Selective neural regulation of epinephrine and norepinephrine cells in the adrenal medulla -- cardiovascular implications

Evaluation of sub-chronic toxicity of melamine via systematic or oral delivery in adult zebrafish based on behavioral endpoints

Melamine-tainted products have been found in the market and raised issues about food safety. Recent studies done in rodents and humans demonstrated the toxicities of melamine, especially in causing kidney damage and bladder stone formation. However, very few studies assessed its behavior toxicity in organisms, including fish. Therefore, in this study, the researchers aim to determine whether sub-chronic exposure to melamine via oral and systematic administration could induce behavioral abnormality in zebrafish. After 14 days of systematic exposure to melamine at doses of 0.1 and 10 ppm levels, zebrafish were subjected to multiple behavioral assays. Results from both exposure routes showed that melamine indeed slightly increased fish locomotion and altered their exploratory behaviors in the novel tank assay. Furthermore, tightened shoaling formation was also displayed by the treated fish in the waterborne exposure group. However, melamine exposure did not cause any obvious alterations in fish behaviors during other behavioral tests. In addition, in comparison with previously published data on the behavior toxicities of several solvents in zebrafish, our phenomic analysis suggests the relatively low behavior toxicities of melamine via either systematic exposure or oral administration to zebrafish compared to those solvents. Nevertheless, our data indicate that the potential neurotoxicity of chronic low-dose melamine should not be ignored.

Adaptive remodeling of the stimulus-secretion coupling: Lessons from the 'stressed' adrenal medulla

Stress is part of our daily lives and good health in the modern world is offset by unhealthy lifestyle factors, including the deleterious consequences of stress and associated pathologies. Repeated and/or prolonged stress may disrupt the body homeostasis and thus threatens our lives. Adaptive processes that allow the organism to adapt to new environmental conditions and maintain its homeostasis are therefore crucial. The adrenal glands are major endocrine/neuroendocrine organs involved in the adaptive response of the body facing stressful situations. Upon stress episodes and in response to activation of the sympathetic nervous system, the first adrenal cells to be activated are the neuroendocrine chromaffin cells located in the medullary tissue of the adrenal gland. By releasing catecholamines (mainly epinephrine and to a lesser extent norepinephrine), adrenal chromaffin cells actively contribute to the development of adaptive mechanisms, in particular targeting the cardiovascular system and leading to appropriate adjustments of blood pressure and heart rate, as well as energy metabolism. Specifically, this chapter covers the current knowledge as to how the adrenal medullary tissue remodels in response to stress episodes, with special attention paid to chromaffin cell stimulus-secretion coupling. Adrenal stimulus-secretion coupling encompasses various elements taking place at both the molecular/cellular and tissular levels. Here, I focus on stress-driven changes in catecholamine biosynthesis, chromaffin cell excitability, synaptic neurotransmission and gap junctional communication. These signaling pathways undergo a collective and finely-tuned remodeling, contributing to appropriate catecholamine secretion and maintenance of body homeostasis in response to stress.

Developmental role of PHD2 in the pathogenesis of pseudohypoxic pheochromocytoma

Despite a general role for the HIF hydroxylase system in cellular oxygen sensing and tumour hypoxia, cancer-associated mutations of genes in this pathway, including PHD2, PHD1, EPAS1 (encoding HIF-2α) are highly tissue-restricted, being observed in pseudohypoxic pheochromocytoma and paraganglioma (PPGL) but rarely, if ever, in other tumours. In an effort to understand that paradox and gain insights into the pathogenesis of pseudohypoxic PPGL, we constructed mice in which the principal HIF prolyl hydroxylase, Phd2, is inactivated in the adrenal medulla using TH-restricted Cre recombinase. Investigation of these animals revealed a gene expression pattern closely mimicking that of pseudohypoxic PPGL. Spatially resolved analyses demonstrated a binary distribution of two contrasting patterns of gene expression among adrenal medullary cells. Phd2 inactivation resulted in a marked shift in this distribution towards a Pnmt-/Hif-2α+/Rgs5+ population. This was associated with morphological abnormalities of adrenal development, including ectopic TH+ cells within the adrenal cortex and external to the adrenal gland. These changes were ablated by combined inactivation of Phd2 with Hif-2α, but not Hif-1α. However, they could not be reproduced by inactivation of Phd2 in adult life, suggesting that they arise from dysregulation of this pathway during adrenal development. Together with the clinical observation that pseudohypoxic PPGL manifests remarkably high heritability, our findings suggest that this type of tumour likely arises from dysregulation of a tissue-restricted action of the PHD2/HIF-2α pathway affecting adrenal development in early life and provides a model for the study of the relevant processes.

