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We examined the functional organization of the parabrachial complex (PB) by mapping the cardiovascular and respiratory responses to PB microstimulation in anesthetized rats. The PB was explored with 100 microns resolution, at threshold doses of electrical current (5 microA) and glutamate (10-500 pmols), and the locations of stimulation sites were identified by small iontophoretic or pressure injections of biocytin or Phaseolus vulgaris leucoagglutinin. Threshold doses of either L-glutamate or electrical current pulses caused pressor-tachycardic responses that mapped to the outer edge of the external lateral subnucleus while depressor bradycardic responses were elicited from stimuli near the dorsal lateral subnucleus. Pressor responses persisted in paralyzed, ventilated animals and were thus not dependent upon concomitant respiratory changes. Cardiac arrhythmias sometimes occurred during large pressor responses and during augmented breaths that occurred during or following PB stimulation. These observations indicate that the PB contains at least two distinct neuronal systems that are potently and opposingly involved in cardiovascular control. The locations of the sites giving the most potent responses implicate specific ascending and descending pathways as substrates for the cardiovascular responses.
Chamberlin et al. (Tue,) studied this question.