Melatonin differentially modulates the expression and function of the hMT₁ and hMT₂ melatonin receptors upon prolonged withdrawal

Melatonin is synthesized and released following a circadian rhythm and reaches its highest blood levels during the night. It relays signals of darkness to target tissues involved in regulating circadian and seasonal rhythms. Here, we report the expression of human melatonin receptors type 1 and 2 (hMT₁ and hMT₂, respectively) in Chinese hamster ovary (CHO) cells following exposure to melatonin treatments mimicking the amplitude (400pM) and duration (8hr) of the nightly melatonin peak and upon withdrawal. Exposure of CHO-MT₁ cells to melatonin (400pM) for 0.5, 1, 2, 4, and 8hr significantly increased specific 2-[¹²⁵I]iodomelatonin (500pM) binding to hMT₁ melatonin receptors upon 16-hr withdrawal. However, the same treatment did not affect the expression of hMT₂ melatonin receptors. The increase in specific 2-[¹²⁵I]iodomelatonin (500pM) binding (162±29%, N=3, P<0.05) 16hr after melatonin withdrawal was parallel to increases in hMT₁ melatonin receptor mRNA (231±33%, N=4, P<0.05). This effect was due to an increase in the total number of hMT₁ receptors [B_max 833±97fmol/mg protein (N=3), control; 1449±41fmol/mg protein (N=3), treated], with no change in binding affinity. The melatonin-mediated increase in MT₁ melatonin receptor expression upon withdrawal was not mediated through either a direct effect of the hormone in the promoter's vector or in the rate of mRNA degradation. In conclusion, melatonin differentially regulates the expression of its own receptors, which may have important implications in the transduction of dark signals in vivo.