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Blockage of Mouse Muscle Nicotinic Receptors by Serotonergic Compounds
Author(s) -
GarcíaColunga Jesús,
Miledi Ricardo
Publication year - 1999
Publication title -
experimental physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.925
H-Index - 101
eISSN - 1469-445X
pISSN - 0958-0670
DOI - 10.1111/j.1469-445x.1999.01837.x
Subject(s) - serotonergic , methysergide , acetylcholine , nicotinic agonist , chemistry , ketanserin , serotonin , acetylcholine receptor , spiperone , xenopus , endocrinology , biophysics , inhibitory postsynaptic potential , 5 ht receptor , desensitization (medicine) , medicine , pharmacology , receptor , biology , biochemistry , gene
SUMMARY Xenopus 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 mM), 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 μM 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, IC 50 , of 5‐HT acting on non‐desensitizing currents elicited by 250 nM ACh was 247 ± 26 μM and the Hill coefficient was [sim]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, EC 50 , 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 [sim]0.75, whereas for the other serotonergic agents tested it was [sim]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 ACh‐current more potently than 5‐HT. We conclude that 5‐HT and serotonergic agents inhibit, in a non‐competitive 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.