Analysis and differentiation of the mechanism in the development of drug tolerance

Isolated rat hepatocytes and 9000 g rat liver supernatant as metabolic systems for the study of the pharmacokinetics of barbiturates

1. A method for metabolic studies with isolated rat hepatocytes has been developed. The rate of metabolism of a number of barbiturates has been studied both in hepatocytes and in 9000 g rat liver supernatant suspension. 2. The half-lives and clearance values, relative to heptabarbital, determined in hepatocytes were similar to the values obtained with isolated perfused rat liver. The values obtained with the 9000 g supernatant preparation were not. 3. In respect of metabolites formed, the metabolism of barbiturates in hepatocytes is comparable to that of the intact rat or liver perfusion. The metabolism in the 9000 g supernatant, however, is not.

Determination of halothane-induced sleeping time in the rat: effect of prior administration of centrally active drugs

A method is described for the determination of halothane-induced sleeping time in the rat. 2 The sleeping time exhibited a diurnal variation which was due, at least in part, to a change in the sensitivity of the central nervous system (CNS) to the anaesthetic. 3 Tolerance to halothane did not develop in rats repeatedly exposed to the anaesthetic over a period of over 48 hours. 4 Repeated sleeping time determinations have been used to follow changes in the sensitivity of the CNS to the anaesthetic occurring with time. 5 A tolerance to halothane was induced by pretreatment of rats with doses of amylobarbitone, pentobarbitone or meprobamate sufficient to keep animals anaesthetized for approximately 12 hours. This tolerance was followed by a period of halothane-hypersensitivity. 6 Halothane-tolerant animals awakened with higher brain halothane concentrations and were also tolerant to intracerebroventricularly administered pentobarbitone. 7 Halothane-hypertensive rats awakened with lower brain halothane concentrations and were also hypersensitivity to intracerebroventricularly administered pentobarbitone. 8 The possibility that the induction of cross-tolerance to halothane may be indicative of a drug's potential to produce dependence is discussed.

A study of the factors affecting the sleeping time following intracerebroventricular administration of pentobarbitone sodium: effect of prior administration of centrally active drugs

1 Injection of pentobarbitone sodium into a lateral cerebral ventricle of rats produced a loss of righting reflex. The duration of anaesthesia was dose-dependent.2 The optimum dose of pentobarbitone to allow study of the factors affecting the sleeping time was considered to be 650 mug injected in 25 mul water.3 In a study of the effect of age and sex on the sleeping time, the youngest rats used (88 g body weight) were found to be the most sensitive to barbiturate. Female rats were more sensitive than male animals.4 The duration of anaesthesia was not affected by induction or inhibition of hepatic drug-metabolizing enzyme activity.5 Prior administration (acute) of central nervous system depressant drugs shortened the latent period and prolonged the duration of sleep. Prior administration of stimulant drugs antagonized the effect of pentobarbitone.6 Animals withdrawn following chronic administration of a number of drugs, barbitone, barbitone/bemegride mixture, Mandrax (methaqualone: diphenhydramine; 10: 1), chlordiazepoxide, nitrazepam, chlorpromazine or ethanol, exhibited a significant tolerance to intracerebroventricularly administered pentobarbitone.7 Withdrawal of amphetamine, morphine, methyprylon or diazepam did not result in tolerance to intracerebroventricularly administered pentobarbitone.8 Chronic administration of all drugs except amphetamine and morphine induced a tolerance to intraperitoneally administered hexobarbitone (100 mg/kg).9 The usefulness of sleeping time determination following intracerebroventricular administration of pentobarbitone as an assessment of central nervous system excitability is discussed. It is concluded that this method gives a valid indication of the sensitivity of the central nervous system to barbiturate and of the level of excitability in general. The method is particularly applicable in situations where the activity of hepatic drug-metabolizing enzyme activity may be altered.