Blockage of Mouse Muscle Nicotinic Receptors by Serotonergic Compounds

Abstract

summaryXenopus laevis oocytes were used to analyse the effects of serotonin (5-hydroxytryptamine, 5_HT) and serotonergic agents on ionic currents elicited by the activation of mammalian muscle nicotinic acetylcholine receptors (AChRs). 5_HT as well as other serotonergic agents, such as ketanserin, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), methysergide, spiperone, or fluoxetine alone (up to 1 mÒ), did not elicit membrane currents in Xenopus oocytes expressing AChRs, but they reversibly reduced the current elicited by acetylcholine (ACh-current). Serotonin was applied before, together with or after ACh application, and its effects were examined on desensitizing and non-desensitizing ACh-currents. 5_HT reduced the amplitude and accelerated the desensitization of the desensitizing currents. In contrast, non-desensitizing currents were reduced in amplitude but their time course was not significantly affected. With the same concentration of 5_HT the inhibition was stronger on desensitizing than on non-desensitizing ACh-currents. For example, 100 ìÒ 5_HT reduced the peak of a desensitizing ACh-current to 0·48 ± 0·06 (peak current ratio) and after 40 s the current was reduced to a ratio of 0·25 ± 0·04, whereas a non-desensitizing ACh-current was reduced to a ratio of 0·66 ± 0·01. All the serotonergic agents tested inhibited the ACh-currents rapidly and reversibly, suggesting that they are acting directly on the AChRs. The half-inhibitory concentration, IC50, of 5_HT acting on nondesensitizing currents elicited by 250 nÒ ACh was 247 ± 26 ìÒ and the Hill coefficient was •0·88, suggesting a single site for the interaction of 5_HT with the receptor. It appears that 5_HT inhibits AChRs non-competitively because neither the half-effective concentration of ACh, EC50, for ACh-current nor the Hill coefficient were affected by 5_HT. Furthermore, the extent of inhibition of 5_HT on AChRs did not depend on the nicotinic agonist (suberyldicholine, ACh or nicotine). The inhibition of AChRs by serotonergic agents was voltage-dependent. The electrical distance of the binding site for 5_HT was •0·75, whereas for the other serotonergic agents tested it was •0·22, suggesting that ketanserin, 8-OH-DPAT, methysergide, spiperone and fluoxetine act within the ion channel, but at a site more external than that for 5_HT. These substances inhibited the AChcurrent more potently than 5_HT. We conclude that 5_HT and serotonergic agents inhibit, in a noncompetitive manner, the ACh-current in muscle AChRs by blocking the open receptor-channel complex. Moreover, 5_HT appears to promote the desensitized state of the receptor when the current is elicited by high ACh concentrations. introduction The muscle nicotinic acetylcholine receptor (AChR), a neurotransmitter-gated ion channel, is a member of a gene superfamily that includes GABAA, glycine and 5_HT× receptors (Cockcroft et al. 1995). Structurally, the AChR is a pentamer composed of four different subunits (á, â, ã or å, and ä) with a stoichiometry of áµâãä (Karlin & Akabas, 1995; Unwin, ...

Voltage‐dependent inhibition of recombinant NMDA receptor‐mediated currents by 5‐hydroxytryptamine

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