Activation of α7 nicotinic acetylcholine receptors persistently enhances hippocampal synaptic transmission and prevents Aß-mediated inhibition of LTP in the rat hippocampus

Changes in hippocampal volume, synaptic plasticity and amylin sensitivity in an animal model of type 2 diabetes are associated with increased vulnerability to amyloid-beta in advancing age

Type-2 diabetes (T2D) is a metabolic disorder that is considered a risk factor for Alzheimer's disease (AD). Cognitive impairment can arise due to hypoglycemia associated with T2D, and hyperamylinemia associated with insulin resistance can enhance AD pathology. We explored whether changes occur in the hippocampus in aging (6-12 months old) female V-Lep○b-/- transgenic (tg) mice, comprising an animal model of T2D. We also investigated whether an increase in vulnerability to Aβ (1-42), a known pathological hallmark of AD, is evident. Using magnetic resonance imaging we detected significant decreases in hippocampal brain volume in female tg-mice compared to wild-type (wt) littermates. Long-term potentiation (LTP) was impaired in tg compared to wt mice. Treatment of the hippocampus with Aβ (1-42) elicited a stronger debilitation of LTP in tg compared to wt mice. Treatment with an amylin antagonist (AC187) significantly enhanced LTP in wt and tg mice, and rescued LTP in Aβ (1-42)-treated tg mice. Taken together our data indicate that a T2D-like state results in an increased vulnerability of the hippocampus to the debilitating effects of Aβ (1-42) and that effects are mediated in part by changes in amylin receptor signaling.

α7 nicotinic acetylcholine receptors induce long-term synaptic enhancement in the dorsal but not ventral hippocampus

Agents that positively modulate the activity of α7nAChRs are used as cognitive enhancers and for the treatment of hippocampus-dependent functional decline. However, it is not known whether the expression and the effects of α7nAChRs apply to the entire longitudinal axis of the hippocampus equally. Given that cholinergic system-involving hippocampal functions are not equally distributed along the hippocampus, we comparatively examined the expression and the effects of α7nAChRs on excitatory synaptic transmission between the dorsal and the ventral hippocampal slices from adult rats. We found that α7nAChRs are equally expressed in the CA1 field of the two segments of the hippocampus. However, activation of α7nAChRs by their highly selective agonist PNU 282987 induced a gradually developing increase in field excitatory postsynaptic potential only in the dorsal hippocampus. This long-term potentiation was not reversed upon application of nonselective nicotinic receptor antagonist mecamylamine, but the induction of potentiation was prevented by prior blockade of α7nAChRs by their antagonist MG 624. In contrast to the long-term synaptic plasticity, we found that α7nAChRs did not modulate short-term synaptic plasticity in either the dorsal or the ventral hippocampus. These results may have implications for the role that α7nAChRs play in specifically modulating functions that depend on the normal function of the dorsal hippocampus. We propose that hippocampal functions that rely on a direct α7 nAChR-mediated persistent enhancement of glutamatergic synaptic transmission are preferably supported by dorsal but not ventral hippocampal synapses.

Antibacterial Efficacy of N-(4-methylpyridin-2-yl) Thiophene-2-Carboxamide Analogues against Extended-Spectrum-β-Lactamase Producing Clinical Strain of Escherichia coli ST 131

