Nicotinic acetylcholine receptors on hippocampal neurons: distribution on the neuronal surface and modulation of receptor activity

Cholinergic Modulation of Dendritic Signaling in Hippocampal GABAergic Inhibitory Interneurons

Dendrites represent the "reception hub" of the neuron as they collect thousands of different inputs and send a coherent response to the cell body. A considerable portion of these signals, especially in vivo, arises from neuromodulatory sources, which affect dendritic computations and cellular activity. In this context, acetylcholine (ACh) exerts a coordinating role of different brain structures, contributing to goal-driven behaviors and sleep-wake cycles. Specifically, cholinergic neurons from the medial septum-diagonal band of Broca complex send numerous projections to glutamatergic principal cells and GABAergic inhibitory neurons in the hippocampus, differentially entraining them during network oscillations. Interneurons display abundant expression of cholinergic receptors and marked responses to stimulation by ACh. Nonetheless, the precise localization of ACh inputs is largely unknown, and evidence for cholinergic modulation of interneuronal dendritic signaling remains elusive. In this article, we review evidence that suggests modulatory effects of ACh on dendritic computations in three hippocampal interneuron subtypes: fast-spiking parvalbumin-positive (PV⁺) cells, somatostatin-expressing (SOM⁺) oriens lacunosum moleculare cells and vasoactive intestinal polypeptide-expressing (VIP⁺) interneuron-selective interneurons. We consider the distribution of cholinergic receptors on these interneurons, including information about their specific somatodendritic location, and discuss how the action of these receptors can modulate dendritic Ca²⁺ signaling and activity of interneurons. The implications of ACh-dependent Ca²⁺ signaling for dendritic plasticity are also discussed. We propose that cholinergic modulation can shape the dendritic integration and plasticity in interneurons in a cell type-specific manner, and the elucidation of these mechanisms will be required to understand the contribution of each cell type to large-scale network activity.

Multiple Roles for Cholinergic Signaling from the Perspective of Stem Cell Function

Stem cells have extensive proliferative potential and the ability to differentiate into one or more mature cell types. The mechanisms by which stem cells accomplish self-renewal provide fundamental insight into the origin and design of multicellular organisms. These pathways allow the repair of damage and extend organismal life beyond that of component cells, and they probably preceded the evolution of complex metazoans. Understanding the true nature of stem cells can only come from discovering how they are regulated. The concept that stem cells are controlled by particular microenvironments, also known as niches, has been widely accepted. Technical advances now allow characterization of the zones that maintain and control stem cell activity in several organs, including the brain, skin, and gut. Cholinergic neurons release acetylcholine (ACh) that mediates chemical transmission via ACh receptors such as nicotinic and muscarinic receptors. Although the cholinergic system is composed of organized nerve cells, the system is also involved in mammalian non-neuronal cells, including stem cells, embryonic stem cells, epithelial cells, and endothelial cells. Thus, cholinergic signaling plays a pivotal role in controlling their behaviors. Studies regarding this signal are beginning to unify our understanding of stem cell regulation at the cellular and molecular levels, and they are expected to advance efforts to control stem cells therapeutically. The present article reviews recent findings about cholinergic signaling that is essential to control stem cell function in a cholinergic niche.

The α7 nicotinic acetylcholine receptors regulate hippocampal adult-neurogenesis in a sexually dimorphic fashion

Disruption in cholinergic signaling has been linked to many environmental and/or pathological conditions known to modify adult neurogenesis. The α7 nAChRs are in the family of cys-loop receptor channels which have been shown to be neuroprotective in adult neurons and are thought to be critical for survival and integration of immature neurons. However, in developing neurons, poor calcium buffering may cause α7 nAChR activation to be neurotoxic. To investigate whether the α7 nAChR regulates neurogenesis in the hippocampus, we used a combination of mouse genetics and imaging to quantify neural stem cell (NSC) densities located in the dentate gyrus of adult mice. In addition, we considered whether the loss of α7 nAChRs had functional consequences on a spatial discrimination task that is thought to rely on pattern separation mechanisms. We found that the loss of α7 nAChRs resulted in increased neurogenesis in male mice only, while female mice showed increased cell divisions and intermediate progenitors but no change in neurogenesis. Knocking out the α7 nAChR from nestin⁺ NSCs and their progeny showed signaling in these cells contributes to regulating neurogenesis. In addition, male, but not female, mice lacking α7 nAChRs performed significantly worse in the spatial discrimination task. This task was sexually dimorphic in wild-type mice, but not in the absence of α7 nAChRs. We conclude that α7 nAChRs regulate adult neurogenesis and impact spatial discrimination function in male, but not female mice, via a mechanism involving nestin⁺ NSCs and their progeny.

Cholinergic modulation of the hippocampal region and memory function

Acetylcholine (ACh) plays an important role in memory function and has been implicated in aging-related dementia, in which the impairment of hippocampus-dependent learning strongly manifests. Cholinergic neurons densely innervate the hippocampus, mediating the formation of episodic as well as semantic memory. Here, we will review recent findings on acetylcholine's modulation of memory function, with a particular focus on hippocampus-dependent learning, and the circuits involved. In addition, we will discuss the complexity of ACh actions in memory function to better understand the physiological role of ACh in memory. This is an article for the special issue XVth International Symposium on Cholinergic Mechanisms.

Effect of galantamine on attention in patients with Alzheimer's disease combined with cerebrovascular disease

AIM

Patients with Alzheimer's disease (AD) and cerebrovascular disease (CVD) show greater attentional deficits compared with AD patients without CVD. The aim of the present study was to investigate the effect of galantamine on attention in AD patients with CVD.

METHODS

In this open trial, 1512 patients with AD and CVD were recruited from 71 nationwide hospitals. The patients were given galantamine for 16 weeks. The primary outcome measure was the score on the Attention Questionnaire Scale (AQS), which measures the patients' attention in their daily lives. The secondary outcome measures were the scores on the Korean Mini-Mental State Examination, the Clinical Dementia Rating scale and the Global Deterioration Scale. Efficacy measures were calculated both at baseline and at the end of the treatment (week 16).

RESULTS

The responders rate on the AQS (change of the AQS from baseline >0) was 60.6% in AD patients with CVD. At the end of the treatment, both the AQS (15.0 ± 5.7 vs 16.3 ± 5.8, P < 0.001) and the Korean Mini-Mental State Examination scores (17.8 ± 4.8 vs 18.1 ± 5.1, P < 0.001) showed a significant improvement relative to the baseline performance. The Clinical Dementia Rating (1.25 ± 0.59 vs 1.22 ± 0.63 P = 0.025) and Global Deterioration Scale (3.82 ± 0.94 vs 3.76 ± 0.96, P = 0.002) scores also showed a significant decrease at the end of the treatment.

