adrenal Cushing's syndrome, when ACTH is suppressed, was previously unknown and was referred to as being "autonomous." More recently, several investigators have shown that some cortisol and other steroid-producing adrenal tumors or hyperplasias are under the control of ectopic (or aberrant, illicit, inappropriate) membrane hormone receptors. These include ectopic receptors for gastric inhibitory polypeptide (GIP), beta-adrenergic agonists, or LH/hCG; a similar outcome can result from altered activity of eutopic receptors, such as those for vasopressin (V1-AVPR), serotonin (5-HT4), or possibly leptin. The presence of aberrant receptors places adrenal cells under stimulation by a trophic factor not negatively regulated by glucocorticoids, leading to increased steroidogenesis and possibly to the proliferative phenotype. The molecular mechanisms responsible for the abnormal expression and function of membrane hormone receptors are still largely unknown. Identification of the presence of these illicit receptors can eventually lead to new pharmacological therapies as alternatives to adrenalectomy, now demonstrated by the long-term control of ectopic P-AR- and LH/hCGR-dependent Cushing's syndrome by propanolol and leuprolide acetate. Further studies will potentially identify a larger diversity of hormone receptors capable of coupling to G proteins, adenylyl cyclase, and steroidogenesis in functional adrenal tumors and probably in other endocrine and nonendocrine tumors.

Influence of morphine treatment in pregnant rats on the mineralocorticoid activity of the adrenals in their neonates

Exposure of pregnant rats to morphine, from day 11 to day 18 of gestation, was previously reported to induce both an adrenal atrophy and hypoactivity of the glucocorticoid function in newborns at term, but did not affect, in vitro, the responsiveness of those glands to adrenocorticotrophin hormone (ACTH) concerning corticosterone release. Moreover, these effects were mediated by maternal hormones from the adrenal glands. In the present work, we investigated the effects of a prenatal morphine exposure on the mineralocorticoid activity of the adrenals in neonates. The first aim of the present study was to determine in these newborns 1) the adrenal and plasma aldosterone concentrations at birth time and during the early postnatal period 2) the plasma levels of Na+ and K+ at birth time, 3) the in vitro responsiveness of the newborn adrenals to angiotensin II (A(II)) and ACTH. The second aim of our study was to investigate the mineralocorticoid activity of the adrenals in newborns from adrenalectomized mothers treated with morphine during gestation. According to present data morphine given to intact mothers induced in newborns a severe adrenal atrophy but increased adrenal aldosterone content and plasma aldosterone level. However, prenatal morphine was unable to affect significantly Na+/K+ ratio in both mothers and newborns. In vitro, the adrenals of neonates from morphine-treated mothers were unresponsive to An and ACTH for promoting aldosterone release; in contrast, aldosterone secretion was significantly stimulated by high potassium levels (55 mEq). Maternal adrenalectomy performed one day before the beginning of morphine treatment prevented morphine-induced adrenal atrophy but was unable to affect significantly the adrenal mineralocorticoid function of the offspring. Such data suggest that a prenatal morphine exposure stimulated both aldosterone synthesis and release in neonates. However, this basal hyperfunction did not appear to be coupled with an enhanced adrenal responsivity to AII or ACTH. Prenatal morphine-induced hyperactivity of the mineralocorticoid function of the newborn adrenals, which drastically contrast with hypoactivity of the glucocorticoid one, was independent of adrenal factors from maternal origin.

Hyperinnervation during adrenal regeneration influences the rate of functional recovery

