Failure to induce rapid eye movement sleep by dark pulses in pigmented inbred rat strains

Studies of albino Lewis rats, pigmented Brown Norway rats, and their F2 backcross progeny have demonstrated that the ability to trigger rapid eye movement (REM) sleep by turning off cage lights (dark pulses) is associated with albinism in these rat strains. Other studies have shown that pigmented inbred rats show REM sleep induction in the dark portion of short light:dark cycles or skin temperature changes. In the present study, these same pigmented breeds, Dark Agouti and hooded Long-Evans rats, were subjected to 5-min dark pulses and failed to show any evidence of REM sleep triggering. In fact, they showed trends towards REM sleep suppression during dark pulses. These results extend the finding that dark pulse triggering of REM sleep, readily evoked in albino rats, does not appear in pigmented rat strains.