Medullary lateral tegmental field: an important source of basal sympathetic nerve discharge in the cat

We used blockade of excitatory amino acid (EAA) neurotransmission in the medullary lateral tegmental field (LTF) and rostral ventrolateral medulla (RVLM) to assess the roles of these regions in the control of inferior cardiac sympathetic nerve discharge (SND) and mean arterial pressure (MAP) in urethan-anesthetized, baroreceptor-denervated cats. Bilateral microinjection of a non-N-methyl-D-aspartate (NMDA)-receptor antagonist [1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo-[f]quinoxaline-7-sulfonamide (NBQX)] into the LTF significantly decreased SND to 46 +/- 4% of control (as demonstrated with power-density spectral analysis) and MAP by 16 +/- 6 mmHg. In contrast, bilateral microinjection of an NMDA-receptor antagonist [D(-)-2-amino-5-phosphonopentanoic acid (D-AP5)] into the LTF did not decrease SND or MAP. These results demonstrate that the LTF is an important synaptic relay in the pathway responsible for basal SND in the cat. Bilateral microinjection of NBQX or D-AP5 into the RVLM significantly decreased power in SND to 48 +/- 5 or 61 +/- 5% of control, respectively, and reduced MAP by 15 +/- 2 or 8 +/- 4 mmHg, respectively. These data indicate that EAA-mediated synaptic drive to RVLM-spinal sympathoexcitatory neurons accounts for a significant component of their basal activity.