The effects of nicotine on attention, information processing, and short-term memory in patients with dementia of the Alzheimer type

Alzheimer's disease and the basal forebrain cholinergic system: relations to beta-amyloid peptides, cognition, and treatment strategies

Alzheimer's disease (AD) is the most common form of degenerative dementia and is characterized by progressive impairment in cognitive function during mid- to late-adult life. Brains from AD patients show several distinct neuropathological features, including extracellular beta-amyloid-containing plaques, intracellular neurofibrillary tangles composed of abnormally phosphorylated tau, and degeneration of cholinergic neurons of the basal forebrain. In this review, we will present evidence implicating involvement of the basal forebrain cholinergic system in AD pathogenesis and its accompanying cognitive deficits. We will initially discuss recent results indicating a link between cholinergic mechanisms and the pathogenic events that characterize AD, notably amyloid-beta peptides. Following this, animal models of dementia will be discussed in light of the relationship between basal forebrain cholinergic hypofunction and cognitive impairments in AD. Finally, past, present, and future treatment strategies aimed at alleviating the cognitive symptomatology of AD by improving basal forebrain cholinergic function will be addressed.

Sustained attention deficit in bipolar disorder is not a working memory impairment in disguise

Euthymic patients with bipolar disorder have been reported to show persistent deficits in sustained attention. However, the sustained attention task which was used also placed demands on working memory. Bipolar disorder patients in the euthymic state were therefore compared with healthy controls on two measures of sustained attention with and without a working memory component. Signal detection methodology was applied to the results. Euthymic patients with bipolar disorder were particularly impaired at detecting targets in the sustained attention task without a working memory component. This deficit was still apparent in a sub-group of patients who were not currently receiving lithium medication. By contrast, performance in the sustained attention task involving working memory task was not significantly different in the two groups. Sustained attention deficits apparent during the euthymic period of bipolar disorder cannot be explained in terms of working memory impairment and represents a reduced inherent capacity rather than a change in response bias. Deficits in sustaining attention may help explain the difficulties in psychological and occupational functioning in bipolar disorder patients during remission.

Cholinergic nicotinic systems in Alzheimer's disease: prospects for pharmacological intervention

Within the last few years, research into the cause and progression of Alzheimer's disease has made significant advances. Although there is still no preventative treatment or cure for this neurodegenerative illness, the development of drugs that may alleviate some of the cognitive symptoms associated with it is advancing. Cholinesterase inhibitors are at present the most effective form of treatment and have shown significant overall response rates in clinical trials. However, although some patients show substantial improvement when treated with this class of drugs, there is considerable variability in the amount of benefit gained in different individuals in terms of their cognitive and behavioural functioning. Furthermore, unfortunately some patients gain little or no benefit from these drugs. It would therefore be of great advantage to explore alternative therapeutic possibilities. This article reviews the potential involvement of the nicotinic cholinergic system in Alzheimer's disease and discusses the possibility of nicotinic pharmacotherapy. Substantial evidence indicates the involvement of the nicotinic cholinergic system in the pathology of Alzheimer's disease. Drugs targeting these sites may not only have a positive effect on cognitive function, but also have additional therapeutic benefits in terms of restoring the hypoactivity in the excitatory amino acid pyramidal system and even slowing the emergence of Alzheimer's disease pathology. The conclusion of this review is that nicotinic treatments are an important potential source of new therapeutic interventions in Alzheimer's disease.

Neural Transplantation in Animal Models of Dementia

Neural transplantation provides a powerful novel technique for investigating the neurobiological basis and potential strategies for repair of a variety of neurodegenerative conditions. The present review considers applications of this technique to dementia. After a general introduction (section 1), attempts to replace damaged neural systems by transplantation are considered in the context of distinct animal models of dementia. These include grafting into aged animals (section 2), into animals with neurotransmitter-selective lesions of subcortical nuclei, in particular involving basal forebrain cholinergic systems (section 3), and into animals with non-specific lesions of neocortical and hippocampal systems (section 4). The next section considers the alternative use of grafts as a source of growth/trophic factors to inhibit degeneration and promote regeneration in the aged brain (section 5). Finally, a number of recent studies have employed transplanted tissues to model and study the neurodegenerative processes associated with ageing and Alzheimer's disease taking place within the transplant itself (section 6). It is concluded (section 7) that although neural transplantation does not offer any immediate prospect of therapeutic repair in clinical dementia, the technique does offer a powerful neurobiological tool for studying the neuropathological processes involved in both spontaneous degeneration and specific diseases of ageing. New understandings derived from neural transplantation may be expected to lead to rational development of novel strategies to inhibit the neurodegenerative process and to promote regeneration in the aged brain.

Nicotine reduces the secretion of Alzheimer's beta-amyloid precursor protein containing beta-amyloid peptide in the rat without altering synaptic proteins

Alzheimer's disease (AD) is characterized by cerebrovascular deposition of the amyloid beta-peptide (A beta), which is derived from a larger beta-amyloid precursor protein (beta APP). Altered metabolism of beta APP, resulting in increased A beta production, appears central in the neuropathology of AD. The processing of the holoprotein beta APP by different "secretase" enzymes results in three major carboxyl-truncated species. One species, which results from the cleavage of beta APP by gamma-secretase, is secreted into the cerebrospinal fluid (CSF) and is called sAPP gamma as it contains an intact A beta domain. Moreover, AD is characterized by cholinergic dysfunction and the loss of synaptic proteins. Reports of an inverse relation between nicotine intake, due to cigarette smoking, and the incidence of AD prompted us to investigate the effects of nicotine on beta APP processing and synaptic proteins in rats and in cell culture. Nicotine, 1 and 8 mg/kg/day, doses commensurate with cigarette smoking, and a higher but well tolerated dose, respectively, was administered over 14 days to rats. Levels of sAPP in the CSF sample were evaluated by Western blot analysis. The higher dose significantly increased levels of total sAPP; however, both doses significantly reduced sAPP gamma, which contains the amyloidogenic portion of A beta. These actions were blocked by nicotinic receptor antagonism. Nicotinic antagonists alone had no effect on either total sAPP or sAPP gamma levels in CSF. Nicotine did not significantly change the intracellular levels of total beta APP in rat brain extracts, which is consistent with neuronal cell culture data. Similarly, levels of vesicular protein, such as synaptophysin, and presynaptic terminal protein SNAP-25 were unaffected by nicotine treatment both in vivo and in cell culture experiments. Taken together, these results suggest that nicotine modifies beta APP processing away from the formation of potentially amyloidogenic products, without altering the levels of synaptic proteins, and that this can potentially offer therapeutic potential for Alzheimer's disease.

Brain-selective stimulation of nicotinic receptors by TC-1734 enhances ACh transmission from frontoparietal cortex and memory in rodents

The authors have described the effect of TC-1734, a brain-selective nicotinic acetylcholine receptor (nAChR) agonist, on acetylcholine (ACh) release in the frontoparietal cortex of rats and on cognitive function in mice. Oral administration of TC-1734 (5, 10 and 20 mg/kg) stimulated ACh release in a dose-dependent manner, as measured by transversal microdialysis. The maximal effect on the amplitude of ACh release was observed at a dose of 10 mg/kg (about 70% above baseline), whereas the maximal effect on the duration of ACh release was observed at the dose of 20 mg/kg. By contrast, oral administration of nicotine (1, 2.5 and 5 mg/kg) did not stimulate ACh release in a dose-dependent manner but produced the same maximal effect on the amplitude of ACh release (about 50% above baseline) at all the doses tested. The ability of both TC-1734 (10 mg/kg) and nicotine (1 mg/kg) to increase ACh levels was antagonized by mecamylamine (1 mg/kg s.c.), suggesting a specific nicotine receptor-mediated effect of both agonists. No tolerance to TC-1734- and nicotine-stimulated ACh release was observed after repeated treatment with TC-1734 (10 mg/kg) or nicotine (1 mg/kg) for 4 days. TC-1734 (1 mg/kg p.o.) improved memory in the object recognition test in mice, and this effect was antagonized by mecamylamine (2.5 mg/kg i.p.). Taken together, these results show that TC-1734 stimulates nAChR in the brain to induce an increase of ACh release in the cortex of rats and enhance memory in mice.

