Showing papers by "Paul Pévet published in 2014"

Daily regulation of body temperature rhythm in the camel (Camelus dromedarius) exposed to experimental desert conditions

Abstract: In the present work, we have studied daily rhythmicity of body temperature (Tb) in Arabian camels challenged with daily heat, combined or not with dehydration. We confirm that Arabian camels use heterothermy to reduce heat gain coupled with evaporative heat loss during the day. Here, we also demonstrate that this mechanism is more complex than previously reported, because it is characterized by a daily alternation (probably of circadian origin) of two periods of poikilothermy and homeothermy. We also show that dehydration induced a decrease in food intake plays a role in this process. Together, these findings highlight that adaptive heterothermy in the Arabian camel varies across the diurnal light–dark cycle and is modulated by timing of daily heat and degrees of water restriction and associated reduction of food intake. The changed phase relationship between the light–dark cycle and the Tb rhythm observed during the dehydration process points to a possible mechanism of internal desynchronization during the process of adaptation to desert environment. During these experimental conditions mimicking the desert environment, it will be possible in the future to determine if induced high-amplitude ambient temperature (Ta) rhythms are able to compete with the zeitgeber effect of the light–dark cycle.

Like melatonin, agomelatine (S20098) increases the amplitude of oscillations of two clock outputs: melatonin and temperature rhythms.

Abstract: Depression and biological rhythms disturbances are strongly associated. Agomelatine is an antidepressant with melatoninergic MT1-MT2 agonist and serotoninergic 5-HT2c antagonist properties. Both melatonin and 5-HT are known to modulate circadian rhythmicity controlled by the endogenous clock located in the suprachiasmatic nuclei (SCN). The aim of the present study was to compare the effect of an acute injection of agomelatine (Ago), melatonin (MLT) or an antagonist 5-HT2c (S32006), on the rhythms of two robust clock outputs: the pineal MLT secretion and the body temperature rhythm (Tc). Daily endogenous MLT profiles were measured using transpineal microdialysis over 4 consecutive days in rats maintained on a 12 h light/12 h dark cycle. Simultaneously, Tc was recorded. The drugs were injected subcutaneously at three doses (1, 2.5 or 5 mg/kg) at the onset of darkness. Both Ago and MLT, at the dose of 2.5 mg/kg, increased the amplitude of the peak of MLT secretion and this effect was observed 2 d after injection. Moreover, both drugs induced a dose-dependent advance of the rhythm onset which resulted in lengthening of the MLT peak. S32006 had no effect on the rhythm of MLT. Ago, MLT and S32006 increased the amplitude of the rhythm of Tc. These data suggest a central action of Ago, directly on the SCN, via melatoninergic receptors responsible for both the increased amplitude of MLT rhythm and the phase advance. The increase in the amplitude of the body temperature could involve both MLT agonist and/or 5-HT2c antagonist properties of Ago.

The Circannual Clock in the European Hamster: How Is It Synchronized by Photoperiodic Changes?

Abstract: In a seasonal environment, mammals time their reproductive phase so that the offspring are born in spring and summer. Two strategies have evolved to ensure accurate seasonal timing of reproduction, but both share a common Zeitgeber, the seasonal changes in photoperiod. The reproductive axis might be directly controlled, as in photoperiodic species, which require photoperiodic input to show seasonal changes in reproductive competence. In contrast, in circannual species photoperiodic changes act indirectly, namely, on an endogenous circannual clock that then times reproduction. This circannual clock generates self-sustained rhythms with a period length of about 1 year, and photoperiodic information is only needed to synchronize these rhythms. Concerning the mechanism that imparts the photoperiodic message internally, so far no differences between photoperiodic and circannual mammals have been reported. Recent results however, strongly suggest that the circannual European hamster (Cricetus cricetus) uses a fundamentally different mechanism than photoperiodic species. In the latter, photoperiod induces a change in the reproductive state via a melatonin-dependent pathway, whereas the circannual clock of the European hamsters can be synchronized via a melatonin-independent pathway. Instead, a circadian mechanism based on a specific organizational state is involved. Juvenile European hamsters use probably both pathways: the melatonin-dependent photoperiodic pathway for the short-term timing in the year of birth and the melatonin-independent circannual pathway for long-term timing of the seasonal events in the next year.