Mediation of baroreceptor inhibition of sympathetic nerve activity via both a brainstem and spinal site in rats.

1. The possible involvement of a spinal site of sympatho-inhibitory action in the baroreceptor reflex pathways was investigated by determining the effect of phenylephrine-induced, baroreceptor-mediated inhibition on a spinally evoked excitatory response of renal nerve activity before, during and after removal of tonic descending excitatory drive from the rostral ventrolateral medulla (RVLM). 2. Stimulation of descending excitatory axons at the C4 vertebral level evoked a highly reproducible excitatory response (mean variance, 2.2 +/- 0.6%) in a renal sympathetic nerve with a latency of 72 +/- 5.7 ms and duration of 211 +/- 32 ms. 3. Following baroreceptor activation, the magnitude of this spinally evoked response was reduced by 33.5 +/- 4.2% compared with control spinal response. 4. To remove tonic descending excitatory drive, glycine was microinjected bilaterally into the RVLM (RVLM block); this briefly and reversibly abolished spontaneous sympathetic nerve activity. 5. Following RVLM block, the spinally evoked response in a renal sympathetic nerve was inhibited by 16.9 +/- 4.4% during baroreceptor activation. 6. Intrathecal administration of the glycine antagonist strychnine to the lower thoracic segments of the spinal cord virtually abolished this inhibition. 7. It was concluded that baroreceptor inhibition of sympathetic activity occurs at a spinal site as well as a supraspinal one. Glycine is a likely mediator of the inhibition at the spinal site.