Control of IsAHP in mouse hippocampus CA1 pyramidal neurons by RyR3-mediated calcium-induced calcium release

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.

The Role of Ryanodine Receptors in Regulating Neuronal Activity and Its Connection to the Development of Alzheimer's Disease

Research into the early impacts of Alzheimer's disease (AD) on synapse function is one of the most promising approaches to finding a treatment. In this context, we have recently demonstrated that the Abeta42 peptide, which builds up in the brain during the processing of the amyloid precursor protein (APP), targets the ryanodine receptors (RyRs) of mouse hippocampal neurons and potentiates calcium (Ca²⁺) release from the endoplasmic reticulum (ER). The uncontrolled increase in intracellular calcium concentration ([Ca²⁺]_i), leading to the development of Ca²⁺ dysregulation events and related excitable and synaptic dysfunctions, is a consolidated hallmark of AD onset and possibly other neurodegenerative diseases. Since RyRs contribute to increasing [Ca²⁺]_i and are thought to be a promising target for AD treatment, the goal of this review is to summarize the current level of knowledge regarding the involvement of RyRs in governing neuronal function both in physiological conditions and during the onset of AD.

Hippocampal dendritic spines express the RyR3 but not the RyR2 ryanodine receptor isoform

The hippocampus is a brain region implicated in synaptic plasticity and memory formation; both processes require neuronal Ca²⁺ signals generated by Ca²⁺ entry via plasma membrane Ca²⁺ channels and Ca²⁺ release from the endoplasmic reticulum (ER). Through Ca²⁺-induced Ca²⁺ release, the ER-resident ryanodine receptor (RyR) Ca²⁺ channels amplify and propagate Ca²⁺ entry signals, leading to activation of cytoplasmic and nuclear Ca²⁺-dependent signaling pathways required for synaptic plasticity and memory processes. Earlier reports have shown that mice and rat hippocampus expresses mainly the RyR2 isoform, with lower expression levels of the RyR3 isoform and almost undetectable levels of the RyR1 isoform; both the RyR2 and RyR3 isoforms have central roles in synaptic plasticity and hippocampal-dependent memory processes. Here, we describe that dendritic spines of rat primary hippocampal neurons express the RyR3 channel isoform, which is also expressed in the neuronal body and neurites. In contrast, the RyR2 isoform, which is widely expressed in the neuronal body and neurites of primary hippocampal neurons, is absent from the dendritic spines. We propose that this asymmetric distribution is of relevance for hippocampal neuronal function. We suggest that the RyR3 isoform amplifies activity-generated Ca²⁺ entry signals at postsynaptic dendritic spines, from where they propagate to the dendrite and activate primarily RyR2-mediated Ca²⁺ release, leading to Ca²⁺ signal propagation into the soma and the nucleus where they activate the expression of genes that mediate synaptic plasticity and memory.

Subcellular localization of hippocampal ryanodine receptor 2 and its role in neuronal excitability and memory

Ryanodine receptor 2 (RyR2) is abundantly expressed in the heart and brain. Mutations in RyR2 are associated with both cardiac arrhythmias and intellectual disability. While the mechanisms of RyR2-linked arrhythmias are well characterized, little is known about the mechanism underlying RyR2-associated intellectual disability. Here, we employed a mouse model expressing a green fluorescent protein (GFP)-tagged RyR2 and a specific GFP probe to determine the subcellular localization of RyR2 in hippocampus. GFP-RyR2 was predominantly detected in the soma and dendrites, but not the dendritic spines of CA1 pyramidal neurons or dentate gyrus granular neurons. GFP-RyR2 was also detected within the mossy fibers in the stratum lucidum of CA3, but not in the presynaptic terminals of CA1 neurons. An arrhythmogenic RyR2-R4496C^+/− mutation downregulated the A-type K⁺ current and increased membrane excitability, but had little effect on the afterhyperpolarization current or presynaptic facilitation of CA1 neurons. The RyR2-R4496C^+/− mutation also impaired hippocampal long-term potentiation, learning, and memory. These data reveal the precise subcellular distribution of hippocampal RyR2 and its important role in neuronal excitability, learning, and memory.

