The ontogenesis of biological rhythms

The periodic synthesis of cAMP in D. discoideum amoebae shares with other biological rhythms the property of appearing at a precise moment 

Moreover, as seen in chapter 7, the cAMP signalhng system undergoes a series of successive transitions in the hours that follow starvation. At first insensitive to extracellular cAMP stimuli, the system becomes excitable in that it amplifies in a pulsatory manner cAMP signals whose amplitude exceeds a threshold. A few hours later, the system becomes capable of generating in an autonomous, periodic manner pulsatile signals of cAMP. This capability, however, lasts for only several hours before cells lose their oscillatory properties and become excitable again. Thus, any explanation of the appearance of the cAMP rhythm in the course of development must account for the entire sequence of transitions absence of relay relay oscillations. This multiplicity of transitions puts additional constraints on the theoretical explanation but thereby strengthens it if it can account for the entirety of the sequence. 

Analogous variations could underlie the appearance of bursting oscillations in the neuron R15 in the course of Aplysia development (Ohmori, 1981). In a similar manner, the electrical activity of cardiac cells could acquire its periodic character at a precise moment in embryogenesis (DeHaan, 1980 Fukii, Hirota Kamino, 1981) this bifurcation would signal the onset of the heart beat. 

Temporal coding in intercellnlar conunnnication from cAMP signalling in Dictyostelium to pnisatile hormone secretion 

1975 Juliani Klein, 1978) that cAMP signals of 5 min periodicity induce the synthesis of proteins required for the intercellular communication machinery (adenylate cyclase, phosphodiesterase, cAMP receptor,. ..). Constant signals or cAMP pulses delivered every 2 min are without effect (Wurster, 1982), like cAMP stimuli applied in a random manner (Nanjundiah, 1988). 

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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. 

The onset of cAMP oscillations in Dictyostelium as a model for the ontogenesis of biological rhythms... 

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. 

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