Evidence that endogenous substance-P (SP) is involved in the maintenance of the growth and steroidogenic capacity of rat adrenal zona glomerulosa

Steroidogenic cell microenvironment and adrenal function in physiological and pathophysiological conditions

The human adrenal cortex is a complex organ which is composed of various cell types including not only steroidogenic cells but also mesenchymal cells, immunocompetent cells and neurons. Intermingling of these diverse cell populations favors cell-to-cell communication processes involving local release of numerous bioactive signals such as biogenic amines, cytokines and neuropeptides. The resulting paracrine interactions play an important role in the regulation of adrenocortical cell functions both in physiological and pathophysiological conditions. Especially, recent evidence indicates that adrenocortical cell microenvironment is involved in the pathogenesis of adrenal disorders associated with corticosteroid excess. The paracrine factors involved in these intraadrenal regulatory mechanisms may thus represent valuable targets for future pharmacological treatments of adrenal diseases.

The neuropeptide substance P regulates aldosterone secretion in human adrenals

Aldosterone, produced by the adrenals and under the control of plasma angiotensin and potassium levels, regulates hydromineral homeostasis and blood pressure. Here we report that the neuropeptide substance P (SP) released by intraadrenal nerve fibres, stimulates aldosterone secretion via binding to neurokinin type 1 receptors (NK1R) expressed by aldosterone-producing adrenocortical cells. The action of SP is mediated by the extracellular signal-regulated kinase pathway and involves upregulation of steroidogenic enzymes. We also conducted a prospective proof-of-concept, double blind, placebo-controlled clinical trial aimed to investigate the impact of the NK1R antagonist aprepitant on aldosterone secretion in healthy male volunteers (EudraCT: 2008-003367-40, ClinicalTrial.gov: NCT00977223). Participants received during two 7-day treatment periods aprepitant (125 mg on the 1^st day and 80 mg during the following days) or placebo in a random order at a 2-week interval. The primary endpoint was plasma aldosterone levels during posture test. Secondary endpoints included basal aldosterone alterations, plasma aldosterone variation during metoclopramide and hypoglycaemia tests, and basal and stimulated alterations of renin, cortisol and ACTH during the three different stimulatory tests. The safety of the treatment was assessed on the basis of serum transaminase measurements on days 4 and 7. All pre-specified endpoints were achieved. Aprepitant decreases aldosterone production by around 30% but does not influence the aldosterone response to upright posture. These results indicate that the autonomic nervous system exerts a direct stimulatory tone on mineralocorticoid synthesis through SP, and thus plays a role in the maintenance of hydromineral homeostasis. This regulatory mechanism may be involved in aldosterone excess syndromes.

Adrenal aldosterone production is regulated by plasma angiotensin and potassium levels. Here the authors report that the neuropeptide substance P stimulates aldosterone production via neurokinin type 1 receptors (NK1R), and report a proof-of-concept placebo controlled clinical trial showing that a NK1R antagonist decreases aldosterone levels.

Trophic factors in the carotid body

The aim of the present study is to provide a review of the expression and action of trophic factors in the carotid body. In glomic type I cells, the following factors have been identified: brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, artemin, ciliary neurotrophic factor, insulin-like growth factors-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha and -beta1, interleukin-1beta and -6, tumour necrosis factor-alpha, vascular endothelial growth factor, and endothelin-1 (ET-1). Growth factor receptors in the above cells include p75LNGFR, TrkA, TrkB, RET, GDNF family receptors alpha1-3, gp130, IL-6Ralpha, EGFR, FGFR1, IL1-RI, TNF-RI, VEGFR-1 and -2, ETA and ETB receptors, and PDGFR-alpha. Differential local expression of growth factors and corresponding receptors plays a role in pre- and postnatal development of the carotid body. Their local actions contribute toward producing the morphologic and molecular changes associated with chronic hypoxia and/or hypertension, such as cellular hyperplasia, extracellular matrix expansion, changes in channel densities, and neurotransmitter patterns. Neurotrophic factor production is also considered to play a key role in the therapeutic effects of intracerebral carotid body grafts in Parkinson's disease. Future research should also focus on trophic actions on carotid body type I cells by peptide neuromodulators, which are known to be present in the carotid body and to show trophic effects on other cell populations, that is, angiotensin II, adrenomedullin, bombesin, calcitonin, calcitonin gene-related peptide, cholecystokinin, erythropoietin, galanin, opioids, pituitary adenylate cyclase-activating polypeptide, atrial natriuretic peptide, somatostatin, tachykinins, neuropeptide Y, neurotensin, and vasoactive intestinal peptide.

