Nicotine impairs acquisition of radial maze performance in rats

Molecular Insights Into Memory-Enhancing Metabolites of Nicotine in Brain: A Systematic Review

Background: The alleged procognitive effects of nicotine and its metabolites in brain are controversial. Objective: Here, we review the pharmacologically active metabolites of nicotine in brain and their effects on neuronal mechanisms involving two main cognitive domains, i.e., learning and memory. Methods: We searched Embase, Medline via PubMed, Scopus, and Web of Science databases for entries no later than May 2018, and restricted the search to articles about nicotine metabolites and cognitive behavior or cognitive mechanisms. Results: The initial search yielded 425 articles, of which 17 were eligible for inclusion after application of exclusion criteria. Of these, 13 were experimental, two were clinical, and two were conference papers. Conclusions: The results revealed three pharmacologically active biotransformations of nicotine in the brain, including cotinine, norcotinine, and nornicotine, among which cotinine and nornicotine both had a procognitive impact without adverse effects. The observed effect was significant only for cotinine.

Chronic nicotine administration does not alter cognitive or mood associated behavioural parameters

Nicotine, the major specific alkaloid in tobacco smoke, exhibits widespread pharmacological effects and may contribute to deterioration in behaviour. The present study thus examined the effects of its chronic administration on some cognitive and mood associated behaviours. Adult rats weighing between 150 and 200g were randomly divided into 4 groups each of 5 females and 5 males. Three groups were administered graded doses of nicotine at 0.25, 2 and 4mg/kg body weight via subcutaneous injections. One group served as control and received normal saline (vehicle for nicotine). Behavioural tests were performed using the Y-maze, elevated-plus maze (EPM) and tail suspension tests (TST) at various time points. Nicotine produced no significant effect in spontaneous alternation on Y-maze, nor on six parameters scored on EPM (open arm entries, time spent in open arms, time per open arm entries, open/closed arm quotient, closed arm entries, and total arm entries), and also no significant effect on immobility time in TST. This lack of effects was observed to be independent of sex and dose administered. The study shows that nicotine does not produce long-term changes in some cognitive and mood associated behaviours, thus suggesting it could be well tolerated even following chronic administration.

Chronic nicotine improves cognitive performance in a test of attention but does not attenuate cognitive disruption induced by repeated phencyclidine administration

RATIONALE

Nicotine-induced cognitive enhancement may be a factor maintaining tobacco smoking, particularly in psychiatric populations suffering from cognitive deficits. Schizophrenia patients exhibit higher smoking rates compared with the general population, suggesting that attempts to self-medicate cognitive schizophrenia deficits may underlie these high smoking levels.

OBJECTIVES

The present study explored pro-cognitive effects of nicotine in a model of schizophrenia-like cognitive dysfunction to test this self-medication hypothesis.

MATERIALS AND METHODS

We investigated whether chronic nicotine (3.16 mg/kg/day, base) would attenuate the performance disruption in the five-choice serial reaction time task (5-CSRTT, a task assessing various cognitive modalities, including attention) induced by repeated administration of phencyclidine (PCP), an N-methyl-D-aspartate receptor antagonist that induces cognitive deficits relevant to schizophrenia.

RESULTS

Chronic nicotine administration shortened 5-CSRTT response latencies under baseline conditions. Nicotine-treated rats also made more correct responses and fewer omissions than vehicle-treated rats. Replicating previous studies, repeated PCP administration (2 mg/kg, 30 min before behavioral testing for two consecutive days followed 2 weeks later by five consecutive days of PCP administration) decreased accuracy and increased response latencies, premature responding, and timeout responding. Chronic nicotine did not attenuate these PCP-induced disruptions.

CONCLUSIONS

Chronic nicotine had pro-cognitive effects by itself, supporting the hypothesis that cognitive enhancement may contribute to tobacco smoking. At the doses of nicotine and PCP used, however, no support was found for the hypothesis that the beneficial effects of nicotine on cognitive deficits induced by repeated PCP administration, assessed in the 5-CSRTT, are larger than nicotine effects in the absence of PCP.