CONCLUSIONS

Galantamine is effective in improving attention in the daily lives of AD patients with CVD. Geriatr Gerontol Int 2017; 17: 1661-1668.

The 15q13.3 deletion syndrome: Deficient α(7)-containing nicotinic acetylcholine receptor-mediated neurotransmission in the pathogenesis of neurodevelopmental disorders

Array comparative genomic hybridization (array CGH) has led to the identification of microdeletions of the proximal region of chromosome 15q between breakpoints (BP) 3 or BP4 and BP5 encompassing CHRNA7, the gene encoding the α7-nicotinic acetylcholine receptor (α7nAChR) subunit. Phenotypic manifestations of persons with these microdeletions are variable and some heterozygous carriers are seemingly unaffected, consistent with their variable expressivity and incomplete penetrance. Nonetheless, the 15q13.3 deletion syndrome is associated with several neuropsychiatric disorders, including idiopathic generalized epilepsy, intellectual disability, autism spectrum disorders (ASDs) and schizophrenia. Haploinsufficient expression of CHRNA7 in this syndrome has highlighted important roles the α7nAChR plays in the developing brain and normal processes of attention, cognition, memory and behavior throughout life. Importantly, the existence of the 15q13.3 deletion syndrome contributes to an emerging literature supporting clinical trials therapeutically targeting the α7nAChR in disorders such as ASDs and schizophrenia, including the larger population of patients with no evidence of haploinsufficient expression of CHRNA7. Translational clinical trials will be facilitated by the existence of positive allosteric modulators (PAMs) of the α7nAChR that act at sites on the receptor distinct from the orthosteric site that binds acetylcholine and choline, the receptor's endogenous ligands. PAMs lack intrinsic efficacy by themselves, but act where and when the endogenous ligands are released in response to relevant social and cognitive provocations to increase the likelihood they will result in α7nAChR ion channel activation.

The effects of galantamine treatment on attention and its relationship with cognition and activities of daily living in patients with mild to moderate Alzheimer's disease

BACKGROUND AND PURPOSE

The positive effects of galantamine on cognition and activities of daily living (ADL) in Alzheimer's disease (AD) are thought to be mediated via improvements in attention. The purpose of this study was to determine the effect of galantamine on attention in AD patients using a computerized attention test and to elucidate the relationship between improvements in attention and change in cognition and ADL.

METHODS

In this multicenter, open-label, prospective study, patients with mild to moderate AD received galantamine and then submitted to computerized attention tests, the Alzheimer's Disease Assessment Scale-cognitive subscale, and instrumental ADL (IADL) at baseline, 4 weeks, and 12 weeks. The differences in reaction time on computerized tests were explored relative to the changes in cognition and IADL.

RESULTS

After 12 weeks of taking the trial medication there was a significant reduction from baseline levels in the choice reaction time (baseline, 5,216±3,650 sec; 12 weeks, 4,139±2,920 sec; p<0.01) and the simple reaction time (baseline, 1,089±782 sec; 12 weeks, 908±606 sec; p<0.01). Correlation analyses of changes in choice or simple reaction times relative to cognition and ADL measures yielded no significant associations. The improvement in attention observed at 4 weeks of galantamine treatment was not associated with any significant changes in outcome measures at the end of trial.

CONCLUSIONS

This study found no significant association between the improvement in attention after treatment with galantamine and changes in cognition and ADL in patients with mild to moderate AD, despite the significant improvement in attention over the course of the treatment.

The role of nicotinic receptors in shaping and functioning of the glutamatergic system: a window into cognitive pathology

The involvement of the cholinergic system in learning, memory and attention has long been recognized, although its neurobiological mechanisms are not fully understood. Recent evidence identifies the endogenous cholinergic signaling via nicotinic acetylcholine receptors (nAChRs) as key players in determining the morphological and functional maturation of the glutamatergic system. Here, we review the available experimental and clinical evidence of nAChRs contribution to the establishment of the glutamatergic system, and therefore to cognitive function. We provide some clues of the putative underlying molecular mechanisms and discuss recent human studies that associate genetic variability of the genes encoding nAChR subunits with cognitive disorders. Finally, we discuss the new avenues to therapeutically targeting nAChRs in persons with cognitive dysfunction for which the α7-nAChR subunit is an important etiological mechanism.

Synaptic pattern of nicotinic acetylcholine receptor α7 and β2 subunits on the direction-selective retinal ganglion cells in the postnatal mouse retina

Direction-selective retinal ganglion cells (DS RGCs) respond strongly to a stimulus that moves in their preferred direction, but respond weakly or do not respond to a stimulus that moves in the opposite or null direction. DS RGCs are sensitive to acetylcholine, and starburst amacrine cells (SACs) make cholinergic synapses on DS RGCs. We studied the distributions of nicotinic acetylcholine receptor (nAChR) α7 and β2 subunits on the dendritic arbors of DS RGCs to search for anisotropies that contribute to the directional preferences of DS RGCs. The DS RGCs from the retinas of postnatal mice (postnatal day P5, P10, and P15) were injected with Lucifer yellow, and injected cells were identified by their dendritic morphology. The dendrites of the DS RGCs were labeled with antibodies for either the nAChR α7 or β2 subunit as well as postsynaptic density protein-95 (PSD-95), visualized by confocal microscopy, and reconstructed from high-resolution confocal images. The distribution of nAChR subunits on the dendritic arbors in both the ON and OFF layers of the RGCs revealed an asymmetrical pattern on early postnatal day P5. However, the distributions of nAChR subunits on the dendritic arbors were not asymmetric on P10 and P15. Our results therefore provide anatomical and developmental evidence suggesting that the nAChR α7 and β2 subunits may involve in the early direction-selectivity formation of DS RGCs in the mouse retina.

