Selective nicotinic receptor antagonists: effects on attention and nicotine-induced attentional enhancement

Neuropharmacological Potential of Diterpenoid Alkaloids

This study provides a narrative review of diterpenoid alkaloids (DAs), a family of extremely important natural products found predominantly in some species of Aconitum and Delphinium (Ranunculaceae). DAs have long been a focus of research attention due to their numerous intricate structures and diverse biological activities, especially in the central nervous system (CNS). These alkaloids originate through the amination reaction of tetra or pentacyclic diterpenoids, which are classified into three categories and 46 types based on the number of carbon atoms in the backbone structure and structural differences. The main chemical characteristics of DAs are their heterocyclic systems containing β-aminoethanol, methylamine, or ethylamine functionality. Although the role of tertiary nitrogen in ring A and the polycyclic complex structure are of great importance in drug-receptor affinity, in silico studies have emphasized the role of certain sidechains in C13, C14, and C8. DAs showed antiepileptic effects in preclinical studies mostly through Na⁺ channels. Aconitine (1) and 3-acetyl aconitine (2) can desensitize Na⁺ channels after persistent activation. Lappaconitine (3), N-deacetyllapaconitine (4), 6-benzoylheteratisine (5), and 1-benzoylnapelline (6) deactivate these channels. Methyllycaconitine (16), mainly found in Delphinium species, possesses an extreme affinity for the binding sites of α7 nicotinic acetylcholine receptors (nAChR) and contributes to a wide range of neurologic functions and the release of neurotransmitters. Several DAs such as bulleyaconitine A (17), (3), and mesaconitine (8) from Aconitum species have a drastic analgesic effect. Among them, compound 17 has been used in China for decades. Their effect is explained by increasing the release of dynorphin A, activating the inhibitory noradrenergic neurons in the β-adrenergic system, and preventing the transmission of pain messages by inactivating the Na⁺ channels that have been stressed. Acetylcholinesterase inhibitory, neuroprotective, antidepressant, and anxiolytic activities are other CNS effects that have been investigated for certain DAs. However, despite various CNS effects, recent advances in developing new drugs from DAs were insignificant due to their neurotoxicity.

Dissociating the involvement of muscarinic and nicotinic cholinergic receptors in object memory destabilization and reconsolidation

The content of long-term memory is neither fixed nor permanent. Reminder cues can destabilize consolidated memories, rendering them amenable to change before being reconsolidated. However, not all memories destabilize following reactivation. Characteristics of a memory, such as its age or strength, impose boundaries on destabilization. Previously, we demonstrated that presentation of salient novel information at the time of reactivation can readily destabilize resistant object memories in rats and this form of novelty-induced destabilization is dependent upon acetylcholine (ACh) activity at muscarinic receptors (mAChRs). In the present study, we sought to determine if this same mechanism for initiating destabilization of resistant object memories is present in mice and further expand our understanding of the mechanisms through which ACh modulates object memory destabilization by investigating the role of nicotinic receptors (nAChRs). We provide evidence that in mice mAChRs are necessary for destabilizing object memories that are readily destabilized and those that are resistant to destabilization. Conversely, nAChRs were found to be necessary only when memories are readily destabilized. We then investigated the role of both receptors in the reconsolidation of destabilized object memory traces and determined that nAChRs, but not mAChRs, are necessary for object memory reconsolidation. Together, these results suggest that nAChRs may play a more selective role in the re-storage of object memories following destabilization and that ACh acts through mAChRs to act as an override signal to initiate destabilization of resistant object memories following reactivation with novelty. These findings expand our current understanding of the role of ACh in the dynamic storage of long-term memory.

Cross-species evidence that nicotine widens the attentional window

RATIONALE

The ability to spread attention over items or locations is as important for everyday functioning as the ability to focus narrowly. Little is known about neuronal processes involved in broad monitoring, but indirect evidence suggests a role of nicotinic acetylcholine receptors (nAChRs).

OBJECTIVE

The present study tested whether the prototypical nAChR agonist nicotine enhances the ability of humans and rodents to maintain a broad attentional window.

METHODS

Fifty-three never-smokers wearing a nicotine (7 mg/24 h) or placebo patch performed an attention task requiring detection of stimuli presented randomly in one of four peripheral locations, with a central cue predicting the target location or indicating the need to spread attention over all locations. Nineteen rats performed the 5-choice serial reaction time task requiring detection of stimuli presented randomly in a horizontal array of five locations. Performance after nicotine (0.1 and 0.2 mg/kg) or vehicle administration was analyzed as a function of target location eccentricity.

RESULTS

In human subjects, nicotine caused greater reaction time reduction when all locations were monitored than when a single location was cued. In rats, nicotine attenuated the decline in stimulus detections and the increase in omission errors with greater target location eccentricity.

CONCLUSIONS

The findings represent cross-species evidence that nAChR agonism facilitates the ability to spread attention broadly. This suggests that nAChR hypofunction may be central to broad monitoring deficits as seen, for example, in schizophrenia. The homology of findings between the rodent and the human paradigm contributes to validating a translational strategy for treatment development.

Effects of nicotinic antagonists on working memory performance in young rhesus monkeys

Acetylcholine plays a pivotal neuromodulatory role in the brain, influencing neuronal activity and cognitive function. Nicotinic receptors, particularly α7 and α4β2 receptors, modulate firing of dorsolateral prefrontal (dlPFC) excitatory networks that underlie successful working memory function. Minimal work however has been done examining working memory following systemic blockade of nicotinic receptor systems in nonhuman primates, limiting the ability to explore interactions of other neuromodulatory influences with working memory impairment caused by nicotinic antagonism. In this study, we investigated working memory performance after administering three nicotinic antagonists, mecamylamine, methyllycaconitine, and dihydro-β-erythroidine, in rhesus macaques tested in a spatial delayed response task. Surprisingly, we found that no nicotinic antagonist significantly impaired delayed response performance compared to vehicle. In contrast, the muscarinic antagonist scopolamine reliably impaired delayed response performance in all monkeys tested. These findings suggest there are some limitations on using systemic nicotinic antagonists to probe the involvement of nicotinic receptors in aspects of dlPFC-dependent working memory function, necessitating alternative strategies to understand the role of this system in cognitive deficits seen in aging and neurodegenerative disease.

