Splitting of locomotor circadian rhythmicity in hamsters is facilitated by pinealectomy

Emerging preclinical interest concerning the role of circadian function in Parkinson's disease

The importance of circadian function in the aetiology, progression and treatment of Parkinson's disease is a topic of increasing interest to the scientific and clinical community. While clinical studies on this theme are relatively new and limited in number there are many preclinical studies which explore possible circadian involvement in Parkinson's disease and speculate as to the mechanism by which clinical benefit can be derived by manipulating the circadian system. The present review explores the sequelae of circadian related studies from a historical perspective and reveals mechanisms that may be involved in the aetiology and progression of the disease. A systematic review of these studies also sets the stage for understanding the basic neuroscientific approaches which have been applied and provides new direction from which circadian function can be explored.

The contribution of the pineal gland on daily rhythms and masking in diurnal grass rats, Arvicanthis niloticus

Melatonin is a hormone rhythmically secreted at night by the pineal gland in vertebrates. In diurnal mammals, melatonin is present during the inactive phase of the rest/activity cycle, and in primates it directly facilitates sleep and decreases body temperature. However, the role of the pineal gland for the promotion of sleep at night has not yet been studied in non-primate diurnal mammalian species. Here, the authors directly examined the hypothesis that the pineal gland contributes to diurnality in Nile grass rats by decreasing activity and increasing sleep at night, and that this could occur via effects on circadian mechanisms or masking, or both. Removing the pineal gland had no effect on the hourly distribution of activity across a 12:12 light-dark (LD) cycle or on the patterns of sleep-like behavior at night. Masking effects of light at night on activity were also not significantly different in pinealectomized and control grass rats, as 1h pulses of light stimulated increases in activity of sham and pinealectomized animals to a similar extent. In addition, the circadian regulation of activity was unaffected by the surgical condition of the animals. Our results suggest that the pineal gland does not contribute to diurnality in the grass rat, thus highlighting the complexity of temporal niche transitions. The current data raise interesting questions about how and why genetic and neural mechanisms linking melatonin to sleep regulatory systems might vary among mammals that reached a diurnal niche via parallel and independent pathways.

Effects of photoperiod on daily locomotor activity, energy expenditure, and feeding behavior in a seasonal mammal

The objective of this study was to determine whether the previously observed effects of photoperiod on body weight in Siberian hamsters were due to changes in the daily patterns of locomotor activity, energy expenditure, and/or feeding behavior. Adult males were monitored through a seasonal cycle using an automated comprehensive laboratory animal monitoring system (CLAMS). Exposure to a short-day photoperiod (SD; 8:16-h light-dark cycle) induced a significant decline in body weight, and oxygen consumption (Vo(2)), carbon dioxide production (Vco(2)), and heat production all decreased reaching a nadir by 16 wk of SD. Clear daily rhythms in locomotor activity, Vo(2), and Vco(2) were observed at the start of the study, but these all progressively diminished after prolonged exposure to SD. Rhythms in feeding behavior were also detected initially, reflecting an increase in meal frequency but not duration during the dark phase. This rhythm was lost by 8 wk of SD exposure such that food intake was relatively constant across dark and light phases. After 18 wk in SD, hamsters were transferred to a long-day photoperiod (LD; 16:8-h light-dark cycle), which induced significant weight gain. This was associated with an increase in energy intake within 2 wk, while Vo(2), Vco(2), and heat production all increased back to basal levels. Rhythmicity was reestablished within 4 wk of reexposure to long days. These results demonstrate that photoperiod impacts on body weight via complex changes in locomotor activity, energy expenditure, and feeding behavior, with a striking loss of daily rhythms during SD exposure.

