Effects of the nicotinic receptor blocker mecamylamine on radial-arm maze performance in rats

The Role of Microglia in Sex- and Region-Specific Blood-Brain Barrier Integrity During Nicotine Withdrawal

Background

Smoking is the largest preventable cause of death and disease in the United States, with <5% of quit attempts being successful. Microglia activation and proinflammatory neuroimmune signaling in reward neurocircuitry are implicated in nicotine withdrawal symptomology. Microglia are integral regulators of blood-brain barrier (BBB) functionality as well; however, whether the effects of nicotine withdrawal on microglia function impact BBB integrity is unknown.

Methods

Mice were treated with chronic nicotine (12 mg/kg/day) and subjected to 48 hours nicotine withdrawal. Regional BBB permeability, together with messenger RNA and protein expression of tight junction proteins, were assessed. PLX5622 chow was used to deplete microglia to evaluate the role of microglia in regulating BBB integrity and nicotine withdrawal symptomology.

Results

Female mice had higher baseline BBB permeability in the prefrontal cortex and hippocampus than males. Nicotine withdrawal further exacerbated the BBB permeability selectively in the prefrontal cortex of females. These effects were concurrent with prefrontal cortex alterations in a subset of tight junction proteins with increased proinflammatory responses following nicotine withdrawal in females. Depletion of microglia via PLX5622 treatment prevented all these molecular effects and attenuated withdrawal-induced anxiety-like behavior in female mice.

Conclusions

These results are the first to show sex differences in regional BBB permeability during nicotine withdrawal. This represents a possible link to both the reduced smoking cessation success seen in women and women's increased risk for smoking-related neurovascular disorders. Furthermore, these findings open an avenue for sex-specific therapeutics that target microglia and BBB dysfunction during nicotine withdrawal in women.

Microglia morphology and proinflammatory signaling in the nucleus accumbens during nicotine withdrawal

Smoking is the largest preventable cause of death and disease in the United States. However, <5% of quit attempts are successful, underscoring the urgent need for novel therapeutics. Microglia are one untapped therapeutic target. While previous studies have shown that microglia mediate both inflammatory responses in the brain and brain plasticity, little is known regarding their role in nicotine dependence and withdrawal phenotypes. Here, we examined microglial changes in the striatum-a mesolimbic region implicated in the rewarding effects of drugs and the affective disruptions occurring during withdrawal. We show that both nicotine and withdrawal induce microglial morphological changes; however, proinflammatory effects and anxiogenic behaviors were observed only during nicotine withdrawal. Pharmacological microglial depletion during withdrawal prevented these effects. These results define differential effects of nicotine and withdrawal on inflammatory signaling in the brain, laying the groundwork for development of future smoking cessation therapeutics.

Interaction of nicotinic acetylcholine receptors with dopamine receptors in synaptic plasticity of the mouse insular cortex

The insular cortex plays essential roles in nicotine addiction. However, much is still unknown about its cellular and synaptic mechanisms responsible for nicotine addiction. We have previously shown that in layer 5 pyramidal neurons of the mouse insular cortex, activation of the nicotinic acetylcholine receptors (nAChRs) suppresses synaptic potentiation through enhancing GABAergic synaptic transmission, although it enhances both glutamatergic and GABAergic synaptic transmission. In the present study, we examined whether dopamine receptors might contribute to the nicotine-induced inhibition of synaptic potentiation. The nicotine-induced inhibition of synaptic potentiation was decreased in the presence of a D1 dopamine receptor antagonist SCH23390 irrespective of the presence of a D2 dopamine receptor antagonist sulpiride, suggesting that D1 dopamine receptors are involved in nicotine-induced inhibition. We also investigated how dopamine receptors might contribute to the nAChR-induced enhancement of glutamatergic and GABAergic synaptic transmission. The nAChR-induced enhancement of GABAergic synaptic transmission was decreased in the presence of SCH23390 irrespective of the presence of sulpiride, whereas that of glutamatergic synaptic transmission was not altered in the presence of SCH23390 and sulpiride. These results suggest that D1 dopamine receptors are involved in the nAChR-induced enhancement of GABAergic synaptic transmission while dopamine receptors are not involved in that of glutamatergic synaptic transmission. These observations indicate that the interaction between nAChRs and D1 dopamine receptors plays critical roles in synaptic activities in layer 5 pyramidal neurons of the mouse insular cortex. These insular synaptic changes might be associated with nicotine addiction.

Cholinergic modulation of spatial learning, memory and navigation

Spatial learning, including encoding and retrieval of spatial memories as well as holding spatial information in working memory generally serving navigation under a broad range of circumstances, relies on a network of structures. While central to this network are medial temporal lobe structures with a widely appreciated crucial function of the hippocampus, neocortical areas such as the posterior parietal cortex and the retrosplenial cortex also play essential roles. Since the hippocampus receives its main subcortical input from the medial septum of the basal forebrain (BF) cholinergic system, it is not surprising that the potential role of the septo-hippocampal pathway in spatial navigation has been investigated in many studies. Much less is known of the involvement in spatial cognition of the parallel projection system linking the posterior BF with neocortical areas. Here we review the current state of the art of the division of labour within this complex 'navigation system', with special focus on how subcortical cholinergic inputs may regulate various aspects of spatial learning, memory and navigation.

Mutually augmenting interactions of dextromethorphan and sazetidine-A for reducing nicotine self-administration in rats

A variety of nicotinic drug treatments have been found to decrease nicotine self-administration. However, interactions of drugs affecting different nicotinic receptor subtypes have not been much investigated. This study investigated the interactions between dextromethorphan, which blocks nicotinic α3β2 receptors as well as a variety of other receptors with sazetidine-A which is a potent and selective α4β2 nicotinic receptor partial agonist with desensitizing properties. This interaction was compared with dextromethorphan combination treatment with mecamylamine, which is a nonspecific nicotinic channel blocker. Co-administration of dextromethorphan (either 0.5 or 5 mg/kg) and lower dose of sazetidine-A (0.3 mg/kg) caused a significant reduction in nicotine SA. With regard to food-motivated responding, 3 mg/kg of sazetidine-A given alone caused a significant decrease in food intake. However, the lower 0.3 mg/kg sazetidine-A dose did not significantly affect food-motivated responding even when given in combination with the higher 5 mg/kg dextromethorphan dose which itself caused a significant decrease in food motivated responding. Interestingly, this higher dextromethorphan dose significantly attenuated the decrease in food motivated responding caused by 3 mg/kg of sazetidine-A. Locomotor activity was increased by the lower 0.3 mg/kg sazetidine-A dose and decreased by the 5 mg/kg dextromethorphan dose. Mecamylamine at the doses (0.1 and 1 mg/kg) did not affect nicotine SA, but at 1 mg/kg significantly decreased food-motivated responding. None of the mecamylamine doses augmented the effect of dextromethorphan in reducing nicotine self-administration. These studies showed that the combination of dextromethorphan and sazetidine-A had mutually potentiating effects, which could provide a better efficacy for promoting smoking cessation, however the strength of the interactions was fairly modest.