Capsaicin-sensitive nerve fibers: a potential extra-ACTH mechanism participating in adrenal regeneration in rats

Pituitary-derived factors, including ACTH, have been widely implicated in initiating adrenal regeneration. However, recent work has demonstrated that adrenal regeneration is also modulated by adrenal nerves that extensively reinnervate the regenerating adrenal. Moreover, transection of the splanchnic nerve removes sensory calcitonin gene-related peptide (CGRP) and preganglionic sympathetic vesicular acetylcholine transporter (VAChT)-positive fibers from the regenerating gland and delays regeneration. However, it is not known whether the splanchnic nerve effects on adrenal regeneration are mediated by the CGRP-positive or VAChT-positive innervation. The present studies use the drug capsaicin, a neurotoxin selective for a subset of primary afferent neurons, to specifically remove CGRP-positive fibers from the adrenal gland and assess subsequent effects on the recovery of adrenal mass and function after surgical enucleation. Male, Sprague-Dawley rats were anesthetized and treated with capsaicin (vs. vehicle) periaxonally to the thoracic splanchnic nerve (33 mM, 15 minutes) or systemically (30-100 mg/kg for 4 days, s.c.). After 7-12 days of recovery, rats received right adrenalectomy and left adrenal enucleation. At 14 and 21 days postenucleation, prestress and poststress plasma and adrenals glands were collected; adrenals were weighed and fixed for immunolabeling of CGRP-positive nerve fibers. Periaxonal capsaicin treatment decreased adrenal CGRP content prior to surgical enucleation; however, reinnervation by CGRP-positive fibers was not prevented and regeneration was not affected. Systemic capsaicin treatment attenuated the reinnervation by CGRP-positive fibers and increased the rate, but not extent, of adrenal regeneration. These results support the hypothesis that adrenal innervation represents an extra-ACTH mechanism to modulate the rate of adrenal regeneration.

Ranakinin, a naturally occurring tachykinin, stimulates phospholipase C activity in the frog adrenal gland

We have previously shown that the frog adrenal gland is innervated by a dense network of fibers containing ranakinin, one of the endogenous tachykinins in the amphibian Rana ridibunda, and we have found that ranakinin stimulates in vitro corticosteroid secretion by frog adrenal tissue. To elucidate the mechanism of action of ranakinin on the frog adrenal gland, we investigated the effect of ranakinin on cAMP formation and polyphosphoinositide metabolism. Incubation of frog adrenal explants with various tachykinins, including ranakinin, substance P, neurokinin A, or neurokinin B, did not produce any significant modification of cAMP concentrations. In contrast, ranakinin induced a time- and dose-dependent stimulation of inositol phosphate formation with a concomitant decrease in membrane polyphosphoinositides. Pretreatment of the tissue slices with the phospholipase C inhibitor U-73122 or with pertussis toxin completely abolished the stimulatory effect of ranakinin on inositol phosphate formation. Prolonged administration of U-73122 to perifused frog adrenal explants markedly attenuated the ranakinin-evoked stimulation of corticosterone and aldosterone secretion. Taken together, these data indicate that in the frog adrenal gland, ranakinin has no effect on the adenylyl cyclase system, but enhances polyphosphoinositide hydrolysis. The stimulatory action of ranakinin on inositol phosphate formation and corticosteroid secretion is mediated through activation of a phospholipase C positively coupled to a pertussis toxin-sensitive G protein.

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.

Neuropeptide K and neurokinin A stimulate CRH and ACTH release by rat adrenal medulla in vitro

Tachykinins are a family of peptides that are able to modulate the activity of the hypothalamo-pituitary CRH-ACTH system. Mammalian tachykinins include neurokinin A (NKA), neurokinin B (NKB), neuropeptide K (NPK), and substance P (SP). We investigated by RIA the effects of tachykinins on the release of CRH and ACTH by rat adrenal medulla in vitro. NKA and NPK concentration-dependently enhanced the release of both CRH and ACTH, NPK being more active than NKA. NKB exerted only a minor stimulatory action exclusively on CRH release, and SP was ineffective. The stimulatory effect of both NKA and NPK on ACTH release was blocked by the CRH receptor antagonist alpha-helical-CRH, thereby suggesting that the increase in ACTH secretion is consequent to the stimulation of CRH release. These findings indicate that NKA and NPK are stimulators not only of the central (hypothalamo-pituitary), but also of the peripheral (intramedullary) branch of the CRH-ACTH system.

The possible role of endogenous substance P in the modulation of the response of rat pituitary-adrenal axis to stresses

The role played by endogenous substance P (SP) in the regulation of hypothalamo-pituitary-adrenal (HPA) axis was investigated in the rat. Normal and ether-stressed (2 min ether-vapor inhalation) or cold-stressed (20 min at 4 degrees C) animals were given a bolus subcutaneous injection of 100 nmol spantide (SPA) a specific antagonist of SP; their blood concentrations of ACTH, aldosterone (ALDO) and corticosterone (B) were measured by specific RIA, 1, 2 or 4 h after the injection. SPA did not evoke significant changes in the basal plasma levels of the three hormones. Ether and cold stresses markedly raised the blood concentrations of ACTH, ALDO and B, being maximal response observed after 1 or 2 h. SPA notably enhanced the responses of the three hormones to ether stress. SPA magnified ALDO and B responses to cold stress, but it notably depressed ACTH one. In light of these findings, it may be concluded that (i) endogenous SP does not affect basal activity of rat HPA axis, but it exerts an inhibitory action on its response to the stresses, especially the ether-inhalation one: and (ii) different mechanisms are involved in the cold and ether stress-induced activation of the HPA axis.

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.