A circadian rhythm of hippocampal theta activity in the mouse.

Hippocampal theta activity dominates the cortical EEG of the mouse during certain behaviors. We have therefore been able to study the circadian distribution of hippocampal theta activity by means of chronic EEG implantation and computerized EEG state scoring. Observations in six mice indicate consistent and significant circadian patterns of theta-dominated EEG, both during wakefulness (theta-dominated wake, or TDW) and during sleep (REM sleep). The probability of REM rises gradually to a maximum during the sleep period and then falls abruptly at activity onset and then falls gradually. The complementary circadian patterns of REM and TDW suggest that they may be two episodes of each coincide remarkably, as do their circadian distributions. The probability of TDW rises to a very high level at activity onset and then falls gradually. The complementary circadian patterns of REM and TDW suggest that they may be two halves of a single circadian rhythm of theta probability. This concept would be relevant in interpreting the abnormally phase-advanced pattern of REM sleep observed in human depressives.