Hippocampal alpha 7 and alpha 4 beta 2 nicotinic receptors and working memory

Nicotine and other nicotinic receptor agonists have been found in a variety of studies to improve memory, while nicotinic receptor blockade can impair memory. The critical neural mechanisms for nicotinic involvement with memory are still under investigation. Initial evidence supports the involvement of the ventral hippocampus. Lesions in this area block nicotine-induced memory improvement and mecamylamine-induced impairment. Local ventral hippocampal application of the nicotinic channel blocker mecamylamine impairs memory in the 8-arm radial maze. Both alpha 4 beta 2 and alpha 7 nicotinic receptors seem to be involved. Ventral hippocampal infusions of high doses of the alpha 4 beta 2 nicotinic antagonist dihydro-beta-erythrodine (DH beta E) and the alpha 7 nicotinic antagonist methyllycaconitine (MLA) impair memory performance on the 8-arm radial maze. However, high doses of these drugs may limit specificity and they cause preconvulsant effects, which in themselves may affect memory. The current study used the more challenging 16-arm radial maze to determine the effects of lower doses of these drugs on memory and to differentiate effects on working and reference memory. Adult female Sprague-Dawley rats were trained on a working and reference memory task in the 16-arm radial maze and then were implanted with bilateral chronic guide cannulae directed to the ventral hippocampus. After recovery from surgery, the rats received acute intrahippocampal infusions of dose combinations of DH beta E and MLA. In the first study, DH beta E (0 and 6.75 microg/side) and MLA (0, 6.75, 13.5 and 27 microg/side) were administered in a counter-balanced order. In the second study, lower doses of DH beta E (0, 1.6375, 3.275 and 6.75 microg/side) were administered alone or with MLA (0 and 6.75 microg/side) in a counter-balanced order. In the first study, DH beta E caused a significant increase in both working and reference memory errors. MLA at a dose of 27 microg/side caused a significant increase in working memory errors, but this dose had no significant effect on reference memory errors. Interestingly, no additive effects were seen with combined administration of DH beta E and MLA in this study, and at the doses used, no effects were seen on response latency. In the second study, lower doses of DH beta E did not cause a significant deficit in working memory performance. Co-administration of MLA with these subthreshold doses did precipitate a memory impairment. The current results confirm the specificity of the memory deficits caused by these drugs. These results support the involvement of alpha 4 beta 2 and alpha 7 nicotinic receptors in the ventral hippocampus as being critical for memory function.

The levels of norharman are high enough after smoking to affect monoamineoxidase B in platelets

Epidemiological studies suggest that smoking reduces the risk for Parkinson's disease. It has been hypothesized that inhibition of monoamineoxidase contributes to this action. The present study examined the contribution of the beta-carbolines norharman, an inhibitor of monoamineoxidase B, and harman, an inhibitor of monoamineoxidase A, which are present in high concentrations in tobacco smoke to the protective action. Nineteen active smokers and five nonsmokers smoked one and two cigarettes. The levels of norharman and harman increased in plasma from smokers and nonsmokers. Ex vivo saturation kinetic experiments revealed that the baseline affinity constant of monoamineoxidase in platelets from smokers was higher than that of nonsmokers in contrast to the maximum turnover rate, which did not differ. Acute smoking affected the monoamineoxidase in nonsmokers only. It is discussed that inhibition of both isoforms of monoamineoxidase is necessary for the neuroprotection and that both norharman and harman play an important role.

Differential roles for nicotinic and muscarinic cholinergic receptors in sustained visuo-spatial attention? A study using a 5-arm maze protocol in mice

A 5-arm maze was used to measure sustained visuo-spatial attention in C57Bl/6 mice and test the hypothesis of differential functional roles for central nicotinic and muscarinic receptors in mediating task performance. Mice were first trained to acquire the basic visual discrimination task in which, on each trial, one randomly chosen arm among the five open arms was baited and remained lit until an arm-choice was made. Mice were then submitted to attentional testing in which trials using light signals of 2, 1 or 0.5 s were intermixed to evaluate the decrement in correct responses as a function of the decrease in light signal duration and thus, to construct a reference curve for the attentional performance of C57Bl/6 mice. Mice were then divided into four groups and received, in rotation, over four pharmacological sessions according to a Latin-square design, i.p. injections of either mecamylamine (4.0 mg/kg), scopolamine (0.8 mg/kg), the combination of mecamylamine and scopolamine or saline, 20 min before re-testing. Injection of cholinergic antagonists produced decreases in percentage of correct responses, which were systematically associated with significant increases in choice latencies. Mecamylamine produced slight disruption, whereas scopolamine and the combined treatment both produced severe disruption. In conclusion, whereas both nicotinic and muscarinic cholinergic antagonists disrupt performance in the attentional task, the increase in response latencies entails that correct responding becomes more dependent on the working memory processes and thus compromises conclusions as to a selective disruption of attention.

Smoking/Nicotine Affects the Magnitude and Onset of Lateralized Readiness Potentials

AbstractSmoking/nicotine improves cognitive performance for a variety of tasks. In most cases, reaction time (RT) is generally shorter after smoking/nicotine. While there may be some slight facilitation of stimulus-evaluation processing, most of the RT effects of nicotine appear to take place following the response-selection stage. This study investigated possible effects (in smokers) of smoking/nicotine on response preparation and execution processes using the lateralized readiness potential (LRP). On each trial, a warning stimulus preceded an imperative stimulus by 1.2s. The warning stimulus completely specified the correct response to the imperative stimulus. The study was completed in two morning sessions in which 4 cigarettes were smoked in each session. The nicotine yield of the cigarettes varied between sessions (0.05mg or 1.1mg). Maximum amplitudes of both the stimulus and response-locked LRPs were larger in the 1.1 mg session. For both stimulus- and response-locked LRPs, smoking the 1.1 mg cigarette (but not the 0.05 mg cigarette) shortened onset latency. However, the magnitude of the effect was much larger for the stimulus-locked LRPs, suggesting that response preparation is facilitated by smoking/nicotine to a greater degree than response execution.

