DIURNAL EXPRESSION OF CLOCK GENES IN PINEAL GLAND AND BRAIN AND PLASMA LEVELS OF MELATONIN AND CORTISOL IN ATLANTIC SALMON PARR AND SMOLTS

In Atlantic salmon, the preadaptation to a marine life, i.e., parr-smolt transformation, and melatonin production in the pineal gland are regulated by the photoperiod. However, the clock genes have never been studied in the pineal gland of this species. The aim of the present study was to describe the diurnal expression of clock genes (Per1-like, Cry2, and Clock) in the pineal gland and brain of Atlantic salmon parr and smolts in freshwater, as well as plasma levels of melatonin and cortisol. By employing an out-of-season smolt production model, the parr-smolt transformation was induced by subjecting triplicate groups of parr to 6 wks (wks 0 to 6) under a 12?h:12?h light-dark (LD) regime followed by 6 wks (wks 6 to 12) of continuous light (LL). The measured clock genes in both pineal gland and brain and the plasma levels of melatonin and cortisol showed significant daily variations in parr under LD in wk 6, whereas these rhythms were abolished in smolts under LL in wk 12. In parr, the pineal Per1-like and Cry2 expression peaked in the dark phase, whereas the pineal Clock expression was elevated during the light phase. Although this study presents novel findings on the clock gene system in the teleost pineal gland, the role of this system in the regulation of smoltification needs to be studied in more detail. (Author correspondence: ti.hu@hotmail.com)     

Daily Oscillation in Melatonin Synthesis in The Turkey Pineal Gland and Retina: Diurnal and Circadian Rhythms

The aim of the present study was to examine arylalkylamine N‐acetyltransferase (AANAT) activity and melatonin content in the pineal gland and retina as well as the melatonin concentration in plasma of the turkey (Meleagris gallopavo), an avian species in which several physiological processes, including reproduction, are controlled by day length. In order to investigate whether the analyzed parameters display diurnal or circadian rhythmicity, we measured these variables in tissues isolated at regular time intervals from birds kept either under a regular light‐dark (LD) cycle or under constant darkness (DD). The pineal gland and retina of the turkey rhythmically produced melatonin. In birds kept under a daily LD cycle, melatonin levels in the pineal gland and retina were high during the dark phase and low during the light phase. Rhythmic oscillations in melatonin, with high night‐time concentrations, were also found in the plasma. The pineal and retinal melatonin rhythms mirrored oscillations in the activity of AANAT, the penultimate enzyme in the melatonin biosynthetic pathway. Rhythmic oscillations in AANAT activity in the turkey pineal gland and retina were circadian in nature, as they persisted under conditions of constant darkness (DD). Transferring birds from LD into DD, however, resulted in a potent decline in the amplitude of the AANAT rhythm from the first day of DD. On the sixth day of DD, pineal AANAT activity was still markedly higher during the subjective dark than during the subjective light phase; whereas, AANAT activity in the retina did not exhibit significant oscillations. The results indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The findings suggest that environmental light may be of primary importance in the maintenance of the high‐amplitude melatonin rhythms in the turkey.     

The Endogenous Melatonin (MT) Signal Facilitates Reentrainment of the Circadian System to Light-Induced Phase Advances by Acting Upon MT2 Receptors

The indolamine melatonin is an important rhythmic endocrine signal in the circadian system. Exogenous melatonin can entrain circadian rhythms in physiology and behavior, but the role of endogenous melatonin and the two membrane-bound melatonin receptor types, MT1 and MT2, in reentrainment of daily rhythms to light-induced phase shifts is not understood. The present study analyzed locomotor activity rhythms and clock protein levels in the suprachiasmatic nuclei (SCN) of melatonin-deficient (C57BL/6J) and melatonin-proficient (C3H/HeN) mice, as well as in melatonin-proficient (C3H/HeN) mice with targeted deletion of the MT1, MT2, or both receptors, to determine effects associated with phase delays or phase advances of the light/dark (LD) cycle. In all mouse strains and genotypes, reentrainment of locomotor activity rhythms was significantly faster after a 6-h phase delay than a 6-h phase advance. Reentrainment after the phase advance was, however, significantly slower than in melatonin-deficient animals and in mice lacking functional MT2 receptors than melatonin-proficient animals with intact MT2 receptors. To investigate whether these behavioral differences coincide with differences in reentrainment of clock protein levels in the SCN, mPER1, mCRY1 immunoreactions were compared between control mice kept under the original LD cycle and killed at zeitgeber time 04 (ZT04) or at ZT10, respectively, and experimental mice subjected to a 6-h phase advance of the LD cycle and sacrificed at ZT10 on the third day after phase advance. This ZT corresponds to ZT04 of the original LD cycle. Under the original LD cycle, the numbers of mPER1- and mCRY1-immunoreactive cell nuclei were low at ZT04 and high at ZT10 in the SCN of all mouse strains and genotypes investigated. Notably, mouse strains with intact melatonin signaling and functional MT2 receptors showed a significant increase in the number of mPER1- and mCRY1-immunoreactive cell nuclei at the new ZT10 as compared to the former ZT04. These data suggest the endogenous melatonin signal facilitates reentrainment of the circadian system to phase advances on the level of the SCN molecular clockwork by acting upon MT2 receptors. (Author correspondence: M.Pfeffer@em.uni-frankfurt.de)     

