The formalin test in mice: the influence of ambient temperature

The ambient temperature had a confounding influence on the licking response in the formalin test. No effect was demonstrated in the early phase. In the late phase, the licking activity was much lower at 20 degrees C than at 25 degrees C. Both the intensity and the duration of the response were increased by increasing the ambient temperature from 20 degrees C to 28 degrees C. 5,6-Dihydroxytryptamine lesions of descending serotonergic pathways induced an increase in the nociceptive response at an ambient temperature of 20 degrees C, while the response was no different from control values at 25 degrees C. Differences in paw skin temperature may explain these temperature dependent effects. It was concluded that control of the ambient temperature is necessary to obtain reliable results in the formalin test, late phase.