A test of the cognitive-enhancing potential of low-dose mecamylamine in healthy non-smokers

RATIONALE

The beneficial effects of nicotinic acetylcholine receptor (nAChR) agonists on cognitive performance have been widely shown. Paradoxically, recent preclinical studies employing extremely low doses of nAChR antagonists have also found cognitive enhancement, perhaps pointing to a novel treatment mechanism for cognitive deficits.

OBJECTIVES

The aim was to test whether low doses of the nAChR antagonist mecamylamine would benefit performance in human volunteers.

METHODS

The study employed a double-blind within-subject design. Over four separate days, healthy adult non-smokers (n = 23) were tested with placebo and three trace doses of mecamylamine (0.25-1 mg, p.o.), adjusted for body weight. Participants performed three computerized tasks: a task of spatial selective attention and stimulus detection, the rapid visual information processing task (RVIPT) taxing sustained attention and working memory, and a change detection short-term memory task. Subjective state and vital signs were assessed repeatedly.

RESULTS

Mecamylamine did not improve performance in any of the tasks. Any trends that were observed instead pointed toward performance impairment. Mecamylamine also had no effects on subjective state or vital signs.

CONCLUSIONS

The present results do not support the hypothesized cognitive-enhancing potential of low doses of mecamylamine. Contrary to preclinical reports, these findings speak against low-dose nAChR antagonism as a novel avenue for treating cognitive deficits.

Effect of acute pesticide exposure on bee spatial working memory using an analogue of the radial-arm maze

Pesticides, including neonicotinoids, typically target pest insects by being neurotoxic. Inadvertent exposure to foraging insect pollinators is usually sub-lethal, but may affect cognition. One cognitive trait, spatial working memory, may be important in avoiding previously-visited flowers and other spatial tasks such as navigation. To test this, we investigated the effect of acute thiamethoxam exposure on spatial working memory in the bumblebee Bombus terrestris, using an adaptation of the radial-arm maze (RAM). We first demonstrated that bumblebees use spatial working memory to solve the RAM by showing that untreated bees performed significantly better than would be expected if choices were random or governed by stereotyped visitation rules. We then exposed bees to either a high sub-lethal positive control thiamethoxam dose (2.5 ng-1 bee), or one of two low doses (0.377 or 0.091 ng-1) based on estimated field-realistic exposure. The high dose caused bees to make more and earlier spatial memory errors and take longer to complete the task than unexposed bees. For the low doses, the negative effects were smaller but statistically significant, and dependent on bee size. The spatial working memory impairment shown here has the potential to harm bees exposed to thiamethoxam, through possible impacts on foraging efficiency or homing.

Blockade of cholinergic transmission elicits somatic signs in nicotine-naïve adolescent rats

High doses of the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine can elicit somatic signs resembling those associated with nicotine withdrawal in nicotine-naïve adult rats. Understanding this phenomenon, and its possible modulation by acute nicotine and age, could inform the use of mecamylamine as both an experimental tool and potential pharmacotherapy for tobacco dependence and other disorders. This study evaluated the ability of high-dose mecamylamine to elicit somatic signs in adolescent rats, and the potential for acute nicotine pretreatment to potentiate this effect as previously reported in adults. Single or repeated injections of mecamylamine (1.5 or 3.0 mg/kg, s.c.) elicited somatic signs in nicotine-naïve adolescents, but this effect was not influenced by 2 h pretreatment with acute nicotine (0.5 mg/kg, s.c.). In an initial evaluation of the effects of age in this model, mecamylamine (2.25 mg/kg, s.c.) elicited somatic signs in nicotine-naïve adolescents and adults. This effect was modestly enhanced following acute nicotine injections in adults but not in adolescents, even when a higher nicotine dose (1.0 rather than 0.5 mg/kg, s.c.) was used in adolescents to account for age differences in nicotine pharmacokinetics. These studies are the first to show that mecamylamine elicits somatic signs in nicotine-naïve adolescent rats, an effect that should be considered when designing and interpreting studies examining effects of high doses of mecamylamine in adolescents. Our findings also provide preliminary evidence that these signs may be differentially modulated by acute nicotine pretreatment in adolescents versus adults.

Decreasing nicotinic receptor activity and the spatial learning impairment caused by the NMDA glutamate antagonist dizocilpine in rats

Nicotinic systems have been shown by a variety of studies to be involved in cognitive function. Nicotinic receptors have an inherent property to become desensitized after activation. The relative role of nicotinic receptor activation vs. net receptor inactivation by desensitization in the cognitive effects of nicotinic drugs remains to be fully understood. In these studies, we tested the effects of the α7 nicotinic receptor antagonist methyllycaconitine (MLA), the α4β2 nicotinic receptor antagonist dihydro-β-erythroidine (DHβE), the nonspecific nicotinic channel blocker mecamylamine and the α4β2 nicotinic receptor desensitizing agent sazetidine-A on learning in a repeated acquisition test. Adult female Sprague-Dawley rats were trained on a repeated acquisition learning procedure in an 8-arm radial maze. MLA (1-4mg/kg), DHβE (1-4mg/kg), mecamylamine (0.125-0.5mg/kg) or sazetidine-A (1 and 3mg/kg) were administered in four different studies either alone or together with the NMDA glutamate antagonist dizocilpine (0.05 and 0.10mg/kg). MLA significantly counteracted the learning impairment caused by dizocilpine. The overall choice accuracy impairment caused by dizocilpine was significantly attenuated by co-administration of DHβE. Low doses of the non-specific nicotinic antagonist mecamylamine also reduced dizocilpine-induced repeated acquisition impairment. Sazetidine-A reversed the accuracy impairment caused by dizocilpine. These studies provide evidence that a net decrease in nicotinic receptor activity can improve learning by attenuating learning impairment induced by NMDA glutamate blockade. This adds to evidence in cognitive tests that nicotinic antagonists can improve cognitive function. Further research characterizing the efficacy and mechanisms underlying nicotinic antagonist and desensitization induced cognitive improvement is warranted.

