Presynaptic calcium stores contribute to nicotine-elicited potentiation of evoked synaptic transmission at CA3-CA1 connections in the neonatal rat hippocampus

RyR2-dependent modulation of neuronal hyperactivity: A potential therapeutic target for treating Alzheimer's disease

Increasing evidence suggests that simply reducing β-amyloid (Aβ) plaques may not significantly affect the progression of Alzheimer's disease (AD). There is also increasing evidence indicating that AD progression is driven by a vicious cycle of soluble Aβ-induced neuronal hyperactivity. In support of this, it has recently been shown that genetically and pharmacologically limiting ryanodine receptor 2 (RyR2) open time prevents neuronal hyperactivity, memory impairment, dendritic spine loss and neuronal cell death in AD mouse models. By contrast, increased RyR2 open probability (Po) exacerbates the onset of familial AD-associated neuronal dysfunction and induces AD-like defects in the absence of AD-causing gene mutations. Thus, RyR2-dependent modulation of neuronal hyperactivity represents a promising new target for combating AD.

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.

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.

Limiting RyR2 Open Time Prevents Alzheimer's Disease-Related Neuronal Hyperactivity and Memory Loss but Not β-Amyloid Accumulation

Neuronal hyperactivity is an early primary dysfunction in Alzheimer’s disease (AD) in humans and animal models, but effective neuronal hyperactivity-directed anti-AD therapeutic agents are lacking. Here we define a previously unknown mode of ryanodine receptor 2 (RyR2) control of neuronal hyperactivity and AD progression. We show that a single RyR2 point mutation, E4872Q, which reduces RyR2 open time, prevents hyperexcitability, hyperactivity, memory impairment, neuronal cell death, and dendritic spine loss in a severe early-onset AD mouse model (5xFAD). The RyR2-E4872Q mutation upregulates hippocampal CA1-pyramidal cell A-type K⁺ current, a well-known neuronal excitability control that is downregulated in AD. Pharmacologically limiting RyR2 open time with the R-carvedilol enantiomer (but not racemic carvedilol) prevents and rescues neuronal hyperactivity, memory impairment, and neuron loss even in late stages of AD. These AD-related deficits are prevented even with continued β-amyloid accumulation. Thus, limiting RyR2 open time may be a hyperactivity-directed, non-β-amyloid-targeted anti-AD strategy.

Yao et al. show that genetically or pharmacologically limiting the open duration of ryanodine receptor 2 upregulates the A-type potassium current and prevents neuronal hyperexcitability and hyperactivity, memory impairment, neuronal cell death, and dendritic spine loss in a severe early-onset Alzheimer’s disease mouse model, even with continued accumulation of β-amyloid.

Multiple sources of internal calcium stores mediate ethanol-induced presynaptic inhibitory GABA release in the central nucleus of the amygdala in mice

RATIONALE

Ethanol can enhance GABA release in various brain regions via presynaptic mechanisms. However, the presynaptic action of ethanol on inhibitory GABA release is still not well understood.

OBJECTIVES

Since calcium is required for neurotransmitter release from presynaptic terminals, the purpose of this study was to investigate the role of both internal and external calcium signaling in ethanol-induced enhancement of GABA release within the central amygdala nucleus (CeA) in acute brain slice preparations.

METHODS

Whole-cell patch clamp electrophysiology was used to record miniature GABA_A receptor-mediated inhibitory postsynaptic currents (mIPSCs) from CeA neurons. Ethanol-enhanced mIPSCs were recorded in the presence of antagonists that regulate internal and external calcium-mediated processes.

RESULTS

Bath-applied ethanol dose-dependently increased the mean frequency of mIPSCs without altering mIPSC amplitude. Ethanol-induced increases in mIPSC frequency were antagonized by dantrolene, 2-APB, and the endoplasmic reticulum calcium pump (SERCA) antagonists thapsigargin and cyclopiazonic acid (CPA). Blocking calcium release from mitochondria or via exocytosis with ruthenium red also attenuated mIPSCs while frequency was not altered in the presence of a non-selective calcium channel blocker cadmium. The L-type calcium blocker nifedipine, but not its analogue nimodipine, blocked ethanol-induced enhancement in CeA neurons.

CONCLUSIONS

These results demonstrate ethanol-induced presynaptic release of GABA is mediated by internal calcium stores and by disrupting neurotransmitter exocytosis within the CeA, a critical brain area involved in drugs of abuse and alcohol addiction.