Modified Snake α-Neurotoxin Averts β-Amyloid Binding to α7 Nicotinic Acetylcholine Receptor and Reverses Cognitive Deficits in Alzheimer's Disease Mice

Alzheimer's disease (AD) is the most common cause of senile dementia and one of the greatest medical, social, and economic challenges. According to a dominant theory, amyloid-β (Aβ) peptide is a key AD pathogenic factor. Aβ-soluble species interfere with synaptic functions, aggregate gradually, form plaques, and trigger neurodegeneration. The AD-associated pathology affects numerous systems, though the substantial loss of cholinergic neurons and α7 nicotinic receptors (α7AChR) is critical for the gradual cognitive decline. Aβ binds to α7AChR under various experimental settings; nevertheless, the functional significance of this interaction is ambiguous. Whereas the capability of low Aβ concentrations to activate α7AChR is functionally beneficial, extensive brain exposure to high Aβ concentrations diminishes α7AChR activity, contributes to the cholinergic deficits that characterize AD. Aβ and snake α-neurotoxins competitively bind to α7AChR. Accordingly, we designed a chemically modified α-cobratoxin (mToxin) to inhibit the interaction between Aβ and α7AChR. Subsequently, we examined mToxin in a set of original in silico, in vitro, ex vivo experiments, and in a murine AD model. We report that mToxin reversibly inhibits α7AChR, though it attenuates Aβ-induced synaptic transmission abnormalities, and upregulates pathways supporting long-term potentiation and reducing apoptosis. Remarkably, mToxin demonstrates no toxicity in brain slices and mice. Moreover, its chronic intracerebroventricular administration improves memory in AD-model animals. Our results point to unique mToxin neuroprotective properties, which might be tailored for the treatment of AD. Our methodology bridges the gaps in understanding Aβ-α7AChR interaction and represents a promising direction for further investigations and clinical development.

Evidence for positive allosteric modulation of cognitive-enhancing effects of nicotine by low-dose galantamine in rats

Cognitive-enhancing effects of nicotinic acetylcholine receptor (nAChR) agonists may be of therapeutic potential in disease states characterized by nAChR hypofunction; however, effects tend to be of small magnitude and unlikely clinical significance. The co-administration of a nAChR positive allosteric modulator (PAM) may enable larger effects by potentiating nAChR responses to an agonist. The acetylcholinesterase (AChE) inhibitor galantamine is a nAChR PAM at a low dose range. A recent clinical study testing effects of a single small dose of galantamine found evidence for synergistic effects with nicotine on one of several cognitive measures. In that study, residual AChE inhibition may have obscured interactions on other measures. The present study aimed at examining small galantamine doses devoid of AChE inhibitory activity in a rodent model of attention. The effects of galantamine (0.03-0.25 mg/kg s.c.) were tested in the presence and absence of nicotine (0.1 mg/kg s.c.) in rats performing the 5-Choice Serial Reaction Time Task, employing a within-subject factorial design. There were no effects on response accuracy of either nicotine or galantamine alone. However, the combination of nicotine and 0.06 mg/kg of galantamine significantly enhanced accuracy. AChE activity assays confirmed that, at this dose, galantamine was devoid of AChE inhibitory activity in the brain. The results suggest that cognitive-enhancing effects of nicotine may be potentiated or uncovered by an extremely small dose of galantamine, well below its typical therapeutic range in humans. Furthermore, these findings provide a general proof-of-principle for a nAChR agonist and PAM combination strategy for cognitive enhancement.

α7 nicotinic acetylcholine receptors as therapeutic targets in schizophrenia: Update on animal and clinical studies and strategies for the future

Schizophrenia is a devastating mental illness and its effective treatment is among the most challenging issues in psychiatry. The symptoms of schizophrenia are heterogeneous ranging from positive symptoms (e.g., delusions, hallucinations) to negative symptoms (e.g., anhedonia, social withdrawal) to cognitive dysfunction. Antipsychotics are effective at ameliorating positive symptoms in some patients; however, they are not reliably effective at improving the negative symptoms or cognitive impairments. The inability to address the cognitive impairments is a particular concern since they have the greatest long-term impact on functional outcomes. While decades of research have been devoted to the development of pro-cognitive agents for schizophrenia, to date, no drug has been approved for clinical use. Converging behavioral, neurobiological, and genetic evidence led to the identification of the α7-nicotinic acetylcholine receptor (α7-nAChR) as a therapeutic target several years ago and there is now extensive preclinical evidence that α7-nAChR ligands have pro-cognitive effects and other properties that should be beneficial to schizophrenia patients. However, like the other pro-cognitive strategies, no α7-nAChR ligand has been approved for clinical use in schizophrenia thus far. In this review, several topics are discussed that may impact the success of α7-nAChR ligands as pro-cognitive agents for schizophrenia including the translational value of the animal models used, clinical trial design limitations, confounding effects of polypharmacy, dose-effect relationships, and chronic versus intermittent dosing considerations. Determining the most optimal pharmacologic strategy at α7-nAChRs: agonist, positive allosteric modulator, or potentially even receptor antagonist is also discussed. article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Evidence for positive allosteric modulation of cognitive-enhancing effects of nicotine in healthy human subjects

RATIONALE

Cognitive benefits of nicotinic acetylcholine receptor (nAChR) agonists are well established but have generally been of small magnitude and uncertain clinical significance. A way of raising the effect size may be to facilitate agonist-induced responses by co-administering a nAChR positive allosteric modulator (PAM).

OBJECTIVE

The aim was to test whether galantamine, a PAM at several nAChR subtypes, can potentiate the cognitive-enhancing effects of nicotine.

METHODS

Twenty-six adult never-smokers were treated, in a double-blind counterbalanced sequence, with nicotine (7 mg/24 h, transdermally) and galantamine (4 mg, p.o.) combined, nicotine alone, galantamine alone, and double placebo. A low dose of galantamine was chosen to minimize acetylcholinesterase inhibition, which was verified in blood assays. In each condition, participants were tested with three cognitive tasks.

RESULTS

Nicotine significantly improved reaction time (RT) and signal detection in a visuospatial attention task and the Rapid Visual Information Processing Task. Galantamine did not modulate these effects. A trend toward RT reduction by galantamine correlated with acetylcholinesterase inhibition. In a change detection task, there were no effects of nicotine or galantamine alone on accuracy or RT. However, both drugs combined acted synergistically to reduce RT. This effect was not associated with acetylcholinesterase inhibition.

CONCLUSIONS

A pattern consistent with allosteric potentiation of nicotine effects by galantamine was observed on one of six performance measures. This may reflect specific nAChR subtype involvement, or additional pharmacological actions of galantamine may have overshadowed similar interactions on other measures. The finding suggests that allosteric potentiation of nAChR agonist-induced cognitive benefits is possible in principle.

