Adrenergic responses of baroreceptive cells in the nucleus tractus solitarii of the rat: a microiontophoretic study.

Anatomical and pharmacological evidence suggests a role for catecholamines (CAs) in the modulation of the baroreceptor reflex within the nucleus tractus solitarii (NTS). Single neurons in the NTS of the rat were studied for their responses to activation of the baroreceptor reflex and to iontophoretic administration of dopamine, norepinephrine (NE), and epinephrine (EPI) to determine the relationship between the effects of baroreflex activation and CA application on baroreceptive neurons in the vagal sensory nucleus. Of 269 cells studied, 104 (38.7%) exhibited decreases and 41 cells (15.2%) showed increases in firing rate in response to baroreflex activation, while the remaining 124 neurons showed no response. All 3 CAs inhibited spike activity in the majority (68.5%) of NTS cells. These inhibitory effects on spontaneous firing were observed regardless of the response profile of a particular neuron to baroreflex activation. The inhibitory effects of NE and EPI on NTS neuronal activity were specifically blocked by the alpha-adrenergic receptor antagonist tolazoline, but not by the beta-adrenergic antagonist sotalol. These results indicate that CAs may interact at several sites within the NTS to influence baroreflex integration, and that the effects of NE and EPI on neuronal activity are mediated by an alpha-adrenergic receptor.