Morphine and Fentanyl Interactions with Thiopental in Relation to Movement Response to Noxious Stimulation

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Abstract
The effects of morphine-thiopetztal and fentanyl-thiopental combinations on the movement response caused by tail clamping a w e studied in rats. Doses that prevented movement response when the agents were given singly and when the agents were given in combination were determined by a probit procedure and compared with isobolgyraphic analysis. With doses of the above agents sufficient to block the movement response to fail clamping (ED50 values: 4.7 (3.3–6.6) mg/kg intravenously for morphine; 8.3 (5.8–11.3) μg/kg intavenously for fentanyl; and 18.8 (17.9–19.7) mg/kg intravenously for thiopental) both fentanyl and, to a lesser extent, morphine have a less than additive or an antagonistic interaction with thiopental. This antagonism is a relative one that does not increase the requirement for one agent upon the addition of another agent.