Effects of nitrous oxide on glycinergic transmission in rat spinal neurons

We investigated the effects of nitrous oxide (N₂O) on glycinergic inhibitory whole-cell and synaptic responses using a "synapse bouton preparation," dissociated mechanically from rat spinal sacral dorsal commissural nucleus (SDCN) neurons. This technique can evaluate pure single- or multi-synaptic responses from native functional nerve endings and enable us to accurately quantify how N₂O influences pre- and postsynaptic transmission. We found that 70 % N₂O enhanced exogenous glycine-induced whole-cell currents (I_Gly) at glycine concentrations lower than 3 × 10^-5 M, but did not affect I_Gly at glycine concentrations higher than 10^-4 M. N₂O did not affect the amplitude and 1/e decay-time of both spontaneous and miniature glycinergic inhibitory postsynaptic currents recorded in the absence and presence of tetrodotoxin (sIPSCs and mIPSCs, respectively). The decrease in frequency induced by N₂O was observed in sIPSCs but not in mIPSCs, which was recorded in the presence of both tetrodotoxin and Cd²⁺, which block voltage-gated Na⁺ and Ca²⁺ channels, respectively. N₂O also decreased the amplitude and increased the failure rate and paired-pulse ratio of action potential-evoked glycinergic inhibitory postsynaptic currents. N₂O slightly decreased the Ba²⁺ currents mediated by voltage-gated Ca²⁺ channels in SDCN neurons. We found that N₂O suppresses glycinergic responses at synaptic levels with presynaptic effect having much more predominant role. The difference between glycinergic whole-cell and synaptic responses suggests that extrasynaptic responses seriously modulate whole-cell currents. Our results strongly suggest that these responses may thus in part explain analgesic effects of N₂O via marked glutamatergic inhibition by glycinergic responses in the spinal cord.

Xenon modulates synaptic transmission to rat hippocampal CA3 neurons at both pre- and postsynaptic sites

KEY POINTS

Xenon (Xe) non-competitively inhibited whole-cell excitatory glutamatergic current (I_Glu ) and whole-cell currents gated by ionotropic glutamate receptors (I_AMPA , I_KA , I_NMDA ), but had no effect on inhibitory GABAergic whole-cell current (I_GABA ). Xe decreased only the frequency of glutamatergic spontaneous and miniature excitatory postsynaptic currents and GABAergic spontaneous inhibitory postsynaptic currents without changing the amplitude or decay times of these synaptic responses. Xe decreased the amplitude of both the action potential-evoked excitatory and the action potential-evoked inhibitory postsynaptic currents (eEPSCs and eIPSCs, respectively) via a presynaptic inhibition in transmitter release. We conclude that the main site of action of Xe is presynaptic in both excitatory and inhibitory synapses, and that the Xe inhibition is much greater for eEPSCs than for eIPSCs.

ABSTRACT

To clarify how xenon (Xe) modulates excitatory and inhibitory whole-cell and synaptic responses, we conducted an electrophysiological experiment using the 'synapse bouton preparation' dissociated mechanically from the rat hippocampal CA3 region. This technique can evaluate pure single- or multi-synapse responses and enabled us to accurately quantify how Xe influences pre- and postsynaptic aspects of synaptic transmission. Xe inhibited whole-cell glutamatergic current (I_Glu ) and whole-cell currents gated by the three subtypes of glutamate receptor (I_AMPA , I_KA and I_NMDA ). Inhibition of these ionotropic currents occurred in a concentration-dependent, non-competitive and voltage-independent manner. Xe markedly depressed the slow steady current component of I_AMPA almost without altering the fast phasic I_AMPA component non-desensitized by cyclothiazide. It decreased current frequency without affecting the amplitude and current kinetics of glutamatergic spontaneous excitatory postsynaptic currents and miniature excitatory postsynaptic currents. It decreased the amplitude, increasing the failure rate (Rf) and paired-pulse rate (PPR) without altering the current kinetics of glutamatergic action potential-evoked excitatory postsynaptic currents. Thus, Xe has a clear presynaptic effect on excitatory synaptic transmission. Xe did not alter the GABA-induced whole-cell current (I_GABA ). It decreased the frequency of GABAergic spontaneous inhibitory postsynaptic currents without changing the amplitude and current kinetics. It decreased the amplitude and increased the PPR and Rf of the GABAergic action potential-evoked inhibitory postsynaptic currents without altering the current kinetics. Thus, Xe acts exclusively at presynaptic sites at the GABAergic synapse. In conclusion, our data indicate that a presynaptic decrease of excitatory transmission is likely to be the major mechanism by which Xe induces anaesthesia, with little contribution of effects on GABAergic synapses.

