Aim: 

The aim of this study was to investigate the effect of photoperiod and melatonin (Mel) in vitro on Siberian hamster (Phodopus sungorus) splenocyte proliferation and intracellular calcium concentration ([Ca²⁺]_i).

Methods: 

Spleens were isolated at midday and midnight from Siberian hamster adult males kept in long (LD 16:8) or short (SD 8:16) photoperiods. Erythrocyte-free spleen leukocytes were incubated without or in presence of Mel and/or a mitogen phytohaemagglutinin (PHA). Freshly-isolated splenocytes were used for [Ca²⁺]_i spectrofluorometric measurement using fluorescent dye Fura-2AM. Additionally, cells were cultured in vitro (for 72 h) and proliferation was assessed by incorporation of [³H]-thymidine.

Results: 

In splenocytes obtained from hamsters housed in both photoperiods Mel increased [Ca²⁺]_i but only when used in pharmacological concentrations. Additionally, it enhanced a stimulatory effect of PHA on splenocyte [Ca²⁺]_i. Although Mel decreased PHA-induced in vitro proliferation in cultured splenocytes isolated from hamsters kept in both photoperiods, this effect was time of day-dependent, i.e. melatonin-sensitive were either "diurnal" or "nocturnal" splenocytes from LD or SD hamsters, respectively. Moreover, Mel inhibited spontaneous proliferation of nocturnal SD-splenocytes.

Conclusions: 

The first time measured effect of exogenous Mel on hamster spleenocyte [Ca²⁺]_i suggests lack of correlation with that on their proliferation in vitro, but seems to be photoperiod (thus endogenous Mel)-dependent phenomenon. It suggests a possible involvement of differences in Mel receptor expression, associated with time of day and photoperiod. Subsequent investigations to elucidate these observations are in progress.

Supported by the Polish MSHE grants No 2P04C 080 27 and 2P04C 090 27