Towards an understanding of neurophysiological self-regulation in early childhood: A heuristic and a new approach

Self-regulation in early childhood encompasses both "top down," volitional processes, as well as the "bottom up" activity of three neurophysiological systems: the parasympathetic nervous system (PNS), the sympathetic nervous system (SNS), and the hypothalamic-pituitary-adrenal (HPA) axis. In this paper we briefly review the structure, function, and early development of each of these systems and then explain why neurophysiological self-regulation is most accurately defined as a function of their joint activity. We note that while there are a number of predictive models that employ this definition, the field would benefit from a straightforward heuristic and aligned methods of visualization and analysis. We then present one such heuristic, which we call neurophysiological space, and outline how it may facilitate a new, collaborative approach to building a better understanding of self-regulation in early childhood. We conclude with a presentation of early education as one setting in which our heuristic and methods could be applied.

Carbon-Fiber Nanoelectrodes for Real-Time Discrimination of Vesicle Cargo in the Native Cellular Environment

Carbon-fiber microelectrodes have proven to be an indispensable tool for monitoring exocytosis events using amperometry. When positioned adjacent to a cell, a traditional microdisc electrode is well suited for quantification of discrete exocytotic release events. However, the size of the electrode does not allow for intracellular electrochemical measurements, and the amperometric approach cannot distinguish between the catecholamines that are released. In this work, carbon nanoelectrodes were developed to permit selective electrochemical sampling of nanoscale vesicles in the cell cytosol. Classical voltammetric techniques and electron microscopy were used to characterize the nanoelectrodes, which were ∼5 μm long and sharpened to a nanometer-scale tip that could be wholly inserted into individual neuroendocrine cells. The nanoelectrodes were coupled with fast-scan cyclic voltammetry to distinguish secretory granules containing epinephrine from other catecholamine-containing granules encountered in the native cellular environment. Both vesicle subtypes were encountered in most cells, despite prior demonstration of populations of chromaffin cells that preferentially release one of these catecholamines. There was substantial cell-to-cell variability in relative epinephrine content, and vesicles containing epinephrine generally stored more catecholamine than the other vesicles. The carbon nanoelectrode technology thus enabled analysis of picoliter-scale biological volumes, revealing key differences between chromaffin cells at the level of the dense-core granule.

Spatial and activity-dependent catecholamine release in rat adrenal medulla under native neuronal stimulation

Neuroendocrine chromaffin cells of the adrenal medulla in rat receive excitatory synaptic input through anterior and posterior divisions of the sympathetic splanchnic nerve. Upon synaptic stimulation, the adrenal medulla releases the catecholamines, epinephrine, and norepinephrine into the suprarenal vein for circulation throughout the body. Under sympathetic tone, catecholamine release is modest. However, upon activation of the sympathoadrenal stress reflex, and increased splanchnic firing, adrenal catecholamine output increases dramatically. Moreover, specific stressors can preferentially increase release of either epinephrine (i.e., hypoglycemia) or norepinephrine (i.e., cold stress). The mechanism for this stressor-dependent segregated release of catecholamine species is not yet fully understood. We tested the hypothesis that stimulation of either division of the splanchnic selects for epinephrine over norepinephrine release. We introduce an ex vivo rat preparation that maintains native splanchnic innervation of the adrenal gland and we document experimental advantages and limitations of this preparation. We utilize fast scanning cyclic voltammetry to detect release of both epinephrine and norepinephrine from the adrenal medulla, and report that epinephrine and norepinephrine release are regulated spatially and in a frequency-dependent manner. We provide data to show that epinephrine is secreted preferentially from the periphery of the medulla and exhibits a higher threshold and steeper stimulus-secretion function than norepinephrine. Elevated stimulation of the whole nerve specifically enhances epinephrine release from the peripheral medulla. Our data further show that elimination of either division from stimulation greatly attenuated epinephrine release under elevated stimulation, while either division alone can largely support norepinephrine release.

Neurobiological underpinnings of dogs' human-like social competence: How interactions between stress response systems and oxytocin mediate dogs' social skills

Domestic dogs (Canis familiaris) have been suggested as a natural model for human social cognition, possessing social skills that are in many ways functionally analogous to those of young humans. Researchers have debated the origins of dogs' human-like social competence and the underlying cognitive mechanisms, but only recently have researchers begun to explore their neurobiological underpinnings. In this review, findings from behavioral studies are integrated with what is known about the biological basis of dogs' human-directed social competence, with an emphasis on how stress-mediating systems, particularly the hypothalamic-pituitary-adrenal (HPA) axis, interact with oxytocin and underlying neural systems to facilitate dogs' interspecific social-cognitive abilities. The working model presented in this paper offers a biological explanation for many of the inconsistent findings from past work on social cognition in dogs and generates questions for future research in the field of canine social competence.

