Role of paraventricular nucleus (PVH) in baroreflex-mediated changes in lumbar sympathetic nerve activity and heart rate.

Electrophysiological and neuroanatomical studies indicate that reciprocal connections between the paraventricular nucleus (PVH) and medullary sites are involved in cardiovascular regulation. To determine whether the PVH is involved in the regulation of baroreflex responses, lumbar sympathetic nerve activity (LSNA) and heart rate (HR) changes were recorded in response to increases in arterial pressure (produced by bolus doses of phenylephrine i.v.) prior to, during, and 60 min following the injection of lidocaine (2% lidocaine, 200 nl) bilaterally in the PVH of chloralose-anesthetized rabbits. Baseline blood pressure, HR, and LSNA did not change in response to the administration of lidocaine in the PVH. The magnitude of 'baroreflex responses' in HR and LSNA were expressed as the ratios of maximal changes in these parameters divided by the corresponding maximal changes in blood pressure. Application of lidocaine to the PVH produced a significantly greater decrease in LSNA (greater than 50%) compared to prelidocaine responses to baroreceptor stimulation. This increase in baroreflex response returned to the normal level during the recovery phase. In contrast, there was no significant change in the response of the HR to baroreceptor stimulation. Administration of norepinephrine into the PVH, intended to simulate possible changes in noradrenergic function, did not affect either LSNA or HR responses to baroreceptor stimulation. Interruption of neural activity in the PVH, augmented the inhibitory response of LSNA but not HR to baroreceptor stimulation. These results indicate that changes in peripheral sympathetic nerve activity and HR in response to baroreceptor activation may be affected differentially by specific forebrain structures such as PVH.