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Cobalt chloride (CoCl2) modifies mitochondrial permeability and has a hypoxic-mimetic effect; thus, the compound induces tolerance to ischemia and increases resistance to a number of injury types. The aim of the present study was to investigate the effects of CoCl2 hypoxic preconditioning for three weeks on thermonociception, somatic and visceral inflammatory pain, locomotor activity and coordination in mice. A significant pronociceptive effect was observed in the hot plate and tail flick tests after one and two weeks of CoCl2 administration, respectively (P<0.001). Thermal hyperalgesia (Plantar test) was present in the first week, but recovered by the end of the experiment. Contrary to the hyperalgesic effect on thermonociception, CoCl2 hypoxic preconditioning decreased the time spent grooming the affected area in the second phase of the formalin test on the orofacial and paw models. The first phase of formalin-induced pain and the writhing test were not affected by CoCl2 preconditioning. Thus, the present study demonstrated that CoCl2 preconditioning has a dual effect on pain, and these effects should be taken into account along with the better-known neuro-, cardio- and renoprotective effects of CoCl2.

IntroductionMethylene blue (MB), a widely used inhibitor of NO activity/production, is also a reduction-oxidation agent that can act both as a powerful antioxidant and as an enhancer of the electron transport chain. Furthermore, it prevents formation of mitochondrial oxygen free radicals and promotes oxygen consumption (Atamna et al. 2010,,Rojas et al. 2012). PurposeThe aim of the study was to investigate the effects of chronic (14 days) MB administration on experimental pain in mice. MethodsSixteen Swiss male mice were divided into 2 groups: control group (n=8) and MB group (n=8); both groups received daily injections, for 14 days, either with saline 20 μl i.p. (control group) or with MB 5 mg/kg b.w. i.p (MB group). Nociceptive tests (tail flick and hot plate) as well as mechanical and thermal withdrawal thresholds were measured every two days before MB/ saline administration. Before and two hours after the last dose was administered (day 14), each group was evaluated for the nociceptive tests and heat/mechanical hyperalgesia; results were compared with paired Student's t test. After nociceptive tests, the mice received 20 μl of 5% formalin into the upper right lip and the intensity of the orofacial pain was assessed. The results were compared with those from saline group using unpaired Student's t test. ResultsChronic administration of MB increased tail flick and hot plate latencies (p=0.03, p=0.02). We also noted an increase in the reaction time for thermal hyperalgesia assessed by Hargreaves method (p=0.04). As for the formalin-induced orofacial pain, MB produced a significant analgesic effect on both phases (p=0.03). ConclusionOur study demonstrates that chronic administration of MB has analgesic effects on acute nociception as well as on the orofacial inflammatory pain; further studies must be conducted in order to elucidate the mechanism by which the methylene blue exerts its antinociceptive effect.

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