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

The protective effect of methanolic extract of Verbascum cheiranthifolium and Biebersteinia multifida DC on hippocampus damage induced by diazinon in male Wistar rats: An experimental study

Diazinon (DZN) an organophosphate (OP), with the most important mechanism of action of DZN being induction of oxidative stress (OS) and inhibition of the enzyme acetylcholinesterase (AChE). Verbascum cheiranthifolium (VER) and Biebersteinia multifida (BM) belong to the Scrophulariaceae and Biebersteiniaceae family respectively. These plants are widely used in Iranian traditional medicine due to their beneficial effects. Thus, this research aimed to appraise the protective effects of the methanolic extract of the VER and BM on changes in the level of expression of α7 and α4 subunits of nicotinic acetylcholine receptors (nAChRs) in hippocampus (HPC) of DZN-treated rats. In this research, 36 male Wistar rats were used and randomly divided into six groups: Control, DZN (40 mg/kg), VER (1 g/kg), DZN+VER (40 mg/kg+1 g/kg), BM (150 mg/kg), and DZN+BM (40 mg/kg+150 mg/kg). At the end of treatment periods, the animals of all groups underwent the Morris water maze (MWM) test. The rats were anesthetized, and blood sampling was performed. Eventually, the brain was removed for histological study and evaluation of OS parameters. The results indicated that DZN increased the extent of expression of nAChRs in the HPC and significantly inhibited cholinesterase (ChEs) activity plus OS parameters. Also, in MWM, the time to find the platform was significantly longer in the DZN group, while the time and the distance in the probe test were lower than in the control groups. VER and BM extract in the treatment groups simultaneously improved the extent of expression of nAChRs, ChEs activity, as well as the parameters of OS and spatial memory significantly. In conclusion, our results support the neuroprotective properties of VER and BM extract versus DZN in rats. Accordingly, the extracts of VER and BM may be useful as an approach for the treatment of learning disorders and memory enhancement.

The role of astrocytic α7 nicotinic acetylcholine receptors in Alzheimer disease

The ongoing search for therapeutic interventions in Alzheimer disease (AD) has highlighted the complexity of this condition and the need for additional biomarkers, beyond amyloid-β (Aβ) and tau, to improve clinical assessment. Astrocytes are brain cells that control metabolic and redox homeostasis, among other functions, and are emerging as an important focus of AD research owing to their swift response to brain pathology in the initial stages of the disease. Reactive astrogliosis - the morphological, molecular and functional transformation of astrocytes during disease - has been implicated in AD progression, and the definition of new astrocytic biomarkers could help to deepen our understanding of reactive astrogliosis along the AD continuum. As we highlight in this Review, one promising biomarker candidate is the astrocytic α7 nicotinic acetylcholine receptor (α7nAChR), upregulation of which correlates with Aβ pathology in the brain of individuals with AD. We revisit the past two decades of research into astrocytic α7nAChRs to shed light on their roles in the context of AD pathology and biomarkers. We discuss the involvement of astrocytic α7nAChRs in the instigation and potentiation of early Aβ pathology and explore their potential as a target for future reactive astrocyte-based therapeutics and imaging biomarkers in AD.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

Prenatal exposure to artificial food colorings alters NMDA receptor subunit concentrations in rat hippocampus

Exposure to artificial food color additives (AFCAs) has been implicated in the etiology of certain childhood hyperactivity and learning disabilities. N-methyl-D-aspartate receptors and alpha-7 nicotinic acetylcholine receptor (α7 nAChR) are involved in learning and memory. We administered a mixture of AFCAs (erythrosine, ponceau 4R, allura red AC, sunset yellow FCF, tartrazine, amaranth, brilliant blue, azorubine, and indigotine) to female rats during gestation to investigate the effects of prenatal exposure to AFCAs on neurobehavior, spatial learning, and memory in their offspring. We also investigated whether AFCAs modulate NR2A, NR2B, and α7 nAChR protein levels in their offsprings' hippocampi. Although spatial learning and memory were not altered, the offspring of rats exposed to AFCAs exhibited decreased motivation and increased despair-related behavior. NR2A and NR2B protein levels were significantly reduced in female offspring in the experimental group (p < 0.05), whereas α7 nAChR level was not significantly altered. Our results suggest that prenatal exposure to AFCAs may lead to sex-dependent alterations in glutamatergic signaling which may continue into adolescence.

Nicotinic receptors as targets for therapeutic discovery

Nicotinic acetylcholine receptors (nAChRs) represent a class of therapeutic targets with the potential to impact numerous diseases and disorders where significant unmet medical needs remain. The latter include cognitive and neurodegenerative diseases; psychotic disorders, such as schizophrenia; acute nociceptive, neuropathic and inflammatory pain; affective disorders, such as depression and inflammation, where nAChR subtypes modulate key cellular pathways involved in anti-inflammatory processes as well as cell survival. Our increased understanding of the heterogeneity of nAChR targets is defining the relationship of biologic effects to specific receptor subtypes, which in turn, will allow further refinement of desired therapeutic activities. Both preclinical and clinical evidence support the notion that novel compounds targeting specific nAChR subtypes will offer increased potency and efficacy, longer lasting effects, fewer side effects and a more rapid onset of action and less dependence, compared with existing therapies. Clinical proof-of-concept is rapidly emerging and will solidify the position of this new therapeutic approach.

Nicotinic receptors, memory, and hippocampus

Nicotinic acetylcholine receptors (nAChRs) modulate the neurobiological processes underlying hippocampal learning and memory. In addition, nicotine's ability to desensitize and upregulate certain nAChRs may alter hippocampus-dependent memory processes. Numerous studies have examined the effects of nicotine on hippocampus-dependent learning, as well as the roles of low- and high-affinity nAChRs in mediating nicotine's effects on hippocampus-dependent learning and memory. These studies suggested that while acute nicotine generally acts as a cognitive enhancer for hippocampus-dependent learning, withdrawal from chronic nicotine results in deficits in hippocampus-dependent memory. Furthermore, these studies demonstrated that low- and high-affinity nAChRs functionally differ in their involvement in nicotine's effects on hippocampus-dependent learning. In the present chapter, we reviewed studies using systemic or local injections of acute or chronic nicotine, nAChR subunit agonists or antagonists; genetically modified mice; and molecular biological techniques to characterize the effects of nicotine on hippocampus-dependent learning.

