Diurnal rhythmicity and hypothalamic deficits in glucose utilization in aged ovariectomized rats

Aging is associated with a loss of cyclic gonadotropin release in female animals. This deficit may reflect dampened circadian rhythmicity of neuroendocrine events and/or altered function in hypothalamic nuclei important to regulation of cyclic female reproduction. The purpose of this study was to determine if diurnal periodicity and glucose metabolism in the hypothalamus are altered with age and whether such changes could help to explain the age-related deficits in gonadotropin release. Young (3-4-month-old) and old (18-21-month-old) rats were ovariectomized and subjected to the 2-deoxy-D-1-14C-glucose technique to measure rates of cerebral glucose utilization (GU), an index of neural function (Sokoloff et al., 1977) in various brain areas and in the pineal gland. We measured GU during the light (1400 hours) and the dark (2200 hours) in 17 anatomical regions including the following hypothalamic areas: medial preoptic nucleus, suprachiasmatic preoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, arcuate nucleus, and median eminence. Serum concentrations of luteinizing hormone (LH) and prolactin were measured in the same rats to determine the effect of age on both of these hormones. Diurnal periodicity of GU was observed in the suprachiasmatic nucleus and the pineal gland in young and old rats. Although there was no age difference in GU of the pineal gland, GU was reduced during the light and dark in the suprachiasmatic nucleus and all other hypothalamic areas examined except the suprachiasmatic preoptic nucleus and the median eminence. Ovariectomy induced an attenuated increase in concentrations of LH in old, compared to young rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Day and night rhythms in the methylation of N-acetylserotonin/5-hydroxytryptophol in the pineal gland of male rats of different ages

Summary Next to a night rhythm of the methylation of N-acetylserotonin/5-hydroxytryptophol the presence of a daytime rhythm could also be established. Rhythmicity was studied in May during the night and in June during daytime in 21, 42 and 70 days old male Wistar rats. In 21 days old rats, moderate HIOMT activity was observed from 12 p.m.–4 a.m. In rats aged 42 and 70 days HIOMT activity was increased showing a peak at 4 a.m. In September this night maximum is observed at 12 p.m. in rats, aged 42 days. This points to the presence of a seasonal change in HIOMT rhythmicity during the night.During daytime a moderate HIOMT activity is present, which reaches a maximum at 2 p.m. in the 21 and 42 day old animals, while in the adult 70 days old rats activity starts to increase at 2 p.m. probably reaching a maximum at 6 p.m.     

Calbindin D28K protein cells in a primate suprachiasmatic nucleus: localization, daily rhythm and age-related changes

In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian pacemaker. The SCN controls daily rhythms and synchronizes the organism to its environment and especially to photic signals. Photic signals via the retinohypothalamic tract reach the ventral part of the SCN, where the majority of calbindin-containing neurons are located. Calbindin cells seem important for the control of circadian rhythmicity. As ageing leads to marked changes in the expression of circadian rhythms, we investigated in the mouse lemur, a nocturnal primate, age-related changes in the oscillation of calbindin protein expression in SCN neurons. We used immunohistochemistry and quantitative analysis of calbindin expression in the SCN of adult and aged mouse lemurs. In this primate, a dense cluster of calbindin-positive neurons was found in the ventral part of the SCN. In adult animals, calbindin-positive SCN neurons did not exhibit daily rhythms in their number or intensity, but exhibited significant daily variations in the percentage of cells with a calbindin-positive nucleus, characterized by high values during the daytime and low values during the night. Immunoreactive intensity peaked in the middle of the daytime. Calbindin expression in the nuclei of calbindin cells in the SCN tends to be modified by ageing. The amplitude of daily variation in calbindin expression was damped, with a lower immunointensity during the daytime and a delayed decrease during the night. These changes may affect the ability of the SCN to transmit rhythmic information to other SCN cells and thereby modify the synchronization of the different cell populations in the SCN.     

Autoradiographic assessment of pineal gland glucose utilization and capillary permeability in the unanesthetized rat

Pineal gland glucose utilization (GU) and capillary permeability (CP) were measured in unanesthetized rats, using complementary quantitative autoradiographic techniques. GU values within the pineal tissue were homogeneously distributed around 70 mumol of glucose/100 g each min, i.e., they were approximately 30% lower than in the cortical gray structures. The blood-to-brain transfer constant of [14C]-alpha-aminoisobutyric acid, as an index of CP, was up to ten orders of magnitude higher than that for the rest of the brain. These measurements were carried out at that point in the circadian rhythm that corresponds to the minimum level of neurosecretory activity of the pineal gland.     

