In the current study, we investigated a potential role for the pancreatic peptide insulin in mediating seasonal energetic trade-offs with immunity in Siberian hamsters. Specifically, we asked: (i) whether exogenous administration of insulin alters humoural and innate immune responses and (ii) whether this effect is photoperiod-dependent. As expected, short-day animals displayed gonadal regression and marked reductions in total body fat relative to long-day hamsters. Exogenous insulin increased KLH-specific IgG production, but only in short-day hamsters; insulin did not affect humoural immunity in long-day animals. Anti-KLH IgM followed a similar, non-significant, trend. In contrast, insulin had no effect on innate immunity (i.e. bacterial killing ability) regardless of photoperiodic condition. Serum leptin was reduced in short-day animals, but was not altered in response to insulin treatment. The findings from the present study support the idea that insulin can influence immune responses in hamsters and the effects of insulin on immunity can vary according to photoperiodic status. In addition, these results suggest that insulin acts as a peripheral signal of energy availability and probably mediates, at least in part, energetic trade-offs between immunity and other physiological systems (e.g. reproduction).
Blood glucose levels were significantly lower 1 h after insulin injection, suggesting that hamsters are physiologically responsive to hormone. Short-day hamsters appear to be more sensitive to the insulin signal as blood glucose levels in short-day animals were significantly lower than long-day animals given insulin. In contrast, there was no significant difference in glucose levels between long-day and short-day control hamsters. A similar pattern was observed with food intake across experimental treatments. Consistent with previous findings in this species (Bartness et al. 1995
), long-day insulin-treated hamsters consumed significantly more food than all other groups. In contrast, an increase in humoural immune response was observed in insulin-treated short-day hamsters, but not long-day hamsters. Thus, insulin-induced immunoenhancement was not probably a direct result of increased food intake. These findings suggest that insulin-induced immunoenhancement is not merely a product of abundant energy, but is also dependent on the photoperiodic state of the animal. Previous work in this and other species suggested that short-, but not long-day conditions can buffer immune suppression from an energetic stressor, suggesting long-day hamsters may be more sensitive to energy changes (Demas et al. 1997b
; Zysling et al. 2006
). However, where energy abundance is concerned, this may not be the case. Under stressful winter conditions, an animal may stand to profit from upregulating immunity if it receives a signal of immediate energy.
Changes in insulin levels alter a wide range of metabolic processes, which may alter immune responses. For example, insulin can influence circulating levels of leptin (Saladin et al. 1995
; Warne et al. 2009
) and can alter the metabolic actions of adipocytes (Muller et al. 1997
). Leptin, in turn, can affect humoural immune responses in a variety of mammalian species, including hamsters (Lord et al. 1998
; Drazen et al. 2001
). Thus, the effects of insulin on immunity may be indirect, via changes in leptin levels. Serum leptin concentrations were determined in the present study to assess whether the observed effects on immunity are caused by altered leptin levels. Leptin levels differed according to photoperiod treatment and, consistent with previous studies, were significantly lower in short-day hamsters (Drazen et al. 2000
). Insulin treatment, however, had no effect on serum leptin levels. Because leptin concentrations were lower in short-day hamsters and insulin increased
antibody responses in these animals, leptin is not probably playing a role in the photoperiod-dependent effects of insulin on humoural immunity.
Although metabolic factors other than leptin cannot be ruled out, insulin may act directly on peripheral immune tissues to enhance immune responses in short-day hamsters. Like most tissues, peripheral lymphoid tissue possesses insulin receptors (Gavin et al. 1973
). Interestingly, the insulin receptor is expressed on resting neutrophils, monocytes and B lymphocytes, but not T lymphocytes; activation of T cells, however, results in marked upregulation of insulin receptors on these cells (Viardot et al. 2007
). Consistent with this idea, lymphocyte proliferation and activation is altered by in vitro
insulin administration (Viardot et al. 2007
), suggesting that insulin can act directly on lymphocytes to modulate acquired immune responses.
Alternatively, insulin may act indirectly, via the central nervous system, to affect immunity. Such a mechanism has been suggested as part of the effects of leptin on humoural immunity (Demas 2002
). Previous research has demonstrated that central insulin receptor expression is reduced under short-day conditions in the arcuate nucleus (ARC) and short-day animals are presumably less sensitive to the insulin signal (Tups et al. 2006
). Similar findings of short-day decreases in insulin receptors have been reported in Japanese quail in the infundibular nucleus, a structure homologous to the mammalian ARC (Anraku et al. 2007
). In contrast to previous findings, short-day hamsters in the present study appear more
sensitive to exogenous insulin (e.g. display largest decrease in blood glucose levels and increase in immunoglobulin response), suggesting possible upregulation of insulin receptors, either centrally or in peripheral tissues (e.g. lymphoid tissues). According to a tissue-specific analysis of the insulin receptor expression in the mouse transcriptome, the insulin receptor is a ubiquitous housekeeping gene: there is no significant difference between expression in the hypothalamus compared with lymphoid organs (Su et al. 2002
). To our knowledge, no studies have examined photoperiodic changes in insulin receptors on lymphoid tissues in this or other seasonally breeding species.
Previous studies in Siberian hamsters have reported a photoperiod-dependent decrease in immune responses. Specifically, short-day hamsters have a lower antibody response than long-day animals (Drazen et al. 2001
; Demas et al. 2002
). In the current study, the expected decrease in immune response in short-day hamsters, however, was not observed; short-day levels of anti-KLH immunoglobulins did not differ between long- and short-day hamsters. This is somewhat surprising given that short-day hamsters exhibited all other typical photoperiod-induced characteristics including decreased body mass, food intake, adipose tissue mass and gonadal regression (Bartness 1996
). Although it is not known why immunity was not altered in short-day hamsters, it is possible that daily injections and handling acted as repeated stressors, potentially altering immune responses and masking expected photoperiodic changes in immunity. Male hamsters typically have elevated glucocorticoid levels in short days and exhibited greater immune responses to restraint stress (Bilbo & Nelson 2003
). Unlike the effects on antibody levels, exogenous insulin had no effect on innate immunity as assessed by bacterial killing ability. Innate immune responses tend to remain uniform. Innate responses use intrinsic microbial molecules, pathogen-associated molecular patterns (PAMPs), to bind foreign molecules (Medzhitov & Janeway 2002
). Alteration in the ability to recognize PAMPs is commonly deleterious to pathogen recognition (Beutler 2004
). Therefore, we might not expect to see a difference in innate immunity even under conditions where a signal of additional energy is involved.
Regardless of the precise mechanisms of action, the present results support the idea that insulin alters humoural immune responses, either directly or indirectly, in Siberian hamsters. Further research is required to determine the specific mechanisms mediating this effect. Taken together, these results suggest that insulin serves as an important peripheral signal linking energy availability and immunity. Exogenous insulin can enhance immune responses in short-day animals and, like leptin, can enhance energy availability owing to increased food intake. These findings support the idea that reduced energy availability can lead to changes in immunity which, in turn, can affect disease susceptibility. In addition, these results suggest an important role for insulin in regulating energetic trade-offs among competing physiological systems and provide an important step towards understanding the neuroendocrine mechanisms regulating seasonal changes in mammalian immunity.