Circadian Rhythms of Melatonin and Sex Steroids in a Nocturnal Bird,Indian Spotted Owlet Athene brama During Reproductively Active and Inactive Phases

The epiphyseal neurohormone melatonin (MEL) exhibits circadian cyclicity, as noted extensively in diurnal vertebrates although very little information is available regarding nocturnal species. We have studied the MEL rhythmicity with 24-hour periodicity in a tropical nocturnal bird, Indian spotted owlet Athene brama, which possesses a well-developed pineal organ. We performed our study during two crucial reproductive phases (active and inactive), when the pineal gland activity in owlet exists in inverse states, i.e., inactive and active respectively. Independent of sex, the circadian rhythm of plasma MEL in owlets showed a two-peak cyclicity with a smaller peak at around 1400 h and the higher one at about 0200 h, while the lowest value was found at 1000 h. The night (0200 h) peak activity of plasma MEL in owlet has a resemblance with the earlier findings in diurnal birds and strongly suggests that independent of species habit the peak activity of MEL is invariably dark dependent. However, the daytime peak of MEL may be due to the daytime hiding nature of this nocturnal bird. Interestingly, it was also noted that the hours of peak activity of MEL (1400 and 0200 h) were the same during both of the reproductive phases, though the environmental day length was longer and ambient temperature was higher during the reproductively inactive phase. During daytime these birds hide in a dark burrow where, in general, the intensities of light and temperature are less, and the amplitude of variations of these factors is not prominent. Hence, the seasonal variations in these oscillatory components may not have affected the entrainment of the owlet pineal oscillator, which regulates the daily MEL rhythm in a similar pattern during both the studied phases. On the other hand, a single circadian peak (around 1000 h) circadian cyclicity of gonadal steroids (i.e., testosterone in the male and estradiol and progesterone in the female) showed an inverse relationship with plasma MEL. Possibly, MEL regulates the daily steroidogenic status in owlets by an inhibitory influence.     

Time and reproductive phase-dependent effects of exogenous melatonin on the pineal gland and ovary of a nocturnal bird, the Indian spotted owlet, Athene brama

In a tropical nocturnal bird, the Indian spotted owlet, Athene brama, the intraperitonial injection of an identical amount (20 mg/100 g b. wt/day) of exogenous melatonin (MEL) for 15 consecutive days increased the pineal weight and plasma MEL level in sexually active birds while it decreased them in inactive birds more potently when injected in the evening (18.30-19.30 h) rather than the morning (0500-0600 h). On the other hand, more efficiently than the morning hour treatment, the evening hour MEL injection decreased the ovary weight and plasma estradiol and progesterone levels both in sexually active and inactive birds, but more potently in active than inactive birds. Thus, the exogenous MEL showed the time and reproductive phase dependent effects on the pineal gland and the ovary of this nocturnal bird.     

A reproductive phase-dependent effect of dietary L-tryptophan on pineal gland and gonad of a nocturnal bird, Indian spotted owlet Athene brama

Unlike other temperate owls, Indian spotted owlet Athene brama possesses a well-developed pineal gland that secrets moderate amount of hydroxy- (serotonin) and methoxy- (melatonin) indoles in circulation. However, in this study, we have reported the response of this endocrine gland to exogenous L-Tryptophan (precursor of the above indoles), and also its effect on gonads of this nocturnal bird. During breeding phase or pineal inactive phase (March), oral treatment of L-Trp (0.5 mg/100 g Bwt/day) significantly increased the pineal gland wt and plasma melatonin (MEL) level, while decreased the gonadal wt and plasma sex steroids levels (estradiol and progesterone in female and testosterone in male). Interestingly, during reproductively quiescent phase or pineal active phase (August), similar amount of L-Trp significantly decreased the plasma MEL level, while increased the above sex steroid levels in plasma. Finally, the results show a clear reproductive phase-dependent inverse effect of L-Trp on pineal gland and gonads for both sexes of the spotted owlets, and suggest that the therapeutic use of this amino acid would be a great advantage for controlling the reproduction of these economically important birds.     