Circadian variations of adrenergic receptors in the mammalian pineal gland: A review

Summary Pineal adrenergic receptor numbers show circadian variations in both rat and Syrian hamster. In the rat pineal -adrenergic receptor density reaches peak values either late in the light phase or at middark; the differences in the circadian phase seem related to the light:dark cycle to which the animals are exposed. No circadian rhythm of pineal -adrenergic receptors is documented in intact rats. In the Syrian hamster pineal -adrenergic receptor density is high throughout the light phase and drops to minimal values at the time of the nocturnal peak of melatonin production. The circadian rhythm of pineal -adrenergic receptor numbers runs parallel to the -adrenergic receptor variation, but is less pronounced.In the rat, pineal melatonin production is rapidly induced by -adrenergic agonists at any time during a 24-hour period, even when the pinealocyte -adrenergic receptor number is lowest (early in the light phase). In contrast, the Syrian hamster pineal seems most responsive to -adrenergic agonists in the late night while being less responsive during the day when -adrenergic receptor density is high. Interestingly, the human pineal gland is also not especially responsive to adrenergic stimulation during the light phase, possibly making the Syrian hamster pineal a better model than the rat pineal for determining neural/pineal interactions in humans. Comparison of the circadian variations in pineal adrenergic receptors leads to the conclusion that the functional differences between rat and hamster pineal are probably not explicable in terms of the adrenergic receptors, but are caused most likely by (a) intracellular mechanism(s) beyond the adrenergic receptors.     

Influence of some pteridines on pineal 5-methoxy-indole synthesis in male wistar rats periodically exposed to either white or red light

In previous investigations the change of circadian rhythmicity in pineal melatonin/5-methoxytryptophol synthesis of rats periodically exposed to red light was similar to that in pineals of rats incubated with pterin-6-aldehyde. These experiments were, however, performed with rats of different age and in different periods of the year. In the present study these two factors influencing pineal indole metabolism have been combined the experiments being carried out in rats aged 28 days and during the same day in the month of January. It was observed that under influence of red light the peak of melatonin/5-methoxytryptophol synthesis shifted towards daytime, whereas incubation with pterin-6-aldehyde did not cause such a shift. If under different experimental conditions the mean amount of melatonin/5-methoxytryptophol which was formed over a 24 hour period was compared, it appeared that pineals of rats exposed to white light incubated with reduced neopterin but not pineals incubated with pterin-6-aldehyde behave in this respect similar to pineals of rats exposed to red light. However, if the ratio between melatonin/5-methoxytryptophol and 5-methoxytryptamine is calculated pineals of white light exposed rats incubated in pterin-6-aldehyde behaved very similar to the pineals of rats exposed to red light. Although the role of pteridines remains obscure, it appears that the parameters 2. circadian rhythmicity and 2. the amount of 5-methoxyindoles and 3. the ratio between these indole derivatives might be of importance in analyzing their physiological effects. The influence of application of light of different wavelengths and year rhythmicity is discussed.     

Response of pineal serotonin N-acetyltransferase activity in male guinea pigs exposed to light pulses at night

Summary Serotonin N-acetyltransferase (NAT), which is crucial for the formation of melatonin, undergoes a typical day/night rhythm in the pineal gland with low levels during daytime and high levels at night. Short pulses of light given at night have been shown to rapidly depress NAT activity in some species, but not in others, the reasons for this difference being unclear. As diurnality and nocturnality of the experimental animals may play a role and since diurnally active animals have been little investigated in this respect, in the present study the diurnally active guinea pig was investigated. Male guinea pigs kept under a lighting regimen of LD 1212 (lights off at 1700 hrs) were killed between 1200 or 1300 hrs and between 0000 and 0200 hrs, at night in the dark or after exposure to 10 or 45 min of light. The results obtained show that the day/night difference of NAT activity is about 2-fold. 10 min or 45 min of light given at night significantly depress pineal NAT activity. Re-exposure to darkness for 1 hr of animals previously given light for 10 min leads to restoration of NAT activity. These findings together with data from the literature suggest that it does not appear to be the activity pattern (diurnality versus nocturnality) of an animal nor the amplitude of the day/night difference of pineal NAT activity that account for the suppressibility or non-suppressibility of pineal NAT activity by light at night.     

