Inhibition of 5-HT3 receptor cation channels by ifenprodil in excised patches of N1E-115 cells.
The patch-clamp technique was applied in out-side-out patches of N1E-115 mouse neuroblastoma cells to investigate the effects of ifenprodil [(+/-) erythreo-ifenprodil tartratel, a drug with neuroprotective properties in cerebral ischemia, on the inward currents through 5-HT3 receptor channels. A high time resolution was achieved by using a rapid solution exchange system (exchange rate <1 ms). Ifenprodil inhibited the peak currents evoked by 30 microM 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 microM) and it was only slightly less potent when it was applied during the 45 s prior to 5-HT only (IC50=29 microM). When applied in this manner, ifenprodil also produced a concentration-dependent increase of the onset time constant (tauON) of the 5-HT (30 microM)-induced currents. When the drug was exclusively co-applied with 5-HT, ifenprodil was least potent in inhibiting the peak currents (IC50=98 microM), and it had no effect on the current onset kinetics. All protocols of ifenprodil application accelerated current inactivation as reflected by a decrease of the current inactivation time constant (tauOFF). All effects of ifenprodil were reversible after washout periods of 2-5 min. In conclusion, the potency of ifenprodil in inhibiting the inward current through 5-HT3 receptor channels is strongly dependent on the application protocol: presence of the drug before the agonist-induced activation of the 5-HT3 receptor channels is necessary for a relatively potent inhibition of the 5-HT-induced peak current and is a prerequisite for the prolongation of tauON; in addition, a weak but fast inhibitory effect on the current amplitude and decay constant of the 5-HT-induced current was revealed by the experiments in which 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 mechanism only.
Barann
M
M
Institut für Pharmakologie und Toxikologie, Universität Bonn, Germany.
Bönisch
H
H
Urban
B W
BW
Göthert
M
M
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D016166:Free Radical Scavengers / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D010880:Piperidines / Q000494:pharmacology*; D011985:Receptors, Serotonin / Q000187:drug effects*; D044406:Receptors, Serotonin, 5-HT3; D012701:Serotonin / Q000494:pharmacology; D015222:Sodium Channels / Q000187:drug effects* / Q000502:physiology; D014407:Tumor Cells, Cultured / Q000166:cytology / Q000187:drug effects / Q000502:physiology
D000317:Adrenergic alpha-Antagonists; D016166:Free Radical Scavengers; D010880:Piperidines; D011985:Receptors, Serotonin; D044406:Receptors, Serotonin, 5-HT3; D015222:Sodium Channels; D012701:Serotonin; C010739:ifenprodil