Carbon Monoxide as a Potential Therapeutic Agent: A Molecular Analysis of Its Safety Profiles

Carbon monoxide (CO) is endogenously produced in mammals, with blood concentrations in the high micromolar range in the hemoglobin-bound form. Further, CO has shown therapeutic effects in various animal models. Despite its reputation as a poisonous gas at high concentrations, we show that CO should have a wide enough safety margin for therapeutic applications. The analysis considers a large number of factors including levels of endogenous CO, its safety margin in comparison to commonly encountered biomolecules or drugs, anticipated enhanced safety profiles when delivered via a noninhalation mode, and the large amount of safety data from human clinical trials. It should be emphasized that having a wide enough safety margin for therapeutic use does not mean that it is benign or safe to the general public, even at low doses. We defer the latter to public health experts. Importantly, this Perspective is written for drug discovery professionals and not the general public.

Comparison of the Rostral Epidural Rete Mirabile and the Patterns of Its Blood Supply in Selected Suiformes and Hippopotamuses

The rostral epidural rete mirabile (rete mirabile epidurale rostrale) is built of considerable small arterial vessels that anastomose with each other. This structure is formed as a result of the division of a large arterial vessel into many small arteries. Furthermore, on the other side of the rete mirabile, these small arteries are joined together to form one large artery, through which blood flows out of the rete mirabile. This system participates in decreasing the temperature of the brain and thus protects the body from thermal stress. Moreover, it influences body-water balance and has a crucial role in the retrograde transfer of neuropeptides. The goal of this study was to describe the rostral epidural rete mirabile and pathways that provide blood to this structure as well as compare it in selected Suiformes and hippopotamuses. The study was performed on desert warthogs (Phacochoerus aethiopicus), Eurasian wild boars (Sus scrofa), collared peccaries (Pecari tajacu), pygmy hippopotamuses (Choeropsis liberiensis), and common hippopotamuses (Hippopotamus amphibius). Preparations were made using the latex method and corrosion cast. An elongated shape characterizes its anatomy with a much wider rostral part than caudal part in the Eurasian wild boars, desert warthogs, and collared peccaries. The main source of blood was the branch to the rostral epidural rete mirabile branched off from the internal carotid artery. Moreover, blood enters the rete by the caudal branch and rostral branch to the rostral epidural rete mirabile. In hippopotamuses, the major source of blood was the rostral branches to the rostral epidural rete mirabile.

Carbon monoxide: a critical physiological regulator sensitive to light

The mechanism by which humans absorb therapeutic light in winter seasonal and nonseasonal depression is unknown. Bright-light-induced release and generation of blood-borne gasotransmitters such as carbon monoxide (CO) may be one mechanism. Here, 24 healthy female volunteers had peripheral blood samples drawn. Samples were collected in a dimly lit room and protected from light exposure. Samples were analyzed for CO concentrations by gas chromatography after 2 h of continuous exposure to darkness vs. bright white light. In a similar confirmatory study, 11 additional volunteers had samples analyzed for CO concentrations after 2 h of continuous exposure to gentle rocking in darkness vs. in bright white light. In the first study, light-unexposed peripheral blood had a mean CO concentration of 1.8 ± 0.4 SD ppm/g. Identically treated samples with 2 h of rocking and exposure to bright white light at illuminance 10,000 lux had a mean CO of 3.6 ± 1.2 ppm/g (p < 0.0001). Post hoc analysis of that study showed that time of day was significantly inversely associated with increase in CO concentration under bright light vs. dark (p < 0.04). In a smaller confirmatory study of 11 healthy female volunteers, after 2 h of rocking, light-unexposed peripheral blood had a mean CO of 1.4 ± 0.5 SD ppm/g. Identically treated blood samples with 2 h of exposure to bright white light at illuminance 10,000 lux had a mean CO of 2.8 ± 1.7 ppm/g (p < 0.02). In conclusion, bright-light exposure robustly increases human blood CO in vitro. This supports the putative role of CO as a physiological regulator of circadian rhythms and light's antidepressant effects. This human evidence replicates earlier data from a preclinical in vivo model. This effect may be stronger in the morning than in the afternoon.

