Baroreceptor inhibition of sympathetic activity at a spinal site.

In chloralose-urethane anaesthetized cats and in one unanesthetized decerebrate cat, the possibility of a baroreceptor pathway exerting an inhibitory effect directly on the sympathetic preganglionic neurone (SPN) was examined. Stimulation of excitatory points in the brain stem and spinal cord evoked responses in T3 white ramus (T3WR) which could be markedly depressed (80-100%) by raising the pressure in an isolated carotid sinus. Conditioning-testing experiments indicated that the onset of this inhibition had a long latency (100-250 msec). Responses in T3WR evoked by stimulating both 'fast' (5 m/sec) and 'slow' (2-3 m/sec) bulbospinal pathways were similarly affected by baroreceptor stimulation. This inhibition could be much reduced (40-100%) by topical application to T3 segment of phentolamine (1-2 microgram). Baroreceptor activation also caused a complete cessation of the discharge evoked by microelectrophoretic application of glutamate to single antidromically identified SPN. The data support the suggestion of a direct action on SPN of a bulbospinal baroreceptor inhibitory pathway.