Temporal processing of aortic nerve evoked activity in the nucleus of the solitary tract.

1. Temporal processing of heterogenous afferent signals by nucleus of the solitary tract (NTS) neurons has been previously characterized. Experiments were performed in 26 pentobarbital-sodium-anesthetized male Sprague-Dawley rats to characterize the temporal processing of evoked activity in NTS neurons with the use of the aortic nerve, which contains exclusively arterial baroreceptor afferent fibers. 2. Extracellular single-cell activity was examined in the NTS during electrical stimulation of the aortic nerve with the use of a conditioning-test paradigm. 3. Results were obtained from 49 neurons, 22 of which were characterized as receiving monosynaptic input from aortic nerve afferents. The average number of evoked potentials per aortic nerve stimulation was 1.1 +/- 0.1 (SE) for the monosynaptic neurons and 1.2 +/- 0.2 for the polysynaptic neurons. Spontaneous activity averaged 3.7 +/- 0.7 Hz. No neuron exhibited an obvious pulse-rhythmic discharge. The average peak onset latency for monosynaptic cells of 17 +/- 2 ms (range 3-31 ms) was significantly (P < 0.05) shorter than the average of 26 +/- 1 ms (range 13-38 ms) for the polysynaptic cells. The average onset latency variability was also less in monosynaptic compared with polysynaptic cells (4 +/- 1 ms vs. 8 +/- 1 ms; P < 0.05). 4. Neurons characterized as receiving a monosynaptic input from the aortic afferents generally did not exhibit time-dependent inhibition. Significant inhibition was observed only at a conditioning-test interval of 50 ms, when the average test response was 79 +/- 8% of control. In contrast, the average response following a 50-ms conditioning-test interval for neurons receiving polysynaptic input from the aortic nerve was only 32 +/- 8% of control. Significant inhibition was observed at conditioning-test intervals of up to 200 ms. 5. At a conditioning-test interval of 50 ms, only 5 of 22 monosynaptic neurons were inhibited by > 50%. Mean arterial pressure during the conditioning-test procedure was significantly lower for these neurons than for the 17 cells that were inhibited by < 50%. This suggests that the level of activity in convergent afferent input might influence the magnitude of time-dependent inhibition. 6. There was an essentially linear recovery from time-dependent inhibition evident in polysynaptic neurons that were tested at all conditioning-test intervals, suggesting a single mechanism of variable duration. Results reported here are consistent with current theory that time-dependent inhibition is mediated by disfacilitation. 7. The results demonstrate that NTS neurons receiving monosynaptic input from the aortic depressor nerve infrequently exhibit time-dependent inhibition. This could allow for the original, unmodified afferent information to be dispersed to subsequent neurons. In contrast, neurons receiving polysynaptic input undergo time-dependent inhibition similar to that which has been reported for other afferent inputs. This could allow for differential degrees of fidelity in the transfer of the afferent information to specific efferent pathways. Therefore the temporal pattern of firing in individual baroreceptor afferents could play a critical role in the function of the arterial baroreflex and therefore in the regulation of blood pressure.