Bilirubin as a signaling molecule

For long time bilirubin was only considered as a potentially dangerous sign of liver diseases, but it now appears clear that it is also a powerful signaling molecule. Together with potent antioxidant activities that were only reported in the last few decades, many other biological effects have now been clearly described. These include especially profound inhibitory effects on almost all effectors of the immune system, with their clinical consequences in the bilirubin-mediated protection against autoimmune and inflammatory diseases. Separate from these, bilirubin activates various nuclear and cytoplasmic receptors, resembling the endocrine activities of actual hormonal substances. This is true for the "classical" hepatic nuclear receptors, including the aryl hydrocarbon receptor, or the constitutive androstane receptor; and also for some lesser-explored receptors such as peroxisome proliferator-activated receptors α and γ; Mas-related G protein-coupled receptor; or other signaling molecules including fatty acid binding protein 1, apolipoprotein D, or reactive oxygen species. All of these targets have broad metabolic effects, which in turn may offer protection against obesity, diabetes mellitus, and other metabolic diseases. The (mostly experimental) data are also supported by clinical evidence. In fact, data from the last three decades have convincingly demonstrated the protective effects of mildly elevated serum bilirubin concentrations against various "diseases of civilization." Additionally, even tiny, micromolar changes of serum bilirubin concentrations have been associated with substantial alteration in the risks of these diseases. It is highly likely that all of the biological activities of bilirubin have yet to be exhaustively explored, and thus we can expect further clinical discoveries about this evolutionarily old molecule into the future.

Retinal venous blood carbon monoxide response to bright light in male pigs: A preliminary study

The physical mechanism by which light is absorbed in the eye and has antidepressant and energizing effects in Seasonal Affective Disorder and other forms of psychiatric major depression is of scientific interest. This study was designed to explore one specific aspect of a proposed humoral phototransduction mechanism, namely that carbon monoxide (CO) levels increase in retinal venous blood in response to bright light. Eleven mature male pigs approximately six months of age were kept for 7days in darkness and fasted for 12h prior to surgery. Following mild sedation, anesthesia was induced. Silastic catheters were inserted into the dorsal nasal vein through the angular vein of the eye to reach the ophthalmic sinus, from which venous blood outflowing from the eye area was collected. The animals were exposed to 5000lx of fluorescent-generated white light. CO levels in the blood were analyzed by gas chromatography before and after 80min of light exposure. At baseline, mean CO levels in the retinal venous blood were 0.43±0.05(SE)nmol/ml. After bright light, mean CO levels increased to 0.54±0.06nmol/ml (two-tailed t-test p<0.05). This study provides preliminary mammalian evidence that acute bright light exposure raises carbon monoxide levels in ophthalmic venous blood.

The Gonadotropins Subunits, GNRH and GNRH Receptor Gene Expression and Role of Carbon Monoxide in Seasonal Breeding Animals

Seasonality in reproduction occurs mainly in wild species and it is the result of natural selection. Signals to start or finish the period of reproductive activity, both environmental and hormonal depend on the neuroendocrine pathway - synthesis and secretion of pituitary hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), under the control of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons. Variable frequency of GnRH pulses is not only the main factor governing primary and preovulatory release of gonadotropins, but it can also play a role in the specific transcriptional activity of gonadotropin subunit genes (LHβ, FSHβandCga). However, changes in release of GnRH pulse pattern do not explain the preferential stimulation of the synthesis and secretion of gonadotropins in the annual reproductive cycle. In this regulation also a GnRH independent mechanism participates. It seems that the main factor responsible for the occurrence of the seasonal modulation of reproduction in sheep and other mammals, is significant changes in response of GnRH systems to gonadal steroids. The effect of carbon monoxide on regulation of the hypothalamic-pituitary-gonadal axis has not been studied to date. There is substantial evidence to suggest that it may play a role in the transduction of information on day length. The presence of heme oxygenase-2 in hypothalamic areas important for regulation of pituitary secretory function and in the pituitary itself suggests that carbon monoxide, the product of this enzyme, may participate in the regulation of hormone secretion by the pineal gland.

