Effects of systemic and intracerebroventricular administration of mecamylamine, a nicotinic cholinergic antagonist, on spatial memory in rats

Effects of central nicotinic cholinergic receptor blockade produced by chlorisondamine on learning and memory performance in rats

The effects of chronic nicotinic receptor blockade on the performance of learning and memory tasks were determined using chlorisondamine, a compound which produces central nicotinic cholinergic receptor blockade that lasts for several weeks after a single icv administration. Chlorisondamine treatment did not affect the acquisition of spatial information in the Morris water maze or in the radial arm maze, tasks in which performance is reportedly disrupted by acute administration of the nicotinic antagonist, mecamylamine. Chlorisondamine also did not affect performance in the inhibitory avoidance task and did not alter the memory enhancement found in this task after post-training administration of nicotine. Mecamylamine, however, completely blocked the memory-enhancing effects of nicotine. In contrast to the differential ability to chlorisondamine and mecamylamine to block nicotine's memory-enhancing effects, these antagonists produced comparable blockade of nicotine's effects on open field behavior. It is unlikely that the different effects of systemically administered mecamylamine and centrally administered chlorisondamine on nicotine-induced memory enhancement are due to mecamylamine's peripheral effects, since hexamethonium, a peripherally active nicotinic antagonist, did not block nicotine-induced memory enhancement. The different pattern of effects of mecamylamine and chlorisondamine may be related to compensatory mechanisms being selectively induced by chronic blockade produced by chlorisondamine and not by acute blockade produced by mecamylamine. Alternatively, different effects of these two nicotinic cholinergic antagonists on the performance of learning and memory tasks might be related to selective actions of these compounds at nicotinic receptor subtypes or at nonnicotinic receptors.

Differential effects of scopolamine and mecamylamine on working and reference memory in the rat

The effects of the muscarinic antagonist scopolamine (0.1-0.6 mg/kg, IP) and the nicotinic antagonist mecamylamine (1-10 mg/kg) were compared in T-maze alternation and discrimination tasks in the rat. Scopolamine dose dependently disrupted performance on the alternation task and potentiated the increase in errors made in controls when the delay between forced and choice runs was increased from 0 to 30 s. Mecamylamine disrupted performance at the 10-mg/kg dose only and dose dependently inhibited the increase in errors made in controls when the delay between forced and choice runs was increased to 30 s. In simple T-maze discrimination, only the 0.6-mg/kg dose of scopolamine disrupted performance of the task, while mecamylamine at both 5 and 10 mg/kg disrupted task performance. These results confirm that working memory tasks are more sensitive to central muscarinic blockade than reference memory tasks. They also demonstrate that in delay conditions working memory performance is enhanced following central nicotinic blockade while reference memory performance is disrupted. This suggests that centrally active muscarinic and nicotinic antagonists have dissociable effects on memory processes in the rat.

Blockade of hippocampal nicotinic receptors impairs working memory but not reference memory in rats

In a three-panel runway task, intrahippocampal injection of the nicotinic receptor antagonist, mecamylamine (10 and 18 micrograms/side), significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in a test of working memory. This increase in errors also occurred after rats were given IP mecamylamine (10 mg/kg). Mecamylamine did not affect the number of errors in a test of reference memory whether it was given at doses up to 18 micrograms/side intrahippocampally or up to 10 mg/kg IP. These results suggest that mechanisms mediated by hippocampal nicotinic receptors play a role in working memory but not in reference memory.

The neuropharmacological and neurochemical basis of place learning in the Morris water maze

The Morris water maze (MWM) offers several advantages over other methods of studying the neurochemical basis of learning and memory, particularly with respect to its ability to dissociate deficits in memory formation from deficits in sensory, motor, motivational and retrieval processes. The contributions of nearly all of the major neurotransmitter systems have been investigated and consistent patterns have emerged. Normal function in glutamatergic and cholinergic systems is necessary for spatial learning, as blockade of NMDA receptors and cholinergic hypofunction prevents spatial learning but does not impair recall. Peptides such as adrenal and sex hormones and somatostatin may also be necessary for spatial learning. In contrast, activity in either GABAergic or opioidergic systems impairs spatial learning, though by quite different means. GABAergic activity prevents memory function, whereas opioidergic activity reduces motivation. Normal monoaminergic activity is necessary for normal performance in the MWM, but not for spatial learning per se. However, noradrenergic and serotonergic systems may enhance cholinergic-mediated mnemonic processes. Further research into the relative contributions of different receptor subtypes as well as interactions between neurochemical systems should provide significant advances in our understanding of the neural basis of learning and memory in mammals.

Nicotinic systems and cognitive function

Nicotinic acetylcholine receptors have been found to be important for maintaining optimal performance on a variety of cognitive tasks. In humans, nicotine-induced improvement of rapid information processing is particularly well documented. In experimental animals nicotine has been found to improve learning and memory on a variety of tasks, while the nicotinic antagonist mecamylamine has been found to impair memory performance. Nicotine has been found to be effective in attenuating memory deficits resulting from lesions of the septohippocampal pathway or aging in experimental animals. Nicotinic receptors are decreased in the cortex of patients with Alzheimer's disease. Preliminary studies have found that some aspects of the cognitive deficit in Alzheimer's disease can be attenuated by nicotine. Nicotine may prove to be useful therapeutic treatment for this and other types of dementia.