PARABRACHIAL NUCLEUS PROJECTION TO THE AMYGDALA IN THE RAT - ELECTROPHYSIOLOGICAL AND ANATOMICAL OBSERVATIONS

The amygdala, an important limbic forebrain centre, is the recipient o f projections from a number of autonomic brainstem nuclei including th e pontine parabrachial nucleus. This study examined the influence of e lectrical stimulation of the parabrachial nucleus on the excitability of amygdala neurons and their response to two cardiovascular stimuli, namely baroreceptor activation and the administration of systemic angi otensin II. We also defined the chemical identity of some amygdala neu rons that receive parabrachial nucleus projections by combining the tr ansport of the anterograde tracer Phaseolus vulgaris leucoagglutinin i njected into the parabrachial nucleus with immunocytochemical labellin g of neurotensin and galanin profiles within the amygdala. In urethane -anesthetized rats, stimulation of parabrachial nucleus evoked four ba sic types of synaptic responses in amygdala cells: (1) a short duratio n (< 100 ms) excitation in 75 of 167 neurons, (2) a longer duration (> 100 ms) excitatory response in 36 neurons, (3) an inhibitory response in 32 cells, and (4) more complex responses consisting of excitation- inhibition or inhibition-excitation sequences in the remainder of the cells. Thirty-seven of 72 amygdala neurons activated synaptically by p arabrachial nucleus stimulation also responded to baroreceptor activat ion or intravenous angiotensin II. Anatomical data revealed the presen ce of Phaseolus vulgaris leucoagglutinin labelled terminals predominan tly within the lateral, medial, and capsular subdivisions of the centr al nucleus of amygdala. Phaseolus vulgaris leucoagglutinin varicositie s and boutons were observed apposed to the neurotensin and galanin neu ronal perikarya within the central nucleus of amygdala. The electrophy siological results provide a framework whereby parabrachial nucleus ef ferents influence the activity of amygdala neurons that are responsive to cardiovascular stimuli. Furthermore, the anatomical data indicate that a portion of the parabrachial nucleus input is directed toward ga lanin and neurotensin neurons within the central nucleus of amygdala.