Rats grouping and the circadian and ultradian synchronization of their carbon dioxide emission by a light-dark 12:12 h alternation.

Carbon dioxide emission (VCO2) was continuously recorded during 19 consecutive days in 25 Sprague Dawley young male rats placed in the same "respiratory chamber", grouped by 5 (G) and then separated (S). All rats were in controlled environmental conditions (20 degrees C temperature, humidity, ventilation, food and water ad libitum) and submitted to a light (100 lux)-dark alternation (LD 12:12). The curves obtained with the respiratory chamber CO2 concentration sampled every 20 minutes were analyzed for circadian periods, amplitudes, phases, ultradian peak oscillation intervals and amplitudes, and VCO2 time variations at L-->D and D-->L light transitions. Analysis of variance and t test show circadian amplitudes significantly (P < 0.001) higher (by 40.9%) than in S; moreover, ultradian peak amplitudes were higher in G than in S (by 78.0% in L and 105.8% in D). The circadian and ultradian (tau > 40 min) period intervals were not significantly different in G and in S. Circadian phase differences between L-->D and D-->L were significantly greater in S (by 50.3 min) but not in G. Light transitions did not significantly modify ultradian phases in G and in S. This data shows a better LD 12:12 synchronization in G than in S, resulting mostly from an increased respiratory amplitude modulation due to interindividual interactions.