Analysis of in vitro glucose utilization in a circadian pacemaker model.

An in vitro glucose utilization method, based upon 14C-2-deoxyglucose kinetics in brain slices, has been used to study circadian rhythms in hypothalamic slices containing the suprachiasmatic nucleus (SCN). Spontaneous SCN metabolic activity in vitro is similar to that observed in vivo with higher metabolic rates in subjective daytime and lower rates during subjective night. However, in vitro SCN metabolic activity during late subjective day is above that seen when glucose utilization is measured in vivo, suggesting that an inhibitory influence normally active in vivo is lost during slice isolation. Incubation of slices containing SCN in the presence of TTX exposes a TTX-insensitive component of metabolic activity in early subjective day, supporting prior suggestions that glucose utilization by the circadian oscillator continues in the absence of Na(+)-dependent action potentials. Studies with high Mg2+ concentrations are consistent with the hypothesis that most metabolic activity above the basal level observed with the glucose utilization method is related to synaptic activity. Pharmacological studies of the SCN brain slice model with radiotracers offer potential for analysis of both circadian rhythmicity and neural regulation.     

Age-related changes in GABA and benzodiazepine receptor binding in rat brain are influenced by sampling time

1. This study examined the saturation binding of tritiated gamma-aminobutyric acid [( 3H]GABA) and [3H]diazepam in brain membranes from young (3 month-old) and aged (21-23 month-old) Long Evans male rats killed at two time points in the 24-hour cycle. 2. The daytime density of low-affinity GABA binding sites was significantly (p less than 0.05) lower in cortical membranes from aged animals. There were no differences between young and old rats in low-affinity GABA binding at night, or in high-affinity GABA binding at either time point. 3. Diazepam binding was significantly lower in the brains of aged animals killed during the daytime. There was no differences at night, when diazepam binding in young animals declined to match that of aged animals. 4. There were no differences in the affinities of either GABA or diazepam binding sites. 5. These findings indicate that sampling time significantly influences age-associated changes in the densities of low-affinity GABA and diazepam binding sites. Therefore, the effects of age on brain receptor binding parameters should be measured at several points in the 24-hour light/dark cycle in order to control for possible age-related changes in binding rhythmicity.     

Melatonin synthesis in the pineal gland of the Richardson's ground squirrel (Spermophilus richardsonii): Influence of age and insulin-induced hypoglycemia

Summary The nocturnal rises in pineal N-acetyltransferase (NAT) activity and melatonin levels were compared in young (25–35 days old) and adult (at least 1 year old) Richardson's ground squirrels. When expressed as NAT activity per pineal gland, the nighttime rise in the activity of this enzyme was less in young than in the adult animals; conversely, the melatonin content of the pineal glands of young animals was higher at one point (4 a.m., 8 hours after darkness onset) when compared to that in adult squirrels. When data were expressed relative to total protein, the NAT and melatonin rhythms in the pineals of young and adult animals were very similar. The effect of insulin-induced hypoglycemia on both daytime and nighttime NAT and melatonin levels in the pineal gland of the Richardson's ground squirrel was also assessed. Low daytime levels of these constituents were not influenced by the administration of 10 units insulin, a treatment which caused a marked drop in circulating glucose levels. At night, when pineal NAT and melatonin levels were high insulin injection had a very modest stimulatory effect on NAT activity (one point was elevated above saline injected controls) while melatonin levels remained unchanged by the treatment. These findings in the ground squirrel in reference to insulin-induced hypoglycemia, and Stressors in general, appear to differ from those in the rat where stress can have a substantial influence or both low daytime and high nighttime levels of pineal NAT and melatonin.     

