Melanopsin - Effects On Light Entrainment

Effects On Light Entrainment

Experiments have shown that entrainment to light, by which periods of behavioral activity or inactivity (sleep) are synchronized with the light-dark cycle, is not as effective in melanopsin knockout mice. Entrainment is lost entirely when melanopsin-expressing cells are killed, as these cells are also required for transmission of rod-cone light information. Mice lacking rods and cones still exhibit circadian entrainment, but also show reduced response to light. Such mice can, however, distinguish between visual patterns. The pupillary reflex is also retained in mice lacking rods and cones but has reduced sensitivity, identifying a crucial input from the rods and cones. Without melanopsin, rods, and cones, mice fail to entrain to circadian rhythms and the pupillary reflex is lost.

ipRGCs are responsible for the ability of some blind people to entrain to the 24-hour light/dark cycles despite loss of image-forming vision. These people have intact retinohypothalamic tracts that allow signaling from the ipRGCs to the suprachiasmatic nucleus. Moreover, ipRGCs have a role in conventional vision; ipRGCs allow mice without rods or cones to show non-circadian light responses and to encode illumination in the visual cortex over a million-fold range.