Development in the fields of natural-product-derived and synthetic small molecules is in stark contrast to the ongoing demand for novel antimicrobials to treat life-threatening infections caused by extended-spectrum β-lactamase producing Escherichia coli (ESBL E. coli). Therefore, there is an interest in the antibacterial activities of synthesized N-(4-methylpyridin-2-yl) thiophene-2-carboxamides (4a–h) against ESBL-producing E. coli ST131 strains. A blood sample was obtained from a suspected septicemia patient and processed in the Bactec Alert system. The isolate’s identification and antibacterial profile were determined using the VITEK 2® compact system. Multi-locus sequence typing of E. coli was conducted by identifying housekeeping genes, while ESBL phenotype detection was performed according to CLSI guidelines. Additionally, PCR was carried out to detect the blaCTX-M gene molecularly. Moreover, molecular docking studies of synthesized compounds (4a–h) demonstrated the binding pocket residues involved in the active site of the β-lactamase receptor of E. coli. The result confirmed the detection of E. coli ST131 from septicemia patients. The isolates were identified as ESBL producers carrying the blaCTX-M gene, which provided resistance against cephalosporins and beta-lactam inhibitors but sensitivity to carbapenems. Among the compounds tested, 4a and 4c exhibited high activity and demonstrated the best fit and interactions with the binding pocket of the β-lactamase enzyme. Interestingly, the maximum of the docking confirmations binds at a similar pocket region, further strengthening the importance of binding residues. Hence, the in vitro and molecular docking studies reflect the promising antibacterial effects of 4a and 4c compounds.

Ca2+ Dyshomeostasis Disrupts Neuronal and Synaptic Function in Alzheimer's Disease

Ca²⁺ homeostasis is essential for multiple neuronal functions and thus, Ca²⁺ dyshomeostasis can lead to widespread impairment of cellular and synaptic signaling, subsequently contributing to dementia and Alzheimer's disease (AD). While numerous studies implicate Ca²⁺ mishandling in AD, the cellular basis for loss of cognitive function remains under investigation. The process of synaptic degradation and degeneration in AD is slow, and constitutes a series of maladaptive processes each contributing to a further destabilization of the Ca²⁺ homeostatic machinery. Ca²⁺ homeostasis involves precise maintenance of cytosolic Ca²⁺ levels, despite extracellular influx via multiple synaptic Ca²⁺ channels, and intracellular release via organelles such as the endoplasmic reticulum (ER) via ryanodine receptor (RyRs) and IP₃R, lysosomes via transient receptor potential mucolipin channel (TRPML) and two pore channel (TPC), and mitochondria via the permeability transition pore (PTP). Furthermore, functioning of these organelles relies upon regulated inter-organelle Ca²⁺ handling, with aberrant signaling resulting in synaptic dysfunction, protein mishandling, oxidative stress and defective bioenergetics, among other consequences consistent with AD. With few effective treatments currently available to mitigate AD, the past few years have seen a significant increase in the study of synaptic and cellular mechanisms as drivers of AD, including Ca²⁺ dyshomeostasis. Here, we detail some key findings and discuss implications for future AD treatments.

Does kynurenic acid act on nicotinic receptors? An assessment of the evidence

As a major metabolite of kynurenine in the oxidative metabolism of tryptophan, kynurenic acid is of considerable biological and clinical importance as an endogenous antagonist of glutamate in the central nervous system. It is most active as an antagonist at receptors sensitive to N-methyl-D-aspartate (NMDA) which regulate neuronal excitability and plasticity, brain development and behaviour. It is also thought to play a causative role in hypo-glutamatergic conditions such as schizophrenia, and a protective role in several neurodegenerative disorders, notably Huntington's disease. An additional hypothesis, that kynurenic acid could block nicotinic receptors for acetylcholine in the central nervous system has been proposed as an alternative mechanism of action of kynurenate. However, the evidence for this alternative mechanism is highly controversial, partly because at least eight earlier studies concluded that kynurenic acid blocked NMDA receptors but not nicotinic receptors and five subsequent, independent studies designed to repeat the results have failed to do so. Many studies considered to support the alternative 'nicotinic' hypothesis have been based on the use of analogs of kynurenate such as 7-chloro-kynurenic acid, or putatively nicotinic modulators such as galantamine, but a detailed analysis of the pharmacology of these compounds suggests that the results have often been misinterpreted, especially since the pharmacology of galantamine itself has been disputed. This review examines the evidence in detail, with the conclusion that there is no confirmed, reliable evidence for an antagonist activity of kynurenic acid at nicotinic receptors. Therefore, since there is overwhelming evidence for kynurenate acting at ionotropic glutamate receptors, especially NMDAR glutamate and glycine sites, with some activity at GPR35 sites and Aryl Hydrocarbon Receptors, results with kynurenic acid should be interpreted only in terms of these confirmed sites of action.