A mouse model containing a GFP-tagged ryanodine receptor 2 (RyR2) has shed light on the precise subcellular localization of hippocampal RyR2 and mechanisms underlying neuronal excitability, learning, and memory.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Kv7 and Kir6 Channels Shape the Slow AHP in Mouse Dentate Gyrus Granule Cells and Control Burst-like Firing Behavior

The slow afterhyperpolarizing potential (sAHP) can silence a neuron for hundreds of milliseconds. Thereby, the sAHP determines the discharge behavior of many types of neurons. In dentate granule cells (DGCs), serving as a filter into the hippocampal network, mostly tonic or adapting discharge properties have been described. As under standard whole-cell recording conditions the sAHP is inhibited, we reevaluated the intrinsic functional phenotype of DGCs and the conductances underlying the sAHP, using gramicidine-perforated patch-clamp technique. We found that in 97/113 (86%) of the DGCs, a burst of action potentials (APs) to excitation ended by a large sAHP, despite continued depolarization. This result suggests that burst-like firing is the default functional phenotype of DGCs and that sAHPs are important for it. Indeed, burst-like firing DGCs showed a significantly higher sAHP-current (I_sAHP) amplitude compared to spike-frequency adapting cells (16/113 = 14%). The I_sAHP was mediated by K_v7 and K_ir6 channels by pharmacological inhibition using XE991 and tolbutamide, although heterogeneously among DGCs. The percent inhibition of I_sAHP by these compounds also correlated with the AP number and AP burst length. Application of 100 µM nickel after XE991 and tolbutamide detected a third conductance contributing to burst-like firing and the sAHP, most likely mediated by T-type calcium channels. Lastly, medial perforant path-dentate gyrus long-term potentiation was amplified by XE991 and tolbutamide. In conclusion, the sAHP shapes intrinsic burst-like firing which, under physiological circumstances, could be controlled via cholinergic afferents and ATP metabolism.

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.

Calcium-induced calcium release and type 3 ryanodine receptors modulate the slow afterhyperpolarising current, sIAHP, and its potentiation in hippocampal pyramidal neurons

The slow afterhyperpolarising current, sI_AHP, is a Ca²⁺-dependent current that plays an important role in the late phase of spike frequency adaptation. sI_AHP is activated by voltage-gated Ca²⁺ channels, while the contribution of calcium from ryanodine-sensitive intracellular stores, released by calcium-induced calcium release (CICR), is controversial in hippocampal pyramidal neurons. Three types of ryanodine receptors (RyR1-3) are expressed in the hippocampus, with RyR3 showing a predominant expression in CA1 neurons. We investigated the specific role of CICR, and particularly of its RyR3-mediated component, in the regulation of the sI_AHP amplitude and time course, and the activity-dependent potentiation of the sI_AHP in rat and mouse CA1 pyramidal neurons. Here we report that enhancement of CICR by caffeine led to an increase in sI_AHP amplitude, while inhibition of CICR by ryanodine caused a small, but significant reduction of sI_AHP. Inhibition of ryanodine-sensitive Ca²⁺ stores by ryanodine or depletion by the SERCA pump inhibitor cyclopiazonic acid caused a substantial attenuation in the sI_AHP activity-dependent potentiation in both rat and mouse CA1 pyramidal neurons. Neurons from mice lacking RyR3 receptors exhibited a sI_AHP with features undistinguishable from wild-type neurons, which was similarly reduced by ryanodine. However, the lack of RyR3 receptors led to a faster and reduced activity-dependent potentiation of sI_AHP. We conclude that ryanodine receptor-mediated CICR contributes both to the amplitude of the sI_AHP at steady state and its activity-dependent potentiation in rat and mouse hippocampal pyramidal neurons. In particular, we show that RyR3 receptors play an essential and specific role in shaping the activity-dependent potentiation of the sI_AHP. The modulation of activity-dependent potentiation of sI_AHP by RyR3-mediated CICR contributes to plasticity of intrinsic neuronal excitability and is likely to play a critical role in higher cognitive functions, such as learning and memory.

