Effects of concurrent manipulations of nicotinic and muscarinic receptors on spatial and passive avoidance learning

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 effects of tacrine and zacopride on the performance of adult rats in the working memory task

1. The present study investigated the effects of tacrine (an inhibitor of acetylcholinesterase) and zacopride (the antagonist of 5-HT3 receptors) on the performance of adult rats in a continuous operant delayed non-matching to position task assessing spatial working memory. 2. Adult rats had a decline in the percent correct responses at the longest delays (16 and 30 sec) in this task. Tacrine (1.0 mg/kg) or zacopride (0.0025, 0.05, 1.0 mg/kg) did not increase the percent correct responses at any time delays. The higher dose of tacrine reduced behavioural activity (e.g. the decreased number of trials completed and increased sample press latency) of rats during memory testing, and it slightly increased choice accuracy across all the delays. 3. The combination of zacopride (1.0 mg/kg) and tacrine (1.0 mg/kg) increased the percent correct responses at the shortest delays, but not at the longest delays. 4. These results indicate a non-mnemonic improvement in the accuracy performance of rats, and they suggest that the effects of acute, systemic administrations of zacopride (which is thought to increase the release of acetylcholine) or/and tacrine (which inhibits the breakdown of acetylcholine) do not improve spatial working/short-term memory in rats.

Improving effect of acetylcholine receptor agonists on a deficit of 2-deoxyglucose uptake in cerebral cortical and hippocampal slices in aged and AF64A-treated rats

The aim of the present study was to determine whether the facilitation of 2-deoxyglucose (2-DG) uptake in the cerebral and hippocampal slices by nicotinic and muscarinic receptor agonists is compromised in the aged rat brain. For this, the effects of the nicotinic receptor agonist nicotine, the muscarinic receptor agonists oxotremorine and McN-A-343, and the ACh esterase inhibitors physostigmine and NK247 on 2-DG uptake in the brain slices of young (2-month-old) and aged (24-26-month-old) rats were tested. The decrements of 2-DG uptake in the cortical slices of aged rats were significantly attenuated by treatment with oxotremorine, nicotine and amiridine. In contrast, the metabolic responsivity of hippocampal slices to these drugs was reduced. To assess whether age-related changes in 2-DG uptake may be due to deficits in cholinergic function, we tested these drugs on the decrements of 2-DG uptake in ethylcholine aziridinium (a neurotoxic analog of choline) injected rats. The reductions of 2-DG uptake by injection of ethylcholine aziridinium was attenuated by oxotremorine but not by physostigmine. The present results reveal that metabolic decrements in the cerebral cortex from aged or ethylcholine aziridinium-injected rats were attenuated by muscarinic and nicotinic receptor agonists, suggesting that the muscarinic and nicotinic receptor mechanism in the cerebral cortex may be involved in cholinergic drug-induced functional recovery in aged rats.

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.

Effects of an inhibitor of GABA-aminotransferase (gamma-vinyl-GABA) on the spatial navigation deficit induced by nicotinic blockade

1. The present study investigated whether stimulation of the GABA-ergic system affects spatial navigation (water-maze, WM) deficit induced by nicotinic blockade (mecamylamine). 2. The effects of various doses of gamma-vinyl-GABA (GVG: 50, 150 and 300 mg/kg) and mecamylamine (2.5 and 10 mg/kg) were examined alone and in combination. 3. GVG at the dose 150 mg/kg alone did not impair the performance of rats in the WM task. 4. Mecamylamine at the dose 2.5 and 10 mg/kg clearly impaired the performance of rats in WM task. 5. When the two drugs were co-administered, no interaction between mecamylamine and GVG was observed. 6. Combined nicotinic and muscarinic blockade did not interact as well with GVG administration. 7. Our results do not provide support for any interaction between cholinergic and GABA-ergic mechanisms.

D-cycloserine, a modulator of the N-methyl-D-aspartate receptor, improves spatial learning in rats treated with muscarinic antagonist

The results of the present study indicate that D-cycloserine, a partial agonist at the glycine binding site, which is a positive modulation site of the N-methyl-D-aspartate receptor, could improve dose dependently the acquisition of a water maze task in rats treated with scopolamine, a muscarinic antagonist. The low to moderate doses of D-cycloserine which improved learning did not affect swimming speed in scopolamine-treated rats. The higher dose did not improve learning, but it increased swimming speed in scopolamine-treated rats. These results show a dissociation between impaired acquisition and abnormal behavioral activity in scopolamine-treated rats in the water maze task. Furthermore, the present results suggest that D-cycloserine can act as a cognitive enhancer at the appropriate doses.