Post-training intrahippocampal infusion of nicotine prevents spatial memory retention deficits induced by the cyclo-oxygenase-2-specific inhibitor celecoxib in rats

Recently, we demonstrated that intrahippocampal infusion of the cyclo-oxygenase (COX)-2-specific inhibitor celecoxib impaired spatial memory retention in the Morris water maze. In the present work, we investigated the effects of nicotine, infused in the rat dorsal hippocampus several minutes after infusion of celecoxib, on memory retention in the Morris water maze. Rats were trained for 3 days; each day included two blocks, and each block contained four trials. Test trials were conducted 48 h after surgery. As expected, bilateral intrahippocampal infusion of celecoxib (19 microg/side; 0.1 m) increased escape latency and travel distance in rats, indicating significant impairment of spatial memory retention. We also examined the effects of bilateral infusion of nicotine (0.5, 1.0 and 2.0 microg/side) on memory retention. Infusion of 1 microg nicotine significantly decreased escape latency and travel distance but not swimming speed, compared with controls, suggesting memory retention enhancement by nicotine at this concentration. In separate experiments, bilateral infusion of nicotine, infused 5 min after 0.1 m (19 microg/side) celecoxib infusion, was associated with escape latency, travel distance and swimming speed profiles very similar to those in control animals. Brain tissue sections from several of these animals were subjected to immunohistochemical staining analysis with anti-COX-2 antibodies. Quantification analysis by optical density measurements showed that the celecoxib infusion reduced the immunoreactivity of COX-2-containing neurons in the CA1 area of the hippocampus compared with controls, although this reduction was not significant. However, infusion of a combination of celecoxib and nicotine significantly increased this immunoreactivity compared with levels in control and celecoxib-infused groups. These results suggest that nicotine prevented or reversed the adverse effects of celecoxib on spatial memory retention and protected or restored the immunostaining pattern of COX-2 neurons in the rat dorsal hippocampus.

Nicotine improves learning and memory in rats: morphological evidence for acetylcholine involvement

It has been suggested that nicotine improves rapid information processing (learning and memory) tasks. However, it is not clear which aspects of cognition actually underlie these improvements because relatively less attention has been given to nicotinic cholinergic systems compared to muscarinic systems. The authors therefore studied the effects of nicotine on the learning and memory performance by a step-through passive avoidance task. Nicotine (0.4 mg/kg) was administered s.c. single dose (acute group), once a day for 3 days (subchronic group) or 21 days (chronic group). Nicotine treated and control rats were trained in one trial learning step-through passive avoidance task, where retention latencies were carried out 1 h, 24 h, and 3 days after learning trial. Treatment with nicotine before training session prolonged the latencies significantly (p < .01). Control group, acute, subacute and chronic nicotine treatment groups showed latencies 4.75 +/- 0.6, 69.4 +/- 14, 116.2 +/- 30, and 118.5 +/- 23 s, respectively. In addition, to prove the actual contribution of nicotinic cholinergic system in improvement of learning and memory processing, histological methods that permit the visualization and quantification of ACh levels were used. Electron microscopic evaluation revealed increased numbers of Ach-containing vesicles especially in hippocampus in chronic nicotine-treated rats; although frontal and temporal cortex in addition to hippocampus showed increment in Ach vesicles in a lesser extent in all nicotine treatment groups. These results indicate that long-term nicotine treatment can be important for improving cognitive function in regard to increased cholinergic activity.

Nicotine as a modulator of behavior: beyond the inverted U

Nicotine is the crucial component in tobacco that underlies smoking behavior; however, the effects of nicotine can vary in both human and animal studies. Recent data from knockout mouse studies, neurotransmitter release studies and electrophysiological experiments support the hypothesis that conflicting behavioral effects elicited by nicotine can result from the activation of different subtypes of nicotinic acetylcholine receptors and the stimulation of antagonistic neuronal pathways. Thus, small differences in the activation state, connectivity or sensitivity of neuronal pathways among individuals might result in large differences in behavioral responses to nicotine. An understanding of the molecular and cellular processes that oppose nicotine reinforcement will be crucial for the development of new interventions to initiate smoking cessation or to prevent the transition from occasional smoking to dependence.