Effects of blockade of α4β2 and α7 nicotinic acetylcholine receptors on cue-induced reinstatement of nicotine-seeking behaviour in rats

Exposure to environmental stimuli conditioned to nicotine consumption critically contributes to the high relapse rates of tobacco smoking. Our previous work demonstrated that non-selective blockade of nicotinic acetylcholine receptors (nAChRs) reversed the cue-induced reinstatement of nicotine seeking, indicating a role for cholinergic neurotransmission in the mediation of the conditioned incentive properties of nicotine cues. The present study further examined the relative roles of the two major nAChR subtypes, α4β2 and α7, in the cue-induced reinstatement of nicotine seeking. Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion, free base) on a fixed-ratio 5 schedule of reinforcement. A nicotine-conditioned cue was established by associating a sensory stimulus with each nicotine infusion. After nicotine-maintained responding was extinguished by withholding the nicotine infusion and its paired cue, reinstatement test sessions were conducted with re-presentation of the cue but without the availability of nicotine. Thirty minutes before the tests, the rats were administered the α4β2-selective antagonist dihydro-β-erythroidine (DHβE) and α7-selective antagonist methyllycaconitine (MLA). Pretreatment with MLA, but not DHβE, significantly reduced the magnitude of the cue-induced reinstatement of responses on the active, previously nicotine-reinforced lever. In different sets of rats, MLA altered neither nicotine self-administration nor cue-induced reinstatement of food seeking. These results demonstrate that activation of α7 nAChRs participates in the mediation of the conditioned incentive properties of nicotine cues and suggest that α7 nAChRs may be a promising target for the development of medications for the prevention of cue-induced smoking relapse.

Tropisetron suppresses collagen synthesis in skin fibroblasts via α7 nicotinic acetylcholine receptor and attenuates fibrosis in a scleroderma mouse model

OBJECTIVE

There is increasing evidence that serotonin (5-hydroxytryptamine [5-HT]) and distinct 5-HT receptors are involved in the pathogenesis of systemic sclerosis. The aim of this study was to test the hypothesis that tropisetron, a routinely used antiemetic agent previously characterized as a 5-HT(3/4) receptor-modulating agent, can directly affect collagen synthesis in vitro and attenuate experimentally induced fibrosis in vivo.

METHODS

Functional in vitro studies were performed using human dermal fibroblasts (HDFs). Signal transduction studies included immunofluorescence analysis, Western immunoblotting, promoter reporter assays, cAMP/Ca(2+) measurements, and use of pharmacologic activators and inhibitors. Gene silencing was performed using small interfering RNA. Putative receptors of tropisetron were detected by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescence. The murine model of bleomycin-induced scleroderma was used to assess the antifibrogenic and antifibrotic effects of tropisetron in vivo. Collagen expression in vitro, ex vivo, and in situ was determined by real-time RT-PCR analysis, Western immunoblotting, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and immunohistochemical analysis.

RESULTS

Tropisetron suppressed collagen synthesis induced by transforming growth factor β1 (TGFβ1). This effect was independent of 5-HT(3/4) receptor but was mediated via α7 nicotinic acetylcholine receptor (α7nAChR). Suppression of TGFβ1-induced collagen synthesis occurred via an unknown molecular mechanism not involving modulation of the Smad, cAMP, Akt, c-Jun, or MAPK pathway. In vivo, tropisetron not only prevented skin fibrosis but also reduced the collagen content in established dermal fibrosis induced by bleomycin.

CONCLUSION

Tropisetron directly reduces collagen synthesis in HDFs via an α7nAChR-dependent mechanism. The antifibrogenic and antifibrotic effects of this agent observed in a mouse model of bleomycin- induced scleroderma indicate the future potential of tropisetron in the treatment of fibrotic diseases such as scleroderma.

Age dependent nicotinic influences over dopamine neuron synaptic plasticity

The dopamine (DA) system of the ventral midbrain plays a critical role as mammals learn adaptive behaviors driven by environmental salience and reward. Addictive drugs, including nicotine, exert powerful influences over the mesolimbic DA system by activating and desensitizing nicotinic acetylcholine receptors (nAChRs) in a subtype-dependent manner. Nicotine induces synaptic plasticity at excitatory synapses onto DA neurons, thereby sending elevated DA signals that participate during the reinforcement of addictive behaviors. While humans and animals of any developmental age are potentially vulnerable to these drug-induced effects, evidence from clinical and epidemiological studies indicates that adolescents have an increased risk of addiction. Although this risk arises from a complex set of variables including societal and psychosocial influences, a contributing factor involves age dependent sensitivity to addictive drugs. One aspect of that sensitivity is drug-induced synaptic plasticity at excitatory synapses onto the dopamine neurons in the ventral midbrain. A single, acute exposure to addictive drugs, including nicotine, produces long-term potentiation (LTP) that can be quantified by measuring the shift in the subtypes of ionotropic glutamate receptors mediating evoked synaptic transmission. This change in glutamatergic transmission is expressed as an increased ratio of AMPA receptors to NMDA receptors at glutamatergic synapses. Age-related differences in the excitability and the nicotine sensitivity within the midbrain dopamine system may contribute to the greater risk of nicotine addiction in adolescent animals and humans.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

Effects of galantamine on measures of attention: results from 2 clinical trials in Alzheimer disease patients with comparisons to donepezil

Deficits in attention are present early in the course of Alzheimer disease (AD). Acetylcholine receptors are appealing molecular targets for intervention as cholinergic pathways are involved in the neurobiology of attention. For this reason, measures of attention were included in 2 independent, multicenter, randomized, parallel, controlled trials in subjects with AD comparing the effects of galantamine, an acetylcholinesterase inhibitor and postulated nicotinic receptor modulator, and donepezil, an acetylcholinesterase inhibitor. The attention battery of the Cognitive Drug Research computerized assessment system was used in both trials. Small magnitude, positive signals were observed for simple and choice reaction times for both compounds. Attention task performance tended to improve early for galantamine-treated subjects. A consistent temporal pattern of improvement was not observed in donepezil-treated subjects. Quantitative findings appeared more pronounced in subjects with moderate AD. Galantamine's proposed action as a nicotinic receptor modulator may bear on these findings. Improved attention may have positive effects on cognitive and functional outcomes for AD patients, although this hypothesis requires further study and validation.

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.

