Nicotinic cholinergic signaling in hippocampal astrocytes involves calcium-induced calcium release from intracellular stores

Modulation of presynaptic store calcium induces release of glutamate and postsynaptic firing

Action potential-independent transmitter release is random and produces small depolarizations in the postsynaptic neuron. This process is, therefore, not thought to play a significant role in impulse propagation across synapses. Here we show that calcium flux through presynaptic neuronal nicotinic receptors leads to mobilization of store calcium by calcium-induced calcium release. Recruitment of store calcium induces vesicular release of glutamate in a manner consistent with synchronization across multiple active zones in the CA3 region of the rat hippocampus. This modulation of action potential-independent release of glutamate is sufficient to drive the postsynaptic pyramidal cell above its firing threshold, thus providing a mechanism for impulse propagation.

Higher expression of alpha7 nicotinic acetylcholine receptors in human fetal compared to adult brain

Neuronal nicotinic acetylcholine receptors are thought to be involved in regulation of several processes during neurogenesis of the brain. In this study the expression of the alpha7 nicotinic acetylcholine receptor subtype was investigated in human fetal (9-11 weeks of gestation), middle-aged (28-51 years) and aged (68-94 years) medulla oblongata, pons, frontal cortex, and cerebellum. The specific binding of the alpha7 receptor antagonist [(125)I]alpha-bungarotoxin was significantly higher in fetal than in both middle-aged and aged medulla oblongata and aged pons. No significant decrease in [(125)I]alpha-bungarotoxin binding sites was observed from fetal to adult cortex and cerebellum. The alpha7 mRNA expression was significantly higher in all fetal brain regions investigated, except for aged cortex, than in corresponding middle-aged and aged tissue. The high expression of alpha7 nicotinic acetylcholine receptors in fetal compared to adult brain supports the view that these receptors play an important role during brain development.

Presence and functional significance of presynaptic ryanodine receptors

Ca(2+)-induced Ca(2+) release (CICR) mediated by sarcoplasmic reticulum resident ryanodine receptors (RyRs) has been well described in cardiac, skeletal and smooth muscle. In brain, RyRs are localised primarily to endoplasmic reticulum (ER) and have been demonstrated in postsynaptic entities, astrocytes and oligodendrocytes where they regulate intracellular Ca(2+) concentration ([Ca(2+)](i)), membrane potential and the activity of a variety of second messenger systems. Recently, the contribution of presynaptic RyRs and CICR to functions of central and peripheral presynaptic terminals, including neurotransmitter release, has received increased attention. However, there is no general agreement that RyRs are localised to presynaptic terminals, nor is it clear that RyRs regulate a large enough pool of intracellular Ca(2+) to be physiologically significant. Here, we review direct and indirect evidence that on balance favours the notion that ER and RyRs are found in presynaptic terminals and are physiologically significant. In so doing, it became obvious that some of the controversy originates from issues related to (i) the ability to demonstrate conclusively the physical presence of ER and RyRs, (ii) whether the biophysical properties of RyRs are such that they can contribute physiologically to regulation of presynaptic [Ca(2+)](i), (iii) how ER Ca(2+) load and feedback gain of CICR contributes to the ability to detect functionally relevant RyRs, (iv) the distance that Ca(2+) diffuses from plasma membranes to RyRs to trigger CICR and from RyRs to the Active Zone to enhance vesicle release, and (v) the experimental conditions used. The recognition that ER Ca(2+) stores are able to modulate local Ca(2+) levels and neurotransmitter release in presynaptic terminals will aid in the understanding of the cellular mechanisms controlling neuronal function.

A beta-induced TNF-alpha expression and acetylcholine action in mouse glial cells

The brains in patients with Alzheimer's disease show chronic inflammatory responses characterized by activated glial cells and increased expression of cytokines. It is of interest to determine whether acetylcholine (ACh) affects Abeta-induced cytokine expression in the glial cells. Since it has been shown that alpha7 subunits of nicotinic ACh receptors are expressed in glial cells and that Abeta(1-42) binds to alpha7, we examined the effects of cholinergic agonists, carbachol, nicotine and oxotremorine-M, on Abeta-induced TNF-alpha expression in mouse glial cells. We did not observe any regulatory effects of ACh on Abeta-induced TNF-alpha transcription in the glial cells. We discuss the pathophysiological roles of ACh in glial cells in the brains of patients with Alzheimer's disease.