Electroacupuncture attenuates learning and memory impairment via activation of α7nAChR-mediated anti-inflammatory activity in focal cerebral ischemia/reperfusion injured rats

Studies have reported that electroacupuncture (EA) may reduce learning and memory impairment following cerebral ischemic injury. However, the precise mechanism of action remains unclear. In the present study, the attenuation of focal cerebral ischemia/reperfusion injury by EA in rats was investigated. EA at the Baihui (DU 20) and Shenting (DU 24) acupoints was demonstrated to significantly improve performance in the Morris water maze task, with shortened latency time and increased frequency of passing the platform. Molecular analysis revealed that EA activated the expression of α7 nicotinic acetylcholine receptors (α7nAChR) in the hippocampus. In addition, EA led to a decreased expression of the microglia/macrophage marker Iba1 and the astrocyte marker glial fibrillary acidic protein in the hippocampus. EA treatment also led to decreased production of the inflammatory cytokines tumor necrosis factor-α and interleukin-1β. Treatment with methyllycaconitine, an α7nAChR antagonist, attenuated the improvement of learning and memory following EA treatment and the inhibitory effects of EA on glial cell activation and inflammatory cytokine production. In conclusion, the findings of the present study demonstrate that EA is able to improve learning and memory function following cerebral ischemic injury via activation of α7nAChR, which significantly decreases the neuroinflammatory response.

Altered filamin A enables amyloid beta-induced tau hyperphosphorylation and neuroinflammation in Alzheimer's disease

Alzheimer’s disease (AD) is a neurodegenerative disease with proteopathy characterized by abnormalities in amyloid beta (Aβ) and tau proteins. Defective amyloid and tau propagate and aggregate, leading to eventual amyloid plaques and neurofibrillary tangles. New data show that a third proteopathy, an altered conformation of the scaffolding protein filamin A (FLNA), is critically linked to the amyloid and tau pathologies in AD. Altered FLNA is pervasive in AD brain and without apparent aggregation. In a striking interdependence, altered FLNA is both induced by Aβ and required for two prominent pathogenic signaling pathways of Aβ. Aβ monomers or small oligomers signal via the α7 nicotinic acetylcholine receptor (α7nAChR) to activate kinases that hyperphosphorylate tau to cause neurofibrillary lesions and formation of neurofibrillary tangles. Altered FLNA also enables a persistent activation of toll-like-receptor 4 (TLR4) by Aβ, leading to excessive inflammatory cytokine release and neuroinflammation. The novel AD therapeutic candidate PTI-125 binds and reverses the altered FLNA conformation to prevent Aβ’s signaling via α7nAChR and aberrant activation of TLR4, thus reducing multiple AD-related neuropathologies. As a regulator of Aβ’s signaling via α7nAChR and TLR4, altered FLNA represents a novel AD therapeutic target.

Neuromodulation of the Feedforward Dentate Gyrus-CA3 Microcircuit

The feedforward dentate gyrus-CA3 microcircuit in the hippocampus is thought to activate ensembles of CA3 pyramidal cells and interneurons to encode and retrieve episodic memories. The creation of these CA3 ensembles depends on neuromodulatory input and synaptic plasticity within this microcircuit. Here we review the mechanisms by which the neuromodulators aceylcholine, noradrenaline, dopamine, and serotonin reconfigure this microcircuit and thereby infer the net effect of these modulators on the processes of episodic memory encoding and retrieval.