Circadian pacemakers in vertebrates

The identification and isolation of circadian pacemaker cells is of critical importance to studies of circadian clocks at all phylogenetic levels. In the vertebrate classes, a few structures of diencephalic origin have been implicated as potential sites but for only two, the avian pineal and the mammalian suprachiasmatic nucleus (SCN), has a pacemaker role in addition to oscillatory behaviour been demonstrated by the transfer of pacemaker properties from one organism to another. Studies of the mammalian system in particular have benefited from the ability to restore circadian function using transplantation of tissue from the SCN and from the availability of a hamster period mutant, tau, that allows donor-derived and host-derived rhythms to be distinguished easily. Initial cross-genotype transplantation studies and the subsequent creation of circadian chimeras expressing two phenotypes simultaneously demonstrated the pacemaker capability of the SCN, and demonstrated the relative autonomy of this nucleus as a pacemaking structure. Despite an abundance of information regarding the anatomy, physiology and pharmacology of these nuclei, the identity of the pacemaker cells and their methods of communication with each other and the organism remain obscure. None the less, it is possible under certain conditions to create chimeras with two clocks that interact. The behaviour of these animals provides a unique opportunity to study the nature and timing of pacemaker communication.

Pinealectomy blocks modulation of active avoidance by central vasopressin application in rats

The inter-relationship between central vasopressin and the pineal gland in the modulation of active avoidance behavior was investigated. In sham-operated (SO) rats, intracerebroventricular (i.c.v) application of 10 ng arginine vasopressin (AVP) after both the last acquisition and the first extinction trials prolonged the extinction of the active avoidance response; application of 50 ng of the V1 antagonist, d(CH2)5Tyr(Me)AVP (AAVP) was without effect in both experiments. In contrast to the SO in pinealectomized (PX) rats neither AVP nor AAVP influenced the extinction of the avoidance response. Intraseptal infusion of 200 pg AVP or 5 ng AAVP either after the last acquisition or the first extinction trial was without effect in both SO and PX rats. Comparison of the acquisition trials revealed no differences between SO and PX rats.

Septal vasopressin induced preservation of social recognition in rats was abolished by pinealectomy

The role of intraseptal vasopressin (AVP) and the pineal gland in the modulation of social memory was investigated. For social recognition, male pinealectomised (Px) and sham-operated (SO) rats were confronted with juveniles for 4 min, and injected with either 200 pg AVP or 5 ng of its V1 receptor antagonist d(CH2)5Tyr(Me)AVP (AAVP) into the mediolateral septum. Re-exposure to the same and a different juvenile took place after 30 or 120 min. In SO rats, the social memory was facilitated after injection of AVP (120 min) and impaired after AAVP application (30 min). In Px rats, however, neither AVP nor AAVP administration influenced the social memory. Comparison between SO and Px control groups treated with artificial cerebrospinal fluid did not reveal any differences as to social recognition responses. After subcutaneous administration of 250 microg melatonin (substitution for pinealectomy) the social discrimination responses in Px rats after peptide application were influenced in a manner identical to that noted in SO animals. These findings suggest that the modulation of social memory by intraseptal AVP is dependent on an intact pineal function.

S20098 affects the free-running rhythms of body temperature and activity and decreases light-induced phase delays of circadian rhythms of the rat

Mammalian endogenous circadian rhythms are entrained to the environmental day-night cycle by light exposure. Melatonin is involved in this entrainment by signaling the day-night information to the endogenous circadian pacemaker. Furthermore, melatonin is known to affect the circadian rhythm of body temperature directly. A striking property of the endogenous melatonin signal is its synthesis pattern, characterized by long-term elevated melatonin levels throughout the night. In the present study, the influence of prolonged treatment with the melatonin agonist S20098 during the activity phase of free-running rats was examined. This was achieved by giving S20098 in the food. The free-running body temperature and activity rhythms were studied. The present study shows that enhancement of the melatonin signal, using S20098, affected the free-running rhythm by gradual phase advances of the start of the activity phase, consequently causing an increase in length of the activity phase. A well-known feature of circadian rhythms is its time-dependent sensitivity for light. Light pulse exposure of an animal housed under continuous dark conditions can cause a phase shift of the circadian pacemaker. Therefore, in a second experiment, the influence of melatonin receptor stimulation on the sensitivity of the pacemaker to light was examined by giving the melatonin agonist S20098 in the food during 1 day prior to exposure to a 60-min light pulse of 0, 1.5, 15, or 150 lux given at circadian time (CT) 14. S20098 pretreatment caused a diminished light pulse-induced phase shift when a light pulse of low light intensity (1.5 lux) was given. S20098 treatment via the food was sufficient to exert chronobiotic activity, and S20098 treatment resulting in prolonged overstimulation of melatonin receptors is able to attenuate the effect of light on the circadian timing system.