Antagonizing α7 nicotinic receptors with methyllycaconitine (MLA) potentiates receptor activity and memory acquisition

α7 nicotinic acetylcholine receptors (α7nAChRs) have been targeted to improve cognition in different neurological and psychiatric disorders. Nevertheless, no α7nAChR activating ligand has been clinically approved. Here, we investigated the effects of antagonizing α7nAChRs using the selective antagonist methyllycaconitine (MLA) on receptor activity in vitro and cognitive functioning in vivo. Picomolar concentrations of MLA significantly potentiated receptor responses in electrophysiological experiments mimicking the in vivo situation. Furthermore, microdialysis studies showed that MLA administration substantially increased hippocampal glutamate efflux which is related to memory processes. Accordingly, pre-tetanus administration of low MLA concentrations produced longer lasting potentiation (long-term potentiation, LTP) in studies examining hippocampal plasticity. Moreover, low doses of MLA improved acquisition, but not consolidation memory processes in rats. While the focus to enhance cognition by modulating α7nAChRs lies on agonists and positive modulators, antagonists at low doses should provide a novel approach to improve cognition in neurological and psychiatric disorders.

Rollercoaster ride of kynurenines: steering the wheel towards neuroprotection in Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) is associated with cerebral cognitive deficits exhibiting two cardinal hallmarks: accruement of extracellular amyloid plaques and intracellular neurofibrillary tangles composed of hyperphosphorylated tau protein. The currently accessible therapeutic armamentarium merely provides symptomatic relief. Therefore, the cry for prospective neuroprotective strategies seems to be the need of the hour. Areas covered: This review comprehensively establishes correlation between kynurenine pathway (KP) metabolites and AD with major emphasis on its two functionally contrasting neuroactive metabolites i.e. kynurenic acid (KYNA) and quinolinic acid (QUIN) and enlists various clinical studies which hold a potential for future therapeutics in AD. Also, major hypotheses of AD and mechanisms underlying them have been scrutinized with the aim to brush up the readers with basic pathology of AD. Expert opinion: KP is unique in itself as it holds two completely different domains i.e. neurotoxic QUIN and neuroprotective KYNA and disrupted equilibrium between the two has a hand in neurodegeneration. KYNA has long been demonstrated to be neuroprotective but lately being disparaged for cognitive side effects. But we blaze a trail by amalgamating the pharmacological mechanistic studies of KYNA in kinship with α7nAChRs, NMDARs and GABA which lends aid in favour of KA.

Neuropharmacology of attention

Early philosophers and psychologists defined and began to describe attention. Beginning in the 1950's, numerous models of attention were developed. This corresponded with an increased understanding of pharmacological approaches to manipulate neurotransmitter systems. The present review focuses on the knowledge that has been gained about these neurotransmitter systems with respect to attentional processing, with emphasis on the functions mediated within the medial prefrontal cortex. Additionally, the use of pharmacotherapies to treat psychiatric conditions characterized by attentional dysfunction are discussed. Future directions include developing a more comprehensive understanding of the neural mechanisms underlying attentional processing and novel pharmacotherapeutic targets for conditions characterized by aberrant attentional processing.

Intermittent stimulation in the nucleus basalis of meynert improves sustained attention in rhesus monkeys

Sustained attention is essential in important behaviors in daily life. Many neuropsychiatric disorders are characterized by a compromised ability to sustain attention, making this cognitive domain an important therapeutic target. In this study, we tested a novel method of improving sustained attention. Monkeys were engaged in a continuous performance task (CPT) while the nucleus basalis of Meynert (NB), the main source of cholinergic innervation of the neocortex, was stimulated. Intermittent NB stimulation improved the animals' performance by increasing the hit rate and decreasing the false alarm rate. Administration of the cholinesterase inhibitor donepezil or the muscarinic antagonist scopolamine alone impaired performance, whereas the nicotinic antagonist mecamylamine alone improved performance. Applying NB stimulation while mecamylamine or donepezil were administered impaired CPT performance. Methylphenidate, a monoaminergic psychostimulant, was applied in conjunction with intermittent stimulation as a negative control, as it does not directly modulate cholinergic output. Methylphenidate also improved performance, and it produced further improvement when combined with NB stimulation. The additive effect of the combination suggested NB stimulation altered behavior independently from methylphenidate effects. We conclude that basal forebrain projections contribute to sustained attention, and that intermittent NB stimulation is an effective way of improving performance.

Cholinergic Overstimulation Attenuates Rule Selectivity in Macaque Prefrontal Cortex

Acetylcholine is released in the prefrontal cortex (PFC) and is a key modulator of cognitive performance in primates. Cholinergic stimulation has been shown to have beneficial effects on performance of cognitive tasks, and cholinergic receptors are being actively explored as promising targets for ameliorating cognitive deficits in Alzheimer's disease. We hypothesized that cholinergic stimulation of PFC during performance of a cognitive task would augment neuronal activity and neuronal coding of task attributes. We iontophoretically applied the general cholinergic receptor agonist carbachol onto neurons in dorsolateral PFC (DLPFC) of male rhesus macaques performing rule-guided prosaccades and antisaccades, a well established oculomotor task for testing cognitive control. Carbachol application had heterogeneous effects on neuronal excitability, with both excitation and suppression observed in significant proportions. Contrary to our prediction, neurons with rule-selective activity exhibited a reduction in selectivity during carbachol application. Cholinergic stimulation disrupted rule selectivity regardless of whether it had suppressive or excitatory effects on these neurons. In addition, cholinergic stimulation excited putative pyramidal neurons, whereas the activity of putative interneurons remained unchanged. Moreover, cholinergic stimulation attenuated saccade direction selectivity in putative pyramidal neurons due to nonspecific increases in activity. Our results suggest excessive cholinergic stimulation has detrimental effects on DLPFC representations of task attributes. These findings delineate the complexity and heterogeneity of neuromodulation of cerebral cortex by cholinergic stimulation, an area of active exploration with respect to the development of cognitive enhancers.SIGNIFICANCE STATEMENT The neurotransmitter acetylcholine is known to be important for cognitive processes in the prefrontal cortex. Removal of acetylcholine from prefrontal cortex can disrupt short-term memory performance and is reminiscent of Alzheimer's disease, which is characterized by degeneration of acetylcholine-producing neurons. Stimulation of cholinergic receptors is being explored to create cognitive enhancers for the treatment of Alzheimer's disease and other psychiatric diseases. Here, we stimulated cholinergic receptors in prefrontal cortex and examined its effects on neurons that are engaged in cognitive behavior. Surprisingly, cholinergic stimulation decreased neurons' ability to discriminate between rules. This work suggests that overstimulation of acetylcholine receptors could disrupt neuronal processing during cognition and is relevant to the design of cognitive enhancers based on stimulating the cholinergic system.