The rat adrenal cortex has the uncommon ability to demonstrate morphological and functional regeneration after injury-induced loss of cortical tissue. Peripheral nerves are involved in tissue regeneration and healing after injury, implying that nerves may also be involved in modulating the regeneration of the adrenal cortex. Studies were initiated to assess changes in adrenal innervation during cortical tissue regeneration subsequent to adrenal enucleation. Innervation of regenerating adrenals was assessed from 3 to 62 days postenucleation by immunohistofluorescent detection of neuronal markers for primary afferent, preganglionic sympathetic, and postganglionic sympathetic fibers. The regenerating adrenal contained few nerves at 3 days postenucleation, but became differentially innervated, with extensive innervation by nerve fibers positive for calcitonin gene-related peptide (CGRP), tyrosine hydroxylase (TH), neuropeptide Y (NPY), and neuronal nitric oxide synthase (nNOS). In contrast, there was only minimal innervation by nerve fibers positive for vasoactive intestinal peptide. By 14 days postenucleation, the CGRP-, TH-, and NPY-positive innervation included areas of hyperinnervation in the capsule, cortex, and central inflammatory site of the regenerating gland. In addition, many chromaffin cells were present at all time points postenucleation. Quantification of the regenerating gland content of CGRP, norepinephrine, epinephrine, and nNOS verified the immunohistofluorescent observations. The period of extensive innervation correlated temporally with the time (3-30 days) during which the regenerating glands recovered steroidogenic function. Moreover, splanchnic nerve transection at the time of adrenal enucleation decreased the innervation by CGRP-positive and vesicular acetylcholine transporter-positive fibers and delayed regeneration. These results support the hypothesis that adrenal innervation modulates tissue regeneration and functional recovery of the enucleated adrenal gland.

Actions of neuropeptide Y on the rat adrenal cortex

Although several studies have demonstrated the presence of neuropeptide Y (NPY) in nerves supplying the mammalian adrenal cortex, its function in this tissue remains unclear, with reports of both stimulatory and inhibitory effects on aldosterone secretion apparently depending on the tissue preparation used. In the present study the effects of NPY on rat adrenal capsular tissue were investigated. NPY significantly stimulated aldosterone secretion in a dose-dependent manner, and this effect was abolished by atenolol, a beta1-adrenergic antagonist. NPY also stimulated the release of catecholamines from intact rat adrenal capsular tissue with the same dose-dependent relationship as the stimulation of aldosterone release. These observations suggest that the actions of NPY may be mediated by the local release of catecholamines from chromaffin cells within adrenal capsular tissue, as we have previously described for vasoactive intestinal peptide. The second part of this study concerned the NPY receptor subtype mediating the actions of NPY on the adrenal cortex. It was found that peptide YY stimulated aldosterone release with a comparable potency to NPY, whereas pancreatic polypeptide (PP) was without effect. The Y1 selective NPY analog Leu31Pro34NPY had a greater effect on aldosterone release than the Y2 selective analog NPY18-36. Studies using the specific Y1 receptor antagonist BIBP 3226 showed significant attenuation of the aldosterone response to NPY, but no effect on the response to added norepinephrine. Binding studies carried out using [125I]NPY revealed the presence of a single population of NPY-binding sites with a Kd of 12.25 nmol/liter and a binding capacity of 623 fmol/mg protein. Competition studies revealed displacement of [125I]NPY specific binding by NPY, peptide YY, and Leu31Pro34NPY, but not by other peptides. Messenger RNA analysis revealed the presence of messenger RNA coding for both the Y1 receptor and the Y4 receptor, but not the other subtypes. Taken together these data suggest that the effects of NPY on the rat adrenal cortex are mediated by the Y1 receptor subtype.

Immune-endocrine interactions in the mammalian adrenal gland: facts and hypotheses

Several cytokines, which are the major mediators of the inflammatory responses, are well-known to stimulate the hypothalamopituitary corticotropin-releasing hormone (CRH)/adrenocorticotropic hormone (ACTH) system, thereby evoking secretory responses by the adrenal cortex. Many of these cytokines, including interleukin-1 (IL-1), IL-2, IL-6, tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (INF-gamma) are synthesized in the adrenal gland by both parenchymal cells and resident macrophages, and the release of some of them (e.g., IL-6 and TNF-alpha) is regulated by the main agonists of steroid hormone secretion (e.g., ACTH and angiotensin-II) and bacterial endotoxins. Adrenocortical and adrenomedullary cells are provided with specific receptors for IL-1, IL-2, and IL-6. IL-1 and TNF-alpha directly inhibit aldosterone secretion of zona glomerulosa cells, whereas IL-6 enhances it. IL-2, IL-3, IL-6, and INF-alpha are able to directly stimulate glucocorticoid production by zona fasciculata and zona reticularis cells, whereas IL-1 exerts an analogous effect through an indirect mechanism involving the stimulation of catecholamine release by chromaffin cells and/or the activation of the intramedullary CRH/ACTH system; again, TNF-alpha depresses glucocorticoid synthesis. IL-6 raises androgen secretion by inner adrenocortical layers. IL-1 enhances the proliferation of adrenocortical cells, and findings suggest that cytokines may control the apoptotic deletion of senescent zona reticularis cells. The relevance of the intraadrenal cytokine system in the fine-tuning of the secretion and growth of the adrenal cortex under normal conditions remains to be explored. However, indirect proof is available that local immune-endocrine interactions may play an important role in modulating adrenal responses to inflammatory and immune challenges and stresses.

