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Abstract

Djungarian hamster (P. sungorus) and Roborovskii hamster (P. roborovskii) belong to the same genus of phodopus. Roborovskii hamster shows high locomotor activity and low level of dopamine (DA) in the brain. Administration of L-tyrosine, a precursor of DA, decreases locomotor activity in Roborovskii hamsters. However, the amino acid metabolism in relation to the hyperactivity is not yet well known. In the present study, L- and D-amino acid concentrations in the brain, liver, and plasma in Djungarian and Roborovskii hamsters were investigated during day and night times to explain the possible difference in hyperactivity between them. Most of the examined amino acids were higher in the night time when hamsters are active compared to those in day time. L- and D-tyrosine concentrations were higher in the liver of Roborovskii hamsters than in Djungarian hamsters. Furthermore, brain concentration of D-tyrosine was higher in the Roborovskii than in Djungarian hamsters, but no significant difference was observed for L-tyrosine concentrations between the two species. These results suggest that the conversion of L-tyrosine to D-tyrosine in the brain of Roborovskii hamster may be higher than in Djungarian hamster, which may cause low DA concentration and hyperactivity in Roborovskii hamster. On the other hand, L- and D-serine, which are known as sedative factors, were lower in Roborovskii hamsters than Djungarian hamster. These results suggest that species-specific regulation in amino acid metabolism may contribute to hyperactivity in Roborovskii hamsters.

Animals and housing conditions

Male Djungarian hamsters (n = 14) and Roborovskii hamsters (n = 14), 3 weeks of age, were purchased from a local dealer similarly as reported previously (Kabuki et al. , ,[], ). We used the minimum number of animals in each group considering the statistical power of sample size and reared under controlled environments. They were housed in a group of three or four animals per cage and freely given access to a standard diet (MF; Oriental Yeast, Tokyo, Japan) and water. A 12-h day/night cycle was maintained throughout the experiment, with light on at 08:00 and off at 20:00. Room temperature was maintained at 23 ± 1°C and humidity at 60%. This study was conducted according to the guidelines for animal experiments in the Faculty of Agriculture and on the Graduate Course of Kyushu University, and to Law No. 105 and Notification No. 6 of the government.

Tissue preparation

After an acclimation period of 8 days, the brains of Djungarian hamsters and Roborovskii hamsters were obtained during the day time (14:00) when hamsters are resting and the night time (02:00) when hamsters are active (n = 7 in each group). Hamsters were decapitated under anesthesia with isoflurane (Escain®, Mylan, Osaka, Japan), and trunk blood was collected each time. The whole brains were carefully taken out from the skulls and placed on a cold glass dish. The livers were taken, and washed with saline. The plasma separated from brood, which was centrifuged at 4°C, 3000 × g for 20 min. The tissue samples were frozen in liquid nitrogen, and all the samples were stored at −80°C until analysis.

Results

In the brain, both L- and D-tyrosine as well as L- and D-serine levels were significantly (P < 0.05) higher at night than day time. D-Tyrosine level was significantly (P < 0.01) higher in Roborovskii than in Djungarian hamsters (Figure B), but no significant difference was found in L-tyrosine between the two species (Figure A). Both L- and D-serine levels were significantly (P < 0.01) higher in Djungarian than Roborovskii hamsters (Figure C,D).