Ongoing ultradian activity rhythms in the common vole, Microtus arvalis, during deprivations of food, water and rest

The timing mechanism underlying ultradian (2-3 h) activity patterns in the common vole, Microtus arvalis, was studied using behavioural deprivation experiments. These were aimed at distinguishing between a homeostatic control mechanism, in which the rhythmic behaviour itself is part of the causal loop, and a clock mechanism, independent of the behaviour. In 175 experiments, deprivation of food during 3 ultradian cycles in (subjective) daytime did not result in significant changes in the ultradian periodicity of attempts to obtain the food, compared with ad lib. access to food and water. A minor, but significant increase in ultradian activity time (α') occurred in the course of the deprivation, but this was compensated by a shorter ultradian rest (ρ'). These results were obtained both in intact animals (n = 24), which showed ultradian and circadian rhythmicity in behaviour, and in animals (n = 21) with electrolytic lesions aimed at the suprachiasmatic nuclei (SCN), which lacked the circadian modulation of behaviour. Simultaneous deprivation of water and food in 8 voles without circadian rhythmicity during 40 experiments also did not lead to any change in the ultradian periodicity of feeding attempts. Rest deprivation was studied in 5 SCN lesioned voles, by forcing running wheel activity to continue following spontaneous running. Thus, the experimental activity bout α' was artificially lengthened to 2-9 h in 67 experiments. The onset of the subsequent rest episodes occurred independent of the duration of the preceding α'. The duration of ρ' was dependent on the preceding, experimental α' in a periodic fashion. The interval experimental τ' (= lengthened α' + following ρ') was equal to one, two or three times the control τ' (obtained on non-experimental days). This result fits the prediction of a clock model and is in conflict with a monotonic increase of ρ' with α', as expected in a homeostatic, restorative process. It is concluded that the ultradian timing of activity in the common vole can be explained neither by homeostatic hunger or thirst mechanisms nor by homeostatic rest/activity regulation. The results strongly suggest an independent clock system generating ultradian feeding rhythms in the common vole.