Impaired muscarinic regulation of excitatory synaptic transmission in the APPswe/PS1dE9 mouse model of Alzheimer's disease

Cholinergic hypothesis and amyloid cascade hypothesis are mainly proposed for Alzheimer's disease; however, the relationship between these hypotheses is poorly understood. To address the question of whether amyloid beta-peptide pathology affects cholinergic neurotransmission, we examined the effect of a cholinesterase inhibitor, physostigmine, on field excitatory postsynaptic potentials (EPSPs) evoked by single-pulse stimulation in the CA1 region of the hippocampus of various APPswe/PS1dE9 transgenic mice with different degrees of amyloid beta-peptide pathology. Reduced field EPSPs by physostigmine in transgenic mice at 3 months of age, when the mice had negligible amyloid beta-peptide levels and no amyloid beta-peptide deposits, were indistinguishable from those in age-matched wild-type mice. In contrast, reduced field EPSPs by physostigmine in transgenic mice at 5 months of age, when the mice had low amyloid beta-peptide levels and subtle amyloid beta-peptide deposits, were significantly lower than those in age-matched wild-type mice. Next, we characterized acetylcholine receptors, which play important roles in cholinergic neurotransmission, because physostigmine resulted in increased acetylcholine levels in the synaptic cleft. Different reductions of field EPSPs by physostigmine between transgenic and wild-type mice at 5 months of age were not affected by a nicotinic receptor antagonist, mecamylamine; however, reduced field EPSPs by physostigmine in both transgenic and wild-type mice were restored to basal levels by a muscarinic receptor antagonist, atropine. These results indicate that cholinergic modulation of glutamatergic transmission is already impaired at the onset of the formation of amyloid beta-peptide deposits, and muscarinic receptor dysfunction is one of the causes of this impairment.

Imidacloprid induces neurobehavioral deficits and increases expression of glial fibrillary acidic protein in the motor cortex and hippocampus in offspring rats following in utero exposure

Imidacloprid, a neonicotinoid, is one of the fastest growing insecticides in use worldwide because of its selectivity for insects. The potential for neurotoxicity following in utero exposure to imidacloprid is not known. Timed pregnant Sprague-Dawley rats (300-350 g) on d 9 of gestation were treated with a single intraperitoneal injection (i.p.) of imidacloprid (337 mg/kg, 0.75 x LD50, in corn oil). Control rats were treated with corn oil. On postnatal day (PND) 30, all male and female offspring were evaluated for (a) acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activity, (b) ligand binding for nicotinic acetylcholine receptors (nAChR) and muscarinic acetylcholine receptors (m2 mAChR), (c) sensorimotor performance (inclined plane, beam-walking, and forepaw grip), and (d) pathological alterations in the brain (using cresyl violet and glial fibrillary acidic protein [GFAP] immunostaining). The offspring of treated mothers exhibited significant sensorimotor impairments at PND 30 during behavioral assessments. These changes were associated with increased AChE activity in the midbrain, cortex and brainstem (125-145% increase) and in plasma (125% increase). Ligand binding densities for [3H]cytosine for alpha4beta2 type nAchR did not show any significant change, whereas [3H]AFDX 384, a ligand for m2mAChR, was significantly increased in the cortex of offspring (120-155% increase) of imidacloprid-treated mothers. Histopathological evaluation using cresyl violet staining did not show any alteration in surviving neurons in various brain regions. On the other hand, there was a rise in GFAP immunostaining in motor cortex layer III, CA1, CA3, and the dentate gyrus subfield of the hippocampus of offspring of imidacloprid-treated mothers. The results indicate that gestational exposure to a single large, nonlethal, dose of imidacloprid produces significant neurobehavioral deficits and an increased expression of GFAP in several brain regions of the offspring on PND 30, corresponding to a human early adolescent age. These changes may have long-term adverse health effects in the offspring.

Nicotinic acetylcholine receptors and nicotinic cholinergic mechanisms of the central nervous system

Subtypes of neuronal nicotinic acetylcholine receptors (nAChRs) are constructed from numerous subunit combinations that compose channel-receptor complexes with varied functional and pharmacological characteristics. Structural and functional diversity and the broad presynaptic, postsynaptic, and nonsynaptic locations of nAChRs underlie their mainly modulatory roles throughout the mammalian brain. Presynaptic and preterminal nicotinic receptors enhance neurotransmitter release, postsynaptic nAChRs contribute a small minority of fast excitatory transmission, and nonsynaptic nAChRs modulate many neurotransmitter systems by influencing neuronal excitability. Nicotinic receptors have roles in development and synaptic plasticity, and nicotinic mechanisms participate in learning, memory, and attention. Decline, disruption, or alterations of nicotinic cholinergic mechanisms contribute to dysfunctions such as epilepsy, schizophrenia, Parkinson's disease, autism, dementia with Lewy bodies, Alzheimer's disease, and addiction.

Blockade of Ca2+-activated K+ channels by galantamine can also contribute to the potentiation of catecholamine secretion from chromaffin cells

Galantamine is a drug in clinical use for the treatment of Alzheimer's disease, but its mechanism(s) of action remains controversial. Here we addressed the question whether galantamine could potentiate neurotransmitter release by inhibiting small conductance Ca2+ -activated K+ channels (KCa2). Galantamine potentiated catecholamine secretory responses induced by 10 s pulses of acetylcholine and high [K+]o applied to fast-superfused bovine adrenal chromaffin cell populations. Catecholamine release was significantly enhanced by galantamine although we did not find concentration dependence in the range 0.1-1 microM. The KCa2 channel blocker apamin (0.3 microM) occluded the potentiating effects of galantamine on acetylcholine-evoked secretion. Like apamin, galantamine also modified the firing of action potentials, but to a lesser extent. In addition, 1 microM galantamine reduced by 41% the KCa2 current without modifying the voltage-dependent Ca2+ currents. These results constitute the first direct evidence that galantamine can potentiate neurotransmitter release by blocking KCa2 channels, in addition to its already demonstrated capacity to mildly block acetylcholinesterase or potentiate allosterically nicotinic receptors.

Mouse strain-specific changes in nicotinic receptor expression with age

The onset and severity of age-related loss of neuronal nicotinic acetylcholine receptor (nAChR) expression in the mammalian hippocampus can vary considerably between individuals. We have examined the expression of four nAChR subunits (nAChR alpha4, nAChR alpha5, nAChR alpha7 and nAChR beta4) in the dorsal hippocampus of adult (12-14 months) and aged (24-28 month) animals from two-mouse strains (CBA/J and C57BL/6). The expression of nAChR alpha4 was selectively diminished with age in both strains, and there was a significant loss of nAChR alpha7 in CA1 of aged CBA/J, but not C57BL/6. There was no change in nAChR alpha5 expression with age whereas nAChR beta4 preferentially diminished in the C57BL/6 CA1 region and remained the same or slightly increased in the aged CBA/J. Coincident with the loss of neuronal nAChR alpha4 in the CBA/J strain was a significant age-related increase of nAChR alpha4 staining of astrocytes, most notably in the stratum radiatum. These results suggest that mouse strains of different genetic backgrounds undergo dissimilar age-related changes in the expression of nAChRs.