Different pharmacokinetics of nicotine following intravenous administration of nicotine base and nicotine hydrogen tartarate in rats

Pharmacokinetics of nicotine was studied in rats following intravenous (i.v.) administration of nicotine base (NB) and nicotine hydrogen tartarate salt (NS) at a nicotine dose of 1 mg/kg. The area under the plasma concentration-time curve (AUC), mean residence time (MRT), systemic clearance (CL), distribution volume at steady state (V(ss)) and terminal plasma half-life (T(1/2,beta)) of nicotine were compared between NB and NS. Compared to NS, NB exhibited higher and sustained plasma nicotine levels, thereby yielding significantly (P<0.05) larger AUC (66.3 vs. 27.7 microg ml/min), MRT (165.7 vs. 58.3 min), T(1/2,beta) (144.2 vs. 51.4 min) and a lower CL (18.3 vs. 46.3 ml/min per kg). The V(ss) was comparable between the two compounds. The metabolic conversion to cotinine from NS was threefold larger than that from NB. The plasma protein binding and distribution to blood cells were comparable between the compounds. The apparent partition coefficient (APC) of NS decreased as a function of its concentration, while that of NB remained nearly constant. Particles of different mean sizes were observed for the 1% (w/v) aqueous solutions of NS (388.6 nm) and NB (123.8 nm). Different metabolism and/or elimination between NB and NS appear to be mainly responsible for their different pharmacokinetics.

A study of the nicotinic agonist SIB-1553A on locomotion and attention as measured by the five-choice serial reaction time task

SIB-1553A is a novel ligand with reputed agonist selectivity at nicotinic receptors containing the beta(4) subunit. As such, it represents an interesting pharmacological tool with which to probe the function of nicotine receptor subtypes. In the present studies, we compared SIB-1553A with nicotine in its ability to stimulate locomotion and to enhance attention in rats as assessed using the five-choice serial reaction time task (5-CSRTT). In nicotine-naive rats, SIB-1553A (10-40 mg/kg) induced a comparable increase in locomotion to nicotine (0.4 mg/kg), whereas in nicotine-sensitised rats, an enhanced locomotor response was seen to nicotine (0.4 mg/kg) but not to SIB-1553A (10-80 mg/kg). Similarly, chronic treatment with either SIB-1553A or nicotine did not lead to a cross-sensitised locomotor response. Unlike nicotine, SIB-1553A-induced locomotion was insensitive to antagonism by either mecamylamine (1 mg/kg) or DH beta E (3 mg/kg), suggesting a non-nicotinic mechanism. In young and aged rats, nicotine (0.4 mg/kg) enhanced attention as demonstrated by an increase in response accuracy and speed. SIB-1553A (3-10 mg/kg) did not mimic any of these changes and at the highest dose tended to disrupt performance. These results lend further support to the involvement of a high affinity site, possibly alpha(4)beta(2), in the locomotor and attentional-enhancing properties of nicotine.

Ventral hippocampal alpha 7 nicotinic receptor blockade and chronic nicotine effects on memory performance in the radial-arm maze

Chronic nicotine administration has been shown to significantly improve working memory. Nicotinic involvement in memory function critically involves the ventral hippocampus. Local ventral hippocampal infusions of the nicotinic antagonists mecamylamine, dihydro-beta-erythroidine (DH beta E) and methyllycaconitine (MLA) significantly impair working memory. The impairment caused by hippocampal infusion of the alpha 4 beta 2 antagonist DH beta E is reversed by chronic systemic nicotine. This study determined the interaction of chronic systemic nicotine with acute ventral hippocampal infusions of the alpha 7 antagonist MLA. Adult female Sprague-Dawley rats were trained on an 8-arm radial maze working memory task. Then they underwent ventral hippocampal cannulation and received sc implants of minipumps delivering nicotine (0 or 5 mg/kg/day for 28 days). Acute ventral hippocampal infusions of MLA (0, 4.88, 14.64 and 43.92 microg/side) were given during 3-4 weeks of chronic nicotine. MLA caused a significant dose-related memory impairment. In the rats not receiving nicotine, the 14.64 and 43.92 microg/side MLA doses caused significant memory impairment. Chronic systemic nicotine exposure did not block the MLA-induced memory impairment. Comparing the current results with MLA with previous results with DH beta E, equimolar ventral hippocampal DH beta E more effectively impaired memory than MLA, but the DH beta E-induced impairment was more effectively reversed by chronic systemic nicotine administration.

Treatment of cognitive dysfunction associated with Alzheimer's disease with cholinergic precursors. Ineffective treatments or inappropriate approaches?

The observations of the loss of cholinergic function in neocortex and hippocampus in Alzheimer's disease (AD) developed the hypothesis that replacement of cholinergic function may be of therapeutic benefit to AD patients. The different approaches proposed or tested included intervention with acetylcholine (ACh) precursors, stimulation of ACh release, use of muscarinic or nicotinic receptor agonists and acetylcholinesterase (AChE) or cholinesterase (ChE) inhibition. Inhibition of endogenous ACh degradation through ChE inhibitors and precursor loading were treatments more largely investigated in clinical trials. Of the numerous compounds in development for the treatment of AD, AChE and ChE inhibitors are the most clinically advanced, although clinical trials conducted to date did not always confirm a significant benefit of these drugs on all symptom domains of AD. The first attempts in the treatment of AD with cholinergic precursors did not confirm a clinical utility of this class of compounds in well controlled clinical trials. However, cholinergic precursors most largely used such as choline and phosphatidylcholine (lecithin) were probably not suitable for enhancing brain levels of ACh. Other phospholipids involved in choline biosynthetic pathways such as CDP-choline, choline alphoscerate and phosphatidylserine clearly enhanced ACh availability or release and provided a modest improvement of cognitive dysfunction in AD, these effects being more pronounced with choline alphoscerate. Although some positive results cannot be generalized due to the small numbers of patients studied, they probably would justify reconsideration of the most promising molecules in larger carefully controlled trials.

Sex differences in brain and behavior: emphasis on nicotine, nitric oxide and place learning

Although males and females are unmistakably different, the recognition of sex as a key variable in science and medicine is considered a revolution in some circles. Sex differences transcend reproductive functions, are evident in the structural and functional organization of the brain, and are reflected in group differences in cognitive abilities and behavior. Males and females have different neural organizational patterns for information processing and different strategies in problem solving. Research on sex differences not only provides descriptive data, but also allows us to elucidate mechanisms that underlie our behavior. In this review, sex differences in the central actions of nicotine (an addictive substance) and nitric oxide, and performance on active avoidance and place learning tasks are discussed as examples, and biobehavioral approaches relating to these topics are presented.

Cognitive screening and neuropsychological assessment in early Alzheimer's disease

Cognitive screening and detailed neuropsychological assessment provide a reliable means of detecting dementia in its earliest stages, tracking the progression of cognitive decline over time, and aiding in the differential diagnosis of various dementing disorders. In addition, recent studies have shown that mild cognitive changes, and particularly declines in memory function, are evident in the "preclinical" phase of Alzheimer's disease and may help to identify elderly individuals who are likely to develop dementia in the near future. Until effective and easily obtainable biological markers for detecting the onset and progression of Alzheimer's disease are developed, neuropsychological assessment will continue to have an important role in the dementia evaluation.

Ligands for in vivo imaging of nicotinic receptor subtypes in Alzheimer brain

The neuronal nicotinic acetylcholine receptors (nAChR) are involved in functional processes in brain including cognitive function and memory. A severe loss of the nAChRs has been detected in brain of patients with Alzheimer's disease (AD). There is a great interest to image nAChRs noninvasive for detection of receptor impairments even at a presymptomatic stage of AD as well for monitoring outcome of drug treatment. (S) [11C]Nicotine, has so far been the only nAChR ligand used in positron emission tomography (PET) studies for visualizing nAChRs in human brain. In order to develop PET/SPECT nAChRs ligands for detection of subtypes of nAChRs nicotine analogues, epibatidine and A-85380 compounds have been characterized in vitro and investigated in vivo. Epibatidine and A-85380 have been found to have higher specific signals and more favorable kinetic parameters than nicotine and its analogues. The epibatidine and A-85380 compounds can also be radiolabeled with high specific radioactivity, show affinities for the nAChRs in the pM range and readily cross the blood-brain barrier. In addition they reversibly bind to the nAChRs and show low non-specific binding and moderately fast metabolism. Due to a probably high alpha4beta2 nAChR selectivity combined with low toxicity, the A-85380 analogs presently seem to be the most promising nAChR ligand imaging of subtypes of nAChRs in human brain.

