Both immune and reproductive (i.e., pregnancy and lactation) functions are energetically demanding processes that fluctuate seasonally. Winter is energetically demanding because energy availability is limited and energy demands are high. The goal of the present study was to assess the trade-off of resources between immunity and reproduction by examining antibody production during pregnancy and lactation, when energy requirements are high. Female Siberian hamsters (Phodopus sungorus) were held in either long or short days, and anti-keyhole limpet hemocyanin antibody production was assessed in pregnant, lactating, or nulliparous animals. Pregnancy and lactation both suppressed humoral immunity (primary antibody production) in long-day Siberian hamsters. However, although antibody production was reduced in short-day females compared with long-day animals, pregnancy and lactation did not reduce antibody body production any further. Interestingly, the size and mass of the litters produced by females in both long and short days did not differ. Taken together, these results suggest that the processes of pregnancy and lactation are costly enough to reduce humoral immunity, and they suggest that immunity may be compromised as a result of energy allocation to these reproductive processes.
Prairie voles (Microtus ochrogaster), like most rodent species, exhibit a phenotypic polymorphism in reproductive response to winter conditions or to short day lengths in the laboratory. Laboratory studies on seasonally breeding species have traditionally focused on the role of photoperiod in modulating reproduction and other seasonal adaptions. However, because animals use proximate environmental factors in addition to photoperiod to phase seasonal adaptions with the appropriate time of year, the present study investigated the interaction of photoperiod and temperature on reproductive function and the gonadotropin-releasing hormone (GnRH) neuronal system. Male prairie voles were housed in either long (LD 16:8) or short (LD 8:16) photoperiods. Voles in each photoperiodic condition were also exposed to either mild (20 degrees C) or low (8 degrees C) temperatures. After 10 weeks, voles were killed and their brains were processed using in situ hybridization for mRNA for proGnRH. The results suggest that GnRH synthesis is not affected by exposure to a single inhibitory proximate factor (i.e. short days or low temperatures alone), even when reproduction is inhibited, whereas a combination of inhibitory proximate factors leads to a decrease in GnRH synthesis (i.e. fewer neurones staining for mRNA for proGnRH). These data suggest that the neuroendocrine mechanisms regulating seasonal alterations in reproductive function are likely to differ between harsh and mild winters.
Individuals of species inhabiting temperate and boreal latitudes optimize the timing of energetically costly processes by curtailing nonessential energetically demanding processes when environmental conditions are not favourable. One proximate environmental variable used to fine-tune moment-to-moment changes in reproductive physiology and behaviour is food intake. The neuroendocrine mechanisms by which food restriction leads to the cessation of reproduction in seasonally breeding rodent species remain largely unspecified. The present study sought to determine the effects of extended food restriction on the gonadotropin releasing hormone (GnRH) neuronal system. Male prairie voles (Microtus ochrogaster) were either fed ad libitum or were exposed to either 1, 2 or 3 weeks of moderate (70% of daily mean) food restriction. In accordance with previous studies of food restriction, gross reproductive organ masses and body mass were unaffected by food deprivation. Although 1 week of food restriction did not result in alterations in the GnRH neuronal system, food restriction for 2 weeks was associated with increased GnRH-immunoreactive (GnRH-ir) neurone soma size. Three weeks of food restriction resulted in a pronounced increase in GnRH-ir neurone numbers, as well as an increase in fibre intensity in the main fibre pathway to the median eminence. Taken together, these findings suggest that extended food restriction leads to modifications in the GnRH neuronal system, providing a means for temporary cessation of reproduction without gross alterations in reproductive physiology. This transient change in the hypothalmo-pituitary-gonadal axis, without pronounced changes in reproductive organ morphology, likely provides a mechanism for the rapid reinitiation of breeding in nature when local conditions provide adequate food availability.
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