Intracellular Calcium Responses Encode Action Potential Firing in Spinal Cord Lamina I Neurons

Maladaptive plasticity of neurons in lamina I of the spinal cord is a lynchpin for the development of chronic pain, and is critically dependent on intracellular calcium signaling. However, the relationship between neuronal activity and intracellular calcium in these neurons is unknown. Here we combined two-photon calcium imaging with whole-cell electrophysiology to determine how action potential firing drives calcium responses within subcellular compartments of male rat spinal cord lamina I neurons. We found that single action potentials generated at the soma increase calcium concentration in the somatic cytosol and nucleus, and these calcium responses invade dendrites and dendritic spines by active backpropagation. Calcium responses in each compartment were dependent on voltage-gated calcium channels, and somatic and nuclear calcium responses were amplified by release of calcium from ryanodine-sensitive intracellular stores. Grouping single action potential-evoked calcium responses by neuron type demonstrated their presence in all defined types, as well as a high degree of similarity in calcium responses between neuron types. With bursts of action potentials, we found that calcium responses have the capacity to encode action potential frequency and number in all compartments, with action potential number being preferentially encoded. Together, these findings indicate that intracellular calcium serves as a readout of neuronal activity within lamina I neurons, providing a unifying mechanism through which activity may regulate plasticity, including that seen in chronic pain.SIGNIFICANCE STATEMENT Despite their critical role in both acute pain sensation and chronic pain, little is known of the fundamental physiology of spinal cord lamina I neurons. This is especially the case with respect to calcium dynamics within these neurons, which could regulate maladaptive plasticity observed in chronic pain. By combining two-photon calcium imaging and patch-clamp electrophysiological recordings from lamina I neurons, we found that action potential firing induces calcium responses within the somatic cytosol, nucleus, dendrites, and dendritic spines of lamina I neurons. Our findings demonstrate the presence of actively backpropagating action potentials, shifting our understanding of how these neurons process information, such that calcium provides a mechanism for lamina I neurons to track their own activity.

Regulation of intrinsic excitability: Roles for learning and memory, aging and Alzheimer's disease, and genetic diversity

Plasticity of intrinsic neuronal excitability facilitates learning and memory across multiple species, with aberrant modulation of this process being linked to the development of neurological symptoms in models of cognitive aging and Alzheimer's disease. Learning-related increases in intrinsic excitability of neurons occurs in a variety of brain regions, and is generally thought to promote information processing and storage through enhancement of synaptic throughput and induction of synaptic plasticity. Experience-dependent changes in intrinsic neuronal excitability rely on activity-dependent gene expression patterns, which can be influenced by genetic and environmental factors, aging, and disease. Reductions in baseline intrinsic excitability, as well as aberrant plasticity of intrinsic neuronal excitability and in some cases pathological hyperexcitability, have been associated with cognitive deficits in animal models of both normal cognitive aging and Alzheimer's disease. Genetic factors that modulate plasticity of intrinsic excitability likely underlie individual differences in cognitive function and susceptibility to cognitive decline. Thus, targeting molecular mediators that either control baseline intrinsic neuronal excitability, subserve learning-related intrinsic neuronal plasticity, and/or promote resilience may be a promising therapeutic strategy for maintaining cognitive function in aging and disease. In this review, we discuss the complementary relationship between intrinsic excitability and learning, with a particular focus on how this relationship varies as a function of age, disease state, and genetic make-up, and how targeting these factors may help to further elucidate our understanding of the role of intrinsic excitability in cognitive function and cognitive decline.