Heterogeneity across brain regions and neurotransmitter interactions with nicotinic effects on memory function

Nicotinic acetylcholine receptors have been shown in many studies to be critically involved in memory function. The precise roles these receptors play depend on the receptor subtype, their anatomic localization, their interactions with other parts of the neural systems underlying cognition and the particular domain of cognitive function. Nicotinic agonists can significantly improve learning, memory, and attention. Nicotinic receptors in the hippocampus are innervated by cholinergic projections from the medial septum and diagonal band. Local infusions of either α7 or α4β2 nicotinic antagonists into either the dorsal or ventral hippocampus produce amnestic effects in rats navigating about a radial arm maze. There is cholinergic innervation of nicotinic receptors in other components of the limbic system as well. In the basolateral amygdala and the anterior thalamus, similar amnestic effects of nicotinic α7 and α4β2 antagonists are seen. Interestingly, there are no additive amnestic effects observed in these limbic areas when α7 and α4β2 receptor antagonists are combined. The particular expression patterns of α7 and α4β2 nicotinic receptors in these limbic and cortical areas may explain this nonadditivity, but further research is needed to determine the specific cause of this phenomenon. Nicotinic receptor mechanisms in the limbic system play an important role in cognitive impairment for a variety of neurological disorders, including Alzheimer's disease and schizophrenia. Alzheimer's disease results in a dramatic decrease in hippocampal nicotinic receptor density, affecting α4β2 receptor expression most prominently. In schizophrenia, there are anomalies in α7 nicotinic receptor expression, which seem to be crucial for the cognitive impairment of the disorder. Chronic nicotine exposure, such as seen with tobacco use, results in an increase in nicotinic receptor density in the limbic system. This effect appears to be related to the desensitization of nicotinic receptors seen after agonist application. Open questions remain concerning the role of desensitization versus activation of nicotinic receptors in cognitive improvement.

Characterization of RO5126946, a Novel α7 nicotinic acetylcholine receptor-positive allosteric modulator

Both preclinical evidence and clinical evidence suggest that α7 nicotinic acetylcholine receptor activation (α7nAChR) improves cognitive function, the decline of which is associated with conditions such as Alzheimer's disease and schizophrenia. Moreover, allosteric modulation of α7nAChR is an emerging therapeutic strategy in an attempt to avoid the rapid desensitization properties associated with the α7nAChR after orthosteric activation. We used a calcium assay to screen for positive allosteric modulators (PAMs) of α7nAChR and report on the pharmacologic characterization of the novel compound RO5126946 (5-chloro-N-[(1S,3R)-2,2-dimethyl-3-(4-sulfamoyl-phenyl)-cyclopropyl]-2-methoxy-benzamide), which allosterically modulates α7nAChR activity. RO5126946 increased acetylcholine-evoked peak current and delayed current decay but did not affect the recovery of α7nAChRs from desensitization. In addition, RO5126946's effects were absent when nicotine-evoked currents were completely blocked by coapplication of the α7nAChR-selective antagonist methyl-lycaconitine. RO5126946 enhanced α7nAChR synaptic transmission and positively modulated GABAergic responses. The absence of RO5126946 effects at human α4β2nAChR and 5-hydroxytryptamine 3 receptors, among others, indicated selectivity for α7nAChRs. In vivo, RO5126946 is orally bioavailable and brain-penetrant and improves associative learning in a scopolamine-induced deficit model of fear conditioning in rats. In addition, procognitive effects of RO5126946 were investigated in the presence of nicotine to address potential pharmacologic interactions on behavior. RO5126946 potentiated nicotine's effects on fear memory when both compounds were administered at subthreshold doses and did not interfere with procognitive effects observed when both compounds were administered at effective doses. Overall, RO5126946 is a novel α7nAChR PAM with cognitive-enhancing properties.

A hippocampal nicotinic acetylcholine alpha 7-containing receptor complex is linked to memory retrieval in the multiple-T-maze in C57BL/6j mice

The link between the cholinergic and serotonergic system in cognitive function is well-documented. There is, however, limited information on spatial memory and this formed the rationale to carry out a study with the aim to show a specific link between nicotinic and serotonergic receptor complexes rather than the corresponding subunits, to spatial memory retrieval in a land maze. A total of 46 mice were used and divided into two groups, trained and untrained (yoked) in the multiple-T-Maze (MTM) and following training during the first four days, probe trials for memory retrieval were performed on days 8, 16 and 30. Six hours following scarification, hippocampi were taken for the analysis of native receptor complex levels using blue-native gels followed by immunoblotting with specific antibodies. 5-HT1A-, 5-HT7-, nAChα4- and nACh-α7-containing receptor complexes were observed and were paralleling memory retrievals and receptor complex levels were shown to be significantly different between trained and yoked animals. Only levels of a nicotinic acetylcholine α7 receptor-containing complex at an apparent molecular weight of approximately 480kDa were shown to be linked to memory retrieval on day 8 but not to retrievals on days 16 and 30 when memory extinction has taken place. Correlation between nAChα4-, 5-HT1A- and 5-HT7-containing receptors and latencies on day 16 may point to a probable link in extinction mechanisms. A series of the abovementioned receptor complexes were correlating among each other probably indicating a serotonergic/cholinergic network paralleling spatial memory formation.

Forgetfulness during aging: an integrated biology

Age-related impairments in memory are often attributed to failures, at either systems or molecular levels, of memory storage processes. A major characteristic of changes in memory with increasing age is the advent of forgetfulness in old vs. young animals. This review examines the contribution of a dysfunction of the mechanisms responsible for modulating the maintenance of memory in aged rats. A memory-modulating system that includes epinephrine, acting through release of glucose from liver glycogen stores, potently enhances memory in young rats. In old rats, epinephrine loses its ability to release glucose and loses its efficacy in enhancing memory. Brain measures of extracellular levels of glucose in the hippocampus during memory testing show decreases in glucose in both young and old rats, but the decreases are markedly greater in extent and duration in old rats. Importantly, the old rats do not have the ability to increase blood glucose levels in response to arousal-related epinephrine release, which is retained and even increased in aged rats. Glucose appears to be able to reverse fully the increased rate of forgetting seen in old rats. This set of findings suggests that physiological mechanisms outside of the brain, i.e. changes in neuroendocrine functions, may contribute substantially to the onset of rapid forgetting in aged animals.

Complex relationships of nicotinic receptor actions and cognitive functions

Nicotine has been shown in a variety of studies to improve cognitive function including learning, memory and attention. Nicotine both stimulates and desensitizes nicotinic receptors, thus acting both as an agonist and a net antagonist. The relative roles of these two actions for nicotine-induced cognitive improvement have not yet been fully determined. We and others have found that acute nicotinic antagonist treatment can improve learning and attention. Nicotine acts on a variety of nicotinic receptor subtypes. The relative role and interactions of neuronal nicotinic receptor subtypes for cognition also needs to be better characterized. Nicotine acts on nicotinic receptors in a wide variety of brain areas. The role of some of these areas such as the hippocampus has been relatively well studied but other areas like the thalamus, which has the densest nicotinic receptor concentration are still only partially characterized. In a series of studies we characterized nicotinic receptor actions, anatomic localization and circuit interactions, which are critical to nicotine effects on the cognitive functions of learning, memory and attention. The relative role of increases and decreases in nicotinic receptor activation by nicotine were determined in regionally specific studies of the hippocampus, the amygdala, the frontal cortex and the mediodorsal thalamic nucleus with local infusions of antagonists of nicotinic receptor subtypes (α7 and α4β2). The understanding of the functional neural bases of cognitive function is fundamental to the more effective development of nicotinic drugs for treating cognitive dysfunction.