Nicotine withdrawal-induced inattention is absent in alpha7 nAChR knockout mice

RATIONALE

Smoking is the leading cause of preventable death in the USA, but quit attempts result in withdrawal-induced cognitive dysfunction and predicts relapse. Greater understanding of the neural mechanism(s) underlying these cognitive deficits is required to develop targeted treatments to aid quit attempts.

OBJECTIVES

We examined nicotine withdrawal-induced inattention in mice lacking the α7 nicotinic acetylcholine receptor (nAChR) using the five-choice continuous performance test (5C-CPT).

METHODS

Mice were trained in the 5C-CPT prior to osmotic minipump implantation containing saline or nicotine. Experiment 1 used 40 mg kg-1 day-1 nicotine treatment and tested C57BL/6 mice 4, 28, and 52 h after pump removal. Experiment 2 used 14 and 40 mg kg-1 day-1 nicotine treatment in α7 nAChR knockout (KO) and wildtype (WT) littermates tested 4 h after pump removal. Subsets of WT mice were killed before and after pump removal to assess changes in receptor expression associated with nicotine administration and withdrawal.

RESULTS

Nicotine withdrawal impaired attention in the 5C-CPT, driven by response inhibition and target detection deficits. The overall attentional deficit was absent in α7 nAChR KO mice despite response disinhibition in these mice. Synaptosomal glutamate mGluR5 and dopamine D4 receptor expression were reduced during chronic nicotine but increased during withdrawal, potentially contributing to cognitive deficits.

CONCLUSIONS

The α7 nAChR may underlie nicotine withdrawal-induced deficits in target detection but is not required for response disinhibition deficits. Alterations to the glutamatergic and dopaminergic pathways may also contribute to withdrawal-induced attentional deficits, providing novel targets to alleviate the cognitive symptoms of withdrawal during quit attempts.

The effects of nicotine in the neonatal quinpirole rodent model of psychosis: Neural plasticity mechanisms and nicotinic receptor changes

Neonatal quinpirole (NQ) treatment to rats increases dopamine D2 receptor sensitivity persistent throughout the animal's lifetime. In Experiment 1, we analyzed the role of α7 and α4β2 nicotinic receptors (nAChRs) in nicotine behavioral sensitization and on the brain-derived neurotrophic factor (BDNF) response to nicotine in NQ- and neonatally saline (NS)-treated rats. In Experiment 2, we analyzed changes in α7 and α4β2 nAChR density in the nucleus accumbens (NAcc) and dorsal striatum in NQ and NS animals sensitized to nicotine. Male and female Sprague-Dawley rats were neonatally treated with quinpirole (1mg/kg) or saline from postnatal days (P)1-21. Animals were given ip injections of either saline or nicotine (0.5mg/kg free base) every second day from P33 to P49 and tested on behavioral sensitization. Before each injection, animals were ip administered the α7 nAChR antagonist methyllycaconitine (MLA; 2 or 4mg/kg) or the α4β2 nAChR antagonist dihydro beta erythroidine (DhβE; 1 or 3mg/kg). Results revealed NQ enhanced nicotine sensitization that was blocked by DhβE. MLA blocked the enhanced nicotine sensitization in NQ animals, but did not block nicotine sensitization. NQ enhanced the NAcc BDNF response to nicotine which was blocked by both antagonists. In Experiment 2, NQ enhanced nicotine sensitization and enhanced α4β2, but not α7, nAChR upregulation in the NAcc. These results suggest a relationship between accumbal BDNF and α4β2 nAChRs and their role in the behavioral response to nicotine in the NQ model which has relevance to schizophrenia, a behavioral disorder with high rates of tobacco smoking.

Stimulation of nicotinic acetylcholine alpha7 receptors rescue schizophrenia-like cognitive impairments in rats

Alpha7 nicotinic acetylcholine receptor (α7 nAChR) dysfunction plays an important role in schizophrenia. Positive allosteric modulators of α7 nAChR have emerged as a promising therapeutic approach to manage cognitive deficits that are inadequately treated in schizophrenic patients. The aim of the present study was to evaluate the ability of type I (CCMI) and type II (PNU120596) α7 nAChR positive allosteric modulators to counteract MK-801-induced cognitive and sensorimotor gating deficits. The activity of these compounds was compared with the action of the α7 nAChR agonist A582941. CCMI, PNU120596 and A582941 reversed the sensorimotor gating impairment evoked by MK-801 based on the prepulse inhibition of the startle response. Additionally, no MK-801-evoked working memory deficits were observed with α7 nAChR ligand pretreatment as assessed in a discrete paired-trial delayed alternation task. However, these compounds did not affect the rats' attentional performances in the five-choice serial reaction time test. The α7 nAChR agents demonstrated a beneficial effect on sensorimotor gating and some aspects of cognition tested in a rat model of schizophrenia. Therefore, these results support the use of α7 nAChR positive allosteric modulators as a potential treatment strategy in schizophrenia.

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.

Strain dependency of the effects of nicotine and mecamylamine in a rat model of attention

RATIONALE

Processes of attention have a heritable component, suggesting that genetic predispositions may predict variability in the response to attention-enhancing drugs. Among lead compounds with attention-enhancing properties are nicotinic acetylcholine receptor (nAChR) agonists.

OBJECTIVES

This study aims to test, by comparing three rat strains, whether genotype may influence the sensitivity to nicotine in the 5-choice serial reaction time task (5-CSRTT), a rodent model of attention.

METHODS

Strains tested were Long Evans (LE), Sprague Dawley (SD), and Wistar rats. The 5-CSRTT requires responses to light stimuli presented randomly in one of five locations. The effect of interest was an increased percentage of responses in the correct location (accuracy), the strongest indicator of improved attention.

RESULTS

Nicotine (0.05-0.2 mg/kg s.c.) reduced omission errors and response latency and increased anticipatory responding in all strains. In contrast, nicotine dose-dependently increased accuracy in Wistar rats only. The nAChR antagonist mecamylamine (0.75-3 mg/kg s.c.) increased omissions, slowed responses, and reduced anticipatory responding in all strains. There were no effects on accuracy, which was surprising giving the clear improvement with nicotine in the Wistar group.

