Lack of Tolerance to an Amphetamine-Barbiturate Mixture and to its Components

Abstract

VARIOUS mixtures of amphetamine and a barbiturate produce effects on behaviour which are greater than those produced by any dose of either constituent drug alone. In particular it has been shown that a mixture of 15 mg/kg amylobarbitone sodium and 0.75 mg/kg amphetamine sulphate (ratio, 20 : 1 by weight, and therefore containing a greater proportion of barbiturate than ‘Drinamyl’) markedly increases the number of entries into the arms of a Y-shaped runway by female rats¹. This dose of the mixture has also facilitated the learning of a bar-press response² and of a T-maze (our unpublished results). The therapeutic use of drugs as well as behavioural experiments often require repeated administration of the same drug; for example, for the experiment with the T-maze (our unpublished results) the animals had to be drugged and tested daily for 45 days. It is therefore important to know whether tolerance develops to the drugs after repeated administration.