Light is a rich source of information for animals. The spectral composition of light in the sea is an indication of the depth, since short-wavelength light pene-trates deeper than long-wavelength light. Day-night transitions are marked by fast changes of light intensity and spectral composition (twilight). The length of day-time is indicative of the season at non-tropical latitudes. And finally, moonlight signals quasi-monthly cycles. All this relevant information can be extracted from the environment using simple irradiance detectors, not associated with vision.
In animals, physiology and behaviour are regulated according to daily, monthly and seasonal rhythms. These rhythms are organized by neuronal networks subdi-vided into three components: core internal clocks, constituted by internal oscillators;
input pathways, to synchronize the clock to the environment; and output signals.
Light is the main input to the clocks, since environmental illuminance is the most robust and “reliable” source of time information.
At any level of the tree of life, light cycles regulates several physiological pro-cesses, like metabolism, growth, metamorphosis, reproduction and even cell cycle (Schultz and Kay, 2003; Kohsaka and Bass, 2006; Bradshaw and Holzapfel, 2007;
Hunt and Sassone-Corsi, 2007). In many instances, this is achieved by coupling cir-cadian rhythms, photoreception or both to hormonal release (Morgan and Hazlerigg, 2008; Nakane and Yoshimura, 2010). Metabolic networks and circadian rhythms are intimately interlocked: genes with a direct role in metabolic control belong to cir-cadian oscillators, while food itself contributes to the entrainment of central and peripheral clocks. These mechanisms are probably very important for adaptation
and optimal energy storage and utilization (Kohsaka and Bass, 2006; Yang et al., 2006).
Behavioural rhythmicity is also widespread in the animal kingdom. The existence of sleep-like behaviours has been documented in vertebrates and protostomes (Al-lada and Siegel, 2008). In many cases, rhythmic behaviour is directly associated to metabolic cycles and energy balance (Green et al., 2008).
The fundamental importance of rhythmic processes explains why mechanisms for illuminance detection and circadian rhythms are present in most animals, and probably why photoreception evolved in the first place (Nilsson, 2009).
2.2.1. Illuminance detection and circadian rhythms in protostomes
The genetics of circadian clocks has been dissected in Drosophila. The core circa-dian oscillator is constituted by transcriptional feedback loops and rhythmic phos-phorylation of the two DNA-binding heterodimers Clock/Cycle (Clk/Cyc), mostly active during the late day, and Period/Timeless (Per/Tim), mostly active at night (fig. 2.3A; for details, see Allada and Chung 2010; Peschel and Helfrich-Förster 2011). These genes are expressed in about 150 neurons of the adult Drosophila brain;
some of these neurons are located in the accessory medulla, which is considered the true circadian pacemaker of insects (Helfrich-Förster, 2004), while others are found in the insect neurosecretory centers, the pars intercerebralis (PI) and the pars lateralis (PL). The most important output of the circadian system is the neuropep-tide pigment-dispersing factor (PDF), expressed in several clock neurons including the accessory medulla; other transmitter systems, like serotonin, acetylcholine, his-tamine, GABA and several neuropeptides, have also been implicated downstream the clock (Yuan et al., 2005; Johard et al., 2009; Peschel and Helfrich-Förster, 2011).
Multiple pathways participate to the light entrainment, to ensure a sophisticated temporal fine-tuning of behaviour. The blue-light receptor cryptochrome (dCRY) is expressed in a subset of clock neurons, and when activated by light it triggers the degradation of the tim protein. Moreover, subsets of the Drosophila clock neurons are directly innervated by the visual system (Helfrich-Förster et al., 2001; Collins and Blau, 2007; Dubruille and Emery, 2008). At larval stages, the cholinergic larval eye called Bolwig organ innervates some pdf+ bsh+1 cells in the prospective accessory medulla (Jones and McGinnis, 1993). After metamorphosis, the Bolwig organ trans-forms in a new photoreceptive structure, called Hofbauer-Buchner (HB) eyelet, but
1The Drosophila bsh gene is the homolog of the vertebrate bsx.
clk
Figure 2.3: Circadian organization in Drosophila. A. The molecular oscillators of the Drosophila clock.
