EVIDENCE FOR A CRITICAL ROLE OF NITRIC-OXIDE IN THE TONIC EXCITATION OF RABBIT RENAL SYMPATHETIC PREGANGLIONIC NEURONS

1. A large proportion of sympathetic preganglionic neurones contain ni tric oxide synthase. The purpose of this study was to determine the ef fects of facilitation and inhibition of nitric oxide synthesis within the lower thoracic spinal cord (which contains the majority of renal p reganglionic neurones) on renal sympathetic nerve activity (rSNA). 2. In anaesthetized rabbits, rSNA was recorded before and after intrathec al injection (50 mu l of 0.5 m solution) of either L-arginine, a precu rsor of nitric oxide, or N-omega-nitro-L-arginine methyl ester (L-NAME ), an inhibitor of nitric oxide synthase, into the lower thoracic spin al cord. Spinal cord sections were also stained for the presence of NA DPH diaphorase, a marker of nitric oxide synthesizing neurones. 3. A h igh density of NADPH diaphorase-containing neurones was found within t he intermediolateral cell column of the lower thoracic spinal cord. 4. Intrathecal injection of L-arginine and L-NAME resulted in a large in crease (113 +/- 25%) and decrease (43 +/- 8%), respectively, in rSNA. In contrast, injection of the inactive isomers D-arginine and D-NAME h ad no significant effect on rSNA. 5. The results indicate that endogen ous nitric oxide in the lower thoracic spinal cord (1) has a potent ex citatory action on renal sympathetic preganglionic neurones, and (2) h elps to maintain the tonic activity of renal sympathetic nerves under resting conditions.