Influence of temperature on mammalian clocks in the context of the

In homeothermic animals, body core temperature is maintained within narrow limits by homeostatic regulation. At the same time it exhibits daily fluctuations imposed by the circadian master clock. The observed rhythm of body temperature in homeotherms results from a continuous interplay between these two regulatory processes and contribues to tune physiological processes over the 24h period. The rhythm of diurnal animals is characterized by higher temperature during the day, whereas the rhythm of nocturnal animals exhibits higher values of temperatures during the night. Although the average core body temperature is close to 37°C in most mammals, the amplitude, the wave form, and the range of the oscillations vary in a species–specific fashion. For example, the differences between maximum and minimum are rather low in humans (0.7-1.2°C), intermediate in mice and rats

(1.5-3°C), and relatively high in shrew (4.2-6°C) (Refinetti, 2010a).

The homeostatic control of body temperature necessitates the function of thermoregulatory centers located within the preoptic anterior hypothalamic area (POA) and thermosensors found deep in the body, in the skin and in the brain. The POA contains warm sensitive (WS) neurons that act both as direct sensors of local temperature and as central integrators of the thermal informations coming from cutaneous and visceral thermosensors. Signals emanating from the POA then instigate both behavioral and autonomous body temperature responses.

Several other brain regions, including the dorsomedial nucleus of the hypothalamus (DMH), the periaqueductal gray matter of the mid-brain (PAG) and the nucleus raphe pallidus (RPa) in the medulla, have been found to be involved in the thermoregulatory pathways that mediate these responses (Figure 19A) (Benarroch, 2007). Local temperature rises or input from A

POA, preoptic anterior hypothalamic area; DMH, dorsomedial nucleus of the hypothalamus; RPa, nucleus raphe pallidus; PAG, periaqueductal gray matter

B

Figure 19. A. Central pathways involved in thermoregulatory responses. An increase in core temperature activates warm sensitive (WS) neurons of the POA and triggers responses for heat loss via still poorly defined pathways, and tonic inhibition of cold response neurons of the DMH, PAG and the RPa. Cold exposure results in decrease in WS activity and disinhibition of DMH, PAG, and RPa neurons that initiate responses for heat production. The midbrain PAG serve as a relay implicated in both cold and heat responses. B.

Schematic representation of the dependence of the activity of cold-activated (blue) and heat-cold-activated (red) thermoTRP channels on temperature. These Figures are taken

peripheral warm sensors increase the activity of WS neurons that in turn trigger heat defense responses (vasodilatation and sweating), while input from peripheral cold sensors leads to a decrease of WS neurons activity that in turn triggers cold defense responses (vasoconstriction and increase heat generation by shivering). These two opposing activities continuously counterbalance each other at a core temperature “set point”, resulting in slight fluctuations of core temperature around this point (Boulant, 2006). These complex thermally driven mechanisms necessitate a constant monitoring of internal and external temperature by peripheral thermosensory neurons. Such temperature-sensitive neurons are located in the dorsal root ganglia (DRG) in the trunk and in the trigeminal ganglia at the level of the brainstem, and extend to the skin, the viscera and vasculature, where stimuli are detected.

This information is then transmitted to the POA via the spinal cord. Thermosensory neurons express a group of molecules called thermo TRP channels that are a subclass of transient receptor potential (TRP) cation channels. These include the heat-activated TRPV1-V4, M2, M4, and M5 and the cold-activated TRPM8 and A1. Each thermo TRP has unique characteristics, highlighted by distinct temperature thresholds of activation. Alltogether, they cover a very wide range of temperatures, in an overlapping fashion (Figure 19B). These TRP channels involved in thermal transduction are also expressed in non-neuronal tissues, such as skin, tongue, trachea, liver or heart (Dhaka et al., 2006). This raises the possibility that non-neuronal cells could also directly sense temperature variations and initiate a local response or relay the information to thermosensitive afferents.

Furthermore, the circadian control of body temperature is probably due to a rhythmic input from the SCN to the hypothalamic thermoregulatory centers. Both heat loss and heat production are controlled by the SCN, and they therefore oscillate. Autonomic heat loss responses are activated during the circadian phase of low body temperature, and heat production responses are activated during the phase of high body temperature. The rhythm of heat production is phase-advanced with respect to the rhythm of heat loss. Thus, core body

temperature increases as long as heat production surpasses heat loss; thereafter it starts to fall (Figure 20) (Refinetti, 2010a; Weinert, 2010). The circadian rhythm of core body temperature can be influenced by different endogenous factors, including locomotor activity, age, and/or body size.

