Molecular cloning and pharmacological characterization of a Bombyx mori tyramine receptor selectively coupled to intracellular calcium mobilization

Tyramine (TA) is a biogenic amine in invertebrates. cDNA encoding the TA receptor (TAR) BmTAR2 was cloned from the nerve tissue of the silkworm Bombyx mori. The receptor's functional and pharmacological properties were examined in BmTAR2-transfected HEK-293 cells. In [3H]TA binding assays, BmTAR2 showed considerably higher affinity for TA than for other biogenic amines, with an IC50 value of 57.5 nM. Moreover, TA induced a dose-dependent increase in intracellular Ca2+ concentration ([Ca2+]i) in cells, with an EC50 value of 11.6 nM, whereas octopamine and dopamine increased [Ca2+]i only at concentrations above 100 μM. A few antagonists were found to inhibit the TA-induced increases in [Ca2+]i; the rank order of potency was yohimbine > chlorpromazine > mianserin. TA showed no effect on intracellular cAMP concentration. The data indicate that BmTAR2 belongs to the second class of TARs, which are selectively coupled to intracellular Ca2+ mobilization. RT-PCR analysis revealed that BmTAR2 was expressed predominantly in the nervous tissue of B. mori larvae, suggesting that TA has neurotransmitter and neuromodulatory roles that are mediated by BmTAR2.