Intracellular Ca2+ Release and Synaptic Plasticity: A Tale of Many Stores

Ca2+ is an essential trigger for most forms of synaptic plasticity. Ca2+ signaling occurs not only by Ca2+ entry via plasma membrane channels but also via Ca2+ signals generated by intracellular organelles. These organelles, by dynamically regulating the spatial and temporal extent of Ca2+ elevations within neurons, play a pivotal role in determining the downstream consequences of neural signaling on synaptic function. Here, we review the role of three major intracellular stores: the endoplasmic reticulum, mitochondria, and acidic Ca2+ stores, such as lysosomes, in neuronal Ca2+ signaling and plasticity. We provide a comprehensive account of how Ca2+ release from these stores regulates short- and long-term plasticity at the pre- and postsynaptic terminals of central synapses.

Beyond the Channel: Metabotropic Signaling by Nicotinic Receptors

The α7 nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel (LGIC) that plays an important role in cellular calcium signaling and contributes to several neurological diseases. Agonist binding to the α7 nAChR induces fast channel activation followed by inactivation and prolonged desensitization while triggering long-lasting calcium signaling. These activities foster neurotransmitter release, synaptic plasticity, and somatodendritic regulation in the brain. We discuss here the ability of α7 nAChRs to operate in ionotropic (α7ⁱ) and metabotropic (α7^m) modes, leading to calcium-induced calcium release (CICR) and G protein-associated inositol trisphosphate (IP₃)-induced calcium release (IICR), respectively. Metabotropic activity extends the spatial and temporal aspects of calcium signaling by the α7 channel beyond its ionotropic limits, persisting into the desensitized state. Delineation of the ionotropic and metabotropic properties of the α7 nAChR will provide definitive indicators of moment-to-moment receptor functional status that will, in turn, spearhead new drug development.

Cadmium increases the sensitivity of adolescent female mice to nicotine-related behavioral deficits

This study investigates spatial and nonspatial working memory, anxiety related behavior, and motor activities in cadmium and/or nicotine exposed female adolescent mice. P28 female adolescent mice (albino strain) were divided into four groups of five (n = 5) mice each. A set of mice (Nic) received subcutaneous nicotine (2.0 mg/kg) while a separate set (Cd) was treated with 2.0 mg/kg cadmium (subcutaneous). For the combined treatments of cadmium and nicotine, we administered 2.0 mg/kg Nicotine and 2.0 mg/kg of Cd. Subsequently, a separate group of animals (n = 5; control) received normal saline. The total duration of treatment for all groups was 28 days (P28–P56). At P56, the treatment was discontinued, after which the animals were examined in behavioural tests. Nicotine and cadmium increased the metabolism and food intake in the female adolescent mice. This also corresponded to an increase in weight when compared with the control. However, a combined nicotine-cadmium treatment induced a decline in weight of the animals versus the control. Also, nicotine administration increased the motor function, while cadmium and nicotine-cadmium treatment caused a decline in motor activity. Both nicotine and cadmium induced a reduction in memory index; however, nicotine-cadmium treatment induced the most significant decrease in nonspatial working memory.

Nicotine-Cadmium Interaction Alters Exploratory Motor Function and Increased Anxiety in Adult Male Mice

In this study we evaluated the time dependence in cadmium-nicotine interaction and its effect on motor function, anxiety linked behavioural changes, serum electrolytes, and weight after acute and chronic treatment in adult male mice. Animals were separated randomly into four groups of n = 6 animals each. Treatment was done with nicotine, cadmium, or nicotine-cadmium for 21 days. A fourth group received normal saline for the same duration (control). Average weight was determined at 7-day interval for the acute (D1-D7) and chronic (D7-D21) treatment phases. Similarly, the behavioural tests for exploratory motor function (open field test) and anxiety were evaluated. Serum electrolytes were measured after the chronic phase. Nicotine, cadmium, and nicotine-cadmium treatments caused no significant change in body weight after the acute phase while cadmium-nicotine and cadmium caused a decline in weight after the chronic phase. This suggests the role of cadmium in the weight loss observed in tobacco smoke users. Both nicotine and cadmium raised serum Ca²⁺ concentration and had no significant effect on K⁺ ion when compared with the control. In addition, nicotine-cadmium treatment increased bioaccumulation of Cd²⁺ in the serum which corresponded to a decrease in body weight, motor function, and an increase in anxiety.