Circadian analysis of mononuclear cells in the rat following pinealectomy and superior cervical ganglionectomy

The role of the pineal gland and adrenocorticosteroids in circadian rhythmicity of immune function was investigated in Sprague-Dawley rats that either had been pinealectomized (PX) or had undergone superior cervical ganglionectomy (SCGX), which functionally denervates the pineal. For both experiments, conducted between August and October, rats were entrained to a 10-h light:14-h dark cycle and fed ad libitum. The circadian rhythm in monocytes (ANOVA, p less than .01 for Control + Sham + PX samples) tended to parallel that of plasma corticosteroid levels (ANOVA, p less than .01) with the peak during early dark and the nadir at early light. In comparison, suppressor/cytotoxic T cells and B cells tended to be more frequent during early light with minimum levels during early dark (ANOVA, p less than .05 and .01, respectively, for Control + Sham + PX samples). Natural killer (NK) activity in control animals varied significantly (ANOVA, p less than .05) over the light:dark cycle with peak activity during early dark in the SCGX experiment, but was only slightly elevated during late dark in the PX experiment. PX animals exhibited increased frequency of NK cells, but only in samples collected during the day (t test, p less than .05). This effect was not observed in sham-operated PX controls and in SCGX animals. Neither PX nor SCGX had a significant (ANOVA) effect on plasma corticosterone levels. These observations in the rat are consistent with findings in mice and humans that corticosteroids play an important general role in circadian rhythmicity of immune functions. In contrast, the present study suggests that pineal gland influence of immune function(s) is more specific and that pineal gland interaction with NK cells has a circadian component.     

Development of circadian rhythmicity and photoperiodic response in subdivisions of the rat suprachiasmatic nucleus

To ascertain whether the circadian rhythmicity of the ventrolateral (vl) suprachiasmatic nucleus (SCN) develops concurrently with that of the dorsomedial (dm) SCN and when the rhythmicity starts to respond to day length, i.e., to the photoperiod, rats with their offspring were maintained under either a long photoperiod with 16 h of light and 8 h of darkness per day (LD 16:8) or under a short, LD 8:16 photoperiod. The rhythms of spontaneous c-Fos immunoreactivity in the dm-SCN and of the light-induced c-Fos immunoreactivity in the vl-SCN were studied in the pups. In 3- and 10-day-old rats, the dm-SCN rhythm in spontaneous c-Fos immunoreactivty was already well expressed but a response to a photoperiod similar to that in adult rats has not yet been developed. The vl-SCN gate for insensitivity of c-Fos production to light at certain times was detected in 10-day but not yet in 3-day-old rats: in the latter, light exposure at any daytime induced high c-Fos immunoreactivity. In the 10-day-old pups, similarly as with adult rats, the gate was shorter under LD 8:16 than under LD 16:8, but the difference in the gate duration between the short and the long photoperiod did not yet attain that of adult animals. The data indicate that the circadian rhythmicity may develop sooner in the dm-SCN, than in the vl-SCN, whereas the photoperiodic response may develop sooner in the vl-SCN.     

Ageing and the diurnal expression of mRNAs for vasoactive intestinal peptide and for the VPAC2 and PAC1 receptors in the suprachiasmatic nucleus of male rats

Ageing alters fundamental aspects of circadian rhythmicity in mammals; the effects include reduced rhythm amplitude and alterations in period length and in entrainment to the light/dark cycle. Such changes may reflect disruptions in cellular function within the suprachiasmatic nucleus (SCN), the site of the predominant circadian pacemaker. In the SCN, vasoactive intestinal peptide (VIP)-synthesizing neurones receive various inputs, including retinohypothalamic projections containing pituitary adenylate cyclase activating peptide (PACAP). SCN VIP cells establish connections with local neurones and send efferents beyond the nucleus. Considerable evidence implicates VIP and PACAP in circadian rhythm maintenance and/or entrainment to photic Zeitgebers. These actions involve members of a distinct family of receptors; mRNAs for two such receptors, VPAC2 and PAC1, are present in the SCN. This study used isotopic in situ hybridization to examine the effects of ageing on expression of mRNAs for VIP, VPAC2 and PAC1 in the SCN of male rats under a 12 : 12 h light/dark cycle. Analysis of film autoradiographs from young adult (2-3 months) or aged (19-20 months) rats, at eight time points across the light/dark cycle, showed loss of diurnal rhythmicity and reduced levels for VIP mRNA in the aged group. A diurnal rhythm of VPAC2 receptor mRNA was present in both groups, but its levels were reduced in the aged rats. There were no differences between the two groups for PAC1 receptor mRNA expression. The present results indicate that ageing reduces VIP and VPAC2 receptor mRNA and eliminates diurnal expression of VIP mRNA within the SCN of aged male rats.     