Redrawn from Gallego and Virshup (2007). B. The organization of the adult Drosophila circadian circuits. Clock neurons are distributed in the accessory medulla, and in the pars intercerebralis (PI) and pars lateralis (PL). The clock neurons receive photic input from the eye rhabdomeric photoreceptors (R7/8, R1/6) and from the Hofbauer-Buchner eyelet (H-B). DN: dorsal neurons, LN: lateral neurons, AL: antennal lobe, MB: mushroom body, Ca: calix, CC: central complex, Oc: ocelli. Adapted from Helfrich-Förster (2005).
still makes contacts with the pdf+ neurons of the accessory medulla (Helfrich-Förster et al., 2002; Malpel et al., 2002). In the adult, the HB eyelet, the ocelli and the eyes are all necessary, together with dCRY, to clock entrainment (fig. 2.3B, Helfrich-Förster et al. 2001).
A comparable system is in place in crustaceans (Strauss and Dircksen, 2010). The presumed crustacean pacemaker is located in the eyestalk, thus close to the optic lobes, like in insects. Behaviour and neurosecretion are regulated by clock neurons through different kinds of outputs, like PDH (the homolog of the insect PDF), several other neuropeptides, and serotonin. The entrainment requires cryptochromes, eyes and extraretinal photoreceptors.
The presence of c-ops1+ bmal+ cells in the brain of annelids and some arthropods suggests that the c-opsin-dependent input to the circadian clock might have been present the the last common ancestor of Ecdysozoa, but lost in Drosophila (Arendt et al., 2004; Velarde et al., 2005).
The role of PDF as a clock output might be a conserved feature of protostomes, since pdf orthologs have been found in lophotrochozoans, but not in cnidarians and deuterostomes (Anctil, 2009; Veenstra, 2010, 2011). Clock genes have been cloned
and analyzed in several protostomes, but nothing is known about the neurobiology of clocks outside the few established model systems. It is likely that the Drosophila situation is partially derived, after several instances of gene loss (discussed later).
2.2.2. Illuminance detection and circadian rhythms in vertebrates
The vertebrate circadian system has been dissected in great detail for the mouse.
The circadian oscillator is based on a transcriptional-translational feedback loop, which involves the DNA-binding heterodimers Clock/Bmal (bmal is the ortholog of the Drosophila cycle) and Per/Cry (fig. 2.4A).
The main circadian oscillator of the mouse brain is the suprachiasmatic nucleus (SCN), located in the hypothalamus. The SCN receives photic information from the ipRGCs, and innervates several brain nuclei (mostly in the hypothalamus) to control hormonal release and locomotor rhythmic activity, like sleep-wake cycles (fig. 2.4B).
Besides the SCN, the olfactory bulb and the retina have self-sustained indepen-dent oscillators. Several other brain nuclei are semiautonomous and slave oscillators (circadian oscillator that are synchronized by the master clock), meaning that most of the brain activity follows circadian rhythms (fig. 2.4D, Dibner et al. 2010). Fi-nally, peripheral circadian oscillators exist in other tissues outside the CNS, like the liver. These oscillators are not entrained by light directly, but are synchronized to the master clock through neuroendocrine pathways (Dibner et al., 2010).
The hormone melatonin is the main systemic output of the vertebrate circadian system. Melatonin is produced in several tissues, but only the pineal gland releases melatonin in the circulation. The activity of the pineal gland is controlled by the SCN through a multisynaptic pathway, involving the autonomic nervous system (fig. 2.4B).
The structure of the circadian system in anamniotes is different, because more self-sustained oscillators exist in the brain, and the role of the SCN has a less dominant role in circadian organization.
