GABA-mediated baroreceptor inhibition of reticulospinal neurons.

A total of 45 cells with spinal projections, pulse-synchronous discharges, and extreme sensitivity to changes in arterial pressure were recorded in the retrofacial portion of nucleus paragigantocellularis lateralis (PGCL) in halothane-anesthetized rats. Every unit tested was inhibited by iontophoretic applications of gamma-aminobutyric acid (GABA), glycine, and taurine and excited by glutamate or the GABA receptor antagonist bicuculline methiodide. The inhibitory effect of GABA was antagonized by bicuculline methiodide but not by strychnine; the effect of glycine was eliminated by strychnine but unaffected by bicuculline. Iontophoretically applied bicuculline completely antagonized the time-locked reduction in unit activity resulting from increases in arterial pressure while strychnine was ineffective. 1-Glutamate, applied in a dose that produced a larger increase in the unit discharge rate than bicuculline, did not significantly alter the magnitude of the inhibition associated with increases in arterial pressure. It is concluded that GABA or a related substance mediates the inhibition of PGCL reticulospinal neurons resulting from the activation of arterial baroreceptors.