Neuroscience. Boosting working memory

Working out which areas of the brain become activated during the formation of working (short-term) memory has been greatly helped by functional magnetic resonance imaging (fMRI). In a Perspective, Robbins et al. discuss new findings (Furey et al.) with fMRI that reveal how working memory is enhanced by the drug physostigmine, which increases cholinergic function in the brain.

Effect of subtype selective nicotinic compounds on attention as assessed by the five-choice serial reaction time task

Nicotine can improve attentional functioning in humans, and a number of studies have recently demonstrated that under specific task conditions, nicotine can also improve attention in the rat. Neuronal nicotinic receptors comprise combinations of alpha(2-9) and beta(2-4) subunits, arranged to form a pentameric receptor, with the principal CNS subtypes currently believed to be alpha(4)beta(2) and a homomeric alpha(7) receptor. In the present studies, we attempted to delineate the particular nicotinic receptor subtype(s) contributing to the effects of nicotine on attention by assessing various nicotinic ligands on performance in the five-choice serial reaction time task (5-CSRTT). In rats performing below criterion (<80% correct, >20% omissions to a 1-s visual stimulus), subchronic dosing with nicotine (0.2 mg/kg sc) and the alpha(4)beta(2) agonist SIB 1765F (5 mg/kg sc) increased correct responding and decreased response latencies across the treatment week; whereas the alpha(7) agonist AR-R 17779 (20 mg/kg sc) was without effect. In subjects meeting the criterion, the competitive high affinity (including alpha(4)beta(2)) nicotine receptor antagonist DHbetaE (1-10 mg/kg sc) and the alpha(7) antagonist methyllycaconitine (MLA: 5-10 mg/kg i.p.) did not disrupt performance, whereas at the highest dose, the non-competitive antagonist mecamylamine (0.3-3 mg/kg sc) decreased accuracy and increased response latencies. These changes bore some similarities to those of pre-feeding and the non-competitive NMDA antagonist dizocilpine (0.03-0.06 mg/kg sc), suggesting that mecamylamine-induced performance disruption may relate to non-nicotinic receptor effects. In subjects chronically treated with nicotine, acute nicotine challenge (0.4 mg/kg sc) significantly increased accuracy whilst having no effect on any other performance measures. Finally, in these same nicotine pre-treated rats, the decrease in latency and increase in premature responses induced by nicotine (0.2 mg/kg sc) to a target stimulus of 150 ms was fully antagonised by DHbetaE (3 mg/kg sc) but not MLA (5 mg/kg i.p.). These results suggest that alpha(7) receptors do not play a role in any of the behavioural effects of nicotine observed in the 5-CSRTT, whereas a high affinity site, perhaps alpha(4)beta(2), is more likely involved.

TC-2559: a novel orally active ligand selective at neuronal acetylcholine receptors

TC-2559 [(E)-N-Methyl-4-[3-(5-ethoxypyridin)yl]-3-buten-1-amine] is a novel nicotinic agonist markedly more selective than recently reported novel nicotinic receptor ligands (selectivity ratio for central nervous system (CNS) to peripheral nervous system (PNS)>4000). TC-2559 competes effectively with [3H]-nicotine binding (K(i)=5 nM) but not with [125I]-bungarotoxin (>50,000 nM). Dopamine release from striatal synaptosomes and ion flux from thalamic synaptosomes indicate that TC-2559 is potent and efficacious in the activation of CNS receptors and significantly reduced glutamate-induced neurotoxicity in vitro. TC-2559 has no detectable effects on muscle and ganglion-type nicotinic acetylcholine receptors at concentrations up to 1 mM. TC-2559 significantly attenuates scopolamine-induced cognitive deficits in a step-through passive avoidance task. Acute and repeated oral dosing of TC-2559 enhances performance in a radial arm maze task. In contrast to the effects of equimolar concentrations of (-) nicotine, TC-2559 does not induce hypothermia and locomotor activity is not enhanced following repeated daily administration of 14 days. TC-2559 has a markedly enhanced CNS-PNS selectivity ratio and an intra-CNS selectivity as evidenced by the improved cognition without increased locomotor activity. The in vitro and in vivo studies in the present study suggest that TC-2559 has the desired profile to be further evaluated as a potential therapeutic agent for neurodegenerative diseases.

A 5-arm maze enables parallel measures of sustained visuo-spatial attention and spatial working memory in mice

A 5-arm maze has been developed to provide parallel tests of sustained visuo-spatial attention and spatial working memory in mice. C57Bl/6 mice were trained to select, either by immediate response (attention) or by delayed-matching response (working memory), one target arm among the five open arms. For attention testing, mice were first trained to acquire the basic task in which one randomly selected baited arm remained lit until a choice was made. Criterion of >80% correct with a response latency <5 s was attained in 52-56 trials. Following this, attention was tested by using trials wherein light signal durations of 2, 1 or 0.5 s were intermixed to vary attentional load. In the working memory test, mice were submitted to a forced visit to a randomly selected baited arm during a presentation phase. Following a variable retention interval (R.I.), mice were replaced into the maze and rewarded for choosing this arm. Criterion of >80% correct was attained in 35-40 trials and mice exhibited high levels of retention for R.I.s up to 4 h. Results validate the 5-arm maze for evaluation of both sustained visuo-spatial attention and spatial working memory in mice. Both the tasks are rapidly acquired and the 20% chance level provides high resolution for evaluating performance. This comparative strategy allows to dissociate attention and memory and to reveal deficits in these processes during ageing or in knockout strains. The high level of retention performance over R.I.s of 4 h enables studies using pharmacological treatments differentially affecting the acquisition, encoding, retention or retrieval phases of working memory. Furthermore, functional brain imaging studies may be used to identify neuronal networks which are differentially activated during these distinct phases.

Protractive effects of chronic treatment with an acutely sub-toxic regimen of diisopropylflurophosphate on the expression of cholinergic receptor densities in rats

Individuals chronically exposed to low levels of organophosphate insecticides may present with subtle impairments in cognition. In addition, low level diisopropylflurophosphate (DFP) exposure (0.25 mg/kg per day for 2 weeks) in rats resulted in protracted working memory impairment [29]. The current studies attempt to show a temporal relationship between the DFP-induced impairment in performance of a spatial memory task and the protracted decrease in the expression of cholinergic receptors and acetylcholinesterase in specific brain regions. Cholinergic receptors labeled with the ligands [(3)H]epibatidine and [(3)H]AFDX-384 were affected to a much greater extent and for a longer period of time than were both acetylcholinesterase activities and cholinergic receptors labeled with [(3)H]QNB. Pre-testing administration of nicotine was shown to completely reverse this DFP-induced impairment in memory-related task performance. Additionally, prophylaxis with pyridostigmine bromide (PB) caused DFP-treated animals to exhibit near normal levels of memory-related task performance. These results are consistent with the development of a protracted phase of learning impairment to sub-acute DFP exposure, which may involve the loss of hippocampal nicotinic receptors, and may be prevented or reversed by PB or nicotine, respectively.

