Biological rhythms communication

Some of the main examples of biological rhythms of nonelectrical nature are discussed below, among which are glycolytic oscillations (Section III), oscillations and waves of cytosolic Ca + (Section IV), cAMP oscillations that underlie pulsatile intercellular communication in Dictyostelium amoebae (Section V), circadian rhythms (Section VI), and the cell cycle clock (Section VII). Section VIII is devoted to some recently discovered cellular rhythms. The transition from simple periodic behavior to complex oscillations including bursting and chaos is briefly dealt with in Section IX. Concluding remarks are presented in Section X. 

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The possible occurrence in the Dictyostelium system of complex oscillatory phenomena such as birhythmicity, bursting and chaos is discussed in chapter 6. The appearance of periodic behaviour in the course of development provides a model for the ontogenesis of biological rhythms. This aspect is treated in chapter 7. Finally, an additional interest of the intercellular communication system of Dictyostelium amoebae is that it allows us to address the question of the physiological function of the periodic phenomenon. This question is dealt with in chapter 8, where the discussion is extended to the role of pulsatile hormone secretion in higher organisms. 

Biological rhythms occur only under precise conditions, and variations in a control parameter can bring about their disappearance. In a symmetrical manner, the variation of such a parameter can lead to the appearance of a rhythm in the course of development. There is no example as yet where the molecular basis of the ontogenesis of a biological rhythm is known in detail. The rhythm of intercellular communication in the slime mould Dictyostelium discoideum provides us with a prototype for the study of this question. 

These results are of general significance for the study of biological rhythms as they show how the continuous variation of certain control parameters can lead to the emergence of a rhythm in the course of development of an organism. Here, the level of certain proteins augments once the amoebae begin to synthesize the components of their intercellular communication system after starvation. As soon as the concentration of the cAMP receptor and the activity of enzymes such as adenylate cyclase and phosphodiesterase reach a critical value, oscillations appear spontaneously. Rinzel Baer (1988) have shown, however, that a certain delay separates the time at which the parameters cross their bifurcation values and the moment at which oscillations... 

An alternative, more detailed model for circadian PER oscillations in Drosophila has been developed independently be Abbot et al. (1995) (M. Rosbasch, personal communication results of that study were first presented at the 4th conference of the Society for Research on Biological Rhythms held in May 1994 in Amelia Island, Florida). That model, which is also based on the negative feedback exerted by PER on per transcription, takes into account a larger number of phosphorylated residues and focuses on the role of PER phosphorylation in delaying the entry of the protein into the nucleus (Curtin et al. 1995). 

D. melanogaster, PDF is localized in a few specialized neurons which have their cell bodies in the optic lobe and some processes in the accessory medulla of the optic lobe, an area which is known to house the neurons of master circadian pacemakers responsible for daily locomotoiy rhythms [72, 73]. It is from these and a number of other observations that PDFs are thought to be the communicator of these pacemaker cells to spread the message of the biological clock, thus acting as peptidergic neurotransmitters [74]. The PDFs do not affect pigment movement in insects. 

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