Role of tachykinins in the regulation of the hypothalamo-pituitary-adrenal axis

Tachykinins are a family of neuropeptides, which act by binding to three main subtypes of G protein-coupled receptors, named NK1, NK2 and NK3. Tachykinins are contained in both nerve fibers and secretory cells of the hypothalamo-pituitary-adrenal (HPA) axis, and evidence indicates that they take part in the functional control of it. Tachykinins involved in this function include substance P (SP), neuropeptide K and its derivative neurokinin A (NKA), and neurokinin B, which preferentially bind to NK1, NK2 and NK3 receptors, respectively. NK1 agonists exert an inhibitory effect on the hypothalamo pituitary CRH/ACTH system, while NK2 and perhaps NK3 agonists stimulate it, thereby controlling the secretion and growth of the adrenal cortex via circulating ACTH. Intra-adrenal tachykinins may also affect the cortex function. Their direct action on adrenocortical cells is doubtful and probably pharmacologic in nature, but several investigations suggest that tachykinins indirectly stimulate the cortex by acting on medullary chromaffin cells, which in turn exert a paracrine control on adrenocortical cells. SP enhances aldosterone production of zona glomerulosa by eliciting catecholamine secretion; neuropeptide K and NKA raise glucocorticoid production of zonae fasciculata and reticularis through the activation of the intramedullary CRH/ACTH system. The relevance of these effects of tachykinins under basal conditions is questionable, although there are indications that SP is involved in the maintenance of a normal growth and steroidogenic capacity of rat zona glomerulosa, and that SP and NKA play an important role in the stimulation of the adrenal growth during the fetal life. In contrast, evidence has been provided that the role of tachykinins, and especially of SP, could become very relevant under paraphysiological (e.g., physical or inflammatory stresses) or pathological conditions (e.g., ACTH-secreting pituitary tumors), when an excess of steroid-hormone production has to be counteracted.

Role of VIP, PACAP, and related peptides in the regulation of the hypothalamo-pituitary-adrenal axis

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are members of a family of regulatory peptides that are widely distributed in the body and share numerous biologic actions. The two peptides display a remarkable amino acid-sequence homology, and bind to a class of G protein-coupled receptors, named PACAP/VIP receptors (PVRs), whose signaling mechanism mainly involves the activation of adenylate-cyclase and phospholipase-C cascades. A large body of evidence suggests that VIP and PACAP play a role in the control of the hypothalamo--pituitary-adrenal (HPA) axis, almost exclusively acting in a paracrine manner, since their blood concentration is very low. VIP and PACAP are contained in both nerve fibers and neurons of the hypothalamus, and VIP, but not PACAP, is also synthesized in the pituitary gland. Both peptides are expressed in the adrenal gland, and especially in medullary chromaffin cells. All the components of the HPA axis are provided with PVRs. VIP and PACAP enhance pituitary ACTH secretion, VIP by eliciting the hypothalamic release of CRH and potentiating its secretagogue action, and PACAP by directly stimulating pituitary corticotropes. Through this central mechanism, VIP and PACAP may increase mineralo- and glucocorticoid secretion of the adrenal cortex. VIP but not PACAP also exerts a weak direct secretagogue action on adrenocortical cells by activating both PVRs and probably a subtype of ACTH receptors. VIP and PACAP raise aldosterone production via a paracrine indirect mechanism involving the stimulation of medullary chromaffin cells to release catecholamines, which in turn enhance the secretion of zona glomerulosa cells via a beta-adrenoceptor-mediated mechanism. PACAP appears to be able to evoke a glucocorticoid response through the activation, at least in the rat, of the intramedullary CRH/ACTH system. The relevance of these effects of VIP and PACAP under basal conditions is questionable, although there are indications that endogenous VIP is involved in the maintenance of the normal growth and steroidogenic capacity of rat adrenal cortex. However, indirect evidence suggests that these peptides might play a relevant role under paraphysiological conditions (e.g., in the mediation of HPA axis responses to cold and inflammatory stresses) or may be somehow involved in the pathogenesis of Cushing disease or some case of hyperaldosteronism associated with secreting pheochromocytomas.