CONCLUSIONS

The findings suggest strain differences in the attention-enhancing effects of nicotine, which would indicate that genetic predispositions predict variability in the efficacy of nAChR compounds for enhancing attention. The absence of effect of mecamylamine on response accuracy may suggest a contribution of nAChR desensitization to the attention-enhancing effects of nicotine.

nAChR dysfunction as a common substrate for schizophrenia and comorbid nicotine addiction: Current trends and perspectives

INTRODUCTION

The prevalence of tobacco use in the population with schizophrenia is enormously high. Moreover, nicotine dependence is found to be associated with symptom severity and poor outcome in patients with schizophrenia. The neurobiological mechanisms that explain schizophrenia-nicotine dependence comorbidity are not known. This study systematically reviews the evidence highlighting the contribution of nicotinic acetylcholine receptors (nAChRs) to nicotine abuse in schizophrenia.

METHODS

Electronic data bases (Medline, Google Scholar, and Web of Science) were searched using the selected key words that match the aims set forth for this review. A total of 276 articles were used for the qualitative synthesis of this review.

RESULTS

Substantial evidence from preclinical and clinical studies indicated that dysregulation of α7 and β2-subunit containing nAChRs account for the cognitive and affective symptoms of schizophrenia and nicotine use may represent a strategy to remediate these symptoms. Additionally, recent meta-analyses proposed that early tobacco use may itself increase the risk of developing schizophrenia. Genetic studies demonstrating that nAChR dysfunction that may act as a shared vulnerability factor for comorbid tobacco dependence and schizophrenia were found to support this view. The development of nAChR modulators was considered an effective therapeutic strategy to ameliorate psychiatric symptoms and to promote smoking cessation in schizophrenia patients.

CONCLUSIONS

The relationship between schizophrenia and smoking is complex. While the debate for the self-medication versus addiction vulnerability hypothesis continues, it is widely accepted that a dysfunction in the central nAChRs represent a common substrate for various symptoms of schizophrenia and comorbid nicotine dependence.

α7 nicotinic ACh receptor-deficient mice exhibit sustained attention impairments that are reversed by β2 nicotinic ACh receptor activation

BACKGROUND AND PURPOSE

Disruptions of executive function, including attentional deficits, are a hallmark of a number of diseases. ACh in the prefrontal cortex regulates attentive behaviour; however, the role of α7 nicotinic ACh receptor (α7nAChR) in attention is contentious.

EXPERIMENTAL APPROACH

In order to probe attention, we trained both wild-type and α7nAChR knockout mice on a touch screen-based five-choice serial reaction time task (5-CSRT). Following training procedures, we then tested sustained attention using a probe trial experiment. To further differentiate the role of specific nicotinic receptors in attention, we then tested the effects of both α7nAChR and β2nAChR agonists on the performance of both wild-type and knockout mice on the 5-CSRT task.

KEY RESULTS

At low doses, α7nAChR agonists improved attentional performance of wild-type mice, while high doses had deleterious effects on attention. α7nAChR knockout mice displayed deficits in sustained attention that were not ameliorated by α7nAChR agonists. However, these deficits were completely reversed by the administration of a β2nAChR agonist. Furthermore, administration of a β2nAChR agonist in α7nAChR knockout mice elicited similar biochemical response in the prefrontal cortex as the administration of α7nAChR agonists in wild-type mice.

CONCLUSIONS AND IMPLICATIONS

Our experiments reveal an intricate relationship between distinct nicotinic receptors to regulate attentional performance and provide the basis for targeting β2nAChRs pharmacologically to decrease attentional deficits due to a dysfunction in α7nAChRs.

Positive allosteric modulation of alpha 7 nicotinic acetylcholine receptors enhances recognition memory and cognitive flexibility in rats

A wide body of preclinical and clinical data suggests that alpha 7 nicotinic acetylcholine receptors (α7-nAChRs) may represent useful targets for cognitive improvement in schizophrenia and Alzheimer׳s disease. A promising recent approach is based on the use of positive allosteric modulators (PAMs) of α7-nAChRs due to their several advantages over the direct agonists. Nevertheless, the behavioural effects of this class of compounds, particularly with regard to higher-order cognitive functions, have not been broadly characterised. The aim of the present study was to evaluate the procognitive efficacies of type I and type II α7-nAChRs PAMs, N-(4-chlorophenyl)-[[(4-chlorophenyl)amino]methylene]-3-methyl-5-isoxazoleacet-amide (CCMI) and N-(5-Chloro-2,4-dimethoxyphenyl)-N'-(5-methyl-3-isoxazolyl)urea (PNU-120596) in the novel object recognition task (NORT), attentional set-shifting task (ASST) and five-choice serial reaction time task (5-CSRTT) in rats. Additionally, the effects of galantamine, an acetylcholinesterase inhibitor that also allosterically modulates nAChRs, were assessed. We report that CCMI (0.3-3mg/kg), PNU-120596 (0.3-3mg/kg) and galantamine (1-3mg/kg) attenuated the delay-induced impairment in NORT performance and facilitated cognitive flexibility in the ASST. Methyllycaconitine (3mg/kg) blocked the actions of CCMI, PNU-120596 and galantamine in the NORT and ASST, suggesting that the procognitive effects of these compounds are α7-nAChRs-dependent. However, none of the compounds tested affected the rats' attentional performance in the 5-CSRTT. The present findings confirm and extend the observations indicating that the positive allosteric modulation of α7-nAChRs enhances recognition memory and cognitive flexibility in preclinical tasks. Therefore, the present study supports the utility of α7-nAChRs PAMs as a potential cognitive enhancing therapy.

Negative affective states and cognitive impairments in nicotine dependence

Smokers have substantial individual differences in quit success in response to current treatments for nicotine dependence. This observation may suggest that different underlying motivations for continued tobacco use across individuals and nicotine cessation may require different treatments in different individuals. Although most animal models of nicotine dependence emphasize the positive reinforcing effects of nicotine as the major motivational force behind nicotine use, smokers generally report that other consequences of nicotine use, including the ability of nicotine to alleviate negative affective states or cognitive impairments, as reasons for continued smoking. These states could result from nicotine withdrawal, but also may be associated with premorbid differences in affective and/or cognitive function. Effects of nicotine on cognition and affect may alleviate these impairments regardless of their premorbid or postmorbid origin (e.g., before or after the development of nicotine dependence). The ability of nicotine to alleviate these symptoms would thus negatively reinforce behavior, and thus maintain subsequent nicotine use, contributing to the initiation of smoking, the progression to dependence and relapse during quit attempts. The human and animal studies reviewed here support the idea that self-medication for pre-morbid and withdrawal-induced impairments may be more important factors in nicotine addiction and relapse than has been previously appreciated in preclinical research into nicotine dependence. Given the diverse beneficial effects of nicotine under these conditions, individuals might smoke for quite different reasons. This review suggests that inter-individual differences in the diverse effects of nicotine associated with self-medication and negative reinforcement are an important consideration in studies attempting to understand the causes of nicotine addiction, as well as in the development of effective, individualized nicotine cessation treatments.

