Thymoquinone, the main constituent of Nigella sativa, affects adenosine receptors in asthmatic guinea pigs

OBJECTIVES For determining the mechanism of anti-asthmatic effect of thymoquinone, this investigation evaluated the effect of thymoquinone in the presence of selective A2A and A2B adenosine receptor antagonists (ZM241385 and MRS1706, respectively). MATERIALS AND METHODS Seventy guinea pigs were randomly divided to 7 groups; control (C), sensitized with ovalbumin (S), sensitized groups pretreated with thymoquinone (S+TQ), ZM241385 (S+Anta A2A), MRS1706 (S+Anta A2B), thymoquinone and antagonists (S+Anta A2A+TQ and S+Anta A2B+TQ). Thymoquinone and each of these antagonists with 3 mg/kg dose were injected i.p. on 10(th) day of sensitization protocol. Tracheal responsiveness (TR) to methacholine and ovalbumin (OA), and total and differential cell count in lung lavage fluid (LLF) in different groups were measured. RESULTS Increased EC50 and LLF neutrophil count and decreased TR to methacholine and OA, LLF eosinophil and basophil counts were observed in S+TQ group compared to S group (P<0.001 to P<0.05). Significant decrease in EC50 (P<0.01), LLF neutrophil, lymphocyte and monocyte count (P<0.001 for all) and significant increase in TR to OA (P<0.01), LLF total WBC (P<0.01) and eosinophil count (P<0.001) were observed in S+A2A group compared to S+TQ group. There was significant increase in LLF eosinophil and monocyte counts in S+Anta A2B group compared with S+TQ group (P<0.001 for both). In S+TQ+Anta A2A group, there was significant increase in LLF eosinophil (P<0.001) and significant decrease in LLF neutrophil (P<0.01) and monocyte (P<0.001) counts compared with S+TQ group. CONCLUSION Thymoquinone affects adenosine receptors, which suggest that some of its anti-inflammatory effects may be mediated by these receptors.