Humoral phototransduction: light transportation in the blood, and possible biological effects

In our measurements plasma and, especially, the main plasma protein, albumin, exhibits a long-lasting light-induced luminescence, which should be capable of transporting light along the blood circulation. Moreover, albumin shows intense fluorescence, with emission at 337 nm, which is controlled by bilirubin. Furthermore, it is known that tryptophan decarboxylase, the last step of serotonin formation, is directly activated by light, with a maximum at 337 nm. As a hypothesis, we propose that light-induced luminescence of plasma components, such as albumin and free radicals, transports ambient light along the blood vessels. This emission could have photochemical and photobiological effects, e.g., photomodulation of enzymes. Albumin fluorescence emission could stimulate serotonin formation at 337 nm, modulated by bilirubin. Such mechanisms could be involved in the action of light therapy on serotonin formation, melatonin suppression and circadian rhythms, both in the pathophysiology of seasonal affective disorder and major depression, and in blood pressure regulation via photovasorelaxation. The proposed model can be called humoral phototransduction.

The Heme Catabolic Pathway and its Protective Effects on Oxidative Stress‐Mediated Diseases

Bilirubin, the principal bile pigment, is the end product of heme catabolism. For many years, bilirubin was thought to have no physiological function other than that of a waste product of heme catabolism--useless at best and toxic at worst. Although hyperbilirubinemia in neonates has been shown to be neurotoxic, studies performed during the past decade have found that bilirubin has a number of new and interesting biochemical and biological properties. In addition, there is now a strong body of evidence suggesting that bilirubin may have a beneficial role in preventing oxidative changes in a number of diseases including atherosclerosis and cancer, as well as a number of inflammatory, autoimmune, and degenerative diseases. The results also suggest that activation of the heme oxygenase and heme catabolic pathway may have beneficiary effects on disease prevention either through the action of bilirubin or in conjunction with bilirubin. If so, it may be possible to therapeutically induce heme oxygenase, increase bilirubin concentrations, and lower the risk of oxidative stress-related diseases.

Erythrocyte response to near-infrared radiation

The effects of NIR (near-infrared radiation 700-2,000 nm) on bovine erythrocytes in plasma was studied as a continuation of earlier studies. Cell shape was observed and the changes of ratio of hemolysis and electrokinetic potential measured as a function of irradiation time. After 10 min of irradiation, the shape of erythrocyte cells was mainly echinocytic. When these cells were incubated at 311 K for 24 h they regained their initial shape, but fresh erythrocytes that were irradiated for 30 min and aged in vitro did not. These phenomena are due to: (1) the absorption of NIR excitation by hemoglobin; the primary photochemical process being the photo-dissociation of oxyhemoglobin to deoxyhemoglobin. Resulting shape and ratio of hemolysis, structural changes and oxidative stress follow higher deoxyhemoglobin concentration. (2) The absorption of the NIR excitation by proteins, water and lipids. After NIR absorption the membrane surface dehydrates, leading to enhanced protonation and dissociation of hydrogen-bonded complexes. This in turn leads to a change in electrokinetic potential.

Extraocular light exposure does not phase shift saliva melatonin rhythms in sleeping subjects

Preliminary work in humans suggests that extraocular light can shift circadian phase. If confirmed, extraocular light may be of therapeutic benefit in the treatment of circadian-related sleep disorders with the advantage over ocular exposure that it can be administered while subjects are asleep. In sleeping subjects, however, the effect of extraocular light exposure on circadian phase has yet to be fully tested. Likewise, there is limited data on the acute effects of extraocular light on sleep and body temperature that may influence its clinical utility Thirteen subjects [3F, 10M; mean (SD) age = 22.1 (3.0)y] participated in a protocol that totaled 7 nights in the laboratory consisting of a screening phase measurement night followed 1 week later by two counterbalanced experimental sessions each of 3 consecutive nights (habituation, treatment, and posttreatment phase measurement night) separated by 4 days. Saliva was collected for melatonin measurement every half hour from 1800 to 0300 h on the screening night and both the posttreatment phase measurement nights. On the treatment nights, continuous measures of rectal temperature and polysomnographic sleep were collected and overnight urine for measurement of total nocturnal urinary 6-sulphatoxymelatonin excretion. To test for the phase-delaying effects of extraocular light, subjects received either placebo or extraocular light (11,000 lux) behind the right knee from 0100 to 0400 h. Treatment had no significant effect on the onset of saliva melatonin secretion, phase of nocturnal core body temperature, or urinary 6-sulfatoxymelatonin excretion, but a small increase was observed in wakefulness over the light administration period. In summary, extraocular light was not shown to delay circadian phase but was shown to increase wakefulness. The authors suggest that the present protocol has limited application as a treatment for circadian-related sleep disorders.