Attention

The ability to focus one's attention on important environmental stimuli while ignoring irrelevant stimuli is fundamental to human cognition and intellectual function. Attention is inextricably linked to perception, learning and memory, and executive function; however, it is often impaired in a variety of neuropsychiatric disorders, including Alzheimer's disease, schizophrenia, depression, and attention deficit hyperactivity disorder (ADHD). Accordingly, attention is considered as an important therapeutic target in these disorders. The purpose of this chapter is to provide an overview of the most common behavioral paradigms of attention that have been used in animals (particularly rodents) and to review the literature where these tasks have been employed to elucidate neurobiological substrates of attention as well as to evaluate novel pharmacological agents for their potential as treatments for disorders of attention. These paradigms include two tasks of sustained attention that were developed as rodent analogues of the human Continuous Performance Task (CPT), the Five-Choice Serial Reaction Time Task (5-CSRTT) and the more recently introduced Five-Choice Continuous Performance Task (5C-CPT), and the Signal Detection Task (SDT) which was designed to emphasize temporal components of attention.

Nicotinic receptors and attention

Facilitation of different attentional functions by nicotinic acetylcholine receptor (nAChR) agonists may be of therapeutic potential in disease conditions such as Alzheimer's disease or schizophrenia. For this reason, the neuronal mechanisms underlying these effects have been the focus of research in humans and in preclinical models. Attention-enhancing effects of the nonselective nAChR agonist nicotine can be observed in human nonsmokers and in laboratory animals, suggesting that benefits go beyond a reversal of withdrawal deficits in smokers. The ultimate aim is to develop compounds acting with greater selectivity than nicotine at a subset of nAChRs, with an effects profile narrowly matching the targeted cognitive deficits and minimizing unwanted effects. To date, compounds tested clinically target the nAChR subtypes most abundant in the brain. To help pinpoint more selectively expressed subtypes critical for attention, studies have aimed at identifying the secondary neurotransmitter systems whose stimulation mediates the attention-enhancing properties of nicotine. Evidence indicates that noradrenaline and glutamate, but not dopamine release, are critical mediators. Thus, attention-enhancing nAChR agents could spare the system central to nicotine dependence. Neuroimaging studies suggest that nAChR agonists act on a variety of brain systems by enhancing activation, reducing activation, and enhancing deactivation by attention tasks. This supports the notion that effects on different attentional functions may be mediated by distinct central mechanisms, consistent with the fact that nAChRs interact with a multitude of brain sites and neurotransmitter systems. The challenge will be to achieve the optimal tone at the right subset of nAChR subtypes to modulate specific attentional functions, employing not just direct agonist properties, but also positive allosteric modulation and low-dose antagonism.

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.

Nicotinic acetylcholine receptors in attention circuitry: the role of layer VI neurons of prefrontal cortex

Cholinergic modulation of prefrontal cortex is essential for attention. In essence, it focuses the mind on relevant, transient stimuli in support of goal-directed behavior. The excitation of prefrontal layer VI neurons through nicotinic acetylcholine receptors optimizes local and top-down control of attention. Layer VI of prefrontal cortex is the origin of a dense feedback projection to the thalamus and is one of only a handful of brain regions that express the α5 nicotinic receptor subunit, encoded by the gene chrna5. This accessory nicotinic receptor subunit alters the properties of high-affinity nicotinic receptors in layer VI pyramidal neurons in both development and adulthood. Studies investigating the consequences of genetic deletion of α5, as well as other disruptions to nicotinic receptors, find attention deficits together with altered cholinergic excitation of layer VI neurons and aberrant neuronal morphology. Nicotinic receptors in prefrontal layer VI neurons play an essential role in focusing attention under challenging circumstances. In this regard, they do not act in isolation, but rather in concert with cholinergic receptors in other parts of prefrontal circuitry. This review urges an intensification of focus on the cellular mechanisms and plasticity of prefrontal attention circuitry. Disruptions in attention are one of the greatest contributing factors to disease burden in psychiatric and neurological disorders, and enhancing attention may require different approaches in the normal and disordered prefrontal cortex.

Effects of systemically applied nAChRα7 agonists and antagonists on light-induced phase shifts of hamster circadian activity rhythms

Many physiological systems in mammals are linked to the body's master circadian rhythm in the sleep/wake cycle and dysfunctions in this rhythm has been associated with neurological diseases such as major depression, Alzheimer's Disease and schizophrenia. There is some evidence that nicotinic cholinergic input to the master circadian pacemaker, the suprachiasmatic nucleus, may modulate circadian activity rhythms, but data employing in vivo preparations is sparse. Therefore we examined the ability of intraperitoneally applied nicotinic agonists and antagonists relatively selective for the α7 nicotinic receptor to modulate light-induced phase shifts of hamster circadian wheel running rhythms. Hamsters were maintained in constant darkness and exposed to light pulses early and late in their active period, mimicking dusk and dawn respectively, which elicited phase delays and advances of their circadian wheel running rhythms. The α7 receptor antagonists bPiDDB (0.03-3mg/kg) and methyllacaconitine (0.1-1mg/kg) inhibited both light- induced phase advances and delays of circadian wheel running rhythms by as much as 75% versus vehicle injections. In contrast, systemic injections of the α7 agonists PHA 543613 and ABT107, both at 0.156-2.5mg/kg, had no effect on light induced phase advances or delays. Further, α7 nicotinic receptors were identified in the hamster suprachiasmatic nucleus using an antibody that recognizes α7 nicotinic receptors. These results clearly identify the ability of α7 nicotinic receptor antagonists to inhibit light-entrainment of the hamster circadian pacemaker. Therefore, nicotinic compounds may be useful for the treatment of circadian dysfunction associated with neurological diseases.

