The expression of MT1 and MT2 melatonin receptor mRNA in several rat tissues

The mechanisms that mediate the various effects of melatonin in mammalian tissues are not always known. Therefore, the aim of this study was to investigate whether MT1 and MT2 melatonin receptors are expressed in certain tissues of the rat. The expression of MT1 and MT2 melatonin receptor mRNA was determined using a real-time quantitative RT-PCR method. In addition, we examined whether mRNA for either subtype of receptor shows any difference in the expression between midnight and noon, similar to the changes in melatonin concentrations in plasma and tissue samples. MT1 and MT2 melatonin receptor mRNAs were found in the rat hypothalamus, retina and small intestine. We also showed a low expression of MT2 mRNA in the rat liver and heart SA node. In the heart apex and the Harderian gland, no appearance of either of the receptor mRNAs was detectable. A significant difference in the expression of MT1 mRNA between day and night was found in the hypothalamus. In conclusion, our findings suggest that at least some effects of melatonin are mediated through membrane MT1 and MT2 receptors in the hypothalamus, the retina and the small intestine. Down-regulation of receptors might be one reason for the difference in the hypothalamic MT1 melatonin receptor mRNA expression between midnight and noon. In the liver and the heart SA node, the physiological significance of possible MT2 receptors remains unclear. According to our negative midnight and noon results in the Harderian gland and heart apex melatonin may exert its effect on these tissues by a non-receptor mechanism.