5-HT3 RECEPTORS IN OUTSIDE-OUT PATCHES OF N1E-115 NEUROBLASTOMA-CELLS- BASIC PROPERTIES AND EFFECTS OF PENTOBARBITAL

A fast solution exchange system (Dilger and Brett, 1990; Biophysics Jo urnal 57: 723-731) with an exchange rate <1 msec was used to study 5-H T3 (5-HT; 5-hydroxytryptamine) receptor-mediated currents in superfuse d outside-out patches of NIE-115 mouse neuroblastoma cells. At negativ e membrane potentials, 5-HT induced inward currents in a concentration -dependent manner (IC50 = 3.8 mu M, Hill coefficient = 1.8). The mean peak current at a near-maximally effective 5-HT concentration of 30 mu M was 20.6 pA. The 5-HT3 receptor antagonist ondansetron (0.3 nM) rev ersibly inhibited the 5-HT (30 mu M) signal by approximately 50%. The currents induced during application of 30 mu M 5-HT for 2 sec were cha racterized by inward rectification, a monophasic onset (tau(ON) = 37.5 msec) and, after reaching a peak, a monophasic decay (desensitization ; tau(OFF) = 391 msec). Onset and decay were slower at lower 5-HT conc entrations. The recovery of fully desensitized patches required a wash out period of 45 sec. Pentobarbital inhibited 5-HT-induced (30 mu M) c urrents in a concentration-dependent manner. The maximally obtainable inhibition with a given pentobarbital concentration was reached alread y when it was exclusively coapplied with 5-HT (IC50 = 135 mu M, Hill c oefficient = -0.7), since additional preexposure for at least 45 sec d id not alter the concentration-response curve of pentobarbital. In con clusion, outside-out patches of NIE-115 cells are suitable to study th e kinetic properties of 5-HT3 receptor channels. The results obtained in this model with pentobarbital are compatible with the suggestion th at the inhibitory action of pentobarbital on 5-HT3 receptors is depend ent on the agonist-activated (open) channel. (C) 1997 Elsevier Science Ltd.