Diurnal variations in tonic pain reactions in mice

Diurnal changes in the behavioural reactions to subcutaneous formalin injec tion (20 mu l, 1%) into the dorsum of an hindpaw were examined in female CB A/J mice aged 70-75 days, maintained in a 12/2 dark/light cycle (light on a t 07.00 h; light off at 19.00 h). Mice showed higher pain scores, as expres sed 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 (7.00-1 0.00). 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 respon se 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 aft er the injection was similar to, or lower than, that found in mice tested i n the morning under white or red light, respectively. In another group of f emale CBA/J mice tested in the hotplate apparatus (at a temperature of 52 d egrees), paw-lick latencies were significantly higher in mice tested at dar k during the night, whereas jump (escape) latencies were higher in the morn ing. These results demonstrate different diurnal variations in the reaction s to brief or prolonged noxious stimulation in mice, with greater responses to tonic pain at the onset of the dark phase. (C) 2000 Elsevier Science In c. All rights reserved.