Restricted daytime feeding attenuates reentrainment of the circadian melatonin rhythm after an 8-h phase advance of the light-dark cycle

It is well established that in the absence of photic cues, the circadian rhythms of rodents can be readily phase-shifted and entrained by various nonphotic stimuli that induce increased levels of locomotor activity (i.e., benzodiazepines, a new running wheel, and limited food access). In the presence of an entraining light-dark (LD) cycle, however, the entraining effects of nonphotic stimuli on (parts of) the circadian oscillator are far less clear. Yet, an interesting finding is that appropriately timed exercise after a phase shift can accelerate the entrainment of circadian rhythms to the new LD cycle in both rodents and humans. The present study investigated whether restricted daytime feeding (RF) (1) induces a phase shift of the melatonin rhythm under entrained LD conditions and (2) accelerates resynchronization of circadian rhythms after an 8-h phase advance. Animals were adapted to RF with 2-h food access at the projected time of the new dark onset. Before and at several time points after the 8-h phase advance, nocturnal melatonin profiles were measured in RF animals and animals on ad libitum feeding (AL). In LD-entrained conditions, RF did not cause any significant changes in the nocturnal melatonin profile as compared to AL. Unexpectedly, after the 8-h phase advance, RF animals resynchronized more slowly to the new LD cycle than AL animals. These results indicate that prior entrainment to a nonphotic stimulus such as RF may "phase lock" the circadian oscillator and in that way hinder resynchronization after a phase shift.

Suprachiasmatic pacemaker organization analyzed by viral transynaptic transport

The suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian pacemaker, is a paired structure with two subdivisions, a ventral core receiving photic input and a dorsal shell receiving non-photic input. Rhythmicity is thought to be generated by individual SCN neurons which are coupled to achieve synchrony [D.K. Welsh, D.E. Logothetis, M. Meister, S.M. Reppert, Individual neurons dissociated from rat suprachiasmatic nucleus express independently phased circadian firing patterns, Neuron, 14 (1995) 697-706]. Normally, the core and shell, and the nuclei on each side, act in unison to transmit rhythmicity to effector systems. It is not known how coupling between neurons in the two subdivisions, and between the two SCNs, takes place. In the present study, we analyze the intrinsic, commissural, and efferent projections of the SCN using the swine herpesvirus (pseudorabies virus, PRV) as a tool for transynaptic analysis of circuits and small iontophoretic injections of the conventional tracer horseradish peroxidase (HRP) conjugated to fluorescein. We find that the core and shell each project through commissural efferents to homologous contralateral areas. The core projects densely to shell but we find little reciprocal innervation. The two subdivisions project to different hypothalamic areas, with the core projecting to the lateral subparaventricular zone and shell to the dorsomedial hypothalamic nucleus and medial subparaventricular zone. These data are the first demonstration that connections within the SCN, and from the SCN to effector regions, are topographically organized and lend insight into the flow of information through and out of the pacemaker.