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.

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.

Involvement of the cholinergic system of CA1 on harmane-induced amnesia in the step-down passive avoidance test

β-carboline alkaloids such as harmane (HA) are naturally present in the human food chain. They are derived from the plant Peganum harmala and have many cognitive effects. In the present study, effects of the nicotinic system of the dorsal hippocampus (CA1) on HA-induced amnesia and exploratory behaviors were examined. One-trial step-down and hole-board paradigms were used to assess memory retention and exploratory behaviors in adult male mice. Pre-training (15 mg/kg) but not pre-testing intraperitoneal (i.p.) administration of HA decreased memory formation but did not alter exploratory behaviors. Moreover, pre-testing administration of nicotine (0.5 µg/mouse, intra-CA1) decreased memory retrieval, but induced anxiogenic-like behaviors. On the other hand, pre-test intra-CA1 injection of ineffective doses of nicotine (0.1 and 0.25 µg/mouse) fully reversed HA-induced impairment of memory after pre-training injection of HA (15 mg/kg, i.p.) which did not alter exploratory behaviors. Furthermore, pre-testing administration of mecamylamine (0.5, 1 and 2 µg/mouse, intra-CA1) did not alter memory retrieval but fully reversed HA-induced impairment of memory after pre-training injection of HA (15 mg/kg, i.p.) which had no effect on exploratory behaviors. In conclusion, the present findings suggest the involvement of the nicotinic cholinergic system in the HA-induced impairment of memory formation.

Behavioral animal models to assess pro-cognitive treatments for schizophrenia

Cognitive dysfunction is a core aspect of schizophrenia that constitutes a major obstacle toward reintegration of patients into society. Although multiple cognitive deficits are evident in schizophrenia patients, no medication is currently approved for their amelioration. Although consensus clinical test batteries have been developed for the assessment of putative cognition enhancers in patients with schizophrenia, parallel animal tests remain to be validated. Having no approved treatment for cognitive symptoms means no positive control can be used to examine pharmacological predictive validity of animal models. Thus, focus has been placed on animal paradigms that have demonstrable construct validity for the cognitive domain being assessed.This review describes the growing arsenal of animal paradigms under development that have putative construct validity to cognitive domains affected in schizophrenia. We discuss (1) the construct validity of the paradigms; (2) compounds developed to investigate putative treatment targets; and (3) manipulations used to first impair task performance. Focus is placed on the paradigm design, including how the use of multivariate assessments can provide evidence that main effects of treatment are not confounded by extraneous effects.

Targeting the nicotinic alpha7 acetylcholine receptor to enhance cognition in disease

A promising drug target currently under investigation to improve cognitive deficits in neuropsychiatric and neurological disorders is the neuronal nicotinic alpha7 acetylcholine receptor (α7nAChR). Improving cognitive impairments in diseases such as Alzheimer's (AD) and schizophrenia remains a large unmet medical need, and the α7nAChR has many properties that make it an attractive therapeutic target. The α7nAChR is a ligand gated ion channel that has particularly high permeability to Ca(2+) and is expressed in key brain regions involved in cognitive processes (e.g., hippocampus). The α7nAChRs are localized both pre-synaptically, where they can regulate neurotransmitter release, and post-synaptically where they can activate intracellular signaling cascades and influence downstream processes involved in learning and memory. In particular, activation of the α7nAChR with small molecule agonists enhances long-term potentiation, an in vitro model of synaptic plasticity, and improves performance across multiple cognitive domains in rodents, monkeys, and humans. Positive allosteric modulation of the α7nAChR offers an alternate approach to direct agonism that could prove to be particularly beneficial in certain disease populations where smoking nicotine is prevalent (e.g., schizophrenia) and could interfere with an orthosteric agonist approach. The current review focuses on the neurobiology of the α7nAChR, its role in cognition and the development status of some of the most promising molecules advancing for the treatment of cognitive dysfunction in disease.

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.

Acetylcholine and attention

Historically, ACh has been implicated in learning and short-term memory functions. However, more recent studies have provided support for a role of cortical ACh in attentional effort, orienting and the detection of behavioral significant stimuli. The current review article summarizes studies in animals and humans which have investigated the role of ACh in attention and cognition. An attempt has been made to differentiate between brain regions involved in attentional processes versus those important for other cognitive functions. To this purpose, various experimental methods and interventions were used. Animal behavioral studies have injected the selective immunotoxin IgG-saporin to induce specific cholinergic lesions, employed electrochemical techniques such as microdialysis, or have administered cholinergic compounds into discrete parts of the brain. Human studies that give some indication on the link between central cholinergic signaling and cognition are obviously confined to less invasive, imaging methods such as fMRI. The brain areas that are deemed most important for intact attentional processing in both animals and humans appear to be the (pre)frontal, parietal and somatosensory (especially visual) regions, where ACh plays a vital role in the top-down control of attentional orienting and stimulus discrimination. In contrast, cholinergic signaling in the septohippocampal system is suggested to be involved in memory processes. Thus, it appears that the role of ACh in cognition is different per brain region and between nicotinic versus muscarinic receptor subtypes.

In vitro binding characteristics of [3H]AZ11637326, a novel alpha7-selective neuronal nicotinic receptor agonist radioligand

AZ11637326 (5'-(2-fluoro[3,4,5(-3)H3]phenyl)-spiro[1-azabicyclo [2.2.2]octane-3,2'(3'H)-furo[2,3-b]pyridine) is a potent partial agonist at the human alpha7 neuronal nicotinic receptor with sub-nanomolar affinity for the human and rat alpha7 [(125)I]alpha-bungarotoxin binding sites. In a search for novel agonist radioligands and imaging ligands for the alpha7 nicotinic receptor, [(3)H]AZ11637326 was synthesized and its in vitro membrane binding properties were characterized. [(3)H]AZ11637326 bound to halpha7-HEK membranes with high specificity (>95%), high affinity (230 pM) and a B(max) of 460 fmol/mg. The rank order of affinity of nicotinic standards determined with [(3)H]AZ11637326 strongly correlated with those determined using the classical alpha7 antagonist [(125)I]alpha-bungarotoxin, indicating that [(3)H]AZ11637326 bound to halpha7-HEK membranes with an alpha7 nicotinic-like pharmacology. The K(i) values for the standards were on average 2.3-fold lower affinity than determined using the prototypical alpha7 nicotinic antagonist [(125)I]alpha-bungarotoxin. Because [(3)H]AZ11637326 specific binding is rapid and reversible, the K(i) values determined using this ligand are more accurate estimates of affinity than those determined using the kinetically sluggish [(125)I]alpha-bungarotoxin. [(3)H]AZ11637326 also bound to a high affinity (510 pM), nicotine-sensitive site on rat hippocampal membranes with an average B(max) of 55 fmol/mg. With rat hippocampal membranes, the nicotine-sensitive fraction of total binding was sub-optimal for a radioligand (~50%), yet the potencies and rank order of the K(i) values for standards were consistent with an alpha7 nicotinic pharmacology. Overall, these studies indicate that [(3)H]AZ11637326 is a useful new in vitro probe of the alpha7 nicotinic receptor agonist site and support its potential utility for in vivo receptor occupancy studies.