Human adrenocortical NCI-H295 cells express VIP receptors. Steroidogenic effect of vasoactive intestinal peptide (VIP)

VIP receptors are frequently overexpressed by various endocrine tumors. In this study the expression of VIP receptors in the human adrenocortical carcinoma cell line NCI-H295 and their involvement in the regulation of steroidogenesis was investigated. NCI-H295 cells express VIP1 and VIP2 receptors as demonstrated by RT-PCR, whereas they do not express VIP itself. The receptors are functionally coupled to steroidogenesis since VIP (10(-9) M to 10(-6) M) exerted a dose-dependent stimulatory effect on the release of aldosterone, cortisol, and DHEA. VIP increased ACTH-stimulated releases of aldosterone and cortisol. The proliferation rate of NCI-H295 cells was not affected by VIP. These data show that NCI-H295 cells express both forms of the VIP receptor and that VIP is involved in an ACTH-independent regulation of steroidogenesis in the adrenal tumor cells.

Innervation of the adrenal cortex, its physiological relevance, with primary focus on the noradrenergic transmission

The current knowledge of the catecholaminergic innervation of the mammalian adrenal cortex is summarized, and macro- and microscopic neuromorphology, including the central nervous system connections of the adrenal cortex, is briefly discussed. Morphological and functional data on the catecholaminergic (i.e., noradrenergic) innervation of the adrenal cortex are reviewed. Experimental data suggest that in addition to the regulation of adrenal blood flow, the noradrenergic innervation has a primary influence on zona glomerulosa cells possibly via beta 1 adrenergic and dopaminergic receptors (DA2 subtype via inhibiting T-type Ca2+ channels) It is concluded that the local, modulatory effect of noradrenergic nerve fibres, terminating in the close vicinity of the zona glomerulosa cells, on the systemic renin-angiotensin-aldosterone and other peptide cascade may be influenced by neuropeptides, particularly neuropeptide Y and vasoactive intestinal peptide.

Morphological and functional studies of the paracrine interaction between cortex and medulla in the adrenal gland

Within the last years it has become evident that besides the hypothalamo-pituitary-adrenal axis, extrapituitary mechanisms exist that regulate the activity of the adrenal cortex. In this context, intra-adrenal regulatory mechanisms play an important role. Several secretory products from adrenomedullary cells are able to influence adrenocortical steroidogenesis. Since the main blood flow within the adrenal is directed centripetally from the cortex to the medulla, chromatin cells should act on cortical cells in a paracrine manner. The morphological prerequisite for this regulatory pathway is seen in the close apposition of the two tissues. Within the mammalian adrenal, the two endocrine tissues are interwoven to an astonishing degree with cortical cells located within the medulla and vice versa. It is concluded from morphological and functional studies that paracrine interactions between cortex and medulla play an important role in the regulation of adrenocortical steroidogenesis.

The role of endothelins in the paracrine control of the secretion and growth of the adrenal cortex

Endothelins (ETs) are a family of vasoactive peptides (ET-1, ET-2, and ET-3) mainly secreted by vascular endothelium and widely distributed in the various body systems, where they play major autocrine/paracrine regulatory functions, acting via two subtypes of receptors (ETA and ETB): Adrenal cortex synthesizes and releases ETS and expresses both ETA and ETB. Zona glomerulosa possesses both ETA and ETB, whereas zona fasciculata/reticularis is almost exclusively provided with ETB. ETS exert a strong mineralocorticoid and a less intense glucocorticoid secretagogue action, mainly via ETB receptors. ETS also appear to enhance the growth and steroidogenic capacity of zona glomerulosa and to stimulate its proliferative activity. This trophic action of ETS is likely to be mediated mainly by ETA receptors. The intraadrenal release of ETS undergoes a multiple regulation, with the rise in blood flow rate and the local release of nitric oxide being the main stimulatory factors. Data are also available that indicate that ETS may also have a role in the pathophysiology of primary aldosteronism caused by adrenal adenomas and carcinomas.