Rapid recovery of pineal function after partial denervation: a possible role for heteroneuronal uptake of transmitter in modulating synaptic efficacy

The pineal gland is innervated by sympathetic neurons whose cell bodies are located in the two superior cervical ganglia and whose axons reach the gland via the two internal carotid nerves (ICNs). Bilateral decentralization of the superior cervical ganglia, produced by lesioning both cervical sympathetic trunks (CSTs), abolishes the circadian rhythm in the activity of the pineal enzyme serotonin N-acetyltransferase (NAT). We have examined the effects on NAT activity of unilaterally cutting the ICN or the CST. During the first night after either operation, nocturnal NAT activity was reduced by 75% compared to controls. However, during the second night after unilaterally cutting the ICN, NAT activity was restored to control values, and normal enzyme activity was seen in these lesioned animals for up to 1 month after this operation. On the other hand, following unilateral decentralization of one superior cervical ganglion, enzyme activity was reduced for at least 5 months. The high enzyme activity in animals with one ICN cut was abolished by cutting the contralateral CST, indicating that the recovery of NAT activity depended on the remaining intact sympathetic neurons. Electrical stimulation of the intact ICN during the daytime in animals in which the contralateral ICN was cut produced an increase in pineal NAT activity which was greater than the increase seen when similar stimulation was performed in sham-operated animals or in animals in which the contralateral superior cervical ganglion had been decentralized. The time course of the recovery of nocturnal NAT activity after unilateral denervation of the pineal gland was similar to the time course of the decrease in norepinephrine uptake sites in the gland.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pineal and associated neuroendocrine rhythms

(1) The mammalian pineal gland exhibits a number of biosynthetic rhythms. (2) Pineal serotonin levels are highest during the day and lowest at night. (3) Conversely, the activity of serotonin N-acetyltransferase, which converts serotonin to N-acetylserotonin, is greatly increased at night over daytime values. (4) As a consequence, N-acetylserotonin content increases in the pineal gland at night as does the product of its metabolism, melatonin. (5) Either indoleamines, such as melatonin, or polypeptides, such as arginine vasotocin, may be the pineal substances which influence the secretion of hormones from other endocrine glands. (6) Besides the obvious circadian rhythms within the pineal gland there are indications that the activity of this organ also changes on a seasonal basis, being more active during the winter than during the summer. (7) One physiological niche of the pineal gland may be to act as an intermediary between seasonal photoperiodic changes and the neuroendocrin reproductive axis. (8) In long day breeders, the pineal is believed tobe most active during the winter and, as a result, its hormonal products suppress sexual physiology. (9) This ensures that animals can only breed during restricted periods of the year.     

Effects of ageing on the behavioural responses to dopamine agonists: decreased yawning and locomotion, but increased stereotypy

Sensorimotor function and the behavioural responses to a range of doses of subcutaneous apomorphine were assessed in mature (6-8 months) and old (23-26 months) Sprague-Dawley rats of comparable weight. In addition, the locomotor activity response of 12-month-old and 24-month-old rats to continuous infusions (14 days by osmotic minipump) of a selective dopamine D2 agonist. (+)-4-propyl-9-hydroxynaphthoxazine (PHNO, 10 micrograms/h) was investigated. Measures of spontaneous locomotor activity and motor coordination revealed impairments in the aged animals. Low doses of apomorphine (10-50 micrograms/kg), which preferentially activate dopamine autoreceptors, induced yawning, chewing mouth movements and penile grooming. The frequency of yawning and duration of penile grooming were significantly decreased in the old animals. In contrast, 200 micrograms/kg of apomorphine induced stereotyped sniffing and licking or gnawing, and these responses were significantly increased in the aged animals. There was a 25% decrease in striatal dopamine levels in the aged animals in this experiment. PHNO increased the amplitude of the circadian rhythms in locomotor activity exhibited by mature rats, and daytime tolerance to the stimulant effects of PHNO was reversed by stress in these animals. Both of these effects were attenuated in the aged rats. These findings suggest that (1) the dopamine receptors mediating yawning and stereotypy have different anatomical locations (2) ageing is associated with decreased responsiveness to stimulation of dopamine autoreceptors, consequent upon the loss of dopaminergic nerve terminals, and (3) while the functional response to selective stimulation of postsynaptic D2 receptors decreases with age, the postsynaptic response to a mixed D1/D2 agonist increases.     