The role of cortical cholinergic afferent projections in cognition: impact of new selective immunotoxins

Previous investigations aimed at determining the role of corticopetal cholinergic afferents in cognition have relied upon human psychopharmacological studies, neuropsychological analyses of Alzheimer's patients, or psychopharmacological manipulations and excitotoxic lesions in animals. Unfortunately, each approach has its limitations. The interpretation of neuropsychological data relies upon correlations of post-mortem assessments of cholinergic degeneration that may be quite temporally distant from the time of cognitive assessment. In contrast, the use of animals allows direct manipulations of the cholinergic system and the establishment of causal relationships between acetylcholine and cognitive function but is limited by the selectivity of the toxins and drugs available to manipulate the system. The recent introduction of immunotoxins to lesion cortical cholinergic pathways with greater selectivity has allowed the effective testing of these hypotheses of cholinergic functions in cognition. Previous neuropsychological, psychopharmacological and excitotoxic lesion data are reviewed and compared to results produced using the more selective immunotoxins to provide an update to the current hypotheses of the role of corticopetal cholinergic afferents in cognitive function. Additionally, the conceptual and methodological cost and benefits of the methods of infusion used to produce lesions with these immunotoxins is assessed.

Ventral hippocampal alpha4beta2 nicotinic receptors and chronic nicotine effects on memory

Chronic nicotine administration has been repeatedly shown to facilitate working memory function in rats on the radial-arm maze. The critical neural mechanisms for this effect are still being discovered. The nicotinic nature of the chronic nicotine induced memory improvement is supported by the finding that it is blocked by chronic mecamylamine co-infusion. The hippocampus also appears to be critically important. Hippocampal ibotenic acid lesions block the effect. Within the hippocampus, we have found that the alpha4beta2 nicotinic receptor subtype is involved in memory functioning. Acute ventral hippocampal infusions of the alpha4beta2 nicotinic antagonist dihydro-beta-erythroidine (DHbetaE) significantly decreased working memory performance in the radial-arm maze. The aim of the current study was to determine the importance of alpha4beta2 receptors within the ventral hippocampus for the memory enhancing effects of chronic nicotine treatment. Adult female Sprague-Dawley rats were trained on the 8-arm radial maze and were cannulated bilaterally in the ventral hippocampus. Osmotic minipumps administering chronic nicotine at a rate of 5 mg per kg per day were also implanted in the nicotine treatment rats. Control rats received saline-only minipumps. For a period of 4 weeks after surgery, each rat received bilateral hippocampal infusions of 0, 2, 6 and 18 microg per side of DHbetaE and tested for memory performance on the radial-arm maze. Radial-arm maze choice accuracy was impaired by acute hippocampal DHbetaE infusion in a dose-related fashion. This acute hippocampal DHbetaE-induced choice accuracy impairment was eliminated by chronic systemic nicotine infusion. Chronic nicotine in combination with acute vehicle hippocampal infusion was not seen to alter choice accuracy. Response latency was not found to be altered by acute hippocampal DHbetaE in the absence of chronic nicotine administration, but it did attenuate the response latency reduction induced by chronic nicotine infusion. Wet dog shakes were not found to be affected by hippocampal DHbetaE when given without chronic nicotine. Wet dog shakes were significantly increased by chronic nicotine infusion. Intra-hippocampal DHbetaE significantly potentiated this effect. The results from the current study reinforce the hypothesis that ventral hippocampal alpha4beta2 nicotinic receptors are important for memory function. These receptors may also have a role to play in the development of other aspects of behavior associated with chronic nicotine treatment.

The effects of nicotine on the 13 Hz steady-state visually evoked potential

OBJECTIVES

The high alpha/low beta range of the spontaneous EEG appears to be particularly sensitive to the effects of nicotine. The present study examined the acute effects of nicotine on the topography of the 13 Hz steady-state visually evoked potential (SSVEP).

METHODS

Thirteen moderate smokers participated in a repeated-measures design. The amplitude and latency of the SSVEP elicited by an unstructured sinusoidal 13 Hz flicker following a <0. 05 mg nicotine cigarette were compared to those following a 0.8 mg nicotine cigarette.

RESULTS

The nicotine condition was associated with an increase in the amplitude of the SSVEP, when compared to the placebo condition, and this increase was greatest in central and right parietal regions. The latency of the SSVEP was reduced in the nicotine condition in bilateral frontal and right parietal regions.

CONCLUSIONS

These results are similar to the effects of nicotine seen in studies examining spontaneous EEG, and are consistent with other studies indicating that the 13 Hz SSVEP indexes brain electrical activity in the high alpha/low beta range. The findings are discussed in terms of possible functional significance of nicotine-induced cortical activation in this frequency range.

Genetic dissection of nicotine-related behaviour: a review of animal studies

Nicotine has a broad spectrum of behavioural effects. A considerable body of data has emerged indicating genetic factors regulate the behavioural effects of nicotine. Experimental genetic techniques have been invaluable in generating knowledge on the interrelationship of genetic factors and behavioural responsiveness to nicotine. Three different approaches have been invoked to explore the relationship of genetic factors to response to nicotine. Firstly, the classical genetic tool of inbred lines has been exploited to delineate genetic influences in the effects of nicotine. Secondly, the use of selectively bred lines has been profitably employed to reveal genetic differences in behavioural responses, such as cognition and exploration, to nicotine. These approaches have also provided useful information on the contribution of genetic factors influencing nicotinic receptors function. Finally the molecular genetic technique of gene targetting to create mice with null mutations of specific genes in the central nervous system, which is having a tremendous impact in drug addiction research, has also been employed to gain insight into the molecular and cellular basis of nicotine action. These techniques are proving to be invaluable in dissecting the role of different subunits of the nicotinic acetylcholine receptors on behaviour. This paper provides a survey of the animal studies that have used the above mentioned techniques to gain insight into genetic basis of the behavioural effects of nicotine.

Increased acetylcholine release in the rat medial prefrontal cortex during performance of a visual attentional task

Recent studies have suggested a functional link between cortical cholinergic output and attentional task demands, whereby acetylcholine (ACh) release is regulated according to the outcome of ongoing behaviour. To explore this hypothesis we measured ACh efflux in the rat medial prefrontal cortex (mPFC) during between-session manipulations of the cognitive demands of an attentional task. Rats were trained to detect visual stimuli in a five-choice serial reaction time task (5-CSRTT) which involves sustained and divided attention. Following habituation to tethering and implantation with a microdialysis probe in the mPFC, rats were tested in the 5-CSRTT for three consecutive days, with different lengths of stimulus duration. During performance of the 5-CSRTT we measured robust, reproducible, task-related increases in ACh release in the mPFC across all sessions. Variations of the stimulus duration from the standard 0.5 s resulted in the predicted behavioural effects (reductions and increases in choice accuracy with 0.25 s and 5 s, respectively), but there was no evidence of either greater changes in ACh release in the more demanding condition or smaller changes in the less demanding condition. By contrast, in the session with 5-s stimulus duration there was a positive correlation between prefrontal cortical ACh efflux and the total number of trials completed. In summary, the present study shows that ACh efflux in the rat mPFC is increased during performance of a 5-CSRTT, but has found no evidence to support a specific relationship between cholinergic cortical output and attentional performance.

