Direct excitation of mitral cells via activation of alpha 1-noradrenergic receptors in rat olfactory bulb slices

The main olfactory bulb receives a significant modulatory noradrenergic inp ut from the locus coeruleus. Previous in vivo and in vitro studies showed t hat norepinephrine (NE) inputs increase the sensitivity of mitral cells to weak olfactory inputs. The cellular basis for this action of NE is not unde rstood. The goal of this study was to investigate the effect of NE and nora drenergic agonists on the excitability of mitral cells, the main output cel ls of the olfactory bulb, using whole cell patch-clamp recording in vitro. The noradrenergic agonists, phenylephrine (PE, 10 muM), isoproterenol (Isop , 10 muM), and clonidine (3 muM), were used to test for the functional pres ence of alpha1-, beta-, and alpha2-receptors, respectively, on mitral cells . None of these agonists affected olfactory nerve (ON)-evoked field potenti als recorded in the glomerular layer, or ON-evoked postsynaptic currents re corded in mitral cells. In whole cell voltage-clamp recordings, NE (30 muM) induced an inward current (54 +/-7 pA, n=16) with an EC50 of 4.7 muM. Both PE and Isop also produced inward currents (22 +/-4 pA, n=19, and 29 +/-9 p A, n=8, respectively), while clonidine produced no effect (n=6). In the pre sence of TTX (1 muM), and blockers of excitatory and inhibitory fast synapt ic transmission [gabazine 5 muM, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX ) 10 muM, and (+/-)-2-amino-5-phosphonopentanoic acid (APV) 50 muM], the in ward current induced by PE persisted (EC50 = 9 muM), whereas that of Isop w as absent. The effect of PE was also observed in the presence of the Ca2+ c hannel blockers, cadmium (100 muM) and nickel (100 muM). The inward current caused by PE was blocked when the interior of the cell was perfused with t he nonhydrolyzable GDP analogue, GDP betaS, indicating that the alpha1 effe ct is mediated by G-protein coupling. The current-voltage relationship in t he absence and presence of PE indicated that the current induced by PE decr eased near the equilibrium potential for potassium ions. In current-clamp r ecordings from bistable mitral cells, PE shifted the membrane potential fro m the downstate (-52 mV) toward the upstate (-40 mV), and significantly inc reased spike generation in response to perithreshold ON input. These findin gs indicate that NE excites mitral cells directly via alpha1 receptors, an effect that may underlie, at least in part, increased mitral cell responses to weak ON input during locus coeruleus activation in vivo.