Customized rapid subtraction hybridization (RaSH) gene microarrays identify overlapping expression changes in human fetal astrocytes resulting from human immunodeficiency virus-1 infection or tumor necrosis factor-alpha treatment

Genes displaying altered expression as a function of human immunodeficiency virus (HIV)-1 infection of cultured primary human fetal astrocytes (PHFA) were previously identified using a rapid subtraction hybridization (RaSH) method. This scheme identified both known and novel genes displaying elevated expression, astrocyte elevated genes (AEG), and decreased expression, astrocyte suppressed genes (ASG), in PHFA as a consequence of infection with HIV-1 or treatment with HIV-1 envelope glycoprotein (gp120). RaSH also identified both known and novel genes displaying enhanced (HR) or reduced (HS) expression in HIV-1 resistant versus HIV-1 susceptible human T-cell clones. In the present study, a customized microarray approach employing these RaSH-derived genes was used to distinguish overlapping gene expression changes occurring in PHFA as a function of treatment with HIV-1 and the neurotoxic agent tumor necrosis factor (TNF)-alpha. RaSH cDNAs were spotted (microarrayed) on nylon membranes and probed with temporally isolated reverse transcribed cDNAs from HIV-1-infected and TNF-alpha-treated PHFA. This strategy identified genes displaying parallel changes after TNF-alpha treatment as observed following HIV-1 infection. Confirmation of genuine differential expression was achieved by Northern blotting. These studies document that TNF-alpha can induce a set of corresponding changes in specific AEGs and ASGs as does HIV-1 infection in PHFA. Furthermore, this customized microarray approach with RaSH-derived clones represents an efficient and sensitive methodology for elucidating molecular changes in PHFA occurring as a consequence of treatment with pharmacological agents affecting astrocyte physiology.

Insights into glutamate transport regulation in human astrocytes: cloning of the promoter for excitatory amino acid transporter 2 (EAAT2)

Glutamate transport is central to neurotransmitter functions in the brain. Impaired glutamate transport induces neurotoxicity associated with numerous pathological processes, including stroke/ischemia, temporal lobe epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, Huntington's disease, HIV-1-associated dementia, and growth of malignant gliomas. Excitatory amino acid transporter-2 (EAAT2) is a major glutamate transporter in the brain expressed primarily in astrocytes. We presently describe the cloning and characterization of the human EAAT2 promoter, demonstrating elevated expression in astrocytes. Regulators of EAAT2 transport, both positive and negative, alter EAAT2 transcription, promoter activity, mRNA, and protein. These findings imply that transcriptional processes can regulate EAAT2 expression. Moreover, they raise the intriguing possibility that the EAAT2 promoter may be useful for targeting gene expression in the brain and for identifying molecules capable of modulating glutamate transport that could potentially inhibit, ameliorate, or prevent various neurodegenerative diseases.

Differential nicotinic acetylcholine receptor subunit expression in the human hippocampus

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels composed of alpha and beta subunits with specific structural, functional and pharmacological properties. In this study the distribution of alpha3, alpha4, alpha7, beta2 and beta4 nAChR subunits in the human hippocampus was investigated using immunohistochemistry. Most pyramidal neurons, pre-alpha cells of the entorhinal cortex and dentate granule cells were immunoreactive for all subunits. Small islands of alpha7 immunoreactive cells were present in the outer presubiculum. alpha4 and beta2, and alpha3, alpha4 and beta2 immunoreactive fibre tracts were present in the stratum radiatum and subiculum, respectively, suggesting nAChRs may play a role in modulating inputs to the hippocampus via Schaffer collaterals and along the perforant pathway. Some astrocytes were immunoreactive for alpha3, alpha7 and beta4 subunits. Immunoreactivity to all subunits was noted in association with blood vessels. These results indicate the involvement of multiple nAChR subtypes in the modulation of both neuronal and non-neuronal functions in the human hippocampus.

Bergmann glia GABA(A) receptors concentrate on the glial processes that wrap inhibitory synapses

We studied the cellular and subcellular distribution of GABA(A) receptors in the Bergmann glia and Purkinje cells in the molecular layer of the cerebellum by using electron microscopy postembedding immunogold techniques. Gold particles corresponding to alpha2 and gamma1 immunoreactivity were localized in Bergmann glia processes that wrapped Purkinje cell somata, dendritic shafts, and some dendritic spines. The gold particles were mainly located on the glial plasma membrane or intracellularly but near the plasma membrane. The density of gold particles corresponding to alpha2 and gamma1 GABA(A) receptor subunits was 4.3-fold higher in the glial processes wrapping Purkinje cell somata than in the glial processes wrapping Purkinje cell dendritic spines. Moreover, the Bergmann glia GABA(A) receptors were often located in close proximity to the type II GABAergic synapses made by the basket cell axons on Purkinje cell somata. These GABAergic synapses were enriched in neuronal GABA(A) receptors containing alpha1 and beta2/3 subunits. Unexpectedly, 2.8% of the Purkinje cell dendritic spines also showed immunoreactivity for the neuronal alpha1 or beta2/3 subunits, which were located on the spine in type I synapses or extrasynaptically. Double-labeling immunogold experiments showed that approximately 50% of the dendritic spines that were immunolabeled with the neuronal GABA(A) receptors were wrapped by Bergmann glia processes containing glial GABA(A) receptors. These results are consistent with a role of the Bergmann glial GABA(A) receptors in sensing GABAergic synaptic function.