Nicotine uses neuron-glia communication to enhance hippocampal synaptic transmission and long-term memory

Nicotine enhances synaptic transmission and facilitates long-term memory. Now it is known that bi-directional glia-neuron interactions play important roles in the physiology of the brain. However, the involvement of glial cells in the effects of nicotine has not been considered until now. In particular, the gliotransmitter D-serine, an endogenous co-agonist of NMDA receptors, enables different types of synaptic plasticity and memory in the hippocampus. Here, we report that hippocampal long-term synaptic plasticity induced by nicotine was annulled by an enzyme that degrades endogenous D-serine, or by an NMDA receptor antagonist that acts at the D-serine binding site. Accordingly, both effects of nicotine: the enhancement of synaptic transmission and facilitation of long-term memory were eliminated by impairing glial cells with fluoroacetate, and were restored with exogenous D-serine. Together, these results show that glial D-serine is essential for the long-term effects of nicotine on synaptic plasticity and memory, and they highlight the roles of glial cells as key participants in brain functions.

Regulation of hippocampal inhibitory circuits by nicotinic acetylcholine receptors

The hippocampal network comprises a large variety of locally connected GABAergic interneurons exerting a powerful control on network excitability and which are responsible for the oscillatory behaviour crucial for information processing. GABAergic interneurons receive an important cholinergic innervation from the medial septum-diagonal band complex of the basal forebrain and are endowed with a variety of muscarinic and nicotinic acetylcholine receptors (mAChRs and nAChRs) that regulate their activity. Deficits in the cholinergic system lead to the impairment of high cognitive functions, which are particularly relevant in neurodegenerative pathologies such as Alzheimer's and Parkinson's diseases as well as in schizophrenia. Here, we highlight some recent advances in the mechanisms by which cholinergic signalling via nAChRs regulates local inhibitory circuits in the hippocampus, early in postnatal life and in adulthood. We also discuss recent findings concerning the functional role of nAChRs in controlling short- and long-term modifications of synaptic efficacy. Insights into these processes may provide new targets for the therapeutic control of pathological conditions associated with cholinergic dysfunctions.

Mechanisms involved in nicotinic acetylcholine receptor-induced neurotransmitter release from sympathetic nerve terminals in the mouse vas deferens

Prejunctional nicotinic acetylcholine receptors (nAChRs) amplify postganglionic sympathetic neurotransmission, and there are indications that intraterminal Ca²⁺ stores might be involved. However, the mechanisms by which nAChR activation stimulates neurotransmitter release at such junctions is unknown. Rapid local delivery (picospritzing) of the nAChR agonist epibatidine was combined with intracellular sharp microelectrode recording to monitor spontaneous and field-stimulation-evoked neurotransmitter release from sympathetic nerve terminals in the mouse isolated vas deferens. Locally applied epibatidine (1 µM) produced ‘epibatidine-induced depolarisations’ (EIDs) that were similar in shape to spontaneous excitatory junction potentials (SEJPs) and were abolished by nonselective nAChR antagonists and the purinergic desensitizing agonist α,β-methylene ATP. The amplitude distribution of EIDs was only slightly shifted towards lower amplitudes by the selective α7 nAChR antagonists α-bungarotoxin and methyllcaconitine, the voltage-gated Na⁺ channel blocker tetrodotoxin or by blocking voltage-gated Ca²⁺ channels with Cd²⁺. Lowering the extracellular Ca²⁺ concentration reduced the frequency of EIDs by 69%, but more surprisingly, the Ca²⁺-induced Ca²⁺ release blocker ryanodine greatly decreased the amplitude (by 41%) and the frequency of EIDs by 36%. Ryanodine had no effect on electrically-evoked neurotransmitter release, paired-pulse facilitation, SEJP frequency, SEJP amplitude or SEJP amplitude distribution. These results show that activation of non-α7 nAChRs on sympathetic postganglionic nerve terminals induces high-amplitude junctional potentials that are argued to represent multipacketed neurotransmitter release synchronized by intraterminal Ca²⁺-induced Ca²⁺ release, triggered by Ca²⁺ influx directly through the nAChR. This nAChR-induced neurotransmitter release can be targeted pharmacologically without affecting spontaneous or electrically-evoked neurotransmitter release.

