Human Melatonin Production Decreases With Age

The purpose of this study was to investigate the effects of time of year and demographic variables on the amplitude of melatonin production in normal human subjects. Melatonin production was estimated by measuring the overnight excretion of its major urinary metabolite, 6-hydroxymelatonin. Urine was collected on three consecutive nights in the summer from a sample of 60 normal subjects balanced for sex and age. The collections were repeated in a subgroup during the winter. Melatonin production clearly declined with age but was not influenced by other demographic variables or by season of the year.     

One-hour exposure to moderate illuminance (500 lux) shifts the human melatonin rhythm

Abstract: Salivary melatonin levels were measured in 12 healthy volunteers in order to determine whether a moderate light intensity, which suppresses the nocturnal rise of melatonin, was able to shift the melatonin rhythm. The samples were collected at 1-hr intervals under lighting of < 100 lux (experiment 1) or < 10 lux (experiment 2). The control melatonin profiles were determined during the first night. In the second night the subjects were exposed to light of 500 lux for 60 min during the rising phase of melatonin synthesis. The third series of samples was collected during the third night. The mean decrease of melatonin levels by the exposure to light was 56% of the prelight concentrations. The melatonin onset times were delayed significantly (about 30 min) the night after the exposure to light. The melatonin offset times tended to be delayed in experiment 2. The shifts of the melatonin offset correlated positively with the amount of the melatonin suppression. The results suggest that a relatively small and short lasting light-induced interruption of melatonin synthesis may affect the melatonin rhythm in humans.     

Light Suppression of Melatonin in the Squirrel Monkey (Saimiri sciureus)

This study examined plasma melatonin levels and the suppressant effect of light on melatonin production in the squirrel monkey. Monkeys were maintained on a 12:12 light-dark cycle (LD 12:12) with lights on from 07:00 to 19:00. Plasma levels of melatonin were determined by gas chromatography negative chemical ionization mass spectrometry. Melatonin levels at 00:00 (99.5 +/- 18.9 pg/ml) were significantly higher than at 02:00 (57.21 +/- 7.7 pg/ml; Student's t = 2.859; P less than or equal to 0.021). Baseline values at 02:00 were compared with levels at the same time of day after exposure to 2 hours of 200 lux of light (30.6 +/- 13.1 pg/ml), which caused an average suppression of 54.8% in melatonin levels. One animal did not show light suppression. Results indicated that the squirrel monkey suppressant response to light, as well as baseline values of melatonin, varied between animals.     

Effect of various acute 60 Hz magnetic field exposures on the nocturnal melatonin rise in the adult Djungarian hamster

ABSTRACT: Acute exposure to a 1 Gauss 60 Hz magnetic field for 15 min beginning 2 hr before darkness delays and blunts the nighttime melatonin rhythm in some but not all studies. To determine whether other exposure parameters (dose, mode, or time) influence the nocturnal melatonin rise, adult Djungarian hamsters reared in long days (16L: 8D, lights off at 1000–1800 hr) were acutely exposed to a 60 Hz continuous magnetic field (15 min of 1 or 0.1 Gauss) beginning 4 hr before or 4 hr after lights off. Other hamsters were exposed to a 60 Hz intermittent magnetic field (15 or 60 min of a 1 Gauss field, 1 min on then 1 min off) between 1 and 2 hr before lights off. In sham-exposed controls, i.e., hamsters simultaneously placed in an adjacent coil system but without current, pineal and serum melatonin concentrations increased from a low baseline (1 hr after lights off) to concentrations that were typical of the nighttime peak by 3 hr after darkness. Acute exposure to the 0.1 or 1 Gauss continuous magnetic field for 15 min at either 4 hr before or 4 hr after lights off did not disrupt the nocturnal rise in pineal or serum melatonin. Similarly, onset of the melatonin rhythm was not suppressed by intermittent magnetic field exposures compared to that in sham controls. Thus, several magnetic field exposure paradigms failed to alter the rising phase of the melatonin rhythm in pineal gland content or in circulation. These findings indicate that the biological clock mechanism that mediates photoperiodic time measurement in this seasonally breeding rodent is resistant to a variety of acute continuous or intermittent magnetic field exposures.     