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.

Is modulation of nicotinic acetylcholine receptors by melatonin relevant for therapy with cholinergic drugs?

Melatonin, the darkness hormone, synchronizes several physiological functions to light/dark cycle. Besides the awake/sleep cycle that is intuitively linked to day/night, daily variations in memory acquisition and innate or acquired immune responses are some of the major activities linked to melatonin rhythm. The daily variation of these complex processes is due to changes in specific mechanisms. In the last years we focused on the influence of melatonin on the expression and function of nicotinic acetylcholine receptors (nAChRs). Melatonin, either "in vivo" or "in vitro", increases, in a selective manner, the efficiency of alpha-bungarotoxin (alpha-BTX)-sensitive nAChRs. Melatonin's effect on receptors located in rat sympathetic nerve terminals, cerebellum, skeletal muscle and chick retina, was tested. We observed that melatonin is essential for the development of alpha-BTX-sensitive nAChRs, and important for receptor maintenance in aging models. Taking into account that both melatonin and alpha-7 nAChRs (one of the subtypes sensitive to alpha-BTX) are involved in the development of Alzheimer's disease, here we discuss the possibility of a therapeutic strategy focused on both melatonin replacement and its potential association with cholinergic drugs.

Positive allosteric modulation of alpha7 neuronal nicotinic acetylcholine receptors: lack of cytotoxicity in PC12 cells and rat primary cortical neurons

BACKGROUND AND PURPOSE

alpha7-Nicotinic acetylcholine receptors (alpha7 nAChRs) play an important role in cognitive function. Positive allosteric modulators (PAMs) amplify effects of alpha7 nAChR agonist and could provide an approach for treatment of cognitive deficits in neuropsychiatric diseases. PAMs can either predominantly affect the apparent peak current response (type I) or increase both the apparent peak current response and duration of channel opening, due to prolonged desensitization (type II). The delay of receptor desensitization by type II PAMs raises the possibility of Ca2+-induced toxicity through prolonged activation of alpha7 nAChRs. The present study addresses whether type I and II PAMs exhibit different cytotoxicity profiles.

EXPERIMENTAL APPROACH

The present studies evaluated cytotoxic effects of type I PAM [N-(4-chlorophenyl)]-alpha-[(4-chlorophenyl)-aminomethylene]-3-methyl-5-isoxazoleacet-amide (CCMI) and type II PAM 1-[5-chloro-2,4-dimethoxy-phenyl]-3-[5-methyl-isoxazol-3-yl]-urea (PNU-120596), or 4-[5-(4chloro-phenyl)-2-methyl-3-propionyl-pyrrol-1-yl]-benzenesulphonamide (A-867744). The studies used cultures of PC12 cells and primary cultures of rat cortical neuronal cells.

KEY RESULTS

Our results showed that neither type I nor type II PAMs had any detrimental effect on cell integrity or cell viability. In particular, type II PAMs did not affect neuron number and neurite outgrowth under conditions when alpha7 nAChR activity was measured by Ca2+ influx and extracellular signal-regulated kinases 1 and 2 phosphorylation, following exposure to alpha7 nAChR agonists.

CONCLUSIONS AND IMPLICATIONS

This study demonstrated that both type I and type II alpha7 nAChR selective PAMs, although exhibiting differential electrophysiological profiles, did not exert cytotoxic effects in cells endogenously expressing alpha7 nAChRs.

Alpha7 nAChR-mediated activation of MAP kinase pathways in PC12 cells

The alpha7 nicotinic acetylcholine receptor (alpha7 nAChR) plays a fundamental role in Ca(2+)-dependent activation of signaling pathways that can modulate intracellular events involved in learning and memory. Activation of extracellular signal-regulated kinase-1 and -2 (ERK1/2) are well documented Ca(2+) signaling events, but these have not been well characterized in response to alpha7 nAChR-selective ligands. The present study examined activation of ERK1/2 and explored pathways leading to CREB phosphorylation utilizing alpha7 nAChR-selective ligands in PC12 cells endogenously expressing alpha7 nAChRs. Robust concentration-dependent increase in ERK1/2 phosphorylation was triggered by structurally diverse alpha7 nAChR agonists such as nicotine, choline, GTS-21, SSR-180711A and PNU-282987 in the presence of the positive allosteric modulator (PAM) PNU-120596. This effect was attenuated by selective alpha7 nAChR antagonists or by chelation of extracellular Ca(2+). ERK1/2 phosphorylation was also attenuated by inhibitors of calmodulin-dependent protein kinase II (CaMKII), p38 MAP kinase and mitogen-activated protein kinase kinase1/2 (MEK1/2), indicating the involvement of these kinases upstream of ERK1/2. This was confirmed by direct measurement of p38 MAPK and MEK1/2 phosphorylation. These data suggest that alpha7 nAChR agonist-triggered Ca(2+) transient in PC12 cells induces activation of CaMKII, leading to sequential phosphorylation of p38 MAPK, MEK1/2, ERK1/2 and CREB. Such mechanisms may endow the alpha7 nAChRs with roles in modulating Ca(2+)-dependent intracellular second messenger events implicated in diverse aspects of cognition.

Nicotinic receptors in the habenula: importance for memory

The habenula is an epithalamic structure through which descending connections pass from the telencephalon to the brainstem, putting it in a key location to provide feedback control over the brainstem monoaminergic projections ascending to the telencephalon. Habenular nuclei lesions have been shown to impair memory function. The habenular nuclei have high concentrations of nicotinic receptors. In this study we assessed the role of habenular nicotinic receptors for working memory. Adult female Sprague-Dawley rats were trained on a 16-arm maze to assess spatial working and reference memory. All rats had at least 18 sessions of training and then had bilateral chronic infusion cannulae placed into the lateral habenula nucleus. These cannulae were each connected to a slow delivery osmotic minipump that chronically infused mecamylamine 100 microg/side/day (n=9) or vehicle (aCSF) for controls (n=15) for a period of 4 weeks. Both mecamylamine-infused and control rats were acutely injected (s.c.) with nicotine (0, 0.2 or 0.4 mg/kg) in a repeated measures counterbalanced design twice at each dose during the chronic local infusion period. There was a significant (P<0.025) mecamylaminexnicotine interaction effect on memory performance. Without nicotine injection the chronic habenular mecamylamine infusion caused a significant (P<0.05) increase in total memory errors. The 0.4 mg/kg nicotine dose significantly (P<0.005) reversed the mecamylamine-induced memory impairment, returning performance back to levels seen in rats with control aCSF habenular infusions. The current study determined that nicotinic receptors in the lateral habenular nucleus are important for spatial memory function. Descending projections from the telencephalon through the habenula to brainstem nuclei using nicotinic receptors appear to be a key pathway for memory processing.

