Intrahippocampal injections of gallamine impair learning of a memory task

Recent evidence has suggested that gallamine may act as an antagonist on presynaptic M2 muscarinic receptors which regulate acetylcholine release. The possibility that gallamine may potentiate learning by enhancing the release of acetylcholine was examined using a behavioral paradigm known to depend on cholinergic activity. Rats were cannulated bilaterally for intrahippocampal (i.h.) injections and adapted to a T-maze before the series of injections was initiated. The effects of bilateral i.h. injections of saline or gallamine (10 micrograms total) were examined on the learning of a win-stay, paired-run task in a T-maze. During the first few sessions, both saline- and gallamine-treated animals achieved below 50% correct responses. Both groups of animals gradually improved performance on the task although, contrary to expectations, gallamine-treated animals did not learn as rapidly as saline-injected animals. To verify the deleterious effects of gallamine on learning, the experiment was repeated using a second group of rats. Similar results were observed; gallamine-treated animals learned to perform the task at a slower rate than the saline-injected controls. The results suggest that treatment strategies with cholinomimetics may impair, rather than enhance performance after long-term administrations.