Binocular contributions to the responsiveness and integrative capacity of the circadian rhythm system to light.

The retinohypothalamic tract (RHT), a monosynaptic retinal projection to the SCN, is the major path by which light entrains the circadian system to the external photoperiod. The circadian system of rodents effectively integrates or counts photons, and the magnitude of the rhythm phase response is proportional to the total energy of the photic stimulus. In the present studies, responsiveness to light and integrative capacity of the circadian system were tested in hamsters after reduction of retinal photoreceptor input by 50%.

Functional analysis of the role of the median raphe as a regulator of hamster circadian system sensitivity to light.

The retinohypothalamic tract, a monosynaptic retinal projection to the suprachiasmatic nucleus (SCN), is the path by which light entrains the circadian system to the external photoperiod. Serotonergic neurons in the mesencephalic median raphe nucleus (MnR) also give rise to a major SCN afferent projection. The present study was designed to determine the extent to which MnR serotonergic projections regulate sensitivity of the circadian rhythm system to light.

Crossed and uncrossed retinal projections to the hamster circadian system.

The hamster suprachiasmatic nucleus (SCN), site of the circadian clock, has been thought to be equally and completely innervated by each retina. This issue was studied in animals that had received an injection of the tracer cholera toxin subunit B (CTb) conjugated to Alexa 488 into the vitreous of one eye, with CTb-Alexa 594 injected into the other. Retinal projections to the SCN and other nuclei of the circadian system were simultaneously evaluated by using confocal laser microscopy.