Diurnal variations in tonic pain reactions in mice

Diurnal changes in the behavioural reactions to subcutaneous formalin injection (20 microl, 1%) into the dorsum of an hindpaw were examined in female CBA/J mice aged 70-75 days, maintained in a 12/12 dark/ light cycle (light on at 07.00 h; light off at 19.00 h). Mice showed higher pain scores, as expressed by the amount of time spent licking the injected paw and by the number of flinching episodes, when tested under red light at the beginning of the dark phase (19.00-22.00: Dark group) than when tested either under white or red light at the beginning of the light phase of the diurnal cycle (7.00-10.00). The increases in pain reactions at night were found both during the first (0-10 min) and the second (11-55 min) phase of the behavioural response to formalin injection. They were not due to aspecific increases in motor behaviour, since self-grooming actually decreased in the Dark group during the second phase of the response, and the amount of locomotor activity after the injection was similar to, or lower than, that found in mice tested in the morning under white or red light, respectively. In another group of female CBA/J mice tested in the hotplate apparatus (at a temperature of 52 degrees), paw-lick latencies were significantly higher in mice tested at dark during the night, whereas jump (escape) latencies were higher in the morning. These results demonstrate different diurnal variations in the reactions to brief or prolonged noxious stimulation in mice, with greater responses to tonic pain at the onset of the dark phase.