Photoperiodic responsiveness in Mongolian gerbils. Department of Psychology, Shiga University, Hiratsu, Otsu

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1 The Japanese Journal of Animal Psychology, 48, 2, (1998) Short report Photoperiodic responsiveness in Mongolian gerbils TAKASHI USUI and NOBUO IBUKA1 Department of Psychology, Shiga University, Hiratsu, Otsu Abstract Photoperiodic response to a short photoperiod (SP, LD 8 : 16) was reported in Mongolian gerbils (Meriones unguiculatus). The change of testicular and epididymal weight as well as body weight was measured as a function of SP exposure of LD 8 : 16. The present findings demonstrated that Mongolian gerbils could discriminate between a long photoperiod (LP, LD 16 : 8) and a short photoperiod (SP, LD 8 : 16). Their testes as well as epididymides regressed and spontaneously redeveloped under SP exposure. Mongolian gerbils were determined from the present findings to be classified as a photoperiodic species. Key words : photoperiodism, gerbils, testes, day-length. When conducting a behavioral and a physiological experiment, we need to know the characteristics of species, such as diurnal or nocturnal, single or gregarious, surfacedwelling or cave-dwelling and photoperiodic or not. In some cases ignorance of such a trait of a species leads to erroneous interpretation of the data and may even be dangerous. It has been known in several rodent species, such as Siberian hamster, Turkish hamster and vole, that the testes regress after exposure to a photoperiod between 12 and 14h of light per day (Duncan, Goldman, DiPinto, & Stetson, 1985 ; Elliott, 1976 ; Hoffman, 1982 ; Hong & Stetson, 1986 ; Rhodes, 1989). The testes of the Syrian hamster (Mesocricetus auratus) will regress after exposure to a short photoperiod than 12.5 hours/day. common laboratory author. Phone & Fax : ; of less On the other hand, the rat (Rattus norvegicus) is considered to be a typical example of a non -photoperiodic species (Wallen & Turek, 1981 ; Wallen, DeRosch, Thebert, Losee -Olson, & Turek, 1987) : It does not show major changes in the reproductive system after exposure to a short photoperiod (SP). Although Mongolian gerbils (Meriones unguiculatus) have been used in many kinds of experiment, their photoperiodic responsiveness was not systematically investigated (Devries, Ferreira, & Glass, 1989). In this short communication we tried to determine whether Mongolian gerbils are rat-type or hamster-type in photoperiodic responsiveness. In order to do so, the change of testicular weight after exposure to LD 8: 16 was measured. Materials and Methods Animals and Procedure Forty-seven male Mongolian gerbils (Meriones unguiculatus) were purchased from Shimizu Animal Supplier in Kyoto. They were exposed to a long photoperiod (LP, LD16 : 8, lights on from 0500 to 2100) for 10 weeks 177

2 from the day of arrival, and were given ad lib access to food (Oriental Rat Mouse Hamster Chow) and water. Then they were divided into 5 groups (n=10 in LSO, n= 8 in LS6, n= 10 in LS9, n=10 in LS12 and n=9 in LS20) matched for body weight, and transferred to a short photoperiod (SP, LD 8 : 16, lights on from 0900 to 1700) for the predetermined period. The animals were designated as LSO, LS6, LS9, LS12 and LS20, respectively because they were sacrificed after the 0-, 6-, 9-, 12- and 20-week exposure to the LD 8 : 16 short photoperiod. The animals were housed in groups of five in opaque polycarbonate cages (39 x 34 x 18 cm) with wire mesh tops. Wood chips were used as cage bedding. Three cages were placed in a black light-tight chamber (130 x 50 x 40 cm) and illuminated by two 10W white differs. T-test for correlated means was used in the analysis of body weight. Data were handled using a commercial statistical computer program (Shinohara, 1983). p <.05 was considered to be statistically significant. Results Mean testicular weights per 50 g body weight after exposure to SP are shown for all groups in Figure 1. ANOVA clearly showed a high statistical difference among the 5 groups (F [4, 42] = 20.64, p <.001). Although the animals that had been in SP for 12 weeks showed decreases in gonadal weight compared to the control animals (LSO) (p >.05), the biggest decline occurred during the first 6 weeks after SP exposure (p <.05). Testicular weight in LS9 reached minimal levels after 9 weeks of SP exposure in comparison with fluorescent lights that provided approximately 400 lux at the cage floor level. Continuous ventilation of the chamber was provided by a fan. The room temperature was kept at 22 } 2 C. After exposure to the LD 8: 16 short photoperiod for the predetermined period, the animals were sacrificed using an overdose of sodium pentobarbital anesthesia (200 mg/kg). Their testes were then removed and immediately weighed in mg. Body weight of each individual was recorded between 1100 and 1400 at weekly intervals throughout testing. Statistical Analysis Data were analyzed using one way-analysis of variance to detect a statistical difference between the 5 groups for testicular weight. In the analysis of testicular weight we adopted testicular weight per 50 g body weight of each animal. For post-hoc analysis Ryan's multiple comparison method was used to show which group Figure 1. Regression and recrudescence of testes after SP exposure. Ordinates : Mean testicular weight in mg per 50 g body weight. Bars indicate SEMs. 178