Cannabidiol inhibits synaptic transmission in rat hippocampal cultures and slices via multiple receptor pathways

BACKGROUND AND PURPOSE

Cannabidiol (CBD) has emerged as an interesting compound with therapeutic potential in several CNS disorders. However, whether it can modulate synaptic activity in the CNS remains unclear. Here, we have investigated whether CBD modulates synaptic transmission in rat hippocampal cultures and acute slices.

EXPERIMENTAL APPROACH

The effect of CBD on synaptic transmission was examined in rat hippocampal cultures and acute slices using whole cell patch clamp and standard extracellular recordings respectively.

KEY RESULTS

Cannabidiol decreased synaptic activity in hippocampal cultures in a concentration-dependent and Pertussis toxin-sensitive manner. The effects of CBD in culture were significantly reduced in the presence of the cannabinoid receptor (CB(1) ) inverse agonist, LY320135 but were unaffected by the 5-HT(1A) receptor antagonist, WAY100135. In hippocampal slices, CBD inhibited basal synaptic transmission, an effect that was abolished by the proposed CB(1) receptor antagonist, AM251, in addition to LY320135 and WAY100135.

CONCLUSIONS AND IMPLICATIONS

Cannabidiol reduces synaptic transmission in hippocampal in vitro preparations and we propose a role for both 5-HT(1A) and CB(1) receptors in these CBD-mediated effects. These data offer some mechanistic insights into the effects of CBD and emphasize that further investigations into the actions of CBD in the CNS are required in order to elucidate the full therapeutic potential of CBD.

Partial Disinhibition Is Required for Transition of Stimulus-Induced Sharp Wave–Ripple Complexes Into Recurrent Epileptiform Discharges in Rat Hippocampal Slices

Sharp wave–ripple complexes (SPW-Rs) in the intact rodent hippocampus are characterized by slow field potential transients superimposed by close to 200-Hz ripple oscillations. Similar events have been recorded in hippocampal slices where SPW-Rs occur spontaneously or can be induced by repeated application of high-frequency stimulation, a standard protocol for induction of long-lasting long-term potentiation. Such stimulation is reminiscent of protocols used to induce kindling epilepsy and ripple oscillations may be predictive of the epileptogenic zone in temporal lobe epilepsy. In the present study, we investigated the relation between recurrent epileptiform discharges (REDs) and SPW-Rs by studying effects of partial removal of inhibition. In particular, we compared the effects of nicotine, low-dose bicuculline methiodide (BMI), and elevated extracellular potassium concentration ([K+]o) on induced SPW-Rs. We show that nicotine dose-dependently transformed SPW-Rs into REDs. This transition was associated with reduced inhibitory conductance in CA3 pyramidal cells. Similar results were obtained from slices where the GABAergic conductance was reduced by application of low concentrations of BMI (1–2 μM). In contrast, sharp waves were diminished by phenobarbital. Elevating [K+]o from 3 to 8.5 mM did not transform SPW-Rs into REDs but significantly increased their incidence and amplitude. Under these conditions, the equilibrium potential for inhibition was shifted in depolarizing direction, whereas inhibitory conductance was significantly increased. Interestingly, the propensity of elevated [K+]o to induce seizure-like events was reduced in slices where SPW-Rs had been induced. In conclusion, recruitment of inhibitory cells during SPW-Rs may serve as a mechanism by which hyperexcitation and eventually seizure generation might be prevented.

beta-Amyloid activates presynaptic alpha7 nicotinic acetylcholine receptors reconstituted into a model nerve cell system: involvement of lipid rafts

