Sensitivity and integration in a visual pathway for circadian entrainment in the hamster (Mesocricetus auratus).

Abstract

SUMMARY1. Light-induced phase shifts of the circadian rhythm of wheel-running activity were used to measure the photic sensitivity of a circadian pacemaker and the visual pathway that conveys light information to it in the golden hamster (Mesocricetus auratus). The sensitivity to stimulus irradiance and duration was assessed by measuring the magnitude of phase-shift responses to photic stimuli of different irradiance and duration. The visual sensitivity was also measured at three different phases of the circadian rhythm.2. The stimulus-response curves measured at different circadian phases suggest that the maximum phase-shift is the only aspect of visual responsivity to change as a function of the circadian day. The half-saturation constants (G.) for the stimulus-response curves are not significantly different over the three circadian phases tested. The photic sensitivity to irradiance (1/ao) appears to remain constant over the circadian day.3. The hamster circadian pacemaker and the photoreceptive system that subserves it are more sensitive to the irradiance of longer-duration stimuli than to irradiance of briefer stimuli. The system is maximally sensitive to the irradiance of stimuli of 300 s and longer in duration. A quantitative model is presented to explain the changes that occur in the stimulus-response curves as a function of photic stimulus duration.4. The threshold for photic stimulation of the hamster circadian pacemaker is also quite high. The threshold irradiance (the minimum irradiance necessary to induce statistically significant responses) is approximately 1011 photons cm-2 s-5 for optimal stimulus durations. This threshold is equivalent to a luminance at the cornea of 0-1 cdm2.5. We also measured the sensitivity of this visual pathway to the total number of photons in a stimulus. This system is maximally sensitive to photons in stimuli between 30 and 3600 s in duration. The maximum quantum efficiency of photic integration occurs in 300 s stimuli.6. These results suggest that the visual pathways that convey light information to t Present address:

The mouse Clock mutation reduces circadian pacemaker amplitude and enhances efficacy of resetting stimuli and phase–response curve amplitude

“…To determine whether photic entrainment may have been altered by the Clock mutation, we determined the resetting effects of single light pulses given to mice free running in constant darkness. Contrary to that seen in other nocturnal rodents (16,17), we found that short light pulses (Ͻ1 h in duration) were only weakly effective in phase-shifting circadian activity rhythms in C57BL͞6J mice. In addition, the variance of the phase shifts in the response to short-duration light pulses was high.…”

Responses of Suprachiasmatic Nucleus Neurons to Light and Dark Adaptation: Relative Contributions of Melanopsin and Rod–Cone Inputs

“…6). The melanopsin-based photoresponse provides the SCN with a signal that is sustained and only weakly affected by light adaptation, in line with the canonical ability of the circadian system to integrate irradiance over long time periods (Nelson and Takahashi, 1991;Dkhissi-Benyahya et al, 2007). Studies in Opn 4 Ϫ/Ϫ mice clearly demonstrate the contribution of melanopsin in photic entrainment and light-induced phase shifts (Panda et al, 2002;Ruby et al, 2002;Hattar et al, 2003;Dkhissi-Benyahya et al, 2007).…”

The Adult Visual Cortex Expresses Dynamic Synaptic Plasticity That Is Driven by the Light/Dark Cycle

“…This latter group conversely showed no recovery of FP elevation back to dark levels when light exposure was maintained into the subsequent night period. The magnitude of the light-induced phase advance of the circadian cycle in the late night is dependent on light intensity and on the duration of light exposure (Nelson and Takahashi, 1991;Meijer et al, 1992) and reflects a suppression of locomotor activity (Pittendrigh and Daan, 1976).…”