Abstract
Pain research is mostly based on experimental assays that use animal models, which may allow deciphering the physiopathology of this condition and to propel drug discovery. The formalin nociception test is considered one of the most predictive approaches to study acute pain in rodents. This test permits monitoring pain-related responses (i.e., itch) caused by a subcutaneous injection of an inflammatory agent, namely 2.5% formalin solution, in the hind paw. After the injection, two distinct time periods or phases of licking/biting behaviour occur, which are separated by a quiescent period. Importantly, these phases differ in duration and underlying mechanisms. Hence, the initial acute phase (phase I), commonly recorded for 5 min just after formalin administration, reflects acute peripheral pain, probably due to direct activation of nociceptors through TRPA1 channels. On the other hand, the phase II, which starts after the quiescent period (5-15 min) and is commonly recorded for 15-30 min, is due to the ongoing inflammatory input and central nociceptive sensitization. Here, we describe in detail the protocol used to perform a reliable and reproducible formalin test in mice.
Highlights
[Background] The formalin test is an experimental assay that permits determining mice nocifensive behaviour
The test was originally described in the late seventies (Dubuisson and Dennis, 1977), and, initially, it consisted of the injection of 50 μl of 5% formalin in the dorsal surface of one forepaw of a rat or a cat
The formalin test allows the study of two different kinds of pain typologies: i) acute peripheral pain mediated by the direct activation of nociceptors through TRPA1 channels, and ii) inflammatory and central nociceptive
Summary
[Background] The formalin test is an experimental assay that permits determining mice nocifensive behaviour. The mice response (i.e., licking and flinching) is assessed after a subcutaneous injection of formalin normally in the plantar hind paw (Tjolsen et al, 1992). A 2.5% formalin (in physiological saline solution) injected in the mid-plantar surface of the rodent hind paw is commonly used to induce nociceptive responses (i.e., paw ‘flinching’ and licking) lasting for [45-90] min (Tjolsen et al, 1992).
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