Cholinergic modulation of the medial prefrontal cortex: the role of nicotinic receptors in attention and regulation of neuronal activity

Acetylcholine (ACh) release in the medial prefrontal cortex (mPFC) is crucial for normal cognitive performance. Despite the fact that many have studied how ACh affects neuronal processing in the mPFC and thereby influences attention behavior, there is still a lot unknown about how this occurs. Here we will review the evidence that cholinergic modulation of the mPFC plays a role in attention and we will summarize the current knowledge about the role between ACh receptors (AChRs) and behavior and how ACh receptor activation changes processing in the cortical microcircuitry. Recent evidence implicates fast phasic release of ACh in cue detection and attention. This review will focus mainly on the fast ionotropic nicotinic receptors and less on the metabotropic muscarinic receptors. Finally, we will review limitations of the existing studies and address how innovative technologies might push the field forward in order to gain understanding into the relation between ACh, neuronal activity and behavior.

Zebrafish model systems for developmental neurobehavioral toxicology

Zebrafish offer many advantages that complement classic mammalian models for the study of normal development as well as for the teratogenic effects of exposure to hazardous compounds. The clear chorion and embryo of the zebrafish allow for continuous visualization of the anatomical changes associated with development, which, along with short maturation times and the capability of complex behavior, makes this model particularly useful for measuring changes to the developing nervous system. Moreover, the rich array of developmental, behavioral, and molecular benefits offered by the zebrafish have contributed to an increasing demand for the use of zebrafish in behavioral teratology. Essential for this endeavor has been the development of a battery of tests to evaluate a spectrum of behavior in zebrafish. Measures of sensorimotor plasticity, emotional function, cognition and social interaction have been used to characterize the persisting adverse effects of developmental exposure to a variety of chemicals including therapeutic drugs, drugs of abuse and environmental toxicants. In this review, we present and discuss such tests and data from a range of developmental neurobehavioral toxicology studies using zebrafish as a model. Zebrafish provide a key intermediate model between high throughput in vitro screens and the classic mammalian models as they have the accessibility of in vitro models and the complex functional capabilities of mammalian models.

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.

Reward learning as a potential target for pharmacological augmentation of cognitive remediation for schizophrenia: a roadmap for preclinical development

Rationale: Impaired cognitive abilities are a key characteristic of schizophrenia. Although currently approved pharmacological treatments have demonstrated efficacy for positive symptoms, to date no pharmacological treatments successfully reverse cognitive dysfunction in these patients. Cognitively-based interventions such as cognitive remediation (CR) and other psychosocial interventions however, may improve some of the cognitive and functional deficits of schizophrenia. Given that these treatments are time-consuming and labor-intensive, maximizing their effectiveness is a priority. Augmenting psychosocial interventions with pharmacological treatments may be a viable strategy for reducing the impact of cognitive deficits in patients with schizophrenia.

Objective: We propose a strategy to develop pharmacological treatments that can enhance the reward-related learning processes underlying successful skill-learning in psychosocial interventions. Specifically, we review clinical and preclinical evidence and paradigms that can be utilized to develop these pharmacological augmentation strategies. Prototypes for this approach include dopamine D1 receptor and α7 nicotinic acetylcholine receptor agonists as attractive targets to specifically enhance reward-related learning during CR.

Conclusion: The approach outlined here could be used broadly to develop pharmacological augmentation strategies across a number of cognitive domains underlying successful psychosocial treatment.

The functional tumor necrosis factor-α (308A/G) polymorphism modulates attentional selection in elderly individuals

There has been increasing interest in understanding the role of inflammatory processes for cognitive functions in aging using molecular genetic approaches. Though this has mostly been evaluated in pathological aging, little is known about the relevance for cognitive functions in healthy aging in humans. On the basis of behavioral data and neurophysiological data (event-related potentials and time-frequency decomposition) we show that the A-allele of the functional tumor necrosis factor (TNF)-α -308 A/G polymorphism confers dysfunction in a number of cognitive processes: prolonged attentional selection indexed by a delayed P1/N1 complex, an increased P3a, which is interpreted as an enhanced distractibility by nonrelevant stimuli and compromised response selection mechanisms, as indexed by a reduced frontocentral N2. Time-frequency analyses show that allelic variations further exert their effects by modulating alpha and beta frequency oscillations. On a neurobiological level, these effects might be because of the interaction of TNF-α with glutamatergic neural transmission by which TNF-α is known to boost apoptotic mechanisms in elderly individuals.

Cognitive function during nicotine withdrawal: Implications for nicotine dependence treatment

Nicotine withdrawal is associated with deficits in neurocognitive function including sustained attention, working memory, and response inhibition. Several convergent lines of evidence suggest that these deficits may represent a core dependence phenotype and a target for treatment development efforts. A better understanding of the mechanisms underlying withdrawal-related cognitive deficits may lead to improve nicotine dependence treatment. We begin with an overview of the neurocognitive effects of withdrawal in rodent and human models, followed by discussion of the neurobehavioral mechanisms that are thought to underlie these effects. We then review individual differences in withdrawal-related neurocognitive effects including genetics, gender, and psychiatric comorbidity. We conclude with a discussion of the implications of this research for developing improved therapies, both pharmacotherapy and behavioral treatments, that target cognitive symptoms of nicotine withdrawal. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Glucose attenuates impairments in memory and CREB activation produced by an α4β2 but not an α7 nicotinic receptor antagonist

Glucose improves memory for a variety of tasks when administered to rats and mice near the time of training. Prior work indicates glucose may enhance memory by increasing the synthesis and release of the neurotransmitter acetylcholine in the brain. To investigate if specific acetylcholine receptor subtypes may mediate some of the memory-enhancing actions of glucose, we examined the effects of subtype-specific nicotinic acetylcholine receptor antagonists on memory in Fischer-344 rats and also examined the ability of glucose to reverse drug-induced impairments. Pre-training peripheral injections of methyllycaconitine (MLA) or dihydro-beta-erythroidine (DHβE), which are specific α7 and α4β2 nicotinic receptor antagonists, respectively, dose-dependently impaired retention latencies in an inhibitory avoidance task when tested 7-days but not 1 h after training. Immediate post-training glucose injections attenuated the impairments, but were more effective in attenuating the DHβE-induced impairments. Likewise, peripheral or direct intrahippocampal injections of MLA or DHβE dose-dependently impaired spatial working memory scores on a spontaneous alternation task. Concurrent administration of glucose reversed DHβE- but not MLA-induced impairments. CREB phosphorylation downstream of cholinergic signaling was assessed 30 min after spontaneous alternation testing and intrahippocampal drug infusions. Both MLA and DHβE impaired hippocampal CREB phosphorylation; glucose reversed DHβE- but not MLA-induced deficits. The effectiveness of glucose in reversing DHβE- but not MLA-induced impairments in behavioral performance and CREB phosphorylation suggests that activation of α7 receptors may play an important role in memory enhancement by glucose.

