Azadirachta indica (Meliaceae) extracts have been reported to exhibit anti-inflammatory and antinociceptive properties. However, the activities of azadirachtin, a limonoid and the major bioactive compound found in the extracts, have been poorly investigated in animal models. In the present study, we investigated the effects induced by azadirachtin in experimental models of pain and inflammation in mice. Carrageenan-induced paw edema and fibrovascular tissue growth induced by subcutaneous cotton pellet implantation were used to investigate the anti-inflammatory activity of azadirachtin in mice. Zymosan-induced writhing and hot plate tests were employed to evaluate the antinociceptive activity. To explore putative mechanisms of action, the level of tumor necrosis factor-α in inflammatory tissue was measured and the effect induced by opioidergic and serotonergic antagonists was evaluated. Previous per os (p. o.) administration of azadirachtin (120 mg/kg) significantly reduced the acute paw edema induced by carrageenan. However, the concomitant increase of the paw concentration of tumor necrosis factor-α induced by this inflammatory stimulus was not reduced by azadirachtin. In addition to inhibiting the acute paw edema induced by carrageenan, azadirachtin (6, 60, and 120 mg/kg) inhibited the proliferative phase of the inflammatory response, as demonstrated by the reduced formation of fibrovascular tissue growth. Azadirachtin (120 mg/kg) also inhibited the nociceptive response in models of nociceptive (hot plate) and inflammatory (writhing induced by zymosan) pain. The activity of azadirachtin (120 mg/kg) in the model of nociceptive pain was attenuated by a nonselective opioid antagonist, naltrexone (10 mg/kg, i. p.), but not by a nonselective serotonergic antagonist, cyproheptadine. In conclusion, this study demonstrates the activity of azadirachtin in experimental models of nociceptive and inflammatory pain, and also in models of acute and chronic inflammation. Finally, multiple mechanisms, including the inhibition of the production of inflammatory mediators and activation of endogenous opioid pathways, may mediate azadirachtin activities in experimental models of inflammation and pain.

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The research on sex differences in nociception and antinociception as well as sex and gender differences in pain and analgesia is a maturing field. There is a vast literature showing experimental and clinical pain suppressive effects induced by minocycline, especially in inflammatory pain. However, as far as we know, possible qualitative or quantitative sex differences in those effects remained to be examined. By employing the formalin test, which has two phases of experimental pain behavior that models nociceptive pain (i.e., first phase) and inflammatory pain (i.e., second phase), we initially evaluated the effect induced by minocycline in female or male C57BL/6 mice. The treatment reduced the second phase of licking behavior in both females and males, and the effects were quantitatively similar in both sexes. Likewise, the same sex-independent effect was observed in Swiss mice, suggesting a genotype-unspecific sex-independent effect. While minocycline is already being tested in clinical trials, this appears to be the first preclinical investigation of sex differences in the experimental pain suppressive effects induced by this widely studied drug. The independence of sex in the antinociceptive effect induced by minocycline may be hopefully translated to gender-independent analgesic effects, which would be surely promising in a therapeutic paradigm.

Nicorandil (2-nicotinamide ethyl nitrate), an antianginal drug characterized by the coupling of nicotinamide with a nitric oxide (NO) donor, activates guanylyl cyclase and opens ATP-dependent K(+) channels. In the present study, we investigated the effects induced by per os (p.o.) administration of nicorandil (12.5, 25 or 50 mg/kg) or equimolar doses (corresponding to the highest dose of nicorandil) of N-(2-hydroxyethyl) nicotinamide (NHN), its main metabolite, or nicotinamide in the model of nociceptive response induced by formaldehyde in mice. Nicorandil, but not NHN or nicotinamide, inhibited the second phase of the nociceptive response. This activity was observed when nicorandil was administered between 30 and 120 min before the injection of formaldehyde. Ipsilateral intraplantar injection of nicorandil (125, 250 or 500 μg/paw) did not inhibit the nociceptive response. After p.o. administration of nicorandil (50 mg/kg), peak plasma concentrations of this compound and NHN were observed 0.63 and 4 h later, respectively. Nicotinamide concentrations were not increased after administration of nicorandil. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 1 or 2 mg/kg), a guanylyl cyclase inhibitor, partially attenuated the antinociceptive activity of nicorandil. However, this activity was not changed by glibenclamide (30 or 60 mg/kg), an inhibitor of ATP-dependent K(+) channels. In conclusion, we demonstrated the antinociceptive activity of nicorandil in a model of pain that exhibits both a nociceptive and an inflammatory profile. This activity is not mediated by nicotinamide or NHN. The coupling of an NO-donor to nicotinamide results in a compound with an increased potency. The NO-cGMP pathway, but not ATP-dependent K(+) channels, partially mediates the antinociceptive activity of nicorandil.