REST-ACTIVITY RHYTHM OF THE BLIND MOLE-RAT SPALAX-EHRENBERGI UNDER DIFFERENT LIGHTING CONDITIONS

The mole rat is a solitary, subterranean and photoperiodic rodent. We investigated its rest activity behavior under several lighting conditi ons, complemented our observations with light-induced c-fos expression , and compared the activity behavior of two chromosomal forms (2n = 58 and 60). The 26 mole rats had a clear overall preference for activity in the light or dark period, but prolonged recordings in five individ uals showed that the initial preference was not stable in the nocturna l animals; they became diurnal. A 6-h advance of the light-dark (LD) c ycle induced a shift of activity and the previous LD preference was re established. The large daily variability of activity onset did not all ow this study to determine whether the animals were entrained to the L D cycle upon release into constant darkness (DD) or whether activity h ad been masked by light. The period of the motor activity rhythm in DD free ran in more than 50% of the animals. No differences in activity were observed between the two karyotypes. Immunohistochemistry for c-f os expression in the nucleus suprachiasmaticus at different circadian times showed that c-fos was induced only in animals exposed to a 1-h l ight pulse during the subjective night, but not during the subjective day or in control animals in the absence of a light pulse. The large i ntra- and inter-individual variability in daily motor activity both in LD and in DD suggest only a weak photic entrainment of the circadian clock to light of approximately 100 lux, and possibly a weak regulatio n of behavior by the circadian clock. (C) 1998 Elsevier Science B.V. A ll rights reserved.