132 Cellular Mechanisms of Baroreceptor Integration at the Nucleus Tractus Solitarius MICHAEL C. ANDRESEN, MARK W. DOYLE, YOUNG-HO JIN, AND TIMOTHY W. BAILEY Department of Physiology and Pharmacology, Oregon Health Sciences University, Portland, Oregon 97201-3098, USA ABSTRACT: The autonomic nervous system makes important contributions to the homeostatic regulation of the heart and blood vessels through arterial baroreflexes, and yet our understanding of the central nervous system mecha- nisms is limited. The sensory synapse of baroreceptors in the nucleus tractus solitarius (NTS) is unique because its participation is obligatory in the barore- flex. Here we describe experiments targeting this synapse to provide greater understanding of the cellular mechanisms at the earliest stages of the barore- flex. Our approach utilizes electrophysiology, pharmacology, and anatomical tracers to identify and evaluate key elements of the sensory information pro- cessing in NTS. KEYWORDS: Baroreceptor; Baroreflex; Glutamate; Visceral sensory; Nucleus of the solitary tract; Synaptic processing INTRODUCTION The autonomic nervous system provides rapid adjustments of the heart and blood vessels as part of the homeostatic regulation of the cardiovascular system and basic integrated life support. 1 One of the most important classes of cardiovascular auto- nomic reflexes is the arterial baroreflex. 2 Over the past 100 years, despite an im- proved general understanding of baroreflex control, detailed information concerning the central nervous system (CNS) nuclei and the mechanisms that they utilize to con- stitute these baroreflexes remains, for the most part, only available as a rough outline. 3 Considerable research identifies the nucleus of the tractus solitarius (NTS) as the site of the first baroreceptor contacts with CNS neurons. 4–6 Thus, the second-order neurons of the baroreceptor reflex lie generally within the dorsal medial portions of the caudal NTS and constitute the beginning of these reflex pathways within the CNS (FIG. 1). Arterial baroreceptors and other cardiovascular afferents converge onto NTS along with other visceral sensory afferents including prominent represen- tations from the respiratory and gastrointestinal systems (broken line box, FIG. 1). 3 These baroreceptor synapses are obligatory and represent a common point in the re- flex pathway through which blood pressure information flows before diverging to a Address for correspondence: Dr. Michael C. Andresen, Department of Physiology and Phar- macology, Oregon Health Sciences University, Portland, Oregon 97201-3098. Voice: 503-494- 5831; fax: 503-494-4352. andresen@OHSU.edu