How Hamsters Keep Time: The 6 PM to 6 AM Light-Sensitive Period

A recent review of the pineal literature revealed that when hamsters are exposed to 24-hour light:dark (LD) cycles with less than 12 hours of darkness (summerlike, SLD), the nightly period of pineal melatonin synthesis (PPMS) begins close to the midpoint of the dark period ("midnight") and ends at lights-on irrespective of the length of the dark period or time of day presented. New evidence based on the onset of behavioral estrus every 4 days indicated that the 24-hour hamster clock controlling timing of estrus (4:30 PM) and the PPMS has a 12-hour light-sensitive period (LSP) set to 6 PM-6 AM in LD 16:8 (dark 8 PM-4 AM, SLD) by a balance in opposing actions of evening and morning light [Alleva: Pineal Research Reviews, Volume 5, Alan R. Liss, Inc., New York, 1987]. Present experiments focus on how this balance is maintained. When lights-off was advanced to 6 PM in SLDs ranging from LD 12.5:11.5 (dark 6:15 PM-5:45 AM) to 18:6 (dark 9 PM-3 AM), the onset of estrus later that day was advanced in every SLD. However, when lights-on was delayed to or beyond 6 AM, the onset of estrus was unaffected. Thus, the balance is maintained by a resistive force (blocking without a delaying action) of evening light and an advancing action of morning light. In this balancing process all evening light from 6 PM to lights-off but only the first 5 minutes or less of morning light were involved. The advancing action of morning light was characterized in LD 13:11 and 18:6 by imposing on the night before estrus a 5:30 PM-6:30 AM dark period scanned with a 15-minute light pulse. Shifts in onset of estrus later that day were plotted vs. time of the light pulse. The resulting phase response curves (PRCs) were similar and comprised only an advancing curve, which rose about 10 PM, peaked at 2 AM, and returned gradually to normal at 6 AM. In contrast, a PRC obtained from LD 12:12 (dark 6 PM-6 AM) was sinelike, comprising a 6 PM-9 PM delaying curve followed by an advancing curve similar to those from SLD. An hypothesis based on these findings is presented to explain how hamsters would keep constant AM-PM time throughout summer.     

Pineal rhythm of N-acetyltransferase activity and melatonin in the male badger, Meles meles L, under natural daylight: Relationship with the photoperiod

The rhythmicity of melatonin secretion and of pineal NAT activity was compared in male badger kept in natural daylight during two distinctly different photoperiods (January and June). The hormone and its enzyme follow the same pattern with a nighttime elevation and a low level during the day, demonstrating the presence of a nyctohemeral rhythm. The high correlation found between the NAT activity and the melatonin concentration suggests that NAT is the rate-limiting enzyme in melatonin synthesis in the badger. Peak amplitudes were similar under the two photoperiods. Melatonin secretion occurred in the first part of the night irrespective of the photoperiod. The rhythm of melatonin secretion is modified by the photoperiod. The duration of high nighttime levels varies; it is longer (8 h) when the night is long (16 h) in January, and shorter (6 h) when the night is short (8 h) in June. In the badger, differences in the duration of high level melatonin at night may reflect variations in day length and convey to the animal the photoperiodic information.     