Melatonin's role in vertebrate circadian rhythms

The circadian secretion of melatonin by the pineal gland and retinae is a direct output of circadian oscillators and of the circadian system in many species of vertebrates. This signal affects a broad array of physiological and behavioral processes, making a generalized hypothesis for melatonin function an elusive objective. Still, there are some common features of melatonin function. First, melatonin biosynthesis is always associated with photoreceptors and/or cells that are embryonically derived from photoreceptors. Second, melatonin frequently affects the perception of the photic environment and has as its site of action structures involved in vision. Finally, melatonin affects overt circadian function at least partially via regulation of the hypothalamic suprachiasmatic nucleus (SCN) or its homologues. The mechanisms by which melatonin affects circadian rhythms and other downstream processes are unknown, but they include interaction with a class of membrane-bound receptors that affect intracellular processes through guanosine triphosphate (GTP)-binding protein second messenger systems. Investigation of mechanisms by which melatonin affects its target tissues may unveil basic concepts of neuromodulation, visual system function, and the circadian clock.

Effects of a daylight cycle reversal on locomotor activity in several inbred strains of mice

There is some evidence of melatonin implication in the nycthemeral regulation of running activity rhythm in rodents. Because some inbred strains of mice such as C57BL/6 and BALB/c have been generally found to present no nocturnal melatonin peak, in contrast to others such as C3H/He and CBA mice, the aim of this study was to examine the adaptation of daily locomotor activity to a light/dark cycle phase shift in these four strains. An apparatus consisting of two boxes connected by a tunnel was used to record spontaneous locomotor activity, defined as the number of transitions between the two boxes. Locomotor activity was monitored continuously during 3 days before and 14 days after a 12-h phase delay of the light/dark cycle. Results essentially showed that the adaptation of the locomotor activity rhythm to the phase shift was faster in C57BL/6 and BALB/c mice than in C3H/He and CBA mice. This could be related, at least in part, to the differences in melatonin synthesis between the former strains and the latter ones. Although melatonin nocturnal peak is not necessary to a daylight regulation of circadian functions in rodents, it could be considered as an endocrine message that takes part in the anticipation of the following light/dark cycle.

Entrainment of rat circadian rhythms by the melatonin agonist S-20098 requires intact suprachiasmatic nuclei but not the pineal

S-20098 is a potent nonindolic melatonin agonist that has been shown to entrain free-running circadian rhythms. The current experiments examined the role of the suprachiasmatic nuclei (SCN) and of the pineal gland in the entrainment of circadian rhythms by S-20098. First, daily injections of S-20098 (1 and 10 mg/kg s.c.) were administered to SCN- and sham-lesioned rats. At both dose levels, circadian effects were noted in all sham-lesioned animals. Locomotor activity and body temperature rhythms in 3 of 5 sham-lesioned rats were entrained by the daily injections. In SCN-lesioned rats, S-20098 had no synchronizing or entraining effects at either dose level. These results show that S-20098 exerts its entraining effects on circadian rhythms via the circadian pacemaker located in the SCN. Second, the effects of daily injections of S-20098 (10 mg/kg s.c.) were examined in pinealectomized, sham-pinealectomized, and intact rats. All rats receiving S-20098, irrespective of surgical treatment, showed circadian changes. Rhythms in 81% of these animals entrained to daily administration of the compound, indicating that entrainment induced by S-20098 does not depend on an intact pineal. When injected with 10 mg/kg S-20098, 69% of rats, irrespective of surgical treatment, showed long-term modifications of free-running period that still were evident several weeks after administration ceased. If confirmed, this finding may have therapeutic implications in humans regarding the optimal mode and administration of S-20098 in a clinical setting.

Effect of cage size on ultradian locomotor rhythms of laboratory mice

The effect of cage size on spontaneous locomotor rhythms of laboratory mice was studied under simulated light-dark (12:12) cycles. On-line image analysis of bodily displacement yielded a locomotor signal over a period of 3 days. Continuous wavelet transform was applied to the signal, and ensemble averaging of eight mice revealed in the time-frequency plot bouts of increased motor activities. Notably, there were two bouts in the dark corresponding to ultradians of periods below 5 h: a first bout at the dark onset (at 0.6-1.0 cycle/h), and a second bout during the second half of the dark period (at 0.4-0.7 cycle/h). These increases of activity were more intense and distinct when the animals were kept inside the larger cage. Furthermore, the first bout disappeared when the animals were kept in the small cage for 3 days.

