Ann M. Schreihofer - US grants

Acute blood loss produces a constellation of physiological responses that aid in the restoration of cardiovascular homeostasis. These compensatory responses include increased constriction of blood vessel and cardiac output brought about, in part, by a stimulation of the sympathetic nervous. However, beyond a critical level of blood loss a seemingly paradoxical reversal of the automatic compensations occurs, leading to a dramatic decrease in arterial pressure. This decompensatory phase of the hemorrhage response is associated with an inhibition of some sympathetic nerves and a decrease in peripheral vascular resistance. Hemorrhage-induced sympathoinhibition is triggered by the activation of cardiac vagal afferents that project to the nucleus tractus solitarius (NTS), but the central mechanism underlying this response remains unknown. Hemorrhage- induced sympahtoinhibition may be produced by a inhibition of neurons in the rostral ventrolateral medulla (RVLM) that provide the major source of tonic excitatory drive to sympathetic vasoconstrictor nerves. Several reflexes that are also triggered by afferents to NTS inhibit sympathetic vasomotor tone (e.g. baroreceptor reflex and Bexold-Jarish reflex)by inhibiting RVLM presympathetic neurons via the excitation of a GABAerigic projection from the caudal ventrolateral medulla (CVLM). However, in contrast to these other reflexes, centrally-acting opiates appears to be crucial for hemorrage-induced sympathoinhibition.. Opiate receptor blockade centrally but not peripherally can reverse hemorrhage-induced sympathoinhibition, but the central mechanism underlying the opiate contribution to this response is not known. Because opiates can inhibit RVLM presympathetic neurons directly, hemorrhage-induced sympathoinhibition may occur by a release of opiates into RVLM that is independent of an inhibilitory projection from V|CVLM. This application has two goals: 1) to reveal the central circuitry for hemorrhage-induced symphathoinhibition, and 2) to determine the role of opiate peptides in this process.

Essential hypertension is a leading form of cardiovascular disease that greatly increases the risks of morbidity and mortality. Many forms of essential hypertension are associated with augmented sympathetic nerve activity (SNA), although the basis of the sympatho-activation is not well understood. Obstructive sleep apnea is present in 30-50% of patients with essential hypertension, and the majority of patients with obstructive sleep apnea develop elevated SNA and arterial pressure (AP). Sleep apnea, characterized by chronic intermittent hypoxia, also alters central respiratory drive and the respiratory-related regulation of SNA. Cardio- respiratory integration appears to be at the heart of deficits observed with sleep apnea, because the elevated SNA and AP with sleep apnea are partially alleviated by altering nighttime breathing. Unfortunately, there is a paucity of information regarding the link between central respiratory drive and the elevated SNA that occurs with chronic intermittent hypoxia. The long range goals of this research are to elucidate central neural circuits that regulate the SNA that maintains AP and pinpoint alterations that may lead to elevated SNA and AP. This SNA is driven by neurons in the rostral ventrolateral medulla (RVLM), and the RVLM is powerfully restrained by GABAergic neurons in the caudal ventrolateral medulla (CVLM). The role of GABAergic CVLM neurons in the baroreflex control of SNA is established, but baro-activated GABAergic CVLM neurons also tonically inhibit the RVLM independent of baroreceptor inputs. In the previous period of this project we showed that central respiratory neurons provide multiple inputs to baro-activated GABAergic CVLM neurons in rats, although the sources are unknown. We also showed the CVLM is essential for evoking respiratory-related sympathetic responses to acute activation of peripheral chemoreceptors by hypoxia. These observations suggest that the CVLM is an important site for cardio-respiratory integration. The SNA that regulates AP is influenced by central respiratory neurons, but mechanisms underlying cardio-respiratory integration in the CNS are not understood. In Aims 1 and 2 of this renewal we will perform electrophysiological experiments in anesthetized rats to determine whether two central respiratory nuclei that project to the CVLM, the Kolliker-Fuse and pre- Botzinger nuclei, influence the activity of baro-activated CVLM neurons, and whether these inputs are selective for particular CVLM neurons or phases of the respiratory cycle. In addition, we will determine whether these inputs impact hypoxia-induced changes in CVLM neuronal activity and SNA. In Aims 3 and 4 we will determine whether regulation of the CVLM is altered by chronic intermittent hypoxia, as a model for obstructive sleep apnea. These studies will provide novel information regarding a powerful baroreceptor-independent influence upon the CVLM neurons that are likely to influence the RVLM, SNA, and AP. In addition, these studies will further our understanding of the impact of cardio-respiratory integration upon the regulation of blood pressure in health and hypertension.