Calcium stores regulate excitability in cultured rat hippocampal neurons

Extracellular calcium ions support synaptic activity but also reduce excitability of central neurons. In the present study, the effect of calcium on excitability was explored in cultured hippocampal neurons. CaCl₂ injected by pressure in the vicinity of a neuron that is bathed only in MgCl₂ as the main divalent cation caused a depolarizing shift in action potential threshold and a reduction in excitability. This effect was not seen if the intracellular milieu consisted of Cs⁺ instead of K-gluconate as the main cation or when it contained ruthenium red, which blocks release of calcium from stores. The suppression of excitability by calcium was mimicked by caffeine, and calcium store antagonists cyclopiazonic acid or thapsigargin blocked this action. Neurons taken from synaptopodin-knockout mice show significantly reduced efficacy of calcium modulation of action potential threshold. Likewise, in Orai1 knockdown cells, calcium is less effective in modulating excitability of neurons. Activation of small-conductance K (SK) channels increased action potential threshold akin to that produced by calcium ions, whereas blockade of SK channels but not big K channels reduced the threshold for action potential discharge. These results indicate that calcium released from stores may suppress excitability of central neurons. NEW & NOTEWORTHY Extracellular calcium reduces excitability of cultured hippocampal neurons. This effect is mediated by calcium-gated potassium currents, possibly small-conductance K channels. Release of calcium from internal stores mimics the effect of extracellular calcium. It is proposed that calcium stores modulate excitability of central neurons.

Specificity in the interaction of high-voltage-activated Ca2+ channel types with Ca2+-dependent afterhyperpolarizations in magnocellular supraoptic neurons

Magnocellular oxytocin (OT) and vasopressin (VP) neurons express an afterhyperpolarization (AHP) following spike trains that attenuates firing rate and contributes to burst patterning. This AHP includes contributions from an apamin-sensitive, medium-duration AHP (mAHP) and from an apamin-insensitive, slow-duration AHP (sAHP). These AHPs are Ca²⁺ dependent and activated by Ca²⁺ influx through voltage-gated Ca²⁺ channels. Across central nervous system neurons that generate Ca²⁺-dependent AHPs, the Ca²⁺ channels that couple to the mAHP and sAHP differ greatly, but for magnocellular neurosecretory cells this relationship is unknown. Using simultaneous whole cell recording and Ca²⁺ imaging, we evaluated the effect of specific high-voltage-activated (HVA) Ca²⁺ channel blockers on the mAHP and sAHP. Block of all HVA channels via 400 μM Cd²⁺ inhibited almost the entire AHP. We tested nifedipine, conotoxin GVIA, agatoxin IVA, and SNX-482, specific blockers of L-, N-, P/Q-, and R-type channels, respectively. The N-type channel blocker conotoxin GVIA (1 μM) was the only toxin that inhibited the mAHP in either OT or VP neurons although the effect on VP neurons was weaker by comparison. The sAHP was significantly inhibited by N-type block in OT neurons and by R-type block in VP neurons although neither accounted for the entirety of the sAHP. Thus the mAHP appears to be elicited by Ca²⁺ from mostly N-type channels in both OT and VP neurons, but the contributions of specific Ca²⁺ channel types to the sAHP in each cell type are different. Alternative sources to HVA channels may contribute Ca²⁺ for the sAHP. NEW & NOTEWORTHY Despite the importance of afterhyperpolarization (AHP) mechanisms for regulating firing behavior of oxytocin (OT) and vasopressin (VP) neurons of supraoptic nucleus, which types of high-voltage-activated Ca²⁺ channels elicit AHPs in these cells was unknown. We found that N-type channels couple to the medium AHP in both cell types. For the slow AHP, N-type channels contribute in OT neurons, whereas R-type contribute in VP neurons. No single Ca²⁺ channel blocker abolished the entire AHP, suggesting that additional Ca²⁺ sources are involved.

The role of ryanodine receptor type 3 in a mouse model of Alzheimer disease

Dysregulated endoplasmic reticulum (ER) calcium (Ca²⁺) signaling is reported to play an important role in Alzheimer disease (AD) pathogenesis. The role of ER Ca²⁺ release channels, the ryanodine receptors (RyanRs), has been extensively studied in AD models and RyanR expression and activity are upregulated in the brains of various familial AD (FAD) models. The objective of this study was to utilize a genetic approach to evaluate the importance of RyanR type 3 (RyanR3) in the context of AD pathology.

