Acute tolerance to barbiturate in the rat.

3 experimental approaches to the quantitation of acute barbiturate tolerance have been compared in the rat. There was no difference between the brain hexobarbitone or barbitone concentration found at the time of loss of righting reflex compared with the concentration found on return of the righting reflex following the period of anaesthesia produced by a single i.p. injection of the drug. However, tolerance was induced by a 7 hr infusion of pentobarbitone which kept rats anaesthetized for approximately 8 hr. Such rats awakened with a significantly higher brain pentobarbitone concentration compared with rats awakening after a single i.p. injection. Repeated i.p. injections of pentobarbitone, sufficient to keep animals anaesthetized for 12 hr, also induced a tolerance to pentobarbitone, as indicated by a reduced sleeping time and higher brain barbiturate concentration on awakening following intracerebroventricularly administered pentobarbitone injected 12 hr after the last i.p. injection. The possible relationship between acute cellular tolerance and physical dependence is discussed.