The site of paracetamol action remains controversial Some pu

The site of paracetamol action remains controversial. Some publications are in favor of a central action (Barrière et al., 2013, Dogrul et al., 2012, Mallet et al., 2010, Pickering et al., 2006), while others report a peripheral action (Dani et al., 2007, Ferreira et al., 1978). FAAH is a ubiquitous enzyme (Deutsch et al., 2002); therefore, after systemic paracetamol administration, AM404 can be formed in the central nervous system (Barrière et al., 2013, Högestätt et al., 2005, Mallet et al., 2010) and in the periphery, especially in blood (Ruggieri et al., 2008). The FAAH inhibition performed in previous experiments by genetic or pharmacological tools was systemic. Thus, the contribution of peripheral versus central involvement of FAAH was indistinguishable. We addressed this issue by challenging the effect of paracetamol after a systemic administration of specific FAAH inhibitors URB597 or URB937. In MK0752 sale to URB597, which reaches the brain, URB937 cannot cross the blood–brain barrier (Clapper et al., 2010; Moreno-Sanz et al., 2011). A dose of URB compounds that inhibited FAAH (Clapper et al., 2010) without affecting inflammatory pain thresholds in our conditions was first investigated. The comparison of the impact of brain-permeant (URB597) and brain-impermeant (URB937) FAAH inhibitors on paracetamol effect will identify the respective involvement of central and/or peripheral FAAH in paracetamol action.
Materials and methods All experiments conformed to the guidelines of the Committee for Research and Ethical Issues of IASP (Zimmermann, 1983) and approved by our regional ethics committee for animal experiments of the Auvergne region in France (CEMEA Auvergne; CEEA-02). Pharmacological experiments were carried out using male C57BL6/J mice (20–24 g) obtained from Charles River Laboratories (L'Arbresle, France). Wild type littermate (FAAH+/+) and FAAH-deficient (FAAH−/−) C57BL/6 mice were used in other experiments. FAAH-deficient mice, originally generated by Cravatt et al. (2001) were bred in the animal house. The FAAH genotype of genitors was confirmed by PCR. The purchased animals were housed for one week prior starting the experiments in order to acclimatize them and they were kept under standard conditions (21–22 °C; 12/12 h light/dark cycle; 55% humidity) with food and water ad libitum.
Results
Discussion A genetic strategy was first used to explore the contribution of FAAH in paracetamol action in an inflammatory pain context. A preliminary experiment confirmed that FAAH−/− mice submitted to the paw immersion test exhibited different nociceptive thresholds compared to their littermates for temperatures above 50 °C as previously shown (Cravatt et al., 2004, Cravatt et al., 2001, Lichtman et al., 2004). We thus used a 46 °C temperature to explore paracetamol action in FAAH−/− mice which, as in the cited studies, did not discriminate a nociceptive threshold difference between FAAH+/+ and FAAH−/− mice. Regarding mechanical stimuli evaluated by von Frey filaments, no difference has been observed between the two genotypes as already reported (Chang et al., 2006). In both pain modalities, the anti-allodynic and anti-hyperalgesic actions of paracetamol were abolished in FAAH−/− mice, generalizing the concept that FAAH is the key enzyme for paracetamol analgesic action in either a naive context (Mallet et al., 2010) or a pathophysiological context (current study). However, one study reported that one gene one polypeptide hypothesis was not the case in a visceral pain test (Soukupová et al., 2010). Yet, we have to wonder if this screening nonspecific visceral pain model, which unfortunately is sensitive to non-analgesic compounds (Hendershot and Forsaith, 1959), is relevant to study the mechanism of action of analgesic drugs and especially paracetamol. Moreover, paracetamol is not recommended for visceral pain either by the American College of Gastroenterology or by the British Society of Gastroenterology (American College of Gastroenterology Task Force on Irritable Bowel Syndrome, 2009; Spiller et al., 2007). Interestingly, the FAAH requirement was studied regarding other effects of paracetamol. For example, the anxiolytic-like effect of paracetamol observed in the mouse elevated plus-maze test was suppressed by a co-administration of URB597 (Zaitone et al., 2012). However, Ayoub et al. (2004) demonstrated that another effect of paracetamol in mice, hypothermia, was not modified in FAAH−/− mice nor in mice pre-treated with URB597. Thus, FAAH involvement in paracetamol actions, marked for its analgesic action, might differ depending on its pharmacological effect.