Effects of light on low nocturnal bilirubin in winter depression: a preliminary report

BACKGROUND

The light-absorbing pigments involved in the induction of treatment of winter depression are unknown. It has been proposed that circulating bilirubin serves as a photoreceptor, in part because of its similarity to the chromophore of phytochrome, a primary time-setting plant molecule.

METHODS

We measured nocturnal bilirubin levels in nine patients with winter depression, and seven age- and gender-matched normal comparison volunteers.

RESULTS

Nocturnal bilirubin levels were lower in patients than in controls (p <.02), increased in both groups during the night (p <.0001), and increased in patients after 2 weeks of morning light treatment (p =.0009), which was accompanied by clinical improvement.

CONCLUSIONS

Low nocturnal bilirubin levels may be associated with winter seasonal depression.

Enhancement of REM sleep during extraocular light exposure in humans

This study examined the effects on sleep of light administered to an extraocular site. A 3-h photic stimulus was applied to the popliteal region during sleep in 14 human subjects. Each subject also underwent a control stimulus condition during a separate laboratory session. The proportion of rapid eye movement (REM) sleep during the 3-h light administration session increased by an average of 31% relative to the control condition. The frequency but not the duration of REM episodes was altered during light exposure, thereby shortening the REM/non-REM (NREM) cycle length. No other sleep stages were significantly affected during light administration nor was sleep architecture altered after the light-exposure interval. These results confirm that extraocular light is transduced into a signal that is received and processed by the human central nervous system. In addition, they expand to a novel sensory modality previous findings that REM sleep can be enhanced by sensory stimulation.

Light augments FOS protein induction in brain of short-term enucleated hamsters

FOS protein is synthesized in neuronal nuclei in response to a variety of environmental stimuli and has been used as a marker of stimulus-specific brain function. The present studies were initiated to examine the effects of ultraviolet light on the induction of FOS protein immunoreactivity (FOS-IR) in several brain regions of adult male hamsters. Experiment 1 confirmed previous observations of FOS-IR induced in visual cortex in response to ultraviolet light. However, protein was also induced by ultraviolet or white light in a variety of other areas and induction occurred in both sighted and enucleated animals. Therefore, experiments were conducted to evaluate the effects of a 514 nm light on FOS-IR induction in blind or sighted animals. Experiments 2 and 3 were performed during the early subjective night and mid-subjective day, respectively, using animals about 4 days after bilateral enucleation or sham surgery. In Experiment 2, light and enucleation independently and interactively resulted in increased FOS-IR neuronal nuclei counts. In Experiment 3, there was a main effect of enucleation and an interaction between enucleation and light condition, but no main effect of light. In Experiment 4, conducted during the early subjective night using animals enucleated 60 days earlier, there were neither effects of light or enucleation. The results support the view that, under certain conditions related to subjective time of day and time since enucleation, light can act through unknown extraocular mechanisms to modify brain activity. Further, short term enucleation itself induces widespread alteration in brain function as indicated by increased FOS-IR expression. The results specifically do not support a role for extraretinal photoreception with respect to direct circadian rhythm regulation.

Extraocular phototransduction and circadian timing systems in vertebrates

It is widely accepted that, for organisms with eyes, the daily regulation of circadian rhythms is made possible by light transduction through those organs. Yet, it has been demonstrated repeatedly in recent years that ocular light receptors that mediate vision, at least in mammals, are not the same photoreceptors involved in circadian regulation. Moreover, it has been recognized for many years that circadian regulation can occur in organisms without eyes. In fact, extraocular circadian phototransduction (EOCP) appears to be a phylogenetic rule for the vast majority of species. EOCP has been reported in every nonmammalian species studied to date. In mammals, however, the story is very different. This paper presents findings from studies that have examined specifically the capacity for EOCP in vertebrate species. In addition, the literature addressing noncircadian aspects of extraocular phototransduction is briefly discussed. Finally, possible mechanisms underlying EOCP are discussed, as are some of the implications of the presence, or absence, of EOCP across phylogeny.

Stimulation of reactive oxygen species production by an antidepressant visible light source

BACKGROUND

The mechanism by which visible light stimulates chronobiological phase-shifting or antidepressant effects in humans is unknown.

METHODS

Normal human NIH/3T3 nonpigmented fibroblasts were irradiated with a visible light source (SunRay) used in the treatment of winter seasonal depression. Electron spin resonance was assessed before and after 10 min of illumination at 2 mW/cm(2) (illuminance of 3700 lux), with and without the presence of 5 microL of 0.0214 mg/mL vitamin C.