Daily injections of melatonin entrain the circadian activity rhythms of nocturnal rats but not diurnal chipmunks

It is well known that daily injections of melatonin entrain the free-running rhythms of nocturnal rodents (rats and hamster) and diurnal sauropside (birds and lizard). Here, we asked whether daily injections of melatonin entrain the free-running rhythm of the chipmunk, a diurnal rodent, and, if they do, is the phase relationship between the time of injection and onset of the activity interval similar to that in sauropside rather than that of nocturnal rodents? Contrary to our expectations, daily injections of melatonin did not entrain the free-running rhythm in 9 of 10 chipmunks, even when a high dose of melatonin (1 mg/kg, b.wt.) was used. These results indicate that the entraining effect of daily injections of melatonin on free-running rhythm varies among mammalian species.

Organization of circadian rhythmicity and suprachiasmatic nuclei in malnourished rats

The present study was aimed at characterizing the effects of low-protein malnutrition (6% casein) on the circadian rhythm of drinking behavior and on suprachiasmatic nuclei immunohistochemistry in Sprague-Dawley rats. Recordings were started at 30 days of age under a 12:12-h light-dark (LD) cycle. At age 150 days, recordings were continued under constant dim red light, and finally the latency to entrain to complete and skeleton photoperiods was established. At the end of the recordings rats were processed for histological analysis. Compared with their controls, malnournished rats exhibited 1) splitting of rhythmicity under LD that 2) condensed to one component in constant dim red light, 3) delayed entrainment to skeleton photoperiod, and 4) precocious entrainment under complete photoperiod. Immunohistochemical analysis showed mainly a decrease in the immunohistochemical detection of vasoactive intestinal polypeptide and glial fibrillar acid protein cells in malnourished animals. These results indicate that in malnourished rats there is a decrease 1) in the coupling force among the oscillators and 2) in the strength of the phase lock between the oscillators and the light-dark cycle.

A microdialysis study on pineal melatonin rhythms in rats after an 8-h phase advance: new characteristics of the underlying pacemaker

This study describes the use of the microdialysis technique to elucidate specific properties of the circadian pacemaking system in the hypothalamus, by measurement of melatonin production in the pineal gland. Melatonin has appeared to be a reliable marker of the pacemaker activity, which is influenced by the light/dark cycle. A phase shift in the light/dark cycle was applied to perturb the rhythm generating system. An 8-h phase advance resulted in the disappearance of melatonin production over two days, with basal levels comparable to normal daytime levels. In the subsequent return of rhythmic melatonin production, new clock characteristics could be revealed, due to the high time-resolution measurements of microdialysis. While half of the animals still did not show any rhythmicity, the other half of the animals regained rhythmicity with entrained onset of melatonin production, while the offset was variable and not stably entrained to lights on. Ten days after the shift, the system had completely recovered and all animals regained normal rhythmicity, in phase with the new light/dark cycle. The results are interpreted in terms of the two-oscillator model, with one oscillator reacting with a phase advance and the other with a phase delay to adapt to the phase shift.

Low-cost automatic activity data recording system

We describe a low-cost, high quality device capable of monitoring indirect activity by detecting touch-release events on a conducting surface, i.e., the animal's cage cover. In addition to the detecting sensor itself, the system includes an IBM PC interface for prompt data storage. The hardware/software design, while serving for other purposes, is used to record the circadian activity rhythm pattern of rats with time in an automated computerized fashion using minimal cost computer equipment (IBM PC XT). Once the sensor detects a touch-release action of the rat in the upper portion of the cage, the interface sends a command to the PC which records the time (hours-minutes-seconds) when the activity occurred. As a result, the computer builds up several files (one per detector/sensor) containing a time list of all recorded events. Data can be visualized in terms of actograms, indicating the number of detections per hour, and analyzed by mathematical tools such as Fast Fourier Transform (FFT) or cosinor. In order to demonstrate method validation, an experiment was conducted on 8 Wistar rats under 12/12-h light/dark cycle conditions (lights on at 7:00 a.m.). Results show a biological validation of the method since it detected the presence of circadian activity rhythm patterns in the behavior of the rats.

