The interaction between atropine and some lipid-soluble barbiturates in rats

The effect of atropine pretreatment on the CNS sensitivity to five lipid-soluble barbiturates was studied in rats. Atropine in a dose of 8 mg/kg intraperitoneally or saline was given 1.5 hr before an EEG-threshold determination with the tested barbiturates. The barbiturate was infused by constant rate through a tail vein. The dose of barbiturate needed to induce a burst suppression period in the EEG which is one second or longer was used as the threshold dose. Atropine pretreatment decreased the threshold dose significantly for two N-methylated barbiturates, hexobarbital (enhexymalum NFN) and methohexital (enallynymalum NFN), but not for thiopental (thiomebumalum NFN), pentobarbital (mebumalum NFN) or amobarbital (pentymalum NFN). No clear effects of atropine pretreatment could be found in the ensuing anaesthesia times after threshold determinations. These results indicate that there are differences in mechanisms of action between barbiturates. One of these mechanisms is related to the cholinergic system in the CNS.