The impact of postsynaptic calcium on synaptic transmission--its role in long-term potentiation

Participation of Ca2+-Calmodulin-Dependent Protein Kinase II in the Antidepressant-Like Effects of Melatonin

Melatonin (N-acetyl-5-methoxytryptamine) is an indoleamine secreted by the pineal gland during the dark phase of the photoperiod. Its main function is the synchronization of different body rhythms with the dark-light cycle. Research on melatonin has significantly advanced since its discovery and we now know that it has considerable significance in various physiological processes, including immunity, aging, and reproduction. Moreover, in recent years evidence of the pharmacological possibilities of melatonin has increased. Indoleamine, on the other hand, has antidepressant-like effects in rodents, which may be mediated by the activation of calcium-calmodulin-dependent kinase II (CaMKII) and are also related to the regulation of neuroplasticity processes, including neurogenesis, synaptic maintenance, and long-term potentiation. Remarkably, patients with major depression show decreased levels of circulating melatonin in plasma. This review presents evidence of the antidepressant-like effects of melatonin in preclinical models and the participation of CaMKII in these actions. CaMKII's role in cognition and memory processes, which are altered in depressive states, are part of the review, and the effects of melatonin in these processes are also reviewed. Furthermore, participation of CaMKII on structural and synaptic plasticity and the effects of melatonin are also described. Finally, the advantages of using melatonin in combination with other antidepressants such as ketamine for neuroplasticity are described. Evidence supports that CaMKII is activated by melatonin and downstream melatonin receptors and may be the common effector in the synergistic effects of melatonin with other antidepressants. SIGNIFICANCE STATEMENT: This review compiled evidence supporting that melatonin causes antidepressant-like effects in mice through calmodulin kinase II stimulation of downstream melatonin receptors as well as the participation of this enzyme in neuroplasticity, memory, and cognition. Finally, we describe evidence about the effectiveness of antidepressant-like effects of melatonin in combination with ketamine.

Aberrant activation of hippocampal astrocytes causes neuroinflammation and cognitive decline in mice

Reactive astrocytes are associated with neuroinflammation and cognitive decline in diverse neuropathologies; however, the underlying mechanisms are unclear. We used optogenetic and chemogenetic tools to identify the crucial roles of the hippocampal CA1 astrocytes in cognitive decline. Our results showed that repeated optogenetic stimulation of the hippocampal CA1 astrocytes induced cognitive impairment in mice and decreased synaptic long-term potentiation (LTP), which was accompanied by the appearance of inflammatory astrocytes. Mechanistic studies conducted using knockout animal models and hippocampal neuronal cultures showed that lipocalin-2 (LCN2), derived from reactive astrocytes, mediated neuroinflammation and induced cognitive impairment by decreasing the LTP through the reduction of neuronal NMDA receptors. Sustained chemogenetic stimulation of hippocampal astrocytes provided similar results. Conversely, these phenomena were attenuated by a metabolic inhibitor of astrocytes. Fiber photometry using GCaMP revealed a high level of hippocampal astrocyte activation in the neuroinflammation model. Our findings suggest that reactive astrocytes in the hippocampus are sufficient and required to induce cognitive decline through LCN2 release and synaptic modulation. This abnormal glial-neuron interaction may contribute to the pathogenesis of cognitive disturbances in neuroinflammation-associated brain conditions.

Targeting N-Methyl-d-Aspartate Receptors in Neurodegenerative Diseases

N-methyl-d-aspartate receptors (NMDARs) are the main class of ionotropic receptors for the excitatory neurotransmitter glutamate. They play a crucial role in the permeability of Ca2+ ions and excitatory neurotransmission in the brain. Being heteromeric receptors, they are composed of several subunits, including two obligatory GluN1 subunits (eight splice variants) and regulatory GluN2 (GluN2A~D) or GluN3 (GluN3A~B) subunits. Widely distributed in the brain, they regulate other neurotransmission systems and are therefore involved in essential functions such as synaptic transmission, learning and memory, plasticity, and excitotoxicity. The present review will detail the structure, composition, and localization of NMDARs, their role and regulation at the glutamatergic synapse, and their impact on cognitive processes and in neurodegenerative diseases (Alzheimer's, Huntington's, and Parkinson's disease). The pharmacology of different NMDAR antagonists and their therapeutic potentialities will be presented. In particular, a focus will be given on fluoroethylnormemantine (FENM), an investigational drug with very promising development as a neuroprotective agent in Alzheimer's disease, in complement to its reported efficacy as a tomography radiotracer for NMDARs and an anxiolytic drug in post-traumatic stress disorder.