Beta amyloid (Abeta) plays a central role in the pathogenesis of Alzheimer's disease. Abeta is the major constituent of senile plaques, but there is a significant presence of Abeta in the brain in soluble forms. The results of functional studies indicate that soluble Abeta interacts with the alpha7 nicotinic acetylcholine receptor (nAChR) complex with apparent high affinity. However, conflicting data exist as to the nature of the Abeta-alpha7 nAChR interaction, and whether it is the result of specific binding. Moreover, both agonist-like and antagonist-like effects have been reported. In particular, agonist-like effects have been observed for presynaptic nAChRs. Here, we demonstrate Abeta(1-42)-evoked stimulatory changes in presynaptic Ca(2+) level via exogenous alpha7 nAChRs expressed in the axonal varicosities of differentiated hybrid neuroblastoma NG108-15 cells as a model, presynaptic system. The Abeta(1-42)-evoked responses were concentration-dependent and were sensitive to the highly selective alpha7 nAChR antagonist alpha-bungarotoxin. Voltage-gated Ca(2+) channels and internal Ca(2+) stores were both involved in Abeta(1-42)-evoked increases in presynaptic Ca(2+) following activation of alpha7 nAChRs. In addition, disruption of lipid rafts by cholesterol depletion led to substantially attenuated responses to Abeta(1-42), whereas responses to nicotine were largely intact. These results directly implicate the nicotinic receptor complex as a target for the agonist-like action of pico- to nanomolar concentrations of soluble Abeta(1-42) on the presynaptic nerve terminal, including the possible involvement of receptor-associated lipid rafts. This interaction probably plays an important neuromodulatory role in synaptic dynamics.

Developmental aspects of the cholinergic system

Beyond its importance in sustaining or modulating different aspects of the activity of the central nervous system (CNS), the cholinergic system plays important roles during development. In the current review, we focus on the developmental aspects associated with major components of the cholinergic system: Acetylcholine, choline acetyltransferase, vesicular acetylcholine transporter, high-affinity choline transporter, acetylcholinesterase, nicotinic and muscarinic receptors. We describe when and where each one of these components is first identified in the CNS and the changes in their levels that occur during the course of prenatal and postnatal development. We also describe how these components are relevant to many events that occur during the development of the CNS, including progenitor cells proliferation and differentiation, neurogenesis, gliogenesis, neuronal maturation and plasticity, axonal pathfinding, regulation of gene expression and cell survival. It will be noticed that evidence regarding the developmental aspects of the cholinergic system comes mostly from studies that used agonists, such as nicotine, and antagonists, such as hemicholinium-3. Studies using immunohistochemistry and genetically altered mice also provided valuable information.

Activation of nicotinic acetylcholine receptors enhances a slow calcium-dependent potassium conductance and reduces the firing of stratum oriens interneurons

A large variety of distinct locally connected GABAergic cells are present in the hippocampus. By releasing GABA into principal cells and interneurons, they exert a powerful control on neuronal excitability and are responsible for network oscillations crucial for information processing in the brain. Here, whole-cell patch clamp recordings in current and voltage clamp mode were used to study the functional role of nicotinic acetylcholine receptors (nAChRs) on the firing properties of stratum oriens interneurons in hippocampal slices from transgenic mice expressing enhanced green fluorescent protein in a subpopulation of GABAergic cells containing somatostatin (GIN mice). Unexpectedly, activation of nAChRs by nicotine or endogenously released acetylcholine strongly enhanced spike frequency adaptation. This effect was blocked by apamin, suggesting the involvement of small calcium-dependent potassium channels (SK channels). Nicotine-induced reduction in firing frequency was dependent on intracellular calcium rise through calcium-permeable nAChRs and voltage-dependent calcium channels activated by the depolarizing action of nicotine. Calcium imaging experiments directly showed that nicotine effects on firing rate were correlated with large increases in intracellular calcium. Furthermore, blocking ryanodine receptors with ryanodine or sarcoplasmic-endoplasmic reticulum calcium ATPase with thapsygargin or cyclopiazonic acid fully prevented the effects of nicotine, suggesting that mobilization of calcium from the internal stores contributed to the observed effects. By regulating cell firing, cholinergic signalling through nAChRs would be instrumental for fine-tuning the output of stratum oriens interneurons and correlated activity at the network level.

