Abstract
Purpose:
In mammals, the master circadian pacemaker resides in the suprachiasmatic nucleus (SCN) of the hypothalamus and synchronizes peripheral circadian oscillators distributed throughout the body to produce a proper temporal order between individual oscillators. Daily, the SCN aligns its circadian clock to the light-dark cycle through retinal input from intrinsically photosensitive retinal ganglion cells (ipRGCs). ipRGCs are capable of driving circadian photoentrainment, either through the intrinsic melanopsin-based phototransduction pathway or using the extrinsic rod/cone pathway. Recent results from our laboratory show that a molecularly defined sub-population of ipRGCs totaling only 200 cells is sufficient to drive circadian photoentrainment. The goal of this study is to determine whether these 200 cells are sufficient for rod/cone input to drive light-dependent circadian behaviors by deleting the melanopsin phototransduction pathway in these 200 ipRGCs.
Methods:
The majority of ipRGCs express a POU domain transcription factor, Brn3b, which is important for RGC differentiation. We utilized our melanopsin Cre line in combination with a mouse line that expresses diphtheria toxin A subunit in a Cre-dependent manner under the Brn3b promoter to specifically ablate Brn3b-positive ipRGCs in vivo, without influencing conventional RGCs or Brn3b-negative ipRGCs.
Results:
We found that without melanopsin, rod/cone input through Brn3b-negative ipRGCs (200 cells) is sufficient to allow circadian photoentrainment at high light intensities. Interestingly, animals that contain only melanopsin-deleted Brn3b-negative ipRGCs display an advanced phase angle of entrainment to the light-dark cycle, where their activity onset occurs earlier compared to control groups, similar to the early sleep onset experienced in some humans known as advanced sleep phase syndrome or ASPS. In addition, using light-dark cycles that model eastward and westward transmeridian travel, we show that these animals respond differentially to advanced (westward) versus delayed (eastward) dark onset.
Conclusions:
This study is the first to dissociate the ability of an animal model to differentially photoentrain to an eastward versus westward travel. This opens a new avenue to understand how eastward and westward travel adjustments differ. One idea is that direct effect of light on behavior (masking) may contribute to delayed dark onset entrainment in mice.
Keywords: 648 photoreceptors •
688 retina