Competitive and non-competitive effects of 5-hydroxyindole on 5-HT3 receptors in N1E-115 neuroblastoma cells.
1. Effects of 5-hydroxyindole (5-OH-indole) (the aromatic moiety of 5-hydroxytryptamine (5-HT)) on 5-HT-evoked ion current and the nature of these effects on 5-HT3 receptors have been investigated in whole-cell voltage clamp and radioligand binding experiments on cultured N1E-115 mouse neuroblastoma cells. 2. The amplitude of 10 microM 5-HT-evoked ion currents was enhanced up to 150% of the control value by increasing concentrations up to 10 mM 5-OH-indole with half maximum effect of 0.8 mM. At concentrations between 10 mM and 50 mM, 5-OH-indole blocked the 5-HT-evoked ion current. Both the enhancement and the block by 5-OH-indole were accompanied by a marked slowing of the kinetics of decay of the 5-HT-evoked inward currents. 3. The blocking effect was surmounted when the 5-HT concentration was raised from 10 microM to 100 microM. Conversely, the increase in amplitude and the slowing of the decay of the 5-HT-evoked ion current induced by 1 mM 5-OH-indole were not reversed by the same increase of 5-HT concentration. 4. The binding of the selective antagonist [3H]-GR65630 to 5-HT3 receptors was displaced by 5-OH-indole in a concentration-dependent manner with a pKi of 1.96. In saturation binding experiments 10 mM 5-OH-indole reduced the affinity of [3H]-GR65630, whereas the total number of binding sites remained unaffected. 5. It is concluded that the blocking effect of high concentrations of 5-OH-indole is due to a competitive interaction with the antagonist recognition sites of 5-HT3 receptors, whereas the potentiating effect of lower concentrations of 5-OH-indole appears to be mediated by a distinct non-competitive interaction.
Kooyman
A R
AR
Research Institute of Toxicology, Utrecht University, The Netherlands.
van Hooft
J A
JA
Vanderheijden
P M
PM
Vijverberg
H P
HP
D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D001932:Brain Neoplasms / Q000378:metabolism*; D004594:Electrophysiology; D007093:Imidazoles / Q000494:pharmacology; D007211:Indoles / Q000494:pharmacology*; D007700:Kinetics; D051379:Mice; D009447:Neuroblastoma / Q000378:metabolism*; D011869:Radioligand Assay; D011985:Receptors, Serotonin / Q000187:drug effects / Q000378:metabolism*; D012701:Serotonin / Q000494:pharmacology; D012702:Serotonin Antagonists; D014407:Tumor Cells, Cultured
D007093:Imidazoles; D007211:Indoles; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; C059068:GR 65630; C026785:5-hydroxyindole; D012701:Serotonin