Alzheimer's disease is associated with a selective increase in alpha7 nicotinic acetylcholine receptor immunoreactivity in astrocytes

Alzheimer's disease (AD) and dementia with Lewy bodies (DLB) are common forms of dementia in the elderly associated with cholinergic dysfunction, including reductions in nicotinic acetylcholine receptors (nAChRs). In AD, astrocytes are implicated in the formation of senile plaques, one of the core pathological features. Using immunohistochemistry, we have investigated astrocytic expression of the two major nicotinic receptor alpha subunits in the human hippocampus and entorhinal cortex. alpha7, but not alpha4, subunit immunoreactivity was associated with astrocytes. An increase in the proportion of astrocytes expressing alpha7 immunoreactivity was observed in AD compared with age-matched controls. A similar increase was not evident in DLB. Elevated alpha7 nAChRs on astrocytes in AD may contribute to alterations in calcium homeostasis and nitric oxide production, which in turn could affect beta-amyloid-mediated inflammatory processes in AD.

Bergmann glia GABA(A) receptors concentrate on the glial processes that wrap inhibitory synapses

We studied the cellular and subcellular distribution of GABA(A) receptors in the Bergmann glia and Purkinje cells in the molecular layer of the cerebellum by using electron microscopy postembedding immunogold techniques. Gold particles corresponding to alpha2 and gamma1 immunoreactivity were localized in Bergmann glia processes that wrapped Purkinje cell somata, dendritic shafts, and some dendritic spines. The gold particles were mainly located on the glial plasma membrane or intracellularly but near the plasma membrane. The density of gold particles corresponding to alpha2 and gamma1 GABA(A) receptor subunits was 4.3-fold higher in the glial processes wrapping Purkinje cell somata than in the glial processes wrapping Purkinje cell dendritic spines. Moreover, the Bergmann glia GABA(A) receptors were often located in close proximity to the type II GABAergic synapses made by the basket cell axons on Purkinje cell somata. These GABAergic synapses were enriched in neuronal GABA(A) receptors containing alpha1 and beta2/3 subunits. Unexpectedly, 2.8% of the Purkinje cell dendritic spines also showed immunoreactivity for the neuronal alpha1 or beta2/3 subunits, which were located on the spine in type I synapses or extrasynaptically. Double-labeling immunogold experiments showed that approximately 50% of the dendritic spines that were immunolabeled with the neuronal GABA(A) receptors were wrapped by Bergmann glia processes containing glial GABA(A) receptors. These results are consistent with a role of the Bergmann glial GABA(A) receptors in sensing GABAergic synaptic function.

Calcium rise in cultured neurons from medial septum elicits calcium waves in surrounding glial cells

One prerequisite for understanding the physiological relevance of intercellular calcium waves in glia is the examination of mechanisms that trigger these waves. Here, we show that stimulation of cultured septal neurons to produce a large and sustained calcium rise in the soma can initiate calcium waves in surrounding glial cells. The initiation of calcium waves is dependent on calcium influx through voltage-gated calcium channels on the neuron. The waves are not due to direct stimulation of the glial cells or to loss of neuronal membrane integrity. Mechanism of wave initiation is distinct from that involved in wave propagation and does not involve glutamate or acetylcholine release. Communication via gap junctions, or nitric oxide production, is not involved in the initial signaling between a stimulated neuron and the surrounding astrocytes. Suramin, a blocker of P2 receptors blocked the waves but failed to abolish the responses in glial cells immediately surrounding the stimulated neuron. Our results suggest that patterns of calcium rises on neurons, like those seen in glutamate cytotoxicity, can cause calcium waves in surrounding glial cells.

Nicotinic receptor signaling in nonexcitable cells

The finding that neuronal nicotinic acetylcholine receptors (nAChRs) are present in non-neuronal cells both within and outside the nervous system raises some interesting issues. The mechanisms underlying receptor signaling and its downstream consequences in these cells remain to be elucidated. Factors controlling the release of acetylcholine and the extent of its diffusion are likely to be different for these cells than for traditional neuronal synapses. Recent advances on the physiologic functions of some of these cell types have provided a better insight into possible functional roles for nAChRs in nonexcitable cells. The presence of nAChRs on these cells also implies a broader scope for the actions of nicotine that needs to be considered from a clinical viewpoint. Revealing the potential physiologic roles for nAChRs on nonexcitable cells is likely to provide a more complete understanding of cholinergic signaling.

