Nicotinic and muscarinic interactions and choice accuracy in the radial-arm maze

Muscarinic acetylcholine (ACh) systems have long been known to be necessary for accurate performance in cognitive tests. Nicotinic ACh systems have been shown to be involved as well. However, there is only a limited amount of information concerning the interactions of these two branches of the ACh transmitter system. The current study was conducted to investigate the improvement in choice accuracy caused by muscarinic and nicotinic agonists and how it is affected by antagonists of these systems. Adult female Sprague-Dawley strain rats (N = 11) were trained on a working memory task in an 8-arm radial maze. Acute injections of the muscarinic and nicotinic agonists, pilocarpine (PILO, 1.0 mg/kg) and nicotine (NIC, 0.2 mg/kg), were made alone or in combination with the muscarinic and nicotinic antagonists, scopolamine (SCOP, 0.1 mg/kg) and mecamylamine (MEC, 10 mg/kg). NIC administration caused a significant improvement in choice accuracy compared with saline (p less than 0.01) and PILO caused a marginally significant improvement in choice accuracy (p less than 0.06). The combination of these nicotinic and muscarinic agonists did not cause an additive improvement. However, the improvement caused by either agonist was reversed by both nicotinic or muscarinic antagonists. This reversal was more complete for NIC than PILO despite the fact that NIC caused a greater improvement than PILO. These results suggest that muscarinic and nicotinic components of the ACh system, which are both important for cognitive function, interact in important ways. These interactions may be critical to consider when devising treatments for cognitive dysfunction associated with cholinergic hypofunction such as with Alzheimer's disease.