Medullary lateral tegmental field: an important synaptic relay in the baroreceptor reflex pathway of the cat

This study was designed to test the hypothesis that the medullary lateral t egmental field (LTF) is an important synaptic relay in the baroreceptor ref lex pathway controlling sympathetic nerve discharge (SND) of urethan-anesth etized cats. We determined the effects of blockade of excitatory amino acid -mediated neurotransmission in the LTF on three indexes of baroreceptor ref lex function: cardiac-related power in SND, strength of linear correlation (coherence value) of SND to the arterial pulse (AP), and inhibition of SND during increased arterial pressure produced by abrupt obstruction of the ab dominal aorta. Bilateral microinjection of D-(-)-2-amino-5-phosphonopentano ic acid, an N-methyl-D-aspartate (NMDA) receptor antagonist, abolished card iac-related power and coherence of SND to the AP, and it prevented inhibiti on of SND during aortic obstruction. These data support the view that NMDA receptor-mediated neurotransmission in the LTF is critical for baroreceptor reflex control of SND. Bilateral microinjection of 1,2,3,4-tetrahydro-6-ni tro-2,3-dioxobenzo-[f]- quinoxaline-7-sulfonamide,a non-NMDA receptor antag onist, decreased cardiac-related power and total power in the 0- to 6-Hz ba nd of SND; however, the AP;SND coherence value remained high, and inhibitio n of SND during aortic obstruction was preserved. These data imply that non -NMDA receptor-mediated neurotransmission in the LTF is involved in setting the level of excitatory drive to sympathetic nerves.