Hippocampal infusions of MARCKS peptides impair memory of rats on the radial-arm maze

In vitro hippocampal studies by Gay et al. (2008) demonstrated that a myristoylated alanine-rich C kinase substrate (MARCKS) peptide comprising the phosphorylation site or effector domain of the protein acts as a powerful inhibitor of alpha7 nicotinic acetylcholine receptors (nAChRs), which are known to be critically involved in memory function. However, behavioral consequences of hippocampal MARCKS peptide infusions have not been investigated. The purpose of the current study was to determine if local infusions in the rat ventral hippocampus of long (comprising amino acids 151-175) and short (amino acids 159-165) forms of MARCKS peptides could affect memory performance in the 16-arm radial maze. Our results demonstrated a dramatic impairment of both working (changing) and reference (constant) memory with MARCKS(151-175) only. The shorter MARCKS peptide did not affect memory performance. This is in line with in vitro results reported by Gay et al. (2008) that long, but not short, MARCKS peptides inhibit alpha7 nAChRs. We also found that the effect of the MARCKS(151-175) peptide was dose-dependent, with a robust memory impairment at 10 microg/side, and smaller inconsistent effects at lower doses. Our present behavioral study, together with the earlier in vitro study by Gay et al. (2008), suggests that effector domain MARCKS peptides could play a significant role in memory regulation and impairment.

Nicotinic antagonist effects in the mediodorsal thalamic nucleus: regional heterogeneity of nicotinic receptor involvement in cognitive function

Nicotine has been found in many studies to improve cognitive function. However, some studies have not found this effect and others have seen nicotine-induced impairments. Systemic administration bathes the brain with drugs. However, the brain is quite intricately organized with various regions playing very different roles in the bases of cognitive function. We have examined the role of nicotinic receptors in a variety of brain areas for memory. In the hippocampus and amygdala, local infusions of both alpha7 and alpha4beta2 antagonists methyllyaconitine (MLA) and dihydro-beta-erythroidine (DHbetaE) significantly impair memory. In the current studies we locally infused acute and chronic doses of MLA and DHbetaE into the mediodorsal thalamic nucleus and tested memory function on a 16-arm radial maze. The rats also received systemic nicotine to determine the impact of more generalized nicotine effects. Since nicotinic treatments are being developed for cognitive impairment of schizophrenia, interactions were studied with the antipsychotic drug clozapine. In the acute study, the 6.75 microg/side of DHbetaE improved working memory. Co-administration of MLA reversed the DHbetaE-induced improvement. Chronic DHbetaE infusions into the mediodorsal thalamic nucleus also improved working memory. Systemic nicotine reversed this effect. Clozapine had no significant interaction. Nicotinic alpha4beta2 receptors in the mediodorsal thalamic nucleus appear to play an opposite role with regard to working memory than those in the hippocampus and amygdala. Heterogeneity in response to nicotinic drugs given systemically may be due to anatomically distinct nicotinic systems in the brain and their unique roles in the neural bases of cognitive function.

Chronic underactivity of medial frontal cortical beta2-containing nicotinic receptors increases clozapine-induced working memory impairment in female rats

Nicotinic receptor decreases in the frontal cortex and hippocampus are important mediators of cognitive impairment in both schizophrenia and Alzheimer's disease. Drug treatments for these diseases should take into account the impacts of compromised brain function on drug response. This study investigated the impact of compromised nicotinic receptor activity in the frontal cortex in rats on memory function. Since both Alzheimer's disease and schizophrenia can involve psychosis, antipsychotic drugs are often given. The impacts of antipsychotic drugs on cognitive function have been found to be quite variable. It is the hypothesis of this and previous studies that the cognitive effects of antispychotic drugs on cognitive function depend on the integrity of brain systems involved in cognition. Previously in studies of the hippocampus, we found that chronic inhibition of beta2-containing nicotinic receptors with dihydro-beta-erythrodine (DHbetaE) impaired working memory and that this effect was attenuated by the antipsychotic drug clozapine. In contrast, chronic hippocampal alpha7 nicotinic receptor blockade with methyllycaconitine (MLA) potentiated the clozapine-induced memory impairment which is seen in rats without compromised nicotinic receptor activity. The current study determined medial frontal cortical alpha7 and beta2-containing nicotinic receptor involvement in memory and the interactions with antipsychotic drug therapy with clozapine. Chronic DHbetaE and MLA infusion effects and interactions with systemic clozapine were assessed in female rats tested for memory on the radial-arm maze. Antipsychotic drug interactions with chronic systemic nicotine were investigated because nicotinic procognitive treatment has been proposed. The same local infusion DHbetaE dose that impaired memory with hippocampal infusion did not impair memory when infused in the medial frontal cortex. Frontal DHbetaE infusion potentiated clozapine-induced memory impairment, whereas previously the memory impairment caused by hippocampal DHbetaE infusion was attenuated by clozapine. Frontal cortical MLA infusions at a dose that previously was found to potentiate the clozapine-induced memory impairment with hippocampal infusion had no significant effect when infused into the medial frontal cortex. The location and subtype of nicotinic receptor underactivity are critical determinates for clozapine effects on memory. Patients with hippocampal beta2-containing nicotinic receptor loss may be well treated with clozapine therapy, while those with frontal cortical beta2-containing receptor loss may have a potentiated memory impairment caused by clozapine.

Using the MATRICS to guide development of a preclinical cognitive test battery for research in schizophrenia

Cognitive deficits in schizophrenia are among the core symptoms of the disease, correlate with functional outcome, and are not well treated with current antipsychotic therapies. In order to bring together academic, industrial, and governmental bodies to address this great 'unmet therapeutic need', the NIMH sponsored the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative. Through careful factor analysis and consensus of expert opinion, MATRICS identified seven domains of cognition that are deficient in schizophrenia (attention/vigilance, working memory, reasoning and problem solving, processing speed, visual learning and memory, verbal learning and memory, and social cognition) and recommended a specific neuropsychological test battery to probe these domains. In order to move the field forward and outline an approach for translational research, there is a need for a "preclinical MATRICS" to develop a rodent test battery that is appropriate for drug development. In this review, we outline such an approach and review current rodent tasks that target these seven domains of cognition. The rodent tasks are discussed in terms of their validity for probing each cognitive domain as well as a brief overview of the pharmacology and manipulations relevant to schizophrenia for each task.

