Influence of deuterium oxide on circadian activity rhythms of hamsters: Role of the suprachiasmatic nuclei

The period of the free-running circadian activity rhythm of Syrian hamsters was measured before and during treatment with 10% deuterium oxide (D₂O). Deuteration increased period length by approximately 0.5 h per cycle both pre- and postoperatively in hamsters sustaining complete, incomplete or no unilateral lesions of the suprachiasmatic nuclei (SCN). Neither coupling between the bilaterally paired SCN, nor elimination of 50% of SCN tissue affected period length during D₂O treatment. However, variability of the response to D₂O was much greater in lesioned than in intact hamsters. We propose that a small percentage of the normal complement of SCN neurons is sufficient to permit full responsiveness of the circadian system to D₂O and that there is substantial redundancy in the neural system that responds to deuterium. Stability of the circadian system appears to be increased by the full complement of SCN neurons.