Bruno Pitrosky

Organization of rat circadian rhythms during daily infusion of melatonin or S20098, a melatonin agonist

Daily administration of melatonin or S20098, a melatonin agonist, is known to entrain the free-running circadian rhythms of rats. The effects of the duration of administration on entrainment were studied. The animals demonstrated free-running circadian rhythms (running-wheel activity, body temperature, general activity) in constant darkness. Daily infusions of melatonin or S20098 for 1, 8, or 16 h entrained the circadian rhythms to 24 h. Two daily infusions of 1 h (separated by 8 h) entrained the activity peak within the shorter time interval. The entraining properties of melatonin and S20098 were similar and were affected neither by pinealectomy nor by infusion of 1- or 8-h duration. However, with 16-h infusion, less than half of the animals became entrained. Once entrained, the phase angle between the onset of infusion and the rhythms (onset of activity or acrophase of body temperature) increased with the duration of infusion. Before entrainment, the free-running period increased with the duration of infusion, an effect that was not predictable from the phase response curve.Daily administration of melatonin or S20098, a melatonin agonist, is known to entrain the free-running circadian rhythms of rats. The effects of the duration of administration on entrainment were studied. The animals demonstrated free-running circadian rhythms (running-wheel activity, body temperature, general activity) in constant darkness. Daily infusions of melatonin or S20098 for 1, 8, or 16 h entrained the circadian rhythms to 24 h. Two daily infusions of 1 h (separated by 8 h) entrained the activity peak within the shorter time interval. The entraining properties of melatonin and S20098 were similar and were affected neither by pinealectomy nor by infusion of 1- or 8-h duration. However, with 16-h infusion, less than half of the animals became entrained. Once entrained, the phase angle between the onset of infusion and the rhythms (onset of activity or acrophase of body temperature) increased with the duration of infusion. Before entrainment, the free-running period increased with the duration of infusion, an effect that was not predictable from the phase response curve.

Circadian Organization in a Diurnal Rodent, Arvicanthis ansorgei Thomas 1910: Chronotypes, Responses to Constant Lighting Conditions, and Photoperiodic Changes

Little information is available on circadian organization in diurnal mammals. In the present study, the daily patterns of wheel-running activity were described in a diurnal rodent, Arvicanthis ansorgei Thomas 1910, as assessed by karyological analysis. Among 108 animals born in the colony and studied under a 12:12 light-dark cycle (lights on at 7:00 a.m.), the authors determined the timing of daily activity (i.e., mean onsets and offsets of pattern of locomotor activity) and the level of wheel-running activity performed during daytime versus nighttime. The activity pattern was essentially diurnal in 84% of individuals, 46% being active only during the light period ± 1 h (activity onsets and offsets at 6:20 a.m. and 7:40 p.m., respectively) and 38% being diurnal with a period of nocturnal activity longer than 1 h (activity onsets and offsets at 5:40 a.m. and 9:30 p.m., respectively). Of the 108 animals, 16% expressed a nocturnal activity with diurnal overlaps no longer than 1 h. In 6 diurnal individuals first exposed to constant light and then to constant dim red light, the endogenous period was shortened from 24.6 ± 0.1 to 24.0 ± 0.1 h, respectively. The numbers of wheel revolutions per day and during the active period remained unchanged between the two lighting conditions. In response to different photoperiodic changes from 16:08 to 08:16 light-dark cycles, the phase angle of photic synchronization, estimated by the daily onset of wheel-running activity in 6 diurnal animals, showed marked changes, its timing occurring 2 h before and 0.5 h after the onset of light under short and long photoperiods, respectively. The numbers of wheel revolutions per 24 h and during the active period were modified similarly according to photoperiodic changes. Finally, in 5 diurnal animals exposed to a 12:12 light-dark cycle, the daily pattern of general locomotor activity, determined by telemetry, was not modified by wheel availability. The data indicate that A. ansorgei is an interesting experimental model to understand the regulation of the circadian timing system in day-active species.

