Effect of photoperiod on the development of wheel-running activity rhythms in LEW/Ztm rats.

Wheel-running activity of LEW/Ztm rats is characterized by a multimodal pattern consisting of two activity bouts about 3-5 h apart. In this study we investigated the development of activity rhythms in LEW rats born and raised under three different photoperiods (LD 18:6, LD 12:12, and LD 6:18). Wheel-running activity was measured for 6 weeks in LD and for another 6 weeks in constant darkness (DD). The length of the photoperiod influenced the phase relationship between the two activity bouts only during the first week after weaning. Then, the characteristic activity pattern was established independently of the length of the photoperiod. However, development under long photoperiods (LD 18:6) resulted in a temporary increase in the level of activity and a significant shortening of the free-running period under DD. These results indicate that the multimodal activity pattern displayed by LEW rats is controlled by separate activity oscillators that establish their unique phase relationship early during development.