DRD2/CHRNA5 interaction on prefrontal biology and physiology during working memory

BACKGROUND

Prefrontal behavior and activity in humans are heritable. Studies in animals demonstrate an interaction between dopamine D2 receptors and nicotinic acetylcholine receptors on prefrontal behavior but evidence in humans is weak. Therefore, we hypothesize that genetic variation regulating dopamine D2 and nicotinic acetylcholine receptor signaling impact prefrontal cortex activity and related cognition. To test this hypothesis in humans, we explored the interaction between functional genetic variants in the D2 receptor gene (DRD2, rs1076560) and in the nicotinic receptor α5 gene (CHRNA5, rs16969968) on both dorsolateral prefrontal cortex mediated behavior and physiology during working memory and on prefrontal gray matter volume.

METHODS

A large sample of healthy subjects was compared for genotypic differences for DRD2 rs1076560 (G>T) and CHNRA5 rs16969968 (G>A) on prefrontal phenotypes, including cognitive performance at the N-Back task, prefrontal physiology with BOLD fMRI during performance of the 2-Back working memory task, and prefrontal morphometry with structural MRI.

RESULTS

We found that DRD2 rs1076560 and CHNRA5 rs16969968 interact to modulate cognitive function, prefrontal physiology during working memory, and prefrontal gray matter volume. More specifically, CHRNA5-AA/DRD2-GT subjects had greater behavioral performance, more efficient prefrontal cortex activity at 2Back working memory task, and greater prefrontal gray matter volume than the other genotype groups.

CONCLUSIONS

The present data extend previous studies in animals and enhance our understanding of dopamine and acetylcholine signaling in the human prefrontal cortex, demonstrating interactions elicited by working memory that are modulated by genetic variants in DRD2 and CHRNA5.

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

Role of α7- and α4β2-nAChRs in the neuroprotective effect of nicotine in stress-induced impairment of hippocampus-dependent memory

We have previously shown that nicotine prevents stress-induced memory impairment. In this study, we have investigated the role of α7- and α4β2-nicotinic acetylcholine receptors (nAChRs) in the protective effect of nicotine during chronic stress conditions. Chronic psychosocial stress was induced using a form of rat intruder model. During stress, specific antagonist for either α7-nAChRs [methyllycaconitine (MLA)] or α4β2-nAChRs [dihydro-β-erythroidine (DHβE)] was infused into the hippocampus using a 4-wk osmotic pump at a rate of 82 μg/side.d and 41 μg/side.d, respectively. Three weeks after the start of infusion, all rats were subjected to a series of cognitive tests in the radial arm water maze (RAWM) for six consecutive days or until the animal reached days to criterion (DTC) in the fourth acquisition trial and in all memory tests. DTC is defined as the number of days the animal takes to make no more than one error in three consecutive days. In the short-term memory test, MLA-infused stressed/nicotine-treated rats made similar errors to those of stress and significantly more errors compared to those of stress/nicotine, nicotine or control groups. This finding was supported by the DTC values for the short memory tests. Thus, MLA treatment blocked the neuroprotective effect of nicotine during chronic stress. In contrast, DHβE infusion did not affect the RAWM performance of stress/nicotine animals. These results strongly suggest the involvement of α7-nAChRs, but not α4β2-nAChRs, in the neuroprotective effect of chronic nicotine treatment during chronic stress conditions.

The dopamine D2 receptor gene DRD2 and the nicotinic acetylcholine receptor gene CHRNA4 interact on striatal gray matter volume: evidence from a genetic imaging study

Dopaminergic activity is modulated by acetylcholine with relevance for cognitive functioning, as shown by pharmacological work in a rodent model. In humans, the two transmitter systems' joint effort on cognition has been described on the molecular genetic level: DRD2 rs6277, a single nucleotide polymorphism (SNP) on the dopamine D2 receptor gene and CHRNA4 rs1044396, a SNP on the nicotinic acetylcholine receptor gene interact on visuo-spatial and phonological working memory. The present study uses structural MRI and voxel based morphometry to extend this behavioral work to an intermediate phenotype on the neural level. We found significantly reduced gray matter volume in the right putamen in carriers of the DRD2 C/C and CHRNA4 T/T groups. This genotype combination has previously proven to be beneficial for working memory capacity. Results are in line with the idea that the two genes jointly influence the gating signals from subcortical structures to the prefrontal cortex.

Low-dose nicotine facilitates spatial memory in ApoE-knockout mice in the radial arm maze

Here, we investigated the effects of nicotine on spatial memory in ApoE-knockout (ApoE-KO) and wild-type (WT) mice in a radial arm maze. Training occurred on three consecutive days and the test was performed on day 4, with one trial per day. Then on day 4, animals were administered nicotine (0.1, 0.25, 0.5, and 1.0 mg/kg) or the antagonist of nicotinic receptors (nAChRs) mecamylamine (MEC 2 mg/kg) alone or together with 0.1 mg/kg nicotine. The number of errors in the first eight choices was recorded. The results were that 0.1 mg/kg nicotine decreased errors in ApoE-KO mice, while 0.1 and 0.25 mg/kg nicotine reduced errors in WT mice, indicating that lower doses of nicotine elicit a memory improvement. In contrast, 1.0 mg/kg nicotine increased errors in WT mice, but not in ApoE-KO mice. MEC alone had no noticeable effect on errors in either strain of mice. However, co-administration of 0.1 mg/kg nicotine and MEC increased errors and reduced the effects of nicotine in WT mice, but not in ApoE-KO mice. Our study found a biphasic effect of nicotine in WT mice: it improves spatial memory at lower doses and impairs it at a higher dose. In ApoE-KO mice, nicotine improves memory at a low dose and has no effect at a higher dose, suggesting that the ApoE deficiency may influence the efficacy of nicotine. Moreover, a reversal of nicotinic effects with MEC was seen in WT mice, indicating the likelihood of the involvement of nAChRs in the spatial-memory response to nicotine.

Post-learning REM sleep deprivation impairs long-term memory: reversal by acute nicotine treatment

Rapid eye movement sleep deprivation (REM-SD) is associated with spatial learning and memory impairment. During REM-SD, an increase in nicotine consumption among habitual smokers and initiation of tobacco use by non-smokers have been reported. We have shown recently that nicotine treatment prevented learning and memory impairments associated with REM-SD. We now report the interactive effects of post-learning REM-SD and/or nicotine. The animals were first trained on the radial arm water maze (RAWM) task, then they were REM-sleep deprived using the modified multiple platform paradigm for 24h. During REM-SD period, the rats were injected with saline or nicotine (1mg/kg s.c. every 12h: a total of 3 injections). The animals were tested for long-term memory in the RAWM at the end of the REM-SD period. The 24h post-learning REM-SD significantly impaired long-term memory. However, nicotine treatment reversed the post-learning REM-SD-induced impairment of long-term memory. On the other hand, post-learning treatment of normal rats with nicotine for 24h enhanced long-term memory. These results indicate that post-learning acute nicotine treatment prevented the deleterious effect of REM-SD on cognitive abilities.