Melatonin Rhythms in Quail: Regulation by Photoperiod and Circadian Pacemakers

The profile of melatonin in the eyes, pineal, and blood of Japanese quail was assessed in birds held under LD 16:8 and LD 6: 18 photoperiods. Melatonin levels in all three tissues showed a robust daily rhythm with higher levels occurring at night. The amplitude of the rhythm was depressed and its duration lengthened on LD 6: 18 relative to LD 16:8. The blood melatonin rhythm precisely reflected the rhythms shown by the pineal and eyes, supporting the idea that the blood rhythm is a result of melatonin secretion by both the eyes and pineal.
The ocular melatonin rhythm continued after sectioning of the optic nerve, was reentrainable to a shift in the phase of the LD cycle, and persisted for at least 2 days in constant darkness. It was concluded that either (1) an intraocular circadian clock drives the ocular melatonin rhythm, or (2) an extraocular clock drives the ocular melatonin rhythm via a route other than the efferent innervation (which enters the eye via the optic tract).     

Rats on 22.5-Hr Light:Dark Cycles Have Vaginal Opening Earlier Than Rats on 26-Hr Light:Dark Cycles

Rats on 22.5-hr light:dark cycles had vaginal opening significantly earlier than rats on 26-hr light:dark cycles. This finding might explain the early menarche in blind girls: Their circadian rhythms, like those of rats on 22.5-hr cycles, might be more rapid than normal. Thus a set number of cycles can occur in a shorter time than usual, allowing puberty to take place early.     

Natural selection against a circadian clock gene mutation in mice

Circadian rhythms with an endogenous period close to or equal to the natural light–dark cycle are considered evolutionarily adaptive (“circadian resonance hypothesis”). Despite remarkable insight into the molecular mechanisms driving circadian cycles, this hypothesis has not been tested under natural conditions for any eukaryotic organism. We tested this hypothesis in mice bearing a short-period mutation in the enzyme casein kinase 1ε (tau mutation), which accelerates free-running circadian cycles. We compared daily activity (feeding) rhythms, survivorship, and reproduction in six replicate populations in outdoor experimental enclosures, established with wild-type, heterozygous, and homozygous mice in a Mendelian ratio. In the release cohort, survival was reduced in the homozygote mutant mice, revealing strong selection against short-period genotypes. Over the course of 14 mo, the relative frequency of the tau allele dropped from initial parity to 20%. Adult survival and recruitment of juveniles into the population contributed approximately equally to the selection for wild-type alleles. The expression of activity during daytime varied throughout the experiment and was significantly increased by the tau mutation. The strong selection against the short-period tau allele observed here contrasts with earlier studies showing absence of selection against a Period 2 (Per2) mutation, which disrupts internal clock function, but does not change period length. These findings are consistent with, and predicted by the theory that resonance of the circadian system plays an important role in individual fitness.Circadian clocks are a ubiquitous feature of life on earth, and serve to maintain synchrony of internal physiology with the external 24-h environment. Colin Pittendrigh, one of the founders of chronobiology, hypothesized that natural selection should favor circadian systems to operate in resonance with the external cycle (1, 2). A prediction from this hypothesis is that individuals exhibiting circadian rhythms with frequencies that are not in close resonance with the 24-h cycle should be selected against in nature. The hypothesis was initially supported by laboratory experiments in fly species that lived longer in a 24-h light–dark (LD) cycle than in non-24-h LD cycles (2–4). Stronger support emerged from dyadic competition experiments in batch cultures of cyanobacteria carrying single gene mutations affecting their circadian period (τ). Strains (either wild type or mutant) with a τ similar to the external LD cycle outcompeted strains with a τ different from the Zeitgeber (5, 6). Whether periods out of resonance with the external cycle entail a real fitness deficit in a natural setting has not been tested in any of these systems.The Ck1ε^tau (hereafter defined as the tau mutation) is a gain-of-function mutation (7) that accelerates the cellular dynamics of the circadian PERIOD protein (8, 9) and affects circadian behavior and physiology (10). It was first detected in Syrian hamsters (Mesocricetus auratus), where it causes τ to shorten with ∼2 h for each copy of the mutant allele (11). In mice, the same mutation shortens the circadian cycle to an almost identical extent (10). As a consequence of the accelerated circadian clockwork, both homozygote tau mice and hamsters are unable to entrain to 24-h LD cycles in the laboratory. Because its frequency deviates considerably from the natural 24-h cycle, the tau mutation provides an excellent model to study effects of deviant circadian periods on fitness in a natural setting. Here we report the consequences of deviant circadian rhythms in six replicate outdoor populations of mice. These populations were established with the release of mice, all born to two heterozygote parents, in identical enclosures, with ∼49% mutant tau alleles in a near Mendelian ratio in each pen. We used s.c. transponders to record each individual’s visits to feeders in each enclosure, which allowed us to quantify the rhythm of feeding activity and to keep track of each individual’s presence—and, hence, monitor lifespan, mortality, and the tau allele frequency in each population.     