Attention as a target of intoxication: insights and methods from studies of drug abuse

A symposium was convened to discuss recent developments in the assessment of attention and the effects of drugs and toxic chemicals on attention at the 17th annual meeting of the Behavioral Toxicology Society on May 1, 1999, in Research Triangle Park, NC. Speakers addressed issues including the methodology of assessing cognitive function, the neurobiology of specific aspects of attention, the dual roles of attention as a target of intoxication and as a mediating variable in the development of addiction to psychoactive drugs, the changes in attention that accompany neuropsychological disorders of schizophrenia, senile dementia of the Alzheimer type and attention deficit hyperactivity disorder, and potential therapies for these disorders. This article provides an overview of the objectives of the symposium, followed by summaries of each of the talks given.

On neurodegenerative diseases, models, and treatment strategies: lessons learned and lessons forgotten a generation following the cholinergic hypothesis

Life's Journey If life is indeed a journey, then poetry must be the map that reveals all its topographic possibilitiesellipsis while science is the compass that keeps us from getting lost. -R. T. Bartus, Simple Words for Complex Lives, (c) 1998 In the nearly 20 years since the cholinergic hypothesis was initially formulated, significant progress has been achieved. Initial palliative treatments for Alzheimer's disease (AD) have proven beneficial and have gained FDA approval, the use of animal models for studying AD and other neurodegenerative diseases has achieved wider acceptance, and important insight into the potential causes and pathogenic variables associated with various neurodegenerative diseases continues to increase. This paper reviews the current status of the cholinergic hypothesis in the context of continuing efforts to improve upon existing treatments for AD and explores the role that animal models might continue to play. Using the benefit of hindsight, particular emphasis is placed on an analysis of the approaches, strategies, and assumptions regarding animal models that proved useful in developing the initial treatments and those that did not. Additionally, contemporary issues of AD are discussed within the context of the cholinergic hypothesis, with particular attention given to how they may impact the further refinement of animal models, and the development of even more effective treatments. Finally, arguments are presented that, despite the deserved enthusiasm and optimism for identifying means of halting the pathogenesis of AD, a clear need for more effective palliative treatments will continue, long after successful pathogenic treatments are available. This review, therefore, focuses on issues and experiences intended to: (a) facilitate further development and use of animal models for AD and other neurodegenerative diseases, and (b) accelerate the identification of newer, even more effective treatments.

Prolonged delivery of nicotine in rats via nasal administration of proliposomes

In order to achieve a prolonged delivery of nicotine to the systemic circulation, proliposomes containing nicotine base (NB-proliposomes) or nicotine hydrogen tartarate salt (NS-proliposomes) and a mixture of powdered nicotine hydrogen tartarate salt and sorbitol (1:9 mixture, MP) were administered intranasally to rats at a nicotine dose of 1 mg/kg. Proliposomes, lipid-sorbitol mixtures that form liposomes upon contact with water, were prepared according to previously established methods, and the mixture (MP) was prepared by mixing NS powder with sorbitol particles (105-350 micrometer in size). Nasal absorption of nicotine from these formulations was very rapid (i.e. less than 10 min was required to reach plasma peaks) and showed substantially sustained plasma nicotine levels compared to saline solutions of NB and NS, and previously reported nasal nicotine sprays. The AUC values from the proliposomes and MP were comparable to those from the saline solutions of NB and NS. However, the mean residence time (MRT) and plasma half-life (T(1/2beta)) of nicotine in the present study were much larger than those from the saline solutions. Thus, a prolonged delivery of nicotine to systemic circulation via the application of proliposomes or MP intranasally appears feasible. NB-proliposomes exhibited the best characteristics in terms of the area under the plasma concentration (AUC), MRT and T(1/2beta) of nicotine, which was followed by NS-proliposomes and MP. Retarded conversion of proliposomes and MP to liposomal emulsions and solution in the nasal cavity seems responsible, in part, for the sustained plasma nicotine concentrations, since the emulsions and solution yielded very short MRT and T(1/2beta) of nicotine. In addition, reduced metabolism to cotinine from the proliposomes and MP was apparently responsible for the sustained plasma nicotine levels. These dosage forms of nicotine appear to circumvent some of the shortcomings of transdermal patches (i.e. long T(max)) and nasal sprays (i.e. short T(1/2beta) and physicochemical instability).

Nicotine enhances stimulus detection performance of middle- and old-aged rats: a longitudinal study

The effects of nicotine on sustained attention were tested in F344xBN male rats when they were chronologically middle and old aged. The rats (n = 11) were trained in a two-choice, stimulus detection task in which a press of one of two levers was reinforced with food, with the correct lever indicated by the position of a briefly illuminated light. They were tested when they were 24-25 and 34-35 months of age (i.e., at 60-68% and 85-95%, respectively of their expected median life span) after saline or 0.1-0.5 mg/kg doses of nicotine (SC). A significant dose-related improvement in percent correct choices and decrease in choice response times was found at both ages, and there was no significant main effect of age or an age by dose interaction. These results support the position that nicotine can enhance attentional processes in rats throughout their life span. Nicotine and other nicotinic agonists may have efficacy in the treatment of disorders such as Alzheimer's disease.

The nature and staging of attention dysfunction in early (minimal and mild) Alzheimer's disease: relationship to episodic and semantic memory impairment

The development of cholinergic therapies for Alzheimer's disease (AD) has highlighted the importance of understanding the role of attentional deficits and the relationship between attention and memory in the earliest stages of the disease. Variability in the tasks used to examine aspects of attention, and in the disease severity, between studies makes it difficult to determine which aspects of attention are affected earliest in AD, and how attentional impairment is related to other cognitive modules. We tested 27 patients in the early stages of the disease on the basis of the MMSE (minimal 24-30 corresponding to minimal cognitive impairment, very mild or possible AD in other classifications; and mild 18-23) on a battery of attentional tests aimed to assess sustained, divided, and selective attention, plus tests of episodic memory, semantic memory, visuoperceptual and visuospatial function, and verbal short-term memory. Although the mildly demented group were impaired on all attentional tests, the minimally impaired group showed a preserved ability to sustain attention, and to divide attention based on a dual-task paradigm. The minimally demented group had particular problems with response inhibition and speed of attentional switching. Examination of the relationship between attention and other cognitive domains showed impaired episodic memory in all patients. Deficits in attention were more prevalent than deficits in semantic memory suggesting that they occur at an earlier stage and the two were partially independent. Impairment in visuoperceptual and visuospatial functions and verbal short-term memory were the least common. Although attention is impaired early in AD, 40% of our patients showed deficits in episodic memory alone, confirming that amnesia may be the only cognitive deficit in the earliest stages of sporadic AD.

Nicotine, brain nicotinic receptors, and neuropsychiatric disorders

Neuronal nicotinic acetylcholine receptors (nAChRs) represent a large family of ligand-gated cation channels with diverse structures and properties. In contrast to the muscular nAChRs, the physiological functions of neuronal nAChRs are not well defined to date. Behavioral studies indicate that brain nAChRs participate in complex functions such as attention, memory, and cognition, whereas clinical data suggest their involvement in the pathogenesis of certain neuropsychiatric disorders (Alzheimer's and Parkinson's diseases, Tourette's syndrome, schizophrenia, depression, etc.). For the majority of these disorders, the use of nAChRs' agonists may represent either a prophylactic (especially for Alzheimer's and Parkinson's diseases) or a symptomatic treatment. The possible mechanisms underlying these beneficial effects as well as the characteristics and potential therapeutic use of new, subtype-selective nAChRs agonists are presented.