Cholinergic receptor subtypes and their role in cognition, emotion, and vigilance control: an overview of preclinical and clinical findings

RATIONALE

The cholinergic system has long been linked to cognitive processes. Two main classes of acetylcholine (ACh) receptors exist in the human brain, namely muscarinic and nicotinic receptors, of which several subtypes occur.

OBJECTIVES

This review seeks to provide an overview of previous findings on the influence of cholinergic receptor manipulations on cognition in animals and humans, with particular emphasis on the role of selected cholinergic receptor subtypes. Furthermore, the involvement of these receptor subtypes in the regulation of emotion and brain electrical activity as measured by electroencephalography (EEG) shall be addressed since these domains are considered to be important modulators of cognitive functioning.

RESULTS

In regard to cognition, the muscarinic receptor subtypes have been implicated mainly in memory functions, but have also been linked to attentional processes. The nicotinic α7 receptor subtype is involved in working memory, whereas the α4β2* subtype has been linked to tests of attention. Both muscarinic and nicotinic cholinergic mechanisms play a role in modulating brain electrical activity. Nicotinic receptors have been strongly associated with the modulation of depression and anxiety.

CONCLUSIONS

Cholinergic receptor manipulations have an effect on cognition, emotion, and brain electrical activity as measured by EEG. Changes in cognition can result from direct cholinergic receptor manipulation or from cholinergically induced changes in vigilance or affective state.

Evidence for the modality independence of the genetic epistasis between the dopaminergic and cholinergic system on working memory capacity

Working memory (WM) is fractionated into systems for visuospatial and phonological information. Recently, it has been shown that the dopamine d2 receptor gene DRD2 and CHRNA4, the gene coding for the nicotinic acetylcholine receptor's alpha4 subunit, interact epistatically on visuospatial WM capacity. In the present study, we show a similar interaction on phonological WM capacity in N=137 healthy subjects genotyped for two single nucleotide polymorphisms (DRD2 rs6277 and CHRNA4 rs1044396). Given the functional independence of the two systems we hypothesize that the genetic interaction targets the central executive which is the common control process for both systems.

The Association between Dopamine DRD2 Polymorphisms and Working Memory Capacity Is Modulated by a Functional Polymorphism on the Nicotinic Receptor Gene CHRNA4

Working memory capacity is extremely limited and individual differences are heritable to a considerable extent. In the search for a better understanding of the exact genetic underpinnings of working memory, most research has focused on functional gene variants involved in the metabolism of the neurotransmitter dopamine. Recently, there has been investigation of genes related to other neurotransmitter systems such as acetylcholine. The potential relevance of a polymorphism located in the gene coding for the alpha4 subunit of the nicotinic acetylcholine receptor (rs#1044396) has been discussed with respect to working memory, but empirical investigations have provided mixed results. However, pharmacological studies in both rodents and humans have shown that the effect of nicotinic agonists on cognitive functions is mediated by dopamine. We therefore hypothesized that such an interaction can be found on a molecular genetic level as well. In order to test this hypothesis, we genotyped 101 healthy subjects for rs#1044396 and three functional polymorphisms on the dopamine d2 receptor gene (rs#1800497, rs#6277, rs#2283265). These subjects performed a visuospatial working memory task in which memory load was systematically varied. We found a significant interaction between rs#1044396 and a haplotype block covering all three dopaminergic polymorphisms on working memory capacity. This effect only became apparent on higher levels of working memory load. This is the first evidence from a molecular genetic perspective that these two neurotransmitter systems interact on cognitive functioning. The results are discussed with regard to their implication for working memory theories and their clinical relevance for treatment of substance abuse and schizophrenia.

Acute nicotine treatment prevents REM sleep deprivation-induced learning and memory impairment in rat

Rapid eye movement (REM) sleep deprivation (SD) is implicated in impairment of spatial learning and memory and hippocampal long-term potentiation (LTP). An increase in nicotine consumption among habitual smokers and initiation of tobacco use by nonsmokers was observed during SD. Although nicotine treatment was reported to attenuate the impairment of learning and memory and LTP associated with several mental disorders, the effect of nicotine on SD-induced learning and memory impairment has not been studied. Modified multiple platform paradigm was used to induce SD for 24 or 48 h during which rats were injected with saline or nicotine (1 mg kg(-1) s.c.) twice a day. In the radial arm water maze (RAWM) task, 24- or 48-h SD significantly impaired learning and short-term memory. In addition, extracellular recordings from CA1 and dentate gyrus (DG) regions of the hippocampus in urethane anesthetized rats showed a significant impairment of LTP after 24- and 48-h SD. Treatment of normal rats with nicotine for 24 or 48 h did not enhance spatial learning and memory or affect magnitude of LTP in the CA1 and DG regions. However, concurrent, acute treatment of rats with nicotine significantly attenuated SD-induced impairment of learning and STM and prevented SD-induced impairment of LTP in the CA1 and DG regions. These results show that acute nicotine treatment prevented the deleterious effect of sleep loss on cognitive abilities and synaptic plasticity.

The role of nicotinic receptors in the amelioration of cholinesterase inhibitors in scopolamine-induced memory deficits

RATIONALE

Nicotine receptors in the brain are closely related with memory amelioration induced by cholinesterase inhibitors.

OBJECTIVE

The present study was undertaken to clarify the role of nicotinic receptors in the ameliorative effects of cholinesterase inhibitors on scopolamine-induced memory deficit.

METHOD

Drug effects were measured using an eight-arm radial maze with four arms baited. Hippocampal theta rhythm during the radial maze task was also recorded with a polygraph system using a telemetric technique.

RESULTS

Scopolamine (0.5 mg/kg, i.p.) caused a spatial memory deficit as well as an increase in hippocampal theta power during radial maze performance. Pilocarpine, nicotine, physostigmine, and donepezil antagonized the effects of scopolamine. The ameliorative effects of nicotine, physostigmine, and donepezil but not piocarpine on memory performance and hippocampal theta activity were reversed by mecamylamine.

CONCLUSION

These results indicate that nicotinic receptors have an essential role in the ameliorative effects of cholinesterase inhibitors in both scopolamine-induced memory deficit and the increase in hippocampal theta activity.