RESULTS

The fibroblasts showed evidence of production of reactive oxygen species after 10 min of irradiation.

CONCLUSIONS

These in vitro data establish that an antidepressant source of visible light is capable of inducing the production of reactive oxygen species in skin. Such species may participate in signal transduction pathways leading to mood changes.

Bright light exposure of a large skin area does not affect melatonin or bilirubin levels in humans

BACKGROUND

Light treatment through the eyes is effective in alleviating the symptoms of some psychiatric disorders. A recent report suggested that skin light exposure can affect human circadian rhythms. Bilirubin can serve as a hypothetical blood-borne mediator of skin illumination into the brain. We studied whether bright light directed to a large body area could suppress the pineal melatonin secretion or decrease serum total bilirubin in conditions that could be used for therapeutic purposes.

METHODS

Seven healthy volunteers participated in two consecutive overnight sessions that were identical except for a light exposure on the chest and abdomen in the second night from 12:00 AM to 6:00 AM (10,000-lux, 32 W/m(2) cool white for six subjects and 3000-lux, 15 W/m(2) blue light for one subject). Hourly blood samples were collected from 7:00 PM to 7:00 AM for melatonin radioimmunoassays. Bilirubin was measured by a modified diazo method in blood samples taken at 12:00 AM and 6:00 AM and in urine samples collected from 7:00 PM to 11:00 PM and from 11:00 PM to 7:00 AM.

RESULTS

The skin light exposure did not cause any significant changes in serum melatonin or bilirubin levels. The excretion of bilirubin in urine was also the same in both sessions.

CONCLUSIONS

Significant melatonin suppression by extraocular light does not occur in humans. Robust concentration changes of serum total bilirubin do not have a role in mediating light information from the skin to the central nervous system.

Failure of extraocular light to facilitate circadian rhythm reentrainment in humans

Although extraocular light can entrain the circadian rhythms of invertebrates and nonmammalian vertebrates, almost all studies show that the mammalian circadian system can only be affected by light to the eyes. The exception is a recent study by Campbell and Murphy that reported phase shifts in humans to bright light applied with fiber-optic pads behind the knees (popliteal region). We tested whether this extraocular light stimulus could accelerate the entrainment of circadian rhythms to a shift of the sleep schedule, as occurs in shift work or jet lag. In experiment 1, the sleep/dark episodes were delayed 8h from baseline for 2 days, and 3h light exposures were timed to occur before the temperature minimum to help delay circadian rhythms. There were three groups: (1) bright (about 13,000 lux) extraocular light from fiber-optic pads, (2) control (dim light, 10-20 lux), and (3) medium-intensity (about 1000 lux) ocular light from light boxes. In experiment 2, the sleep/dark episodes were inverted, and extraocular light was applied either before the temperature minimum to help delay circadian rhythms or after the temperature minimum to help advance rhythms. Circadian phase markers were the salivary dim light melatonin onset (DLMO) and the rectal temperature minimum. There was no evidence that the popliteal extraocular light had a phase-shifting effect in either experiment. Possible reasons for phase shifts in the Campbell and Murphy study and not the current study include the many differences between the protocols. In the current study, there was substantial sleep deprivation before the extraocular light was applied. There was a large shift in the sleep/dark schedule, rather than allowing subjects to sleep each day from midnight to noon, as in the Campbell and Murphy study. Also, when extraocular light was applied in the current protocol, subjects did not experience a change from sleeping to awake, a change in posture (from lying in bed to sitting in a chair), or a change in ocular light (from dark to dim light). Further research is necessary to determine the conditions under which extraocular light might produce phase shifts in human circadian rhythms.

Nocturnal melatonin secretion is not suppressed by light exposure behind the knee in humans

Ocular light exposure can phase shift circadian rhythms and suppress nocturnal melatonin production. A recent finding suggests that extraocular light can also produce phase shifts in humans. We investigated whether extraocular light could also suppress melatonin secretion in humans. We assayed the salivary melatonin of 16 subjects during a baseline night and an experimental night in dim light (10-20 lux). The experimental night included either: (1) 3-h ocular light exposure (1000 lux, n = 6); (2) 3-h extraocular light exposure behind the knee (13,000 lux, n = 7) or (3) constant dim light exposure (10-20 lux, n = 3). Melatonin suppression occurred with ocular light but not with extraocular light or constant dim light.

