Behavioural effects of different intensities of formalin pain in rats.

The effects of two concentrations of formalin (0.1% and 10%) on Licking, Flexing and Paw-Jerk, and standard measures of activity, were studied in male rats during three experimental conditions: Box, Open-Field and Novel Object. Pain-evoked responses were present in all formalin-injected animals, with greater intensity in the group injected with formalin 10%. In this group Rearing and Olfactory Exploration were reduced with respect to the controls, locomotion was inhibited to the point that it was virtually absent during the last part of the test. In contrast, the lower formalin concentration not only failed to inhibit the locomotor/exploratory behaviours but also appeared to induce a general activation of behaviour, as suggested by the longer durations of Pendulum, the absence of Sleeping-like episodes and the higher number of Approaches to the object found in this group. The results of the present experiment underline the importance of a detailed analysis of behaviour in animal models of pain and support the view that the intensity of pain plays a crucial role in its behavioural effects.