MT1 receptor expression and AA-NAT activity in lymphatic tissue following melatonin administration in male golden hamster

• Melatonin treatment to hamsters increased AA-NAT activity in spleen, thymus & BM.
• Melatonin treatment induces an enhancement in immunity (IL-2, IFN-γ, %SR, GM-CFU).
• Melatonin significantly increased the MT1 receptor expression in lymphatic tissues.

Exogenous melatonin as a marker of the chemical expression of darkness is playing a key role in the synchronization of circadian functions and seasonal biological rhythms. Our study was designed to elucidate whether melatonin treatment can modulate the melatonin synthesis via the rate limiting enzyme arylalkylamine-N-acetyltransferase (AA-NAT) in spleen, thymus and bone marrow thereby the proliferation rate of splenocytes, thymocytes and bone marrow mononuclear cells (BM-MNCs) of golden hamsters. The AA-NAT activity in different lymphoid tissue documented the synthesis of melatonin in those organs. Exogenous melatonin treatment to hamsters enhanced the AA-NAT activity in spleen and thymus along with an increase in the inflammatory response by DTH reactions that could be related to the increased level of interleukin-2 and IFN-γ by T lymphocytes in serum/culture medium, proliferation rate and expression of melatonin membrane receptor MT1. Thus, the relevance of melatonin synthesis by lymphatic tissues might be maintaining surveillance and local defence responses.