α7-nAChR agonist enhances neural plasticity in the hippocampus via a GABAergic circuit

Agonists of the α₇-nicotinic acetylcholine receptor (α₇-nAChR) have entered clinical trials as procognitive agents for treating schizophrenia and Alzheimer's disease. The most advanced compounds are orthosteric agonists, which occupy the ligand binding site. At the molecular level, agonist activation of α₇-nAChR is reasonably well understood. However, the consequences of activating α₇-nAChRs on neural circuits underlying cognition remain elusive. Here we report that an α₇-nAChR agonist (FRM-17848) enhances long-term potentiation (LTP) in rat septo-hippocampal slices far below the cellular EC₅₀ but at a concentration that coincides with multiple functional outcome measures as we reported in Stoiljkovic M, Leventhal L, Chen A, Chen T, Driscoll R, Flood D, Hodgdon H, Hurst R, Nagy D, Piser T, Tang C, Townsend M, Tu Z, Bertrand D, Koenig G, Hajós M. Biochem Pharmacol 97: 576-589, 2015. In this same concentration range, we observed a significant increase in spontaneous γ-aminobutyric acid (GABA) inhibitory postsynaptic currents and a moderate suppression of excitability in whole cell recordings from rat CA1 pyramidal neurons. This modulation of GABAergic activity is necessary for the LTP-enhancing effects of FRM-17848, since inhibiting GABA_A α₅-subunit-containing receptors fully reversed the effects of the α₇-nAChR agonist. These data suggest that α₇-nAChR agonists may increase synaptic plasticity in hippocampal slices, at least in part, through a circuit-level enhancement of a specific subtype of GABAergic receptor.

Allosteric modulation of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are receptors for the neurotransmitter acetylcholine and are members of the 'Cys-loop' family of pentameric ligand-gated ion channels (LGICs). Acetylcholine binds in the receptor extracellular domain at the interface between two subunits and research has identified a large number of nAChR-selective ligands, including agonists and competitive antagonists, that bind at the same site as acetylcholine (commonly referred to as the orthosteric binding site). In addition, more recent research has identified ligands that are able to modulate nAChR function by binding to sites that are distinct from the binding site for acetylcholine, including sites located in the transmembrane domain. These include positive allosteric modulators (PAMs), negative allosteric modulators (NAMs), silent allosteric modulators (SAMs) and compounds that are able to activate nAChRs via an allosteric binding site (allosteric agonists). Our aim in this article is to review important aspects of the pharmacological diversity of nAChR allosteric modulators and to describe recent evidence aimed at identifying binding sites for allosteric modulators on nAChRs.

The effect of α7 nicotinic receptor activation on glutamatergic transmission in the hippocampus

Nicotinic acetylcholine receptors (nAChRs) are expressed widely in the CNS, and mediate both synaptic and perisynaptic activities of endogenous cholinergic inputs and pharmacological actions of exogenous compounds (e.g., nicotine and choline). Behavioral studies indicate that nicotine improves such cognitive functions as learning and memory, however the cellular mechanism of these actions remains elusive. With help from newly developed biosensors and optogenetic tools, recent studies provide new insights on signaling mechanisms involved in the activation of nAChRs. Here we will review α7 nAChR's action in the tri-synaptic pathway in the hippocampus. The effects of α7 nAChR activation via either exogenous compounds or endogenous cholinergic innervation are detailed for spontaneous and evoked glutamatergic synaptic transmission and synaptic plasticity, as well as the underlying signaling mechanisms. In summary, α7 nAChRs trigger intracellular calcium rise and calcium-dependent signaling pathways to enhance glutamate release and induce glutamatergic synaptic plasticity.