Mecamylamine blocks enhancement of reference memory but not working memory produced by post-training injection of nicotine in rats tested on the radial arm maze

The focus of this study was to analyze whether the psychostimulant nicotine would enhance reference and working memory consolidation in rats tested on the 8-arm radial arm maze. Mecamylamine, a nicotine antagonist, was used to attempt to block the enhancement of memory consolidation. All rats were given one training trial/day for 12 consecutive days, and 4 arms were baited. Rats were separated into five groups: the saline-nicotine group received an intraperitoneal (i.p.) injection of saline immediately after each trial followed 15 min later by an subcutaneous (s.c.) injection of nicotine (0.6 mg/kg free base); the nicotine-delay group received an s.c. injection of nicotine 2 h after each training trial, two groups received an i. p. injection of one of two different doses of mecamylamine (2.5 and 6.0 mg/kg) immediately after each trial, which was followed 15 min later by an s.c. nicotine injection, and a control group received an i.p. injection of saline immediately and 15 min after each training trial. Results showed that the saline-nicotine group made fewer reference and working memory errors than the saline- or nicotine-delay groups, but only the effect of nicotine on reference memory was blocked by the higher dose of mecamylamine. It appears from these results that nicotine's effects on reference and working memory may be mediated through different mechanisms.

Anxiogenic effects of nicotine in the dorsal hippocampus are mediated by 5-HT1A and not by muscarinic M1 receptors

After direct administration into the dorsal hippocampus nicotine decreased the time spent in social interaction, without changing locomotor activity, indicating an anxiogenic effect. The possibility that post-synaptic M1 muscarinic receptors mediated this effect was examined by determining whether dorsal hippocampal administration of a specific M1 receptor agonist (McN-A-343) had anxiogenic effects, and whether the anxiogenic effect of nicotine could be reversed by co-administration of the M1 receptor antagonist, pirenzepine. McN-A-343 (0.3, 1.6, 3.2, 15.8 nmol) was without effect on social interaction, and pirenzepine (0.7 and 2.4 nmol) injection into the dorsal hippocampus failed to reverse the decrease in social interaction caused by nicotine (6.3 nmol) injection into this area. However, the decrease in social interaction after nicotine (50 nmol) was completely reversed by the specific 5-HT1A receptor antagonist, WAY 100635 (0.4 nmol) after co-administration of both drugs into the dorsal hippocampus. Thus, the anxiogenic effect of nicotine in this brain region seems to be mediated by 5-HT1A, but not M1, receptors. In contrast to the effect of nicotine in naive animals, those retested after a second injection of 50 nmol did not show a significant anxiogenic effect. The theoretical implications of this are discussed and from a practical point of view this suggests caution in the retesting of animals after central injections.

Nicotine enhancement of contextual fear conditioning

Nicotine has been suggested to have cognitive enhancing effects. The present study examined the effects of nicotine and the nicotinic antagonist mecamylamine on contextual fear conditioning in C57BL/6 mice. The fear conditioning task was chosen because the task examines two types of learning: contextual learning, and conditioned stimulus (CS)-unconditioned stimulus (US) learning. Multiple doses of nicotine were tested and 0.5 mg/kg nicotine, given on both training and testing days, improved contextual learning but had no effect on formation of an auditory CS-US association. No effect was found at lower doses or when nicotine was given on training day only, or testing day only. The nicotinic receptor antagonist mecamylamine (1 and 2 mg/kg) did not alter contextual fear conditioning but mecamylamine did prevent the nicotine-associated increase in contextual learning. A higher dose of nicotine (1 mg/kg, training day only) interfered with contextual conditioning when the context was paired with both the CS and US, but had no effect on the auditory CS-US association. This effect of 1 mg/kg nicotine on contextual learning disappeared when mice were trained without the CS. The present results indicate that nicotine enhancement of contextual fear conditioning is dose-dependent, but the presence of nicotine is required both during training and testing.

Counteractive effects of a partial (sabcomeline) and a full (RS86) muscarinic receptor agonist on deficits in radial maze performance induced by S-AMPA lesions of the basal forebrain and medial septal area

After S-AMPA (8.0 mM) lesions to the nucleus basalis and medial septal regions, at the source of the cortical and hippocampal branches of the forebrain cholinergic projection system, rats displayed long-lasting and relatively stable impairment in long-term reference and short-term working memory in both spatial (place) and associative (cue) radial maze tasks. Treatment with four doses of the partial agonist at the M1 cholinergic muscarinic receptor, sabcomeline (formerly known as SB 202026: 0.01-0.156 mg/kg), substantially reduced working and reference memory errors in both tasks in lesioned rats, in a mainly dose-related manner. These effects were more consistent than those found with the direct muscarinic agonist RS86 (0.05-0.781 mg/kg). The performance of non-lesioned controls was largely unaffected by either treatment. These findings are consistent with previous evidence for cholinergic participation in the radial maze deficits induced by excitotoxic lesions to the forebrain cholinergic projection system. They show that with a relatively selective lesion, which respectively, reduced choline acetyltransferase activity to 36.5 and 22.5% of control level in frontal and dorsolateral cortex, and to 61.8 and 69.2% of control level in dorsal and ventral hippocampus, lesioned rats were responsive to pharmacological treatments aimed to enhance cholinergic function by full or partial agonist activity at M1 receptors. Findings that nicotine (0.1 mg/kg) also reduced radial maze errors in the lesioned animals supports the suggestion that lesion-induced deficits in radial maze performance were amenable to improvement by cholinergic receptor manipulation. However, given the potential adverse side effects of full receptor agonists, which nonselectively target cholinergic receptors throughout the organism, the functional efficacy of sabcomeline, which shows regional selectivity for the central M1 receptor subtypes, suggests that deleterious effects of cholinergic depletion on cognition can be counteracted without incurring the risk of unwanted side effects.

