Short-term potentiation of carotid sinus nerve inputs to neurons in the nucleus of the solitary tract

Reflex studies have shown that the effects of afferent stimulation can persist beyond the period of stimulation. To determine if some form of 'short-term potentiation' occurs during the initial integration of afferent inputs within the nucleus of the solitary tract (NTS), the synaptic responses of NTS neurons to high frequency carotid sinus nerve (CSN) stimulation were examined in anesthetized rats. In extracellular recording experiments, high frequency CSN stimulation (1-3 sec, 100-300 Hz) increased the number of action potentials evoked by 30 CSN stimuli from 31 ± 3 to 38 ± 4 (P < 0.05, n = 11). In this population, evoked discharge was enhanced in six cells, not altered in three cells and reduced in two cells. Spontaneous discharge was increased in the five cells in which it was present (P < 0.05). In intracellular recording experiments, high frequency CSN stimulation increased EPSP amplitude from 5.1 ± 0.4 to 6.1 ± 0.4 mV (P < 0.01, n = 21). In this population, amplitude was enhanced in 13 cells, not altered in four cells and reduced in four cells. The enhanced EPSP occurred in the absence of any change in membrane potential in five cells and during a 3-5 mV depolarization in eight cells. In both the intra- and extra-cellular experiments, the effects of high frequency stimulation were over within 5 min. The results indicate that brief, intense periods of visceral afferent activation can alter the responses of NTS neurons to subsequent afferent inputs.