The rat formalin test: Can it predict neuropathic pain treatments?

Widely used, the formalin test is a tonic model of continuous pain resulting from formalin-induced tissue injury. It is a useful model, particularly for the screening of novel compounds, since it encompasses inflammatory, neurogenic, and central mechanisms of nociception [5,6]. Reports can vary widely with regard to concentration of formalin (itself a 37% dilute solution of formaldehyde) as well as the recording and characterisation of resultant pain behaviours in rats [4,5,6]. Therefore the effect of 1.25, 2.5, and 5.0% formalin on nociceptive behaviours was evaluated by measuring two commonly reported pain-like behaviours – flinching and licking/biting [1,2,3], associated with formalin intraplantar injection. Three clinically effective neuropathic pain treatments were tested (Gabapentin, Duloxetine, and Mexiletine) [1,2,3] to determine if these could reduce formalin-induced pain-like behaviours. An electronic recording method (Noldus Observer 5.0) was explored as an alternative to manual methods using observation and stop clocks. This allowed simultaneous recording of licking/biting and flinching behaviours.