Integration and saturation within the circadian photic entrainment pathwayof hamsters

The sensitivity of the visual pathway that subserves circadian entrainment was measured in hamsters after prior stimulation and using trains of multip le pulses. Immediately after subsaturating stimulation in the late subjecti ve night, there was a significant decrease in responsiveness that persisted for at least 1 h. The reduced responsiveness was not due to light adaptati on (shifting of the stimulus-response curve) but rather to response saturat ion, which appeared to reduce the sensitivity to subsequent stimulation and limit the maximum response of the pacemaker. The system, therefore, integr ates the total number of photons delivered in two light stimuli separated i n time by up to 1 h. The responsiveness was also,measured using stimulus tr ains containing 10-1,000 individual pulses of equal irradiance and equal to tal photons. Results suggest that this pathway is responsive to the total p hotons delivered in all of the stimuli and is not responsive to light onset s or offsets associated with individual stimuli. These data outline several fundamental characteristics of phase shifting for the circadian photic ent rainment pathway in hamsters. Knowledge of these characteristics is importa nt For designing and interpreting results of future studies to dissect the cellular and molecular nature of the mammalian circadian clock and for unde rstanding how visual information effects the cellular clock during entrainm ent.