Photoperiodic effects on seasonal physiology, reproductive status and hypothalamic gene expression in young male F344 rats

Abstract

Seasonal or photoperiodically sensitive animals respond to altered day length with changes in physiology (growth, food intake and reproductive status) and behaviour to adapt to predictable yearly changes in the climate. Typically, different species of hamsters, voles and sheep are the most studied animal models of photoperiodism. Although laboratory rats are generally considered nonphotoperiodic, one rat strain, the inbred Fischer 344 (F344) rat, has been shown to be sensitive to the length of daylight exposure by changing its physiological phenotype and reproductive status according to the season. The present study aimed to better understand the nature of the photoperiodic response in the F344 rat. We examined the effects of five different photoperiods on the physiological and neuroendocrine responses. Young male F344 rats were held under light schedules ranging from 8 h of light/day to 16 h of light/day, and then body weight, including fat and lean mass, food intake, testes weights and hypothalamic gene expression were compared. We found that rats held under photoperiods of ≥ 12 h of light/day showed increased growth and food intake relative to rats held under photoperiods of ≤ 10 h of light/day. Magnetic resonance imaging analysis confirmed that these changes were mainly the result of a change in lean body mass. The same pattern was evident for reproductive status, with higher paired testes weight in photoperiods of ≥ 12 h of light/day. Accompanying the changes in physiological status were major changes in hypothalamic thyroid hormone (Dio2 and Dio3), retinoic acid (Crabp1 and Stra6) and Wnt/β-Catenin signalling genes (sFrp2 and Mfrp). Our data demonstrate that a photoperiod schedule of 12 h of light/day is interpreted as a stimulatory photoperiod by the neuroendocrine system of young male F344 rats.

Keywords: F344 rat; body weight; hypothalamic gene expression; photoperiod; reproduction.

Figure 1

Effect of photoperiod on body weight, food intake and testes weight. (a) Body weight gain and (b) cumulative food intake was significantly lower in L8 and L10 compared to L12, L14 and L16. (c) Paired testes weights of male F344 rats under different photoperiods. For each group, different lowercase letters above bars indicate significant differences (P < 0.05) between photoperiod groups. (d) Paired testes weights as a percentage of body weight. Data are the mean ± SEM; *** P < 0.001.

Figure 2

Effect of photoperiod on body composition. (a) Total lean tissue mass in g and (b) lean tissue mass as a percentage of body weight of F344 rats held under different photoperiods (c) Total fat tissue mass (g) and (d) fat tissue mass as a percentage of body weight in F344 rats held under different photoperiods. Data are the mean ± SEM; P < 0.05, *** P < 0.001.

Figure 3

Effect of photoperiod on hypothalamic thyroid hormone signalling. (a) Dio2 and (b) Dio3 mRNA expression under five different photoperiods. For each group, different lowercase letters above bars indicate significant differences (P < 0.05) between photoperiod groups. Data are shown as fold changes relative to L12 (± SEM). Note that, for clarity, L12 has been set to −1 in (b).

Figure 4

Effect of photoperiod on hypothalamic retinoic acid signalling genes. (a) Crabp1 and (b) Stra6 mRNA expression under five different photoperiods. For each group, different lowercase letters above bars indicate significant differences (P < 0.05) between photoperiod groups. Data are shown as fold changes relative to L12 (± SEM).

Figure 5

Effect of photoperiod on hypothalamic Wnt/β-Catenin signalling. (a) sFrp2 and (b) Mfrp mRNA expression under five different photoperiods. For each group, different lowercase letters above bars indicate significant differences (P < 0.05) between photoperiod groups. Data are shown as fold changes relative to L12 (± SEM).