Effects of daily injections of melatonin on locomotor activity rhythms in rats maintained under constant bright or dim light

It has been demonstrated that daily melatonin injections entrain free-running locomotor activity rhythms in rats kept in constant darkness, and synchronize disrupted circadian patterns of wheel-running activity under constant light. Contrary to these previous observations, our result did not show that daily injections of melatonin synchronize disrupted locomotor activity in rats maintained under constant bright light. On the other hand, daily treatment with melatonin entrained the intact free-running rhythm in rats kept in constant dim light. This entrainment took place only when the time of injection corresponded to the activity onset time, and similar results were obtained in blinded rats. Pinealectomy had no influences on either the free-running rhythm or melatonin-induced entrainment. To examine whether a behavioral feedback mechanism is involved in melatonin-induced entrainment, rats were immobilized for 3 h after each daily melatonin injections. This did not interfere with melatonin-induced entrainment. These results suggest that the mechanism underlying melatonin-induced entrainment of activity rhythms may be different from those in photic and behavioral entrainment.

Melatonin and light induce phase shifts of circadian activity rhythms in the C3H/HeN mouse

This study examines the effect of light pulses and administration of the pineal hormone melatonin on the circadian activity rhythm of C3H/HeN mice. Mice were housed in constant dark in cages equipped with running wheels. Phase shifts in the circadian rhythm of wheel-running activity were measured following treatment with a 15-min pulse of light (300 lux) or administration of vehicle (ethanol/saline) or melatonin (90 micrograms, sc). Light treatment induced phase changes in circadian activity rhythms; specifically, delays during early subjective night (circadian time [CT] 12.5 to CT 18.5) and advances during late subjective night (CT 0.5). A single dose of melatonin administered at various CTs had no consistent effect on free-running circadian activity rhythms. By contrast, melatonin administration for 3 consecutive days at the same clock time induced advances in circadian activity rhythms by more than 1 h when the first dose was administered at CT 10 and induced delays in circadian activity rhythms by up to 1 h when the first dose was administered between CT 24 and CT 2. With the caveat that multi- ple melatonin treatments are required to induce phase shifts, the results suggest that the circadian timing system controlling the rhythm of wheel-running activity in the C3H/HeN mouse is responsive to both light and melatonin.

Melatonin effects on behavior: possible mediation by the central GABAergic system

The best described function of the pineal hormone melatonin is to regulate seasonal reproduction, with its daily production and secretion varying throughout the seasons or the photoperiod. Additionally, a number of behavioral effects of the hormone have been found. This review describes the effects of melatonin in rodent behavior. We focus on: (a) inhibitory effects (sedation, hypnotic activity, pain perception threshold elevation, anti-convulsive activity, anti-anxiety effects); and (b) direct effects on circadian rhythmicity (entrainment, resynchronization, alleviation of jet-lag symptoms, phase-shifting). Most of these effects are clearly time-dependent, with a peak of melatonin activity during the night. One of the possible mechanisms of action for melatonin in the brain is the interaction with the GABAergic system, as suggested by neurochemical and behavioral data. Finally, some pineal hormone effects might be candidates as putative therapies for several human disorders.

