INHIBITION OF 5-HT3 RECEPTOR CATION CHANNELS BY IFENPRODIL IN EXCISEDPATCHES OF N1E-115 CELLS

The patch-clamp technique was applied in outside-out patches of NlE-11 5 mouse neuroblastoma cells to investigate the effects of ifenprodil [ (+/-) erythreo-ifenprodil tartrate], a drug with neuroprotective prope rties in cerebral ischemia, on the inward currents through 5-HT3 recep tor channels. A high time resolution was achieved by using a rapid sol ution exchange system (exchange rate <1 ms), Ifenprodil inhibited the peak currents evoked by 30 mu M 5-HT in a concentration-dependent but voltage-independent manner, The effect was most potent when ifenprodil was continuously applied to the patches 45 s before and during the 2- s administration of 5-HT (IC50=16 mu M) and it was only slightly less potent when it was applied during the 45 s prior to 5-HT only (IC50=29 mu M). When applied in this manner, ifenprodil also produced a concen tration-dependent increase of the onset time constant (tau(ON)) of the 5-HT (30 mu M)-induced currents. When the drug was exclusively co-app lied with 5-HT, ifenprodil was least potent in inhibiting the peak cur rents (IC50=98 mu M), and it had no effect on the current onset kineti cs. All protocols of ifenprodil application accelerated current inacti vation as reflected by a decrease of the current inactivation time con stant (tau(OFF)) All effects of ifenprodil were reversible after washo ut periods of 2-5 min. In conclusion, the potency of ifenprodil in inh ibiting the inward current through 5-HT3 receptor channels is strongly dependent on the application protocol: presence of the drug before th e agonist-induced activation of the 5-HT3 receptor channels is necessa ry for a relatively potent inhibition of the 5-HT-induced peak current and is a prerequisite for the prolongation of tau(ON) in addition, a weak but fast inhibitory effect on the current amplitude and decay con stant of the 5-HT-induced current was revealed by the experiments in w hich ifenprodil was exclusively present during exposure to 5-HT. Three alternatives compatible with the components of the ifenprodil effect have been discussed: (1) different effects of the two enantiomers, (2) action via two different mechanisms, and (3) operation via a single m echanism only.