Differential effects of an NMDA and a non-NMDA receptor antagonist on medullary lateral tegmental field neurons

Abstract

We microiontophoresed an N-methyl-d-aspartate (NMDA) and a non-NMDA receptor antagonist onto medullary lateral tegmental field (LTF) neurons, the naturally occurring discharges of which were correlated to the cardiac-related rhythm in sympathetic nerve discharge (SND) of dialurethane-anesthetized cats. Some of these neurons were classified as sympathoexcitatory, because their firing rate decreased during baroreceptor reflex activation. Microiontophoresis of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo-[ f]quinoxaline-7-sulfonamide (NBQX), a non-NMDA receptor antagonist, reduced the mean firing rates of these neurons (51 ± 8% of control, P < 0.001, n = 20) without affecting their relationship to cardiac-related SND, as indicated by the lack of significant changes in the ratio of peak to background counts in arterial pulse (AP)-triggered histograms of LTF neuronal activity and the AP-LTF coherence value at the frequency of the heartbeat. In contrast, microiontophoresis of D(−)-2-amino-5-phosphonopentanoic acid, an NMDA receptor antagonist, onto LTF neurons reduced the ratio of peak to background counts in AP-triggered histograms to 57 ± 9% of control ( P = 0.002, n = 16) and the AP-LTF coherence value to 25 ± 10% of control ( P = 0.001, n = 10). These data support the view that non-NMDA and NMDA receptors are involved in setting the basal level of activity of LTF sympathoexcitatory neurons and in synchronizing their discharges to the AP, respectively.