Pineal melatonin rhythms in the lizardAnolis carolinensis: effects of light and temperature cycles

Summary Pineal and ocular melatonin was assessed, over 24 h periods, in male lizards (Anolis carolinensis) entrained to 24 h light-dark (LD) cycles and a constant 32 C, and in lizards entrained to both 24 h LD cycles and 24 h temperature cycles (32 C/20 C). At a constant temperature, the duration of the photoperiod has a profound effect on the duration, amplitude, and phase of the pineal melatonin rhythm (Fig. 1). The pineal melatonin rhythm under cyclic temperature peaks during the cool (20 C) phase of the cycle regardless of whether or not the cool phase occurs during the light or dark phase of a LD 1212 cycle (Fig. 3). Under a temperature cycle and constant dim illumination, a pineal melatonin rhythm is observed which peaks during the cool phase of the temperature cycle, but the amplitude of the rhythm is depressed relative to that observed under LD (Fig. 2). Illumination up to 2 h in duration does not suppress the nocturnal melatonin peak in theAnolis pineal (Fig. 4). No melatonin rhythm was observed in the eyes ofAnolis under either 24 h LD cycles and a constant temperature (Fig. 1), or under simultaneous light and temperature cycles (Fig. 3). Ocular melatonin content was, in all cases, either very low or non-detectable.Abbreviations HIOMT hydroxyindole-O-methyltransferase - NAT N-acetyltransferase     

Differential Regulation of Arylalkylamine N-Acetyltransferase Activity in Chicken Retinal Ganglion Cells by Light and Circadian Clock

Retinal ganglion cells (RGCs) contain circadian clocks driving melatonin synthesis during the day, a subset of these cells acting as nonvisual photoreceptors sending photic information to the brain. In this work, the authors investigated the temporal and light regulation of arylalkylamine N-acetyltransferase (AA-NAT) activity, a key enzyme in melatonin synthesis. The authors first examined this activity in RGCs of wild-type chickens and compared it to that in photoreceptor cells (PRs) from animals maintained for 48?h in constant dark (DD), light (LL), or regular 12-h:12-h light-dark (LD) cycle. AA-NAT activity in RGCs displayed circadian rhythmicity, with highest levels during the subjective day in both DD and LL as well as in the light phase of the LD cycle. In contrast, AA-NAT activity in PRs exhibited the typical nocturnal peak in DD and LD, but no detectable oscillation was observed under LL, under which conditions the levels were basal at all times examined. A light pulse of 30–60?min significantly decreased AA-NAT activity in PRs during the subjective night, but had no effect on RGCs during the day or night. Intraocular injection of dopamine (50 nmol/eye) during the night to mimic the effect of light presented significant inhibition of AA-NAT activity in PRs compared to controls but had no effect on RGCs. The results clearly demonstrate that the regulation of the diurnal increase in AA-NAT activity in RGCs of chickens undergoes a different control mechanism from that observed in PRs, in which the endogenous clock, light, and dopamine exhibited differential effects. (Author correspondence: mguido@fcq.unc.edu.ar)     

Expression of the melatonin receptor Mel(1c) in neural tissues of the reef fish Siganus guttatus