Chronic nicotine restores normal Aβ levels and prevents short-term memory and E-LTP impairment in Aβ rat model of Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by increased deposition of beta-amyloid (Aβ) peptides and progressive cholinergic dysfunction in regions of the brain involved in learning and memory processing. In AD, progressive accumulation of Aβ peptide impairs nicotinic acetylcholine receptor (nAChR) function by an unknown mechanism believed to involve α(7)- and α(4)β(2)-nAChR blockade. The three approaches of the current study evaluated the effects of chronic nicotine treatment in the prevention of Aβ-induced impairment of learning and short-term memory. Rat AD model was induced by 14-day i.c.v. osmotic pump infusion of a 1:1 mixture of 300 pmol/day Aβ(1-40)/Aβ(1-42) or Aβ(40-1) (inactive peptide, control). The effect of nicotine (2 mg/(kg day)) on Aβ-induced spatial learning and memory impairments was assessed by evaluation of performance in the radial arm water maze (RAWM), in vivo electrophysiological recordings of early-phase long-term potentiation (E-LTP) in urethane-anesthetized rats, and immunoblot analysis to determine changes in the levels of beta-site amyloid precursor protein (APP)-cleaving enzyme (BACE), Aβ and memory-related proteins. The results indicate that 6 weeks of nicotine treatment reduced the levels of Aβ(1-40) and BACE1 peptides in hippocampal area CA1 and prevented Aβ-induced impairment of learning and short-term memory. Chronic nicotine also prevented the Aβ-induced inhibition of basal synaptic transmission and LTP in hippocampal area CA1. Furthermore, chronic nicotine treatment prevented the Aβ-induced reduction of α(7)- and α(4)-nAChR. These effects of nicotine may be due, at least in part, to upregulation of brain derived neurotropic factor (BDNF).

Neuromodulation by glutamate and acetylcholine can change circuit dynamics by regulating the relative influence of afferent input and excitatory feedback

Substances such as acetylcholine and glutamate act as both neurotransmitters and neuromodulators. As neuromodulators, they change neural information processing by regulating synaptic transmitter release, altering baseline membrane potential and spiking activity, and modifying long-term synaptic plasticity. Slice physiology research has demonstrated that many neuromodulators differentially modulate afferent, incoming information compared to intrinsic and recurrent processing in cortical structures such as piriform cortex, neocortex, and the hippocampus. The enhancement of afferent (external) pathways versus the suppression at recurrent (internal) pathways could cause cortical dynamics to switch between a predominant influence of external stimulation to a predominant influence of internal recall. Modulation of afferent versus intrinsic processing could contribute to the role of neuromodulators in regulating attention, learning, and memory effects in behavior.

Post-training intrahippocampal infusion of nicotine-bucladesine combination causes a synergistic enhancement effect on spatial memory retention in rats

We previously had shown that bilateral intrahippocampal infusion of 1 microg nicotine (but not 0.5 microg dose) led to an improvement in spatial memory retention in the Morris water maze task in male rats. We also reported that a similar type of bilateral infusion of H89, a protein kinase AII (PKA II) inhibitor, caused a deficit in spatial memory retention. In the present study, we wished to test the hypothesis that intrahippocampal infusion of dibutyryl cyclic AMP (DB-cAMP also called bucladesine), a membrane permeable selective activator of PKA, into the CA1 region can cause an improvement in spatial memory in this maze task. Indeed, bilateral infusion of 10 and 100 microM bucladesine (but not 1 and 5 microM doses) led to a significant reduction in escape latency and travel distance (showing an improvement in spatial memory) compared to the control. Also, bilateral infusion of 0.5 microg nicotine or 1 microM bucladesine alone did not lead to an improvement in spatial memory. However, such bilateral infusion of bucladesine at 1 and 5 microM concentrations infused within minutes after 0.5 microg nicotine infusion improved spatial memory retention. Taken together, our data suggest that intrahippocampal bucladesine infusions improve spatial memory retention in male rats and that bucladesine can interact synergistically with nicotine to improve spatial memory.

Molecular studies on the protective effect of nicotine in adult-onset hypothyroidism-induced impairment of long-term potentiation

We have recently shown that chronic nicotine treatment reverses hypothyroidism-induced learning and memory impairment. Chronic nicotine treatment also reverses the hypothyroidism-induced impairment of long-term potentiation (LTP). Analysis of LTP associated key signaling molecules revealed that chronic nicotine treatment prevented the hypothyroidism-induced reduction of the basal phosphotransferase activity of CaMKII and protein levels of P-CaMKII. In addition, the failure of high frequency stimulation to increase the levels of P-CaMKII in hypothyroid rats was reversed by nicotine treatment, suggesting that the neuroprotective effect of nicotine during hypothyroidism involved activation of CaMKII. Furthermore, chronic nicotine treatment reverses the hypothyroidism-induced elevated phosphatase activity and protein levels of calcineurin, a phosphatase that regulates CaMKII activation. We conclude that the neuroprotective effects of nicotine in adult-onset hypothyroidism may result from restoration of CaMKII and calcineurin activity.

Nicotine reverses adult-onset hypothyroidism-induced impairment of learning and memory: Behavioral and electrophysiological studies

Nicotine alleviates cognitive impairment associated with a variety of health conditions. We examined the effect of chronic nicotine treatment on adult-onset hypothyroidism-induced impairment of learning and memory in rats. Hypothyroidism was induced by surgical removal of thyroid glands (thyroidectomy). One month later, chronic nicotine treatment (1 mg/kg sc, twice/day) was instituted for 4-6 weeks. Test of hippocampus-dependent spatial learning and memory in the radial arm water maze showed that hypothyroidism impaired learning as well as short-term and long-term memory retention. Chronic nicotine treatment reversed the hypothyroidism-induced learning and memory impairment. In normal rats, chronic nicotine treatment had no effect on learning and memory. Extracellular recordings from the CA1 region of anesthetized hypothyroid rats showed severe reduction of both early-phase and late-phase long-term potentiation (LTP) magnitude, which was reversed in nicotine-treated hypothyroid rats. These results show that chronic nicotine treatment prevents hypothyroidism-induced impairment of spatial cognition and LTP.

Chronic but not acute nicotine treatment reverses stress-induced impairment of LTP in anesthetized rats

Stress impairs long-term potentiation (LTP) and is a major cause for starting or increasing tobacco smoking. We have previously shown that chronic concurrent nicotine treatment prevents stress-induced LTP impairment. Nicotine reduces stress-induced impairment of LTP, probably, through activation of nicotinic acetylcholine receptors in the hippocampus. Herein, we investigated the effects of acute and chronic nicotine treatments on the chronic-stress-induced impairment of LTP in area CA1 of the hippocampus of urethane-anesthetized rats. Extracellular in vivo recording from the hippocampal area CA1 showed that pre-treatment with nicotine (1 mg/kg; sc twice/day for 2 weeks prior to stress) protected LTP from the inhibitory effect of subsequent chronic psychosocial stress (4 additional weeks concurrently with nicotine). In another series of experiments, 2 weeks of psychosocial stress was followed by 4 weeks of nicotine treatment concurrently with continuing stress. Nicotine treatment reversed established stress-induced impairment of LTP. However, acute nicotine treatment of rats (a single dose of 1 mg/kg; sc.) did not reverse chronic-stress-induced impairment of LTP. The results show that the impairment of LTP during chronic stress can be blocked by chronic, but not acute, nicotine treatment.