Chronic nicotinic agonist and antagonist effects on T-maze alternation

A variety of studies have found that nicotine improves working memory function. However, other studies have either not found improvements or have found nicotine-induced deficits. The demands of the particular memory test may be critical for the expression of the nicotine effects. In several studies, we have found that chronic nicotine administration improves working memory performance in the radial arm maze. Chronic mecamylamine coadministration reversed this effect. The current study was conducted to determine the effects of chronic nicotine and mecamylamine on choice accuracy in a T-maze spatial alternation task. The same dose and duration of nicotine administration that we have previously found to significantly improve choice accuracy in the radial-arm maze was not effective in altering T-maze spatial alternation. The critical difference in task demands may be the presence with T-maze alternation of proactive interference. During a session, a choice alternative repeatedly changes valence from correct to incorrect and back again. In contrast, with the radial-arm maze as run in our studies, in a session the valence of an arm only changes once from correct to incorrect. Previous work with nicotine effects on spatial alternation in an operant task found evidence that nicotine increased the negative effect of proactive interference on performance. In the current study, chronic mecamylamine caused a significant deficit in T-maze spatial alternation. This same dose did not produce a deficit in the radial-arm maze and, in fact, caused an improvement during the first week of administration.

Neural grafting of cholinergic neurons in the hippocampal formation

The cholinergic septohippocampal system plays an important role in spatial learning and memory functions. Transections of the septohippocampal pathway have been shown to result in a near complete loss of cholinergic innervation in the hippocampus and induce severe spatial memory impairments. In this article, we have reviewed the studies which demonstrate the ability of intrahippocampal septal grafts to reinnervate the hippocampal formation and ameliorate spatial learning and memory deficits. Neuroanatomical studies suggest that grafts of cholinergic tissue can innervate the host hippocampal formation in a pattern that mimics that of the normal septohippocampal pathway. This innervation, in turn, is associated with the formation of graft-to-host synaptic connections. Neurochemical studies reveal that intrahippocampal grafts of septal cells can restore choline acetyltransferase activity, acetylcholine synthesis, and high affinity choline uptake in presynaptic terminals of grafted neurons. In addition, these grafts can normalize the upregulation of cholinergic muscarinic receptors seen postsynaptically in the hippocampus following lesions of the septohippocampal pathway. The functional nature of these grafts is also substantiated by electrophysiological recordings which demonstrate stimulus-evoked graft-to-host synaptic transmission as well as the reinstatement of EEG activity typical of septohippocampal connectivity. In addition to graft-to-host connections, behavioral and neurochemical studies also provide evidence for host-to-graft connections that can regulate the activity of grafted cholinergic neurons during the performance of specific behavioral tasks requiring spatial memory function. Together, these studies suggest that grafts of cholinergic neurons from the medial septal nucleus can become anatomically and functionally incorporated into the circuitry of the host hippocampal formation.

Effects of stimulation or blockade of central nicotinic-cholinergic receptors on performance of a novel version of the rat stimulus discrimination task