Galantamine prevents apoptosis induced by beta-amyloid and thapsigargin: involvement of nicotinic acetylcholine receptors

Galantamine is currently used to treat Alzheimer's disease patients; it behaves as a mild blocker of acetylcholinesterase (AChE) and has an allosteric modulating action on nicotinic acetylcholine receptors (nAChRs). In this study, we observed that galantamine prevented cell death induced by the peptide beta-amyloid(1-40) and thapsigargin in the human neuroblastoma cell line SH-SY5Y, as well as in bovine chromaffin cells. The protective effect of galantamine was concentration-dependent in both cell types; maximum protection was produced at 300 nM. The antiapoptotic effect of galantamine at 300 nM, against beta-amyloid(1-40) or thapsigargin-induced toxicity, was reversed by alpha-bungarotoxin. At neuroprotective concentrations, galantamine caused a mild and sustained elevation of the cytosolic concentration of calcium, [Ca2+]c, measured in single cells loaded with Fura-2. Incubation of the cells for 48 h with 300 nM galantamine doubled the density of alpha7 nicotinic receptors and tripled the expression of the antiapoptotic protein Bcl-2. These results strongly suggest that galantamine can prevent apoptotic cell death by inducing neuroprotection through a mechanism related to that described for nicotine, i.e. activation of nAChRs and upregulation of Bcl-2. These findings might explain the long-term beneficial effects of galantamine in patients suffering of Alzheimer's disease.

Release of [3h]-l-glutamate by stimulation of nicotinic acetylcholine receptors in rat cerebellar slices

This is a neurochemical study which shows that nicotine acting through alpha7-containing nicotinic acetylcholine receptors promotes the release of [(3)H]-glutamate from rat cerebellar slices. Release evoked by half maximal concentration of nicotine (100 microM) was blocked by alpha-bungarotoxin and in a calcium-free medium, suggesting an effect mediated by an alpha7 receptor. Dihydro-beta-erythroidine and mecamylamine were effective only at very high concentrations, excluding the participation of heteromeric receptors. The effect of nicotine was partially blocked by inhibitors of glutamatergic receptors DL-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, indicating a glutamate-induced glutamate release. Nicotine-evoked response was dependent on activation of tetrodotoxin sensitive sodium channels. Therefore, here we show that glutamate released by stimulation of alpha7-containing nicotinic receptors, located preterminal and/or postsynaptically, evokes a further glutamate release in adult rat cerebellar slices.

Selective vulnerability of cerebellar granule neuroblasts and their progeny to drugs with abuse liability

Cerebellar development is shaped by the interplay of genetic and numerous environmental factors. Recent evidence suggests that cerebellar maturation is acutely sensitive to substances with abuse liability including alcohol, opioids, and nicotine. Assuming substance abuse disrupts cerebellar maturation, a central question is: what are the basic mechanisms underlying potential drug-induced developmental defects? Evidence reviewed herein suggests that the maturation of granule neurons and their progeny are intrinsically affected by several classes of substances with abuse liability. Although drug abuse is also likely to target directly other cerebellar neuron and glial types, such as Purkinje cells and Bergmann glia, findings in isolated granule neurons suggest that they are often the principle target for drug actions. Developmental events that are selectively disrupted by drug abuse in granule neurons and/or their neuroblast precursors include proliferation, migration, differentiation (including neurite elaboration and synapse formation), and programmed cell death. Moreover, different classes of drugs act through distinct molecular mechanisms thereby disrupting unique aspects of development. For example, drug-induced perturbations in: (i) neurotransmitter biogenesis; (ii) ligand and ion-gated receptor function and their coupling to intracellular effectors; (iii) neurotrophic factor biogenesis and signaling; and (iv) intercellular adhesion are all likely to have significant effects in shaping developmental outcome. In addition to identifying therapeutic strategies for drug abuse intervention, understanding the mechanisms by which drugs affect cellular maturation is likely to provide a better understanding of the neurochemical events that normally shape central nervous system development.

Comparison of alpha7 nicotinic acetylcholine receptor development in the hippocampal formation of C3H and DBA/2 mice

A strain-specific restriction fragment length polymorphism in the alpha7 receptor gene locus has been reported to significantly affect the expression of the alpha7 subtype of nicotinic receptor in adult mouse hippocampus. The goal of the present study was to characterize the development of the alpha7 receptor in hippocampus from two mouse strains (C3H and DBA/2) with different alleles of the alpha7 receptor gene locus by using alpha-bungarotoxin (alpha-BTX) autoradiography. Binding of alpha-BTX was initially detected in fetal C3H mice on embryonic day 13 (E13) in the dorsal portion of the hippocampal anlage. In contrast, alpha-BTX binding was initially detected primarily in hippocampal area CA3 in the DBA/2 strain on E16. Binding of alpha-BTX was absent from the neuroepithelium in both strains. A marked increase in alpha-BTX binding was observed in hippocampal area CA1 and to a lesser extent in area CA3 between E18 and postnatal day 5 (P5) in neonatal C3H mice, an increase that was not observed in the DBA/2 mice. By the end of the first postnatal month, hippocampal alpha-BTX binding appeared adult-like in each strain. These data suggest that variations in the alpha7 receptor gene locus differentially influence the developmental expression of the alpha7 receptor in murine hippocampus. Therefore, the potential influence of the alpha7 receptor on developmental processes such as cell migration, dendritic elaboration and/or axonal connectivity may exhibit strain-selective differences because of the dissimilar time courses of alpha7 receptor expression in C3H and DBA/2 mice.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.

Galantamine is an allosterically potentiating ligand of neuronal nicotinic but not of muscarinic acetylcholine receptors

Galantamine (Reminyl), an approved treatment for Alzheimer's disease (AD), is a potent allosteric potentiating ligand (APL) of human alpha 3 beta 4, alpha 4 beta 2, and alpha 6 beta 4 nicotinic receptors (nAChRs), and of the chicken/mouse chimeric alpha 7/5-hydroxytryptamine3 receptor, as was shown by whole-cell patch-clamp studies of human embryonic kidney-293 cells stably expressing a single nAChR subtype. Galantamine potentiates agonist responses of the four nAChR subtypes studied in the same window of concentrations (i.e., 0.1-1 microM), which correlates with the cerebrospinal fluid concentration of the drug at the recommended daily dosage of 16 to 24 mg. At concentrations >10 microM, galantamine acts as an nAChR inhibitor. The other presently approved AD drugs, donepezil and rivastigmine, are devoid of the nicotinic APL action; at micromolar concentrations they also block nAChR activity. Using five CHO-SRE-Luci cell lines, each of them expressing a different human muscarinic receptor, and a reporter gene assay, we show that galantamine does not alter the activity of M1-M5 receptors, thereby confirming that galantamine modulates selectively the activity of nAChRs. These studies support our previous proposal that the therapeutic action of galantamine is mainly produced by its sensitizing action on nAChRs rather than by general cholinergic enhancement due to cholinesterase inhibition. Galantamine's APL action directly addresses the nicotinic deficit in AD.

