Feeding behavior of lambs in relation to kinetics of 1,8-cineole dosed intravenously or into the rumen

The monoterpene 1,8-cineole is a major constituent of the essential oils that adversely influence intake of sagebrush by herbivores, but little is known about the mechanisms of its action. We investigated the influence of 1,8-cineole on the feeding behavior of two groups of sheep, one group dosed intravenously and the other intraruminally. In the first study, we infused 40 mg/kg BW of 1,8-cineole intravenously into four lambs on wk 1, 2, and 4. In the second, we administered 125 mg/kg BW of 1,8-cineole into the rumen of four lambs as a single-bolus dose in wk 1 and 2. Lambs dosed intravenously spent less time feeding than controls (28 vs. 60 min; P < 0.05), as did lambs dosed intraruminally (35 vs. 60 min; P < 0.05). Dosed lambs ate less than controls during rumen dosing studies (P < 0.05). For the intravenous infusion studies, rates of elimination did not differ among weeks (P < 0.05). For the rumen infusion studies, however, the absorption rate constant increased from 0.035/min to 0.076/min from wk 1 to 2, while the absorption half-life declined from 24 to 10 min (P < 0.05). Maximum plasma concentrations and time to reach maximum plasma concentrations were no faster in wk 2 than wk 1, but the primary elimination rate constant was 2.3 times higher in wk 2 (0.058/min) than in wk 1 (0.025/min) (P < 0.05). Dosed lambs exhibited clinical effects-licking of lips, drowsiness, staggering, and 1,8-cineole-smelling breath-that were much more pronounced with intravenous than rumen infusions. Dosing did not affect the acid-base balance. Collectively, these data suggest 1) rapid absorption and distribution of 1,8-cineole was responsible for initiating satiety, while more prolonged excretion was responsible for the duration of the satiety effect, and 2) lambs more readily adapted to 1,8-cineole in the rumen-dose study than in the intravenous-dose study.