Unconventional ligands and modulators of nicotinic receptors

Evidence gathered from epidemiologic and behavioral studies have indicated that neuronal nicotinic receptors (nAChRs) are intimately involved in the pathogenesis of a number of neurologic disorders, including Alzheimer's disease, Parkinson's disease, and schizophrenia. In the mammalian brain, neuronal nAChRs, in addition to mediating fast synaptic transmission, modulate fast synaptic transmission mediated by the major excitatory and inhibitory neurotransmitters glutamate and GABA, respectively. Of major interest, however, is the fact that the activity of the different subtypes of neuronal nAChR is also subject to modulation by substances of endogenous origin such as choline, the tryptophan metabolite kynurenic acid, neurosteroids, and beta-amyloid peptides and by exogenous substances, including the so-called nicotinic allosteric potentiating ligands, of which galantamine is the prototype, and psychotomimetic drugs such as phencyclidine and ketamine. The present article reviews and discusses the effects of unconventional ligands on nAChR activity and briefly describes the potential benefits of using some of these compounds in the treatment of neuropathologic conditions in which nAChR function/expression is known to be altered.

Regulatory mechanisms that govern nicotinic synapse formation in neurons

Individual cholinoceptive neurons express high levels of different neuronal nicotinic acetylcholine receptor (nAChR) subtypes, and target them to the appropriate synaptic regions for proper function. This review focuses on the intercellular and intracellular processes that regulate nAChR expression in vertebrate peripheral nervous system (PNS) and central nervous system (CNS) neurons. Specifically, we discuss the cellular and molecular mechanisms that govern the induction and maintenance of nAChR expression-innervation, target tissue interactions, soluble factors, and activity. We define the regulatory principles of interneuronal nicotinic synapse differentiation that have emerged from these studies. We also discuss the molecular players that target nAChRs to the surface membrane and the interneuronal synapse.

Nicotine and nicotinic receptors; relevance to Parkinson's disease

The development of nicotinic agonists for therapy in neurodegenerative disorders such as Parkinson's disease is an area currently receiving considerable attention. The rationale for such work stems from findings that reveal a loss of nicotinic receptors in Parkinson's disease brains. These results, coupled with reports that nicotine treatment relieves some of the symptoms of this disorder, provides support for the contention that nicotine and/or nicotinic agonists may be beneficial for acute symptomatic treatment. Moreover, the observation that there is a decreased incidence of Parkinson's disease with tobacco use, possibly due to the nicotine in tobacco products, may imply that such drugs are useful for long-term neuroprotection. However, there are multiple nicotinic receptor populations in the brain with different functional properties. Identification of the subtypes involved in nigrostriatal dopaminergic activity is therefore critical for the rational use of selective therapeutic agents for symptomatic treatment and/or neuroprotection. Accumulating evidence, both in rodents and nonhuman primates now indicate that alpha6* nicotinic receptors are present on nigrostriatal dopaminergic neurons, and furthermore, that receptors containing this subunit may be most vulnerable to nigrostriatal damage, at least in nonhuman primates. These data suggest that nicotinic receptor ligands directed to alpha6* nicotinic receptors might be particularly relevant for Parkinson's disease therapeutics.

Development of depolarization-induced calcium transients in insect glial cells is dependent on the presence of afferent axons

Changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) induced by depolarization have been measured in glial cells acutely isolated from antennal lobes of the moth Manduca sexta at different postembryonic developmental stages. Depolarization of the glial cell membrane was elicited by increasing the external K(+) concentration from 4 to 25 mM. At midstage 5 and earlier stages, less than 20% of the cells responded to 25 mM K(+) (1 min) with a transient increase in [Ca(2+)](i) of approximately 40 nM. One day later, at late stage 5, 68% of the cells responded to 25 mM K(+), the amplitude of the [Ca(2+)](i) transients averaging 592 nM. At later stages, all cells responded to 25 mM K(+) with [Ca(2+)](i) transients with amplitudes not significantly different from those at late stage 5. In stage 6 glial cells isolated from deafferented antennal lobes, i.e., from antennal lobes chronically deprived of olfactory receptor axons, only 30% of the cells responded with [Ca(2+)](i) transients. The amplitudes of these [Ca(2+)](i) transients averaged 93 nM and were significantly smaller than those in normal stage 6 glial cells. [Ca(2+)](i) transients were greatly reduced in Ca(2+)-free, EGTA-buffered saline, and in the presence of the Ca(2+) channel blockers cadmium and verapamil. The results suggest that depolarization of the cell membrane induces Ca(2+) influx through voltage-activated Ca(2+) channels into antennal lobe glial cells. The development of the depolarization-induced Ca(2+) transients is rapid between midstage 5 and stage 6, and depends on the presence of afferent axons from the olfactory receptor cells in the antenna.

