The cholinergic system and spatial learning.

Acetlylcholine (ACh) in the central nervous system is critical for a multitude of functions. Here, we concentrate on declarative memory in humans, and its equivalent episodic-like memory in rodents and highlight current understanding of cholinergic system in these processes. Spatial memory formation represents a simple form of episodic-like memory in rodents that engages the basal forebrain cholinergic system and its target structures. In these, ACh exerts numerous functions. (1) During spatial acquisition learning, ACh efflux into the extracellular space is immediate in hippocampus and cortex; during consolidation of spatial reference memory, ACh levels are low. These requirements explain why ACh receptor blockade during acquisition blocks memory formation, and it is also consonant with the notion that an unspecific enhancement of cholinergic activity during consolidation is detrimental to memory formation. (2) Working and short-term memory for spatial locations engages the nucleus basalis – prefrontal cortex ACh system. ACh activity is trial related and maintained for some time post-training. (3) Striatal cholinergic activity is increased during stimulus–response learning and behavioural flexibility (reversal learning, extinction) providing a possible switch between different behavioural strategies. (4) At present, there is no clear difference between muscarinic and nicotinergic systems with respect to spatial learning. Antagonists of the respective receptors impair memory formation, agonists can reverse these deficits or may, under specific conditions act more like a general cognitive enhancers by way of improving attention. (5) Data reviewed here do not provide conclusive evidence for muscarinic or nicotinic receptors presenting as novel therapeutic targets, and there is no clear indication for ACh derived novel biomarkers for translational medicine. Unresolved and contradictory results are highlighted and discussed.