Endogenous rhythms

The fact that most serotonergic dorsal raphe neurons are dependent on extrinsic excitatory or facilitatory inputs to express their characteristic spontaneous activity may seem to contradict previous studies suggesting that these neurons may function as autonomous pacemakers (42) with an endogenous rhythm (31) attributable to the presence of pacemaker potentials (8). Such a contradiction exists only if one insists that endogenous rhythms and pacemaker potentials must, by definition, be totally autonomous, i.e., completely independent of all extrinsic synaptic or neurohumoral influences. Such a definition would seem too restrictive in view of the fact that some invertebrate neurons display pacemaker potentials only when certain afferent fibers are stimulated (38) or when exposed to certain neurohumoral substances (18,28). 

Trulson, M. E., Howell, G. A., Brandstetter, J. W., Frederickson, M. H., and Frederickson, C. J. (1982) In vitro recording of raphe unit activity Evidence for endogenous rhythms in presumed serotonergic neurons. Life Sci., 31 785-790. 

Overland, L. (1960). Endogenous rhythm in opening and odor of flowers of Cestrum nocturnum. American Journal of Botany 47 378-382. 

In common with all living matter, the brain is subject to rhythmic changes in its functional state. The most common, and most marked rhythms are the 24-h rhythms , so called because they are synchronised by the 24 h light-dark cycle of the environment (for examples of such rhythms see Richter, 1964 Mills, 1966 Friedman and Walker, 1968). Many of these rhythms can be shown to exist solely as a result of environmental factors, and disappear in the absence of any external time cue (Zeitgeber). Others are endogenous rhythms, which under normal circumstances are conditioned to an exact 24 h cycle by external Zeitgebers. However, in the absence of time cues, these rhythms do not disappear, but revert to their true frequency, which only approximates to 24 h. These are the true circadian rhythms. 

Restores hemispheric imbalance Acts as a zeitgeber (resynchronizes abnormai endogenous rhythms)... 

Balasimha D, Ram G, Tewari MN (1977) Cytokinin-coumarin interaction in relation to growth, sulfhydryl, chlorophylls, and peroxidase-activity in Phaseolus radiatus L. seedlings. Biochem Physiol Pllanz 171 49-54 Ball NG, Dyke IJ (1956) The effects of indoie-3-acetic acid and 2 4-dichlorophenoxyacetic acid on the growth rate and endogenous rhythm of intact Avena coleoptiles. J Exp Bot7 25-41... 

Innocenti, A. M., Mazzuca, S., Bitonti, M. B., De Gara, L., Liso, R., and Arrigoni, O., 1994, Endogenous rhythm of ascorbic acid in seedling roots of broad bean. Plant Physiol. Biochem. 32 521-525. 

Note For some important recent papers dealing with the subjects of Chap. 4, in particular with endogenous rhythms of enzyme activities, properties of PEP carboxylase and intracellular malase transport, the Appendix on pp. 199— 201 should be consulted. 

It has long been known that a rhythmic gas exchange in CAM plants can persist under continuous darkness (see Bunning, 1973 see Fig. 5.14). Thus, the gas exchange of certain CAM plants follows a typical circadian endogenous rhythm. The question of rhythms in CAM has previously been reviewed by Wilkins (I960, 1969) and Queiroz (1974). 

In a series of extensive and detailed studies, Queiroz has reported much evidence that both CO2 production and fixation are rhythmic. The circadian rhythm of CO2 exchange is thus explained by Queiroz (1974) in terms of endogenous rhythms of enzyme activity (cf. Chap.4.2.3.2,b). Queiroz s data on enzyme capacity indicate that the rhythm of CO2 fixation is an overt indication of actual P-enolpyruvate carboxylase activity which oscillates synchronously with CO2 fixation (Queiroz, 1974). Similarly, the rhythm of CO2 production follows the enzymic capacity of the endogenously controlled malate enzyme (see Fig. 4.6 for the rhythmic phases of PEP carboxylase, malate enzyme, and malic acid). 

Wilkins, M.B. An endogenous rhythm in the rate of carbon dioxide output of Bryophyllum. I. 

Wilkins, M.B. An endogenous rhythm in the rate of carbon dioxide output of Bryophyllum. III. The effects of temperature changes on the phase and period of the rhythm. Proc. R. Soc. 156,220-241 (1962 a)... 

Wilkins,M.B. (ed.), pp. 647-671. London McGraw-Hill 1969 Wilkinson, M.J., Smith, H. Properties of phosphoenol pyruvate carboxylase from Bryophyllum fedtschenkoi leaves and fluctuation in carboxylase activity during endogenous rhythms of carbon dioxide output. Plant Sci. Lett. 6, 319-324 (1976)... 

Paediatrics Circadian rhythms was assessed during prolonged midazolam infusion in 27 paediatric intensive care unit children under mechanical ventilation [13 ]. This study foxmd that endogenous rhythms in critically ill and sedated children are severely disturbed and desynchronised. It also suggested that midazolam pharmacokinetics were not influenced by the light-dark cycle in children in intensive care xmits (ICUs). 

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