Nicotinic alpha7- or beta2-containing receptor knockout: effects on radial-arm maze learning and long-term nicotine consumption in mice

Classically, it has been thought that high-affinity nicotinic receptors-containing beta2 subunits are the most important receptor subtypes for nicotinic involvement in cognitive function and nicotine self-administration, while low affinity alpha7-containing nicotinic receptors have not been thought to be important. In the current study, we found that knockout of either beta2 or alpha7 subunits caused significant deficits in spatial discrimination in mice. The character of the impairment in the two knockouts was different. The beta2 knockout preferentially impaired cognition in males while the alpha7 caused impairment regardless of sex. Both beta2- and alpha7-containing nicotinic receptors also are important for nicotine self-administration, also in different ways. Most animal model studies of nicotine self-administration are relatively short-term whereas the problem of tobacco addiction is considerably longer-term. To better model the impact of nicotinic receptor subtypes on nicotine self-administration over the long-term, we studied the impact of genetic knockout of low affinity alpha7 receptors vs. high-affinity beta2-containing nicotinic receptors. Mice with knockouts of either of these receptors and their wildtype counter parts were given free access to a choice of nicotine-containing and nicotine-free solution in their home cages on a continuous basis over a period of 5 months. During the first few weeks, the beta2-containing nicotinic receptor knockout mice showed a significant decrease in nicotine consumption relative to wildtype mice, whereas the alpha7 knockout mice did not significantly differ from wildtype controls at the beginning of their access to nicotine. Interestingly, in the longer-term after the first few weeks of nicotine access, the beta2 knockout mice returned to wildtype mouse levels of nicotine consumption, whereas the alpha7 knockout mice developed an emergent decrease in nicotine consumption. The alpha7 receptor knockout-induced decrease in nicotine consumption persisted for the 5-month period of the study. Both alpha7- and beta2-containing nicotinic receptors play critical roles in cognitive function and nicotine self-administration. Regarding cognitive function, beta2-containing receptors are important for maintaining normal sex differences in spatial learning and memory, whereas alpha7 receptors are important for cognitive function regardless of sex. Regarding nicotine self-administration high-affinity beta2-containing nicotinic receptors are important for consumption during the initial phase of nicotine access, but it is the alpha7 nicotinic receptors that are important for the longer-term regulation of nicotine consumption.

Oscillatory bands, neuronal synchrony and hippocampal function: implications of the effects of prenatal choline supplementation for sleep-dependent memory consolidation

Choline supplementation of the maternal diet has long-term facilitative effects on spatial and temporal memory processes in the offspring. To further delineate the impact of early nutritional status on brain and behavior, we examined effects of prenatal-choline availability on hippocampal oscillatory frequency bands in 12 month-old male and female rats. Adult offspring of time-pregnant dams that were given a deficient level of choline (DEF=0.0 g/kg), sufficient choline (CON=1.1 g/kg) or supplemental choline (SUP=3.5 g/kg) in their chow during embryonic days (ED) 12-17 were implanted with an electroencephalograph (EEG) electrode in the hippocampal dentate gyrus in combination with an electromyograph (EMG) electrode patch implanted in the nuchal muscle. Five consecutive 8-h recording sessions revealed differential patterns of EEG activity as a function of awake, slow-wave sleep (SWS) and rapid-eye movement (REM) sleep states and prenatal choline status. The main finding was that SUP rats displayed increased power levels of gamma (30-100 Hz) band oscillations during all phases of the sleep/wake cycle. These findings are discussed within the context of a general review of neuronal oscillations (e.g., delta, theta, and gamma bands) and synchronization across multiple brain regions in relation to sleep-dependent memory consolidation in the hippocampus.

Modulation of hippocampus-dependent learning and synaptic plasticity by nicotine

A long-standing relationship between nicotinic acetylcholine receptors (nAChRs) and cognition exists. Drugs that act at nAChRs can have cognitive-enhancing effects and diseases that disrupt cognition such as Alzheimer's disease and schizophrenia are associated with altered nAChR function. Specifically, hippocampus-dependent learning is particularly sensitive to the effects of nicotine. However, the effects of nicotine on hippocampus-dependent learning vary not only with the doses of nicotine used and whether nicotine is administered acutely, chronically, or withdrawn after chronic nicotine treatment but also vary across different hippocampus-dependent tasks such as the Morris water maze, the radial arm maze, and contextual fear conditioning. In addition, nicotine has variable effects across different types of hippocampal long-term potentiation (LTP). Because different types of hippocampus-dependent learning and LTP involve different neural and molecular substrates, comparing the effects of nicotine across these paradigms can yield insights into the mechanisms that may underlie the effects of nicotine on learning and memory and aid in understanding the variable effects of nicotine on cognitive processes. This review compares and contrasts the effects of nicotine on hippocampus-dependent learning and LTP and briefly discusses how the effects of nicotine on learning could contribute to nicotine addiction.

Comparative effects of the alpha7 nicotinic partial agonist, S 24795, and the cholinesterase inhibitor, donepezil, against aging-related deficits in declarative and working memory in mice

INTRODUCTION

The comparative effects of a newly described specific alpha7 nAChR partial agonist, S 24795, and a cholinesterase inhibitor, donepezil, currently used as a symptomatic Alzheimer's disease treatment were studied in two mouse models of aging-related memory deficits.

MATERIALS AND METHODS

We employed radial arm-maze paradigms assessing short-term working memory (STWM, experiment A) and mnemonic flexibility, a cardinal property of long-term declarative (LTDM, experiment B). Both compounds were administered daily at 0.3 and 1 mg/kg subcutaneously (~3 weeks).

RESULTS

In the STWM experiment, vehicle-treated aged mice displayed a severe and persistent deficit in the retention of successive arm visits in comparison to younger controls. S 24795 at 1 mg/kg (trends at 0.3 mg/kg) and donepezil at 0.3 mg/kg (but not 1 mg/kg) exerted beneficial effects on this deficit: The performance of aged mice treated with these drugs remarkably increased across the testing days and almost reached young adult performance level. In the critical test trials of memory flexibility (i.e., LTDM), in experiment B, S 24795 at 1 mg/kg (trends at 0.3 mg/kg) and donepezil at the dose of 1 mg/kg (but not 0.3 mg/kg) improved aged mice performance.

CONCLUSION

This preclinical demonstration that S 24795 restored specific age-related memory deficits with as much efficacy as donepezil adds to recent literature in highlighting the potential interest of an alpha7 nAChR drug as a symptomatic AD therapeutic.