The effects of p-chlorophenylalanine-induced serotonin synthesis inhibition and muscarinic blockade on the performance of rats in a 5-choice serial reaction time task

The effects of serotonergic dysfunction induced by treatment with p-chlorophenylalanine (PCPA), an inhibitor of serotonin synthesis, and cholinergic dysfunction induced by scopolamine on the performance of adult rats in the 5-choice serial reaction time task measuring selective attention were studied. Food-deprived rats were trained to detect and respond to brief flashes of light presented randomly in one of five locations, until they reached a stable level of performance (about 4 months). Scopolamine 0.2 mg/kg produced a marked variation in the performance but did not, however, induce any consistent impairment in the discriminative accuracy. Other doses of scopolamine (0.05 and 0.1 mg/kg) or N-methyl-scopolamine 0.2 mg/kg, a peripheral muscarinic receptor antagonist, did not affect discriminative accuracy. Furthermore, scopolamine as well as N-methyl-scopolamine produced a number of other performance deficits, such as significantly decreased overall probability of responding and significantly increased response latencies. PCPA treatment induced an almost total depletion (> 99%) of frontal cortical serotonin and its major metabolite 5-HIAA and reduced the frontal cortical concentrations of noradrenaline (-30%) and dopamine (-42%). During baseline testing conditions, there was a trend for the discriminative accuracy to be decreased by PCPA, although this effect failed to reach significance (P = 0.07). Presenting the stimuli at unpredictable intervals or reducing the intensity of the visual stimulus impaired discriminative accuracy in both PCPA-treated and control rats. The decrease in discriminative accuracy induced by PCPA reached statistical significance when the stimuli were presented faster than normally or the intensity of the visual stimulus was reduced. PCPA treatment did not make the rats more susceptible to the effects of scopolamine on discriminative accuracy. However, PCPA treatment also induced a number of other performance deficits, resulting in a decreased overall tendency to respond. In summary, there is a statistically non-significant trend for the discriminative accuracy to be decreased by PCPA treatment under normal testing conditions, and as the discrimination task is made more difficult (stimulus intensity reduction, presentation of the stimuli at faster than normal rates), the deficit in discriminative accuracy produced by PCPA treatment is revealed. The results suggest a role for brain serotonin in the general organization of behavior.

Effects of an inhibitor of GABA-aminotransferase (gamma-vinyl-GABA) on the spatial navigation deficit induced by muscarinic blockade

The present study investigated whether stimulation of the GABAergic system affects spatial navigation [water-maze (WM)] deficit induced by muscarinic blockade (scopolamine). The effects of various doses of gamma-vinyl-GABA (GVG) (50, 150, and 300 mg/kg) and scopolamine (0.4 and 0.1 mg/kg) were examined alone and in combination. GVG at 50 and 150 mg/kg alone did not impair the performance of rats in the WM yask. At 300 mg/kg, GVG caused slight impairment, increasing latency and total distance swim during training trials. Scopolamine at 0.4 mg/kg clearly impaired the performance of rats in the WM task. When the two drugs were coadministered, no interaction between scopolamine and GVG was observed. Our results do not provide support for any interaction between cholinergic muscarinic and GABAergic mechanisms.

Visualization of cholinoceptive neurons in the rat neocortex: colocalization of muscarinic and nicotinic acetylcholine receptors

The present investigation analyzes the cellular distribution of muscarinic and nicotinic acetylcholine receptors in rat neocortex, by use of monoclonal antibodies raised against purified receptor proteins. The degree of colocalization of both types of receptors was determined by way of immunofluorescent double-labeling techniques. For both classes of receptors, pyramidal and nonpyramidal cells were found immunostained and an identical laminar distribution pattern of immunopositive neurons in the rat neocortex became apparent. A striking similarity in distribution of the two cholinergic receptor types was found in the frontal/motor and parietal cortex. Accordingly, we observed a high degree of colocalization of muscarinic and nicotinic acetylcholine receptors within immunopositive cortical neurons. Approximately 90% of the cholinoceptive neurons expressed both types of receptors. The current data demonstrate that (i) the distribution of muscarinic and nicotinic cholinoceptive neurons in the neocortex is present in identical laminar patterns and represent the same type of cells, (ii) both classes of cholinergic receptors are highly colocalized within cholinoceptive neurons, which points at individual neurons as a likely site of interaction between muscarinic and nicotinic acetylcholine receptor-mediated processes.