Pathophysiology and diagnosis of disorders of the adrenal medulla: focus on pheochromocytoma

The principal function of the adrenal medulla is the production and secretion of catecholamines. During stressful challenging conditions, catecholamines exert a pivotal homeostatic role. Although the main adrenomedullary catecholamine, epinephrine, has a wide array of adrenoreceptor-mediated effects, its absence does not cause life-threatening problems. In contrast, excess production of catecholamines due to an adrenomedullary tumor, specifically pheochromocytoma, results in significant morbidity and mortality. Despite being rare, pheochromocytoma has a notoriously bad reputation because of its potential devastating effects if undetected and untreated. The paroxysmal signs and symptoms and the risks of missing or delaying the diagnosis are well known for most physicians. Nevertheless, even today the diagnosis is still overlooked in a considerable number of patients. Prevention and complete cure are however possible by early diagnosis and appropriate treatment but these patients remain a challenge for physicians. Yet, biochemical proof of presence or absence of catecholamine excess has become more easy and straightforward due to developments in assay methodology. This also applies to radiological and functional imaging techniques for locating the tumor. The importance of genetic testing for underlying germline mutations in susceptibility genes for patients and relatives is increasingly recognized. Yet, the effectiveness of genetic testing, in terms of costs and benefits to health, has not been definitively established. Further improvement in knowledge of genotype-phenotype relationships in pheochromocytoma will open new avenues to a more rationalized and personalized diagnostic approach of affected patients.

Reduced calcium current density in female versus male mouse adrenal chromaffin cells in situ

Neuroendocrine adrenal medullary chromaffin cells are a main output of the sympathetic nervous system. Acute stress activates the sympatho-adrenal stress reflex, excites adrenal chromaffin cells, and elicits catecholamine secretion into the circulation. Previous studies have demonstrated that stress-evoked serum catecholamine levels are greater in males. We investigated potential mechanistic bases for this gender dimorphism at the level of the adrenal medulla. We utilized in situ single-cell perforated patch voltage clamp to measure basic electrophysiological parameters that affect cell excitability. We found that chromaffin cells from male and female mice exhibit statistically identical depolarization-evoked calcium currents. However, the resting capacitance, an index of cell surface area, was significantly higher in cells from female mice. Thus the current density in female cells was significantly lower. We found that inhibition of protein kinase C, an enzyme shown to regulate both exocytosis and endocytosis, eliminates the cell surface area gender dimorphism. Finally, we performed kinetic simulations of the secretion process and report a predicted elevated secretory capacity in male cells. Thus, regulation of cell size may act to decrease cell excitability in female cells and may in-part represent the mechanistic basis for increased stress-evoked catecholamine secretion described in males.

Neuropeptide coding of sympathetic preganglionic neurons; focus on adrenally projecting populations

Chemical coding of sympathetic preganglionic neurons (SPN) suggests that the chemical content of subpopulations of SPN can define their function. Since neuropeptides, once synthesized are transported to the axon terminal, most demonstrated chemical coding has been identified using immunoreactive terminals at the target organ. Here, we use a different approach to identify and quantify the subpopulations of SPN that contain the mRNA for pituitary adenylate cyclase activating polypeptide (PACAP) or enkephalin. Using double-labeled immunohistochemistry combined with in situ hybridization (ISH) we firstly identified the distribution of these mRNAs in the spinal cord and determined quantitatively, in Sprague-Dawley rats, that many SPN at the T4-T10 spinal level contain preproPACAP (PPP+, 80 ± 3%, n=3), whereas a very small percentage contain preproenkephalin (PPE+, 4 ± 2%, n=4). A similar neurochemical distribution was found at C8-T3 spinal level. These data suggest that PACAP potentially regulates a large number of functions dictated by SPN whereas enkephalins are involved in few functions. We extended the study to explore those SPN that control adrenal chromaffin cells. We found 97 ± 5% of adrenally projecting SPN (AP-SPN) to be PPP+ (n=4) with only 47 ± 3% that were PPE+ (n=5). These data indicate that adrenally projecting PACAPergic SPN regulate both adrenal adrenaline (Ad) and noradrenaline (NAd) release whereas the enkephalinergic SPN subpopulation must control a (sub) population of chromaffin cells - most likely those that release Ad. The sensory innervation of the adrenal gland was also determined. Of the few adrenally projecting dorsal root ganglia (AP-DRG) observed, 74 ± 12% were PPP+ (n=3), whereas 1 ± 1% were PPE+ (n=3). Therefore, if sensory neurons release peptides to the adrenal medulla, PACAP is most likely involved. Together, these data provide a neurochemical basis for differential control of sympathetic outflow particularly that to the adrenal medulla.