Mixtures of opposing phosphorylations within hexamers precisely time feedback in the cyanobacterial circadian clock

Circadian oscillations are generated by the purified cyanobacterial clock proteins, KaiA, KaiB, and KaiC, through rhythmic interactions that depend on multisite phosphorylation of KaiC. However, the mechanisms that allow these phosphorylation reactions to robustly control the timing of oscillations over a range of protein stoichiometries are not clear. We show that when KaiC hexamers consist of a mixture of differentially phosphorylated subunits, the two phosphorylation sites have opposing effects on the ability of each hexamer to bind to the negative regulator KaiB. We likewise show that the ability of the positive regulator KaiA to act on KaiC depends on the phosphorylation state of the hexamer and that KaiA and KaiB recognize alternative allosteric states of the KaiC ring. Using mathematical models with kinetic parameters taken from experimental data, we find that antagonism of the two KaiC phosphorylation sites generates an ultrasensitive switch in negative feedback strength necessary for stable circadian oscillations over a range of component concentrations. Similar strategies based on opposing modifications may be used to support robustness in other timing systems and in cellular signaling more generally.Circadian clocks are biological timing systems that allow organisms to anticipate and prepare for daily changes in the environment. A hallmark of a circadian oscillator is its ability to drive self-sustained rhythms in gene expression and behavior with a period close to 24 h, even in the absence of environmental cues (1). A general challenge for the biochemical machinery that generates rhythms is to precisely define the duration of the day in the face of perturbations, including fluctuations in the cellular abundance of the molecular components. The importance of maintaining precise circadian timing is underscored by experiments showing that mismatch between the clock period and the rhythms in the external environment results in health problems and fitness defects (2, 3).Although circadian clocks are found across all kingdoms of life, the Kai oscillator from cyanobacteria presents a uniquely powerful model system to study the design principles inherent in the molecular interactions that generate rhythms. A mixture of the purified proteins KaiA, KaiB, and KaiC results in stable oscillations in the phosphorylation state of KaiC in vitro that persist for many days and share many of the properties of circadian clocks in vivo (4–6). In particular, the oscillator can successfully generate near–24-h rhythms over a range of concentrations of the clock proteins both in vivo and in vitro (7–9), so fine-tuning of gene expression is not needed to support a functional clock. Much has been learned about the behavior of the isolated Kai proteins, including the determination of high-resolution crystal structures of all three components (10–12). A critical challenge that remains is to understand how the properties of the Kai proteins are integrated together in the full system to generate precisely timed rhythms.KaiC appears to be the central hub of timing information in the oscillator. Each KaiC molecule consists of two AAA+ family ATPase domains that consume the free energy of ATP hydrolysis to drive oscillations. Like many other members of this family, KaiC forms hexamers, and the enzymatic active sites are formed at the subunit interfaces where nucleotides are bound. The C-terminal, or CII, domain of KaiC has additional phosphotransferase activities that are unusual for the AAA+ family: it can phosphorylate and dephosphorylate two residues near the subunit interface, Ser431 and Thr432 (13). KaiC autokinase and autophosphatase activities occur at the same active site (14, 15). In isolation, KaiC has high phosphatase activity, but the enzyme is pushed toward kinase activity by the activator protein KaiA, which interacts directly with the KaiC C-terminal tail (16, 17). Roughly speaking, kinase activity predominates during the day, and phosphatase activity predominates during the night (18). Thus, understanding the feedback mechanisms that generate a precise time delay between these modes is crucial to understanding timing in the oscillator (19).Inactivation of KaiA and a transition from kinase to phosphatase mode occur when KaiB•KaiC complexes form, closing a negative feedback loop by sequestering KaiA in a ternary complex and leaving it unable to act on other KaiC molecules (20, 21). By temporarily removing KaiA molecules from their activating role, this molecular titration mechanism may act to synchronize the activity of all KaiC hexamers in the reaction (20, 22, 23). Phosphorylation and dephosphorylation proceed in a strongly ordered fashion so that in response to a change in KaiA activity, Thr432 is (de)phosphorylated first, followed later by Ser431 (18, 20, 21). It is known that phosphorylated Ser431 is important for allowing the formation of KaiB•KaiC complexes. However, recent work has made it clear that the binding of KaiB involves both KaiC domains—in particular, the slow ATPase activity of the N-terminal CI domain, which is not phosphorylated, is required for KaiB interaction (24, 25).Because of the importance of precisely timing negative feedback via KaiB•KaiC complex formation for generating appropriate rhythms (22), we wanted to understand the role of phosphorylation of the KaiC hexamer in controlling this process. The involvement of both KaiC domains suggests that information about phosphorylation in CII is communicated allosterically through changes in hexamer structure to the CI domain, potentially through ring–ring stacking interactions (24, 26). We therefore hypothesized that the KaiC phosphorylation sites on each subunit might act as allosteric regulators in the context of a hexameric ring so that phosphorylation of one subunit would alter the ability of all other subunits in the ring to engage with KaiA and KaiB, providing a cooperative mechanism to control the timing of these interactions.We conducted a series of biochemical experiments and perturbations to study the effect of altering the status of each phosphorylation site on the KaiC hexamer. To interpret these results, we then developed a mathematical model analogous to classical models of allosteric transitions in multimeric proteins. We constrain the kinetic parameters in this model using experimental measurements of rate constants, allowing us to compare the predictions of the model directly with data. We conclude that maintenance of circadian timing over a range of protein concentrations requires an effectively ultrasensitive switch in each KaiC hexamer from an exclusively KaiA-binding state to a state that can bind to KaiB as phosphorylation proceeds. This effect requires that KaiC hexamers consist of mixtures of differentially phosphorylated subunits, as would be produced by stochastic autophosphorylation of a hexamer. Ultrasensitivity results from opposing effects of phosphorylation on Thr432 and Ser431 in controlling a concerted transition within a given KaiC hexamer. Including this mechanism in the model is necessary to explain the experimentally observed tolerance of the system to altered protein concentrations.     