In anamniotes, the pineal organ is formed by photoreceptors and projection neu-rons; the photoreceptors have a autonomous clock, thus the melatonin release is controlled locally by the circadian rhythm and the direct photic input. Moreover, the pineal projects to several nuclei of the hypothalamus and the preoptic area, in-cluding the SCN, and participates to the entrainment of the clock (fig. 2.4C).
The organization of the mammalian and the insect circadian systems have been compared at several levels. Despite the neuronal structure of the central pacemakers
Figure 2.4: Circadian organization in vertebrates. A. The molecular oscillators of the mammalian clock. Redrawn from Gallego and Virshup (2007). B. Circadian organization in mammals. The ipRGCs of the retina (Ret) send photic information through the retino-hypothalamic tract (RHT) to entrain the SCN. The SCN projects to several hypothalamic nuclei to regulate circadian processes. One of these nuclei is the periventricular nucleus (PVN), that projects to the intermediolateral nuclei (IML) of the spinal cord through the mesencephalic periaqueductal grey (PAG). The IML innervates the superior cervical ganglia (SCG) that projects to the pineal gland (Pin) via the internal carotid nerve (ICN). The pineal releases melatonin in the circulation, which acts as the systemic signal for darkness. The pres-ence of self-sustained circadian oscillators is indicated in pink, slave oscillators in green. Yellow arrow:
light input to the clock. Adapted from Klein et al. (2010); C. Circadian organization in the teleost fish. Retinal ganglion cells and pineal projection neurons send projections to the hypothalamus, like to the SCN and the preoptic area (POA). The pineal has photoreceptors with a self-sustained circadian oscillator (pink). So the rhythm of melatonin release is regulated in the pineal directly, without any input from the hypothalamus. More semiautonomous and slave oscillators exist in the fish brain (not shown). Yellow arrows: light inputs to the clock. D. Circadian oscillators in the mammalian brain. As above, the presence of self-sustained circadian oscillators is indicated in pink, semiautonomous oscilla-tors in blue and slave oscillaoscilla-tors in green. AMY, amygdala; ARC, arcuate nucleus; BNST, bed nucleus of the stria terminalis; CB, cerebellum; CX, cortex; DG, dentate gyrus; DMH, dorsomedial hypothala-mus; DRN, dorsal raphe nucleus; HB, habenula; Hip, hippocampus; LH, lateral hypothalahypothala-mus; ME, median eminence; MRN, median raphe nucleus; NAc, nucleus accumbens; NTS, nucleus of the solitary tract; OB, olfactory bulb; OVLT, vascular organ of the lamina terminalis; Pi, piriform cortex; Pin, pineal gland; Pit, pituitary gland; PVN, paraventricular nucleus of the hypothalamus; PVT, paraventricular nucleus of the thalamus; Ret, retina; RVLM, rostral ventrolateral medulla; SCN, suprachiasmatic nu-clei; SON, supraoptic nucleus; VLPO, ventrolateral preoptic area; VTA, ventral tegmental area. Image from Dibner et al. (2010).
(SCN and accessory medulla) are similar (Helfrich-Förster, 2004), the main output systems used are different (melatonin and PDF, respectively). In any case, the en-trainment of the clock involves both cryptochromes and opsins; however, mammals (but not all the other vertebrates!) apparently lost both the cryptochrome and the ciliary photoreceptor input, while Drosophila lost the ciliary photoreceptor compo-nent.
Looking at the circadian organization in a slow evolving lophotrochozoan, like Platynereis, will help to reconstruct the ancestral state at the base of Bilateria. The bmal-expressing ciliary photoreceptors of Platynereis brain, probably involved in an-nelid circadian entrainment, need to be compared to the ciliary photoreceptors in-volved in entrainment of the vertebrate clock. For this reason, here I will describe in more detail the non-visual ciliary photoreceptors of vertebrates.