The Role of TASK-3 Two-Pore Domain Potassium Channels in the Entrainment of Mammalian Circadian Rhythms

In mammals light is the principal timing cue for alignment of physiology to the external environment. Illumination from the unrelenting 24-hour day-night cycle enters the biological system and is communicated to the master pacemaker, the suprachiasmatic nucleus (SCN) to drive circadian entrainment.

The decoding of light by the retina and the signalling pathways to and from the SCN rely on neural excitation mechanisms, achieved through changes in membrane potential from a resting state stabilised by K2P channels. With TASK-3 being the most abundant K2P channel in the rodent SCN it is feasible this channel has a crucial role in regulating SCN neural transmission for effective circadian entrainment. This study investigates this role through the use of transgenic TASK-3 KO mice.

In the first experimental chapter I demonstrate the presence of TASK-3 mRNA in the SCN and retina of wild type mice. Further, I reveal a circadian pattern in TASK-3 mRNA expression with significant midday nadir which feasibly influences resting membrane potential (RMP) supporting increased neuronal excitation reported at this time.

The following three chapters explore TASK-3 conductance in behavioural output rhythms via locomotor activity studies under light-dark cycles and in constant darkness. This series of experiments highlights how TASK-3 is essential for effective adjustment to changing light and how loss of this channel reduces light-driven and endogenous activity intensity and rhythm amplitude.

With light entering the circadian system exclusively via the eyes, the role of TASK-3 at the level of the retina is of upmost importance to entrainment. This is investigated in chapter 6 using pupillary light reflex as a measure of retinal sensitivity and decoding capacity. Through manipulation of intensity and wavelength specific classes of photoreceptor are studied for their contribution to this non-image forming response. These experiments show TASK-3 ablation significantly attenuates retinal sensitivity to sub-saturating light in a mechanism likely to be melanopsin-independent.

Finally examination of mRNA expression of core clock genes reveals the role of TASK-3 at the level of the SCN. Here, loss of TASK-3 conductance is shown to alter daily rhythms in several key genes thereby linking the properties of this background leakage channel to the molecular clockwork.

Overall these experiments demonstrate some of the roles TASK-3 conductance plays within the SCN and in output rhythms; and the requirement of this channel within the retina for effective retinal decoding across the visible spectrum over a range of light intensities.