Recovery of deficient cholinergic calcium signaling by adenosine in cultured rat cortical astrocytes

The regulation of the cholinergic calcium signaling in astroglial cells is thought to play a crucial role in the pathogenesis of Alzheimer's disease. We investigated the action of the cell modulator adenosine on acetylcholine (Ach)-mediated intracellular calcium ([Ca(2+)](i)) transients in cultured rat cortical astrocytes using the Ca(2+) imaging technique. The stable adenosine analog 2-chloroadenosine (2ClA) potentiated the [Ca(2+)](i) rise induced by activation of muscarinic Ach receptors by shifting approximately 30-fold the half-effective Ach concentration. This 2ClA effect was maintained upon removal of extracellular Ca(2+), indicating that Ach-induced [Ca(2+)](i) elevation was due mainly to Ca(2+) mobilization from intracellular stores. Pharmacological studies demonstrated that the 2ClA action was mediated by A1 receptors. Incubation with pertussis toxin abrogated the 2ClA effect but left unchanged the [Ca(2+)](i) rise produced by Ach alone. The [Ca(2+)](i) response elicited by Ach alone was abolished upon blockade of muscarinic receptor subtypes that stimulate phospholipase C, whereas the [Ca(2+)](i) elevation generated by the combined action of subthreshold Ach and 2ClA was not affected. Collectively, these results suggest that the impaired cholinergic signaling, the cardinal symptom of Alzheimer's disease, can be reinforced at the second messenger level by an alternative intracellular Ca(2+) mobilizing path, which can be brought into play by the concomitant activation of A1 purinoceptors and muscarinic receptors negatively coupled to adenylyl cyclase.

Identification and cloning of human astrocyte genes displaying elevated expression after infection with HIV-1 or exposure to HIV-1 envelope glycoprotein by rapid subtraction hybridization, RaSH

Neurodegeneration and dementia are common complications of AIDS caused by human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system. HIV-1 target cells in the brain include microglia, infiltrating macrophages and astrocytes, but rarely neurons. Astrocytes play an important role in the maintenance of the synaptic micro-environment and in neuronal signal transmission. To investigate potential changes in cellular gene expression associated with HIV-1 infection of astrocytes, we employed an efficient and sensitive rapid subtraction hybridization approach, RaSH. Primary human astrocytes were isolated from abortus brain tissue and low-passage cells were infected with HIV-1. To identify genes that display both early and late expression modifications after HIV-1 infection and to avoid cloning genes displaying normal cell cycle fluctuations in astrocytes, RNAs were isolated and pooled from 6, 12, 24 h and 3 and 7 day uninfected and infected cells and used for RaSH. Temporal cDNA libraries were prepared from double-stranded cDNAs that were enzymatically digested into small fragments, ligated to adapters, PCR amplified, and hybridized by incubation of tester and driver PCR fragments. By subtracting temporal cDNAs derived from uninfected astrocytes from temporal cDNAs made from HIV-1 infected cells, genes displaying elevated expression in virus infected cells, termed astrocyte elevated genes (AEGs), were identified. Both known and novel AEGs, not reported in current DNA databases, are described that display early or late expression kinetics following HIV-1 infection or treatment with recombinant HIV-1 envelope glycoprotein (gp120). For selected AEGs, expression of their protein products was also tested by Western blotting and found to display elevated expression following HIV-1 infection. The comparable pattern of regulation of the AEGs following HIV-1 infection or gp120 treatment suggest that HIV-1 exposure of astrocytes, even in the absence of productive infection, can induce changes in cellular gene expression.

Intracellular Ca2+ signals evoked by stimulation of nicotinic acetylcholine receptors in SH-SY5Y cells: contribution of voltage-operated Ca2+ channels and Ca2+ stores

Neuronal nicotinic acetylcholine receptors (nAChR) can regulate several neuronal processes through Ca2+-dependent mechanisms. The versatility of nAChR-mediated responses presumably reflects the spatial and temporal characteristics of local changes in intracellular Ca2+ arising from a variety of sources. The aim of this study was to analyse the components of nicotine-evoked Ca2+ signals in SH-SY5Y cells, by monitoring fluorescence changes in cells loaded with fluo-3 AM. Nicotine (30 microm) generated a rapid elevation in cytoplasmic Ca2+ that was partially and additively inhibited (40%) by alpha7 and alpha3beta2* nAChR subtype selective antagonists; alpha3beta4* nAChR probably account for the remaining response (60%). A substantial blockade (80%) by CdCl2 (100 microm) indicates that voltage-operated Ca2+ channels (VOCC) mediate most of the nicotine-evoked response, although the alpha7 selective antagonist alpha-bungarotoxin (40 nm) further decreased the CdCl2- resistant component. The elevation of intracellular Ca2+ levels provoked by nicotine was sustained for at least 10 min and required the persistent activation of nAChR throughout the response. Intracellular Ca2+ stores were implicated in both the initial and sustained nicotine-evoked Ca2+ responses, by the blockade observed after ryanodine (30 microm) and the inositoltriphosphate (IP3)-receptor antagonist, xestospongin-c (10 microm). Thus, nAChR subtypes are differentially coupled to specific sources of Ca2+: activation of nAChR induces a sustained elevation of intracellular Ca2+ levels which is highly dependent on the activation of VOCC, and also involves Ca2+ release from ryanodine and IP3-dependent intracellular stores. Moreover, the alpha7, but not alpha3beta2* nAChR, are responsible for a fraction of the VOCC-independent nicotine-evoked Ca2+ increase that appears to be functionally coupled to ryanodine sensitive Ca2+ stores.