Reproductive phase depedent circadian variation in the pineal biochemical constituents of Indian palm squirrel, Funambulus pennanti

In mammals, pineal gland is intimately concerned with the co-ordination of rhythm physiology. Biochemical characteristics of pineal gland in man and other mammals may provide strong, yet sometimes elusive support for the belief in functional individuality and probable importance of this tiny gland. In seasonal breeding animals, pineal gland function is very much dependent on the reproductive status. Therefore, the aim of this experiment is to note the circadian rhythmicity of different biochemical constituents of pineal gland during active and inactive phases of reproductive cycle of a seasonally breeding rodent, F. pennanti. In the present study, pineal biochemical constituents i.e. protein and cholesterol showed higher values during daytime (1400 h). The plasma melatonin level presented two peaks during active (April; at 1800 h and 0200 h) and inactive (December; at 1400 h and 0200 h) phases of reproductive cycle. The pineal protein, cholesterol and plasma melatonin values in term of basal and peak levels were higher during the reproductive inactive/pineal active phase. Therefore, pineal--also known to have antigonadotropic properties and cholesterol which appears conjugated with pineal serotonin, presented circadian rhythmicity along with the plasma level of melatonin. This rhythmicity noted in present study was dependent on the reproductive and pineal activity status, and might be regulated by the sex steroid receptor present on the pineal gland.     

Colour also matters for nocturnal birds: owlet bill coloration advertises quality and influences parental feeding behaviour in little owls

Chromatic signals of offspring quality have been shown to play a role in parent–offspring communication in diurnal birds, but are assumed to be useless in dim light conditions because colour-based discrimination probably requires more light. A major ecological and evolutionary conundrum in this scenario is why the nestlings of some nocturnal owls display colourful beaks. Here, we test the hypothesis that yellow bill coloration of owlets of the nocturnal little owl Athene noctua may function as a chromatic signal revealing to parents aspects of quality of their offspring. In a first step, we examined physical variation in bill coloration and its covariation with owlet quality. Secondly, we studied parental provisioning in relation to an experimental manipulation of bill coloration of owlets. Bills of owlets showed higher within-nest variation in yellow–red chroma than in brightness. Plasma carotenoid concentration and nestling immunological status were not associated with chromatic or achromatic features of the bill. Interestingly, however, heavier owlets displayed more yellow bills than lighter ones. The effect of bill coloration on parental favouritism changed with brood size. Parents holding large broods preferentially fed owlets with enhanced over reduced yellow bill coloration, whereas those with small broods did not significantly bias feeding in relation to owlet bill coloration. Our results, based on integration of objective spectrophotometric assessment of colour and experimental procedures, confirm that parent little owls use bill coloration to reveal information on owlet body mass to adjust their feeding strategies, thus highlighting the importance of considering potential chromatic signals for a full comprehension of parent–offspring communication processes in nocturnal bird species.     

Nocturnal activity in the green sea turtle alters daily profiles of melatonin and corticosterone

In nature, green turtles (Chelonia mydas) can exhibit nocturnal activity in addition to their typically diurnal activity cycle. We examined whether nocturnal activity in captive and free-living green turtles altered daily plasma profiles of melatonin (MEL) and corticosterone (CORT). In captivity, diurnally active green turtles expressed distinct diel cycles in MEL and CORT; a nocturnal rise was observed in MEL and a diurnal rise was observed in CORT. However, when induced to perform both low- and high-intensity nocturnal activity, captive green turtles exhibited a significant decrease in MEL, compared to inactive controls. In contrast, plasma CORT increased significantly with nocturnal activity, and further, the relative increase in CORT was correlated with the intensity of the nocturnal behavior. In free-living green turtles that performed nocturnal activity including: nesting, mate searching, and feeding/swimming behaviors, plasma profiles in MEL and CORT exhibited relatively little, or no, daily fluctuation. Our findings demonstrate that nocturnal activity in green turtles is often associated with MEL and CORT profiles that resemble those measured during the day. We speculate that these conspicuous changes in MEL and CORT during nocturnal activity could either support or promote behaviors that enable acquisition of transient resources important to the survival and reproductive success of green turtles.     