The golden rabbitfish, Siganus guttatus, is a reef fish exhibiting a restricted lunar-related rhythm in behavior and reproduction. Here, to understand the circadian rhythm of this lunar-synchronized spawner, a melatonin receptor subtype-Mel(1c)-was cloned. The full-length Mel(1c) melatonin receptor cDNA comprised 1747 bp with a single open reading frame (1062 bp) that encodes a 353-amino acid protein, which included 7 presumed transmembrane domains. Real-time PCR revealed high Mel(1c) mRNA expression in the retina and brain but not in the peripheral tissues. When the fish were reared under light/dark (LD 12:12) conditions, Mel(1c) mRNA in the retina and brain was expressed with daily variations and increased during nighttime. Similar variations were noted under constant conditions, suggesting that Mel(1c) mRNA expression is regulated by the circadian clock system. Daily variations of Mel(1c) mRNA expression with a peak at zeitgeber time (ZT) 12 were observed in the cultured pineal gland under LD 12:12. Exposure of the cultured pineal gland to light at ZT17 resulted in a decrease in Mel(1c) mRNA expression. When light was obstructed at ZT5, the opposite effect was obtained. These results suggest that light exerts certain effects on Mel(1c) mRNA expression directly or indirectly through melatonin actions.     

Static and extremely low frequency electromagnetic field exposure: Reported effects on the circadian production of melatonin

The circadian rhythm of melatonin production (high melatonin levels at night and low during the day) in the mammalian pineal gland is modified by visible portions of the electromagnetic spectrum, i.e., light, and reportedly by extremely low frequency (ELF) electromagnetic fields as well as by static magnetic field exposure. Both light and non-visible electromagnetic field exposure at night depress the conversion of serotonin (5HT) to melatonin within the pineal gland. Several reports over the last decade showed that the chronic exposure of rats to a 60 Hz electric field, over a range of field strengths, severely attenuated the nighttime rise in pineal melatonin production; however, more recent studies have not confirmed this initial observation. Sinusoidal magnetic field exposure also has been shown to interfere with the nocturnal melatonin forming ability of the pineal gland although the number of studies using these field exposures is small. On the other hand, static magnetic fields have been repeatedly shown to perturb the circadian melatonin rhythm. The field strengths in these studies were almost always in the geomagnetic range (0.2 to 0.7 Gauss or 20 to 70 μtesla) and most often the experimental animals were subjected either to a partial rotation or to a total inversion of the horizontal component of the geomagnetic field. These experiments showed that several parameters in the indole cascade in the pineal gland are modified by these field exposures; thus, pineal cyclic AMP levels, N-acetyltransferase (NAT) activity (the rate limiting enzyme in pineal melatonin production), hydroxyindole-O-methyltransferase (HIOMT) activity (the melatonin forming enzyme), and pineal and blood melatonin concentrations were depressed in various studies. Likewise, increases in pineal levels of 5HT and 5-hydroxyindole acetic acid (5HIAA) were also seen in these glands; these increases are consistent with a depressed melatonin synthesis. The mechanisms whereby non-visible electromagnetic fields influence the melatonin forming ability of the pineal gland remain unknown; however, the retinas in particular have been theorized to serve as magnetoreceptors with the altered melatonin cycle being a consequence of a disturbance in the neural biological clock, i.e., the suprachiasmatic nuclei (SCN) of the hypothalamus, which generates the circadian melatonin rhythm. The disturbances in pineal melatonin production induced by either light exposure or non-visible electromagnetic field exposure at night appear to be the same but whether the underlying mechanisms are similar remains unknown.     

Daily oscillation in melatonin synthesis in the Turkey pineal gland and retina: diurnal and circadian rhythms

The aim of the present study was to examine arylalkylamine N-acetyltransferase (AANAT) activity and melatonin content in the pineal gland and retina as well as the melatonin concentration in plasma of the turkey (Meleagris gallopavo), an avian species in which several physiological processes, including reproduction, are controlled by day length. In order to investigate whether the analyzed parameters display diurnal or circadian rhythmicity, we measured these variables in tissues isolated at regular time intervals from birds kept either under a regular light-dark (LD) cycle or under constant darkness (DD). The pineal gland and retina of the turkey rhythmically produced melatonin. In birds kept under a daily LD cycle, melatonin levels in the pineal gland and retina were high during the dark phase and low during the light phase. Rhythmic oscillations in melatonin, with high night-time concentrations, were also found in the plasma. The pineal and retinal melatonin rhythms mirrored oscillations in the activity of AANAT, the penultimate enzyme in the melatonin biosynthetic pathway. Rhythmic oscillations in AANAT activity in the turkey pineal gland and retina were circadian in nature, as they persisted under conditions of constant darkness (DD). Transferring birds from LD into DD, however, resulted in a potent decline in the amplitude of the AANAT rhythm from the first day of DD. On the sixth day of DD, pineal AANAT activity was still markedly higher during the subjective dark than during the subjective light phase; whereas, AANAT activity in the retina did not exhibit significant oscillations. The results indicate that melatonin rhythmicity in the turkey pineal gland and retina is regulated both by light and the endogenous circadian clock. The findings suggest that environmental light may be of primary importance in the maintenance of the high-amplitude melatonin rhythms in the turkey.     

