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

Abstract

The effects of inhibiting the neural activity in the pontine A5 region on renal 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% CO(2)) after injections into the A5 region of the GABA receptor agonist muscimol or vehicle solution. Under normoxia, injections of muscimol did not affect resting RSNA or blood pressure but increased the range of the RSNA baroreflex by 24 and 33% at doses of 175 or 875 pmol, respectively, without affecting the reflex gain. Hypoxia alone increased resting RSNA by 63%, as well as the range and gain of the RSNA baroreflex by 53 and 89%, respectively, without affecting blood pressure. However, under hypoxia, muscimol increased resting RSNA by 37 and 47% but decreased the gain of the RSNA baroreflex by 19 and 34% at doses of 175 or 875 pmol, respectively, without affecting the reflex range. The effects of muscimol on RSNA were mediated via changes in the amplitude of the sympathetic bursts, whereas burst frequency remained unaffected. These data suggest that the A5 region has a little tonic influence on RSNA in conscious rabbits but serves to limit the renal sympathetic responses to baroreceptor unloading or chemoreceptor stimulation. The different changes in the baroreflex range and gain evoked by muscimol under normoxia and hypoxia indicate that the A5 modulatory action may depend on the activity of the afferent inputs to this region.

Sympathetic and cardiac baroreflex function in panic disorder

“…GABA plays a critical role in the inhibitory projection between the caudal and rostral ventrolateralmedulla; a projection believed to be the key to the negative feedback homeostatic character of the baroreceptor reflex arc [36]. Recent experimental data support the hypothesis that GABA receptors are of primary importance in determining the gain of the sympathetic baroreflex control to the vasculature [37].…”

Premotor sympathetic neurons of conditioned fear in the rat

“…Activation of A5 neurons also increases the gain of the baroreflex in the splanchnic nerve (Huangfu et al, 1992). On the other hand, inhibition of the A5 area in conscious rabbits does not affect blood pressure or heart rate or renal sympathetic nerve activity, but increases the range of the renal sympathetic nerve activity to baroreceptor unloading (Maiorov et al, 2000).…”

Pontine Mechanisms of Respiratory Control

“…Specifically, A5-area activation increases splanchnic and renal nerve activity but usually decreases lumbar sympathetic nerve activity (87,183,243). However, blocking A5 activity with injections of muscimol does not affect resting renal sympathetic nerve activity in the unanesthetized rabbit (242). Recording A5 neurons identified via antidromic activation of their axons in the spinal cord and by its complete inactivation following systemic injections of clonidine indicates that these cells have a respiratory (post-I)-modulated activity pattern (154,184).…”