Early presynaptic and postsynaptic calcium signaling abnormalities mask underlying synaptic depression in presymptomatic Alzheimer's disease mice

Alzheimer's disease (AD)-linked presenilin (PS) mutations result in pronounced endoplasmic reticulum calcium disruptions that occur before detectable histopathology and cognitive deficits. More subtly, these early AD-linked calcium alterations also reset neurophysiological homeostasis, such that calcium-dependent presynaptic and postsynaptic signaling appear functionally normal yet are actually operating under aberrant calcium signaling systems. In these 3xTg-AD mouse brains, upregulated ryanodine receptor (RyR) activity is associated with a shift toward synaptic depression, likely through a reduction in presynaptic vesicle stores and increased postsynaptic outward currents through small-conductance calcium-activated potassium SK2 channels. The deviant RyR-calcium involvement in the 3xTg-AD mice also compensates for an intrinsic predisposition for hippocampal long-term depression (LTD) and reduced long-term potentiation (LTP). In this study, we detail the impact of disrupted RyR-mediated calcium stores on synaptic transmission properties, LTD, and calcium-activated membrane channels of hippocampal CA1 pyramidal neurons in presymptomatic 3xTg-AD mice. Using electrophysiological recordings in young 3xTg-AD and nontransgenic (NonTg) hippocampal slices, we show that increased RyR-evoked calcium release in 3xTg-AD mice "normalizes" an altered synaptic transmission system operating under a shifted homeostatic state that is not present in NonTg mice. In the process, we uncover compensatory signaling mechanisms recruited early in the disease process that counterbalance the disrupted RyR-calcium dynamics, namely increases in presynaptic spontaneous vesicle release, altered probability of vesicle release, and upregulated postsynaptic SK channel activity. Because AD is increasingly recognized as a "synaptic disease," calcium-mediated signaling alterations may serve as a proximal trigger for the synaptic degradation driving the cognitive loss in AD.

Multiple genes in the 15q13-q14 chromosomal region are associated with schizophrenia

OBJECTIVE

The chromosomal region, 15q13-q14, including the α7 nicotinic acetylcholine receptor gene, CHRNA7, is a replicated region for schizophrenia. This study fine-mapped genes at 15q13-q14 to determine whether the association is unique to CHRNA7.

METHODS

Family-based and case-control association studies were performed on Caucasian-non-Hispanic and African-American individuals from 120 families as well as 468 individual patients with schizophrenia and 144 well-characterized controls. Single-nucleotide polymorphism (SNP) markers were genotyped, and association analyses carried out for the outcomes of schizophrenia, smoking, and smoking in schizophrenia.

RESULTS

Three genes were associated with schizophrenia in both ethnic populations: TRPM1, KLF13, and RYR3. Two SNPs in CHRNA7 were associated with schizophrenia in African-Americans, and a second SNP in CHRNA7 was significant for an association with smoking and smoking in schizophrenia in Caucasians.

CONCLUSION

Results of these studies support association of the 15q13-q14 region with schizophrenia. The broad positive association suggests that more than one 15q gene may be contributing to the disorder, either in combination or through a regulatory mechanism.

Redox sensitive calcium stores underlie enhanced after hyperpolarization of aged neurons: role for ryanodine receptor mediated calcium signaling

A decrease in the excitability of CA1 pyramidal neurons contributes to the age related decrease in hippocampal function and memory decline. Decreased neuronal excitability in aged neurons can be observed as an increase in the Ca(2+)- activated K(+)- mediated post burst afterhyperpolarization (AHP). In this study, we demonstrate that the slow component of AHP (sAHP) in aged CA1 neurons (aged-sAHP) is decreased ∼50% by application of the reducing agent dithiothreitol (DTT). The DTT-mediated decrease in the sAHP was age specific, such that it was observed in CA1 pyramidal neurons of aged (20-25 mo), but not young (6-9 mo) F344 rats. The effect of DTT on the aged-sAHP was blocked following depletion of intracellular Ca(2+) stores (ICS) by thapsigargin or blockade of ryanodine receptors (RyRs) by ryanodine, suggesting that the age-related increase in the sAHP was due to release of Ca(2+) from ICS through redox sensitive RyRs. The DTT-mediated decrease in the aged-sAHP was not blocked by inhibition of L-type voltage gated Ca(2+) channels (L-type VGCC), inhibition of Ser/Thr kinases, or inhibition of the large conductance BK potassium channels. The results add support to the idea that a shift in the intracellular redox state contributes to Ca(2+) dysregulation during aging.

