NORADRENERGIC NEUROTRANSMISSION AT PVN IN LOCUS CERULEUS-INDUCED BAROREFLEX SUPPRESSION IN RATS

We investigated the role of ascending noradrenergic projections from t he locus ceruleus (LC) to the paraventricular nucleus (PVN) of the hyp othalamus in LC-induced suppression of the baroreceptor reflex (ERR) r esponse in adult Sprague-Dawley rats maintained under pentobarbital an esthesia. On the basis of in vivo microdialysis and highperformance li quid chromatography-electrochemical detection, microinjection of L-glu tamate (5 nmol) into the LC resulted in a site-specific increase in no repinephrine (NE) concentration in the dialysate collected from the pa rvocellular subnucleus of the PVN. The temporal course of this increas e in extracellular NE concentration in the PVN coincided with the time course of inhibition elicited by the LC on the ERR response. Microinf usion of NE (10, 50, or 100 nM) into the parvocellular subnucleus of t he PVN by reverse microdialysis also promoted a parallel increase in N E at the PVN and a reduction in the ERR response. inhibition of the ER R response induced by microinjection into the PVN of the alpha(1)-adre noceptor agonist phenylephrine (10 nmol) or chemical activation of the LC was reversed by bilateral PVN microinjection of prazosin (100 pmol ). However, local application to the PVN of the alpha(2)- or beta-adre noceptor agonist guanabenz (10 nmol) or isoproterenol(10 nmol) was ine ffective. Our results suggest that NE released from the LC-PVN noradre nergic projection may participate in LC-induced suppression of the ERR response by activating the alpha 1-adrenoceptors at the parvocellular subnucleus of the PVN.