NITRIC-OXIDE INFLUENCES NEURONAL-ACTIVITY IN THE NUCLEUS-TRACTUS-SOLITARIUS OF RAT BRAIN-STEM SLICES

Nitric oxide (NO) is shown to be synthesized in the central nervous sy stem as well as in vascular endotheliaI cells. However, the physiologi cal role of NO in cardiovascular regulation in the central nervous sys tem remains unclear. The present study examines whether NO plays a rol e in the regulation of neuronal activity in the nucleus tractus solita rius (NTS). Single-unit extracellular recordings were obtained from NT S neurons in slices (400 mu m) of the rat brainstem, which had spontan eous discharges at a frequency of 0.5 to 3 spikes per second. Eighty-o ne neurons were tested for sensitivity to L-arginine, which is the phy siological precursor of NO. L-Arginine (10(-7) to 10(-4) mol/L) increa sed neuronal activity dose dependently in 33 (40.7%) of 81 neurons tes ted, but D-arginine (10(-5) mol/L) did not. The neurons that responded to L-arginine responded to glutamate as well. N-G-Monomethyl-L-argini ne (10(-5) to 3 x 10(-5) mol/L), an inhibitor of the formation of NO, dose-dependently blocked increases in the neuronal activity evoked wit h L-arginine (10(-5) mol/L). Hemoglobin (1.5 mg/L), a trapper of NO, a nd methylene blue (10(-5) mol/L), an inhibitor of guanylate cyclase, a lso blocked increases in the neuronal activity evoked with L-arginine (10(-5) mol/L). Sodium nitroprusside (SNP, 10(-5) to 10(-4) mol/L), wh ich spontaneously produces NO, increased the neuronal activity in the neurons that responded to L-arginine. SNP did not alter the neuronal a ctivity of the neurons that did not respond to L-arginine. Methylene b lue (10(-5) mol/L) blocked increases in the neuronal activity evoked w ith SNP (10(-5) mol/L), but hemoglobin did not. These results suggest that NO is formed from L-arginine in the NTS neurons and that NO incre ases the neuronal activity of adjacent neurons in the NTS through an i ncrease in cGMP.