Protection against β-amyloid-induced synaptic and memory impairments via altering β-amyloid assembly by bis(heptyl)-cognitin

β-amyloid (Aβ) oligomers have been closely implicated in the pathogenesis of Alzheimer’s disease (AD). We found, for the first time, that bis(heptyl)-cognitin, a novel dimeric acetylcholinesterase (AChE) inhibitor derived from tacrine, prevented Aβ oligomers-induced inhibition of long-term potentiation (LTP) at concentrations that did not interfere with normal LTP. Bis(heptyl)-cognitin also prevented Aβ oligomers-induced synaptotoxicity in primary hippocampal neurons. In contrast, tacrine and donepezil, typical AChE inhibitors, could not prevent synaptic impairments in these models, indicating that the modification of Aβ oligomers toxicity by bis(heptyl)-cognitin might be attributed to a mechanism other than AChE inhibition. Studies by using dot blotting, immunoblotting, circular dichroism spectroscopy, and transmission electron microscopy have shown that bis(heptyl)-cognitin altered Aβ assembly via directly inhibiting Aβ oligomers formation and reducing the amount of preformed Aβ oligomers. Molecular docking analysis further suggested that bis(heptyl)-cognitin presumably interacted with the hydrophobic pockets of Aβ, which confers stabilizing powers and assembly alteration effects on Aβ. Most importantly, bis(heptyl)-cognitin significantly reduced cognitive impairments induced by intra-hippocampal infusion of Aβ oligomers in mice. These results clearly demonstrated how dimeric agents prevent Aβ oligomers-induced synaptic and memory impairments, and offered a strong support for the beneficial therapeutic effects of bis(heptyl)-cognitin in the treatment of AD.

Concentration-response relationship of the α7 nicotinic acetylcholine receptor agonist FRM-17874 across multiple in vitro and in vivo assays

Pharmacological activation of α7 nicotinic acetylcholine receptors (α7 nAChRs) may improve cognition in schizophrenia and Alzheimer's disease. The present studies describe an integrated pharmacological analysis of the effects of FRM-17874, an analogue of encenicline, on α7 nAChRs in vitro and in behavioral and neurophysiological assays relevant to cognitive function. FRM-17874 demonstrated high affinity binding to human α7 nAChRs, displacing [(3)H]-methyllacaconitine (Ki=4.3nM). In Xenopus laevis oocytes expressing human α7 nAChRs, FRM-17874 acted as an agonist, evoking inward currents with an EC50 of 0.42μM. Lower concentrations of FRM-17874 (0.01-3nM) elicited no detectable current, but primed receptors to respond to sub-maximal concentrations of acetylcholine. FRM-17874 improved novel object recognition in rats, and enhanced memory acquisition and reversal learning in the mouse water T-maze. Neurophysiological correlates of cognitive effects of drug treatment, such as synaptic transmission, long-term potentiation, and hippocampal theta oscillation were also evaluated. Modulation of synaptic transmission and plasticity was observed in rat hippocampal slices at concentrations of 3.2 and 5nM. FRM-17874 showed a dose-dependent facilitation of stimulation-induced hippocampal theta oscillation in mice and rats. The FRM-17874 unbound brain concentration-response relationship for increased theta oscillation power was similar in both species, exhibited a biphasic pattern peaking around 3nM, and overlapped with active doses and exposures observed in cognition assays. In summary, behavioral and neurophysiological assays indicate a bell-shaped effective concentration range and this report represents the first attempt to explain the concentration-response function of α7 nAChR-mediated pro-cognitive effects in terms of receptor pharmacology.

cAMP-dependent protein kinase inhibits α7 nicotinic receptor activity in layer 1 cortical interneurons through activation of D1/D5 dopamine receptors

KEY POINTS

Protein kinases can modify the function of many proteins including ion channels. However, the role of protein kinase A in modifying nicotinic receptors in the CNS has never been investigated. We showed through whole-cell recordings of layer 1 prefrontal cortical interneurons that α7 nicotinic responses are negatively modulated by protein kinase A. Furthermore, we show that stimulation of dopamine receptors can similarly attenuate α7 nicotinic responses through the activation of protein kinase A. These results suggest how the interaction of the cholinergic and dopaminergic systems may influence neuronal excitability in the brain.

