Editing Suprachiasmatic nucleus (section)

==Circadian rhythms of endothermic (warm-blooded) and ectothermic (cold-blooded) vertebrates==

[[Image:Wiki stranglesnake.jpg|thumb|A thermographic image of an ectothermic snake wrapping around the hand of an endothermic human]]
Information about the direct neuronal regulation of metabolic processes and [[circadian rhythm]]-controlled behaviors is not well known among either [[endothermic]] or [[ectothermic]] [[vertebrates]], although extensive research has been done on the 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 [[homeothermy]] as well as regulating locomotion and other behavioral outputs of the circadian clock within ectothermic vertebrates.<ref name="taka">{{cite journal | vauthors = Buhr ED, Yoo SH, Takahashi JS | title = Temperature as a universal resetting cue for mammalian circadian oscillators | journal = Science | volume = 330 | issue = 6002 | pages = 379–85 | date = October 2010 | pmid = 20947768 | pmc = 3625727 | doi = 10.1126/science.1195262 | bibcode = 2010Sci...330..379B }}</ref> The behavioral differences between both classes of vertebrates when compared to the respective structures and properties of the SCN as well as 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===

In general, external temperature does not influence endothermic animal circadian rhythm because of the ability of these animals to keep their internal body temperature constant through homeostatic thermoregulation; 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 peripheral circadian oscillators may be separate entities from one another despite having a master oscillator within the SCN.<ref name="taka" /> 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.<ref name="taka"/> In ectothermic animals, particularly the [[ruin lizard]], ''Podarcis siculus'', temperature has been shown to affect the circadian oscillators within the SCN.<ref name="mag">{{cite journal | vauthors = Magnone MC, Jacobmeier B, Bertolucci C, Foà A, Albrecht U | title = Circadian expression of the clock gene Per2 is altered in the ruin lizard (Podarcis sicula) when temperature changes | journal = Brain Research. Molecular Brain Research | volume = 133 | issue = 2 | pages = 281–5 | date = February 2005 | pmid = 15710245 | doi = 10.1016/j.molbrainres.2004.10.014 | hdl = 11392/1198011 | url = http://doc.rero.ch/record/4325/files/1_albrecht_cec.pdf }}</ref> This reflects a potential evolutionary relationship among endothermic and ectothermic vertebrates as ectotherms rely on environmental temperature to affect their circadian rhythms and behavior while endotherms have an evolved SCN that is resistant to external temperature fluctuations and uses photoreception as a means for entraining the circadian oscillators within their SCN.<ref name="taka" /> In addition, 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===

Significant research has been conducted on the genes responsible for controlling circadian rhythm, in particular within the SCN. Knowledge of the gene expression of [[Clock gene|''Clock'' (''Clk'')]] and [[PER2|''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.<ref name=":6" /> Studies on [[thermoregulation]] of [[ruin lizard]]s and mice have informed some connections between the neural and genetic components of both vertebrates when experiencing induced hypothermic conditions.<ref name="mag" /> Certain findings have reflected how evolution of SCN both structurally and genetically has resulted in the engagement of characteristic and stereotyped thermoregulatory behavior in both classes of vertebrates.

*'''Mice''': Among vertebrates, it is known that mammals are endotherms that are capable of homeostatic thermoregulation. It has been shown that mice display thermosensitivity within the SCN. However, the regulation of body temperature in [[Hypothermia|hypothermic]] mice is more sensitive to the amount of light in their environment.<ref name="toki" /> Even while fasted, mice in darkened conditions and experiencing hypothermia maintained a stable internal body temperature.<ref name="toki" /> 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 knockout 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.<ref name="toki">{{cite journal | vauthors = Tokizawa K, Uchida Y, Nagashima K | title = Thermoregulation in the cold changes depending on the time of day and feeding condition: physiological and anatomical analyses of involved circadian mechanisms | journal = Neuroscience | volume = 164 | issue = 3 | pages = 1377–86 | date = December 2009 | pmid = 19703527 | doi = 10.1016/j.neuroscience.2009.08.040 | s2cid = 207246725 }}</ref> 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. In terms of structure, the SCNs of lizards have a closer resemblance to those of mice, possessing a dorsomedial portion and a ventrolateral core.<ref name="cas">{{cite journal | vauthors = Casini G, Petrini P, Foà A, Bagnoli P | title = Pattern of organization of primary visual pathways in the European lizard Podarcis sicula Rafinesque | journal = Journal für Hirnforschung | volume = 34 | issue = 3 | pages = 361–74 | date = 1993 | pmid = 7505790 }}</ref> However, genetic expression of the circadian-related ''Per2'' gene in lizards is similar to that in reptiles and birds, despite the fact that birds have been known to have a distinct SCN structure consisting of a lateral and medial portion.<ref name="abe">{{cite journal | vauthors = Abraham U, Albrecht U, Gwinner E, Brandstätter R | title = Spatial and temporal variation of passer Per2 gene expression in two distinct cell groups of the suprachiasmatic hypothalamus in the house sparrow (Passer domesticus) | journal = The European Journal of Neuroscience | volume = 16 | issue = 3 | pages = 429–36 | date = August 2002 | pmid = 12193185 | doi = 10.1046/j.1460-9568.2002.02102.x | s2cid = 15282323 }}</ref> Studying the lizard SCN because of the lizard's small body size and ectothermy is invaluable to understanding how this class of vertebrates modifies its behavior within the dynamics of circadian rhythm, but it has not yet been determined whether the systems of cold-blooded vertebrates were slowed as a result of decreased activity in the SCN or showed decreases in metabolic activity as a result of hypothermia.<ref name="mag"/>