ACTIVATION OF 5-HT3 RECEPTORS EXPRESSED IN XENOPUS OOCYTES DOES NOT INCREASE CYTOPLASMIC CA2+ LEVELS

The wildtype 5-HT3 receptor was expressed in Xenopus oocytes from a cl oned cDNA. Two-electrode voltage-clamp and fura-2 techniques were used simultaneously to monitor the current induced by the activation of th e 5-HT3 receptor and the cytoplasmic Ca2+ concentration ([Ca2+](i)). T he application of serotonin (5-HT; 50 mu M) in a physiological bathing solution containing Ca2+ (1.8 mM) elicited inward currents of up to s imilar to 1.3 mu A at a potential of -90 mV. There was little or no de tectable change in [Ca2+](i). In experiments on oocytes bathing in a n ominally Ca2+-free medium, the injection of (2,4,5)IP3, an analog of i nositol 1,4,5-trisphosphate ((1,4,5)IP3), produced a large increase in [Ca2+](i) levels due to the liberation bf Ca2+ from intracellular sto res. This response was followed by a sustained elevation of [Ca2+](i) upon restoring extracellular Ca2+ (i.e. capacitative Ca2+ entry). Furt hermore, when calcium-permeable heteromeric (NR1/NR2A) N-methyl-D-aspa rtate (NMDA) receptors were expressed in oocytes, the activation of th ese receptors led to a large increase in [Ca2+](i) in a Ca2+-containin g external medium. It is concluded that wildtype 5-HT3 receptors expre ssed in Xenopus oocytes and studied under conditions used here show li ttle or no permeability for Ca2+.