The respiratory effects of the volatile oil of the black seed (Nigella sativa) in guinea-pigs: elucidation of the mechanism(s) of action.

1. The effect of the volatile oil (VO) of the black seed (Nigella sativa) on the respiratory system of the urethane-anaesthetized guinea-pig was investigated and compared with those of its constituent thymoquinone (TQ). 2. Intravenous administration of VO in the dose range (4-32 microliters kg-1) induced dose-dependent increases in the respiratory rate and the intratracheal pressure. 3. The effects of VO were significantly antagonized by treatment of the animals with mepyramine, atropine and reserpine. They were not antagonized by indomethacin, diethyl carbamazine or hydrocortisone. 4. Intravenous administration of TQ in the dose range (1.6-6.4 mg kg-1) induced significant increases in the intratracheal pressure without any effect in the respiratory rate. 5. The results suggested that VO-induced respiratory effects were mediated via release of histamine with direct involvement of histaminergic mechanisms and indirect activation of muscarinic cholinergic mechanisms. 6. Removal of TQ from VO may provide a potential centrally acting respiratory stimulant.