Cholinergic plasticity in the hippocampus

Tests were made for use-dependent plasticity in the cholinergic projections to hippocampus. Transient infusion of the cholinergic agonist carbachol into hippocampal slices induced rhythmic activity that persisted for hours after washout. Comparable effects were obtained with physostigmine, a drug that blocks acetylcholine breakdown and thereby enhances cholinergic transmission. It thus seems that activation of cholinergic synapses induces lasting changes in hippocampal physiology. Two lines of evidence indicated that cholinergic synapses are also the sites at which the plasticity is expressed. First, the induction and expression of the rhythms were not blocked by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid, indicating that a long-term potentiation effect between pyramidal cells was not involved. Second, a muscarinic antagonist (atropine) completely abolished stable rhythmic activity after agonist washout. This result indicates that endogenous cholinergic activity is responsible for the persistence of rhythmic oscillations. These experiments suggest that short periods of intense cholinergic activity induce lasting changes in cholinergic synapses and thus extend such forms of plasticity to beyond the glutamatergic system.

Nicotinic acetylcholine receptors as targets for antidepressants

While the monoamine deficiency hypothesis of depression is still most commonly used to explain the actions of antidepressant drugs, a growing body of evidence has accumulated that is not adequately explained by the hypothesis. This article draws attention to contributions from another apparently common pharmacological property of antidepressant medications--the inhibition of nicotinic acetylcholine receptors (nAChR). Evidence is presented suggesting the hypercholinergic neurotransmission, which is associated with depressed mood states, may be mediated through excessive neuronal nicotinic receptor activation and that the therapeutic actions of many antidepressants may be, in part, mediated through inhibition of these receptors. In support of this hypothesis, preliminary evidence is presented suggesting that the potent, centrally acting nAChR antagonist, mecamylamine, which is devoid of monoamine reuptake inhibition, may reduce symptoms of depression and mood instability in patients with comorbid depression and bipolar disorder. If this hypothesis is supported by further preclinical and clinical research, nicotinic acetylcholine receptor antagonists may represent a novel class of therapeutic agents for treating mood disorders.

Identification of cholinoceptive glycinergic neurons in the mammalian retina

The light-evoked release of acetylcholine (ACh) affects the responses of many retinal ganglion cells, in part via nicotinic acetylcholine receptors (nAChRs). nAChRs that contain beta2alpha3 neuronal nicotinic acetylcholine receptors have been identified and localized in the rabbit retina; these nAChRs are recognized by the monoclonal antibody mAb210. We have examined the expression of beta2alpha3 nAChRs by glycinergic amacrine cells in the rabbit retina and have identified different subpopulations of nicotinic cholinoceptive glycinergic cells using double and triple immunohistochemistry with quantitative analysis. Here we demonstrate that about 70% of the cholinoceptive amacrine cells in rabbit retina are glycinergic cells. At least three nonoverlapping subpopulations of mAb210 glycine-immunoreactive cells can be distinguished with antibodies against calretinin, calbindin, and gamma-aminobutyric acid (GABA)(A) receptors. The cholinergic cells in rabbit retina are thought to synapse only on other cholinergic cells and ganglion cells. Thus, the expression of beta2alpha3 nAChRs on diverse populations of glycinergic cells is puzzling. To explore this finding, the subcellular localization of beta2alpha3 was studied at the electron microscopic level. mAb210 immunoreactivity was localized on the dendrites of amacrines and ganglion cells throughout the inner plexiform layer, and much of the labeling was not associated with recognizable synapses. Thus, our findings indicate that ACh in the mammalian retina may modulate glycinergic circuits via extrasynaptic beta2alpha3 nAChRs.

Increased sensitivity to nicotine-induced seizures in mice expressing the L250T alpha 7 nicotinic acetylcholine receptor mutation

High doses of nicotine, the addictive component of tobacco, induce clonic-tonic seizures in animals. Pharmacological and biochemical data have suggested that alpha 7-containing neuronal nicotinic receptors (nAChRs) contribute to these seizures. To study potential alpha 7 contributions, we examined alpha 7 subunits with a Leu250-to-Thr substitution in the channel domain, which creates a gain-of-function mutation. Previous studies have shown that mice homozygous for the alpha 7 L250T mutation (T/T) die shortly after birth, but animals heterozygous for the mutation (+/T) are viable and grow to adulthood. Hippocampal neurons from the +/T mice exhibited altered alpha 7-type currents with increased amplitudes and slower desensitization kinetics, confirming a partial gain of function for the alpha 7 nAChR. We found that +/T mice were more sensitive to the convulsant effects of nicotine compared with their wild-type (+/+) littermates. Furthermore, although their behavior was normal in basal conditions, +/T mice showed a unique nicotine-induced phenotype, consisting of head-bobbing and paw-tapping movements. Increased sensitivity to nicotine-induced seizures occurred despite a 60% decline in brain alpha 7 nAChR protein levels. There were no changes in the levels of alpha 4, alpha 5, alpha 6, alpha 7, beta 2, and beta 4 mRNA, or in [(125)I]epibatidine and [(3)H]nicotine binding between +/T and +/+ mice. Recent data from our laboratory show that alpha 7-null mice maintain normal sensitivity to nicotine-induced seizures. Hence, these present findings suggest that alterations in the properties rather than absence of alpha 7 nAChRs might affect the mechanisms underlying the convulsive properties of nicotine.

