Electrophysiological evidence for alpha 1- and alpha 2-adrenoceptors in solitary tract nucleus

The effects of microiontophoresis of norepinephrine and epinephrine were examined on the spontaneous neuronal activity of single neurons in the nucleus tractus solitarius (NTS) in urethan-anesthetized rats. Neuronal responses to catecholamine iontophoresis were examined in the absence and presence of the alpha 1-adrenergic antagonist, prazosin, and the alpha 2-antagonist, idazoxan, to characterize the subtypes of alpha-adrenergic receptors mediating catecholamine action in this important autonomic nucleus. Norepinephrine produced inhibitions in firing, which were blocked primarily by idazoxan, whereas epinephrine-induced neuronal inhibitions were blocked by either prazosin or idazoxan. Inhibitions of spontaneous firing were also produced by iontophoresis of methoxamine, an alpha 1-selective agonist, or the alpha 2-selective agonist clonidine in all neurons that were tested. Neuronal responses to methoxamine were blocked selectively by prazosin, whereas the effects of clonidine were antagonized selectively by idazoxan. These data provide evidence at the level of the single unit that catecholamines may affect activity in the NTS via interactions with both alpha 1- and alpha 2-adrenergic receptors.