Single amino acid substitution affects desensitization of the 5-hydroxytryptamine type 3 receptor expressed in Xenopus oocytes

5-Hydroxytryptamine type 3 receptors were expressed in Xenopus oocytes from a cloned cDNA. The peak inward current evoked by 5-hydroxytryptamine (30 μM) was linearly related to the holding potential (-100 to +20 mV) and reversed near 0 mV. The inward current (at -60 mV) declined during the continued presence of 5-hydroxytryptamine with a half-time of about 2 s; this desensitization was 20 times slower in calcium-free solution. Desensitization was markedly different in channels in which Leu²⁸⁶ was changed by site-directed mutagenesis; this residue is thought to lie near the middle of the M2 segment. Desensitization was faster with Phe, Tyr, or Ala in this position and slower with Thr. Phe and Thr substitutions in the equivalent position of the nicotinic acetylcholine receptor have similar effects on desensitization, suggesting that the underlying conformational change might be common to ligand-gated channels.