Effects of chronic sazetidine-A, a selective α4β2 neuronal nicotinic acetylcholine receptors desensitizing agent on pharmacologically-induced impaired attention in rats

RATIONALE

Nicotine and nicotinic agonists have been shown to improve attentional function. Nicotinic receptors are easily desensitized, and all nicotinic agonists are also desensitizing agents. Although both receptor activation and desensitization are components of the mechanism that mediates the overall effects of nicotinic agonists, it is not clear how each of the two opposed actions contributes to attentional improvements. Sazetidine-A has high binding affinity at α4β2 nicotinic receptors and causes a relatively brief activation followed by a long-lasting desensitization of the receptors. Acute administration of sazetidine-A has been shown to significantly improve attention by reversing impairments caused by the muscarinic cholinergic antagonist scopolamine and the NMDA glutamate antagonist dizocilpine.

METHODS

In the current study, we tested the effects of chronic subcutaneous infusion of sazetidine-A (0, 2, or 6 mg/kg/day) on attention in Sprague-Dawley rats. Furthermore, we investigated the effects of chronic sazetidine-A treatment on attentional impairment induced by an acute administration of 0.02 mg/kg scopolamine.

RESULTS

During the first week period, the 6-mg/kg/day sazetidine-A dose significantly reversed the attentional impairment induced by scopolamine. During weeks 3 and 4, the scopolamine-induced impairment was no longer seen, but sazetidine-A (6 mg/kg/day) significantly improved attentional performance on its own. Chronic sazetidine-A also reduced response latency and response omissions.

CONCLUSIONS

This study demonstrated that similar to its acute effects, chronic infusions of sazetidine-A improve attentional performance. The results indicate that the desensitization of α4β2 nicotinic receptors with some activation of these receptors may play an important role in improving effects of sazetidine-A on attention.

Improvement of attentional function with antagonism of nicotinic receptors in female rats

Nicotinic agonists have been shown in a variety of studies to improve cognitive function. Since nicotinic receptors are easily desensitized by agonists, it is not completely clear to what degree receptor desensitization or receptor activation are responsible for nicotinic agonist-induced cognitive improvement. In the current study, the effect of the neuronal nicotinic cholinergic α4β2 receptor antagonist dihydro-β-erythroidine (DHβE) and the α7 nicotinic receptor antagonist methyllycaconitine (MLA) on attentional function was determined. Adult female Sprague-Dawley rats were trained on the visual signal detection task. They were required to discriminate whether or not a light signal occurred on a trial and respond with a lever press on one side after a signal and the opposite side after the absence of a signal in order to receive a food pellet reinforcer. Acute administration of the α4β2 antagonist DHβE improved attentional function either alone or in reversing the attentional impairment caused by the NMDA glutamate antagonist dizocilpine (MK-801). Acute administration of MLA also significantly attenuated the dizocilpine-induced attentional impairment. In previous research we have shown that the α4β2 nicotinic desensitizing agent and partial agonist sazetidine-A also was effective in reversing dizocilpine-induced attentional impairments on the signal detection task and that low doses of the general nicotinic antagonist mecamylamine improved learning and memory. The current studies indicate that blockade of nicotinic receptors can effectively attenuate attentional impairments. Development of drugs that provide a net decrease in nicotinic receptor activity either through antagonism or desensitization could be worth exploring for beneficial effects for treating cognitive impairments.

Effects of AZD3480, a neuronal nicotinic acetylcholine receptor agonist, and donepezil on dizocilpine-induced attentional impairment in rats

BACKGROUND AND RATIONALE

Nicotinic acetylcholine systems play major roles in cognitive function. Nicotine and a variety of nicotinic agonists improve attention, and nicotinic antagonist exposure impairs it. This study was conducted to investigate the effect of a novel nicotinic receptor agonist at α4β2 nicotinic receptors (AZD3480) on attention and reversal of pharmacologically induced attentional impairment produced by the NMDA glutamate antagonist dizocilpine (MK-801).

METHODS

Adult female Sprague-Dawley rats were trained to perform an operant visual signal detection task to a stable baseline of accuracy. The rats were then injected subcutaneously following a repeated measures, counter-balanced design with saline, AZD3480 (0.01, 0.1, and 1 mg/kg), dizocilpine (0.05 mg/kg), or their combinations 30 min before the test. The effect of donepezil on the same pharmacologically induced attentional impairment was also tested. A separate group of rats was injected with donepezil (0.01, 0.1, and 1 mg/kg), dizocilpine (0.05 mg/kg), or their combinations, and their attention were assessed. Saline was the vehicle control.

RESULTS

Dizocilpine caused a significant (p < 0.0005) impairment in percent correct performance. This attentional impairment was significantly (p < 0.0005) reversed by 0.01 and 0.1 mg/kg of AZD3480. AZD3480 by itself did not alter the already high baseline control performance. Donepezil (0.01-1.0 mg/kg) also significantly (p < 0.005) attenuated the dizocilpine-induced attentional impairment.

CONCLUSIONS

AZD3480, similar to donepezil, showed significant efficacy for counteracting the attentional impairment caused by the NMDA glutamate antagonist dizocilpine. Low doses of AZD3480 may provide therapeutic benefit for reversing attentional impairment in patients suffering from cognitive impairment due to glutamatergic dysregulation and likely other attentional disorders.

Nicotine increases brain functional network efficiency

Despite the use of cholinergic therapies in Alzheimer's disease and the development of cholinergic strategies for schizophrenia, relatively little is known about how the system modulates the connectivity and structure of large-scale brain networks. To better understand how nicotinic cholinergic systems alter these networks, this study examined the effects of nicotine on measures of whole-brain network communication efficiency. Resting state fMRI was acquired from fifteen healthy subjects before and after the application of nicotine or placebo transdermal patches in a single blind, crossover design. Data, which were previously examined for default network activity, were analyzed with network topology techniques to measure changes in the communication efficiency of whole-brain networks. Nicotine significantly increased local efficiency, a parameter that estimates the network's tolerance to local errors in communication. Nicotine also significantly enhanced the regional efficiency of limbic and paralimbic areas of the brain, areas which are especially altered in diseases such as Alzheimer's disease and schizophrenia. These changes in network topology may be one mechanism by which cholinergic therapies improve brain function.