The Avian Pineal Gland

The pineal gland plays a key role in the control of the daily and seasonal rhythms in most vertebrate species. In mammals, rhythmic melatonin (MT) release from the pineal gland is controlled by the suprachiasmatic nucleus via the sympathetic nervous system. In most non‐mammalian species, including birds, the pineal gland contains a self‐sustained circadian oscillator and several input channels to synchronize the clock, including direct light sensitivity. Avian pineal glands maintain rhythmic activity for days under in vitro conditions. Several physical (light, temperature, and magnetic field) and biochemical (Vasoactive intestinal polypeptide (VIP), norepinephrine, PACAP, etc.) input channels, influencing release of melatonin are also functional in vitro, rendering the explanted avian pineal an excellent model to study the circadian biological clock. Using a perifusion system, we here report that the phase of the circadian melatonin rhythm of the explanted chicken pineal gland can be entrained easily to photoperiods whose length approximates 24 h, even if the light period is extremely short, i.e., 3L:21D. When the length of the photoperiod significantly differs from 24 h, the endogenous MT rhythm becomes distorted and does not follow the light‐dark cycle. When explanted chicken pineal fragments were exposed to various drugs targeting specific components of intracellular signal transduction cascades, only those affecting the cAMP‐protein kinase‐A system modified the MT release temporarily without phase‐shifting the rhythm in MT release. The potential role of cGMP remains to be investigated.     

EFFECTS OF LIGHT AND MELATONIN TREATMENT ON BODY TEMPERATURE AND MELATONIN SECRETION DAILY RHYTHMS IN A DIURNAL RODENT,THE FAT SAND RAT

Many mammals display predictable daily rhythmicity in both neuroendocrine function and behavior. The basic rest-activity cycles are usually consistent for a given species and vary from night-active (nocturnal), those mostly active at dawn and dusk (i.e., crepuscular), and to day-active (diurnal) species. A number of daily rhythms are oppositely phased with respect to the light/dark (LD) cycle in diurnal compared with nocturnal mammals, whereas others are equally phased with respect to the LD cycle, regardless of diurnality/nocturnality. Pineal produced melatonin (MLT) perfectly matches this phase-locked feature in that its production and secretion always occurs during the night in both diurnal and nocturnal mammals. As most rodents studied to date in the field of chronobiology are nocturnal, the aim in this study was to evaluate the effect of light manipulations and different photoperiods on a diurnal rodent, the fat sand rat, Psammomys obesus. The authors studied its daily rhythms of body temperature (T_b) and 6-sulphatoxymelatonin (6-SMT) under various photoperiodic regimes and light manipulations (acute and chronic exposures) while maintaining a constant ambient temperature of 30°C?±?1°C. The following protocols were used: (A) Control (CON) conditions 12L:12D; (A1) exposure to one light interference (LI) of CON-acclimated individuals for 30?min, 5?h after lights-off; (A2) short photoperiod (SP) acclimation (8L:16D) for 3 wks; (A3) 3 wks of SP acclimation with chronic LI of 15?min, three times a night at 4-h intervals; (A4) chronic exposure to constant dim blue light (470nm, 30 lux) for 24?h for 3 wks (LL). (B) The response to exogenous MLT administration, provided in drinking water, was measured under the following protocols: (B1) After chronic exposure to SP with LI, MLT was provided once, starting 1?h before the end of photophase; (B2) after a continuous exposure to dim blue light, MLT was provided at 15:00?h for 2?h for 2 wks; (B3) to CON animals, MLT was given intraperitoneally (i.p.) at 14:00?h. The results demonstrate that under CON acclimation, Psammomys obesus has robust T_b and 6-SMT daily rhythms in which the acrophase (peak time) of T_b is during the photophase, whereas that of 6-SMT is during scotophase. LI resulted in an elevation of T_b and a reduction of 6-SMT levels. A significant difference in the response was noted between acute and chronic exposure to LI, particularly in 6-SMT levels, which were lower than CON after LI and higher after chronic LI, implying an acclimation process. Constant exposure to blue light abolished T_b and 6-SMT rhythms in all the animals. MLT administration resumed the T_b daily rhythm in these animals, and had a recovery effect on the chronic LI-exposed animals, resulting in a T_b decrease. Altogether, the authors show in this study the different modifications of T_b rhythms and MLT levels in response to environmental light manipulations. These series of experiments may serve as a basis for establishing P. obesus as an animal model for further studies in chronobiology. (Author correspondence: hagitsc@research.haifa.ac.il)     

