Mammals, circadian rhythms

Experimental advances during the last decade have clarified the molecular bases of circadian rhythms, first in Drosophila and Neurospora, and more recently in cyanobacteria, plants, and mammals [95-99]. In nearly all cases investigated so far, it appears that circadian rhythms originate from the negative... 

Meyer-Bernstein EL, Sehgal A 2001 Molecular regulation of circadian rhythms in Drora/ifo/a and mammals. Neuroscientist 7 496-505... 

King DP, Takahashi JS 2000 Molecular genetics of circadian rhythms in mammals. Annu Rev Neurosci 23 713-742... 

Circadian rhythms of behaviour in mammals are robust and precise. For example, in constant darkness and temperature, the circadian rhythm of locomotor activity in laboratory rodents persists indefinitely (Pittendrigh 1993) and is accurate to within a few minutes per day (Pittendrigh Daan 1976a, Vitatema et al 1994). 

In mammals an approximately 24-h (circadian) rhythm controls behavior and affects many physiological functions. As previously mentioned (p. 1800), the brain has its own rhythms, which originate with pacemaker neurons. The heart beats with another neurally established rhythm. The circadian rhythm has a much longer period and, therefore, seems more mysterious. 

It is now known that circadian rhythms are controlled by clock genes which are found in species as wide apart as insects and mammals. It would appear that the clock genes are activated by light falling on the retina. The activated retina neurons then stimulate the retinohypothalamic tract which projects to the suprachiasmatic nucleus and thence to the anterior pituitary. This pathway is responsible for coupling the circadian rhythm with the... 

The vertebrate NPY family contains such prominent members as pancreatic polypeptide (PP), characterized initially from chicken where PP is synthesized and released from the endocrine cells of the pancreatic islets [204], neuropeptide Y (NPY) found in porcine brain [205] and peptide YY (PYY) isolated from porcine intestine [206]. Such peptides are characterized by a chain length of 36 amino acids and Tyr-amide at the C-terminus. Whereas NPY is exclusively expressed in neurons of mammals and is known to control processes such as stimulation of food intake, vasoconstriction, sexual behaviour and circadian rhythm, PP and PPY are synthesized in endocrine cells of the gut and are inhibitors of gut motility and of the secretion of exocrine products from the pancreas. 

The question of how to generate hourly hormonal rhythms pertains of course to the origin of circadian rhythms that are observed in unicellular (Vanden Driessche, 1980 Schweiger et al, 1986 Edmunds, 1988) up to the most evolved organisms (Moore-Ede et al, 1982). In mammals, the suprachiasmatic nuclei contain the neurons that generate these periodicities, so that the key to both circadian rhythms and the GnRH rhythm controlhng reproduction lies in neurons of the hypothalamus. 

Optimal periodic control involves a periodic process, which is characterized by a repetition of its state over a fixed time period. Examples from nature include the circadian rhythm of the core body temperature of mammals and the cycle of seasons. Man-made processes are run periodically by enforcing periodic control inputs such as periodic feed rate to a chemical reactor or cyclical injection of steam to heavy oil reservoirs inside the earth s crust. The motivation is to obtain performance that would be better than that imder optimal steady state conditions. 

Jacobs EH, Yamatodani A, Ummerman H. Is histamine the final neurotransmitter in the entrainment of circadian rhythms in mammals Trends Pharmacol Scl, 21 S), 293-298, 2000 Jalan R, Damink SW, Deutz NE, Lee A, Hayes PC. Moderate hypothermia for uncontrolled intracranial hypertension in acute liver failure. Lcmcet, 354(9185), 1164-1168, 1999 Jalan R, Pollok A, Shah SH, Madhavan K, Simpson KJ. Liver derived pro-infltunmatory cytokines may be important in producing intracranial hypertension in ALE J. Hepatol, 37, 536-538, 2002... 

Caldelas, I., Chimal-Monroy, J., Martinez-Gomez, M. and Hudson, R. (2005) Non-photic circadian entrainment in mammals A brief review and proposal for study during development. Biol. Rhythm Res. 36, 23-37. 

In mammalian tissue culture cells, robust circadian gene expression can be entrained by 12 h temperature cycles with an amplitude of 4 °C (e.g. 37 °C versus 33 °C) (Brown et al 2002). We thus wondered whether physiological temperature rhythms, themselves circadian and with an amplitude of —4 °C in most mammals, could also sustain cyclic clock-gene transcription. To this end, we engineered a... 

Clock oscillation occurs first at a cellular level. The clock genes so far identified in mammals are structurally similar to those in Drosophila (Young Kay 2001). This suggests that mammals and Drosophila utilize similar components to generate circadian ( 24 h) rhythms. Mammalian clock research is now showing whether the core feedback loop of clock genes speculated to be present in Drosophila (Hardin et al 1990) is also conserved in mammals. 

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