ROLE OF ADENOSINE 3'', 5''-MONOPHOSPHATE IN THE REGULATION OF CIRCADIAN OSCILLATION OF SEROTONIN N-ACETYLTRANSFERASE ACTIVITY IN CULTURED CHICKEN PINEAL GLAND

—When pineal glands of 10–12-day-old chicks were organ-cultured in darkness, serotonin N-acetyltransferase activity was low during the daytime, increased at midnight and then decreased to the daytime level the next morning. The pattern of increase and decrease of enzyme activity in cultured pineal glands was comparable to the circadian rhythm of N-acetyltransferase activity in vivo. When pineal glands were kept at a low temperature for 5 h prior to culture, the phase of autonomous rhythm of enzyme activity was delayed. When chicken pineal glands were cultured during the daytime for 6 h, derivatives of adenosine 3′, 5′-monophosphate (cyclic AMP), cholera toxin, a high concentration of KCl and phosphodiesterase inhibitors increased N-acetyltransferase activity 3–7-fold, indicating an involvement of cyclic AMP in the regulation of N-acetyltransferase activity in chicken pineal gland as has been shown in rat pineal gland. When pineal glands were cultured at night in darkness, cholera toxin or a high KCl did not enhance the night-time increase of the enzyme activity. Derivatives of cyclic AMP or phosphodiesterase inhibitors enhanced the autonomous night-time increase of N-acetyltransferase activity in an additive or more than additive manner in cultured pineal glands. These observations suggest that adenylate cyclase of pinealocytes is inactive during daytime, but is activated at night in darkness, which is transduced to the synthesis of N-acetyltransferase molecules. Catecholamines suppressed the basal level and the nocturnal increase of N-acetyltransferase activity via α-adrenergic receptor. The nocturnal increase of enzyme activity was prevented by cycloheximide or actinomycin D. Cocaine, which stabilizes cell membrane potential or light exposure, blocked the nighttime increase of N-acetyltransferase activity in cultured chicken pineal glands.     

Diurnal rhythms of pinealocyte ultrastructure, pineal serotonin content and plasma melatonin level in the domestic pig

The study was conducted to investigate diurnal changes in pinealocyte ultrastructure, pineal serotonin content and plasma melatonin concentration in the domestic pig. The immature pigs (n=24) were kept under a cycle of 12 h light : 12 h dark, with a photophase between 0800 and 2000. During the photophase the animals were exposed to direct sunlight. After four weeks the gilts were slaughtered at 0900, 1400, 2100 and 0200. The pineals were removed and divided into two parts - one for quantitative ultrastructural study (by a point count method) and one for serotonin assay. Simultaneously, blood samples were taken for melatonin assay. The relative volume of mitochondria in pinealocyte perikarya was significantly higher at 1400 than at 0200 and 0900 as well as at 2100 than at 0200. The relative volume of Golgi apparatus was higher at 0900 and 1400 than at 0200. The relative volume of dense bodies of the MBB-1 type in pinealocyte perikarya was significantly lower at 1400 and 2100 than at 0900. In contrast, the relative volume of MBB-2 was higher at 1400 than at 0900 and 0200. The numerical density of DCV in perikarya was significantly higher at 0200 than at 1400. No significant differences were found in rough endoplasmic reticulum, lysosomes and multivesicular bodies. The pineal serotonin content showed a prominent rhythm with the maximum at 1400. The plasma melatonin concentration was significantly higher at 0200 than at 0900, 1400 and 2100. The obtained results demonstrate that both pinealocyte ultrastructure and pineal biochemistry in the pig undergo significant changes in the course of the diurnal rhythm.     

