Locomotor and feeding activity rhythms in a light-entrained diurnal rodent, Octodon degus

The wheel running (WR) and feeding activity (FA) of Octodon degus, a new laboratory rodent characterized by its diurnal habits, were recorded under different lighting conditions. Under 12:12-h light-dark (LD 12:12) cycles, WR activity exhibited a crepuscular pattern with two peaks, M and E, associated with “dawn” and “dusk,” respectively. In both cases, an anticipatory activity was patent, suggesting that, beside the masking effect of LD transitions, both peaks have an endogenous origin. This pattern, which was also observed under a skeleton photoperiod (LD 0.5:11.5), became unimodal after LD 0.5:23.5 and constant darkness (DD) exposure. Simultaneously, FA showed an arrhythmic pattern in most animals, especially under DD, when none of the animals exhibited a significant circadian rhythm. The existence of two groups of oscillators, or two oscillators, would explain most properties of the WR rhythms noted in this species. Our results show that the degu’s temporal feeding strategy seems mainly arrhythmic, whereas its WR pattern is driven by a strongly circadian bimodal rhythm.

Organization of circadian rhythmicity and suprachiasmatic nuclei in malnourished rats

The present study was aimed at characterizing the effects of low-protein malnutrition (6% casein) on the circadian rhythm of drinking behavior and on suprachiasmatic nuclei immunohistochemistry in Sprague-Dawley rats. Recordings were started at 30 days of age under a 12:12-h light-dark (LD) cycle. At age 150 days, recordings were continued under constant dim red light, and finally the latency to entrain to complete and skeleton photoperiods was established. At the end of the recordings rats were processed for histological analysis. Compared with their controls, malnournished rats exhibited 1) splitting of rhythmicity under LD that 2) condensed to one component in constant dim red light, 3) delayed entrainment to skeleton photoperiod, and 4) precocious entrainment under complete photoperiod. Immunohistochemical analysis showed mainly a decrease in the immunohistochemical detection of vasoactive intestinal polypeptide and glial fibrillar acid protein cells in malnourished animals. These results indicate that in malnourished rats there is a decrease 1) in the coupling force among the oscillators and 2) in the strength of the phase lock between the oscillators and the light-dark cycle.

The impact of nocturnal light pulses on the activity pattern of terrestrial snails (Helix lucorum) entrained to a photoperiod of 12 h light: 12 h dark

The effects of nocturnal light pulses, "asymmetrical skeleton photoperiod regimes," on the circadian locomotor activity rhythm in adults of the edible snail Helix lucorum previously entrained to a photoperiod regime of 12 h light : 12 h dark were examined. In all experiments the snails showed a clear bimodal rhythm of behaviour, with peaks at both dusk and dawn. Shifts occurred according to the timing of 2-h light pulses applied in the scotophase and were related to the phase of snails' entrained rhythms. It is suggested that a minimum dark period (up to 10 h) is necessary for H. lucorum to retain its normal daily rhythmicity.