The time course of locomotor activity and body temperature are very similar. In nocturnal mammals, body temperature has been shown to be higher during the night regardless of the activity level, indicating that the daily rhythm of body temperature is not simply a side effect of the activity rhythms. Nevertheless, the locomotor activity may alter the amplitude and the shape of body temperature rhythms (Brown and Refinetti, 1996; Refinetti, 2010a). In mammals, the rhythm of body temperature is not present at birth, because of the global immaturity of the circadian system and of the deficiency of heat conservation, and the proneness to heat loss. Body temperature rhythms thus gradually develop at a species-specific Figure 20. Relationships between body temperature, heat production, and heat loss of a laboratory rat maintained in constant darkness.

Thin lines correspond to actual data collected at 6-minute intervals. Thick lines are cosine waves fit to the data. Dashed vertical lines indicate the acrophases of the rhythms. The last graph at the bottom compares the body temperature rhythm with the heat-balance rhythm (where heat balance is defined as the difference between the normalized values of heat production and heat loss). A phase difference of 6h is found, that is the heat balance rhythm leads the temperature rhythm by 6h. This phase difference is presumably due to thermal inertia of the body and should vary in animals of different sizes.

This figure is issued from (Refinetti, 2010a)

rate. At old age, they are progressively altered, and their ability to synchronize to the periodic environment diminishes. Aging seems to be associated with a reduction of the amplitude in body temperature rhythms and a small advance in the phase angle in its entrainment (Weinert, 2010). Body size significantly influences the amplitude of body temperature rhythms. Indeed, the amplitude was found to be smaller in large as compared to small animals. Conceivably, large bodies may buffer the effects of the circadian oscillations in heat production and loss (thermal inertia) (Refinetti, 2010a).

Although in vivo experiments are ultimately important for the understanding of entrainment and resetting of circadian clocks at the whole organism level, the analysis of the influence of temperature in this context turns out to be a more challenging task. Indeed it is very difficult to separate the effects of temperature from the multiplicity of signals present in the body, such as hormones, neuronal inputs, and food-induced metabolites. Modulation of exogenous factors influencing body temperature, such as food availability or ambient temperature may suggest a role of body temperature rhythm in the synchronization of peripheral clocks. For example, because food ingestion is associated with a rise in temperature (Stephan, 2002), food availability can modulate the amplitude and the shape of body temperature rhythms. Moreover, since restricted feeding is able to change the phase of clock gene expression in peripheral tissues, and since it can provoke dramatic changes in the phase, shape, and amplitude of body temperature profiles, one could postulate that these temperature changes may in turn participate in the resetting of peripheral clocks (Damiola et al., 2000). Remarkably, the chronic administration of methamphetamine² to SCN-lesioned rats is capable of restoring circadian-like rhythms in spontaneous locomotor activity, feeding, and body temperature and to restore the phase relationship of circadian gene expression among peripheral tissues (Honma et al., 1988; Pezuk et al., 2010). Therefore, a role of body temperature rhythms in contributing to the synchronization of peripheral oscillator in

2 Methamphetamine is a strong catecholamine releaser in the central nervous system and is known to affect locomotor activity, the length of alpha (the duration between onset and offset of daily activity), and the period

metamphetamine treated animals is again conceivable. Furthermore, the application of periodic ambient temperature changes has been shown to affect locomotor activity only in some species, albeit not in all individuals of the tested group (Francis and Coleman, 1997;

Refinetti, 2010b). Finally, it has been shown that inverted environmental temperature cycles (light/24°C, dark/37°C) in mice are able to reverse circadian genes expression in peripheral oscillators (liver) without affecting those of the central clock in the SCN. Moreover, this change in clock genes expression is associated with a strong additional peak of body temperature taking place in the late evening/early morning (Brown et al., 2002). Therefore, these studies suggest that both endogenous and imposed exogenous temperature can contribute to the entrainment of peripheral oscillators in homeotherms.

Nevertheless, in these different studies, the changes observed on molecular circadian oscillators cannot be attributed in a compelling fashion to changes in body temperature, since these were always accompanied with changes in locomotor activity and feeding behavior.