In vitro evidence for a blood-borne renin-releasing factor

Role of 5-HT in the regulation of the brain-pituitary-adrenal axis: effects of 5-HT on adrenocortical cells

Serotonin (5-HT) plays a pivotal role in the regulation of the brain-pituitary-adrenal axis. In particular, 5-HT has been shown to control the activity of hypothalamic CRF neurons and pituitary corticotrope cells through activation of 5-HT1A and (or) 5-HT2A/2C receptor subtypes. 5-HT, acting through 5-HT2 receptors, can also trigger the renin-angiotensin system by stimulating renin secretion and consequently can enhance aldosterone production. At the adrenal level, 5-HT produced locally stimulates the secretory activity of adrenocortical cells through a paracrine mode of communication. The presence of 5-HT in the adrenal gland has been demonstrated immunohistochemically and biochemically in various species. In the frog, rat, and pig adrenal gland, 5-HT is synthesized by chromaffin cells, while in the mouse adrenal cortex, 5-HT is contained in nerve fibers. In man, 5-HT is present in perivascular mast cells. In vivo and in vitro studies have shown that 5-HT stimulates corticosteroid secretion in various species (including human). The type of receptor involved in the mechanism of action of 5-HT differs between the various species. In frogs and humans, the stimulatory effect of 5-HT on adrenocortical cells is mediated through a 5-HT4 receptor subtype positively coupled to adenylyl cyclase and calcium influx. In the rat, the effect of 5-HT on aldosterone secretion is mediated via activation of 5-HT7 receptors. Clinical studies indicate that 5-HT4 receptor agonists stimulate aldosterone secretion in healthy volunteers and in patients with corticotropic insufficiency and primary hyperaldosteronism. Local serotonergic control of corticosteroid production may be involved in the physiological control of the activity of the adrenal cortex as well as in the pathophysiology of cortisol and aldosterone disorders.Key words: HPA axis, renin-angiotensin system, adrenal gland, corticosteroid secretion, serotonergic receptors.

Forebrain pathways mediating stress-induced hormone secretion

Exposure to hostile conditions initiates the secretion of several hormones, including corticosterone/cortisol, catecholamines, prolactin, oxytocin, and renin, as part of the survival mechanism. Such conditions are often referred to as "stressors" and can be divided into three categories: external conditions resulting in pain or discomfort, internal homeostatic disturbances, and learned or associative responses to the perception of impending endangerment, pain, or discomfort ("psychological stress"). The hormones released in response to stressors often are referred to as "stress hormones" and their secretion is regulated by neural circuits impinging on hypothalamic neurons that are the final output toward the pituitary gland and the kidneys. This review discusses the forebrain circuits that mediate the neuroendocrine responses to stressors and emphasizes those neuroendocrine systems that have previously received little attention as stress-sensitive hormones: renin, oxytocin, and prolactin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABAA, histamine, and serotonin receptors alter the neuroendocrine stress response. The effects of these drugs are discussed in relation to their effects on forebrain neural circuits that regulate stress hormone secretion. For psychological stressors such as conditioned fear, the neural circuits mediating neuroendocrine responses involve cortical activation of the basolateral amygdala, which in turn activates the central nucleus of the amygdala. The central amygdala then activates hypothalamic neurons directly, indirectly through the bed nucleus of the stria terminalis, and/or possibly via circuits involving brainstem serotonergic and catecholaminergic neurons. The renin response to psychological stress, in contrast to those of ACTH and prolactin, is not mediated by the bed nucleus of the stria terminalis and is not suppressed by benzodiazepine anxiolytics. Stressors that challenge cardiovascular homeostasis, such as hemorrhage, trigger a pattern of neuroendocrine responses that is similar to that observed in response to psychological stressors. These neuroendocrine responses are initiated by afferent signals from cardiovascular receptors which synapse in the medulla oblongata and are relayed either directly or indirectly to hypothalamic neurons controlling ACTH, prolactin, and oxytocin release. In contrast, forebrain pathways may not be essential for the renin response to hemorrhage. Thus current evidence indicates that although a diverse group of stressors initiate similar increases in ACTH, renin, prolactin, and oxytocin, the specific neural circuits and neurotransmitter systems involved in these responses differ for each neuroendocrine system and stressor category.