Effects of acute nicotine on several operant behaviors in rats

The present experiment assessed nicotine's effects on complex cognitive processes using a variety of operant tasks in rats, including incremental repeated acquisition (IRA) to assess learning; conditioned position responding (CPR) to assess auditory, visual, and position discrimination; progressive ratio (PR) to assess motivation; temporal response differentiation (TRD) to assess timing; and differential reinforcement of low response rates (DRL) to assess timing and response inhibition. Acute nicotine administration (0.0, 0.3, 0.42, 0.56, 0.75, and 1.0 mg/kg, IP) increased IRA and CPR response rate without significantly altering accuracy. Nicotine had similar effects on response rate for PR. For TRD, nicotine had a U-shaped dose effect on accuracy, but failed to shift the mode of the TRD response distribution. For DRL, nicotine reduced accuracy and also shifted the mode of the DRL response initiation time distribution to the left. Nicotine produced an inverted U-shaped dose-effect curve for the overall number of "bursting" responses under both of these schedules. The results of this experiment suggest that nicotine can impair performance on some aspects of cognitive-behavioral performance, while simultaneously improving performance on others.

Nicotine for Alzheimer's disease

BACKGROUND

Nicotine is a cholinergic agonist that acts, not only post-synaptically, but also releases pre-synaptic acetylcholine, and in animal models has been shown to reverse spatial memory decline in rats with lesion in the medial septal nucleus and to show recovery on memory in aged monkeys. Nicotine also has effects on other transmitters like serotonin (5HT), dopamine, or GABA. On the other hand, because nicotine has serious adverse effects, especially concerning cardiovascular risks in elderly people, and also on sleep and behavior, there are several important reasons to conduct a systematic review to assess the clinical efficacy and safety of nicotine in patients with AD.

OBJECTIVES

The aim of this review is to determine whether there is evidence of beneficial effect, and to assess its safety profile, when nicotine is used for Alzheimer's disease.

SEARCH STRATEGY

The Cochrane Controlled Trials Register (Issue 2, 99) was searched using the terms nicotin* and alzheimer*. Three references to trials were deemed suitable for inclusion but are awaiting consideration while the investigators are contacted.

SELECTION CRITERIA

All unconfounded, double-blind, randomized trials in which treatment with nicotine patches or administration of nicotine intravenously was administered for more than a day and compared to placebo in people with Alzheimer's disease.

DATA COLLECTION AND ANALYSIS

As no trials were suitable for inclusion, no data have been extracted or pooled in a meta-analysis. One trial is awaiting consideration, and if included in an update of this review, any available data will be incorporated.

MAIN RESULTS

The poor quality of the trials did not allow any synthesis of results across studies. However, the data available in trials considered are compatible with nicotine producing harm, no change or improvement.

REVIEWER'S CONCLUSIONS

This review is not able to provide reliable evidence that nicotine is a useful treatment for Alzheimer's disease.

Autoradiographic comparison of [3H](-)nicotine, [3H]cytisine and [3H]epibatidine binding in relation to vesicular acetylcholine transport sites in the temporal cortex in Alzheimer's disease

The laminar binding distribution of three nicotinic receptor agonists, [3H](-)nicotine, [3H]cytisine, and [3H]epibatidine, and their relation to the [3H]vesamicol binding, which is known to represent the vesicular acetylcholine transport sites, was performed employing in vitro autoradiography on the medial temporal cortex (Brodmann area 21). Autopsied brain tissue from nine Alzheimer patients and seven age-matched controls were used. The binding pattern of the three nicotinic ligands in the normal cortex was in general similar, showing binding maxima in the cortical layers I, III and V. The binding of [3H](-)nicotine, [3H]cytisine, and [3H]epibatidine was lower in the older controls and more uniform throughout the layers as compared with younger controls. There was a significant age-related decrease in the binding of the three nicotinic ligands within the controls (age range: 58 to 89 years; P[3H](-)nicotine = 0.002, P[3H]epibatidine = 0.010, P[3H]cytisine = 0.037). In the older controls, the [3H]epibatidine binding was much decreased as compared with that of [3H](-)nicotine and [3H]cytisine. This may indicate a higher selectivity of [3H]epibatidine for a nicotinic receptor subtype that is particularly affected by aging. The laminar binding pattern of [3H]vesamicol showed one maximum in the outer cortical layers II/III. The [3H]vesamicol binding did not change with aging. The binding of all ligands was significantly decreased in all layers of the temporal cortex in Alzheimer's disease, but the [3H]vesamicol binding decreased only half as much as the nicotinic receptors. Also, choline acetyltransferase activity was percentually more reduced than [3H]vesamicol binding in Alzheimer's disease. The cortical laminar binding pattern of all 3H-ligands was largely absent in the Alzheimer's disease cases. The less severe loss of vesicular acetylcholine transport sites as compared with the loss of the nicotinic receptors and choline acetyltransferase activity may suggest that vesamicol binding sites might be more preserved in presynaptic terminals still existing and thereby expressing compensatory capacity to maintain cholinergic activity.

Sexually dimorphic effect of an acute smoking manipulation on skin resistance but not on heart-rate during a cognitive verbal task

In a two-day, two-session experiment where smokers male and female college-student subjects worked on a cognitive verbal task during either the first or second day, and on a cognitive spatial task on the second or first day, smoking was manipulated as an acute independent variable by requiring 10+ hours of pre-experimental abstention, and providing a cigarette during the 15-minute rest period between the two sessions. Non-smoker female and male subjects underwent the same experiment, and hence served as controls for the effects of this acute-smoking manipulation. Overall adaptation (decreased arousal) to the experiment was manifested in a significant increase in skin resistance level (SRL) in all subjects, but when this adaptation effect was statistically controlled, there was a significant smokers by sex interaction during the verbal task only, such that SRL was increased by the cigarette in males, but decreased in females. In contrast, the same analysis indicated only a marked increase in heart-rate (HR) due to smoking, which was unaffected either by sex or by whether the task was the verbal or the (easier) spatial one. We interpret the SRL results as reflecting a sex difference in the direction of transient psychological arousal, and discuss it in relation to evidence in the literature based on self reports, and to evidence (based on HR in this study and on blood pressure in other studies) on physiological (cardiovascular) arousal. Key Words: Electrodermal activity, heart-rate, psychological vs. physiological, verbal and spatial cognitive tasks.

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.

Reappraising neurotransmitter-based strategies

A number of observations support the hypothesis that a central deficit in acetylcholine (ACh) may be responsible for the initiation of Alzheimer's disease (AD). For example, cholinergic innervation in AD is reduced in areas of the brain important for processing information. Further, reduced concentrations of choline acetyltransferase (ChAT), the enzyme responsible for the synthesis of ACh, correlate with the number of beta-amyloid senile plaques and cognitive dysfunction in AD patients. Consequently, several strategies to increase cholinergic neurotransmission have been developed, including ACh precursors, ACh release enhancers, cholinesterase (ChE) inhibitors and receptor agonists. Although ChE inhibitors appear to be the most promising, tacrine, the first ChE inhibitor to be registered and approved for the treatment of AD, has significant tolerability problems. Thus, ChE inhibitors with improved side-effect profiles have been developed and subsequently awarded marketing approval. However, in addition to the cholinergic system that is the most severely affected neurotransmitter system in AD, other neurotransmitter systems may be involved (e.g. serotonergic, noradrenergic and glutamatergic). Therefore, bifunctional compounds or combinations of drugs may provide additional therapeutic value.

