Subthreshold aortic nerve inputs to neurons in nucleus of the solitary tract

Subthreshold aortic nerve (AN) inputs to neurons receiving a monosynaptic AN-evoked input (MSNs: respond to each of two AN stimuli separated by 5 ms) and neurons receiving a polysynaptic AN input (PSNs) in the nucleus of the solitary tract (NTS) were identified in anesthetized rats. In extracellular recordings from 24 MSNs and 49 PSNs, 12% of MSNs and 29% of PSNs only responded to AN stimulation during the application of excitatory amino acids. In intracellular recordings from 24 MSNs and 22 PSNs, 12% of MSNs and 14% of PSNs responded to AN stimulation with excitatory postsynaptic potentials that did not evoke action potential discharge. Reductions in arterial pres sure produced minimal changes in the spontaneous discharge of suprathreshold AN-evoked neurons, suggesting that these neurons receive excitatory inputs from nonbaroreceptor sources. The results suggest that some baroreflex-related NTS neurons exist in a 'reserve state' and can be changed to an active state or vice versa. This will change the number of neurons involved in baroreflex circuits and provides a novel mechanism for regulating baroreflex function independently of alterations in peripheral afferent input.