TEMPORAL PROCESSING OF AORTIC NERVE EVOKED ACTIVITY IN THE NUCLEUS OFTHE SOLITARY TRACT

1. Temporal processing of heterogeneous afferent signals by nucleus of the solitary tract (NTS) neurons has been previously characterized. E xperiments 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 conta ins exclusively arterial baroreceptor afferent fibers. 2. Extracellula r single-cell activity was examined in the NTS during electrical stimu lation of the aortic nerve with the use of a conditioning-test paradig m. 3. Results were obtained from 49 neurons, 22 of which were characte rized 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 pea k 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 c ells (4 +/- 1 ms vs. 8 +/- 1 ms; P < 0.05). 4. Neurons characterized a s receiving a monosynaptic input from the aortic afferents generally d id not exhibit time-dependent inhibition. Significant inhibition was o bserved only at a conditioning-test interval of 50 ms. when the averag e test response was 79 +/- 8% of control. In contrast, the average res ponse following a 50-ms conditioning-test interval for neurons receivi ng polysynaptic input from the aortic nerve was only 32 +/- 8% of cont rol. Significant inhibition was observed at conditioning-test interval s 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 pres sure during the conditioning-test procedure was significantly lower fo r these neurons than for the 17 cells that were inhibited by <50%. Thi s suggests that the level of activity in convergent afferent input mig ht 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 inte rvals. suggesting a single mechanism of variable duration. Results rep orted here are consistent with current theory that time-dependent inhi bition is mediated by disfacilitation. 7. The results demonstrate that NTS neurons receiving monosynaptic input from the aortic depressor ne rve infrequently exhibit time-dependent inhibition. This could allow f or the original, unmodified afferent information to be dispersed to su bsequent neurons. In contrast, neurons receiving polysynaptic input un dergo time-dependent inhibition similar to that which has been reporte d for other afferent inputs. This could allow for differential degrees of fidelity in the transfer of the afferent information to specific e fferent pathways. Therefore the temporal pattern of firing in individu al baroreceptor afferents could play a critical role in the function o f the arterial baroreflex and therefore in the regulation of blood pre ssure.