Visceral brain-body information transfer

Organisms interact with their environments through various afferent (sensory) and efferent (motor) mechanisms. While the usual environment of interest has been external to the organism, the internal environment is also of fundamental importance. This article briefly reviews many of the interactive mechanisms between the brain and the visceral environment, along with identification of relevant brain structures and linkages related to these peripheral functions (particularly the hypothalamus). Afferent and efferent neural (autonomic nervous system) and chemical (endocrine, immune, and blood-brain barrier and circumventricular organs) pathways are described, and potential unifying principles (emotion and, especially, homeostasis, including allostasis and stress) are identified. The importance of bidirectional (afferent, efferent) communication is emphasized. These systems of visceral brain-body information transfer are major connections between the central nervous system and the body through which and by which many psychosomatic processes occur.

The neurobiology of stress and development

Stress is a part of every life to varying degrees, but individuals differ in their stress vulnerability. Stress is usefully viewed from a biological perspective; accordingly, it involves activation of neurobiological systems that preserve viability through change or allostasis. Although they are necessary for survival, frequent neurobiological stress responses increase the risk of physical and mental health problems, perhaps particularly when experienced during periods of rapid brain development. Recently, advances in noninvasive measurement techniques have resulted in a burgeoning of human developmental stress research. Here we review the anatomy and physiology of stress responding, discuss the relevant animal literature, and briefly outline what is currently known about the psychobiology of stress in human development, the critical role of social regulation of stress neurobiology, and the importance of individual differences as a lens through which to approach questions about stress experiences during development and child outcomes.

Adrenal adrenaline- and noradrenaline-containing cells and celiac sympathetic ganglia are differentially controlled by centrally administered corticotropin-releasing factor and arginine-vasopressin in rats

The adrenal glands and sympathetic celiac ganglia are innervated mainly by the greater splanchnic nerves, which contain preganglionic sympathetic nerves that originated from the thoracic spinal cord. The adrenal medulla has two separate populations of chromaffin cells, adrenaline-containing cells (A-cells) and noradrenaline-containing cells (NA-cells), which have been shown to be differentially innervated by separate groups of the preganglionic sympathetic neurons. The present study was designed to characterize the centrally activating mechanisms of the adrenal A-cells, NA-cells and celiac sympathetic ganglia with expression of cFos (a marker for neural excitation), in regard to the brain prostanoids, in anesthetized rats. Intracerebroventricularly (i.c.v.) administered corticotropin-releasing factor (CRF) induced cFos expression in the adrenal A-cells, but not NA-cells, and celiac ganglia. On the other hand, i.c.v. administered arginine-vasopressin (AVP) resulted in cFos induction in both A-cells and NA-cells in the adrenal medulla, but not in the celiac ganglia. Intracerebroventricular pretreatment with indomethacin (an inhibitor of cyclooxygenase) abolished the CRF- and AVP-induced cFos expression in all regions described above. On the other hand, intracerebroventricular pretreatment with furegrelate (an inhibitor of thromboxane A2 synthase) abolished the CRF-induced cFos expression in the adrenal A-cells, but not in the celiac ganglia, and also abolished the AVP-induced cFos expression in both A-cells and NA-cells in the adrenal medulla. These results suggest that centrally administered CRF activates adrenal A-cells and celiac sympathetic ganglia by brain thromboxane A2-mediated and other prostanoid than thromboxane A2 (probably prostaglandin E2)-mediated mechanisms, respectively. On the other hand, centrally administered AVP activates adrenal A-cells and NA-cells by brain thromboxane A2-mediated mechanisms in rats.

