Excitatory amino-acid receptors contribute to carotid sinus and vagus nerve evoked excitation of neurons in the nucleus of the tractus solitarius

To determine the role of excitatory amino-acid (EAA) receptors in afferent evoked excitation of neurons in the nucleus of the solitarius (NTS), responses of NTS neurons to activation of visceral afferent inputs were examined before and during iontophoretic application of the broad spectrum EAA receptor antagonist kynurenate (KYN). Iontophoretic application of KYN, at doses which attenuated glutamate but not substance P or acetylcholine evoked discharge, inhibited carotid sinus nerve (CSN) and vagus nerve evoked discharge. KYN attenuation of evoked responses was similar whether the evoked input was monosynaptic (CSN evoked discharge reduced by 50 ± 6% (mean ± SE; n = 5); vagus nerve evoked discharge reduced by 45 ± 4%, n = 6) or polysynaptic (CSN evoked discharge reduced by 48 ± 6%, n = 6; vagus nerve evoked discharge reduced by 45 ± 4%, n = 8). Spontaneous action potential discharge rate was reduced during KYN iontophoresis in 6 cells (1.8 ± 0.4 spikes/s vs. 0.7 ± 0.2 spikes/s). Iontophoretic application of a structural analogue of KYN which has no EAA receptor antagonist properties, xanthurenic acid, had no effect on glutamate, CSN or vagus nerve evoked discharge. Iontophoretic application of KYN reduced the action potential discharge evoked by activation of the carotid body chemoreceptors by 52 ± 2% in 5 cells tested. The results demonstrate that excitatory amino-acid receptors are involved in visceral afferent evoked activation of NTS neurons. Furthermore, since both mono- and poly-synaptic inputs were attenuated, these receptors appear to be utilized at multiple levels of afferent integration within NTS.