It is well known that adenine-based purines exert multiple effects on pain transmission. Recently, we have demonstrated that guanine-based purines may produce some antinociceptive effects against chemical and thermal pain in mice. The present study was designed to investigate the antinociceptive effects of intrathecal (i.t.) administration of inosine or guanine in mice. Additionally, investigation into the mechanisms of action of these purines, their general toxicity and measurements of CSF purine levels were performed. Animals received an i.t. injection of vehicle (30mN NaOH), inosine or guanine (up to 600nmol) and submitted to several pain models and behavioural paradigms. Guanine and inosine produced dose-dependent antinociceptive effects in the tail-flick, hot-plate, intraplantar (i.pl.) glutamate, i.pl. capsaicin and acetic acid pain models. Additionally, i.t. inosine inhibited the biting behaviour induced by spinal injection of capsaicin and i.t. guanine reduced the biting behaviour induced by spinal injection of glutamate or AMPA. Intrathecal administration of inosine (200nmol) induced an approximately 115-fold increase on CSF inosine levels. This study provides new evidence on the mechanism of action of extracellular guanine and inosine presenting antinociceptive effects following spinal administration. These effects seem to be related, at least partially, to the modulation of A1 adenosine receptors.

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Background: Allopurinol is a potent inhibitor of the enzyme xanthine oxidase used in the treatment of hyperuricemia and gout. Because it is well known that purines exert multiple affects on pain transmission, we hypothesized that the inhibition of xanthine oxidase by allopurinol could be a valid strategy to treat pain in humans.This study aimed to compare the analgesic efficacy of oral allopurinol versus placebo as an adjuvant therapy in patients displaying fibromyalgia.Methods: This randomized, double-blinded, placebo-controlled study included 60 women with the diagnosis of fibromyalgia. Patients were randomly assigned to receive either oral allopurinol 300 mg (n = 31) or placebo (n = 29) twice daily during 30 days. The patients were submitted to evaluation for pain sensitivity, anxiety, depression, and functional status before treatment, and 15 and 30 days thereafter.Results: Oral administration of allopurinol 300 mg twice daily was ineffective in improving pain scores measured by several tools up to 30 days of treatment (P > 0.05).Additionally, no significant effects of allopurinol over anxiety, depressive symptoms, and functional status of fibromyalgia patients were observed in the present study. Conclusions: Although previous findings indicated that allopurinol could present intrinsic analgesic effects in both animals and humans, this study showed no benefit of the use of oral allopurinol as an adjuvant strategy during 30 days in women displaying fibromyalgia. However, considering previous promising results, new prospective studies are still valid to further investigate allopurinol and more selective purine derivatives in the management of pain syndromes. K E Y W O R D S allopurinol, anxiety, depression, fibromyalgia, pain, purines, xanthine oxidase SignificancePrevious evidence has indicated that allopurinol could display some intrinsic analgesic effects and could be an antinociceptive drug against acute and chronic pain syndromes. However, our randomized controlled clinical trial has shown that allopurinol is not effective in the management of fibromyalgia pain and other fibromyalgia-related outcomes, such as anxiety and depression.