BRAIN RESEARCH ELSEVIER Brain Research 705 (1995) 71-78 Research report Convergence of ventrolateral medulla and aortic baroreceptor inputs onto amygdala neurons Stefanie Roder, John Ciriello * Department of Physiology, Health Sciences Centre, University of Western Ontario, London, OntarioN6A 5C1, Canada Accepted 22 August 1995 Abstract Experiments were done to investigate the effect of stimulation of neurons in ventrolateral medulla (VLM) and aortic baroreceptors on the discharge rate of amygdala neurons. The region of central nucleus of the amygdala (ACe) was explored for spontaneously active single units that altered their discharge rate to electrical stimulation of VLM in the a-chloralose anesthetized rat. Responsive units were also assessed for their response to electrical stimulation of the aortic depressor nerve (ADN). Stimulation of VLM altered the discharge rate of 47% (43/92) of the units tested in and around the region of ACe. Of these units, 60% (26/43) were excited (mean latency, 13.6 _ 3.6 ms) and 40% (17/43) were inhibited (mean latency, 23.1 + 4.3 ms) by VLM stimulation. Of the 43 units that responded to stimulation of VLM, 19 (44%) also responded to ADN stimulation with a mean latency of 32.5 _+ 7.6 ms. These data demonstrate that inputs from VLM and ADN converge onto ACe neurons and suggest that VLM may function as a relay for cardiovascular afferent information to the amygdala. Keywords: Central nucleus of the amygdala; A1 noradrenergic cell group; Limbic system; Cardiovascular regulation 1. Introduction The amygdala, a limbic forebrain structure, has previ- ously been implicated in cardiovascular regulation [15,17,18,21,22,26,27,31,33,38]. In particular, neurons within central nucleus of the amygdala (ACe) have been shown to alter their discharge rate during activation of baroreceptor and chemoreceptor afferent fibers within the carotid sinus and aortic depressor (ADN) nerves in the anesthetized or awake cat [5,6,23,43]. In addition, in the conscious cat neurons in and around the region of ACe have been shown to discharge with a cardiac rhythm [24]. Furthermore, it has been reported that the metabolic activ- ity of neurons in amygdala was altered after activation [13] or transection of ADN in the rat [44]. Cardiovascular inputs may reach ACe via direct projections from nucleus of the solitary tract [29,32,35,36,46], the primary termina- tion site of cardiovascular afferent fibers [8,12], or via indirect projections from nucleus of the solitary tract to other brainstem structures that in turn innervate amygdala [7,20]. Recently, neuroanatomical evidence has been ob- tained demonstrating that neurons within ventrolateral * Corresponding author. Fax: (1) (519) 661-3827. 0006-8993/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0006-8993(95)01 140-4 medulla (VLM), a lower brainstem structure previously shown to be involved in the generation and reflex regula- tion of vasomotor tone [11], project directly to the region of ACe [19,32,37]. VLM neurons have been shown to receive projections from nucleus of the solitary tract [25,29,35,40] and to alter their discharge rate during stimu- lation of cardiovascular afferent fibers [10]. Therefore, the possibility exists that VLM may function as a relay for cardiovascular afferent information to amygdala. In this study, the effect of stimulation of VLM on the discharge rate of neurons in and around the region of ACe was investigated. Extracellular single unit recording experi- ments were done to identify neurons in amygdala that receive inputs from VLM. In addition, the effect of activa- tion of aortic baroreceptor afferent inputs on these ACe neurons responsive to VLM stimulation was determined. 2. Materials and methods 2.1. Surgical procedure Experiments were done in 14 male Wistar rats (335-615 g), anesthetized with t~-chloralose (60 mg/kg, i.v., ini-