Neuropeptides, amines and amino acids as mediators of the sympathetic effects of PVN activation

The aim of the present study was to determine the influence on renal sympathetic nerve activity of the different chemically coded neuronal phenotypes that project from the paraventricular nucleus (PVN) to the spinal cord. Experiments were carried out on male Wistar rats anaesthetised with chloralose and urethane. Changes in renal sympathetic nerve activity were measured following activation of neurones in the PVN with D,L-homocysteic acid (100 nl, 200 mM), before and following intrathecal application of glutamate, vasopressin, oxytocin, dopamine and their receptor antagonists. Excitatory and inhibitory effects on renal sympathetic nerve activity were elicited by PVN stimulation. PVN excitatory effects were mimicked by intrathecal administration of glutamate and vasopressin and selectively antagonised by intrathecal administration of kynurenic acid and a V1a receptor antagonist, respectively. A low dose of dopamine increased renal sympathetic activity and this was selectively antagonised by haloperidol; however, the latter was without effect on PVN excitatory responses. A high dose of dopamine decreased renal sympathetic nerve activity and this was selectively blocked by a D1 dopamine receptor antagonist (SCH 23390), which also antagonised a minority of inhibitory responses obtained from the caudal extension of the PVN. Oxytocin also had two actions in 5 rats it inhibited and in 10 rats it increased renal sympathetic nerve activity, both actions being blocked selectively by oxytocin receptor antagonists. Neither of the PVN effects on renal sympathetic nerve activity appeared to be dependent on oxytocin pathways. Tests with intrathecal administration of bicuculline showed that PVN inhibition of renal sympathetic nerve activity was not dependent on spinal GABA(A) receptor activation. The results show that PVH-induced excitation of sympathetic activity to the kidney is mainly mediated by glutamate or vasopressin neurones whereas dopamine via Dl receptors may mediate some of the PVN inhibitory effects.