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 Journal 57: 723-731) with an exchange rate < 1 msec was used to study 5-HT3 (5-HT; 5-hydroxytryptamine) receptor-mediated currents in superfused outside-out patches of N1E-115 mouse neuroblastoma cells. At negative membrane potentials, 5-HT induced inward currents in a concentration-dependent manner (IC50 = 3.8 microM, Hill coefficient = 1.8). The mean peak current at a near-maximally effective 5-HT concentration of 30 microM was 20.6 pA. The 5-HT3 receptor antagonist ondansetron (0.3 nM) reversibly inhibited the 5-HT (30 microM) signal by approximately 50%. The currents induced during application of 30 microM 5-HT for 2 sec were characterized 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 concentrations. The recovery of fully desensitized patches required a washout period of 45 sec. Pentobarbital inhibited 5-HT-induced (30 microM) currents in a concentration-dependent manner. The maximally obtainable inhibition with a given pentobarbital concentration was reached already when it was exclusively coapplied with 5-HT (IC50 = 135 microM. Hill coefficient = -0.7), since additional preexposure for at least 45 sec did not alter the concentration-response curve of pentobarbital. In conclusion, outside-out patches of N1E-115 cells are suitable to study the kinetic properties of 5-HT3 receptor channels. The results obtained in this model with pentobarbital are compatible with the suggestion that the inhibitory action of pentobarbital on 5-HT3 receptors is dependent on the agonist-activated (open) channel.