The light-dark regimen and cancer development

The role of the modulation of the pineal gland function in development of cancer is discussed in the review. An inhibition of the pineal function with pinealectomy or with the exposure to the constant light regimen stimulates mammary carcinogenesis, whereas the light deprivation inhibits the carcinogenesis. Epidemiological observations on increased risk of breast cancer in night shift workers, flight attendants, radio and telegraph operators and on decreased risk in blind women are in accordance with the results of experiments in rodents. Treatment with pineal indole hormone melatonin inhibits carcinogenesis in pinealectomized rats or animals kept at the standard light/dark regimen (LD) or at the constant illumination (LL) regimen.     

The Daily Melatonin Pattern in Djungarian Hamsters Depends on the Circadian Phenotype

Djungarian hamsters (Phodopus sungorus) bred at the Institute of Halle reveal three different circadian phenotypes. The wild type (WT) shows normal locomotor activity patterns, whereas in hamsters of the DAO (delayed activity onset) type, the activity onset is continuously delayed. Since the activity offset in those hamsters remains coupled to “light-on,” the activity time becomes compressed. Hamsters of the AR (arrhythmic) type are episodically active throughout the 24?h. Previous studies showed that a disturbed interaction of the circadian system with the light-dark (LD) cycle contributes to the phenomenon observed in DAO hamsters. To gain better insight into the underlying mechanisms, the authors investigated the daily melatonin rhythm, as it is a reliable marker of the circadian clock. Hamsters were kept individually under standardized laboratory conditions (LD 14:10, T?=?22°C?±?2°C, food and water ad libitum). WT, DAO (with exactly 5?h delay of activity onset), and AR hamsters were used for pineal melatonin and urinary 6-sulfatoxymelatonin (aMT6s) measurement. Pineal melatonin content was determined at 3 time points: 4?h after “light-off” [D?+?4], 1?h before “light-on” [L???1], and 1?h after “light-on” [L?+?1]). The 24-h profile of melatonin secretion was investigated by transferring the animals to metabolic cages for 27?h to collect urine at 3-h intervals for aMT6s analysis. WT hamsters showed high pineal melatonin content during the dark time (D?+?4, L???1), which significantly decreased at the beginning of the light period (L?+?1). In contrast, DAO hamsters displayed low melatonin levels during the part of the dark period when animals were still resting (D?+?4). At the end of the dark period (L???1), melatonin content increased significantly and declined again when light was switched on (L?+?1). AR hamsters showed low melatonin levels, comparable to daytime values, at all 3 time points. The results were confirmed by aMT6s data. WT hamsters showed a marked circadian pattern of aMT6s excretion. The concentration started to increase 3?h after “light-off” and reached daytime values 5?h after “light-on.” In DAO hamsters, in contrast, aMT6s excretion started about 6?h later and reached significantly lower levels compared to WT hamsters. In AR animals, aMT6s excretion was low at all times. The results clearly indicate the rhythm of melatonin secretion in DAO hamsters is delayed in accord with their delayed activity onset, whereas AR hamsters display no melatonin rhythm at all. Since the regulatory pathways for the rhythms of locomotor activity and melatonin synthesis (which are downstream from the suprachiasmatic nucleus [SCN]) are different but obviously convey the same signal, we conclude that the origin of the phenomenon observed in DAO hamsters must be located upstream of the SCN, or in the SCN itself. (Author correspondence: weinert@zoologie.uni-halle.de)     

Circadian Rhythms of Self-feeding and Locomotor Activity in Zebrafish (Danio Rerio)

