Retention of a 24-hour time memory in Syrian hamsters carrying the 20-hour short circadian period mutation in casein kinase-1ε (ck1ε^tau/tau)

Circadian rhythmic expression of conditioned place avoidance (CPA) was produced in Syrian hamsters homozygous for the circadian short period mutation, tau. In constant dim red light neither the 20. h endogenous period, nor a 20. h place conditioning schedule eliminated the 24. h modulation of CPA behavior described previously for wild type (wt) hamsters and other species. Tau mutants exhibited a 20. h rhythm superimposed on the 24. h modulation. The 20. h component was removed selectively with lesions of the suprachiasmatic nucleus. Wt animals conditioned on a 20. h schedule did not produce a 20. h rhythm, but still expressed the 24. h modulation. The results show that the context entrainable oscillator (CEO) underlying memory for the timing of an unconditioned stimulus, retains a period of about 24. h regardless of clock gene background (tau mutation) and/or the conditioning schedule (24 vs 20. h). Therefore the CEO responsible for time memory is distinct from the biological clock controlling activity; the underlying circadian molecular mechanisms may differ from the ubiquitous transcription-translation feedback oscillator; and time memory itself is not classically conditioned.