Day-night differences in the sensitivity of adrenoceptors in the Syrian hamster pineal gland: an in vivo iontophoretic study

Investigations on the regulation of pineal melatonin synthesis in the Syrian hamster revealed distinct differences compared to this well-understood mechanism in rat. E.g., a circadian profile of pineal norepinephrine (NE) is absent, there is no beta-adrenoceptor sensitivity during daytime and adrenergic receptor supersensitivity is not easily achieved. To elucidate the action of NE on pineal receptor sites, the effects of iontophoretic application of adrenergic compounds on spontaneous electrical discharge rates of pinealocytes were investigated during day- and nighttime. Following application of either NE, isoproterenol or clonidine, cells were activated, inhibited or not affected. Whereas about one-third of the units responded to iontophoretic application of sympathomimetics at daytime, the number of affected cells was doubled during the night. These results demonstrate the involvement of adrenoceptors in the regulation of circadian rhythms in electrophysiological properties of Syrian hamster pinealocytes. Since relatively few cells responded during daytime and inhibitory adrenergic mechanisms dominated at night, the classification of receptors by means of iontophoresis may provide a basis to explain the difficulty to influence pineal melatonin synthesis by sympathomimetics in the Syrian hamster.     

Arginine-vasopressin and vasointestinal polypeptide rhythms in the suprachiasmatic nucleus of the mouse lemur reveal aging-related alterations of circadian pacemaker neurons in a non-human primate

The suprachiasmatic nucleus (SCN) of the hypothalamus, the mammalian circadian pacemaker, is entrained by external cues and especially by photic information. Light is transmitted primarily via the retinohypothalamic tract, which terminates in the ventral part (or core) of the SCN, where vasoactive intestinal polypeptide (VIP)-containing neurons are located. VIP cells are mainly intrinsic and project to the dorsal part (or shell) of the SCN, where neurons containing arginine-vasopressin (AVP) reside. As aging leads to marked changes in the expression of circadian rhythms, we examined in primates whether age-related decay in biological rhythmicity is associated with changes in the oscillation of peptide expression in SCN neurons. We used double immunohistochemistry and quantitative analysis in the SCN of mouse lemurs, which provide a unique model of aging in non-human primates. In adult animals, VIP-positive and AVP-positive SCN neurons exhibited daily rhythms of their number and immunostaining intensity: AVP immunoreactivity peaked during the second part of the day, and VIP peaked during the night. In aged mouse lemurs, the peaks of AVP and VIP immunopositivity were significantly shifted, so that AVP was most intense at the beginning of the night, whereas VIP peaked at the beginning of daytime. The results show that the circadian rhythm of neuropeptides in the SCN is modified by aging in primates, with a differential regulation of the two main peptidergic cell populations. These changes may affect the ability of the SCN to transmit rhythmic information to other neural target sites, and thereby to modify the expression of some biological rhythms.     

Encoding time of day and time of year by the avian circadian system

The most important zeitgeber for seasonal rhythmicity of physiology and behaviour in birds is the annual cycle of photoperiod. Regulatory mechanisms are less well understood in birds than in mammals since photic information can be perceived by photoreceptors in the retina and the pineal gland, as well as in the brain, and photoperiodic time measurement might be performed with reference to at least three autonomous circadian systems, the retina, the pineal gland and a hypothalamic oscillator. In many bird species, the pineal melatonin rhythm plays a central role in circadian organization. Durations of elevated melatonin in the blood reflect night length when animals are kept under natural photoperiodic conditions, as well as under different light/dark schedules in the laboratory. In the house sparrow, time of year is encoded in a particular melatonin signal, being short in duration and high in amplitude in long photoperiods and being long in duration and low in amplitude in short photoperiods, independent of whether the light zeitgeber is natural or artificial or varies in strength. Specific features of the melatonin signal are retained in vivo as well as in vitro when birds or isolated pineal glands are transferred to constant conditions. To regulate daily and seasonal changes of behaviour and physiology, melatonin may act at various target sites, including a complex hypothalamic oscillator that, unlike that in mammals, is not confined to a single cell group in the house sparrow. There is increasing evidence that interactions between two or more components of the songbird circadian pacemaking system are essential to encode and store biologically meaningful information about time, and thus provide the basis for photoperiodic time measurements and after effects in birds.