Expression and functional characterisation of melatonin receptors in the spinal cord of the rat: implications for pain modulation

Recently, a species-dependent distribution of melatonin binding sites have been found in lamina I-V and lamina X of the spinal cord. In order to learn more about the function of spinal melatonin receptors, we investigated (i) the gene expression for melatonin receptor subtypes in lumbar and thoracal spinal cord tissue by means of the reverse-transcriptase polymerase chain reaction (RT-PCR) technique, and (ii) the electrophysiological and pharmacological properties of melatonin receptors heterologously expressed in Xenopus oocytes after injection of spinal cord mRNA by means of the voltage clamp technique. Because ample evidence indicates an antinociceptive effect of melatonin, (iii) the role of spinal melatonin receptors for maintaining mechanical and thermal hyperalgesia was studied in a rat model for postoperative pain. The RT-PCR data revealed that transcripts for MT1 and MT2 melatonin receptors are present in the dorsal and ventral horn of lumbar and thoracal spinal cord tissue. Injection of mRNA from lumbar spinal cord tissue into Xenopus oocytes led to the functional reconstitution of melatonin receptors which activate calcium-dependent chloride inward currents. Melatonin responses were abolished by simultaneous administration of the antagonists, 2-phenylmelatonin and luzindole and were unaffected by the MT2 antagonist 4-phenyl-2-propionamidotetralin. Intrathecal administration of different melatonin doses (10-100 nmol) did not inhibit mechanical or thermal hyperalgesia. However, intrathecal application of a low dose of morphine together with melatonin caused a brief antinociceptive effect suggesting an enhanced morphine analgesia by melatonin. In conclusion, the present study demonstrated for the first time the presence of transcripts of MT1 and MT2 receptors located in the dorsal and ventral horn of the spinal cord. Furthermore, spinal melatonin enhanced the antinociceptive effect of morphine indicating that melatonin acts as a neuromodulator in the spinal cord.