Low-dose mecamylamine improves learning of rats in the radial-arm maze repeated acquisition procedure

The nicotinic antagonist mecamylamine has been widely shown to cause cognitive impairment. However, these effects are mainly seen with high doses. There have been scattered findings that low doses of mecamylamine can have the opposite effect. This may be due to opposite effects of low doses of mecamylamine. In the current study, an extensive dose-effect function of mecamylamine was characterized in the low-dose range. Adult female Sprague-Dawley rats were trained on a repeated acquisition procedure on an automated 8-arm radial maze. Three of the eight arms were designated as correct for any particular session. Five trials per session were run. The number of errors per trial to find the three correct arms was determined. The rats were trained on the repeated acquisition procedure for at least 18 sessions at which time they showed reliable learning each session. Then, the effect of low doses of mecamylamine between 0 and 1 mg/kg were assessed in a repeated measures counterbalanced design. This dose range of mecamylamine did not affect performance on the first trial when the rats were naïve to the array to be learned. On trials 2-5 a significant (p<.025) quadratic dose-effect function was seen over this dose range. The most substantial effect was seen with 0.125 mg/kg of mecamylamine, which caused a significant (p<.05) improvement relative to the saline control condition. The effect diminished with increasing mecamylamine doses and with the 1 mg/kg dose choice accuracy was back to control levels. This study showed that low doses of mecamylamine can effectively improve learning. A U-shaped dose-effect curve was documented. This suggests possible low-dose nicotinic antagonist lines of treatment for cognitive impairment.

Chronic psychosocial stress-induced impairment of hippocampal LTP: possible role of BDNF

Electrophysiological recording reveals that chronic nicotine treatment prevents stress-induced impairment of long-term potentiation (LTP) in the CA1 region of the hippocampus of anesthetized rats. We investigated the molecular mechanism of this action of nicotine in the CA1 region. Immunoblot analysis showed that chronic nicotine treatment (1 mg/kg, 2 times/day) normalized the stress-induced decrease in the basal levels of BDNF, CaMKII (total and phosphorylated; P-CaMKII), and calmodulin. Additionally, nicotine reversed the stress-induced increase in calcineurin basal levels. Chronic nicotine treatment also markedly increased the basal levels of BDNF in naïve rats. Furthermore, high-frequency stimulation (HFS), which increased the levels of P-CaMKII in control as well as nicotine-treated stressed rats, failed to increase P-CaMKII levels in untreated stressed rats. Compared to unstimulated control, the levels of both total CaMKII and calcineurin were increased after HFS in all groups including the stressed, but no changes were detected after HFS in the levels of BDNF and calmodulin. These results indicate that normalization by nicotine of the stress-induced changes in the levels of signaling molecules including BDNF may contribute to the recovery of LTP.

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 improves delayed recognition in schizophrenic patients

RATIONALE

Nicotine has been shown to enhance some aspects of memory, attention and cognition in normal subjects and in some patient populations such as Alzheimer's and Parkinson's disease groups.

OBJECTIVES

Memory disorders are consistently observed in schizophrenic patients, so it is of interest to determine whether nicotine might improve memory performance in these patients.

METHODS

Delayed recognition was assessed using yes/no recognition of visuospatial designs. Working memory was assessed in a delayed match-to-sample paradigm using unfamiliar faces. Nicotine (1.0 mg delivered via nasal spray) was administered to schizophrenic patients and normal volunteers prior to testing in the nicotine condition. Results were compared to a baseline condition in which no nicotine was given.

RESULTS

On both tasks, normal volunteers performed better overall than schizophrenic patients. Significant improvement following nicotine administration was obtained only on the delayed recognition task and only for the subset of schizophrenic patients who were smokers. This improvement reflected a reduction in false alarm rates in the nicotine condition; hit rates were unaffected by nicotine.

CONCLUSIONS

These results suggest that nicotine enhances delayed recognition memory in schizophrenic patients who smoke, but that similar performance enhancement is not observed for working memory.

Norleucine1-Angiotensin IV alleviates mecamylamine-induced spatial memory deficits

The brain angiotensin AT4 receptor subtype has been implicated in cognitive processing. We initially established that intracerebroventricular administration of the nAChR-antagonist mecamylamine (mec) interfered with spatial memory performance in male Sprague-Dawley rats. Next we demonstrated that mec-induced deficits in spatial memory were overcome by the AT4 receptor-agonist Norleucine1-Angiotensin IV (Nle1-Ang IV). Nle1-Ang IV could not, however, compensate for spatial learning impairments precipitated by both mec and the mAChR-antagonist scopolamine. These findings support the importance of the AT4 receptor in cognitive processing and suggest that the ability of Nle1-Ang IV to improve spatial memory deficiencies may be dependant upon the brain cholinergic system.

Acetylcholinesterase induces neuronal cell loss, astrocyte hypertrophy and behavioral deficits in mammalian hippocampus

Previous studies have demonstrated that acetylcholinesterase (AChE) promotes the assembly of amyloid-beta-peptides into neurotoxic amyloid fibrils and is toxic for chick retina neuronal cultures and neuroblastoma cells. Moreover, AChE is present in senile plaques in Alzheimer's disease (AD) brains. Here we have studied the effect of AChE on astrocytes and hippocampal neurons in vivo. Morphological as well as behavioral disturbances were analyzed after intrahippocampal injection of AChE. Rats were trained in the Morris water maze and assayed for behavioral parameters. Neuronal cell loss was found in the upper leaf of the dentate gyrus in rats injected with AChE in comparison with control animals. Glial fibrillary acidic protein immunoreactivity showed astrocytic hypertrophy and the magnitude of the response was associated with neuronal cell loss. Behavioral results show that injection of AChE produces cognitive impairment demonstrated by an altered water maze performance including (i) a higher escape latency score, (ii) a decreased spatial acuity and (iii) a shorter time of swimming in the platform quadrant. These findings indicate that a local increment in neuronal AChE concentration at the mammalian hippocampus, such as those present in amyloid deposits, may play a role in triggering neuropathological and behavioral changes such as those observed in AD brains.

