Role of glutamate receptors in transmission of vagal cardiac input to neurones in the nucleus tractus solitarii in dogs

1.. Vagal afferent input from cardiac mechanoreceptors excites neurones in the nucleus tractus solitarii (NTS), but discharge patterns evoked by physi ological activation of pressure-sensitive cardiac mechanoreceptors have not been studied in, vivo. The role of glutamate receptor subtypes in transmis sion of afferent activity to the NTS neurones has not been determined. The present study therefore has two aims: first, to characterise the discharge patterns of neurones in the NTS that receive pressure-sensitive vagal cardi ac receptor input and second, to determine the roles of ionotropic glutamat e receptor subtypes in the transmission of this putative cardiac mechanorec eptor-related activity to NTS neurones. 2. Pulse-synchronous activity of neurones in the NTS evoked by vagal affere nt input was recorded extracellularly in an anaesthetised dog model using m ultibarrel glass electrodes, which allowed picoejection of the glutamate re ceptor antagonists NBQX or AP5 to block either non-NMDA or NMDA receptors, respectively, during the neuronal recording. Pressure sensitivity of the re corded neurones was examined by monitoring their response to a small increa se in arterial blood pressure. Selective pressure activation of carotid sin us baroreceptors in an isolated sinus or selective denervation of aortic ba roreceptors were used to test for convergent excitation of the neurones by arterial baroreceptors. 3. Pulse-synchronous cardiac-related neuronal activity recorded from neuron es in both the Fight and left NTS was eliminated following section of the l eft (n = 17) or right (n = 1) vagus nerves. No spontaneous, non-pulsatile a ctivity was observed in these neurones before or after vagotomy. Activity t ransmitted via left vagal afferents was found to be sensitive to changes in arterial blood pressure. In these neurones, activity was blocked in 13 of 17 neurones by picoejection of NBQX, with the remainder requiring both NBQX and AP5. None of the cardiac-related neurones responded to activation of c arotid baroreceptors or denervation of aortic baroreceptors, indicating no convergence of activity from carotid baroreceptors or aortic baroreceptors with pressure thresholds of approximately 130 mmHg or less. 4. The results suggest that vagal pressure-sensitive afferent input from ca rdiac mechanoreceptors is transmitted primarily by left vagal afferent fibr es via non-NMDA receptors to neurones in both the ipsilateral and contralat eral NTS. NMDA receptors were also found to have a role in the activation o f a small subpopulation of neurones.