Effects of combined methysergide and mecamylamine/scopolamine treatment on spatial navigation

In the present study, we investigated the effects of a 5-HT2 receptor antagonist, methysergide (2.5, 7.5 and 20 mg/kg), on spatial learning in saline, mecamylamine (10 mg/kg) and scopolamine (0.8 mg/kg) treated rats. Methysergide had no effect on water-maze (WM) spatial learning in rats subjected to saline or mecamylamine pretreatments. However, scopolamine-induced WM learning deficit was augmented by methysergide at doses of 7.5 and 20 mg/kg. These results further suggest (A) that cholinergic and serotonergic systems may interact in the regulation of spatial learning, and (B) that the cholinergic component of this interaction with serotonin2 receptors is mediated by muscarinic receptors, but not by nicotinic receptors.

Effects of concurrent nicotinic antagonist and PCPA treatments on spatial and passive avoidance learning

The present study investigates the effects of concurrent manipulations of nicotinic cholinergic receptors (mecamylamine 10 mg/kg, i.p.) and serotonin neurons (PCPA, 400 mg on each of 4 days) on spatial navigation (water maze, WM) and passive avoidance (PA) performance. PCPA treatment had no effect on WM navigation or PA performance of intact rats, but greatly aggravated mecamylamine induced performance deficit. Either single or combined treatments with hexamethonium (5.0 mg/kg, s.c.) and PCPA had no effect on WM or PA performance. These findings may suggest that nicotinic cholinergic receptors are also importantly involved in the cholinergic-serotonergic regulation of cognitive functioning.

Effects of nicotine on spatial memory deficits in rats with septal lesions

Impaired septohippocampal function has been implicated in the memory deficits associated with Alzheimer's disease (AD), and septal lesions have been used to model the cognitive deficits associated with AD. In this study, we assessed the effects of systemic administration of nicotine on lesion-induced deficits in the acquisition of a spatial discrimination version of the Morris water maze. Rats with radiofrequency lesions of the medial septum were required to learn which of two visible platforms in a pool of water provided a means of escape. On each of the first 4 days of training, the rats received an injection of (-)nicotine (0, 0.1 or 0.3 mg/kg, i.p.) before training. Nicotine markedly improved the performance of septal rats. This enhanced performance was maintained in rats subsequently tested 1 and 15 days later without additional drug treatment. Septal rats initially trained under nicotine were impaired, however, when the platform locations were reversed and training was conducted under saline. Our findings suggest that nicotinic receptor stimulation might be useful in the treatment of cognitive deficits.

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

The effects of both systemic and intracerebroventricular administration of mecamylamine, a nicotinic antagonist, were tested on the Morris water maze performance of rats. In experiment 1, mecamylamine (0, 3, and 10 mg/kg, IP) was administered before daily training sessions on the Morris water maze, a task in which rats use environmental cues to learn the location of an invisible escape platform in a large pool of water. The escape latencies of rats given the higher dose of mecamylamine were significantly longer than the latencies of rats given either saline or the peripherally-acting nicotinic antagonist hexamethonium (10 mg/kg). Analysis of search patterns during a free swim trial conducted in the absence of an escape platform confirmed the disruptive effects of the higher dose of mecamylamine. Similar drug effects were not observed when these rats were trained to a visible platform, and mecamylamine did not affect the retrieval of spatial information in well-trained rats. In experiment 2, similar effects were observed with ICV administration of mecamylamine (0, 10, 30, and 100 micrograms). The two higher doses increased escape latencies during the last day of place training and all three doses significantly impaired performance on a free swim. No significant effects were noted on subsequent training to a visible platform, and only the highest dose marginally impaired the retrieval of spatial information in well-trained animals. Thus, mecamylamine appears to impair the acquisition of spatial information in the Morris water maze but does not affect retrieval of previously acquired spatial information at comparable doses.

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.