Prophylactic treatment of seasonal affective disorder (SAD) by using light visors: bright white or infrared light?

BACKGROUND

Thirty-eight patients with SAD participated in a light visor study addressing two questions. 1. Can the development of a depressive episode be prevented by daily exposure to bright light started before symptom onset in early fall and continued throughout the winter? 2. Does the light have to be visible in order to have beneficial effects?

METHODS

Three groups participated in the study: I (n = 14) received bright white light (2500 lux); II, (n = 15) received infrared light (0.18 lux); III (n = 9, control group) did not receive any light treatment at all.

RESULTS

Infrared light is just as effective as bright white light. Both are more effective than the control condition.

CONCLUSIONS

Light visors can be effectively used to prevent the development of SAD. The fact that exposure to infrared light was as effective as exposure to bright white light questions the specific role of visible light in the treatment of SAD.

No evidence for extraocular photoreceptors in the circadian system of the Syrian hamster

Campbell and Murphy reported recently that 3 h of bright light (13,000 lux) exposure to the area behind the knee caused phase shifts of the circadian rhythms of both body temperature and saliva melatonin in humans. The authors tested the hypothesis that extraocular photoreception is also involved in the circadian system of the Syrian hamster. Hamsters were bilaterally enucleated (eyes removed), and their backs were shaved. Hamsters with stable free-running rhythms in constant darkness were exposed to direct sunlight for 1 or 3 hours during their subjective night. Intact (control) animals showed phase shifts as expected, but the locomotor activity of enucleated animals was unaffected by the exposure to sunlight. The authors also measured the pineal melatonin content after exposure to sunlight. Pineal melatonin content in intact animals declined markedly as expected, but no decline was observed in the enucleated hamsters. The authors conclude that extraocular phototransduction is not capable of shifting the phase of the hamster's locomotor activity rhythm or of suppressing pineal melatonin synthesis.

Role of mouse cryptochrome blue-light photoreceptor in circadian photoresponses

Cryptochromes are photoactive pigments in the eye that have been proposed to function as circadian photopigments. Mice lacking the cryptochrome 2 blue-light photoreceptor gene (mCry2) were tested for circadian clock-related functions. The mutant mice had a lower sensitivity to acute light induction of mPer1 in the suprachiasmatic nucleus (SCN) but exhibited normal circadian oscillations of mPer1 and mCry1 messenger RNA in the SCN. Behaviorally, the mutants had an intrinsic circadian period about 1 hour longer than normal and exhibited high-amplitude phase shifts in response to light pulses administered at circadian time 17. These data are consistent with the hypothesis that CRY2 protein modulates circadian responses in mice and suggest that cryptochromes have a role in circadian photoreception in mammals.

Extraocular circadian phototransduction in humans

Physiological and behavioral rhythms are governed by an endogenous circadian clock. The response of the human circadian clock to extraocular light exposure was monitored by measurement of body temperature and melatonin concentrations throughout the circadian cycle before and after light pulses presented to the popliteal region (behind the knee). A systematic relation was found between the timing of the light pulse and the magnitude and direction of phase shifts, resulting in the generation of a phase response curve. These findings challenge the belief that mammals are incapable of extraretinal circadian phototransduction and have implications for the development of more effective treatments for sleep and circadian rhythm disorders.

Bilirubin, REM sleep, and phototransduction of environmental time cues. A hypothesis

The prevailing hypothesis for phototransduction is that visual (rod or cone) pigments mediate light's primary effects on biological clock systems. Common light-responsive chronobiological behavioral properties of plants and animals and some common molecular structures of plants and animals suggest the possibility that heme moieties and bile pigments in animals mediate some nonvisual influences of light on neuroactive gases and biological rhythms. As plant phytochrome resets the plant biological clock, the similar chromophore in bile pigments is proposed to transduce environmental light zeitgeber signals to endogenous biological clocks. The temporal association of plasma bilirubin and rapid-eye-movement (REM) sleep in populations, the correlation of secretion of biliary bilirubin with REM sleep among 10 different species (Spearman r = 0.89, p < 0.002), and the known responses of bilirubin to light lead to the hypothesis that bilirubin, in particular, plays an evolutionary role in the regulation of REM sleep and in mediating some of light's antidepressant effects.