Chronic nicotine treatment reduces beta-amyloidosis in the brain of a mouse model of Alzheimer's disease (APPsw)

Alzheimer's disease neuropathology is characterised by beta-amyloid plaques and neurofibrillary tangles. Inhibition of beta-amyloid accumulation may be essential for effective therapy in Alzheimer's disease. In this study we have treated transgenic mice carrying the Swedish mutation of human amyloid precursor protein [Tg(Hu.APP695.K670N-M671L)2576], which develop brain beta-amyloid deposits, with nicotine in drinking fluid (200 microg/mL) from 9-14.5 months of age (5.5 months). A significant reduction in amyloid beta peptide 1-42 positive plaques by more than 80% (p < 0.03) was observed in the brains of nicotine treated compared to sucrose treated transgenic mice. In addition, there was a selective reduction in extractable amyloid beta peptides in nicotine treated mice; cortical insoluble 1-40 and 1-42 peptide levels were lower by 48 and 60%, respectively (p < 0.005), whilst there was no significant change in soluble 1-40 or 1-42 levels. The expression of glial fibrillary acidic protein was not affected by nicotine treatment. These results indicate that nicotine may effectively reduce amyloid beta peptide aggregation in brain and that nicotinic drug treatment may be a novel protective therapy in Alzheimer's disease.

Stimulation of non-alpha7 nicotinic receptors partially protects dopaminergic neurons from 1-methyl-4-phenylpyridinium-induced toxicity in culture

Previous work has shown that nicotine treatment protects against nigrostriatal degeneration in rodents, findings that may be of relevance to the decreased incidence of Parkinson's disease in cigarette smokers. In the present studies, we investigated the effect of nicotine against 1-methyl-4-phenylpyridinium-induced toxicity in dopaminergic ventral mesencephalic cultures to identify the nicotinic receptor population that may be involved. [3H]Epibatidine, a ligand that binds to receptors containing alpha2-alpha6 subunits, bound to at least two populations of sites that were up-regulated by nicotine in a time and dose dependent manner. We next examined the effect of nicotine on cultures treated with 1-methyl-4-phenylpyridinium, a neurotoxin that selectively damages nigrostriatal dopaminergic neurons. Pre-treatment with nicotine, at 10(-7)-10(-4) M, partially prevented the toxin-induced decline in dopaminergic cells. Pre-exposure to nicotine for 24 h resulted in optimal protection, suggesting that receptor up-regulation may contribute to the observed neuroprotective effect. Nicotine-mediated protection was blocked by pre-incubation with the nicotinic receptor antagonist d-tubocurarine (10(-4) M), but not the alpha7 receptor-selective antagonist alpha-bungarotoxin (10(-7) M). Our results show that nicotinic receptor activation partially protects nigral dopaminergic neurons from 1-methyl-4-phenylpyridinium-induced toxicity in culture and that this appears to occur through an interaction at non-alpha7 containing receptors.

Synaptically released acetylcholine evokes Ca2+ elevations in astrocytes in hippocampal slices

Recent results have demonstrated the existence of bidirectional communication between glial cells and neurons. We investigated in brain slices whether rat hippocampal astrocytes respond to acetylcholine synaptically released by an extrinsic pathway. We stimulated the stratum oriens/alveus, which contains cholinergic afferents from the septum and diagonal band of Broca, and recorded whole-cell membrane currents and intracellular Ca2+ levels of astrocytes located in the hippocampal stratum oriens. Nerve-fiber stimulation evoked a long-lasting inward current and increased the Ca2+ levels in astrocytes. Both astrocytic responses were abolished by tetrodotoxin or Cd2+ and were increased by 4-aminopyridine, indicating that the responses were attributable to synaptically released neurotransmitter. The inward current was inhibited by glutamate transporter antagonists, indicating that it was attributable to the electrogenic glutamate transporter activity. The synaptically evoked intracellular Ca2+ elevations were not affected by glutamate receptor antagonists but were abolished by atropine, indicating that they were mediated by muscarinic cholinergic receptors. Thapsigargin prevented the Ca2+ elevation but did not modify the inward current, indicating that the Ca2+ signal was attributable to intracellular Ca2+ mobilization. These results indicate that hippocampal astrocytes respond to acetylcholine released by synaptic terminals. The synaptically released acetylcholine acts on muscarinic receptors, mobilizing Ca2+ from the intracellular stores. Different regions in the recorded astrocytes showed independent stimulus-induced Ca2+ variations, suggesting the existence of subcellular domains in the astrocytic responses evoked by the synaptic cholinergic activity. Therefore, our results show the existence of cholinergic neuron-astrocyte signaling and suggest that astrocytes are a target of axonal inputs from different brain areas.