Social defeat increases food intake, body mass, and adiposity in Syrian hamsters

Overeating and increases in body and fat mass are the most common responses to day-to-day stress in humans, whereas stressed laboratory rats and mice respond oppositely. Group housing of Syrian hamsters increases body mass, adiposity, and food intake, perhaps due to social confrontation-induced stress. In experiment 1 we asked, Does repeated social defeat increase food intake, body mass, and white adipose tissue (WAT) mass in Syrian hamsters? Male hamsters subjected to the resident-intruder social interaction model and defeated intermittently 15 times over 34 days for 7-min sessions significantly increased their food intake, body mass, and most WAT masses compared with nondefeated controls. Defeat significantly increased terminal adrenal norepinephrine, but not epinephrine, content. In experiment 2 we asked, Are 15 intermittent resident-intruder interactions necessary to increase body mass and food intake? Body mass and food intake of subordinate hamsters defeated only once were similar to those of nondefeated controls, but four or eight defeats similarly and significantly increased these responses. In experiment 3 we asked, Do intermittent defeats increase adiposity and food intake more than consecutive defeats? Four intermittent or consecutive defeats similarly and significantly increased food intake and body mass compared with nondefeated controls, but only intermittent defeats significantly increased all WAT masses. Consecutive defeats significantly increased mesenteric and inguinal WAT masses. Plasma leptin, but not insulin, concentrations were similarly and significantly increased compared with nondefeated controls. Collectively, social defeat, a natural stressor, significantly increased food intake, body mass, and adiposity in Syrian hamsters and may prove useful in determining mechanisms underlying human stress-induced obesity.

Motor cortical control of cardiovascular bulbar neurones projecting to spinal autonomic areas

There is evidence that the motor cortex is involved in cardiovascular adjustments associated with somatic motor activity, as it has functional connections with the ventrolateral medulla, a brainstem region critically involved in the control of blood pressure and the regulation of plasma catecholamine levels. The ventrolateral medulla sends projections to the spinal intermediolateral nucleus, where preganglionic neurones controlling heart and blood vessels (T2 segment) and adrenal medulla (T8 segment) are found. The aim of the present study was to determine whether electrical stimulation of the rat motor cortex induces cardiovascular responses and Fos expression in ventrolateral medulla neurones projecting to the T2 and T8 segments. After a set of experiments designed to record cardiovascular parameters (blood pressure and plasma catecholamine levels), injections of retrograde tracer (Fluorogold) were performed in the intermediolateral nucleus of two groups of rats, at the T2 or at the T8 segmental levels. Five days later, the motor cortex was stimulated in order to induce Fos expression in the ventrolateral medulla. Stimulation of the motor cortex induced: (1). hypotension and a significant decrease in plasma noradrenaline levels, and (2). a significant increase in the number of the double-labelled neurones in the rostral ventrolateral medulla projecting to T2. These data demonstrate that cardiovascular adjustments, preparatory to, or concomitant with, motor activity may be initiated in the motor cortex and transmitted to cardiac and vasomotor spinal preganglionic neurones, via the ventrolateral medulla.

Role of brain thromboxane A2 in the release of noradrenaline and adrenaline from adrenal medulla in rats

Plasma noradrenaline reflects the release from adrenal medulla and sympathetic nerves; however, the exact mechanisms of adrenal noradrenaline release remain to be elucidated. The present study was designed to characterize the source of plasma noradrenaline induced by centrally administered vasopressin and corticotropin-releasing hormone (CRH) in urethane-anesthetized rats. Intracerebroventricularly administered vasopressin (0.2 nmol/animal) and CRH (1.5 nmol/animal) elevated plasma levels of noradrenaline and adrenaline. Intracerebroventricularly administered indomethacin [1.2 micromol (500 microg)/animal] (an inhibitor of cyclooxygenase) abolished the elevations of both noradrenaline and adrenaline induced by vasopressin and CRH. Intracerebroventricularly administered furegrelate [1.8 micromol (500 microg)/animal] (an inhibitor of thromboxane A(2) synthase) abolished the elevations of both noradrenaline and adrenaline induced by vasopressin, while the reagent only attenuated the elevation of plasma adrenaline evoked by CRH. Acute bilateral adrenalectomy abolished the elevation of both noradrenaline and adrenaline induced by vasopressin, while the procedure reduced only the elevation of adrenaline induced by CRH. These results suggest that the release of noradrenaline from adrenal medulla and sympathetic nerves is mediated by different central mechanisms. The vasopressin-induced noradrenaline release from adrenal medulla is mediated by brain thromboxane A(2)-mediated mechanisms, while the CRH-induced noradrenaline release from sympathetic nerves is mediated by brain prostanoid (other than thromboxane A(2))-mediated mechanisms. The vasopressin- and CRH-induced adrenaline release from adrenal medulla is also mediated by brain thromboxane A(2)-mediated mechanisms in rats.