The Photoperiodic Response in Syrian Hamster Depends upon a Melatonin‐Driven Circadian Rhythm of Sensitivity to Melatonin

The pineal gland, via the daily pattern of melatonin (MEL) secretion, is directly involved in the conduction of photoperiodic information. The duration of MEL secretion is proportional to the duration of the dark period and, whatever the photoperiod is, MEL synthesis occurs 3 or 4 h after the dark onset in Syrian hamsters. In order to determine the relative importance of the duration or the coincidence hypothesis, a daily infusion protocol was used in sexually active pinealectomized hamsters. Long duration of MEL infusion (10 h) completely inhibit testes whereas short duration infusion (5 h) had no effect. When the animals were infused twice within 2 h 30 min separated by 3 h, they presented a complete gonadal atrophy, similar to the one observed with the 10 h infusion. Measurement of plasma MEL during the infusion and seperation periods revealed that MEL reached physiological nighttime values during the infusion period and fell to daytime values 1 h after the end of an infusion period. Thus, the results could not be due to a time additive action of the two MEL pulses. An intermediate response was observed when the 2 signals were applied across the light/dark transition. Gonadal regression did not occur when the 2 periods of infusion were separated by 5 h 30 min. The efficiency of this type of infusion was not dependent on the ambiant photoperiod since similar results were obtained in long and short photoperiods. The infusion was also as effective during the day as well as during the night. These results suggest that there is a rhythm of sensitivity to MEL, based on the coincidence hypotheses, that are important for transmission of photoperiodic information. This rhythm of sensitivity to MEL seems to be entrained by MEL itself, since the efficiency of the two pulses of MEL is not dependent of time of application and/or of photoperiod.

Influence of the Mode of Daily Melatonin Administration on Entrainment of Rat Circadian Rhythms

In previous entrainment studies, melatonin (MEL) was administered by handling the animal, but because such handling may act as a confounding variable, the results from these studies are equivocal. The authors used MEL administration techniques that do not involve direct handling of the animal. Long Evans rats were used, and core body temperature (CBT) and wheel-running activity were recorded. One group of rats received a daily 1-h time-fixed infusion of MEL or the vehicle via a subcutaneous catheter. Animals in a second group had timed access to drinking water involving daily presence of drinking water containing MEL or the vehicle for2hata fixed time of the day. Following entrainment to LD 12:12, both groups were transferred to constant darkness to free-run under vehicle administration. MEL was then administered, and entrainment occurred when activity onset coincided with MEL onset. Under both regimens, entrainment of wheel-running and CBT rhythms showed equal phaserelation to the onset of MEL administration, and free-running reoccurred when MEL was withdrawn. The authors concluded that MEL administration via drinking water and via infusion represent efficient ways to synchronize free-running rhythms in rats.

Role of the thalamic intergeniculate leaflet and its 5-HT afferences in the chronobiological properties of 8-OH-DPAT and triazolam in syrian hamster.

The 5-HT(1A/7) receptor agonist 8-hydroxy-2-[di-n-propylamino]-tetralin (8-OH-DPAT) has chronobiological effects on the circadian system and, in the Syrian hamster, it is known that serotonergic (5-HT) projections connecting the median raphe nucleus to the suprachiasmatic nuclei (SCN) of the hypothalamus are a prerequisite for the expression of 8-OH-DPAT-induced phase advance of locomotor activity rhythm. We examined the possible involvement of the thalamic intergeniculate leaflet (IGL) in the phase-shifting properties of 8-OH-DPAT injections at CT7. Bilateral electrolytic lesions of the IGL blocked phase-shift responses to 8-OH-DPAT of the activity rhythm. Phase changes induced by injections of 8-OH-DPAT at CT7 and triazolam (Tz), a short-acting benzodiazepine, at CT6 were also studied after bilateral chemical lesion of the 5-HT fibres connecting the dorsal raphe nucleus (DR) to IGL. Destruction of 5-HT fibres within the IGL blocked the phase-shift response to Tz, but not the phase-shift response to 8-OH-DPAT. In conclusion, (a) IGL is essential for the phase-shifting effect of peripheral 8-OH-DPAT injections; (b) 5-HT fibres connecting DR to IGL are necessary for the expression of the phase-shifting effect of Tz but not of 8-OH-DPAT.

Effects of different doses and durations of melatonin infusions on plasma melatonin concentrations in pinealectomized Syrian hamsters: Consequences at the level of sexual activity

Abstract: The effect of different doses and durations of melatonin infusions on plasma melatonin concentrations has been studied in pinealectomized Syrian hamsters maintained under short photoperiod at either 7°C or 18°C. The effects of the infusions on plasma melatonin concentrations and on gonadal activity were compared. The results show that the minimal effective quantity of infused melatonin that induced gonadal atrophy was 40 ng/h at 7°C and 20 ng/h at 18°C. An infusion of 8 hr duration per day is necessary to inhibit sexual activity, while an infusion of 6 hr duration was ineffective. This finding suggests that the critical duration of melatonin infusion is between 6 and 8 hr. Despite the various doses of melatonin infused, plasma melatonin concentrations measured in the middle of the infusion period did not differ significantly from concentrations measured in intact animals. This finding suggests that the metabolism of infused melatonin increases as the dose of melatonin increases. Moreover, the different physiological effects observed after the various melatonin infusions cannot be explained by variations in plasma melatonin concentrations.

