Effects of photoperiod, melatonin, and the pineal gland on compensatory gonadal hypertrophy during postnatal development in the marsh rice rat (Oryzomys palustris).

The roles of photoperiod, melatonin, and the pineal gland in regulating the magnitude of compensatory gonadal hypertrophy (CGH) and other reproductive and non-reproductive organ growth during post-weaning development were examined in the marsh rice rat Oryzomys palustris. Juvenile rice rats of both sexes were left gonadally intact (control group) or unilaterally castrated (ULC) and housed on 12L:12D, 14L:10D, or 16L:8D. Within a photoperiod (14L:10D and 16L:8D, but not 12L:12D), growth of the remaining testis, but not the remaining ovary, as well as several additional organs in both sexes were significantly affected, suggesting that the compensatory hypertrophy of the testis is photoperiod-dependent. There was no effect of testis asymmetry on CGH as ULC of either testis in rice rats housed on 14L:10D resulted in a comparable increase of CGH. Melatonin implants in rice rats maintained on 16L:8D had little to no effect (CGH included) on most parameters examined. Both melatonin implants and pinealectomy (separate experiments) in rice rats transferred to 12L:12D prevented short photoperiod-induced effects on CGH, the growth of the reproductive organs and the Harderian glands. Evening melatonin injections had a significant inhibitory effect on the growth of the remaining testis (no CGH was observed) and all other parameters measured. Lastly, ULC did not alter the percentage of males which successfully mated compared to intact animals. Taken together, these data suggest that photoperiod, melatonin, and the pineal gland can affect and regulate reproductive (e.g., CGH in some cases) and non-reproductive growth during postnatal development in the marsh rice rat.

Food availability and photoperiod affect reproductive development and maintenance in the marsh rice rat (Oryzomys palustris).

To examine whether photoperiod and food availability interact to influence reproductive development (Experiment 1), we exposed juvenile male and female rice rats to 16L:8D or 14L:10D and to ad lib, 80% of ad lib, or 60% of ad lib food intake from 3 to 8 weeks of age and recorded body and reproductive organ masses. Absolute paired testis masses were similar in ad lib and 80% of ad lib groups but significantly different than the 60% of ad lib group in both photoperiods. Relative paired testis masses were significantly different in the 80% and 60% of ad lib groups on 16L:8D only. Absolute seminal vesicle masses (SVM) were directly dependent upon the level of food restriction in both photoperiods, but relative SVMs were different only in the 60% of ad lib group. Terminal body masses were also directly dependent upon the level of food restriction and were greater on 16L:8D than on 14L:10D at most levels of food availability. In juvenile females, absolute uterine mass was only affected in the 60% of ad lib group on 14L:10D, while absolute paired ovary masses were affected on both photoperiods in the 60% of ad lib groups only. There was no effect of photoperiod or food on relative uterine and paired ovary masses. Terminal body mass was affected by food intake in both photoperiods. Lastly, in adult males (Experiment 2), photoperiod and food restriction affected reproductive function. Within a photoperiod, there was no effect of food restriction (75% of ad lib) on the testes, seminal vesicles, or testosterone levels in animals housed on 16L:8D, but terminal body mass was significantly reduced. On 12L:12D, however, food restriction significantly decreased testes and SVMs relative to ad lib-fed controls. Testosterone levels were reduced regardless of food availability. There was no effect of food restriction on terminal body mass. These results suggest that multiple potential environmental cues can be utilized to affect gonadal status in both juvenile and adult marsh rice rats.