Alpha-adrenergic influences on neuronal responses to visceral afferent input in the nucleus tractus solitarius.

Studies were made of the effects of the alpha 1-adrenoceptor agonist methoxamine on spontaneous and synaptically evoked activity in the solitary tract nucleus in isolated, perfused slices of the brain of the rat and those effects were compared to the effects of the alpha 2-agonist clonidine. Methoxamine had no effect on the spike activity of 7 out of 8 spontaneously firing neurones, with no response to electrical stimulation of the solitary tract. Among neurones that responded to tract stimulation, those which were otherwise silent (N = 38) showed a decrease of responsiveness to tract input during the infusion of methoxamine, whereas those with spontaneous activity (N = 10) showed mostly increases of both firing rate and responsiveness. The effects of activation of alpha 2-receptors on responsiveness to tract stimulation were the opposite of previously demonstrated effects of activation of alpha 2-receptors, and these opposing effects could be demonstrated in the same neurone. These results suggest that the transmission of visceral sensory information within the nucleus tractus solitarius may be controlled by opposing alpha 1- and alpha 2-receptor-mediated neural systems.