The purpose of this proposal is to characterize the role of the dorsomedial hypothalamic nucleus (DMH) and the medullary raphe pallidus in the Cardiogenic Sympathetic Afferent Reflex (CSAR). BACKGROUND; activation of cardiac spinal sensory afferents during myocardial ischemia causes chest pain and an increase in sympathetic activity which increases heart rate and blood pressure, referred to as the CSAR. During myocardial ischemia and infarction, this sympathetic reflex activity is important for the maintenance of blood pressure and perfusion of vital organs. However, the increased sympathetic activity may also be detrimental because it further increases the oxygen demand of the ischemic myocardium. The activity of sympathetic preganglionic neurons relies on the activity of medullary and supramedullary sites. While the brain stem nuclei, such as the commissural nucleus tractus solitarius and rostral ventrolateral medulla (RVLM) have been show to play an important role in the CSAR, the role of other supramedullary nuclei and the sources that generate sympathetic activity in the CSAR are still not clear. The hypothalamus plays an important role in cardiovascular autonomic regulation. We have previously published work that characterized the role of the paraventricular nucleus (PVN) of the hypothalamus in the CSAR and shown that while acute chemical inhibition of the PVN attenuates the CSAR, it is not mediated by PVN glutamate receptors. We have preliminary data to suggest that the CSAR recovers in rats after chronic ibotenic acid lesion of the PVN. This suggests that there are other supramedullary sites involved. There is good evidence to suggest that the DMH plays an important role in the CSAR. The DMH orchestrates the sympathetic cardiovascular response to stressful conditions and sends direct projections not only to the PVN, but also to the medullary raphe and the RVLM sympathetic pre-autonomic nuclei important in cardiovascular regulation. Activation of DMH neurons with electrical stimulation and microinjection of the glutamate receptor agonists, or the GABAA receptor antagonist bicuculline, induces a profound increase in reflex sympathetic nerve activity and blood pressure in anesthetized and conscious rats. While the DMH plays an important role in the sympathetic regulation of blood pressure, little is known regarding the role of the DMH in the CSAR. The proposed study will provide much needed information on hypothalamic regulation of the autonomic nervous system and a clearer understanding of the role of the DMH and raphe in the CSAR. Our working hypothesis is that ?activation of cardiac sensory afferents activates glutamate receptors on pre-sympathetic DMH neurons that project to the cardiosensitive neurons within the brain stem raphe pallidus?.
Specific Aims; the first specific aim for the proposed research project is to determine the importance of DMH activation in the CSAR. We will test the hypothesis that ?inactivation of the DMH attenuates the CSAR and that this reflex response is mediated by ionotropic glutamate receptors in the DMH?. Our second specific aim is to determine the neuronal firing characteristics of DMH and raphe pallidus neurons in the CSAR. We plan to test the hypothesis that ?stimulation of cardiac sensory nerve endings activates cardiosensitive neurons in the raphe pallidus?. RESEARCH PLAN; to test the reflex sympathetic activation during the CSAR we will use epicardial application of bradykinin to the anterior surface of the left ventricle in the anesthetized rat. To determine the importance of DMH in the CSAR and the role of glutamate receptor subtypes, we will measure the renal sympathetic nerve activity and blood pressure response to epicardial bradykinin before and after hypothalamic nanoinjection of drugs that specifically block glutamate receptor subtypes or overall DMH activity. To characterize the electrophysiological activity of raphe pallidus and raphe-projecting DMH neurons to activation of cardiac visceral afferents we will perform single-unit electrophysiological recordings of these pre-sympathetic neurons.
; the proposed study will provide much needed information on hypothalamic regulation of the autonomic nervous system as well as the central pathways and activity generated in the CSAR. A better understanding of the DMH and raphe in regulating blood pressure after myocardial ischemia will be very important for clarifying mechanisms of sympathetic activation in the CSAR and lead to improving treatments being used to mitigate the extent of myocardial infarction. AREA GRANT JUSTIFICATION: These studies employ surgical and electrophysiological approaches appropriate for undergraduate research on a poorly understood cardiac reflex. In keeping with the goal to incorporate undergraduates into research, students will be directly involved in assisting or performing surgery, necropsy, and histology in an area of research that most undergraduates get very little exposure. Students will be responsible for discrete units of this research that they take on as independent projects of their own. The investigators have a successful history of incorporating undergraduate students in research projects leading to scientific presentations, Honors theses, and peer-reviewed publications.
