Allosteric potentiation of the 5-HT3 receptor-mediated ion current in N1E-115 neuroblastoma cells by 5-hydroxyindole and analogues.

Potentiation of the 5-HT3 receptor-mediated ion current in mouse N1E-115 neuroblastoma cells by 5-hydroxyindole (5-OHi) and three analogues (5-aminoindole, catechol and indole) was examined using whole-cell voltage clamp and single channel patch clamp techniques. The substances tested enhanced the amplitude of the maximum 5-HT-evoked ion current by 12-30%. The rank order (at 1 mM) to potentiate the 5-HT-induced current was: 5-OHi approximately 5-aminoindole approximately catechol > indole. The concentration-effect curve of 5-HT was shifted leftwards by 1 mM 5-OHi, resulting in a two-fold increase of the apparent affinity of 5-HT from 1.4 microM to 0.7 microM, without affecting the Hill coefficient. The time constant of reversal of activation of the 5-HT-induced ion current upon washout of the agonist was delayed by 1 mM 5-OHi from 4.0 sec to 12.8 sec. 5-HT3 receptor-gated single channel events in cell-attached patches in the presence and absence of 1 mM 5-OHi were indistinguishable, apart from a slight increase in the event frequency. The results suggest that 5-OHi and analogues potentiate the 5-HT3 receptor-mediated ion current by delaying agonist dissociation and thereby increase the probability of channel opening. From the increased apparent affinity of 5-HT and the non-surmountability of the potentiating effect, it is concluded that 5-OHi and analogues are allosteric modulators of 5-HT3 receptors.