Transient enhancement of spike-evoked calcium signaling by a serotonergic interneuron

Enhancement of presynaptic Ca(2+) signals is widely recognized as a potential mechanism for heterosynaptic potentiation of neurotransmitter release. Here we show that stimulation of a serotonergic interneuron increased spike-evoked Ca(2+) in a manner consistent with its neuromodulatory effect on synaptic transmission. In the gastropod mollusk, Tritonia diomedea, stimulation of a serotonergic dorsal swim interneuron (DSI) at physiological rates heterosynaptically enhances the strength of output synapses made by another swim interneuron, C2, onto neurons in the pedal ganglion. Using intracellular electrophysiological recording combined with real-time confocal imaging of C2 (loaded with Oregon Green Bapta 1), it was determined that DSI stimulation increases the amplitude of spike-evoked Ca(2+) signals in C2 without altering basal Ca(2+) signals. This neuromodulatory action was restricted to distal neurites of C2 where synapses with pedal neurons are located. The effect of DSI stimulation on C2 spike-evoked Ca(2+) signals resembled DSI heterosynaptic enhancement of C2 synapses in several measures: both decayed within 15 s, both were abolished by the serotonin receptor antagonist, methysergide, and both were independent of DSI's depolarizing actions on C2. A brief puff of serotonin could mimic the enhancement of spike-evoked Ca(2+) signals in the distal neurites of C2, but larger puffs or bath-applied serotonin elicited nonphysiological effects. These results suggest that DSI heterosynaptic enhancement of C2 synaptic strength may be mediated by a local enhancement of spike-evoked Ca(2+) signals in the distal neurites of C2.

Allosteric modulation of alpha 7 nicotinic receptors selectively depolarizes hippocampal interneurons, enhancing spontaneous GABAergic transmission

The role of postsynaptic nicotinic receptors for acetylcholine (nAChRs) in mediating fast neurotransmission processes in the CNS is controversial. Here we have studied the modulation of synaptic transmission by an agonist (choline) and an allosteric modulator (5-OH-indole) of alpha7 nAChRs in rat hippocampal neuronal cultures. Choline evoked a fast inactivating inward current, causing neuron depolarization and action potential discharge, thereby enhancing the spontaneous postsynaptic current activity (sPSCs). This effect was markedly enhanced when both choline and 5-OH-indole were applied together and was blocked by the selective alpha7 nAChR antagonist methyllycaconitine. This choline action was suppressed by the GABA(A) receptor antagonist bicuculline, while the glutamatergic receptor antagonist kynurenic acid had no effect. Frequency, but not amplitude or area, of both excitatory and inhibitory miniature postsynaptic currents (mEPSCs and mIPSCs) were drastically reduced when Ca(2+) influx was blocked by Cd(2+). Additionally, nAChR activation did not modify the mIPSCs. These data suggest that Ca(2+) influx through the highly Ca(2+)-permeablealpha7 nAChRs was insufficient to directly activate neurotransmitter release, suggesting that a tight colocalization of this receptor with secretory hot spots is unlikely. In a few cases, the activation of alpha7 AChRs led to a suppression of spontaneous synaptic transmission. This effect may be related to the potentiation of GABAergic interneurons that inhibit the spontaneous activity of neurons making synapses with the cell under study. We suggest that GABA release is modulated by alpha7 nAChRs. Thus, selective allosteric modulators of alpha7 nAChRs could have potential therapeutic applications in brain disorders such as epilepsy and schizophrenia and in alterations of cognition and sensory processing.

Superoxide-induced potentiation in the hippocampus requires activation of ryanodine receptor type 3 and ERK

Reactive oxygen species (ROS) are required for the induction of long-term potentiation (LTP) and behave as signaling molecules via redox modifications of target proteins. In particular, superoxide is necessary for induction of LTP, and application of superoxide to hippocampal slices is sufficient to induce LTP in area CA1. Although a rise in postsynaptic intracellular calcium is necessary for LTP induction, superoxide-induced potentiation does not require calcium flux through N-methyl-d-aspartate (NMDA) receptors. Ryanodine receptors (RyRs) mediate calcium-induced calcium release from intracellular stores and have been shown to modulate LTP. In this study, we investigated the highly redox-sensitive RyRs and L-type calcium channels as calcium sources that might mediate superoxide-induced potentiation. In agreement with previous studies of skeletal and cardiac muscle, we found that superoxide enhances activation of RyRs in the mouse hippocampus. We identified a functional coupling between L-type voltage-gated calcium channels and RyRs and identified RyR3, a subtype enriched in area CA1, as the specific isoform required for superoxide-induced potentiation. Superoxide also enhanced the phosphorylation of extracellular signal-regulated kinase (ERK) in area CA1, and ERK was necessary for superoxide-induced potentiation. Thus superoxide-induced potentiation requires the redox targeting of RyR3 and the subsequent activation of ERK.