Pineal gland of a nocturnal bird, Indian spotted owlet, Athene brama: morphological and endocrine observations

It has been reported that owls (Strigiformes) do not have a pineal gland. However, our light microscopy study revealed an intermediate form of tubulofollicular and solid-type large pineal gland in a tropical owlet, Athene brama. The epithelial cells forming follicles (6-8) in the distal region and the solid cluster of parenchymal cells of different diameters in the proximal region anteriorly tapered with a long cylindrical stalk and continued into commissural organs and choroid plexus. The intrapineal localization of perivascular nerve fibers and blood vessels clearly explained the sympathetic innervation as well as vascularization of this neuroendocrine gland. Further, electron microscopy revealed a developed intracellular structure of the pinealocytes with a large number of mitochondria, Golgi bodies, and granular as well as clear vesicles in the process terminals. The evidence of intrapinealocyte lipid droplets and dense bodies and a moderate amount of melatonin in plasma (ranging from 100-365 pg/mL) during different reproductive phases finally proved a defined secretory activity of the gland in this tropical, nocturnal bird.     

Repeated exposures to daytime bright light increase nocturnal melatonin rise and maintain circadian phase in young subjects under fixed sleep schedule

Effects of two different light intensities during daytime were examined on human circadian rhythms in plasma melatonin, core body temperature, and wrist activity under a fixed sleep schedule. Sleep qualities as indicated by polysomnography and subjective sleepiness were also measured. In the first week, under dim light conditions ( approximately 10 lx), the onset and peak of nocturnal melatonin rise were significantly delayed, whereas the end of melatonin rise was not changed. The peak level of melatonin rise was not affected. As a result, the width of nocturnal melatonin rise was significantly shortened. In the second week, under bright light conditions ( approximately 5,000 lx), the phases of nocturnal melatonin rise were not changed further, but the peak level was significantly increased. Core body temperature at the initial sleep phase was progressively elevated during the course of dim light exposure and reached the maximum level at the first night of bright light conditions. Subjective sleepiness gradually declined in the course of dim light exposure and reached the minimum level at the first day of bright light. These findings indicate that repeated exposures to daytime bright light are effective in controlling the circadian phase and increasing the peak level of nocturnal melatonin rise in plasma and suggest a close correlation between phase-delay shifts of the onset of nocturnal melatonin rise or body temperature rhythm and daytime sleepiness.     

Seasonal Adreno-Cortical Cycle of a Nocturnal Bird,Indian Spotted Owlet Athene brama: Biochemical and Morphological Observations

It is well reported that the environmental factors along with different endocrine stimulus play a crucial role in maintenance of adrenocortical activity in birds. This study is first to report a detailed seasonal activity cycle of adrenal cortex, particu-larly its secretory physiology in a tropical nocturnal bird, Indian spotted owlet Athene brama. The maximum cortical activity having highest glandular mass, glandular free cholesterol, esterified cholesterol profiles, and peak level of corticosterone in plasma coincided with the long day length, highest temperature and increasing amplitude of relative humidity and rainfall of the early summer month, May. Cortical activity declined to minimum level in August when the ecofactors also declined parallely and hence, the birds entered into partial hibernation. The cortical activity progressed slowly throughout the winter (September–March) to reach maximum level in May. Further, the electron microscopic observations of cortical cell morphology strongly supported the above seasonal activity status of the gland revealing a comparatively large number of mitochondria during May than August, along with lipid filled vacuoles during May but not in August. Besides, assessment of gonadal and pineal hormones in relation with seasonal activity of adrenal cortex presented a parallel relationship with gonad while completely inverse relationship with pineal. Therefore, the study concludes that the seasonal adrenocortical activity of this tropical nocturnal bird might be regulated by multiple factors, particularly by the environmental temperature, humidity/rainfall and photoperiod along with the internal factors at least by gonadal and pineal hormones.     

Seasonal Adreno-Cortical Cycle of a Nocturnal Bird, Indian Spotted Owlet Athene brama: Biochemical and Morphological Observations

