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 μg injected in 25 μl 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 Prior administration of stimulant drugs antagonized the effect of pentobarbitone. 6 Animals withdrawn following chronic administration of a number of drugs, barbitone, barbitone/bemegride mixture, Mandrax (methaqualone: diphenhydramine; 10: 1),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.