Local photic entrainment of the retinal circadian oscillator in the absence of rods, cones, and melanopsin

Abstract

Synchronization of the mammalian master circadian pacemaker to the daily light/dark cycle is mediated exclusively through retinal photoreceptors. The mammalian retina itself is also a self-sustained circadian oscillator. Here we report that the retinal molecular circadian clock can be entrained by lighting cycles in vitro, but that rods, cones, and melanopsin (Opn4) are not required for this entrainment. In vivo, retinas of Opn4;rd1/rd1 mice synchronize to light/dark cycles regardless of the phase of the master circadian pacemakers of the suprachiasmatic nuclei or the behavior of the animal. These data demonstrate that the retina uses a separate mechanism for local entrainment of its circadian clock than for entrainment of organism-level rhythmicity.circadian rhythm M ost mammalian tissues contain autonomous circadian clocks, and these oscillators are synchronized by the "master circadian pacemaker," which is localized in the suprachiasmatic nucleus (SCN) of the hypothalamus (1). The SCN controls the rhythms of diverse physiological functions, including feeding, temperature cycles, and circulating hormones, which in turn synchronize the clocks of tissues throughout the body (2). Following lesions of the SCN, animal behavior becomes arrhythmic and internal synchrony among tissues throughout the body is lost (3-5). Photic entrainment of the mammalian SCN (and thus the animal's behavior) is mediated exclusively through photoreception in the retina (6, 7). Although outer retinal (rod/cone)-based and melanopsinexpressing retinal ganglion cell-based photoreceptors are each sufficient for circadian entrainment of behavior, animals lacking rods, cones, and melanopsin (Opn4) cannot entrain to external light/dark cycles (8-10).The mammalian retina itself is also a self-sustained circadian oscillator (11), and many retinal physiologic functions oscillate with a 24-h period. These include transcription and translation of photoreceptor genes, neurotransmitter synthesis and release, interphotoreceptor coupling, disk shedding in rods, and amplitude of the electroretinogram (ERG) (12-18). More than 1,000 genes in the retina have a significant 24-h diurnal rhythm in transcript abundance (15). When the core circadian clock gene Bmal1 is deleted specifically in the retina (rendering it incapable of free-running rhythmicity), transcriptional and ERG rhythmicity are lost and ERG amplitudes are reduced at all times of day, suggesting that local circadian rhythmicity is required for normal retinal function (15). The circadian clock in the mammalian retina persists in culture, and can be measured ex vivo by neurotransmitter release or by luminescent reporters of circadianly expressed core clock genes (11,19). Importantly, the retinal rhythm of melatonin synthesis can be synchronized directly by light/dark cycles in culture (11,20), demonstrating that the SCN is not necessary for synchronization of retinal rhythms. Here we demonstrate that the retinal clock remains entrained to the light/dark cycle regardless of SCN and behavioral phase...

Neuropsin (OPN5)-mediated photoentrainment of local circadian oscillators in mammalian retina and cornea

“…The retina also manifests a local circadian clock, which regulates many important functions, such as photoreceptor disk shedding, photoreceptor gap-junction coupling, and neurotransmitter release (4)(5)(6). Surprisingly, local retinal photoentrainment does not require the SCN, and it also does not require rods, cones, or ipRGCs (7,8). Thus, the rd1/rd1;Opn4 −/− mouse retina, which has lost essentially all rods and cones due to degeneration and also has an ablated Opn4 gene (3), remains synchronized to light/dark cycles both in vivo and ex vivo (7).…”

“…Surprisingly, local retinal photoentrainment does not require the SCN, and it also does not require rods, cones, or ipRGCs (7,8). Thus, the rd1/rd1;Opn4 −/− mouse retina, which has lost essentially all rods and cones due to degeneration and also has an ablated Opn4 gene (3), remains synchronized to light/dark cycles both in vivo and ex vivo (7).…”

“…We did not examine two other pigments, retinal G protein-coupled receptor (RGR) opsin (23) and peropsin (RRH) (24). RGR opsin participates in retinoid turnover (25,26), whereas RRH is expressed principally in the retinal pigment epithelium (24), a cell layer absent in the photoentrainable ex vivo retina preparation (7).…”

The retinal clock in mammals: role in health and disease

“…Recent data suggest that the photoreceptors regulating light response of retinal rhythms differ from those used by the SCN. 44 …”

“…44 The authors propose that an ultraviolet (UV)-sensitive opsin, neuropsin (OPN5), is the sole opsin required to phase-shift the retinal clock in response to light. 45 Neuropsin is expressed in many vertebrate species and is localized in a subset of mouse RGCs, [45][46][47][48] but it is unclear whether it is expressed in ganglion cells distinct from ipRGCs.…”