Expression and functional characterization of the mt1 melatonin receptor from rat brain in Xenopus oocytes: evidence for coupling to the phosphoinositol pathway

Melatonin-sensitive receptors were expressed in Xenopus laevis oocytes following an injection of mRNA from rat brain. The administration of 0.1 100 μmol/L melatonin to voltage-clamped oocytes activates calcium-dependent chloride currents via a pertussis toxin-sensitive G protein and the phosphoinositol pathway. To determine which melatonin receptor type (mt 1 , MT 2 , MT 3 ) is functionally expressed in the Xenopus oocytes, we used (i) agonists and antagonists of different receptor types to characterize the pharmacological profile of the expressed receptors and (ii) a strategy of inhibiting melatonin receptor function by antisense oligonucleotides. During pharmacological screening administration of the agonists 2-iodomelatonin and 2-iodo-N-butanoyl-5-methoxytryptamine (IbMT) to the oocytes resulted in oscillatory membrane currents, whereas the administration of the MT 3 agonist 5-methoxycarbonylamino-N-acetyltryptamine (GR135,531) exerted no detectable membrane currents. The melatonin response was abolished by a preceding administration of the antagonists 2-phenylmelatonin and luzindole but was unaffected by the MT 3 antagonist prazosin and the MT 2 antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT). In the antisense experiments, in the control group the melatonin response occurred in 45 of 54 mRNA-injected oocytes (83%). Co-injection of the antisense oligonucleotide, corresponding to the mt, receptor mRNA, caused a marked and significant reduction in the expression level (13%; P < 0.001). In conclusion, the results demonstrate that injection of mRNA from rat brain in Xenopus oocytes induced the expression of the mt, receptor which is coupled to the phosphoinositol pathway.