It is well reported that the environmental factors along with different endocrine stimulus play a crucial role in maintenance of adrenocortical activity in birds. This study is first to report a detailed seasonal activity cycle of adrenal cortex, particu-larly its secretory physiology in a tropical nocturnal bird, Indian spotted owlet Athene brama . The maximum cortical activity having highest glandular mass, glandular free cholesterol, esterified cholesterol profiles, and peak level of corticosterone in plasma coincided with the long day length, highest temperature and increasing amplitude of relative humidity and rainfall of the early summer month, May. Cortical activity declined to minimum level in August when the ecofactors also declined parallely and hence, the birds entered into partial hibernation. The cortical activity progressed slowly throughout the winter (September-March) to reach maximum level in May. Further, the electron microscopic observations of cortical cell morphology strongly supported the above seasonal activity status of the gland revealing a comparatively large number of mitochondria during May than August, along with lipid filled vacuoles during May but not in August. Besides, assessment of gonadal and pineal hormones in relation with seasonal activity of adrenal cortex presented a parallel relationship with gonad while completely inverse relationship with pineal. Therefore, the study concludes that the seasonal adrenocortical activity of this tropical nocturnal bird might be regulated by multiple factors, particularly by the environmental temperature, humidity/rainfall and photoperiod along with the internal factors at least by gonadal and pineal hormones.     

Daily cycles in plasma and ocular melatonin in demand-fed sea bass, Dicentrarchus labrax L.

Melatonin is regarded as an internal zeitgeber, involved in the synchronization to light of the daily and seasonal rhythms of vertebrates. To date, plasma and ocular melatonin in fish have been extensively surveyed almost solely in freshwater species – with the exception of some migrating species of salmonids. In the present paper, melatonin levels of a marine species (sea bass, Dicentrarchus labrax L) were examined. In addition, the daily rhythms of the demand-feeding activity of sea bass, a fish species characterized by a dual phasing capacity (i.e. the ability to switch between diurnal and nocturnal behaviour), were investigated before sampling. Sea bass, distributed in 12 groups of four fish and kept under constant water temperature and salinity, were exposed to a 12 h light:12 h dark cycle (200:0 lx, lights on at 0800 hours). After 4 weeks recording, the animals were killed at 0900, 1200, 1400, 1600, 1900, 2100, 2400, 0200, 0400, 0700 and 0900 hours. Actograms of demand-feeding records revealed a nocturnal feeding behaviour, with some cases of spontaneous inversions in phasing. Melatonin levels in plasma peaked in the middle of the dark phase, dropping after lights on. Melatonin in the eye, on the contrary, exhibited an inverse profile, with high levels during daytime and low levels at night. These results suggest that melatonin in the plasma and the eye may act independently on the flexible circadian system of sea bass. Accepted: 30 January 1997     

Nocturnal increase in plasma cGMP levels in humans.

The circadian dynamics of responses to cyclic guanosine 3',5'-monophosphate (cGMP) in in vitro experiments and the stimulating effects of the pineal hormone melatonin on cGMP levels both in vitro and in vivo provoked an investigation into the diurnal pattern of occurrence of this second messenger in human plasma and its correlation with plasma melatonin levels. Plasma cGMP levels were measured in 9 normal human subjects who were over 50 years of age. Samples were obtained hourly through a 20-h period (11 a.m. to 7 a.m.) that included the subjects' habitual hours of nocturnal sleep; physical activity was kept to a minimum during the daylight hours. The area under the time-plasma cGMP concentration curve showed a significant increase during the period of nocturnal sleep compared to that observed during the period of daytime wakefulness. The individual temporal pattern of the nocturnal rise in plasma cGMP differed among the subjects; however, the initial increase typically was observed soon after bedtime. No significant correlation was observed between individual nocturnal plasma melatonin levels and cGMP levels.     

Pineal melatonin: circadian rhythm and variations during the hibernation cycle in the ground squirrel, Spermophilus lateralis

