5-Hydroxyindole potentiates human α7 nicotinic receptor-mediated responses and enhances acetylcholine-induced glutamate release in cerebellar slices

Abstract

The effects of 5-hydroxyindole (5-HI) have been investigated on human α7 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes and GH4 cells, on native α7 nAChRs expressed by IMR-32 cells and on α7 nAChR-mediated events in mossy fibre–granule cell synapses in rat cerebellar slices. In oocytes expressing α7 nAChRs, 5-HI potentiated sub-maximal, 60 μM ACh-induced ion currents in a concentration-dependent manner, the threshold effective concentration being 30 μM. 5-HI itself did not act as an agonist on α7 nAChRs. A maximum potentiation of 12 times the control was observed at 20 mM 5-HI. The effect of 1 mM 5-HI on the concentration–response curve for ACh revealed that 5-HI increased the potency as well as the efficacy of ACh on α7 nAChRs. 5-HI also potentiated α7-mediated increases in intracellular free calcium levels in both mammalian cells heterologously expressing human α7 nAChRs and in human IMR-32 neuroblastoma cells expressing native α7 nAChRs. At mossy fibre–granule cell synapses, application of 1 mM ACh induced glutamate-evoked excitory post-synaptic currents (EPSCs). Co-application of 1 mM 5-HI with 1 mM ACh further increased the frequency of the EPSCs. The ACh-induced release, as well as the 5-HI-induced enhancement of release, were blocked by 1–10 nM methyllycaconitine or 200 nM α-bungarotoxin, demonstrating that both effects were mediated by presynaptic α7 nAChRs. The results demonstrate that responses mediated by α7 nAChRs are strongly potentiated by 5-HI.