Effects of Melatonin and 6-Methoxybenzoxazolinone on Photoperiodic Control of Testis Size in Adult Male Golden Hamsters

Consumption of young plants containing 6-methoxybenzoxazolinone (6-MBOA) appears to play an important role in the initiation of reproduction each spring in wild populations of the montane vole. Following its identification, 6-MBOA has been found to stimulate the reproductive system in a number of rodent species, but the mechanism of action remains unknown. The chemical structure of 6-MBOA is similar to melatonin, which, in addition to its well-known antigonadal effects, can exert a progonadal influence under certain experimental conditions. To determine if 6-MBOA might act as a melatonin agonist, four experiments were conducted to compare the effect of these two compounds on testis size in the golden hamster, a rodent whose responses to melatonin are well characterized. 1) Hamsters exposed to 14 h light per day (14L:10D) received a daily injection of melatonin (25.0 micrograms) or 6-MBOA (17.8 micrograms). 2) Hamsters exposed to 6L:18D received Silastic capsules (50 or 200 mm) containing melatonin or 6-MBOA. 3) Hamsters exposed to 6L:18D received chow containing melatonin (21.1 or 42.2 micrograms/gm chow) or 6-MBOA (15.0 or 30.0 micrograms/gm). 4) Hamsters exposed to 6L:18D received drinking water containing melatonin (15.5 micrograms/ml) or 6-MBOA (11.0 micrograms/ml). Testis widths were determined at 2--3 week intervals, and after 66-73 days testes were removed and weighed. Melatonin significantly influenced testis size in each experiment, but treatment with 6-MBOA had no effect in any of these experimental paradigms, indicating that 6-MBOA does not act as a melatonin agonist in the hamster. However, these results indicate that the consumption of melatonin (and presumably melatonin agonists) could serve as an environmental stimulus for reproductive activity.     