[unreadable] DESCRIPTION (provided by applicant): The brain plays an important role in the regulation of arterial pressure (AP) via the autonomic nervous system. The long range goal of the applicant is to understand how the central nervous system (CNS) controls sympathetic vasomotor tone under conditions of normal AP and how the function is altered with hypertension. Many forms of hypertension are linked with elevated sympathetic nerve activity (SNA), although the basis of this sympatho-activation is not well understood. Normally, SNA is restrained by GABAergic neurons in the caudal ventrolateral medulla (CVLM) to maintain a consistent AP. For the short-term control of SNA and AP, these GABAergic CVLM neurons are an essential link in the central pathway for the baroreceptor reflex. However, GABAergic CVLM neurons also are clearly important for setting the long-term level of SNA independent of the baroreflex. We have observed that in the chronic absence of baroreceptor inputs or their site of termination in the nucleus tractus solitarius (NTS), the restoration of a normal mean AP is associated with a normal CVLM-mediated inhibition of SNA and AP. In contrast, spontaneously hypertensive rats have elevated SNA which may be linked with impaired CVLM-mediated inhibition of SNA and AP. Apart from baroreceptor inputs and the NTS, little is known about the central mechanisms regulating the activity of these powerful GABAergic CVLM neurons. This project will use a combination of state-of-the-art methods in rats (i.e. electrophysiological, neuroanatomical, and molecular approaches) to determine how other areas of the brain regulate the activity of GABAergic CVLM neurons under normotensive and hypertensive conditions. Specifically, we will determine whether identified glutamatergic inputs to the CVLM from the paraventricular nucleus of the hypothalamus and ventrolateral periaqueductal gray activate GABAergic CVLM neurons that project to the rostral ventrolateral medulla (RVLM). We will use chronic arterial baroreceptor denervated rats as a normotensive model to examine inputs to GABAergic CVLM neurons that may be masked by powerful baroreceptor inputs. In addition, we will determine whether the CVLM-mediated inhibition of SNA is impaired in spontaneously hypertensive rats. Collectively, these studies will provide a sophisticated analysis of the mechanisms underlying the brainstem's role in the long-term regulation of sympathetic vasomotor tone and AP and will determine functional changes that occur with chronic baroreceptor denervation and spontaneous hypertension. [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Obesity is a nationwide epidemic and a leading cause of cardiovascular disease. Obese people display hypertension, impaired baroreflex control of arterial pressure (AP), and exaggerated pressor responses to stress, which contribute to end-organ injury and increased morbidity in obese patients. Altered sympathetic regulation of the heart and vasculature is integral to obesity-associated impairment of cardiovascular regulation, but mechanisms underlying deficits in sympathetic control are poorly understood. Obese Zucker rats (OZR) have autonomic deficits analogous to those observed in obese people: increased sympathetic nerve activity (SNA) with hypertension, blunted baroreflex-mediated changes in SNA, and exaggerated increases in SNA and AP with other sympatho-excitatory reflexes. Exaggerated sympatho-excitatory responses persist in the absence of baroreceptor feedback, suggesting additional baroreflex-independent alterations in the control of SNA in OZR. The opposing effects of obesity upon baroreflex versus other sympathetic reflexes are likely due to their disparate underlying mechanisms. Baroreflex-mediated increases in SNA are elicited by a withdrawal of GABAergic inhibition from the caudal ventrolateral medulla (CVLM) to the brainstem neurons that drive SNA in rostral ventrolateral medulla (RVLM). In contrast, other sympatho-excitatory stimuli raise SNA by glutamatergic or angiotensinergic stimulation of the RVLM. We hypothesize that OZR have a dual deficit in sympathetic regulation of cardiovascular function: impaired baroreflex-mediated GABAergic inhibition of the RVLM, AND enhanced sensitivity of RVLM neurons to excitatory stimuli controlling sympathetic vasomotor tone. In Aim 1 we will determine if impaired baroreflexes are due to deficits in baroreceptor afferent function or changes in the brain stem. In Aim 2 we will determine if OZR have a reduced GABAergic inhibition of the RVLM. In Aim 3 we will determine whether excitation of the RVLM with glutamate or angtiotensin II produces larger increases in SNA and AP in OZR, even without baroreflexes. In Aim 4 we will determine whether OZR also have exaggerated sympatho- excitatory responses initiated by the forebrain, which activate SNA exciting the RVLM. This proposal will use state-of-the-art anatomical and electrophysiological measures to provide the first mechanistic understanding of deleterious changes in brain stem control of autonomic regulation associated with obesity.