Multiple brain pathways and receptors underlying tobacco addiction

Over the last 20 years much progress has been made in understanding the pharmacologic basis of tobacco addiction. In particular, the role of nicotine in reinforcing smoking behavior has been studied from a variety of perspectives. This article discusses two important aspects of this topic: (1) brain pathways underlying tobacco addiction; and (2) the actions of nicotine at nicotinic cholinergic receptors. Recent evidence will be reviewed indicating that nicotine reinforces smoking behavior by acting on more than one subtype of nicotinic receptor. Similarly, the role of several brain pathways in tobacco addiction will be considered. Tobacco addiction may thus be seen as a complex neuropsychopharmacological disorder; further progress in smoking cessation treatment may require that we address the multiple molecular and brain components of this addiction.

Nootropic alpha7 nicotinic receptor allosteric modulator derived from GABAA receptor modulators

Activation of brain alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) has broad therapeutic potential in CNS diseases related to cognitive dysfunction, including Alzheimer's disease and schizophrenia. In contrast to direct agonist activation, positive allosteric modulation of alpha7 nAChRs would deliver the clinically validated benefits of allosterism to these indications. We have generated a selective alpha7 nAChR-positive allosteric modulator (PAM) from a library of GABAA receptor PAMs. Compound 6 (N-(4-chlorophenyl)-alpha-[[(4-chloro-phenyl)amino]methylene]-3-methyl-5-isoxazoleacet-amide) evokes robust positive modulation of agonist-induced currents at alpha7 nAChRs, while preserving the rapid native characteristics of desensitization, and has little to no efficacy at other ligand-gated ion channels. In rodent models, it corrects sensory-gating deficits and improves working memory, effects consistent with cognitive enhancement. Compound 6 represents a chemotype for allosteric activation of alpha7 nAChRs, with therapeutic potential in CNS diseases with cognitive dysfunction.

beta2 subunit-containing nicotinic receptors mediate the enhancing effect of nicotine on trace cued fear conditioning in C57BL/6 mice

RATIONALE

Previous research indicates that acute nicotine administration enhances the acquisition of contextual fear conditioning and trace cued fear conditioning. Pharmacological inhibition of alpha4beta2 nicotinic acetylcholine receptors (nAChRs), but not alpha7 nAChRs, blocked the enhancing effect of nicotine on contextual fear conditioning. Similarly, genetic deletion of the beta2 nAChR subunit but not the alpha7 nAChR subunit blocked the enhancing effect of nicotine on contextual fear conditioning.

OBJECTIVES

In the present study, nAChR subunit knockout mice were used to compare the involvement of beta2 subunit-containing nAChRs and alpha7 subunit-containing nAChRs in the effects of nicotine on hippocampus-dependent trace cued fear conditioning and contextual fear conditioning.

METHODS

beta2 nAChR subunit knockout mice, alpha7 nAChR subunit knockout mice, and their wild-type littermates received either nicotine or saline 5 minutes before training and testing. Mice were trained using five conditioned stimulus (CS; 30 s, 85 dB white noise)--trace (30 s)--unconditioned stimulus (US; 2 s footshock) pairings. Freezing to the context and freezing to the CS were assessed 24 h later.

RESULTS

Both contextual and trace cued fear conditioning were enhanced by nicotine administration in wild-type littermates and in alpha7 nAChR subunit knockout mice. In contrast, neither contextual fear conditioning nor trace cued fear conditioning was enhanced by nicotine administration in beta2 nAChR subunit knockout mice.

CONCLUSIONS

These results suggest that beta2 subunit-containing nAChRs but not alpha7 nAChR subunit-containing nAChRs are critically involved in the enhancing effect of nicotine on contextual and trace cued fear conditioning.

Cholinergic modulation of Pavlovian fear conditioning in rats: differential effects of intrahippocampal infusion of mecamylamine and methyllycaconitine

The cholinergic system has consistently been implicated in Pavlovian fear conditioning. Considerable work has been done to localize specific nicotinic receptor subtypes in the hippocampus and determine their functional importance; however, the specific function of many of these subtypes has yet to be determined. An alpha7 nicotinic antagonist methyllycaconitine (MLA) (35 microg), and a broad spectrum non-alpha7 nicotinic antagonist mecamylamine (35 microg) was injected directly into the dorsal hippocampus or overlying cortex either 15 min pre-, 1 min post-, or 6h post-fear conditioning. One week after conditioning, retention of contextual and cue (tone) conditioning were assessed. A significant impairment in retention of contextual fear was observed when mecamylamine was injected 15 min pre- and 1 min post-conditioning. No significant impairment was observed when mecamylamine was injected 6h post-conditioning. Likewise, a significant impairment in retention of contextual fear was observed when MLA was injected 1 min post-conditioning; however, in contrast, MLA did not show any significant impairments when injected 15 min pre-conditioning, but did show a significant impairment when injected 6h post-conditioning. There were no significant impairments observed when either drug was injected into overlying cortex. No significant impairments were observed in cue conditioning for either drug. In general, specific temporal dynamics involved in nicotinic receptor function were found relative to time of receptor dysfunction. The results indicate that the greatest deficits in long-term retention (1 week) of contextual fear are produced by central infusion of MLA minutes to hours post-conditioning or mecamylamine within minutes of conditioning.

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.

The antisaccade task as an index of sustained goal activation in working memory: modulation by nicotine

INTRODUCTION

The antisaccade task provides a laboratory analogue of situations in which execution of the correct behavioural response requires the suppression of a more prepotent or habitual response. Errors (failures to inhibit a reflexive prosaccade towards a sudden onset target) are significantly increased in patients with damage to the dorsolateral prefrontal cortex and patients with schizophrenia. Recent models of antisaccade performance suggest that errors are more likely to occur when the intention to initiate an antisaccade is insufficiently activated within working memory. Nicotine has been shown to enhance specific working memory processes in healthy adults.

MATERIALS AND METHODS

We explored the effect of nicotine on antisaccade performance in a large sample (N = 44) of young adult smokers. Minimally abstinent participants attended two test sessions and were asked to smoke one of their own cigarettes between baseline and retest during one session only.

RESULTS AND CONCLUSION

Nicotine reduced antisaccade errors and correct antisaccade latencies if delivered before optimum performance levels are achieved, suggesting that nicotine supports the activation of intentions in working memory during task performance. The implications of this research for current theoretical accounts of antisaccade performance, and for interpreting the increased rate of antisaccade errors found in some psychiatric patient groups are discussed.

Ventral hippocampal alpha7 and alpha4beta2 nicotinic receptor blockade and clozapine effects on memory in female rats

RATIONALE

Nicotinic systems in the hippocampus play important roles in memory function. Decreased hippocampal nicotinic receptor concentration is associated with cognitive impairment in schizophrenia and Alzheimer's disease.