ABSTRACT

Phosphorylation of ion channels, including nicotinic acetylcholine receptors (nAChRs), by protein kinases plays a key role in the modification of synaptic transmission and neuronal excitability. α7 nAChRs are the second most prevalent nAChR subtype in the CNS following α4β2. Serine 365 in the M3-M4 cytoplasmic loop of the α7 nAChR is a phosphorylation site for protein kinase A (PKA). D1/D5 dopamine receptors signal through the adenylate cyclase-PKA pathway and play a key role in working memory and attention in the prefrontal cortex. Thus, we examined whether the dopaminergic system, mediated through PKA, functionally interacts with the α7-dependent cholinergic neurotransmission. In layer 1 interneurons of mouse prefrontal cortex, α7 nicotinic currents were decreased upon stimulation with 8-Br-cAMP, a PKA activator. In HEK 293T cells, dominant negative PKA abolished 8-Br-cAMP's effect of diminishing α7 nicotinic currents, while a constitutively active PKA catalytic subunit decreased α7 currents. In brain slices, the PKA inhibitor KT-5720 nullified 8-Br-cAMP's effect of attenuating α7 nicotinic responses, while applying a PKA catalytic subunit in the pipette solution decreased α7 currents. 8-Br-cAMP stimulation reduced surface expression of α7 nAChRs, but there was no change in single-channel conductance. The D1/D5 dopamine receptor agonist SKF 83822 similarly attenuated α7 nicotinic currents from layer 1 interneurons and this attenuation of nicotinic current was prevented by KT-5720. These results demonstrate that dopamine receptor-mediated activation of PKA negatively modulates nicotinic neurotransmission in prefrontal cortical interneurons, which may be a contributing mechanism of dopamine modulation of cognitive behaviours such as attention or working memory.

Differential effects of the histamine H(3) receptor agonist methimepip on dentate granule cell excitability, paired-pulse plasticity and long-term potentiation in prenatal alcohol-exposed rats

BACKGROUND

We previously reported that prenatal alcohol-induced deficits in dentate gyrus (DG) long-term potentiation (LTP) are ameliorated by the histamine H3 receptor inverse agonist ABT-239. ABT-239 did not enhance LTP in control rats, suggesting that the possibility of a heightened H3 receptor-mediated inhibition of LTP in prenatal alcohol-exposed (PAE) offspring.

METHODS

To further investigate this mechanism, we examined the effect of methimepip, a selective histamine H3 receptor agonist, on DG granule cell responses and LTP in saccharin control and PAE rats. Long-Evans rat dams voluntarily consumed either a 0 or 5% ethanol solution 4 hours each day throughout gestation. Adult male offspring from these dams were anesthetized with urethane and electrodes implanted into the entorhinal cortical perforant path and the DG.

RESULTS

In control offspring, methimepip reduced the coupling of fast excitatory postsynaptic field potentials to population spikes (E-S coupling), the probability of glutamate release, as measured by paired-pulse ratio (PPR) and diminished DG LTP. Similar reductions in E-S coupling and LTP were observed in saline-treated PAE offspring. In contrast to the control group, methimepip did not exacerbate PAE-induced reductions in E-S coupling or LTP.

CONCLUSIONS

While the effects of methimepip in control offspring were consistent with speculation of a PAE-induced enhancement of H3 receptor-mediated inhibition of E-S coupling and LTP, the absence of an added effect of methimepip in PAE offspring could indicate either an inability to further inhibit these responses with methimepip in PAE rats or the presence of more complex regulatory neural interactions with in vivo recordings in PAE rats. Follow-up studies of H3 receptor-mediated responses in DG slice preparations are under way to provide clearer insights into the role of the H3 receptor regulation of excitatory transmission in PAE rats.