Role of adrenomedullin and related peptides in the regulation of the hypothalamo-pituitary-adrenal axis

Adrenomedullin (ADM) is a hypotensive peptide, originally isolated from human pheochromocytomas, and then found to be widely distributed in the various body systems. ADM derives from preproadrenomedullin, a 185-amino acid residue prohormone, containing at its N-terminal a 20-amino acid sequence, named proadrenomedullin N-terminal 20 peptide (PAMP). ADM and PAMP immunoreactivities have been detected in the hypothalamo-pituitary-adrenal (HPA) axis of humans, rats, and pigs. Adrenal glands possess binding sites for both ADM and PAMP, the former being mainly of the subtype 1 of calcitonin gene-related peptide (CGRP) receptors. ADM exerts a direct inhibitory action on angiotensin II- or potassium-stimulated aldosterone secretion of zona glomerulosa cells. This effect is mediated by the CGRP1 receptor and its mechanism probably involves the blockade of Ca2+ influx. In contrast, ADM enhances aldosterone production by in situ perfused rat adrenals and human adrenal slices (containing medullary chromaffin cells), again through the activation of CGRP1 receptors. This aldosterone secretagogue effect of ADM is blocked by the beta-adrenoceptor antagonist l-alprenolol, thereby suggesting that it is indirectly mediated by the release of catecholamines by chromaffin cells. The effects of ADM on adrenal glucocorticoid release are doubtful and probably mediated by the increase in adrenal blood flow rate and the inhibition of ACTH release by pituitary corticotropes. The concentrations reached by ADM and PAMP in the blood rule out the possibility that they act on the HPA axis as circulating hormones. Conversely, their content in both adrenal and hypothalamo-pituitary complex is consistent with a paracrine mechanism of action, which may play a potentially important role in the regulation of fluid and electrolyte homeostasis.

Vasoactive intestinal peptide is a local regulator of adrenocortical function

Several immunohistochemical studies have shown that vasoactive intestinal peptide (VIP) is present in nerve terminals supplying the adrenal capsule and zona glomerulosa, but its function in this tissue has been unclear. Using the intact perfused rat adrenal preparation we showed that VIP is a vasodilator in this tissue, and stimulates aldosterone and corticosterone secretion. The effects of VIP are dependent on the tissue preparation used, and we have evidence that the effect on aldosterone secretion is secondary to local catecholamine release. Administration of a low sodium diet greatly enhanced the aldosterone response to VIP stimulation. Receptor binding studies reveal an increase in the number of VIP receptors in zona glomerulosa tissue in the low sodium group. We have also investigated the regulation of adrenal tissue content of VIP. The low sodium diet caused an increase in peptide content, while a high sodium diet had the opposite effect. Splanchnic nerve section, on the other hand, had no effect on zona glomerulosa VIP content. These findings suggest that adrenal VIP has a significant role in the regulation of zona glomerulosa function, particularly in response to altered electrolyte balance.

Further investigations on the effects of neuropeptide Y on the secretion and growth of rat adrenal zona glomerulosa

NPY is a regulatory peptide, high levels of which are contained in adrenal glands of several mammals and which is co-released with catecholamines during various stressful conditions. The acute and chronic effects of NPY on adrenocortical secretion and growth were studied in the rat. NPY concentration-dependently increased aldosterone (ALDO), but not corticosterone (B) secretion of adrenal slices (maximal effective concentration was 10(-7) M). Two competitive inhibitors of NPY receptors, named PYX-1 and PYX-2, were found to dose-dependently inhibit ALDO response of adrenal preparations to 10(-7) M NPY; PYX-2 was more efficient than PYX-1, and at a concentration of 10(-5) M completely annulled the effect of 10(-7) M NPY. The acute bolus intraperitoneal (i.p.) injection of NPY (3 nmol/kg) raised plasma ALDO concentration (PAC), but not that of B (PBC); this effect of NPY was blocked by the simultaneous injection of PYX-2 (300 nmol/kg). The prolonged i.p. infusion with NPY (3 nmol/kg/h for 7 days) increased PAC (but not PBC) and induced a marked hypertrophy of the zona glomerulosa (ZG) and its parenchymal cells; dispersed ZG cells obtained from NPY-infused rats displayed a significantly enhanced basal and maximally agonist-stimulated ALDO production. The simultaneous infusion with PYX-2 (300 nmol/kg/h) completely annulled all these effects of NPY. The acute or chronic administration of PYX-2 alone did not evoke any apparent effect on the ZG secretion and growth. In light of these findings the following conclusions can be drawn: (i) NPY is able to stimulate not only the secretion, but also the growth of adrenal ZG in rats, via a receptor-mediated mechanism (since this effect is blocked by PYX-2); (ii) endogenous NPY does not play a prominent role in the physiological maintenance of secretion and growth of rat ZG (since PYX-2 alone is ineffective); (iii) NPY may play a crucial role in the fine tuning of the ZG functions in conditions requiring an increased release of mineralocorticoid hormones.

