Formalin test is used to assess acute pain responses in rodents to a formalin injection of the hind paw. Pain models were initially established in rats and then applied in mice. However, nerve anatomy differs between the species and within strains (Rigaud et al., 2008). This could have important consequences on how tissue areas should be processed after performing these tests.
Acid-sensing ion channels (ASICs) play important roles in pain conditions. ASIC1a contributes to spinal processing of inflammation-related pain behaviors (Duan et al., 2007), and peripheral inflammation increases the expression of ASIC1a in the rat spinal cord (Wu et al., 2004). On the other hands, Psalmotoxin-1 (Pctx-1), a toxin that inhibits ASIC1a, and an ASIC1a antisense oligodeoxynucleotide, have been shown to affect the response to formalin test (Mazzuca et al., 2007). Little is known about ASIC1 protein levels in some models, as protein levels proved difficult to detect.
We decided to analyze ASIC1a protein levels in the mouse acute formalin model at different lumbar segments in the spinal cord of C57BL/6 mice. Our results showed that the biphasic behavior of mice to formalin was accompanied by an increase in ASIC1 levels following a gradient stronger at L3 levels and decreasing towards L5 in formalin-injected mice. This work highlights the role of the ASIC1 channel in pain, focusing by areas to avoid a dilution effect and the potential role of pharmacological therapies aiming to this channel.