Antagonism of acute physostigmine and neostigmine toxicity in mice by hemicholinium-3.

Hemicholinium-3 (HC-3) was administered intraperitoneally to mice concurrently with the intraperitoneal administration of physostigmine or neostigmine. HC-3 increased the LD50 values for both physostigmine and neostigmine but did not alter the effect on brain ACh levels produced by these agents. Since HC-3 does not cross the blood brain barrier after intraperitoneal administration, the antidoting action of HC-3 is peripherally mediated and does not solely involve an inhibition of ACh synthesis. The increase in brain acetylcholine caused by neostigmine was related to a reduction in acetylcholinesterase activity, providing evidence that intraperitoneally administered neostigmine crosses the blood-brain barrier.