Circadian rhythms of Endothermic and Ectothermic Vertebrates
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The information known about the direct neuronal regulation of metabolic processes and circadian rhythm-controlled behaviors is not well known among either group of vertebrates, although extensive research has been done on the Suprachiasmatic Nucleus (SCN) in model animals such as the mammalian mouse and ectothermic reptiles, particularly lizards. The SCN is known to be involved not only in photoreception through innervation from the retinohypothalamic tract, but also in thermoregulation of vertebrates capable of homeostasis, as well as regulating locomotion and other behavioral outputs of the circadian clock within ectothermic vertebrates. [1] The behavioral differences between both classes of vertebrates, when compared to the respective structures and properties of the SCN and various other nuclei proximate to the hypothalamus, provide insight into how these behaviors are the consequence of differing circadian regulation. Ultimately, many neuroethological studies must be done to completely ascertain the direct and indirect roles of the SCN on circadian-regulated behaviors of vertebrates.
The SCN of endotherms and ectotherms
editGenerally, external temperature does not influence endothermic animal behavior or circadian rhythm because of the ability of these animals to keep their internal body temperature constant through homeostatic thermoreguation; however, peripheral oscillators (see Circadian rhythm) in mammals are sensitive to temperature pulses and will experience resetting of the circadian clock phase and associated genetic expression, suggesting how circadian oscillators may be separate entities from one another despite having a master oscillator within the SCN . Furthermore, when individual neurons of the SCN from a mouse were treated with heat pulses, a similar resetting of oscillators was observed, but when an intact SCN was treated with the same heat pulse treatment the SCN was resistant to temperature change by exhibiting an unaltered circadian oscillating phase[1]. In ectothermic animals, particularly the ruin lizard Podacris sicula, temperature has been shown to affect the circadian oscillators within the SCN [2]. This reflects a potential evolutionary relationship among endothermic and ectothermic vertebrates, in how ectotherms rely on environmental temperature to affect their circadian rhythms and behavior and endotherms have an evolved SCN to essentially ignore external temperature and use photoreception as a means for entraining the circadian oscillators within their SCN. Additionally, the differences of the SCN between endothermic and ectothermic vertebrates suggest that the neuronal organization of the temperature-resistant SCN in endotherms is responsible for driving thermoregulatory behaviors in those animals differently from those of ectotherms, since they rely on external temperature for engaging in certain behaviors.
Behaviors controlled by the SCN of vertebrates
editSignificant research has been conducted on the genes responsible for controlling circadian rhythm, particularly within the SCN. Knowledge of the gene expression of Clock (Clk) and Period2 (Per2), two of the many genes responsible for regulating circadian rhythm within the individual cells of the SCN, has allowed for a greater understanding of how genetic expression influences the regulation of circadian rhythm-controlled behaviors. Studies on thermoregulation of ruin lizards and mice have informed some connections between the neural and genetic components of both vertebrates when experiencing induced hypothermic conditions. Certain findings have reflected how evolution of SCN both structurally and genetically has resulted in both classes of vertebrates engaging in characteristic and stereotyped thermoregulatory behavior.
- Mice: Among vertebrates, it is known that mammals are endotherms that are capable of homeostatic thermoregulation. Mice have been shown to have some thermosensitivity within the SCN, although the regulation of body temperature by mice experiencing hypothermia is more sensitive to whether they are in a bright or dark environment; it has been shown that mice in darkened conditions and experiencing hypothermia maintain a stable internal body temperature, even while fasting. In light conditions, mice showed a drop in body temperature under the same fasting and hypothermic conditions. Through analyzing genetic expression of Clock genes in wild-type and knock out strains, as well as analyzing the activity of neurons within the SCN and connections to proximate nuclei of the hypothalamus in the aforementioned conditions, it has been shown that the SCN is the center of control for circadian body temperature rhythm[3]. This circadian control thus includes both direct and indirect influence of many of the thermoregulatory behaviors that mammals engage in to maintain homeostasis.
- Ruin lizards: Several studies have been conducted on the genes expressed in circadian oscillating cells of the SCN during various light and dark conditions, as well as effects from inducing mild hypothermia in reptiles. Structurally, the SCN of lizards have a closer resemblance to that of mice, possessing a dorsomedial portion and a ventrolateral core[4];however, the genetic expression of the circadian related Per2 gene in lizards is similar to reptiles and birds, despite the fact that birds have been known to have a distinct SCN structure consisting of a lateral and medial portion[5]. Studying the lizard SCN because of its small body size and ectothermy is invaluable to understanding how these class of vertebrates modify their behavior within the dynamics of circadian rhythm, but it has not yet been determined if the system of cold-blooded vertebrates was slowed as a result of decreased activity in the SCN or a decrease in metabolic activity as a result of hypothermia[2].
References
edit- ^ a b Buhr, Ethan D., Seung-Hee Yoo, and Joseph S. Takahashi. "Temperature as a Universal Resetting Cue for Mammalian Circadian Oscillators." Science 330 (2010): 379-385.
- ^ a b Magnone, M. C., Jacobmeier, B., Bertolucci, C., Foà, A., & Albrecht, U. (2005). Circadian expression of the clock gene Per2 is altered in the ruin lizard (Podarcis sicula) when temperature changes. Molecular Brain Research, 133, 281-285
- ^ Tokizawa, K., Uchida, Y., & Nagashima, K. (2009). Thermoregulation in the cold changes depending on the time of day and feeding condition: physiological and anatomical analyses of involved circadian mechanisms. Neuroscience, 164, 1377-1386.
- ^ G. Casini, P. Petrini, A. Foa`, P. Bagnoli, Pattern of organization of primary visual pathways in the European lizard Podarcis sicula Rafinesque, J. Hirnforsch. 34 (1993) 361– 374.
- ^ U. Abraham, U. Albrecht, E. Gwinner, R. Brandstatter, Spatial and temporal variation of passer Per2 gene expression in two distinct cell groups of the suprachiasmatic hypothalamus in the house sparrow(Passer domesticus), Eur. J. Neurosci. 16 (2002) 429– 436.