Subthreshold aortic nerve (AN) inputs to neurons in the nucleus of the solitary tract (NTS)

Electrophysiologists typically identify neurons involved in a circuit by action potential response to activation of afferent inputs. To determine if NTS neurons receive subthreshold AN inputs, extracellular recordings combined with iontophoresis or intracellular recordings were performed in pentobarbital anesthetized rats. In the iontophoretic experiments, a constant ejecting current of excitatory amino acid (EAA, 3-20nA) was applied while the electrode was lowered into the NTS. Fourteen NTS neurons (2 monosynaptic, MS, and 12 polysynaptic, PS) which did not respond to AN stimulation in the absence of EAA did respond during application of glutamate (2MS & 4PS), AMPA (6PS), kainate (1PS) and NMDA (1PS). Compared to NTS neurons which responded to AN stimulation in the absence of EAAs (n=21MS, 35PS), more NTS neurons receiving subthreshold AN inputs had no spontaneous discharge (79% vs 38%, p <.05) and a greater onset latency variability (10.5 ±.5 vs 5.0±.8ms, MS, p<.05; 15.7±3.1 vs 9.6±1.2, PS, p<.05). Consistent with the extracellular study, intracellular recordings identified a population of NTS neurons (3MS & 3PS) which responded to AN stimulation with an EPSP which did not evoke action potential discharge. Compared to neurons in which AN stimulation evoked action potential discharge (n=21MS, 19PS), these neurons had more hyperpolarized membrane potentials (68±6 vs 53±2mV, MS, p<01; 64±7 vs 55±2mV, PS, p=.11). Our results indicate at rest in the anesthetized rat some NTS neurons receive subthreshold MS & PS AN inputs. EAAs can shift neurons from a reserve to an active role in the baroreflex independently of any change in the level of afferent input.