Dorsal hippocampal brain receptor complexes linked to the protein synthesis-dependent late phase (LTP) in the rat

In order to link major brain receptor complex levels to in vivo electrically induced LTP, a bipolar stimulation electrode was chronically implanted into the perforant path, while two monopolar recording electrodes were implanted into the dentate gyrus of the dorsal hippocampus. The recording electrode was measuring extracellular excitatory postsynaptic potentials, while the other one measured population spikes. Immunoblotting of native receptor proteins was carried out in the DH based upon blue-native gel electrophoresis and immunoprecipitation followed by mass spectrometrical identification of the NR1-GluA1-GluA2 complex was used to provide evidence for complex formation. The induction of LTP in DH was proven and NMDA receptor complex levels containing NR1, GluA1, GluA2 and GluA3 were modulated by LTP induction. The LTP-associated changes of receptor complex levels may indicate concerted action, interaction and represent a pattern of major brain receptor complexes in the DH following electrical induction of LTP in the rat.

Treadmill exercise prevents learning and memory impairment in Alzheimer's disease-like pathology

Alzheimer's disease (AD) is a neurodegenerative disorder that is characterized by progressive memory loss. In contrast, accumulating evidence suggests a neuroprotective role of regular exercise in aging associated memory impairment. In this study, we investigated the ability of regular exercise to prevent impairments of short-term memory (STM) and early long-term potentiation (E-LTP) in area CA1 of the hippocampus in a rat model of AD (i.c.v. infusion of 250 pmol/day Aβ1-42 peptides). We utilized behavioral assessment, in vivo electrophysiological recording, and immunoblotting in 4 groups of adult Wistar rats: control, treadmill exercise (Ex), β-amyloid-infused (Aβ), and amyloid-infused/treadmill exercised (Ex/Aβ). Our findings indicated that Aβ rats made significantly more errors in the radial arm water maze (RAWM) compared to all other groups and exhibited suppressed E-LTP in area CA1, which correlated with deleterious alterations in the levels of memory and E-LTP-related signaling molecules including calcineurin (PP2B), brain derivedneurotrophic factor (BDNF) and phosphorylated CaMKII (p-CaMKII). Compared to controls, Ex and Ex/Aβ rats showed a similar behavioral performance and a normal E-LTP with no detrimental changes in the levels of PP2B, BDNF, and p- CaMKII. We conclude that treadmill exercise maybe able to prevent cognitive impairment associated with AD pathology.

Neurotransmitter receptor and time dependence of the synaptic plasticity disrupting actions of Alzheimer's disease Aβ in vivo

Many endogenous factors influence the time course and extent of the detrimental effects of amyloid β-protein (Aβ) on synaptic function. Here, we assessed the impact of varying endogenous glutamatergic and cholinergic transmission by pharmacological means on the disruption of plasticity at hippocampal CA3-to-CA1 synapses in the anaesthetized rat. NMDA receptors (NMDARs) are considered critical in mediating Aβ-induced inhibition of long-term potentiation (LTP). However, intracerebroventricular injection of Aβ1-42 inhibited not only NMDAR-dependent LTP but also voltage-activated Ca(2+)-dependent LTP induced by strong conditioning stimulation during NMDAR blockade. On the other hand, another form of NMDAR-independent synaptic plasticity, endogenous acetylcholine-induced muscarinic receptor-dependent long-term enhancement, was not hindered by Aβ1-42. Interestingly, augmenting endogenous acetylcholine activation of nicotinic receptors prior to the injection of Aβ1-42 prevented the inhibition of NMDAR-dependent LTP, whereas the same intervention when introduced after the infusion of Aβ was ineffective. We also examined the duration of action of Aβ, including water soluble Aβ from Alzheimer's disease (AD) brain. Remarkably, the inhibition of LTP induction caused by a single injection of sodium dodecyl sulfate-stable Aβ dimer-containing AD brain extract persisted for at least a week. These findings highlight the need to increase our understanding of non-NMDAR mechanisms and of developing novel means of overcoming, rather than just preventing, the deleterious synaptic actions of Aβ.

