Photoperiod Modulates the Inhibitory Effect of In Vitro Melatonin on Lymphocyte Proliferation in Female Siberian Hamsters

Abstract

In Siberian hamsters (Phodopus sungorus), short days suppress reproductive function and lymphocyte proliferation. To determine whether melatonin influences cell-mediated immunity through a direct action on lymphocyte proliferation, in vitro responsiveness to mitogens and melatonin was assessed in systemic and splenic lymphocytes from adult female Siberian hamsters housed in either long or short days for 13 weeks. Short days provoked reproductive regression and reduced lymphocyte proliferation. Physiological concentrations of melatonin (50 pg/ml) inhibited in vitro proliferation of circulating lymphocytes, whereas higher concentrations (> or = 500 pg/ml) were required to inhibit proliferation of splenic lymphocytes. Immunomodulatory effects of melatonin were restricted to lymphocytes from long-day hamsters-in vitro melatonin had no effect on circulating or splenic lymphocytes from females in short days. Responsiveness to melatonin in short-day lymphocytes may be restrained by the already expanded nightly pattern of melatonin secretion in short days. These data support the hypothesis that melatonin acts directly on lymphocytes from long-day hamsters to suppress blastogenesis.

Blood Levels of Selected Metabolic Factors, Cytokines, and Lymphocyte Subpopulations in Arabian and Thoroughbred Horses During the Longest and Shortest Days of the Year

“…It is apparent that gender differences exist concerning seasonal changes of lymphocyte subpopulations in the horse and this should be investigated further. The data from the present study are partially consistent with the previous studies performed in male and female hamsters [2,38,39]. But, these studies of hamster have typically been conducted under artificial photoperiodic conditions and therefore some physiological variables of the natural photoperiod might be masked by artificial light or artificial darkness [40].…”

“…However, others have reported on seasonal variations of white blood cells including lymphocytes in the horse [36,37]. In addition, it has been reported that in Siberian hamsters, long-day breeders like horse, photoperiodic changes in immune function are observed [2,38,39]. In the present study, we found that a gender-by-day length interaction on lymphocyte subpopulations suggest that only stallions responded to photoperiod whereas mares were not responsive.…”

Photoperiodic time measurement and seasonal immunological plasticity

“…However, in other rodents, splenocytes, bone marrow cells, and lymphoid cells in the circulation all possess functional MEL receptors, and MEL treatments in vitro enhance or suppress immune responses in a cell- and antigen-specific manner (e.g., Pozo et al, 1997). In hamsters, MEL can inhibit or stimulate lymphocyte and splenocyte proliferation in vitro, depending on sex, age and photoperiod (Prendergast et al, 2001; 2002b). This is consistent with other reports in deer mice, in which exogenous MEL in vivo has been reported to enhance splenocyte proliferation in vitro, although MEL treatments did not inhibit primary immunoglobulin M antibody production (Demas & Nelson, 1998).…”

Form over Function?

“…Zhou et al (2002) provide evidence for effects of day length on immune measures (auricular lymphocyte counts) and responses (IL-6 production in sensitized lymphocytes, mitogen-stimulated T-cell blastogenesis). This is, of course, not the first report indicating effects of photoperiod on immune responses in a reproductively photoperiodic species (e.g., , nor is it the first report indicating the absence thereof (e.g., Prendergast et al, 2001). We agree with the statement of Zhou et al that it is important to discover any functional significance to the photoperiodic and melatonin-induced changes in immune measures and responses reported by many authors for several mammalian species (Blom et al, 1994;Demas and Nelson, 1998;Yellon, Teasley, et al, 1999;Drazen et al, 2000;Mann et al, 2000;Prendergast et al, 2001;Prendergast et al, 2002;Yellon and Tran, 2002); however, we do not view the majority of data published to date on effects of photoperiod on immunity as adequate tests of immune function (as described above).…”