Reduced body mass, food intake, and testis size in response to short photoperiod in adult F344 rats

Abstract

Background: Although laboratory rats are often considered classic nonseasonal breeders, peripubertal rats of two inbred strains, F344 and BN, have both reproductive and nonreproductive responses to short photoperiods. Unmanipulated adult rats have not been reported to have robust responses to short photoperiod alone, although several treatments can induce photoperiodic responses in adults. In this study, we tested the hypotheses that unmanipulated F344 rats retain responses to short photoperiod as adults and that they have the necessary elements for an endogenous circannual rhythm of sensitivity to short photoperiod.

Results: Relative to rats kept in long photoperiods (L16:D8), adult F344 rats transferred at 4.5 months of age to short photoperiods (L8:D16) had significantly lower testis size, food intake, and body weight. In a second experiment, newly weaned F344 rats underwent an initial period of inhibition of reproductive maturation, lower food intake, and lower body weight in short photoperiod or intermediate photoperiod (L12:D12) relative to rats in long photoperiod. By 18 weeks of treatment, rats in the two inhibitory photoperiods no longer differed from long photoperiod controls. In short photoperiod, rats underwent a second period of slight reproductive inhibition between weeks 35 and 48, but there was an effect on body weight and slight inhibition of food intake only in an intermediate photoperiod.

Conclusion: Male F344 rats retain photoresponsiveness as adults, with less reproductive inhibition but equivalent nonreproductive responses. There was only weak evidence for an endogenous timer controlling a circannual cycle of sensitivity to short photoperiod.

Figure 1

Running three-point means of estimated testis volume (± SE) (a) and means transformed to a percentage of the LD mean value (b) in adult F344 rats over 16 weeks in LD or SD. Asterisks indicate significant differences at P < 0.05 in tests on both the raw data and the data converted to percentages.

Figure 2

Mean ± SE of change in body weight (a) and mean ± SE of daily food intake (b) in adult F344 rats over 16 weeks in LD or SD. Mean body weight at Week 0 was 333 ± 4 g for each treatment group. Standard error bars are not shown for data points in which error bars were smaller than the symbols. Asterisks indicate significant differences at P < 0.05.

Figure 3

Running three-point means (± SE) of estimated testis volume over 0 – 14 weeks (a) and 18–52 weeks (b), and means transformed to a percentage of the LD mean value over 0 – 14 weeks (c) and 18–52 weeks (d) in F344 rats in LD, INT, or SD. Standard error bars are not shown for data points in which error bars were smaller than the symbols. 'a' indicates a significant difference between the SD and LD groups at P < 0.05, and 'b' indicates a significant difference between the INT and LD groups at P < 0.05. Panels a and b and c and d differ in scale of the y-axis.

Figure 4

Differences in synchrony of testicular regression among rats (a) treated with an abrupt change from LD to SD in Experiment 1 and (b) held in constant SD for 52 weeks in Experiment 2. The x-axis indicates the timing of lowest testis size for individuals relative to the time of the lowest mean testis size in that experiment (time "0").

Figure 5

Means (± SE) of body weight over 0 – 14 weeks (a) and 18–52 weeks (b), and means transformed to a percentage of the LD mean value over 0 – 14 weeks (c) and 18–52 weeks (d) in F344 rats in LD, INT, or SD. Standard error bars are not shown for data points in which error bars were smaller than the symbols. 'a' indicates a significant difference between the SD and LD groups at P < 0.05, and 'b' indicates a significant difference between the INT and LD groups at P < 0.05.

Figure 6

Means (± SE) of food intake over 0 – 52 weeks (a) and means transformed to a percentage of the LD mean value over 0 – 52 weeks (b) in F344 rats in LD, INT, or SD. 'a' indicates a significant difference between the SD and LD groups at P < 0.05, and 'b' indicates a significant difference between the INT and LD groups at P < 0.05. Panels a and b differ in scale of the y-axis.

Figure 7

LH concentration (mean ± SE) in F344 rats at selective time points in LD (gray shading) or SD (black bars). There were no significant differences between treatments.