Adrenomedullin stimulates steroid secretion by the isolated perfused rat adrenal gland in situ: comparison with calcitonin gene-related peptide effects

Adrenomedullin (ADM), a vasodilatatory peptide contained in adrenal medulla, was found to induce a dose-dependent increase in aldosterone (ALDO) and corticosterone (B) release by the in situ perfused rat adrenal gland, along with a rise in the flow rate of the perfusion medium. The minimal effective dose for ALDO response was three and two orders of magnitude less than those able to evoke B and medium flow rate responses. Calcitonin gene-related peptide (CGRP), another vasodilatatory peptide contained in adrenal medulla and showing a slight homology in its amino acid sequence with ADM, elicited similar effects. CGRP (8-37), a specific antagonist of CGRP1 receptors, annulled all the effects of both ADM and CGRP, whereas l-alprenolol, a beta-adrenoceptor antagonist, partially reversed only ALDO response to the peptides. In light of these findings the following conclusions are drawn: i) ADM and CGRP stimulate rat adrenals in vivo to release B by raising blood flow rate; ii) ADM and CGRP enhance ALDO secretion via an indirect mechanism probably requiring the release of catecholamines by medullary chromaffin cells; and iii) the effects of ADM and CGRP on the rat adrenal gland are mediated by a common receptor of the CGRP1 subtype.

Neuropeptides in the adrenal gland: distribution, localization of receptors, and effects on steroid hormone synthesis

In this review we defined and classified the neuropeptides (NPs) related to the adrenal gland, according to Palkovits (Frontiers Neuroendocrinol 10:1 1988). The concentration (RIA) and distribution (immunohistochemistry) of NPs, as well as the localization of the receptors (radioligand studies) were summarized. Direct effects of NPs on aldosterone and corticosterone synthesis obtained by in vivo, in situ perfusion, and in vitro experimental approaches were reviewed. Data (from different rat strains and genders) for 35 NPs are presented.

In vitro steroidogenic effects of neuropeptide Y (NPY1-36), Y1 and Y2 receptor agonists (Leu31-Pro34 NPY, NPY18-36) and peptide YY (PYY) on rat adrenal capsule/zona glomerulosa

The effects of neuropeptide Y (NPY1-36), of two analogs (Leu31-Pro34 NPY and NPY18-36) and of Peptide YY (PYY) on aldosterone and corticosterone secretions by freshly isolated rat adrenal capsule/zona glomerulosa preparations were investigated in vitro. NPY-related peptides (NPY1-36, Leu31-Pro34 NPY, NPY18-36), but not PYY, induced a dose-dependent release of aldosterone at concentrations ranging from 10(-8) to 10(-6) M. All the investigated peptides failed to significantly affect corticosterone secretion in concentrations ranging from 10(-10) to 10(-6) M (NPY1-36, NPY18-36), 10(-11) to 10(-6) M (Leu31-Pro34 NPY) or 10(-9) to 10(-6) M (PYY). Aldosterone secretion by this preparation of isolated adrenal capsule/zona glomerulosa was also significantly stimulated by high potassium levels (55 mEq) or by angiotensin II (AII) in concentrations ranging from 10(-8) to 10(-6) M. Moreover, NPY and Y1 or Y2 receptor agonists were positive aldosterone releasing agents as potent as AII. The present data support the existence of: (1) NPY binding sites of the Y3-like subtype, on rat adrenal capsule/zona glomerulosa. (2) A stimulatory effect of NPY on aldosterone production. So that the NPYergic innervation of the rat adrenal capsule/zona glomerulosa could be implicated in the multifactorial control of aldosterone production.