The role of nicotinic acetylcholine receptors in the mechanisms of anesthesia

Nicotinic acetylcholine receptors are members of the ligand-gated ion channel superfamily, that includes also gamma-amino-butiric-acid(A), glycine, and 5-hydroxytryptamine(3) receptors. Functional nicotinic acetylcholine receptors result from the association of five subunits each contributing to the pore lining. The major neuronal nicotinic acetylcholine receptors are heterologous pentamers of alpha4beta2 subunits (brain), or alpha3beta4 subunits (autonomic ganglia). Another class of neuronal receptors that are found both in the central and peripheral nervous system is the homomeric alpha7 receptor. The muscle receptor subtypes comprise of alphabetadeltagamma (embryonal) or alphabetadeltaepsilon (adult) subunits. Although nicotinic acetylcholine receptors are not directly involved in the hypnotic component of anesthesia, it is possible that modulation of central nicotinic transmission by volatile agents contributes to analgesia. The main effect of anesthetic agents on nicotinic acetylcholine receptors is inhibitory. Volatile anesthetics and ketamine are the most potent inhibitors both at alpha4beta2 and alpha3beta4 receptors with clinically relevant IC(50) values. Neuronal nicotinic acetylcholine receptors are more sensitive to anesthetics than their muscle counterparts, with the exception of the alpha7 receptor. Several intravenous anesthetics such as barbiturates, etomidate, and propofol exert also an inhibitory effect on the nicotinic acetylcholine receptors, but only at concentrations higher than those necessary for anesthesia. Usual clinical concentrations of curare cause competitive inhibition of muscle nicotinic acetylcholine receptors while higher concentrations may induce open channel blockade. Neuronal nAChRs like alpha4beta2 and alpha3beta4 are inhibited by atracurium, a curare derivative, but at low concentrations the alpha4beta2 receptor is activated. Inhibition of sympathetic transmission by clinically relevant concentrations of some anesthetic agents is probably one of the factors involved in arterial hypotension during anesthesia.

Synaptic plasticity and nicotine addiction

Nicotine, the main addictive component of tobacco, activates and desensitizes nicotinic acetylcholine receptors (nAChRs). In that way, nicotine alters normal nicotinic cholinergic functions. Among the myriad of psychopharmacological effects that underlie the addiction process, nicotine influences nAChR participation in synaptic plasticity. This influence has particular importance in the mesocorticolimbic dopamine system, which serves during the reinforcement of rewarding behaviors.

Modulation of nicotinic receptor activity in the central nervous system: a novel approach to the treatment of Alzheimer disease

Impaired cholinergic function in the central nervous system is an early feature of Alzheimer disease (AD). Currently, cholinergic deficit is usually corrected by increasing the amount of acetylcholine in the synapse by inhibiting acetylcholinesterase (AChE). One of the most consistent cholinergic deficits in AD is the reduced expression of nicotinic acetylcholine receptors (nAChR) in the brain. Since these receptors are essential for learning and memory, restoring nicotinic cholinergic function is a promising approach to treating AD. Allosteric modulation of nAChR is a novel approach, which circumvents development of tolerance through long-term use of conventional nicotinic agonists. Allosteric modulators interact with receptor-binding sites distinct from those capable of recognizing the natural agonist. Positive allosteric modulation of nAChR activity has no effect on conductance of single channels; instead, by facilitating channel opening, it potentiates responses evoked by the interaction of the natural agonist with presynaptic and postsynaptic nAChR. Allosteric modulation of nAChR activity could therefore potentially produce a significant benefit in AD. One such allosteric modulator is galantamine. In addition to increasing nAChR activity, galantamine also inhibits AChE. This novel, dual mechanism of action distinguishes galantamine from many other AChE inhibitors. Galantamine has been shown to improve cognitive and daily function for at least 6 months in placebo-controlled trials, and to maintain these functions at baseline levels for at least 12 months in a 6-month open-label extension study. Galantamine has positive effects on nAChR expression, which are likely to contribute to its sustained efficacy in the treatment of AD patients.

Chronic nicotine exposure reduces N-methyl-D-aspartate receptor-mediated damage in the hippocampus without altering calcium accumulation or extrusion: evidence of calbindin-D28K overexpression

Neuronal accumulation of excess Ca2+ has been implicated in cellular death following several forms of physical and chemotoxic insult. Recent studies have suggested that exposure to agonists at brain nicotinic acetylcholine receptors reduces cytotoxic consequences of increased intracellular Ca2+ following some insults. In the present study, the ability of chronic exposure to (-)-nicotine to reduce cytotoxicity and attenuate increases in intracellular Ca2+ caused by exposure to N-methyl-D-aspartate were examined in organotypic cultures of rat hippocampus. Cultures were exposed to nicotine (0.1-10.0 microM) for five days prior to excitotoxic insult with N-methyl-D-aspartate. Exposure to N-methyl-D-aspartate produced concentration-dependent increases in both accumulation of 45Ca and in early and delayed cell death in the CA1, CA3 and dentate gyrus regions of cultures. The CA1 region of the hippocampus displayed the greatest sensitivity to cytotoxic effects of N-methyl-D-aspartate exposure; however, this regional difference was not associated with increased accumulation of 45Ca. Prior exposure to nicotine markedly attenuated N-methyl-D-aspartate-induced early and delayed cell death in each hippocampal region at concentrations as low as 0.1microM. However, nicotine did not alter the initial N-methyl-D-aspartate-stimulated influx of 45Ca or enhance extrusion of accumulated 45Ca measured at several time-points after insult. Five days of exposure to nicotine markedly increased immunoreactivity of the Ca2+ binding protein calbindin-D28K in each region of hippocampal cultures, effects reduced by mecamylamine co-exposure. These findings suggest that the potent protective effects of chronic nicotine exposure against neuronal overexcitation are not likely attributable to attenuations of Ca2+ accumulation, but are likely related to increased buffering of accumulated Ca2+.

Cholinergic modulation of excitatory synaptic transmission in the CA3 area of the hippocampus

Cholinergic innervation of the hippocampus has been implicated in memory formation and retrieval. Here we study cholinergic modulation of excitatory transmission in the CA3 area of the rat hippocampus. We used a combination of optical measurements of presynaptic calcium and electrophysiological measurements of synaptic currents to study associational-commissural (A/C) and mossy fiber (MF) synapses in brain slices. Direct synaptic modulation mediated by ACh receptors is only evident at the A/C synapse, where synaptic inhibition primarily reflects presynaptic calcium channel inhibition mediated by muscarinic receptors. MF synapses can, however, be indirectly modulated by muscarinic receptor activation. Muscarine elevates the firing rate of inhibitory cells, which increases GABA release and inhibits MF synapses by activating presynaptic GABA(B) receptors. Muscarine also depolarizes dentate granule cells and elevates their rate of firing. This leads to synaptic enhancement when combined with the use-dependent facilitation of MF synapses. In addition we were unable to evoke an increase in presynaptic calcium levels in MF boutons with local application of nicotinic receptor agonists. This finding does not support a leading hypothesis for MF modulation in which activation of presynaptic nicotinic receptors enhances transmission directly by elevating presynaptic calcium levels. However, indirect synaptic modulation could arise from nicotinic excitation of inhibitory neurons. Thus, to understand cholinergic modulation within the CA3 region, it is necessary to take into account secondary actions on synapses arising from other chemical messengers released by other cell types and to consider effects on firing patterns of presynaptic cells, which in turn influence release via use-dependent synaptic plasticity.