The dysregulation of intracellular calcium in Alzheimer disease

Alzheimer disease (AD) is the most common neurodegenerative disorder worldwide and is at present, incurable. The accumulation of toxic amyloid-beta (Abeta) peptide aggregates in AD brain are thought to trigger the extensive synaptic loss and neurodegeneration linked to cognitive decline, an idea that underlies the 'amyloid hypothesis' of AD etiology in both the familal (FAD) and sporadic forms of the disease. Mutations causing FAD also result in the dysregulation of neuronal calcium (Ca2+) handling and may contribute to AD pathogenesis, an idea termed the 'calcium hypothesis' of AD. In particular, Ca2+ dysregulation by the endoplasmic reticulum (ER) in AD mouse models results in augmented cytosolic Ca2+ levels which can trigger signalling cascades that are detrimental to neuronal function and health. However, there is growing evidence to suggest that not all forms of Ca2+ dysregulation in AD neurons are harmful and some of them instead may be compensatory. These changes may help modulate neuronal excitability and slow AD pathology, especially in the early stages of the disease. Clearly, a better understanding of how dysregulation of neuronal Ca2+ handling contributes to neurodegeneration and neuroprotection in AD is needed as Ca2+ signalling modulators are targets of great interest as potential AD therapeutics.

Deviant ryanodine receptor-mediated calcium release resets synaptic homeostasis in presymptomatic 3xTg-AD mice

Presenilin mutations result in exaggerated endoplasmic reticulum (ER) calcium release in cellular and animal models of Alzheimer's disease (AD). In this study, we examined whether dysregulated ER calcium release in young 3xTg-AD neurons alters synaptic transmission and plasticity mechanisms before the onset of histopathology and cognitive deficits. Using electrophysiological recordings and two-photon calcium imaging in young (6-8 weeks old) 3xTg-AD and non-transgenic (NonTg) hippocampal slices, we show a marked increase in ryanodine receptor (RyR)-evoked calcium release within synapse-dense regions of CA1 pyramidal neurons. In addition, we uncovered a deviant contribution of presynaptic and postsynaptic ryanodine receptor-sensitive calcium stores to synaptic transmission and plasticity in 3xTg-AD mice that is not present in NonTg mice. As a possible underlying mechanism, the RyR2 isoform was found to be selectively increased more than fivefold in the hippocampus of 3xTg-AD mice relative to the NonTg controls. These novel findings demonstrate that 3xTg-AD CA1 neurons at presymptomatic ages operate under an aberrant, yet seemingly functional, calcium signaling and synaptic transmission system long before AD histopathology onset. These early signaling alterations may underlie the later synaptic breakdown and cognitive deficits characteristic of later stage AD.

Impaired long-term potentiation and enhanced neuronal excitability in the amygdala of Ca(V)1.3 knockout mice

Previously, we demonstrated that mice in which the gene for the L-type voltage-gated calcium channel Ca(V)1.3 is deleted (Ca(V)1.3 knockout mice) exhibit an impaired ability to consolidate contextually-conditioned fear. Given that this form of Pavlovian fear conditioning is critically dependent on the basolateral complex of the amygdala (BLA), we were interested in the mechanisms by which Ca(V)1.3 contributes to BLA neurophysiology. In the present study, we used in vitro amygdala slices prepared from Ca(V)1.3 knockout mice and wild-type littermates to explore the role of Ca(V)1.3 in long-term potentiation (LTP) and intrinsic neuronal excitability in the BLA. We found that LTP in the lateral nucleus (LA) of the BLA, induced by high-frequency stimulation of the external capsule, was significantly reduced in Ca(V)1.3 knockout mice. Additionally, we found that BLA principal neurons from Ca(V)1.3 knockout mice were hyperexcitable, exhibiting significant increases in firing rates and decreased interspike intervals in response to prolonged somatic depolarization. This aberrant increase in neuronal excitability appears to be at least in part due to a concomitant reduction in the slow component of the post-burst afterhyperpolarization. Together, these results demonstrate altered neuronal function in the BLA of Ca(V)1.3 knockout mice which may account for the impaired ability of these mice to consolidate contextually-conditioned fear.