Tetrahydroaminoacridine improves passive avoidance retention defects induced by aging and medial septal lesion but not by fimbria-fornix lesion

The present study examines whether tetrahydroaminoacridine (THA) can improve the deterioration in passive avoidance (PA) retention performance induced by medial septal (MS) and fimbria-fornix (FF) lesions in young rats or by aging. Retention of young MS-lesioned rats was improved by pretraining injection of THA at 3 mg/kg, but not by THA at 1 mg/kg or by either of the posttraining doses of THA (1 and 3 mg/kg). Pretraining injections of THA at 1 or 3 mg/kg had no effect on the PA retention performance of FF-lesioned rats. Age-induced PA failure was alleviated by pretraining administration of THA at 1 and 3 mg/kg. Posttraining injections of THA (1 or 3 mg/kg) had no effect on PA retention performance of aged rats. These results demonstrate that 1) THA may improve hippocampal cholinergic denervation-induced functional deficits and 2) some of the age-related PA deficits may be due to a cholinergic deficit and can be reversed with THA.

Comparison of the effects of acute and chronic ibotenic and quisqualic acid nucleus basalis lesioning

The present study examines the effects of acute (1 month recovery) and chronic (8 month recovery) bilateral quisqualic (quis) and ibotenic (ibo) acid nucleus basalis (NB) lesioning on the activity of cholinergic neurons and on passive avoidance (PA) and water-maze (WM) performance. Our data demonstrate that A: The activity of choline acetyltransferase (ChAT) in cortical tissue and the number of ChAT positive neurons in the NB were decreased 1 and 8 months after quis or ibo NB lesioning. B: Ibo NB lesioning produced a greater nonspecific subcortical cell loss than quis NB lesioning. C: PA retention was impaired by acute and chronic quis and ibo NB lesioning. D: Acute ibo NB lesioning impaired acquisition and reversal learning in WM performance whereas chronic ibo NB lesioning impaired only reversal WM learning. Acute and chronic quis NB lesioning impaired reversal WM learning. The present results suggest that NB cholinergic neurons do not recover spontaneously from excitotoxin-induced damage and that they may be importantly involved in inhibitory avoidance and spatial reversal learning performance.

Joint modulation of neocortical electrical activity by nicotinic and muscarinic receptors

A combination of subthreshold doses of nicotinic and muscarinic antagonist induced an increase in all the spectral components. The spectral amplitude change induced by a large scopolamine dose could not be augmented by either mecamylamine or an additional injection of scopolamine. NB lesions increased frontal slow wave activity (delta and theta amplitudes). Single or combined injections of scopolamine and mecamylamine produced a significantly smaller change in the frontal EEG amplitude values of NB-lesioned rats than in controls.

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.

Effects of THA on passive avoidance and spatial performance in quisqualic acid nucleus basalis-lesioned rats

Bilateral quisqualic acid nucleus basalis (NB) lesions impaired passive avoidance (PA) retention. NB lesions did not impair acquisition performance (stable platform location) in the water maze (WM). However, NB-lesioned rats were impaired in learning the new location of the escape platform in WM. Pretraining injections of tacridine (an anticholinesterase, THA) at 3 mg/kg, but not at 1 mg/kg, slightly improved PA retention performance in NB-lesioned rats. THA (1 or 3 mg/kg) did not alleviate NB lesion-induced WM defect. The results further suggest that loss of NB neurons impair PA acquisition and relearning of the new platform location in WM, and that cholinergic neuron loss may be at least partially involved in the NB lesion-induced performance defect.

Comparison of quisqualic and ibotenic acid nucleus basalis magnocellularis lesions on water-maze and passive avoidance performance

The present study compares water-maze (WM) (reference and working memory) and passive avoidance (PA) (acquisition and retention) deficits induced by ibotenic (ibo) and quisqualic (quis) acid nucleus basalis magnocellularis (NBM) lesions. Ibo lesions produced a large subcortical cell loss and a decrease in frontal cortex (FR) choline acetyltransferase (ChAT) activity. Ibo lesions impaired WM acquisition and PA acquisition and retention performance. Quis NBM lesions were restricted to the ventromedial pallidum, but ChAT activity was decreased in FR. Quis NBM lesions impaired PA acquisition and retention, but had no effect on the reference or working memory WM performance.