Calcium oscillations encoding neuron-to-astrocyte communication

The observation that the excitatory neurotransmitter glutamate released from presynaptic terminals can activate, beside the post-synaptic neuron, the glial cell astrocyte, stimulated glial cell research like no other event since the recognition in the 1980s that astrocytes can express on their membrane many receptors for classical neurotransmitters. The properties and the functional role(s) of such a neuron-to-astrocyte signaling have now become the focus of intense research in neurobiology. Indeed, a growing body of evidence has recently highlighted the ability of astrocytes to work as sophisticated detectors of synaptic activity: by changing the frequency of [Ca(2+)](i) oscillations evoked by the synaptic release of glutamate, these cells display the remarkable capacity to discriminate between different levels and patterns of synaptic activity. Furthermore, the observation that astrocytes increase the frequency of [Ca(2+)](i) oscillations in response to repetitive episodes of high neuronal activity challenges the common concept that memory function in the brain is an exclusive property of neuronal cells. Glutamate-mediated [Ca(2+)](i) elevations can also trigger in astrocytes the release of glutamate that can ultimately affect neuronal transmission. Given the wide role played by glutamate in brain physiology, our view on how the brain operates needs now to be revised taking into account the bi-directional, glutamatergic communication between neurons and astrocytes.

A novel Ca2+-induced Ca2+ release mechanism mediated by neither inositol trisphosphate nor ryanodine receptors

Members of both major families of intracellular Ca(2+) channels, ryanodine and inositol 1,4,5-trisphosphate (IP3) receptors, are stimulated by substantial increases in cytosolic free Ca(2+) concentration ([Ca(2+)]c). They thereby mediate Ca(2+)-induced Ca(2+) release (CICR), which allows amplification and regenerative propagation of intracellular Ca(2+) signals. In permeabilized hepatocytes, increasing [Ca(2+)]c to 10 microM stimulated release of 30+/-1% of the intracellular stores within 60 s; the EC(50) occurred with a free [Ca(2+)] of 170+/-29 nM. This CICR was abolished at 2 degrees C. The same fraction of the stores was released by CICR before and after depletion of the IP3-sensitive stores, and CICR was not blocked by antagonists of IP3 receptors. Ryanodine, Ruthenium Red and tetracaine affected neither the Ca(2+) content of the stores nor the CICR response. Sr(2+) and Ba(2+) (EC(50)=166 nM and 28 microM respectively) mimicked the effects of increased [Ca(2+)] on the intracellular stores, but Ni(2+) blocked the passive leak of Ca(2+) without blocking CICR. In rapid superfusion experiments, maximal concentrations of IP3 or Ca(2+) stimulated Ca(2+) release within 80 ms. The response to IP3 was complete within 2 s, but CICR continued for tens of seconds despite a slow [half-time (t(1/2))=3.54+/-0.07 s] partial inactivation. CICR reversed rapidly (t(1/2)=529+/-17 ms) and completely when the [Ca(2+)] was reduced. We conclude that hepatocytes express a novel temperature-sensitive, ATP-independent CICR mechanism that is reversibly activated by modest increases in [Ca(2+)], and does not require IP3 or ryanodine receptors or reversal of the sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase. This mechanism may both regulate the Ca(2+) content of the intracellular stores of unstimulated cells and allow even small intracellular Ca(2+) signals to be amplified by CICR.