Distinct contribution of cone photoreceptor subtypes to the mammalian biological clock

Ambient light detection is important for the synchronization of the circadian clock to the external solar cycle. Light signals are sent to the suprachiasmatic nuclei (SCN), the site of the major circadian pacemaker. It has been assumed that cone photoreceptors contribute minimally to synchronization. Here, however, we find that cone photoreceptors are sufficient for mediating entrainment and transmitting photic information to the SCN, as evaluated in mice that have only cones as functional photoreceptors. Using in vivo electrophysiological recordings in the SCN of freely moving cone-only mice, we observed light responses in SCN neuronal activity in response to 60-s pulses of both ultraviolet (UV) (λ_max 365 nm) and green (λ_max 505 nm) light. Higher irradiances of UV light led to irradiance-dependent enhancements in SCN neuronal activity, whereas higher irradiances of green light led to a reduction in the sustained response with only the transient response remaining. Responses in SCN neuronal activity decayed with a half-max time of ∼9 min for UV light and less than a minute for green light, indicating differential input between short-wavelength–sensitive and mid-wavelength–sensitive cones for the SCN responsiveness. Furthermore, we show that UV light is more effective for photoentrainment than green light. Based on the lack of a full sustained response in cone-only mice, we confirmed that rapidly alternating light levels, rather than slowly alternating light, caused substantial phase shifts. Together, our data provide strong evidence that cone types contribute to photoentrainment and differentially affect the electrical activity levels of the SCN.

In addition to rods and cones, light is also sensed in the retina by a specialized subset of melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs). ipRGCs incorporate input from rod and cone photoreceptors (1). Two types of cone photoreceptors are present in the murine retina: short-wavelength–sensitive cones (S-cones; λ_max 360 nm), which are maximally sensitive to ultraviolet (UV) light, and mid-wavelength–sensitive cones (M-cones; λ_max 508 nm), which are maximally sensitive to green light (2). The ipRGCs project to the suprachiasmatic nuclei (SCN) of the hypothalamus, hereby subserving photoentrainment of the major pacemaker for circadian rhythms in physiology and behavior (3). The ablation of ipRGCs results in the loss of photoentrainment of circadian rhythms to the environmental light–dark (LD) cycle (1). Although rod and cone photoreceptors are not essential for entrainment of the biological clock to the external LD cycle, both rod and cone photoreceptors influence the SCN, which is evidenced by the experimental finding that mice can entrain to an LD cycle in the absence of melanopsin (4, 5). In addition, recordings in SCN of melanopsin-deficient mice show preservation of sustained light responses in the SCN, the magnitude of which seems unaffected by the absence of melanopsin (6, 7).Whereas rods are capable of driving photoentrainment at a wide range of light intensities (8), the majority of cone-only (Opn4^−/−Gnat1^−/−) mice, which lack melanopsin and functional rod signaling, show surprisingly large interindividual differences in their ability to entrain to LD cycles of white light, and some of them exhibit a positive phase angle of entrainment (9, 10). However, phase-shifting responses in mice lacking M-cones are attenuated (11, 12), which contradicts the reduced ability of Opn4^−/−Gnat1^−/− mice to entrain to an LD cycle. The question is, therefore, to what extent cones contribute to photic entrainment and whether S- and M-cones contribute similarly to the entrainment of the circadian clock.Photoentrainment is dependent on light-induced changes in SCN neuronal activity (13, 14). Typically, SCN neurons respond to light with a transient increase in SCN electrical activity followed by a sustained component throughout light exposure. Together, rod and cone photoreceptors can mediate light responses at the level of the SCN, including both the fast and the sustained components (6, 7, 15). These findings are consistent with rod- and cone-mediated responses recorded in ipRGCs (16–18). In this study, we determined the specific contribution of the S- and M-cone photoreceptors to circadian photoreception. We performed behavioral and in vivo electrophysiological recordings in Opn4^−/−Gnat1^−/− mice to determine the effects of λ_max 365 nm (UV) and λ_max 505 nm (green) light on photoentrainment and on light-induced responses in electrical activity of SCN neurons. Furthermore, we assessed the capability of Opn4^−/−Gnat1^−/− mice to phase shift in response to intermittent monochromatic light pulses aimed to stimulate the UV- or green-sensitive cones.     