Residential maze as a task for testing rats' maze learning ability: effects of hippocampal lesions and cholinergic receptor antagonists

We investigated whether rats can learn mazes by a procedure in which rats were left in the maze (residential maze) in groups for 1 h a day. Water and food locations, which served as the start and goal boxes respectively in the test trial, were at the opposite ends of the maze. On the test trial conducted everyday before the residence period, animals put in the start box showed a significant decrease of the error response into the blind alleys and running time to reach the goal box. Systemic administration of scopolamine (0.25, 0.5 mg/kg), a muscarinic receptor antagonist, dose-dependently increased the number of errors, but mecamylamine, a nicotinic receptor antagonist, had no effect. Bilateral hippocampal lesions retarded both the acquisition and retention of this maze learning. The results suggest that this residential maze procedure is useful for testing maze learning ability in rats.

Genetically dystrophic mdx/mdx mice exhibit decreased response to nicotine in passive avoidance

mdx mice are considered as a genetic homologous of human Duchenne muscular dystrophy. Recent evidence demonstrates that in mouse sympathetic ganglion dystrophin is involved in the stabilization of nicotinic acetylcholine receptor clusters. The purpose of this study was to verify possible effects of dystrophin alterations at the central level. This was assessed by evaluating the response to nicotine administration in mdx and wild-type mice. Thus the effects of post-training nicotine administrations (0.1, 0.25 and 0.5 mg/kg) were tested in mice subjected to a passive avoidance memory task, that measures the ability of mice to remember on test day a shock received 24 h before. Nicotine enhanced memory in wild-type as well as in mdx mice. However, the doses needed to increase memory in mdx were higher than in wild-type. These results are discussed in terms of possible functional changes in central nicotinic acetylcholine receptor in mdx mice.

Interaction between the cholinergic system and CRH in the modulation of spatial discrimination learning in mice

Both cholinergic and CRH systems have been linked to cognitive processes such as learning and memory, and neuroanatomical as well as neurochemical evidence suggests important interactions between these two systems. Moreover, recent reports of pro-mnestic effects of CRH open the possibility that CRH could have beneficial effects in animals with cholinergic dysfunction. In a first experiment, spatial discrimination of C57BL/6 mice treated with various doses of scopolamine (0.5--2.0 mg/kg IP) was tested in a two-choice water maze task. Scopolamine, but not methylscopolamine, impaired accuracy and decreased responsivity. In contrast, similar doses of the nicotinic antagonist mecamylamine had no effect on choice accuracy but altered responsivity, as indicated by increased errors of omission and a reduction in swim speed during early experimental stages. ICV CRH (0.5--1.0 microg) also failed to significantly affect accuracy, but a strong tendency was observed to impair percentage correct responses. Measures of responsivity, such as errors of omission, choice latency and distance traveled, and of thigmotaxis were not significantly affected by CRH. However, initial swim speed was reduced by the peptide. Combined treatment with scopolamine (0.5 mg/kg IP) and CRH (0.5 microg ICV) had only mild, and primarily independent, effects, but overall suggested that concomitant blockade of muscarinic receptors and activation of the CRH system would rather act synergistically to disrupt spatial discrimination learning. Synergistic effects were also observed when animals receiving a combination of mecamylamine (2.0 mg/kg IP) and CRH (0.5 microg ICV) were tested, both in terms of responsivity and thigmotaxis, and there was limited evidence that part of these effects were potentiating. Thus, the cholinergic and CRH systems interact in the modulation of learning, but CRH, contrary to prediction, worsens the impairment caused by cholinergic blockade.

What have we learned about cognition in myasthenia gravis?: a review of methods and results

Most individuals with myasthenia gravis (MG) complain of cognitive impairment, but empirical studies of cognition in MG have produced mixed results. In the present review, we critically examined the methodology and results of previous studies that investigated cognition in MG. Results from our review revealed that none of the studies met at least 50% of criteria under review. The most common shortcomings of previous studies included small sample size, no exclusion for visual difficulties in patients, inadequate assessment of mood, and poor control for prednisone use. Despite these methodological difficulties, mild impairments on measures of learning have been identified. These findings need to be replicated with adequate control of potential confounds before any conclusions can be made regarding cognition in this disease. Suggestions for design of future studies are provided.

Nicotine improves Morris water task performance in rats given medial frontal cortex lesions

The object of this study was to investigate whether nicotine would improve cognitive impairments produced by medial frontal cortex lesions in rats behaviorally tested on the Morris water task (MWT). Rats were assigned to either a lesion or sham group. In the lesion group, animals were given vehicle (peanut oil) treatment or treatment with nicotine for 11 consecutive days before, after, or before and after a medial frontal cortex lesion. Additionally, a sham group was included that was given vehicle both before and after the lesion. Results showed that lesioned rats receiving pre- or post-operative nicotine treatment demonstrated improved Morris task acquisition performance relative to the lesioned group given the vehicle, although a deficit was shown relative to shams. On the probe trial, rats that received a pre- and post-treatment of nicotine demonstrated performance equivalent to shams and had significantly better performance than rats that received nicotine treatment before the lesion and lesioned animals treated with the vehicle. These results demonstrate that nicotine has therapeutic effects in rats that have received cortical injury.

Nicotinic treatment for degenerative neuropsychiatric disorders such as Alzheimer's disease and Parkinson's disease

Nicotinic systems play an important role in the neural basis of working memory and attention. Recent progress in understanding of the structure, function, and distribution of central nervous system (CNS) nicotinic receptors and their pharmacology has opened up new possibilities for novel CNS therapeutics with nicotinic agents. In this paper, we review the theoretical justification and the experimental evidence supporting these developments. We focus on the applications of nicotinic agonists in CNS disorders that are degenerative in nature, namely Parkinson's disease and Alzheimer's disease. We suggest that there is considerable potential for therapeutic applications in the near future. Clinically, two major issues remain: (a) the selectivity of effects, that is, developing compounds which are selective in producing improvement in cognition, motor function, attention, or pain without significant side-effects; and (b) the realistic likelihood of long-term improvements in everyday functioning in people who have degenerative diseases.

Development of nicotinic drug therapy for cognitive disorders

Nicotine, as well as other nicotinic drugs, may provide useful therapeutic treatment for a variety of cognitive impairments including those found in Alzheimer's disease, schizophrenia and attention deficit hyperactivity disorder (ADHD). We have found that nicotine skin patches significantly improve attentional performance in people with these disease states as well as normal nonsmoking adults. Animal models are critical for determining the neurobehavioral bases for nicotinic effects on cognitive function. We have found in lesion and local infusion studies with rats that the hippocampus is an important substrate for nicotinic effects on working memory function. Both alpha7 and alpha4beta2 nicotinic receptors in the hippocampus are involved. Further work has investigated the relationship of nicotinic systems with dopaminergic and glutaminergic systems in the basis of cognitive function. Nicotine has proven to be a useful prototypic compound for the family of nicotinic compounds. It produces cognitive improvements in both animal models and clinical populations. Recent work with more selective nicotinic receptor agonists and antagonists in animal models is providing important information concerning the neural mechanisms for nicotinic involvement in cognitive function and opening avenues for development of safe and effective nicotinic treatments for clinical use.