Chronic treatment of old rats with donepezil or galantamine: effects on memory, hippocampal plasticity and nicotinic receptors

The function of the cholinergic system is known to change during normal aging and in pathological conditions such as Alzheimer's disease. The present study was designed to assess, within the same group of old animals, the behavioral, electrophysiological and neurochemical effects of chronic treatment with agents that increase the function of the cholinergic system through both muscarinic and nicotinic mechanisms. Doses were determined that produced 60% cholinesterase inhibition by donepezil and galantamine for the old rats. This was chosen to be analogous to therapeutic levels achieved for treatment of human Alzheimer's disease patients with these agents. Because of the well-known age-related changes in spatial memory and hippocampal synaptic plasticity, spatial working memory in the radial eight-arm maze and hippocampal long-term potentiation induction and decay, as well as nicotinic receptor density and affinity, were measured in old rats implanted with minipumps that delivered donepezil, galantamine or saline. There was no effect of drug treatment on baseline synaptic transmission or on the threshold or magnitude of long-term potentiation induction. Both drug treatment groups, however, showed significantly extended long-term potentiation decay times at the perforant path-granule cell synapse over the saline control animals, as measured during the week following induction. Both drugs also elevated the number of nicotinic receptors within the hippocampus and neocortex. This is the first demonstration of cholinergic modulation of synaptic plasticity over the time-course of days. Furthermore, the durability of long-term potentiation was significantly, positively correlated with nicotinic receptor binding in the hippocampus. Chronic treatment with donepezil or galantamine had no significant effect on a well-learned spatial working memory task on the radial maze. These data suggest that the therapeutic doses of cholinesterase inhibitors used to treat patients with Alzheimer's disease may have effects on neurophysiology and neurochemistry that are close to the threshold for producing detectable behavioral improvements.

Effect of benzodiazepines on the epithelial and neuronal high-affinity glutamate transporter EAAC1

EAAC1-mediated glutamate transport concentrates glutamate across plasma membranes of brain neurons and epithelia. In brain, EAAC1 provides a presynaptic uptake mechanism to terminate the excitatory action of released glutamate and to keep its extracellular concentration below toxic levels. Here we report the effect of well known anxiolytic compounds, benzodiazepines, on glutamate transport in EAAC1-stably transfected Chinese hamster ovary (CHO) cells and in EAAC1-expressing Xenopus laevis oocytes. Functional properties of EAAC1 agreed well with already reported characteristics of the neuronal high-affinity glutamate transporter (Km D-Asp,CHO cells: 2.23+/-0.15 microM; Km D-Asp,oocytes: 17.01+/-3.42 microM). In both expression systems, low drug concentrations (10-100 microM) activated substrate uptake (up to 200% of control), whereas concentrations in the millimolar range inhibited (up to 50%). Furthermore, the activation was more pronounced at low substrate concentrations (1 microM), and the inhibition was attenuated. The activity of other sodium cotransporters such as the sodium/D-glucose cotransporter SGLT1, stably transfected in CHO cells, was not affected by benzodiazepines. In electrophysiological studies, these drugs also failed to change the membrane potential of EAAC1-expressing Xenopus laevis oocytes. These results suggest a direct action on the glutamate transporter itself without modifying the general driving forces. Thus, in vivo low concentrations of benzodiazepines may reduce synaptic glutamate concentrations by increased uptake, providing an additional mechanism to modulate neuronal excitability.

Neuronal nicotinic receptors in the locust Locusta migratoria. Cloning and expression

We have identified five cDNA clones that encode nicotinic acetylcholine receptor (nAChR) subunits expressed in the nervous system of the locust Locusta migratoria. Four of the subunits are ligand-binding alpha subunits, and the other is a structural beta subunit. The existence of at least one more nAChR gene, probably encoding a beta subunit, is indicated. Based on Northern analysis and in situ hybridization, the five subunit genes are expressed. localpha1, localpha3, and locbeta1 are the most abundant subunits and are expressed in similar areas of the head ganglia and retina of the adult locust. Because Loc<alpha3 binds alpha-bungarotoxin with high affinity, it may form a homomeric nAChR subtype such as the mammalian alpha7 nAChR. Localpha1 and Locbeta1 may then form the predominant heteromeric nAChR in the locust brain. localpha4 is mainly expressed in optic lobe ganglionic cells and localpha2 in peripherally located somata of mushroom body neurons. localpha3 mRNA was additionally detected in cells interspersed in the somatogastric epithelium of the locust embryo, suggesting that this isoform may also be involved in functions other than neuronal excitability. Transcription of all nAChR subunit genes begins approximately 3 days before hatching and continues throughout adult life. Electrophysiological recordings from head ganglionic neurons also indicate the existence of more than one functionally distinct nAChR subtype. Our results suggest the existence of several nAChR subtypes, at least some of them heteromeric, in this insect species.

Brain nicotinic receptors: structure and regulation, role in learning and reinforcement

The introduction, in the late sixties, of the concepts and methods of molecular biology to the study of the nervous system had a profound impact on the field, primarily through the identification of its basic molecular components. These structures include, for example, the elementary units of the synapse: neurotransmitters, neuropeptides and their receptors, but also ionic channels, intracellular second messengers and the relevant enzymes, cell surface adhesion molecules, or growth and trophic factors [21,78,81, 52,79]. Attempts to establish appropriate causal relationships between these molecular components, the actual organisation of neural networks, and a defined behavior, nevertheless, still must overcome many difficulties. A first problem is the recognition of the minimum levels of organisation, from the molecular, cellular, or multicellular (circuit) to the higher cognitive levels, that determine the given physiological and/or behavioral performance under investigation. A common difficulty (and potential source of errors of interpretation) is to relate a cognitive function to a network organization which does not possess the required structural complexity and vice-versa. Another problem is to distinguish, among the components of the system, those which are actually necessary and those which, taken together, suffice for a given behavior to take place. Identification of such a minimal set of building blocks may receive decisive insights from the elaboration of neurally plausible formal models that bring together, within a single and coherent 'artificial organism', the neuronal network, the circulating activity, and the behavior they determine (see [42,43,45,72,30]). In this communication, we shall attempt, still in a preliminary fashion, to bring together: (1) our recent knowledge on the molecular biology of brain nicotinic receptors (nAChRs) and their allosteric properties and (2) integrated behaviors, such as cognitive learning, investigated for instance with delayed-response or passive avoidance tasks that are likely to involve nAChRs in particular at the level of reinforcement (or reward) mechanisms (see [18,29,135]).