This study evaluated the effects of two central nicotinic-cholinergic receptor agonists and an antagonist on performance accuracy of a rat, delayed stimulus discrimination task (DSDT). Rats were trained to discriminate between an auditory and visual stimulus by pressing a right or left lever. To diminish the rat's ability to use mediating spatial strategies to solve the task, computer automated, retractable doors separated the animal from the levers during delay intervals, thus reducing positioning at the lever. After stable baselines were achieved, rats were grouped and administered placebo (saline) and nicotine, lobeline or mecamylamine in a randomized dose series. Each group received two complete series of the selected compound on different occasions. Mecamylamine impaired DSDT accuracy in a dose-dependent manner while optimal doses of nicotine and lobeline significantly improved accuracy. Nicotine differed from lobeline in regard to its interaction with a dose of mecamylamine (1.0 mg/kg) that had not impaired DSDT accuracy. Combined administration of lobeline and mecamylamine was followed by a significantly increased level of DSDT accuracy that was similar to the improvement following administration of lobeline alone. In contrast, combined administration of nicotine and mecamylamine did not result in increased DSDT accuracy. Furthermore, lobeline administration similarly improved accuracy of trials associated with both the light and the tone, while nicotine improved accuracy of trials associated with the light to a much greater degree. These data suggest that the increases in DSDT accuracy associated with lobeline may be expressed through non-nicotinic mechanisms or a nicotinic receptor which is not blocked by mecamylamine.

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

The selective 5-HT3 receptor antagonist, WAY100289, enhances spatial memory in rats with ibotenate lesions of the forebrain cholinergic projection system

The effects of three doses (0.003, 0.03 and 1.0 mg/kg sc) of the 5-HT3 receptor antagonist, WAY 100289, on spatial learning and memory in the water maze were examined in rats before and after ibotenate lesions to the nucleus basalis and medial septal brain regions at the source of cholinergic projections to cortex and hippocampus. The representative cholinergic nicotinic and muscarinic receptor agonists nicotine (0.1 mg/kg) and arecoline (1.0 mg/kg) were also tested for comparison. Both arecoline and nicotine improved initial acquisition in rats before lesioning, in terms of latency to find a hidden platform and accuracy of search strategy. WAY100289 did not affect the performance of normal rats significantly, apart from some non-significant trends towards improvement with the highest dose. However, in animals showing transient navigational deficits in retention and relearning after lesioning, WAY100289 improved performance at all three doses, though ameliorative effects of nicotine and arecoline were more marked also in lesioned rats. These results show that WAY100289 improved spatial learning in animals impaired after lesions to cholinergic projection nuclei, which may reflect an interaction with cholinergic transmission to enhance cognitive function. However, in the present study, WAY100289 appeared to be less effective than direct cholinergic agonists.

Pharmacological consequences of nicotinergic plus serotonergic manipulations

The present study investigates the effects of concurrent manipulations of nicotinic cholinergic receptors (nicotinic cholinergic agonist: nicotine 0.03, 0.1, 0.3 mg/kg, nicotinic cholinergic antagonist: mecamylamine 7.5 mg/kg) and serotonin neurons (p-chlorophenylalanine (PCPA), 400/kg mg on each of 3 days) on spatial navigation (water maze, WM) and passive avoidance (PA) performance. Nicotine did not affect PA performance but at the highest dose slightly impaired WM performance. PCPA did not affect WM navigation or PA performance in saline or nicotine-treated rats. Nicotine restored WM and PA performance defect in mecamylamine pretreated rats. PCPA aggravated the WM defect and decreased the WM performance-improving effect of nicotine in mecamylamine pretreated rats. PCPA did not aggravate the PA performance defect of mecamylamine but completely blocked the PA performance-improving effect of nicotine in mecamylamine pretreated rats. These results suggest that serotonergic and nicotinergic cholinergic systems jointly modulate performance in WM and PA tests.

Increased responsiveness of hippocampal pyramidal neurons to nicotine in aged, learning-impaired rats

The effect of aging upon the responsiveness of hippocampal CA1 pyramidal neurons to nicotine was investigated using electrophysiological techniques in male Fischer 344 rats. Prior to electrophysiological recording, animals were behaviorally tested using the Morris water maze. All 3-6 and 18-21 month rats displayed rapid place learning in this task, while none of the 27-30 month animals learned within the 5-day test period. By contrast, rats of all age groups were able to learn a cue version of the water maze task. Following behavioral testing, the animals were anesthetized with sodium pentobarbital for acute recording. Nicotine was locally applied to electrophysiologically identified CA1 pyramidal neurons using pressure microejection from two-barreled glass microelectrodes. For each neuron, a dose of nicotine was found which elicited a 300-400% increase in basal firing rate. These data were used to construct cumulative dose response curves for populations of neurons tested in 3-6-, 18-21-, and 27-30-month-old animals. An age-related increase in the responsiveness of CA1 pyramidal neurons to locally applied nicotine was observed. The results of this study suggest that an increase in hippocampal CA1 pyramidal cell responsiveness to nicotine could be related to the impaired place learning ability seen with aging.