Project Summary Ann M. Schreihofer Obesity impairs short-term regulation of mean arterial pressure (MAP) by autonomic reflexes, contributing to the destabilization of MAP. Independent of hypertension, increased variability of MAP is a major risk factor for end organ damage and stroke. Treatments that ameliorate hypertension but not elevated variability of MAP leave patients at risk for adverse cardiovascular outcomes. This project uses obese Zucker rats (OZR) to determine cellular and systemic mechanisms that produce altered autonomic reflexes in the setting of metabolic syndrome. Like obese humans, adult OZR become hyperinsulinemic with poor glycemic control. They also develop sympathetically-driven hypertension with diminished baroreflex control of sympathetic nerve activity (SNA) and heart rate (HR) compared to lean Zucker rats. Other sympatho-inhibitory reflexes processed through the nucleus tractus solitarius (NTS) are also impaired in adult OZR, coincident with the development of reduced physiological responses to glutamatergic activation of the NTS. In contrast, glutamatergic activation of the rostral ventrolateral medulla (RVLM) produces enhanced physiological responses coincident with the onset of augmented sympatho-excitatory reflexes. This latter condition occurs independent of impaired baroreflexes and also increases MAP variability. Amelioration of hypertension or impaired glycemic control in adult male OZR each partially restores baroreflex control of HR, although the fates of NTS function and other sympatho- inhibitory reflexes are not known. Furthermore, whether these treatments also dampen augmented RVLM activation and sympatho-excitatory reflexes is unknown. Female OZR develop metabolic syndrome, but impaired baroreflexes emerge later, well beyond the development of hypertension. The efficacy of treatments used in males and the functions of NTS, RVLM, and other sympathetic reflexes are unknown in female OZR. Central hypotheses: In male OZR, poor glycemic control dampens glutamatergic activation of NTS neurons receiving vagal inputs to impair sympatho-inhibitory reflexes, and this state is exacerbated by hypertension. Further, we hypothesize that simultaneous ingestion of excess salt with hyperphagia augments glutamatergic activation of the RVLM to yield exaggerated sympatho-excitatory reflexes that could further destabilize MAP. Although female rats may develop salt-induced sensitization of the RVLM, we hypothesize estrogen enhances NTS function to combat impairment of sympatho-inhibitory reflexes in early stages of metabolic syndrome. We propose to determine how obesity impacts responses of individually recorded NTS and RVLM neurons to inputs from the periphery and forebrain in male and female OZR compared to age-matched LZR. We will also determine whether reducing salt intake, poor glycemic control, or MAP alters NTS and RVLM function coincident with restoration of sympathetic reflexes in OZR. This project will provide novel insights into obesity- related autonomic deficits and determine whether standard treatments for hypertension and hyperglycemia are adequate to restore altered brainstem function and sympathetic reflexes that are not evaluated in the clinic.