Influence of Photoperiod and Exogenous Melatonin on the Maturation and Apoptosis of B Lymphocytes in the Spleen.

We studied the effects of exogenous melatonin (administrated ad libitum at a concentration of 4 mg/liter in drinking water for 4 weeks) on the differentiation of B lymphocytes (PAX-5+) and apoptosis (Casp3+) of spleen cells in outbred laboratory mice kept under conditions of natural photoperiod, constant darkness, or constant illumination. Constant darkness increased the number of B-lymphocyte precursors and caspase-3 lymphocytes in the lymphoid follicles of the spleen. Constant light exposure reduced the number of B lymphocyte precursors, increased the number of Casp3+ lymphocytes and the content of caspase-3 and transcription factor PAX-5 in these cells. Melatonin administration under artificial photoperiods (24/24 and 0/24 regimens) reduced the number of Casp3+ cells and increased the number of PAX-5+ cells in the white pulp.