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

Subthreshold aortic nerve (AN) inputs to neurons receiving a monosynaptic A N-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 resp onded to AN stimulation during the application of excitatory amino acids. I n intracellular recordings from 24 MSNs and 22 PSNs, 12% of MSNs and 14% of PSNs responded to AN stimulation with excitatory postsynaptic potentials t hat did not evoke action potential discharge. Reductions in arterial pressu re 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-rela ted 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 ba roreflex circuits and provides a novel mechanism for regulating baroreflex function independently of alterations in peripheral afferent input.