Variations in pineal melatonin content throughout a 24-hour period and during different phases of the hibernation bout cycle were studied in the golden-mantled ground squirrel (Spermophilus lateralis). In addition to pineal melatonin, the circadian variation in the activities of pineal N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) were also investigated in summer animals maintained at 22 +/- 2 degrees C, on a light:dark (L:D) schedule of 12:12 hr for 1 month (lights on at 08.00 hr). Pineal glands were collected from six animals in each group at 1200, 1600, 2000, 2400, 0200, 0400, and 0800 hr. Changes in pineal melatonin content during the hibernation bout cycle were investigated in ground squirrels housed at 4 +/- .05 degrees C in relative darkness (1.9-3.4 lux; 10:14 LD). Pineal glands were obtained between 12:00 and 18:00 hr from 30 animals during one of three phases of the cycle (deep hibernation, euthermic interbout, and entrance into hibernation). Pineal melatonin was also measured for comparison in six winter euthermic animals that were housed at 22 +/- 2 degrees C, on a L:D schedule of 10:14 hr. Melatonin was measured in individual pineal glands by radioimmunoassay. The daily melatonin rhythm in S. lateralis was characterized by a marked increase in pineal melatonin during the dark phase, in which peak nighttime values were nearly 20-fold greater than daytime basal levels. The daily rhythm for NAT activity paralleled the changes in melatonin, showing a peak activity at 0200 hr that was 45 times greater than mean daytime values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inverse relationship between diurnal rhythms in plasma levels of thyroid hormones and pineal arylalkylamine-N-acetyltransferase (AANAT) activity in an air-breathing fish,Clarias gariepinus

Plasma levels of thyroid hormones (T₄ and T₃) and pineal AANAT activity exhibited prominent diurnal rhythms during all the four phases (quiescent, progressive, breeding, and regressive) of the fish, Clarias gariepinus. During all the four phases, we observed that (i) the acrophase (peak) of the circadian rhythm of both T₃ and T₄ occurred around midday, while the acrophase of AANAT activity rhythm was recorded during midnight, (ii) a significant negative correlation existed between plasma levels of the thyroid hormones and AANAT activity, and (iii) a direct relationship existed between water temperature/daylength and plasma levels of thyroid hormones, and an inverse relationship between water temperature/daylength and AANAT activity. On the basis of these findings, we conclude that an inverse relationship exists between the diurnal rhythms in plasma levels of thyroid hormones and pineal AANAT activity. The observed inverse relationship between the levels of thyroid hormones and pineal AANAT activity seems to be maintained by seasonal changes in water temperature and daylength.     

Circadian Variation of Cyclic AMP in the Rat Pineal Gland

Abstract: This study was carried out to investigate circadian variation of cyclic AMP contents in the rat pineal glands, using the high-energy microwave radiation technique. The pattern of cyclic AMP concentration in the pineal gland showed a distinct circadian variation, with the maximum level at 0200 and the lowest at 1400. The administration of propranolol completely blocked the dark-induced increase in the pineal cyclic AMP level at 0200, and the administration of isoproterenol induced a threefold, rapid increase in the cyclic AMP level at 1400, although it did not change the level at 0200.     

Photic and nonphotic inputs to the diurnal circadian clock

Diurnal animals occupy a different temporal niche from nocturnal animals and are consequently exposed to different amounts of light as well as different dangers. Accordingly, some variation exists in the way that diurnal animals synchronize their internal circadian clock to match the external 24-hour daily cycle. First, though the brain mechanisms underlying photic entrainment are very similar among species with different daily activity patterns, there is evidence that diurnal animals are less sensitive to photic stimuli compared to nocturnal animals. Second, stimuli other than light that synchronize rhythms (i.e. nonphotic stimuli) can also entrain and phase shift daily rhythms. Some of the rules that govern nonphotic entrainment in nocturnal animals as well as the brain mechanisms that control nonphotic influences on rhythms do not appear to apply to diurnal animals, however. Some evidence supports the idea that arousal or activity plays an important role in entraining rhythms in diurnal animals, either during the light (active) or dark (inactive) phases, though no consistent pattern is seen. GABAergic stimulation induces phase shifts during the subjective day in both diurnal and nocturnal animals. In diurnal Arvicanthis niloticus (Nile grass rats), SCN GABA_A receptor activation at this time results in phase delays while in nocturnal animals phase advances are induced. It appears that the effect of GABA at this circadian phase results from the inhibition of period gene expression in both diurnal and nocturnal animals. Nonetheless, the resulting phase shifts are in opposite directions. It is not known what stimuli or behaviours ultimately induce changes in GABA activity in the SCN that result in alterations of circadian phase in diurnal grass rats. Taken together, studies such as these suggest that it may be problematic to apply the principles governing nocturnal nonphotic entrainment and its underlying mechanisms to diurnal species including humans.     

