Circadian regulation of phospholipid metabolism in retinal photoreceptors and ganglion cells

The neural retina is a key component of the vertebrate circadian system that is responsible for synchronizing the central circadian pacemaker to external light-dark (LD) cycles. The retina is itself rhythmic, showing circadian cycles in melatonin levels and gene expression. We assessed the in vivo incorporation of P-32-phosphate and H-3-glycerol into phospholipids of photoreceptor cells (PRCs) and retina ganglion cells (GCs) from chicks in constant illumination conditions (dark: DD or light: LL) over a 24-h period. Our findings showed that in DD there was a daily oscillation in P-32-labeling of total phospholipids synthesized in GCs and axonally transported to the brain. This metabolic fluctuation peaked during the subjective night (zeitgeber time [ZT] 20), persisted for several hours well into the subjective day and declined at subjective dusk (ZT 10-12). PRCs also exhibited an in vivo rhythm of P-32-phospholipid synthesis in DD. This rhythm peaked around ZT 22, continued a few hours into the day and declined by the end of subjective dusk. The major individual species labeled 1 h after P-32 administration was phosphatidylinositol (PI) in both PRCs and GCs. Rhythmic phospholipid biosynthesis was also observed in DD after H-3-glycerol administration, with levels in GCs elevated from midday to early night. PRGs exhibited a similar rhythmic profile with the lowest levels of labeling during midnight. Phosphatidylcholine (PC) accounted for the individual species with the highest ratio of H-3-glycerol incorporation in both cell populations at all phases examined. By contrast, in LL the rhythm of H-3-glycerol labeling of phospholipids damped out in both cell layers. Our findings support the idea that, in constant darkness, the metabolism of retinal phospholipids, including their de novo biosynthesis, is regulated by an endogenous circadian clock.