We applied reverse microdialysis and high performance liquid chromatography (HPLC) analysis to evaluate the participation of presynaptic noradrenergic neurotransmission in the suppression by substance P (SP) of a,-adrenoceptor activity a t the nucleus reticularis gigantocellularis (NRGC) in Sprague-Dawley rats anesthetized with pentobarbital sodium. Microinfusion of SP (600 FWmin) into the NRGC through a stereotaxically positioned microdialysis probe attenuated the hypotensive and bradycardiac actions of the a2-adrenoceptor agonist, guanabenz (100 pgkg, i.v.). This inhibitory effect correlated positively with the time course of elevation in the estimated extracellular concentrations of SP and norepinephrine (NE) in the NRGC. Direct microinfusion of NE (50 nWmin) into the NRGC also lessened the cardiosuppressant effects of guanabenz. These circulatory and NE responses to SP were, however, significantly blunted in rats in which the noradrenergic innervation in the NRGC was depleted with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) pretreatment. Microinfusion of NE into the NRGC of DSP4-pretreated animals restored the attenuation of guanabenzinduced cardiovascular suppression. These results suggest that SP may depress the activity of a2-adrenoceptor a t the NRGC that are involved in circulatory regulation by increasing the extracellular concentration of NE via a presynaptic modulation of noradrenergic neurotransmission. Q 1995 Wiley-Liss, Inc.