Circadian rhythms of melatonin in European starlings exposed to different lighting conditions: relationship with locomotor and feeding rhythms

In passerine birds, the periodic secretion of melatonin by the pineal organ represents an important component of the pacemaker that controls overt circadian functions. The daily phase of low melatonin secretion generally coincides with the phase of intense activity, but the precise relationship between the melatonin and the behavioral rhythms has not been studied. Therefore, we investigated in European starlings (Sturnus vulgaris) (1) the temporal relationship between the circadian plasma melatonin rhythm and the rhythms in locomotor activity and feeding; (2) the persistence of the melatonin rhythm in constant conditions; and (3) the effects of light intensity on synchronized and free-running melatonin and behavioral rhythms. There was a marked rhythm in plasma melatonin with high levels at night and/or the inactive phase of the behavioral cycles in almost all birds. Like the behavioral rhythms, the melatonin rhythm persisted for at least 50 days in constant dim light. In the synchronized state, higher daytime light intensity resulted in more tightly synchronized rhythms and a delayed melatonin peak. While all three rhythms usually assumed a rather constant phase relationship to each other, in one bird the two behavioral rhythms dissociated from each other. In this case, the melatonin rhythm retained the appropriate phase relationship with the feeding rhythm. Accepted: 10 December 1999     

Reproductive phase dependent circadian variation in hypothalamic concentration of serotonin, dopamine and peripheral thyroxine levels in Japanese Quail following 5-HTP and L-DOPA administration at specific time intervals

Temporal phase relations of circadian hypothalamic neurotransmitters are reported to regulate seasonal reproduction in some avian species. Present experiments were designed to study circadian variation in the hypothalamic concentration of neurotransmitters (serotonin and dopamine) and the plasma thyroxine level in sexually active (long day) and inactive (short day) Japanese Quail. A significant circadian cycle was noted in the hypothalamic content of both serotonin and dopamine, but with different patterns. In breeding Quail, peak activity of serotonin and dopamine was noted at 10.00 A.M. and 10.00 P.M. respectively i.e. at the interval of 12 hours. However, during sexually quiescent condition, peaks of both neurotransmitters occurred at 2.00 P.M. i.e. having a 0-hour temporal relationship. During the breeding phase, the plasma thyroxine level showed a biphasic pattern with two circadian peaks at 10.00 A.M. and 10.00 P.M. whereas in the non-breeding condition a single peak was observed at 10.00 A.M. In the second experiment, to study the effect of temporal synergism of neurotransmitter precursor drugs on circadian cycles, two groups of Quail were administered daily with serotonin precursor 5-HTP (5-hydroxytryptophan) and dopamine precursor L-DOPA (L-dihydroxyphenylalanine) (5 mg/100 g body weight) 12 hour (12-hr) and 8 hour (8-hr) apart over a period of 11 days under continuous conditions of light and then transferred to long day length for 15 days when the experiment was terminated. When compared to controls, the 12-hr condition induced breeding while the 8-hr condition led to a non-breeding condition. The circadian pattern of serotonin levels of control and 12-hr Quail was similar to that of a normal sexually active bird, while that of the 8-hr Quail showed the pattern of a sexually inactive bird. The plasma thyroxine level exhibited a biphasic pattern in 12-hr Quail, which was similar to a normally breeding bird, whereas unlike sexually inactive birds, the thyroxine concentration in 8-hr Quail was relatively low and did not show significant cyclicity. Interestingly, the plasma testosterone level of 12-hr Quail followed a more or less similar pattern with peak activities coinciding with that of thyroxine i.e. biphasic in the sexually active condition (12-hr and control) but a single peak in the quiescent (8-hr) condition. These findings suggest that the temporal phase relation of circadian serotonergic and dopaminergic oscillator varies as a function of reproductive status of the bird, and breeding/non-breeding conditions may be induced experimentally by changing the phase relation of these oscillations.