Properties of a T-type Ca2+channel-activated slow afterhyperpolarization in thalamic paraventricular nucleus and other thalamic midline neurons

Burst firing mediated by a low-threshold spike (LTS) is the hallmark of many thalamic neurons. However, postburst afterhyperpolarizations (AHPs) are relatively uncommon in thalamus. We now report data from patch-clamp recordings in rat brain slice preparations that reveal an LTS-induced slow AHP (sAHP) in thalamic paraventricular (PVT) and other midline neurons, but not in ventrobasal or reticular thalamic neurons. The LTS-induced sAHP lasts 8.9 +/- 0.4 s and has a novel pharmacology, with resistance to tetrodotoxin and cadmium and reduction by Ni(2+) or nominally zero extracellular calcium concentration, which also attenuate both the LTS and sAHP. The sAHP is inhibited by 10 mM intracellular EGTA or by equimolar replacement of extracellular Ca(2+) with Sr(2+), consistent with select activation of LVA T-type Ca(2+) channels and subsequent Ca(2+) influx. In control media, the sAHP reverses near E(K(+)), shifting to -78 mV in 10.1 mM [K(+)](o) and is reduced by Ba(2+) or tetraethylammonium. Although these data are consistent with opening of Ca(2+)-activated K(+) channels, this sAHP lacks sensitivity to specific Ca(2+)-activated K(+) channel blockers apamin, iberiotoxin, charybdotoxin, and UCL-2077. The LTS-induced sAHP is suppressed by a beta-adrenoceptor agonist isoproterenol, a serotonin 5-HT(7) receptor agonist 5-CT, a neuropeptide orexin-A, and by stimulation of the cAMP/protein kinase A pathway with 8-Br-cAMP and forskolin. The data suggest that PVT and certain midline thalamic neurons possess an LTS-induced sAHP that is pharmacologically distinct and may be important for information transfer in thalamic-limbic circuitry during states of attentiveness and motivation.

Different involvement of type 1, 2, and 3 ryanodine receptors in memory processes

The administration of the ryanodine receptor (RyR) agonist 4-Cmc (0.003-9 nmol per mouse intracerebroventricularly [i.c.v.]) ameliorated memory functions, whereas the RyR antagonist ryanodine (0.0001-1 nmol per mouse i.c.v.) induced amnesia in the mouse passive avoidance test. The role of the type 1, 2, and 3 RyR isoforms in memory processes was then evaluated by inhibiting the expression of the three RyR proteins in the mouse brain. A selective knockdown of the RyR isoforms was obtained by the i.c.v. administration of antisense oligonucleotides (aODNs) complementary to the sequence of RyR1, RyR2 and RyR3 proteins, as demonstrated by immunoblotting experiments. RyR1 (5-9 nmol per mouse i.c.v.) knockdown mice did not show any memory dysfunction. Conversely, RyR2 (1-7 nmol per mouse i.c.v.) and RyR3 (1-7 nmol per mouse i.c.v.) knockdown animals showed an impairment of memory processes. This detrimental effect was temporary and reversible, disappearing 7 d after the end of the aODN treatment. At the highest effective doses, none of the compounds used impaired motor coordination, as revealed by the rota rod test, nor modified spontaneous mobility and inspection activity, as revealed by the hole-board test. In conclusion, the lack of any involvement of cerebral RyR1 was demonstrated. These findings also showed the involvement of type 2 and type 3 RyR in the modulation of memory functions identifying these cerebral RyR isoforms as critical targets underlying memory processes.