Rivastigmine, a brain-selective acetylcholinesterase inhibitor, ameliorates cognitive and motor deficits induced by closed-head injury in the mouse

The effects of Rivastigmine, a novel centrally-acting anticholinesterase agent, were evaluated on cerebral edema, neurological and motor deficits, and impairment of spatial memory induced in mice by closed-head injury (CHI). Severe injury was induced in the left hemisphere of mice under ether anesthesia. Rivastigmine (1 or 2 mg/kg) or saline (10 ml/kg) was injected SC 5 min later. Rivastigmine (2 mg/kg) reduced cerebral edema by at least 50% (p < 0.01), 24 h after CHI and accelerated the recovery of motor function 7 and 14 days after CHI. Control mice (n = 24), previously trained to find the goal platform in a Morris water maze failed to recall or relearn its position for at least 11 days post-injury. Those given a single injection of Rivastigmine (2 mg/kg) regained their pre-test latencies by the third day after CHI. The neuroprotective effects of Rivastigmine on brain edema, neurological and motor function, and performance in the Morris water maze were completely antagonized by simultaneous SC injection of either scopolamine (0.5 mg/kg) or mecamylamine (2.5 mg/kg). The antagonists alone had no significant effect on any of these parameters. These data show that the reduction by Rivastigmine of the immediate and long-term sequelae of brain injury are mediated by increased cholinergic activity at both muscarinic and nicotinic receptors.

Chronic haloperidol administration does not block acute nicotine-induced improvements in radial-arm maze performance in the rat

Nicotine has been found to improve cognitive performance in a variety of tasks including the radial maze. Nicotine has also been shown to promote the release of a variety of neurotransmitters including dopamine (DA). DA has been found to be important for nicotine's reinforcing effects. DA involvement with nicotine's cognitive effects is unclear. In the current study, the effects of acute nicotine injections (0, 0.1, 0.2, or 0.4 mg/kg) were examined on radial-arm maze performance in rats given chronic infusions the DA antagonist haloperidol (0, 0.2, or 0.6 mg/kg/day). Chronic haloperidol infusion was not found to attenuate the memory improvement caused by acute nicotine injection. In fact, the dose-related nicotine-induced memory improvement was clearer in the haloperidol-treated groups than in controls. This is similar to the effect of nicotine we saw in human subjects given chronic doses of haloperidol. Our previous studies demonstrated significant nicotinic-DA interactions with regard to memory function. The current results suggest that in the DA-nicotinic relationship DA stimulation is not necessary for the memory improvement caused by nicotine.

Mecamylamine-induced impairment of acquisition and retrieval of olfactory conditioning in the honeybee

Mecamylamine, a nicotinic receptor antagonist, was injected into the honeybee brain haemolymph. The effects of the drug were investigated on Pavlovian conditioning of the proboscis extension reflex. The conditioned response was acquired after a one-trial learning session, consisting of an olfactory-conditioned stimulus combined with a gustatory antennal unconditioned stimulus. The drug was injected at different times before or after the learning session in order to dissociate its effects on acquisition, consolidation and retrieval processes. The performance was evaluated in short-delayed recall tasks. To control potential effects on sensory-motor activity, the effects of the drug were also investigated on sensory processes (through olfactory and gustatory functions) and on motor processes of proboscis extension. The results of conditioning experiments showed that pretrial injection induced a decrease of retention performance 1 h after the learning trial. Mecamylamine injected 20 min after the learning session induced a time-dependent impairment of retention performance, as has been shown by the performance level registered from 10 to 80 min after injection. A 5-min post-trial injection had no effect on retention performance. Control experiments did not reveal any effect of mecamylamine on the response reflex of proboscis extension and on responsiveness to olfactory stimuli (geraniol, lavender and vanillin). The absence of effects on sensory perception combined with the amnestic effect induced by pre- or late post-trial injections lead us to conclude that mecamylamine specially impaired acquisition and retrieval processes. The involvement of nicotinic-like receptors in these processes is discussed.

Nicotinic, muscarinic and dopaminergic actions in the ventral hippocampus and the nucleus accumbens: effects on spatial working memory in rats

Acetylcholine (ACh) systems have been widely shown to be important for memory. In particular, ACh hippocampal neurons are critical for memory formation, though ACh innervation of other areas such as the nucleus accumbens may also be important. There has also been increasing interest in ACh and dopaminergic (DA) interactions with regard to short-term spatial memory. In a series of studies, we have found that ACh and DA agonists and antagonists given systemically interact to influence memory. The critical neural loci of these interactions are not currently known. In the present study, we used local infusion techniques to examine the role of ACh and DA transmitter systems in the nucleus accumbens and the ventral hippocampus on radial-arm maze (RAM) working memory performance. Into the nucleus accumbens of rats, we infused the nicotinic ACh agonist nicotine, the nicotinic ACh antagonist mecamylamine, the DA agonist apomorphine, or the DA antagonist haloperidol. Into the ventral hippocampus, we infused nicotine, mecamylamine, the muscarinic ACh agonist pilocarpine, or the muscarinic ACh antagonist, scopolamine. The nicotinic ACh and DA interaction was tested by a hippocampal infusion of mecamylamine alone or together with the DA D2 agonist quinpirole given via subcutaneous injection. The results confirmed that both nicotinic and muscarinic ACh receptors in the ventral hippocampus play a significant role in spatial working memory. Blockade of either nicotinic or muscarinic ACh receptors caused significant impairments in RAM choice accuracy. However, infusion of either nicotinic or muscarinic agonists failed to improve choice accuracy. The interaction of DA D2 systems in different with hippocampal nicotinic blockade than with general nicotinic blockade. Systemic administration of quinpirole potentiated the amnestic effect of mecamylamine infused into the ventral hippocampus, whereas it was previously found to reverse the amnestic effect of systemically administered mecamylamine. In contrast to the significant effects of mecamylamine in the hippocampus, no effects were found after infusion into the nucleus accumbens. Nicotine also was not found to have a significant effect on memory after intra-accumbens infusion. Neither the DA agonist apomorphine nor the DA antagonist haloperidol had a significant effect on memory after infusion into the nucleus accumbens. This study provides support for the involvement of nicotinic and muscarinic receptors in the ventral hippocampus in memory function. Ventral hippocampal nicotinic systems have significant interactions with D2 systems, but these differ from their systemic interactions. In contrast, nicotinic ACh and DA systems in the nucleus accumbens were not found in the current study to be important for working memory performance in the RAM.