Active and passive membrane properties of rat sympathetic preganglionic neurones innervating the adrenal medulla

The intravascular release of adrenal catecholamines is a fundamental homeostatic process mediated via thoracolumbar spinal sympathetic preganglionic neurones (AD-SPN). To understand mechanisms regulating their excitability, whole-cell patch-clamp recordings were obtained from 54 retrogradely labelled neonatal rat AD-SPN. Passive membrane properties included a mean resting membrane potential, input resistance and time constant of -62 +/- 6 mV, 410 +/- 241 MOmega and 104 +/- 53 ms, respectively. AD-SPN were homogeneous with respect to their active membrane properties. These active conductances included transient outward rectification, observed as a delayed return to rest at the offset of the membrane response to hyperpolarising current pulses, with two components: a fast 4-AP-sensitive component (A-type conductance), contributing to the after-hyperpolarisation (AHP) and spike repolarisation; a slower prolonged Ba(2+)-sensitive component (D-like conductance). All AD-SPN expressed a Ba(2+)-sensitive instantaneous inwardly rectifying conductance activated at membrane potentials more negative than around -80 mV. A potassium-mediated, voltage-dependent sustained outward rectification activated at membrane potentials between -35 and -15 mV featured an atypical pharmacology with a component blocked by quinine, reduced by low extracellular pH and arachidonic acid, but lacking sensitivity to Ba(2+), TEA and intracellular Cs(+). This quinine-sensitive outward rectification contributes to spike repolarisation. Following block of potassium conductances by Cs(+) loading, AD-SPN revealed the capability for autorhythmicity and burst firing, mediated by a T-type Ca(2+) conductance. These data suggest the output capability is dynamic and diverse, and that the range of intrinsic membrane conductances expressed endow AD-SPN with the ability to generate differential and complex patterns of activity. The diversity of intrinsic membrane properties expressed by AD-SPN may be key determinants of neurotransmitter release from SPN innervating the adrenal medulla. However, factors other than active membrane conductances of AD-SPN must ultimately regulate the differential ratio of noradrenaline (NA) versus adrenaline (A) release secreted in response to various physiological and environmental demands.

Differential chemoreceptor reflex responses of adrenal preganglionic neurons

Adrenal sympathetic preganglionic neurons (ADR SPNs) regulating the chromaffin cell release of epinephrine (Epi ADR SPNs) and those controlling norepinephrine (NE ADR SPNs) secretion have been distinguished on the basis of their responses to stimulation in the rostral ventrolateral medulla, to glucopenia produced by 2-deoxyglucose, and to activation of the baroreceptor reflex. In this study, we examined the effects of arterial chemoreceptor reflex activation, produced by inhalation of 100% N(2) or intravenous injection of sodium cyanide, on these two groups of ADR SPNs, identified antidromically in urethane-anesthetized, artificially ventilated rats. The mean spontaneous discharge rates of 38 NE ADR SPNs and 51 Epi ADR SPNs were 4.4 +/- 0.4 and 5.6 +/- 0.4 spikes/s at mean arterial pressures of 98 +/- 3 and 97 +/- 3 mmHg, respectively. Ventilation with 100% N(2) for 10 s markedly excited all NE ADR SPNs (+222 +/- 23% control, n = 36). In contrast, the majority (40/48; 83%) of Epi ADR SPNs were unaffected or slightly inhibited by ventilation with 100% N(2) (population response: +6 +/- 10% control, n = 48). Similar results were obtained after injection of sodium cyanide. These observations suggest that the network controlling the spontaneous discharge of NE ADR SPNs is more sensitive to brief arterial chemoreceptor reflex activation than is that regulating the activity of Epi ADR SPNs. The differential responsiveness to activation of the arterial chemoreceptor reflex of the populations of ADR SPNs regulating epinephrine and norepinephrine secretion suggests that their primary excitatory inputs arise from separate populations of sympathetic premotor neurons and that a fall in arterial oxygen tension is not a major stimulus for reflex-mediated adrenal epinephrine secretion.

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.

Sympathetic nervous system and experimental diabetes: role of adrenal medullary hormones

The sympathetic nervous system is of major importance in the regulation of various physiological functions. The present review discusses the mechanisms which control glucose homeostasis and the role of the sympathetic nervous system in experimental diabetes with special emphasis on the role of adrenal medullary hormones, over-activity of the sympathetic nervous system and its relationship to hypertension in the diabetic state and the effect of stress. The chapter also reviews the ability of various drugs and pharmacological agents to produce hyperglycemia in experimental animals and how this information can be used in screening new chemical entities and in differentiating the mode of action of these agents.