Forebrain pathways mediating stress-induced renin secretion

1. All stressors tested so far increase plasma renin levels; among these are exposure to foot-shock, immobilization, forced swimming, head-up tilt, exercise, hypotension haemorrhage and conditioned fear. 2. In old rats (22 months old), conditioned fear stress fails to increase plasma renin concentrations to the same level as in young rats (7 months old). 3. Destruction of cells in the paraventricular hypothalamic nucleus (PVH), either electrolytically or with the cell-selective neurotoxin ibotenic acid, prevents the effect of conditioned fear stress, but not of immobilization, on plasma renin concentration. 4. Ibotenic acid-induced lesions in the central amygdaloid nucleus inhibit conditioned fear stress-induced increases in plasma renin concentrations, but do not reduce the renin response to immobilization. Lesions in lateral amygdaloid nuclei do not reduce the renin response to stressors. 5. Although lesions in the bed nucleus of the stria terminalis (BNST) reduce the adrenocortical response to conditioned fear stress, they do not reduce the effect of stress on plasma renin concentration. 6. Destruction of catecholaminergic terminals in the PVH prevents the effect of conditioned fear stress on plasma renin concentration. 7. Electrolytic lesions in the dorsal raphe nucleus, which is a major site of origin of ascending serotonergic pathways, also inhibit the effect of conditioned fear stress on plasma renin concentration. 8. Activation of serotonin1A (5HT1A) receptors with the anti-anxiety drugs buspirone and ipsapirone reduces the firing rate of serotonergic neurons in the dorsal raphe nucleus in the midbrain and decreases the effect of stress on plasma renin concentrations. In contrast, the benzodiazepine anxiolytic drugs, chlordiazepoxide and midazolam, are ineffective in inhibiting the renin response to stress. 9. Chemical sympathectomy combined with adrenal medullectomy does not prevent the effect of conditioned fear stress on plasma renin concentration, suggesting that the sympathetic system is not the sole mediator of the message from the brain to the kidneys. 10. Combined, these observations suggest that aversive information from the cortex is transmitted via the amygdala to catecholaminergic cells in the medulla and to serotonergic cells in the dorsal raphe that stimulate the PVH to increase the release of renin.

Serotonergic regulation of renin and prolactin secretion

Drugs that, directly or indirectly produce activation of serotonin (5-HT) receptors increase plasma concentrations of both prolactin and renin. The serotonergic regulation of prolactin and renin secretion share several common characteristics. Serotonergic neurons originating in the dorsal raphe and terminating in the hypothalamus stimulate the secretion of both prolactin and renin. Destruction of cells in the hypothalamic paraventricular nucleus (PVN) inhibits both the prolactin and renin responses to 5-HT agonists and 5-HT-releasing drugs. Activation of 5-HT2 receptors increases the secretion of both prolactin and renin, while activation of other 5-HT receptor subtypes has differential effects on these hormones. However, there are also differences between the serotonergic mechanisms that regulate the secretion of prolactin and renin. Activation of 5-HT1A receptors increases the secretion of prolactin but not of renin. In addition, activation of peripheral 5-HT2 receptors stimulates the secretion of renin, while activation of peripheral 5-HT3 receptors increases plasma levels of prolactin but not renin. In humans, the effect of 5-HT-releasing drugs and 5-HT agonists on plasma prolactin concentrations has been studied to a greater extent than effects on most other hormones. In contrast, the renin response to 5-HT agonists and 5-HT releasers has not been well characterized in humans. Because of the important role of the renin-angiotensin system in cardiovascular regulation, studies on the serotonergic regulation of renin release in humans could increase our understanding of cardiovascular disorders associated with altered serotonergic function. Examples include anxiety and consequences of cocaine abuse. In conclusion, comparing the serotonergic regulation of prolactin and renin secretion indicates similarities that might shed light on common brain mechanisms that regulate neuroendocrine function.

Serotonin and stress-induced increases in renin secretion are not blocked by sympathectomy/adrenal medullectomy but are blocked by beta antagonists

The present study examined the role of the sympathetic nervous system as a mediator of the message from the CNS to the kidneys to stimulate the secretion of renin. Two procedures that increase the secretion of renin were tested: administration of the serotonin releaser fenfluramine, which increases renin release without altering blood pressure [53], and subjecting the rats to the 'psychological' stressor of conditioned emotional response (CER) stress. Pretreatment of rats with either the beta antagonist sotalol or the beta 1-selective antagonist atenolol completely prevented the increase in plasma renin activity and concentration caused by fenfluramine (5 mg/kg i.p.) injection. However, chemical sympathectomy with 6-hydroxydopamine (6-OHDA) combined with surgical adrenal medullectomy did not prevent the increase in plasma renin activity and concentration following fenfluramine injection. Since beta-antagonists have been previously shown to prevent the renin response to CER stress, we also tested whether the sympathetic nervous system mediates the renin response to CER stress. Chemical sympathectomy combined with adrenal medullectomy did not prevent the effect of CER stress on renin release. The completeness of the sympathectomy/adrenal medullectomy was verified biochemically by measuring plasma epinephrine and both plasma and renal norepinephrine concentrations. Plasma epinephrine and renal norepinephrine levels were reduced to below 1% of control while plasma norepinephrine was reduced to below 8% of control values. In conclusion, our data support previous reports suggesting that activation of CNS pathways increases the secretion of renin. However, the message from the brain to release renin from the kidneys does not exclusively involve either the sympathetic innervation of the kidneys or adrenal epinephrine. Although beta 1 receptors are involved in mediating this phenomenon, their location or mechanism remains unknown and will be discussed.