METHODS

We modeled in rats the cognitive effects of chronic decrease in hippocampal alpha7 or alpha4beta2 receptors with 4-week continuous bilateral local infusions of the alpha7 nicotinic antagonist methyllycaconitine (MLA) or the alpha4beta2 antagonist dihydro-beta-erythroidine (DHbetaE). The working memory effects of these infusions were assessed by performance on the radial-arm maze. To test the effect of antipsychotic medication, we gave acute injections of clozapine and to determine the impact of nicotine, which is widely used by people with schizophrenia approximately half of the rats received chronic systemic infusions of nicotine.

RESULTS

Chronic ventral hippocampal DHbetaE infusion caused a significant (p < 0.001) working memory impairment. Acute systemic clozapine (2.5 mg/kg) caused a significant (p < 0.005) working memory impairment in rats given control aCSF hippocampal infusions. Clozapine significantly (p < 0.025) attenuated the memory deficit caused by chronic hippocampal DHbetaE infusions. Chronic ventral hippocampal infusions with MLA did not significantly affect the working memory performance in the radial-arm maze, but it did significantly (p < 0.05) potentiate the memory impairment caused by 1.25 mg/kg of clozapine. Chronic systemic nicotine did not significantly interact with these effects.

CONCLUSIONS

The state of nicotinic receptor activation in the ventral hippocampus significantly affected the impact of clozapine on working memory with blockade of alpha7 nicotinic receptors potentiating clozapine-induced memory impairment and blockade of alpha4beta2 receptors reversing the clozapine effect from impairing to improving memory.

Nicotinic-antipsychotic drug interactions and cognitive function

In summary, neuronal nicotinic systems are important for a variety of aspects of cognitive function impacted by antipsychotic drugs. It has been demonstrated that antipsychotic drugs have memory and attentional impairing effects when given to unimpaired subjects. Nicotine can reduce some of these impairments, but antipsychotic drug administration can also attenuate nicotine effects. We have found that nicotinic agonists selective for alpha7 and alpha4beta2 receptor subtypes significantly improve learning and memory. Serotonergic actions of antipsychotic drugs may decrease efficacy of nicotinic co-treatments. When the antipsychotic drug clozapine and nicotine are administered to subjects with cognitive impairments caused by NMDA glutamate receptor blockade or hippocampal dysfunction they can significantly attenuate the attentional and memory impairments. Nicotine has been shown in our studies to reverse the memory impairment caused by acute clozapine-induced memory improvement. Acute risperidone and haloperidol has been shown to attenuate nicotine-induced memory improvement. We have determined the role of hippocampal alpha7 and alpha4beta2 nicotinic receptors in the neural basis of nicotinic antipsychotic interactions. Local acute and chronic hippocampal infusion of either nicotinic alpha7 or alpha4beta2 antagonists cause significant spatial working memory impairment. Chronic hippocampal nicotinic antagonist infusions have served as a model of persistent decreases in nicotinic receptor level seen in schizophrenia and Alzheimer's disease. Clozapine attenuated the memory deficit caused by chronic suppression of hippocampal alpha4beta2 receptors while the amnestic effects of clozapine were potentiated by chronic suppression of hippocampal alpha7 receptors. Nicotinic co-treatment may be a useful adjunct in the treatment of schizophrenia, to attenuate cognitive impairment of schizophrenia. Nicotine as well as selective nicotinic alpha7 and alpha4beta2 receptor agonists significantly improve working memory and attentional function. Nicotine treatment was found to be effective in attenuating the attentional and memory impairments caused by the psychototmimetic NMDA antagonist dizocilpine (MK-801), a model of the cognitive impairment of schizophrenia. Studies of the interactions of antipsychotic drugs with nicotinic agents provided quite useful information concerning possible co-treatment of people with schizophrenia with nicotinic therapy. Nicotine was found to significantly attenuate the memory impairments caused by the antipsychotic drugs clozapine and olanzapine. Interestingly, nicotine-induced cognitive improvement was significantly attenuated by the antipsychotic drug clozapine. One of the principal effects of clozapine is to block 5HT2 receptors. Ketanserin a 5HT2 antagonist significantly attenuated nicotine-induced improvements in attention and memory. Thus it appears that antipsychotic drugs with actions blocking 5HT2 receptors may limit the efficacy of nicotinic co-treatments for cognitive enhancement.

Antisense knockdown of the rat alpha7 nicotinic acetylcholine receptor produces spatial memory impairment

Selective and brain penetrating pharmacological antagonists for use in clarifying a role of alpha7 nicotinic acetylcholine receptors (nAChR) in behavioral paradigms are presently unavailable. Studies in alpha7 knock-out mice (KO) have not revealed convincing changes in behavioral phenotype, in particular measures of cognition that include contextual fear conditioning and spatial memory, which may be due to compensatory developmental changes. Therefore, an antisense oligonucleotide (aON) targeted toward the 3'- and 5'-UTR coding regions of the rat alpha7 nicotinic acetylcholine receptor was used. Following central injection of aON into the lateral ventricle of Long Evans rats for 6 days, treated rats exhibited a significant 42% and 25% decrease in alpha7 nAChR densities in hippocampus and cortex, respectively, as measured by [(3)H]-methyllycaconitine (MLA) binding. There was no change in alpha4beta2 densities measured by [(3)H]-cytisine binding. Acquisition of Morris Water Maze (MWM) performance, a measure of spatial memory, was impaired in aON-treated rats. In addition, a reduction in target platform crossings during a subsequent probe-trial was observed. These data demonstrate the ability of this aON to reduce hippocampal and cortical alpha7 nicotinic receptor densities associated with impaired MWM performance and support the specific involvement of the alpha7 nAChR in spatial learning and memory, a phenotype not affected in alpha7 KO mice.

Memory-related deficits following selective hippocampal expression of Swedish mutation amyloid precursor protein in the rat

The gene encoding for the Swedish double mutation (K595N/M596L) of amyloid precursor protein (APP695Swe) was expressed bilaterally in adult rat hippocampus to determine its long-term effects on memory-related behavior as well as amyloid deposition. Recombinant adeno-associated viral serotype 2 (rAAV2) vectors were injected that contained either non-expressing DNA or cDNA encoding for APP695Swe under control of a chicken beta actin/cytomegalovirus promoter/enhancer. Immunolabeling human APP with the antibody 6E10 was observed throughout the cytoplasm of aspiny and, to a lesser extent, spine-bearing hippocampal neurons 6 and 12 months post-injection of the APP695Swe but not control vector. Abeta1-42 immunolabeling was identified in unusual immunoreactive objects within the hilus of the dentate gyrus and in the granule cell layer, proximal to the injection site. At 12 months post-transduction, rats that received the APP695Swe gene also demonstrated significant deficits in the acquisition and probe components of the spatial-memory-related Morris water task compared to control animals. These behavioral deficits occurred in the absence of any amyloid plaques, gliosis, or FluoroJade labeling of dying neurons. In conclusion, prolonged and localized APP695Swe expression in hippocampal neurons is sufficient to produce memory deficits without plaque formation or neuronal loss.