Sympathetic response to stimulation of the pontine A5 region in conscious rabbits.

Studies in anaesthetized animals have shown that the pontine A5 noradrenergic region plays an important role in the sympathetic control of arterial pressure (AP). The aim of this study was to develop, in conscious rabbits, a technique for microinjections into the A5 region and examine the effects of stimulation of this region on renal sympathetic nerve activity (RSNA). In preliminary mapping experiments on four anaesthetized rabbits, electrical stimulation of the A5 region induced a pressor response ranging between 25 and 75 mmHg while unilateral injection of glutamate (100 nmol) did not change AP. The mapping experiments were used to enable guide cannulae implantation for subsequent microinjections into the A5 region. In six conscious rabbits, unilateral injection of glutamate (100 nmol) caused a consistent increase in RSNA (+45%) but did not change AP. In another eight rabbits, bilateral injection of glutamate (0.3, 3, 30 nmol) into the A5 region dose-dependently increased RSNA by 13%, 30% and 40%, respectively. In four rabbits, angiotensin II (0.3, 3, 30 pmol) injected bilaterally into the A5 region increased RSNA by 5%, 22% and 28%, respectively. In all animals the increase in RSNA was mainly mediated by increasing amplitude of sympathetic synchronized bursts while their frequency remained unchanged. However, both glutamate and angiotensin II did not change AP indicating that the sympathoexcitatory response to the A5 stimulation might be relatively confined to the renal bed. Using a novel microinjection technique developed for conscious rabbits, we found that the A5 region may provide an important excitatory and possibly selective input to the renal sympathetic preganglionic neurons.