Effects of psychoactive drugs on short-term memory in rats and rhesus monkeys

To examine the effects of drugs on short-term memory in animals, the delayed discrimination experiment in rats and the delayed matching to sample experiment in rhesus monkeys were conducted. Nicotine at 0.125 mg/kg, s.c. in rats and at 0.5 mg/kg, s.c. in monkeys increased the percentages of correct choices. Scopolamine at 0.06-0.12 mg/kg, s.c. in rats and at 0.015 mg/kg, s.c. in monkeys decreased the percentages of correct choices. However, supposedly memory-specific, delay-time-dependent disruptive effects by scopolamine were found only in monkeys. Diazepam at 0.5-2 mg/kg, s.c. did not change the correct choices in rats. However, diazepam at 1-4 mg/kg, i.g. decreased the correct choices in monkeys regardless of the delay time. Chlorpromazine at 0.25-1.5 mg/kg, s.c. showed inconsistent effects in rats. In monkeys, chlorpromazine at 0.25-0.5 mg/kg, s.c. had no effect. These results suggested that using both rats and monkeys would be useful for evaluating the effects of drugs on memory.

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.

Effects of cholinergic-rich neural grafts on radial maze performance of rats after excitotoxic lesions of the forebrain cholinergic projection system--II. Cholinergic drugs as probes to investigate lesion-induced deficits and transplant-induced functional recovery

The effects of two doses of muscarinic (arecoline and scopolamine) and nicotinic (nicotine and mecamylamine) cholinergic receptor agonists and antagonists on the radial maze errors of rats, performing poorly after ibotenate lesions to the nucleus basalis and medial septal brain regions, were assessed before and after transplantation of cholinergic-rich and -poor fetal grafts, using tasks which measured short- (working) and long-term (reference) spatial and associative memory. Lesioned rats showed improvement with the agonists, and impairment with the antagonists, at low doses which did not affect the performance of controls; these effects were more marked for working than reference memory, especially in the spatial task. The peripherally acting antagonists N-methylscopolamine and hexamethonium did not affect the performance of control or lesioned rats. Effects of the cholinergic probes were re-examined 16 weeks after grafting, in groups with cholinergic-rich grafts to cortex and/or hippocampus which showed functional recovery, and groups with cholinergic-rich grafts to basal forebrain, or cholinergic-poor grafts to basal forebrain, cortex, and hippocampus, which showed no improvement. All lesioned rats, regardless of site, type, or efficacy of transplant, continued to show marked impairment with the antagonists. Poorly performing grafted animals also showed improvement with the agonists. In rats with behaviourally effective cholinergic-rich grafts, arecoline had no effect, but nicotine substantially increased working and reference memory errors, particularly spatial working memory errors. Rats with grafts in both cortex and hippocampus showed the largest increases in errors after nicotine. These results show that lesioned rats were more sensitive to the bi-directional effects of cholinergic receptor ligands than controls, consistent with a role for acetylcholine in the lesion-induced deficits. The predominant effect of drugs on working memory may also be consistent with disruption of acquisition rather than of storage or retrieval processes in memory, and may be related to impairment of attention. The results further show that, despite behavioural recovery, supersensitive responses to cholinergic drugs were not normalized in rats with cholinergic-rich grafts, and that an additive interaction between graft and host may have occurred in response to nicotine.

Chronic nicotine and withdrawal effects on radial-arm maze performance in rats

Rats were tested for choice accuracy in an eight-arm radial maze during and after chronic administration of nicotine via subcutaneously implanted glass and Silastic capsules. Nicotine administration significantly improved choice accuracy relative to controls. The effect gradually became apparent over the first 2 weeks of exposure and persisted through the third week. Surprisingly, the significant facilitation of the nicotine-treated rats relative to controls continued for 2 weeks after the end of nicotine administration. No effects of nicotine were seen on choice latency or the strategy to make adjacent arm entries.

Reinforced alternation performance is impaired by muscarinic but not by nicotinic receptor blockade in rats

In pre-trained rats, food-rewarded reinforced alternation in a T-maze was markedly impaired by scopolamine, but unaffected by the centrally-active nicotinic antagonist mecamylamine (0.2-6.3 mg/kg s.c.). The nicotinic antagonist chlorisondamine (10 micrograms i.v.t.) was also ineffective, despite demonstrated blockade of nicotine-induced locomotor depression. These results question the extent to which central nicotinic receptors are involved in memory function.