Different adrenal sympathetic preganglionic neurons regulate epinephrine and norepinephrine secretion

Brain stimulation or activation of certain reflexes can result in differential activation of the two populations of adrenal medullary chromaffin cells: those secreting either epinephrine or norepinephrine, suggesting that they are controlled by different central sympathetic networks. In urethan-chloralose-anesthetized rats, we found that antidromically identified adrenal sympathetic preganglionic neurons (SPNs) were excited by stimulation of the rostral ventrolateral medulla (RVLM) with either a short (mean: 29 ms) or a long (mean: 129 ms) latency. The latter group of adrenal SPNs were remarkably insensitive to baroreceptor reflex activation but strongly activated by the glucopenic agent 2-deoxyglucose (2-DG), indicating their role in regulation of adrenal epinephrine release. In contrast, adrenal SPNs activated by RVLM stimulation at a short latency were completely inhibited by increases in arterial pressure or stimulation of the aortic depressor nerve, were unaffected by 2-DG administration, and are presumed to govern the discharge of adrenal norepinephrine-secreting chromaffin cells. These findings of a functionally distinct preganglionic innervation of epinephrine- and norepinephrine-releasing adrenal chromaffin cells provide a foundation for identifying the different sympathetic networks underlying the differential regulation of epinephrine and norepinephrine secretion from the adrenal medulla in response to physiological challenges and experimental stimuli.

Adrenal medullary catecholamine secretion patterns in rats evoked by reflex and direct neural stimulation

Epinephrine (EPI) and norepinephrine (NE), secretion patterns evoked by reflex (to hypotension and hypoglycemia) and direct neural stimulation of the adrenal medulla were measured in pentobarbital anesthetized male Sprague-Dawley rats. Secretion rates were determined by collecting adrenal venous blood. Baseline catecholamine secretion was similar in innervated and denervated glands indicating that there was little tonic sympathetic neural drive to the medulla. Both hydralazine-induced hypotension and insulin-induced hypoglycemia significantly increased catecholamine secretion with the secretion of EPI predominating. Similarly, in response to stimulation of the splanchnic nerve, frequency-related increments in EPI and NE were elicited with EPI release being greater than NE at all frequencies. However, the magnitude of the increase in secretion during splanchnic stimulation at frequencies above 1 hz greatly exceeded the release achieved by reflex stimulation. The results indicate that despite the fact that the stimuli of hypotension and hypoglycemia are integrated by different centers in the brain, the pattern of adrenal release is similar.

Autonomic dysreflexia increases plasma adrenaline level in the chronic spinal cord-injured rat

Autonomic dysreflexia (AD) occurs in up to 80% of quadriplegics and high paraplegics and is characterized by exaggerated sympathetic reflexes which induced paroxysmal hypertension. The aim of this study was to determine if plasma catecholamine levels increased during autonomic dysreflexia in the chronic spinal cord-injured (SCI) rats with special care to adrenaline. Catecholamine samples were collected before, during and 1 h after AD induced hypertension with colorectal distension. Results showed that plasma noradrenaline and adrenaline levels increased respectively 1.5-fold and 5-fold during AD in the chronic SCI rats. This suggests substantial roles for these two hormones in mediating the cardiovascular changes during AD. Knowledge of catecholamine levels during AD may thus aid in determining pathophysiology and potential pharmacologic treatments of this autonomic dysfunction.

Expression of Fos protein in adrenal preganglionic neurons following chemical stimulation of the rostral ventrolateral medulla of the rat

The ventrolateral medulla is known to be involved in the regulation of arterial blood pressure, especially via its connections with sympathetic preganglionic neurons (SPNs) mainly located in the intermediolateral nucleus of the spinal cord. It has been shown that stimulation of the rostral part of the ventrolateral medulla (RVLM) elicits a release of catecholamines from the adrenal medulla. The aim of the present study was to demonstrate the existence of a functional pathway between the RVLM and adrenal SPNs using the combination of a retrograde tract tracing technique (cholera toxin B subunit) with the immunohistochemical detection of Fos protein following the chemical stimulation of RVLM. The data obtained showed that: (1) chemical stimulation of the RVLM induced Fos immunoreactivity in the intermediolateral nucleus and particularly in SPNs projecting to the adrenal medulla; (2) along the thoracic segments T2-T12, 26.1% of retrogradely identified adrenal SPNs were Fos-immunoreactive with the greatest percentage (30.9%) in the T8 segment. These results favored a functional control of the RVLM on adrenal SPNs which may contribute to a substantial activation of the cardiovascular system via the release of adrenal catecholamines.

