Influence of pontine A5 region on renal sympathetic nerve activity in conscious rabbits

The effects of inhibiting the neural activity in the pontine A5 region on r enal sympathetic responses to baroreflex and/or chemoreflex activation were examined in conscious rabbits. Eight rabbits were chronically instrumented with guide cannulas for bilateral microinjections into the A5 area and an electrode for measuring renal sympathetic nerve activity (RSNA). Baroreflex curves were obtained under conditions of normoxia and hypoxia (10% O-2 + 3 % CO2) after injections into the A5 region of the GABA receptor agonist mus cimol or vehicle solution. Under normoxia, injections of muscimol did not a ffect resting RSNA or blood pressure but increased the range of the RSNA ba roreflex by 24 and 33% at doses of 175 or 875 pmol, respectively, without a ffecting the reflex gain. Hypoxia alone increased resting RSNA by 63%, as w ell as the range and gain of the RSNA baroreflex by 53 and 89%, respectivel y, without affecting blood pressure. However, under hypoxia, muscimol incre ased resting RSNA by 37 and 47% but decreased the gain of the RSNA barorefl ex by 19 and 34% at doses of 175 or 875 pmol, respectively, without affecti ng the reflex range. The effects of muscimol on RSNA were mediated via chan ges in the amplitude of the sympathetic bursts, whereas burst frequency rem ained unaffected. These data suggest that the A5 region has a little tonic influence on RSNA in conscious rabbits but serves to limit the renal sympat hetic responses to baroreceptor unloading or chemoreceptor stimulation. The different changes in the baroreflex range and gain evoked by muscimol unde r normoxia and hypoxia indicate that the A5 modulatory action may depend on the activity of the afferent inputs to this region.