Neural circuitry in the regulation of adrenal corticosterone rhythmicity

Adrenal cortical secretion of glucocorticoids is an essential adaptive response of an organism to stress. Although the hypothalamic-pituitary-adrenal axis regulates the adrenal cortex via release of ACTH, there is strong evidence supporting a role for sympathetic innervation in modulating adrenal glucocorticoid secretion. The dissociation between changes in ACTH and glucocorticoids under non-stress and stress conditions has reinforced the concept that neural control of the adrenal cortex acts to modulate steroidogenic responses to circulating ACTH. A dual control of the adrenal cortex has been implicated in the prominent circadian rhythm in glucocorticoids. However, the central neural substrate for circadian changes in glucocorticoids that are mediated by peripheral neural innervation of the adrenal cortex has not been conclusively delineated. The hypothesis to be addressed is that neurons in the paraventricular nucleus of the hypothalamus receive input from the suprachiasmatic nucleus and project to sympathetic preganglionic neurons in the spinal cord to provide inhibitory and excitatory input to the adrenal cortex that drives the circadian rhythm. This review examines anatomical and physiological evidence that forms the basis for this putative neural circuit.     

A role for bursa fabricii and bursin in the ontogeny of the pineal biosynthetic activity in the chicken

Abstract: The tripeptide bursin (Lys-His-Gly-NH₂) is a B cell differentiation hormone derived from the bursa fabricii. The latter is a cloacal diverticulum and the site of B lymphocyte differentiation and selection in aves; also the bursa fabricii is involved in endocrine functions. Herein we demonstrate that in the chicken, the bursa fabricii and bursin are crucial to the ontogeny of both the pineal response to antigenic challenge and pineal circadian synthetic activity. In early embryonically bursectomized chickens, the plasma melatonin response to immunization by porcine thyroglobulin (Tg) was abolished. Also, the amplitudes of both plasma melatonin and pineal N-acetyltransferase (NAT) circadian rhythms were reduced by 50%, whereas the activity of hydroxyindole-O-methyltransferase (HIOMT) remained unchanged. Conversely, administration of either minute amounts (100 pg, 100 fg) or highly dilute (5 × 10⁻²⁷ g) bursin, with the exception of a highest dose (100 μg), to bursaless embryos induced recovery of normal antigen-induced melatonin response and normal amplitudes of melatonin and NAT rhythms. These findings establish that early in embryonic life, the bursa fabricii and its derived signal (bursin) are essential for normal development of pineal synthetic activity and underline the efficacy of very dilute bursin as an informative signal.     

Role of photoperiod and the pineal gland in T cell-dependent humoral immune reactivity in the Siberian hamster

The present study tested the hypothesis that antibody production in response to xenoantigen is modulated by daylength and dependent upon the pineal gland. Alter injection of sheep erythrocytes (SRBC), serum immunoglobulin (Ig) concentrations were 5-fold lower in hamsters in short versus long days. Pinealectomy (Pinx) abolished the nocturnal melatonin rhythm, blocked short-day-mediated testis regression, and eliminated the short-day reduction in Ig production after SRBC treatment. Antibody titers in response to SRBC were equivalently augmented in short-day Pinx and long-day sham hamsters. The results indicate that photoperiodic effects on T cell-dependent humoral immunity are dependent upon the pineal gland. These findings raise the possibility that day length-associated changes in some immune system functions are mediated by the pineal melatonin rhythm.     