Endothelin regulation of adrenal function

1. The goal of the present review is to recount the evidence that endothelin (ET) has a significant influence on the peripheral sympathetic nervous system by regulating the function of the adrenal medulla. 2. The presence of an active ET system in the adrenal medulla has been clearly demonstrated. Endothelin protein, mRNA, binding sites and ET-converting enzyme have been identified in adrenal tissue and medullary chromaffin cells, suggesting that this peptide may contribute to the regulation of adrenal medullary function. 3. Studies investigating the function of ET in the adrenal gland have demonstrated that ET has a stimulatory effect on the adrenal medulla. Endothelin elicits an increase in catecholamine release from perfused intact adrenal glands as well as from cultured chromaffin cells. This effect has been shown to be mediated by ETA and ETB receptors. 4. The mechanism by which ET causes an increase in catecholamine release from the adrenal medulla appears to be independent of cholinergic activation of chromaffin cells. Endothelin has been shown to act directly at chromaffin cells to increase intracellular calcium, which results in catecholamine release. 5. Endothelin can indirectly affect catecholamine release by its effect on adrenal blood flow. Studies indicate that ET has both vasoconstrictor and vasodilator effects in the adrenal gland, which suggests a role for ET in the regulation of adrenal blood flow. Endothelin has also been proposed to participate in the selective contraction of the adrenomedullary veins, which enhances the discharge of catecholamines from the adrenal gland during activation.

Adrenal epinephrine secretion is not regulated by sympathoinhibitory neurons in the caudal ventrolateral medulla

By providing the principal inhibitory regulation of the discharge of sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM), neurons in the caudal ventrolateral medulla (CVLM) play a major role in regulating the level of sympathetic nerve activity (SNA) to cardiovascular targets. To determine whether adrenal medullary secretion of epinephrine (EPI) is also regulated by sympathoinhibitory inputs from the CVLM to the RVLM, we compared levels of plasma EPI obtained after disinhibition of RVLM neurons with levels obtained after inhibition of CVLM neurons, both of which result in sustained elevations in arterial blood pressure (AP), SNA, and heart rate (HR). Plasma norepinephrine (NE) concentrations were significantly elevated following bilateral microinjection either of bicuculline (BIC) into the RVLM or of muscimol into the CVLM of urethane/chloralose-anesthetized, artificially-ventilated rats. In sharp contrast, although plasma EPI concentrations were significantly elevated following disinhibition of neurons in the RVLM, they were unchanged by inhibition of neurons in the CVLM. These results demonstrate that the discharge of sympathetic premotor neurons in the RVLM regulating adrenal secretion of EPI is modulated by a tonic, GABA-ergic inhibition that arises from a source that is different from the sympathoinhibitory neurons in the CVLM that project to RVLM sympathetic premotor neurons controlling vasoconstrictor and cardiac targets.

Adrenal epinephrine and norepinephrine release to hypoglycemia measured by microdialysis in conscious rats

Experiments were conducted in conscious male rats to determine whether hypoglycemia induced by insulin administration preferentially stimulated epinephrine (Epi) or norepinephrine (NE) adrenal medullary chromaffin cells. The release of Epi and NE from the adrenal medulla was continuously monitored using a microdialysis probe of novel design that had been inserted in the adrenal medulla approximately 16 h before the administration of insulin. Following insulin, 3 U/kg i.v., blood glucose declined and dialysate Epi levels rose. No measurable increment in dialysate NE was obtained. Similarly, plasma Epi increased with no detectable change in NE. Patterns of dialysate and plasma catecholamine changes were similar in two groups of animals that had been fed or fasted overnight before insulin treatment. However, the magnitude of the Epi increase was greater in the fasted animals. After recovery of the blood glucose concentration to preinsulin levels, dialysate and plasma catecholamine concentrations returned to control values. These experiments clearly demonstrate that adrenal medullary chromaffin cells that produce Epi are preferentially stimulated in response to insulin-induced hypoglycemia.

Abnormal epinephrine release in young adults with high personal and high parental blood pressures

BACKGROUND

Increased activity of the sympathetic nervous system has been proposed as a cause of high blood pressure (BP) and may be related to diet and body weight. To determine the role of these factors in predisposition to high BP, we studied 100 young adults with high or low BP from families in which both parents had either high or low BP.

METHODS AND RESULTS

Plasma catecholamine, glucose, and insulin levels were measured before and after an oral glucose load. There was a significant correlation between fasting plasma norepinephrine and mean arterial pressure (P=.001). Subjects with high BP, irrespective of parental BP, were heavier (P=.003) and fatter (P=.002) and had a greater rise in plasma insulin (P=.003) following glucose than those with low BP. Offspring with high BP whose parents also had high BP showed an unexpected rise in plasma epinephrine (P=.004) following glucose. This adrenal medullary response was not the result of high parental or high personal BP alone as it was not seen in offspring with low BP whose parents had high BP or in offspring with high BP whose parents had low BP.

CONCLUSIONS

Irrespective of family history, high BP is associated with increased body weight and hyperinsulinemia and reflects personal environment and behavior. However, abnormal epinephrine release is characteristic of the combination of genetic, environmental, and behavioral factors that is associated with high personal BP and a familial predisposition to high BP.