Peripheral p-chloroamphetamine is an unsuitable probe for investigation of central serotoninergic control on renal renin secretion in the rat

Intraperitoneal application of p-chloroamphetamine (PCA) is considered a suitable probe for investigation of central serotoninergic control on renin release in the rat, although it causes several behavioral and autonomic changes including negative water balance (increased urination and loss of body weight). The possibility that PCA-induced renin release is secondary to the alterations in water balance was investigated 1 hour after intraperitoneal PCA in male Wistar (Wi) (Experiment I). Long-Evans (LE) and diabetes insipidus (DI) (Experiment II), DI rats pretreated by the inhibitor of angiotensin I-converting enzyme captopril (Experiment III), and water-loaded or propranolol-pretreated Wi rats (Experiment IV). PCA treatment induced significant body weight loss, increase in hematocrit, stimulation of renin-aldosterone system (RAS) and elevation of plasma creatinine level. A toxic damage of the kidney and liver was documented histologically 72 h after 5 mg/kg PCA in Wi rats. The blockade of PCA-induced stimulation of RAS (by captopril or propranolol) markedly potentiated the attendant negative water balance, whereas positive water balance (oral water load) abolished PCA-induced renin secretion. In conclusion, intraperitoneal PCA is an unsuitable probe for investigation of central serotoninergic control on renin release in the rat since PCA-induced renin release is caused by the attendant negative water balance.

Partial characterization of a renin-releasing factor from plasma and hypothalamus

Previous studies have indicated that administration of the serotonin releaser p-chloroamphetamine HCl produces a dose-dependent increase in renin secretion through a blood-borne renin-releasing factor. The present studies were designed to partially characterize this renin-releasing factor using an in vitro kidney slice method for the bioassay of renin-releasing activity. Plasma from p-chloroamphetamine-treated, nephrectomized rats was used to obtain the renin-releasing factor, which was fractionated by ultrafiltration into fractions of molecular weight ranges of 1000 to 5000, 5000 to 10,000, and 10,000 to 20,000. The molecular weight ranges of the renin-releasing factor was determined to be between 5000 and 10,000. Since previous studies have shown that lesions in the hypothalamus prevent the effect of p-chloroamphetamine on renin secretion, we tested whether a hypothalamic extract can release renin from kidney slices. Addition of extracts of boiled rat hypothalamic tissue to the kidney slices caused an increase in renin release. Addition of cerebellar extracts produced a smaller increase in renin release, whereas addition of pituitary extracts had no effect. Fractionation by ultrafiltration of bovine hypothalamic extract revealed that the fraction with a molecular weight range of 5000 to 10,000 possessed the highest renin-releasing ability. The 1000 to 5000 (molecular weight) fraction possessed a sizeable renin-releasing activity, but the 10,000 to 20,000 fraction had no renin-releasing activity. Both bovine hypothalamus fractions (molecular weights between 1000-5000 and 5000-10,000) and plasma fraction lost their renin-releasing activity after digestion with pronase, suggesting that the renin-releasing factor or factors are peptides. These results suggest that a renin-releasing factor originate in the hypothalamus.

An improved method for the measurement of renin release from coronal, vibratome-cut kidney slices

Our studies were designed to optimize a system for studying renin release in vitro. Our rat kidney slices (400 micron) were cut coronally on a vibratome, and the medullary tissue was dissected from each slice. Previous investigators obtained kidney slices by slicing sagittally, parallel to the cortical surface, with a hand-held microtome. Slicing on a vibratome and removal of the medulla ensured that the slices were of uniform thickness, weight (18.3 +/- 0.52 mg; n = 24), and renin content. Furthermore, one kidney can provide approximately 30 renal slices whereas the previous methods provided only two slices per kidney. The slices were placed in siliconized vials containing a Krebs-Ringer solution (pH 7.4) at 37 degrees C for 30 min. The Krebs-Ringer solution was decanted and replaced with fresh Krebs-Ringer solution for a 30-minute preincubation period. At the end of the 30-min preincubation period, a 0.2-ml sample was taken for the determination of renin release, and the remaining medium was decanted. Fresh Krebs-Ringer solution and the test substances were subsequently added for the incubation period. Throughout the experiment, the vials were maintained at 37 degrees C, and each vial received a gas mixture of 95% O2-5% CO2 via plastic tubing connected to a needle that was inserted into the snap-cap of each vial but did not bubble the medium. The old (bubbling) method for the bioassay of renin yielded values for renin release that were low and had large interexperimental variation (0.23 +/- 0.04 to 1.9 +/- 0.3 ng angiotensin I (AI)/mg kidney/hr; n = 24).(ABSTRACT TRUNCATED AT 250 WORDS)