Melatonin: A pleiotropic molecule that modulates DNA damage response and repair pathways

DNA repair is responsible for maintaining the integrity of the genome. Perturbations in the DNA repair pathways have been identified in several human cancers. Thus, compounds targeting DNA damage response (DDR) hold great promise in cancer therapy. A great deal of effort, in pursuit of new anticancer drugs, has been devoted to understanding the basic mechanisms and functions of the cellular DNA repair machinery. Melatonin, a widely produced indoleamine in all organisms, is associated with a reduced risk of cancer and has multiple regulatory roles on the different aspects of the DDR and DNA repair. Herein, we have mainly discussed how defective components in different DNA repair machineries, including homologous recombination (HR), nonhomologous end‐joining (NHEJ), base excision repair (BER), nucleotide excision repair (NER), and finally DNA mismatch repair (MMR), can contribute to the risk of cancer. Melatonin biosynthesis, mode of action, and antioxidant effects are reviewed along with the means by which the indoleamine regulates DDR at the transduction, mediation, and functional levels. Finally, we summarize recent studies that illustrate how melatonin can be combined with DNA‐damaging agents to improve their efficacy in cancer therapy.     

Absence of Nocturnal Plasma Melatonin Surge Under Long and Short Artificial Photoperiods in the Domestic Sow

The circadian plasma melatonin profile of German Landrace sows was determined at 1-h intervals for 31 h after exposure to each of four different artificial photoperiods. Under 12L:12D, melatonin concentrations in four sows during the light phase ranged from 22 +/- 5.9 to 96 +/- 25.1 pg/ml (mean +/- SEM). During the dark phase (total dark) the individual concentrations increased two- to fivefold over the peak individual light phase values in three sows. This nocturnal surge was of 3.8 h duration and peaked at 190, 294, and 546 pg/ml at 0100 h, which was 5.5 h after the onset of dark. The surge was abolished in these animals after exposure to 16L:8D and could not be reinstated by the subsequent exposure to 8L:16D. During the latter two photoperiods the mean concentrations were consistently less than the maximum mean value of 30.8 +/- 10 pg/ml during the dark phase and 55 +/- 21.9 pg/ml during the light phase. All the sows displayed regular estrous cycles during the study, and the day of the estrous cycle on which the samples were obtained under each photoperiod was not significantly correlated with the presence or absence of the nocturnal surge. A separate experiment using different animals confirmed the nocturnal surge under 12L:12D. In two out of four cycling sows the peak concentrations during the dark phase (122 and 110 pg/ml at 0400 and 0500 h) were two- to sixfold higher than those of the light phase. Absence of the nocturnal surge under long and short daylengths may be a factor contributing to the decline in reproductive performance during the summer and winter months.     

Circadian rhythms of plasma melatonin in the ruin lizard Podarcis sicula: Effects of pinealectomy

Plasma melatonin was measured in lizards (Podarcis sicula) at six different times of day under conditions of constant temperature and darkness. Intact animals showed a circadian rhythm of melatonin with a peak in the subjective night of 207 pg/ml (median) and a trough during the subjective day that was below the minimum detection level of the assay (50 pg/ml). Pinealectomy abolished the circadian rhythm of plasma melatonin; median levels were near or below the minimum detection level at all times sampled. The data suggest that the pineal is the only source of rhythmic blood-borne melatonin in Podarcis sicula, and are consistent with the hypothesis that changes in the free-running period of the locomotor rhythm induced by pinealectomy in this species are due to withdrawal of rhythmic melatonin from the blood.