MICROTINE ULTRADIAN RHYTHM OF ACTIVITY - AN EVALUATION OF DIFFERENT HYPOTHESES ON THE TRIGGERING MECHANISM

Ultradian activity rhythms are a peculiar behavioural pattern, in whic h the 24-h day is divided into several short-term activity cycles. Suc h patterns are particularly prominent in microtine rodents. In this pa per we review experimental findings and proposed hypotheses on the pro ximate triggering mechanism of ultradian rhythmicity. Eight different mechanisms (classified as homeostatic, pacemaker and stochastic hypoth eses) were analysed within the same theoretical model-framework, consi dering feeding, food processing, energy turnover and activity behaviou r. Ultradian activity patterns could consistently be generated by each of the analysed models. None of the models, however, resulted in freq uency dependencies in respect to body weight, metabolic rate, food qua lity and food deprivation, unless additional assumptions were introduc ed. From our model evaluations we concluded that the stochastic model does not represent an independent hypothesis. Hence, only homeostatic and pacemaker hypotheses remained as alternative approaches to the tri ggering mechanism. Although the pacemaker hypothesis appeared more rea sonable, a combination with some feedback mechanism to metabolism seem ed even more likely. Some key experiments are suggested, which could g ive insight into the complex mechanism.