On the Mechanism of Potentiation by Morphine of Thiopental Sleeping Time

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Abstract
Injection of thiopental sodium, 25 or 40 mg/kg, i.p., into normal rats produced a prompt sleep which lasted 8.3 ± 2.9 and 16.8 ± 2.7 min, respectively. Injection of morphine sulphate, 20 mg/kg, i.p., itself produced only mild sedation but prolonged the thiopental induced sleep to 50 and 67.3 min, respectively. At 1 min after onset of sleep the brain thiopental concentration in morphine pretreated animals was significantly lower than in control animals. Furthermore the latter also awoke with higher brain thiopental concentrations; the morphine pretreated animals slept for a longer time despite lower brain thiopental concentration. The t½ of brain thiopental was longer in the morphine pretreated animals, 25 min, than in the control animals, 9 min. Distribution of thiopental in several tissues at various time periods following injection of thiopental revealed some time-related quantitative differences between the two groups but a similar qualitative pattern. It is suggested that morphine lowers the brain threshold for thiopental induced sleep.

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