Entrainment of locomotor activity rhythm in pinealectomized adult Syrian hamsters by daily melatonin infusion.

Melatonin entrains circadian rhythms in several species of rodents, but a role for melatonin as a Zeitgeber in the adult Syrian hamster is debated. The aim of this study was to define the conditions of daily programmed melatonin infusion in which an entrainment of the locomotor activity rhythm is obtained in adult male Syrian hamsters. The animals were pinealectomized, cannulated with a subcutaneous infusion system and submitted to dim red light conditions. They were initially daily infused with vehicle until free-running was established. Then, the animals were divided into three experimental groups, each group corresponding to a specific melatonin dose and infusion duration: (1) 10 microg melatonin/h for 5 h; (2) 30 microg melatonin/h for 5 h; and (3) 50 microg melatonin/h for 1 h. Of the total 64 hamsters, 37 hamsters fully entrained to the melatonin infusion regardless of whether the animals expressed during pre-treatment a free-running period (tau)< or >24 h, 20 animals presented a transient entrainment and seven did not entrain. Of the 37 animals entrained, withdrawal of melatonin re-established free-running rhythms, although often with a different tau compared with that observed during pre-treatment. These results indicate that after a long time of daily infusion, melatonin is able to entrain the free-running rhythm in adult Syrian hamster. The mechanism involved is not known, but the change in tau observed after melatonin treatment in some animals suggests that melatonin, directly or indirectly, affects the functioning of the clock.

Chapter 24 The suprachiasmastic nucleus: the biological clock of all seasons

Publisher Summary The suprachiasmatic nucleus (SCN) plays an essential role in the generation and maintenance of circadian rhythms in mammals. The intrinsic pacemaker activity of the SCN is autonomous in nature and is entrained to the daily 24-hour period primarily by the light/dark cycle. Due to its connections with the pineal gland, the SCN is also known to be indirectly involved in the control of seasonal rhythms. The data presented in this chapter demonstrates that SCN activity is not only cued by the light/dark cycle but also strongly influenced by the photoperiod. Moreover, it appears that integration of the photoperiodic message through the rhythmic secretion of Mel implicates a Mel driven circadian rhythm of sensitivity to Mel. This clearly indicates that the SCN is a biological clock for all seasons regulating photoperiodic changes observed at the physiological and behavioral level. It also indicates that the currently accepted separation between the circadian and seasonal photo-timing system is perhaps not justified on the physiological grounds.

The Photoperiodic Response in Syrian Hamsters Depends upon a Melatonin-Driven Rhythm of Sensitivity to Melatonin

The pineal gland conveys photoperiodic information to the brain through its daily pattern of melatonin (MEL) secretion. The duration of MEL secretion is proportional to the duration of the night. To determine the mechanism by which MEL transduces photoperiod, we used a protocol of daily MEL infusion given to sexually active pinealectomized Syrian hamsters. A long MEL signal (10 h) inhibited sexual activity, whereas a 5-hour infusion had no effect. However, animals given a 2.5-hour infusion twice separated by an interval of 3 h produced complete gonadal atrophy. Changes in the time interval between infusions blocked the potency of the MEL infusion, suggesting a tight temporal relationship between MEL signals. Additionally, the infusions were as effective whether applied during the day or during the night, in both long and short photoperiods. These data suggest that there is a rhythm of sensitivity to MEL involved in the photoperiodic response which is entrained by MEL itself.

S22153, a melatonin antagonist, dissociates different aspects of photoperiodic responses in Syrian hamsters.

In the Syrian hamster, short photoperiod (SP) induces changes in several physiological functions (body mass, reproduction, hibernation), and these responses involve the pineal hormone melatonin. The present study investigated the effects of a melatonin antagonist, S22153, on photoperiodic adaptation of male Syrian hamster. When constantly released from subcutaneous implants, S22153 had no effect on body or testes masses of animals kept in long photoperiod. S22153 decreased the total hibernation duration observed in animals exposed to SP and low temperature. The decrease in hibernation duration was due to a marked reduction in the number and duration of hypothermic bouts. Moreover, S22153 significantly inhibited the increase of interscapular brown adipose tissue (BAT) mass induced by SP. However, neither the gonadal atrophy nor the body mass increase induced by SP were affected by S22153. These results show that S22153 affects only part of the physiological changes controlled by SP and cold. Whether the decreases in BAT mass and hibernation duration are linked still remains an open question.