The pineal gland: photoreception and coupling of behavioral, metabolic, and cardiovascular circadian outputs

Although removal of the pineal gland has been shown to have very little effect on the mammalian circadian system in constant darkness (DD), several recent reports have suggested that the mammalian pineal gland may be more important for circadian organization in nocturnal rodents than was previously believed. Removal of the pineal gland (PINX) facilitates the disruptive effects of constant bright light on wheel-running rhythmicity. This suggests at least two possibilities for the role of the pineal gland in the mammalian circadian system. First, pinealectomized rats may perceive ambient light intensity to be brighter than do sham-operated (SHAM) rats. Second, the pineal gland, probably via its secretion of melatonin, may also be involved in coupling components of the circadian system. Coupling, as we see it, may occur at several levels of organization: (1) between retinohypothalamic afferents and suprachiasmatic nuclei (SCN) oscillatory neurons, (2) among multiple SCN oscillators, (3) between the SCN and their multiple outputs, and/or (4) among the multiple circadian outputs themselves. In this study we show that PINX rats free-run with a longer period in four different light intensities than do SHAM rats. Moreover, the rate of increase of tau is greater among PINX rats than among SHAM rats. This supports the first hypothesis. We also show that in PINX rats the circadian rhythms of wheel running, general activity, body temperature, and heart rate are all more disrupted in constant bright light than are those of SHAM rats, and each rhythmic output is disrupted in parallel. This supports the second hypothesis. Melatonin is probably not involved in coupling presynaptic elements of SCN afferents in the retinohypothalamic tract to pacemaking cells within the SCN, since enucleation has no effect on SCN 2-[125I]iodomelatonin (IMEL) binding. Together the data do not discount either of the two hypotheses but do restrict the possible levels at which the pineal gland is involved in coupling. These data also further support a growing body of literature indicating that the pineal gland and its hormone melatonin play a role in mammalian circadian organization.

Pinealectomized rats entrain and phase-shift to melatonin injections in a dose-dependent manner

Previous work has shown that daily injections of the pineal hormone melatonin (N-acetyl, 5-methoxytryptamine) entrain the free-running locomotor rhythms of rats held in constant darkness (with a median effective dose [ED50] of 5.45 +/- 1.33 micrograms/kg) and in constant bright light. The present experiments determined the dose-response characteristics of entrainment and phase shifting to daily and single melatonin injections in both sham-operated (SHAM) and pinealectomized (PINX) rats. The data indicated an ED50 of 332 +/- 53 ng/kg and 121 +/- 22 ng/kg for SHAM and PINX rats, respectively, during the entrainment experiment. The ED50's for the entrainment experiment were considerably lower than doses previously employed, and much lower than doses employed in reproductive and metabolic studies in rats and hamsters. The data indicated that no partial entrainment occurred; nor were there differences in phase angle, length of activity, or period among all effective doses. Next, a single injection of 1 mg/kg melatonin has previously been shown to cause a phase advance of approximately 45 min when administered at about circadian time (CT) 10. We found that both SHAM and PINX animals phase-advanced, in a dose-dependent manner to a single melatonin injection given at CT 10. The data for the phase-shifting experiment indicated an ED50 of 8.19 +/- 0.572 micrograms/kg and 2.16 +/- 0.326 micrograms/kg for SHAM and PINX animals, respectively, with an average phase advance of 40 min for both groups. Together, the data suggest that the presence of the pineal gland is not necessary for the effects of melatonin on the rat circadian system, and that PINX animals are marginally more sensitive to melatonin than their SHAM controls.

Age, but not pineal status, modulates circadian periodicity of golden hamsters

The pineal gland and its hormone, melatonin, have been implicated in the regulation of rat circadian rhythmicity. The present study was designed to evaluate whether the pineal has a similar role in the hamster, and to clarify whether the marked rhythm responses to constant light (LL) previously seen in serotonin-depleted hamsters might be attributable to a functional pinealectomy. The results demonstrated that young, but not old, hamsters showed loss of the circadian wheel-running rhythm (mostly via splitting) in LL, and that young hamsters had longer circadian periods in LL than old animals. Neither effect was related to the presence of a pineal gland. In LD 14:10 old animals ran much less than young animals, regardless of pineal status, and the suppression of running by LL was greater in old animals. The activity phase duration was only modestly related to age and not to pineal presence. The data support a previous report of an age effect on incidence of rhythm splitting and circadian period length, but do not support the view that the pineal gland helps modulate circadian rhythmicity in the hamster.