Spontaneous ‘regression’ of enhanced immune function in a photoperiodic rodent Peromyscus maniculatus

Abstract

Short days inhibit reproduction and enhance immune function in deer mice (Peromyscus maniculatus). Their reproductive inhibition is sustained by an endogenous timing mechanism: after ca. 20 weeks in short days, reproductive photorefractoriness develops, followed by spontaneous recrudescence of the reproductive system. It is unknown whether analogous seasonal timing mechanisms regulate their immune function or whether enhanced immune function is sustained inde¢nitely under short days. In order to test this hypothesis, we housed adult male deer mice under long (16 h light day 71 ) or short (8 h light day 71 ) day conditions for 32 weeks or under long day conditions for 20 weeks followed by 12 weeks of short days. Mice under the long day conditions remained photostimulated over the 32 weeks, whereas mice housed under the short day conditions exhibited gonadal regression followed by photorefractoriness and spontaneous recrudescence. Mice transferred to short days at week 20 were reproductively photoregressed at week 32. Total splenocytes, relative splenic mass and mitogen-activated splenocyte proliferation were greater in those mice transferred to short days at week 20 than in those mice housed under either long or short day conditions for 32 consecutive weeks, and immune function in mice exposed to short days for 32 weeks was comparable with that of long day animals. These data suggest that short day enhancement of immune function is not inde¢nite. With prolonged (4 32 weeks) exposure to short days, several measures of immune function exhibit`spontaneous' regression, restoring long day-like immunocompetence. The results suggest that formal similarities and, possibly, common substrates exist among the photoperiodic timekeeping mechanisms that regulate seasonal transitions in reproductive and immune function.

Melatonin Regulates Energy Balance and Attenuates Fever in Siberian Hamsters

“…Our study is the first to suggest that melatonin may organize fever expression in an adaptive and ecologically relevant manner. In agreement with a previous report (40), immune responses appear to follow a similar time course to changes within the reproductive axis, with several weeks of exposure to a given photoperiod necessary to elicit changes. Interestingly, the attenuation of fever amplitude in 1-wk MEL-treated hamsters may reflect preliminary changes in immune function after 1 wk of exposure.…”

Seasonal changes in vertebrate immune activity: mediation by physiological trade-offs

“…20-24 weeks) and then 'spontaneously' regrow their testes. Melatonin does not affect immune activity in vivo or in vitro in refractory rodents (Prendergast & Nelson 2001;Prendergast et al 2002b). Latitude of origin is related to responsiveness to photoperiod as deer mice from a high-latitude population, but not mice from a low-latitude population, responded reproductively to photoperiod and exogenous melatonin (Bronson 1985).…”

Short photoperiod-induced ovarian regression is mediated by apoptosis in Siberian hamsters (Phodopus sungorus)

“…For long-day (LD) seasonal breeders, such as Siberian hamsters (Phodopus sungorus), exposure to long photoperiods (. 12.5 h of light per day; short duration of melatonin release) induces and maintains reproductive function, whereas exposure to short photoperiods (, 12.5 h of light per day; long duration of melatonin release) results in the cessation of reproductive function (Hoffmann 1986, Bronson 1989, Knopper & Boily 2000, Prendergast & Nelson 2001. Extended periods of melatonin secretion negatively affect reproductive activity via inhibition of the hypothalamic-pituitary-gonadal (HPG) axis.…”