Alpha 2-adrenergic modulation of synaptic excitability in the rat nucleus tractus solitarius.

These studies examined the effects of the alpha-adrenergic agonist clonidine on spontaneous and synaptically evoked activity in the solitary tract nucleus in superfused rat brain slices. In one group of neurons which showed no spontaneous spike activity (n = 27), clonidine superfusion induced a dose-dependent increase of postsynaptic responsiveness to input from the ipsilateral solitary tract. In another group of neurons which were spontaneously active but unresponsive to tract input (n = 20), clonidine induced a dose-dependent depression of spontaneous discharge. A third group of neurons which were both spontaneously active and responsive to tract input (n = 11) showed primarily a depression of both activities. The neuronal responses to clonidine in all 3 groups were selectively blocked by the alpha 2-adrenoceptor antagonist yohimbine but not by the alpha 1-antagonist prazosin. These results provide insight into the possible actions of endogenous adrenergic systems in the synaptic processing of afferent sensory information within the solitary tract nucleus.