Circadian control of retinal sensitivity

The rotation of the earth around its axis and the sun generates rhythmic daily and seasonal cycles in our world. Organisms have evolved a biological clock to be able to predict and follow environmental cyclic rhythms to control their physiology and behavior for survival. In mammals the master clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus which receives light inputs via retinohypothalamic tract directly from the retina to be entrained to external cycles. Circadian rhythms are maintained by two negative feedback loops involving several clock genes such as Clock, Bmal1, Period, Cryptochrome and Rev-erbα. Apart from the central master clock, there are several peripheral clocks controlling local feature of organs in synchrony with the SCN. One of these peripheral circadian oscillators is the retina which is the one that processes the light cues and may influence the circadian rhythms generated by the SCN.

We are interested how the visual system adjust itselves to the ever changing light environment such that it can optimally extract information from its visual surroundings. Literature indicates that in vertebrates, the retinal circadian clock reconfigure the retinal circuits to optimize cone vision in the day-time and rod vision in the night-time. A circadian intercellular feedback loop of melatonin and dopamine seems to be involved in this process (Dowling, 1986, Doyle et al., 2002). Fundamental retinal processes are under circadian control, including photoreceptor disc shedding (LaVail and Ward, 1978), responses of photoreceptors and On-bipolar cells to light (Manglapus et al., 1998; Barnard et al., 2006), horizontal cell coupling (Dowling & Ehinger, 1978; Hedden & Dowling, 1978), and rod-cone and rod-rod coupling (Ribelayga et al., 2008; Jin and Ribelayga 2016).

Interestingly, although many core clock proteins are expressed in different retina layers, only cones express all the clock proteins (Liu et al., 2012). This poses the question how the circadian processes in the retina are controlled. The goal of this research line is to explore the role of the clock in remodeling of the retinal network that occurs every morning and evening to prepare the retina for the expected light environment.