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.

Expression of the alpha3 nicotinic receptor subunit mRNA in aging and Alzheimer's disease

Changes in the number of high-affinity nicotine binding sites have been widely reported in specific regions of the human brain during aging and in degenerative neurological diseases associated with aging, such as Alzheimer's disease. Nicotinic receptors are highly diverse and a description of the molecular subtypes affected in such conditions has not been achieved to date. To investigate the status of the alpha3 subunit-containing subtypes in such conditions, we assessed by in situ hybridisation the alpha3 mRNA density in the hippocampus, entorhinal cortex and thalamus of Alzheimer's patients and age-matched controls. No significant difference in the expression of the alpha3 mRNA, either qualitative or quantitative, was found between Alzheimer's individuals and controls in any of the analysed areas. This result suggests that the nicotine binding changes occurring in these areas in Alzheimer's patients are not correlated to a variation of the alpha3 mRNA in the same regions. Nevertheless, a negative correlation between the alpha3 mRNA density and the age was observed in the entorhinal cortex of both the Alzheimer's and the normal subjects, suggesting a potentially extensive decay of the alpha3-expressing neurons or loss of alpha3-containing receptors in intact neurons of the entorhinal cortex in the late elderly.

GTS-21, a nicotinic agonist, attenuates multiple infarctions and cognitive deficit caused by permanent occlusion of bilateral common carotid arteries in rats

We examined the effects of GTS-21 [3-(2,4-dimethoxybenzylidene)-anabaseine dihydrochloride], a nicotinic agonist, on histopathological changes of the brain and radial maze learning performance in rats with permanent occlusion of the bilateral common carotid arteries (2VO) and elucidated whether this compound has a protective effect against the neuronal degeneration and spatial cognitive deficit caused by chronic ischemia. Rats were administered GTS-21 (1 and 10 mg/kg, p.o.) or vehicle 24 hr and 30 min before the 2VO operation and then once daily for 2 months after the operation. The 2VO rats given vehicle had multiple infarctions in the cerebral cortex, hippocampus and striatum and rarefaction in the white matter at 2 months after the operation, although the number and distribution of infarctions varied among individual animals. In addition, the 2VO rats given vehicle showed a higher rate of errors in the acquisition trials of the 8-arm radial maze task than sham-operated controls. However, 2VO rats treated with GTS-21 (1 and 10 mg/kg, p.o.) showed significantly decreased neuropathological changes and less errors in the acquisition trials compared to the vehicle-treated 2VO rats. These results indicate that GTS-21 attenuates impairment of spatial cognitive deficit and progressive neuronal degeneration induced by 2VO and suggest that this compound is beneficial for the treatment of neurodegenerative diseases following chronic cerebral hypoperfusion.

Differential changes of nicotinic receptors in the rat brain following ibotenic acid and 192-IgG saporin lesions of the nucleus basalis magnocellularis

The basal forebrain cholinergic neurons are implicated in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD). The nicotinic acetylcholine receptors (nAChRs) have been found to be significantly afflicted in AD. To study the underlying mechanisms for dysfunction of the basal forebrain cholinergic neurons development of suitable animal models is warranted. In this study we investigated the effects of bilateral lesions of the nucleus basalis magnocellularis on nAChRs in the rat brain using the cholinergic system selective immunotoxin 192-IgG saporin and non-selective excitotoxin ibotenic acid. Changes in nAChRs were measured by 3H-cytisine and 3H-epibatidine, two ligands with different selectivity for nAChRs subtypes. In the parietal cortex of ibotenic acid lesioned rates, the choline acetyltransferase activity (ChAT) was decreased by 24% while no changes were detected in the frontal cortex or hippocampus. Similarly, a 40% decrease was observed in the number of nAChRs labelled by 3H-cytisine, but not by 3H-epibatidine, in the parietal cortex, while no changes were found in the frontal cortex or hippocampus. Although the 192-IgG saporin induced lesions reduced the ChAT activity in the frontal cortex, parietal cortex and hippocampus by 77, 50 and 21%, respectively, no changes were observed in the number of nAChRs as studied by 3H-cytisine or 3H-epibatidine. The results indicate a difference in vulnerability of the cortical nAChR subtypes to experimental lesions of the nucleus basalis magnocellularis. The findings in this study suggest that a major portion of the nAChRs might be located on non-cholinergic neurons in the brain.

The effects of nicotine on spatial and non-spatial expectancies in a covert orienting task

The present study examined how nicotine influences shifts of visuo spatial attention in casual smokers at each of three delays after smoking one cigarette: immediately, 1 h and 24 h. Informative peripheral cues were used to exogenously orient attention to the location where an increase or decrease in the size of a peripheral object was most likely to occur. One size change was more likely to occur than the other and the task was choice (expansion/contraction) reaction time. The performance decrement obtained when the target appeared at an uncued location was smallest in sessions run immediately after smoking (when nicotine levels were highest), suggesting that nicotine may increase the ease with which attention can be disengaged from a cued location. This finding confirms previous research which suggests a specific role for the basal forebrain cholinergic system in visual orienting. In contrast, nicotine was not found to affect non-spatial expectancies based on stimulus-response (expansion/contraction) probability. These findings, together with recent converging evidence, strongly support the proposition that different attentional operations are mediated by different neural subsystems.

Conformationally restricted analogues of nicotine and anabasine

A series of conformationally restricted analogues of nicotine has been synthesized and evaluated as agonists of neuronal acetylcholine receptors. Compound 2 (SIB-1663), which selectively activated human recombinant alpha 2 beta 4 and alpha 4 beta 4 nAChRs, was shown to be active in animal models of Parkinson's disease and pain.

Regional distribution of subtypes of nicotinic receptors in human brain and effect of aging studied by (+/-)-[3H]epibatidine

Epibatidine, a potent nicotinic agonist, was used to study the regional distribution of nicotinic acetylcholine receptor binding sites in the human brain. Saturation studies performed in the human temporal cortex with (+/-)-[3H]epibatidine revealed binding to two binding sites with Kd and Bmax values of 0.018 and 4.2 nM, 12.7 and 15.4 fmol/mg protein, respectively. Competition studies with (+/-)-[3H]epibatidine/unlabelled nicotine or [3H]nicotine/unlabelled (+/-)-epibatidine showed binding to two binding sites in the human temporal cortex (Ki=0.16 and 12.6 nM; 0.007 and 0.3 nM, respectively). Similarly, when unlabelled nicotine was used to displace (+/-)-[3H]epibatidine, two binding sites were also revealed in the thalamus and the cerebellum of human brain (Ki=0.065 and 7.7 nM; 0.07 and 12.5 nM, respectively). The regional binding of (+/-)-[3H]epibatidine binding in human brain was somewhat different from that of [3H]nicotine. A proportionally higher binding was observed for (+/-)-[3H]epibatidine in the cerebellum and the thalamus compared to [3H]nicotine, probably reflecting different selectivity to nicotinic receptor subtypes. A marked significant age-related decrease in (+/-)-[3H]epibatidine binding was observed in the frontal and the temporal cortices (-79%, -84%, respectively) of human subjects between 56-85 years of age, which was similar to that of [3H]nicotine (-82%, -79%, respectively). The (+/-)-[3H]epibatidine binding in the cerebellum decreased significantly with age (-77%), while [3H]nicotine binding showed no significant age-related changes in this brain region. The findings indicate that a specifically modulate regional nicotinic receptors in human brain.