To investigate daily feeding rhythms in zebrafish, the authors have developed a new self-feeding system with an infrared photocell acting as a food-demand sensor, which lets small-size fish such as zebrafish trigger a self-feeder. In this paper, the authors used eight groups of 20 fish. Locomotor activity rhythms were also investigated by means of infrared sensors. Under a 12?h:12?h light (L)-dark (D) cycle, zebrafish showed a clear nocturnal feeding pattern (88.0% of the total daily food-demands occurring in the dark phase), concentrated during the last 4?h of the dark phase. In contrast, locomotor activity was mostly diurnal (88.2% of total daily activity occurring in the light phase). Moreover, both feeding and locomotor rhythms were endogenously driven, as they persisted under free-running conditions. The average period length (τ) of the locomotor and feeding rhythms was shorter (τ?=?22.9?h) and longer (τ?=?24.6?h) than 24?h, respectively. During the time that food availability was restricted, fish could only feed during ZT0–ZT12 or ZT12–ZT16. This resulted in feeding activity being significantly modified according to feeding time, whereas the locomotor activity pattern remained synchronized to the LD cycle and did not change during this trial. These findings revealed an independent phasing between locomotor and feeding activities (which were mostly nocturnal or diurnal, respectively), thus supporting the concept of multioscillatory control of circadian rhythmicity in zebrafish. (Author correspondence: javisan@um.es)     

SYNCHRONIZATION TO LIGHT AND RESTRICTED-FEEDING SCHEDULES OF BEHAVIORAL AND HUMORAL DAILY RHYTHMS IN GILTHEAD SEA BREAM (SPARUS AURATA)

Food is not continuously available in the wild, and so most animals show a wide variety of circadian rhythms that can be entrained to feeding time. The aim of this research was to evaluate the effect of time-restricted feeding on the daily rhythms of gilthead sea bream, with food being provided during the day or night under a 12:12?h light-dark (LD) cycle or constant light (LL) conditions. Self-feeding and locomotor activity, as well as daily rhythms of cortisol, glucose, and melatonin, were evaluated. Fish synchronized their feeding behavior to the feeding phase, so that in LD they displayed 78% nocturnal feeding activity under night-feeding and 81% diurnal feeding activity under day-feeding, while under LL-feeding they displayed 72% of their daily activity during the 12?h feeding phase. In contrast, locomotor activity was mostly diurnal (66–71%), regardless of the feeding schedule, and it became arrhythmic under LL. Cortisol showed daily rhythms that peaked at different times, depending on the light and feeding schedule: one peak several hours before feeding under day-feeding and night-feeding, and two peaks under LL-feeding. Glucose displayed low-amplitude variations, with no daily rhythms being detected. Melatonin, however, showed a nocturnal rhythm, regardless of the feeding schedule, while the rhythm became attenuated under LL. Taken together, these results highlight the role of feeding on endocrine and metabolic rhythms, suggesting that feeding behavior should be considered when studying these variables. (Author correspondence: jflopez@um.es)     

Differential response of pineal melatonin levels to light at night in laboratory-raised and wild-captured 13-lined ground squirrels (Spermophilus Tridecemlineatus)

Pineal melatonin levels were compared in laboratory-raised or wild-captured 13-lined ground squirrels (Spermophilus tridecemlineatus) that were either exposed to 10 h of darkness at night or to light which had an irradiance of 400 μW/cm². In laboratory-born squirrels the period of darkness was associated with a gradual rise in pineal melatonin levels with peak values being reached at 0200 h, 6 h after darkness onset. Thereafter, melatonin levels decreased and were back to low daytime levels by 0800 h, 2 h after light onset. The exposure of laboratory-raised animals to an irradiance of 400 μW/cm² during the night totally prevented the nocturnal rise in pineal melatonin levels in these animals. In wild-captured ground squirrels the period of darkness at night was associated with a rapid rise in pineal melatonin such that by 2200 h, 2 h after lights out, peak melatonin values were already attained; additionally, melatonin levels remained high throughout the period of darkness but returned to daytime values by 0800 h. Exposure of wild-captured squirrels to a light irradiance of 400 μW/cm² during the normal dark period was completely incapable of suppressing pineal melatonin levels. The difference in the sensitivity of the pineal gland of laboratory-raised and wild-captured ground squirrels may relate to their previous lighting history.     

Morphologic changes in the pineal gland of rats exposed to continuous darkness

We examined the pineal structure of rats exposed to constant darkness (DD) at light microscopic level. Two groups of rats were exposed to 12:12 light/dark cycle (LD) or DD from their prenatal ontogenesis and then for 3 months after birth. The gland structure of DD rats was observed to have an active appearance. Some of the observed pinealocytes with light nuclei from DD rats were determined to contain double nucleoli. Nuclear area and perimeter of both dark and light types were greater in rats kept in DD than in LD. Rats exposed to DD had more cells with light nuclei and lesser cells with dark ones than rats kept in LD. No significant differences in nuclear characteristics of intermediate type were found between rats kept in LD and those kept in DD. The activity of mammalian pineal can be altered by light conditions to which the animal is exposed.