A Single Amino-acid in the TM1 Domain is an Important Determinant of the Desensitization Kinetics of Recombinant Human and Guinea Pig Alpha-homomeric 5-hydroxytryptamine Type 3 Receptors

Desensitization of ligand-gated ion channels shapes synaptic responses and provides critical neuroprotection at central synapses, yet the molecular mechanisms underlying the desensitization process are poorly understood. Using the whole-cell voltage-clamp technique, we investigated desensitization kinetics of recombinant human and guinea pig alpha-homomeric 5-hydroxytryptamine type 3 (5-HT(3A)) receptors heterologously expressed in human embryonic kidney 293 cells. Human 5-HT(3A) receptors desensitize 3.5 times faster than does the homologous receptor from guinea pigs. By constructing various chimeras and through site-directed mutagenesis, we have identified a single serine in the M1 region of the human 5-HT(3A) receptor sequence (S248) that, when substituted with threonine found in the equivalent guinea pig sequence (T254), conferred guinea pig-like kinetics on the time course of desensitization of the human receptor. Correspondingly, the reverse mutation (guinea pig 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-HT(3A) receptors.