Retention of a 24-hour time memory in Syrian hamsters carrying the 20-hour short circadian period mutation in casein kinase-1[straight epsilon] ( ck1 [straight epsilon] tau/tau )

* Hamsters show peak conditioned avoidance at the time of day of conditioning. * Short period mutants ( tau = 20) retain both 24h and 20h performance peaks. * The 20h response is eliminated by suprachiasmatic nucleus ablation. * Conditioning at 20h intervals is followed by a 24h response performance rhythm. * CPA rhythms are produced by a non-canonical circadian clock mechanism. 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 20h endogenous period, nor a 20h place conditioning schedule eliminated the 24h modulation of CPA behavior described previously for wild type (wt ) hamsters and other species. Tau mutants exhibited a 20h rhythm superimposed on the 24h modulation. The 20h component was removed selectively with lesions of the suprachiasmatic nucleus. Wt animals conditioned on a 20h schedule did not produce a 20h rhythm, but still expressed the 24h modulation. The results show that the context entrainable oscillator (CEO) underlying memory for the timing of an unconditioned stimulus, retains a period of about 24h regardless of clock gene background (tau mutation) and/or the conditioning schedule (24 vs 20h). 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.