Nicotine induces calcium spikes in single nerve terminal varicosities: a role for intracellular calcium stores

While nicotine is known to act at neuronal nicotinic acetylcholine receptors (nAChRs) to facilitate neurotransmitter release, the mechanisms underlying this action are poorly understood. Some of its effects are known to be mediated by presynaptic receptors. In the mouse vas deferens nicotine (10-30 microM) transiently increased the force of neurogenic contraction by 135+/-25%, increased the amplitude of excitatory junction potentials by 74+/-6% and increased the frequency of spontaneous excitatory junction potentials in four out of six preparations. Confocal microscopy and the calcium indicator Oregon Green 488 BAPTA-1 dextran were used to measure calcium concentration changes in the nerve terminals. Nicotine did not affect the action potential-evoked calcium transient but instead triggered small, random fluctuations ("calcium spikes") in intra-varicosity calcium concentrations at an average frequency of 0.09+/-0.02 Hz. These were insensitive to tetrodotoxin at a concentration that blocked action-potential evoked calcium transients (300 nM). They were abolished by the nAChR blocker hexamethonium (100 microM) and by both ryanodine (100 microM) and caffeine (3 mM), agents that modify calcium release from intracellular stores. We propose a novel mechanism whereby nicotine's action at nAChRs triggers calcium-induced calcium release from a ryanodine-sensitive calcium store in nerve terminals. This primes neurotransmitter release mechanisms and enhances both spontaneous and action potential-evoked neurotransmitter release.

The brain metabolite kynurenic acid inhibits alpha7 nicotinic receptor activity and increases non-alpha7 nicotinic receptor expression: physiopathological implications

The tryptophan metabolite kynurenic acid (KYNA) has long been recognized as an NMDA receptor antagonist. Here, interactions between KYNA and the nicotinic system in the brain were investigated using the patch-clamp technique and HPLC. In the electrophysiological studies, agonists were delivered via a U-shaped tube, and KYNA was applied in admixture with agonists and via the background perfusion. Exposure (>/=4 min) of cultured hippocampal neurons to KYNA (>/=100 nm) inhibited activation of somatodendritic alpha7 nAChRs; the IC(50) for KYNA was approximately 7 microm. The inhibition of alpha7 nAChRs was noncompetitive with respect to the agonist and voltage independent. The slow onset of this effect could not be accounted for by an intracellular action because KYNA (1 mm) in the pipette solution had no effect on alpha7 nAChR activity. KYNA also blocked the activity of preterminal/presynaptic alpha7 nAChRs in hippocampal neurons in cultures and in slices. NMDA receptors were less sensitive than alpha7 nAChRs to KYNA. The IC(50) values for KYNA-induced blockade of NMDA receptors in the absence and presence of glycine (10 microm) were approximately 15 and 235 microm, respectively. Prolonged (3 d) exposure of cultured hippocampal neurons to KYNA increased their nicotinic sensitivity, apparently by enhancing alpha4beta2 nAChR expression. Furthermore, as determined by HPLC with fluorescence detection, repeated systemic treatment of rats with nicotine caused a transient reduction followed by an increase in brain KYNA levels. These results demonstrate that nAChRs are targets for KYNA and suggest a functionally significant cross talk between the nicotinic cholinergic system and the kynurenine pathway in the brain.

The brain metabolite kynurenic acid inhibits alpha7 nicotinic receptor activity and increases non-alpha7 nicotinic receptor expression: physiopathological implications

The tryptophan metabolite kynurenic acid (KYNA) has long been recognized as an NMDA receptor antagonist. Here, interactions between KYNA and the nicotinic system in the brain were investigated using the patch-clamp technique and HPLC. In the electrophysiological studies, agonists were delivered via a U-shaped tube, and KYNA was applied in admixture with agonists and via the background perfusion. Exposure (>/=4 min) of cultured hippocampal neurons to KYNA (>/=100 nm) inhibited activation of somatodendritic alpha7 nAChRs; the IC(50) for KYNA was approximately 7 microm. The inhibition of alpha7 nAChRs was noncompetitive with respect to the agonist and voltage independent. The slow onset of this effect could not be accounted for by an intracellular action because KYNA (1 mm) in the pipette solution had no effect on alpha7 nAChR activity. KYNA also blocked the activity of preterminal/presynaptic alpha7 nAChRs in hippocampal neurons in cultures and in slices. NMDA receptors were less sensitive than alpha7 nAChRs to KYNA. The IC(50) values for KYNA-induced blockade of NMDA receptors in the absence and presence of glycine (10 microm) were approximately 15 and 235 microm, respectively. Prolonged (3 d) exposure of cultured hippocampal neurons to KYNA increased their nicotinic sensitivity, apparently by enhancing alpha4beta2 nAChR expression. Furthermore, as determined by HPLC with fluorescence detection, repeated systemic treatment of rats with nicotine caused a transient reduction followed by an increase in brain KYNA levels. These results demonstrate that nAChRs are targets for KYNA and suggest a functionally significant cross talk between the nicotinic cholinergic system and the kynurenine pathway in the brain.

A neuron-glia signalling network in the active brain

Glial cells are active partners of neurons in processing information and synaptic integration. They receive coded signals from synapses and elaborate modulatory responses. The active properties of glia, including long-range signalling and regulated transmitter release, are beginning to be elucidated. Recent insights suggest that the active brain should no longer be regarded as a circuitry of neuronal contacts, but as an integrated network of interactive neurons and glia.