3 USUI & IBUKA Photoperiodic response in gerbils that in LSO (p <.05). Spontaneous recrudescence in testicular size was evident after 12 weeks of SP exposure. After 20 weeks of SP exposure, SP animals had testes that were not significantly different from LP animals (LSO) before exposure to SP. The changes of epididymal weight after SP exposure showed a similar tendency as those of testicular weight. Namely, epididymal weight reached minimal levels after 9 weeks of SP exposure, and complete spontaneous recrudescence in the epididymis was also observed after 20 weeks of SP exposure. The changes of body weight after transfer to SP are shown for each group in Figure 2. The animals of all groups showed similarly slight declines in body weight for a few weeks after SP exposure, but thereafter gradually gained body weight. The animals of LS12 and LS20 rapidly gained body weight after 9 weeks of SP exposure. Body weight immediately before SP exposure was compared to that of the trough value after SP exposure in each group. There was a significant difference between them for each group (t [7] =3.20, p <.05 ; t[ =6.17, p <.001 ; t [9] = 5.50, p <.001 and t [8] =3.93, p <.001, in LS6, LS9, LS12, and LS20, respectively). Discussion The present findings clearly show that Mongolian gerbils can discriminate between a long photoperiod (LP) of LD 16 8 and a short photoperiod (SP) of LD 8 : 16. When they were transferred from LP to SP, their Figure 2. Changes in body weight for the 5 groups over the entire course after SP exposure. 179

4 testes and epididymides regressed as a function of SP exposure for 9 weeks, then spontaneously redeveloped after 20 weeks under SP. Norway rats (Rattus norvegicus), house mice (Mus musculus), and guinea pigs (Cavia porcellus) are very often used in various kinds of experiments. These animals maintain large functional testes after they were kept in SP. Therefore they are usually reported to be nonphotoperiodic species because the manipulation of photoperiod does not affect their reproductive competence (Dempsey, Myers, Young, & Jennison, 1934 ; Nelson, 1990 ; Wallen & Turek, 1981 ; Wallen et al, 1987). On the other hand, the testes of hamsters and voles begin to regress their testes in a few weeks when they are exposed to SP. Testicular weight reaches minimal levels in 12 or 15 weeks after exposure to SP in Syrian hamsters, and completely redevelop after weeks of SP exposure (Ibuka, 1997 ; Reiter, 1987 ; Stetson, Matt, & Watson - Whitmyre, 1976). The time course of regression and recrudescence of testes in Mongolian gerbils was similar to that in Syrian hamsters, but slightly little shorter than that in Syrian hamsters. When Mongolian gerbils were exposed to SP, they revealed not only testicular regression but also a decrease of body weight. However, the body weight of the animals of 4 groups reached minimal levels before testicular weight did. The animals began to gain weight after 9 weeks of SP exposure. The time course for the decrease and recrudescence of body weight proceeded in a parallel fashion to that of the testicular weight, but its phase was advanced in comparison with that of testicular weight. Acknowledgements This research was partly supported by Grant C ( ) of Japanese Ministry of Education, second author. Science, Culture and Sports to the References Dempsey, E. W., Myers, H. J., Young, W. C., & Jennison, D. B Absence of light and the reproductive cycle in the guinea pig. American Journal of Physiology, 109, Devries, M. J., Ferreira, S. A., & Glass, J. D Evidence that short photoperiod-induced gonadal regression in the Mongolian gerbil is mediated by the action of melatonin in the medial hypothalamus. Brain Research, 494, Duncan, M. J., Goldman, B. D., DiPinto, M. N., & Stetson, M. H Testicular function and pelage color have different critical day lengths in the Djungarian hamster, Phodopus sungorus. Endocrinology, 116, Elliott, J. A Circadian rhythms and phtoperiodic time measurement in mammals. Federation Proceedings, 35, Hoffman, K The critical photoperiod in the Djungarian hamster Phodopus sungorus. In J. Aschoff, S. Daan & G.A. Groos (Eds.), Vertebrate circadian systems. Berlin Springer- Verlag. Pp Hong, S. M., & Stetson, M. H Functional maturation of the gonads of Turkish hamsters under various photoperiods. Reproduction, 35, Ibuka, N. Biology of 1997 Comparative discussion of the seasonal adaptation in daily torpor of Djungarian hamsters and hibernation of Syrian hamsters. Japanese Journal of Animal Psychology, 47, Reiter, R. J Mechanism of control of repro- 180

5 USUI & IBUKA : Photoperiodic response in gerbils ductive physiology by the pineal gland and its hormones. In R. J. Reiter & F. Fraschini (Eds.), Advances in pineal research 2. London : John Libbey & Co, Ltd. Pp Nelson, R. J Photoperiodic responsiveness in house mice. Physiology and Behavior, 48, Rhodes, D. H The influence of multiple photoperiods and pinealectomy on gonads, pelage and body weight in male meadow voles, Microtus pennsylvanicus. Comparative Biochemistry and Physiology, A93, Shinohara, K A computer program for behavioral science (in Japanese). Vol. 1. Kyoto : Nakanishiya Shuppan. Pp Stetson, M. H., Matt, K. S., & Watson-Whitmyre, M Photoperiodism and reproduction in golden hamsters : Circadian organization and the termination of photorefractoriness. Biol - ogy of Reproduction, 14, Wallen, E. P., & Turek, F. W Photoperiodicity in the male albino rat. Nature, 289, Wallen, E. P., DeRosch, M. A., Thebert, A., Losee- Olson, S., & Turek, F. W Photoperiodic response in the male laboratory rat. Biology of Reproduction, 37, (Received April 10, 1998 ; accepted June 25, 1998) 181

Effect of One-Second Light Pulses on Testicular Function and Locomotor Activity in the Golden Hamster

BIOLOGY OF REPRODUCTION 28, 557-565 (1983) Effect of One-Second Light Pulses on Testicular Function and Locomotor Activity in the Golden Hamster DAVID J. EARNEST2 and FRED W. TUREK Department of Neurobiology