A single amino-acid in the TM1 domain is an important determinant of the desensitization kinetics of recombinant human and guinea pig alpha-homomeric5-hydroxytryptamine type 3 receptors

Desensitization of ligand-gated ion channels shapes synaptic responses and provides critical neuroprotection at central synapses, yet the molecular me chanisms underlying the desensitization process are poorly understood. Usin g the whole-cell voltage-clamp technique, we investigated desensitization k inetics of recombinant human and guinea pig alpha -homomeric 5-hydroxytrypt amine type 3 (5-HT3A) receptors heterologously expressed in human embryonic kidney 293 cells. Human 5-HT3A receptors desensitize 3.5 times faster than does the homologous receptor from guinea pigs. By constructing various chi meras and through site-directed mutagenesis, we have identified a single se rine in the M1 region of the human 5-HT3A receptor sequence (S248) that, wh en substituted with threonine found in the equivalent guinea pig sequence ( T254), conferred guinea pig-like kinetics on the time course of desensitiza tion of the human receptor. Correspondingly, the reverse mutation (guinea p ig T254S) resulted in a fast, human-like time constant of desensitization. Thus, the primary structure of the M1 region is an important determinant of desensitization kinetics of recombinant 5-HT3A receptors.