Cholinergic coordination of presynaptic and postsynaptic activity induces timing-dependent hippocampal synaptic plasticity

Correlated presynaptic and postsynaptic activity is the key factor in inducing Hebbian plasticity and memory. However, little is known about the physiological events that could mediate such coordination. Correlated cholinergic input induces spike timing-dependent plasticity-like hippocampal synaptic plasticity. Cholinergic receptors are localized to both presynaptic and postsynaptic glutamatergic sites and thus have the potential to coordinate presynaptic and postsynaptic activity to induce plasticity. By directly monitoring presynaptic and postsynaptic activities with genetically encoded calcium indicators in mouse septohippocampal cocultures, we found interactive but independent presynaptic and postsynaptic modulations in the cholinergic-dependent synaptic plasticity. Neither presynaptic nor postsynaptic modulation alone is sufficient, but instead a coordinated modulation at both sites is required to induce the plasticity. Therefore, we propose that correlated cholinergic input can coordinate presynaptic and postsynaptic activities to induce timing-dependent synaptic plasticity, providing a novel mechanism by which neuromodulators precisely modulate network activity and plasticity with high efficiency and temporal precision.

Amyloid β-protein suppressed nicotinic acetylcholine receptor-mediated currents in acutely isolated rat hippocampal CA1 pyramidal neurons

Amyloid β protein (Aβ) is responsible for the deficits of learning and memory in Alzheimer's disease (AD). The high affinity between Aβ and nicotinic acetylcholine receptors (nAChRs) suggests that the impairment of cognitive function in AD might be involved in the Aβ-induced damage of nAChRs. This study investigated the effects of Aβ fragments on nAChR-mediated membrane currents in acutely isolated rat hippocampal pyramidal neurons by using whole-cell patch clamp technique. The results showed that: (1) nonspecific nAChR agonist nicotine, selective α7 nAChR agonist choline, and α4β2 nAChR agonist epibatidine all effectively evoked inward currents in CA1 neurons at normal resting membrane potential, with different desensitization characteristics; (2) acute application of different concentrations (pM-μM) of Aβ25-35, Aβ31-35, or Aβ35-31 alone did not trigger any membrane current, but pretreatment with 1 μM Aβ25-35 and Aβ31-35 similarly and reversibly suppressed the nicotine-induced currents; (3) further, choline- and epibatidine-induced currents were also reversibly suppressed by the Aβ pretreatment, but more prominent for the choline-induced response. These results demonstrate that the functional activity of both α7 and α4β2 nAChRs in the membrane of acutely isolated hippocampal neurons was significantly downregulated by Aβ treatment, suggesting that nAChRs, especially α7 nAChRs, in the brain may be the important biological targets of neurotoxic Aβ in AD. In addition, the similar suppression of nAChR currents by Aβ25-35 and Aβ31-35 suggests that the sequence 31-35 in Aβ molecule may be a shorter active center responsible for the neurotoxicity of Aβ in AD.

Cholinergic modulation of cognitive processing: insights drawn from computational models

Acetylcholine plays an important role in cognitive function, as shown by pharmacological manipulations that impact working memory, attention, episodic memory, and spatial memory function. Acetylcholine also shows striking modulatory influences on the cellular physiology of hippocampal and cortical neurons. Modeling of neural circuits provides a framework for understanding how the cognitive functions may arise from the influence of acetylcholine on neural and network dynamics. We review the influences of cholinergic manipulations on behavioral performance in working memory, attention, episodic memory, and spatial memory tasks, the physiological effects of acetylcholine on neural and circuit dynamics, and the computational models that provide insight into the functional relationships between the physiology and behavior. Specifically, we discuss the important role of acetylcholine in governing mechanisms of active maintenance in working memory tasks and in regulating network dynamics important for effective processing of stimuli in attention and episodic memory tasks. We also propose that theta rhythm plays a crucial role as an intermediary between the physiological influences of acetylcholine and behavior in episodic and spatial memory tasks. We conclude with a synthesis of the existing modeling work and highlight future directions that are likely to be rewarding given the existing state of the literature for both empiricists and modelers.