Synaptic memory and CaMKII

Ca2+/calmodulin-dependent protein kinase II (CaMKII) and long-term potentiation (LTP) were discovered within a decade of each other and have been inextricably intertwined ever since. However, like many marriages, it has had its up and downs. Based on the unique biochemical properties of CaMKII, it was proposed as a memory molecule before any physiological linkage was made to LTP. However, as reviewed here, the convincing linkage of CaMKII to synaptic physiology and behavior took many decades. New technologies were critical in this journey, including in vitro brain slices, mouse genetics, single-cell molecular genetics, pharmacological reagents, protein structure, and two-photon microscopy, as were new investigators attracted by the exciting challenge. This review tracks this journey and assesses the state of this marriage 40 years on. The collective literature impels us to propose a relatively simple model for synaptic memory involving the following steps that drive the process: 1) Ca2+ entry through N-methyl-d-aspartate (NMDA) receptors activates CaMKII. 2) CaMKII undergoes autophosphorylation resulting in constitutive, Ca2+-independent activity and exposure of a binding site for the NMDA receptor subunit GluN2B. 3) Active CaMKII translocates to the postsynaptic density (PSD) and binds to the cytoplasmic C-tail of GluN2B. 4) The CaMKII-GluN2B complex initiates a structural rearrangement of the PSD that may involve liquid-liquid phase separation. 5) This rearrangement involves the PSD-95 scaffolding protein, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), and their transmembrane AMPAR-regulatory protein (TARP) auxiliary subunits, resulting in an accumulation of AMPARs in the PSD that underlies synaptic potentiation. 6) The stability of the modified PSD is maintained by the stability of the CaMKII-GluN2B complex. 7) By a process of subunit exchange or interholoenzyme phosphorylation CaMKII maintains synaptic potentiation in the face of CaMKII protein turnover. There are many other important proteins that participate in enlargement of the synaptic spine or modulation of the steps that drive and maintain the potentiation. In this review we critically discuss the data underlying each of the steps. As will become clear, some of these steps are more firmly grounded than others, and we provide suggestions as to how the evidence supporting these steps can be strengthened or, based on the new data, be replaced. Although the journey has been a long one, the prospect of having a detailed cellular and molecular understanding of learning and memory is at hand.

Effect of arsenic-containing hydrocarbon on the long-term potentiation at Schaffer Collateral-CA1 synapses from infantile male rat

Arsenic-containing hydrocarbons (AsHCs) are common constituents of marine organisms and have potential toxicity to human health. This work is to study the effect of AsHCs on long-term potentiation (LTP) for the first time. A multi-electrode array (MEA) system was used to record the field excitatory postsynaptic potential (fEPSP) of CA1 before and after treatment with AsHC 360 in hippocampal slices from infantile male rats. The element content of Na, K, Ca, Mg, Mn, Cu, Zn, and As in the hippocampal slices were analyzed by elemental mass spectrometry after the neurophysiological experiment. The results showed that low AsHC 360 (1.5 μg As L^-1) had no effect on the LTP, moderate AsHC 360 (3.75-15 μg As L^-1) enhanced the LTP, and high AsHC 360 (45-150 μg As L^-1) inhibited the LTP. The enhancement of the LTP by promoting Ca²⁺ influx was proved by a Ca²⁺ gradient experiment. The inhibition of the LTP was likely due to damage of synaptic cell membrane integrity. This study on the neurotoxicity of AsHCs showed that high concentrations have a strong toxic effect on the LTP in hippocampus slices of the infantile male rat, which may lead to a negative effect on the development, learning, and memory.

NMDA receptor-independent LTP in mammalian nervous system

Long-term potentiation (LTP) of synaptic transmission is a form of activity-dependent synaptic plasticity that exists at most synapses in the nervous system. In the central nervous system (CNS), LTP has been recorded at numerous synapses and is a prime candidate mechanism associating activity-dependent plasticity with learning and memory. LTP involves long-lasting increase in synaptic strength with various underlying mechanisms. In the CNS, the predominant type of LTP is believed to be dependent on activation of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR), which is highly calcium-permeable. However, various forms of NMDAR-independent LTP have been identified in diverse areas of the nervous system. The NMDAR-independent LTP may require activation of glutamate metabotropic receptors (mGluR) or ionotropic receptors other than NMDAR such as nicotinic acetylcholine receptor (α7-nAChR), serotonin 5-HT3 receptor or calcium-permeable AMPA receptor (CP-AMPAR). In this review, NMDAR-independent LTP of various areas of the central and peripheral nervous systems are discussed.

Activity-dependent long-term potentiation of electrical synapses in the mammalian thalamus

Activity-dependent changes of synapse strength have been extensively characterized at chemical synapses, but the relationship between physiological forms of activity and strength at electrical synapses remains poorly characterized and understood. For mammalian electrical synapses comprising hexamers of connexin36, physiological forms of neuronal activity in coupled pairs have thus far only been linked to long-term depression; activity that results in strengthening of electrical synapses has not yet been identified. Here, we performed dual whole-cell current-clamp recordings in acute slices of P11-P15 Sprague-Dawley rats of electrically coupled neurons of the thalamic reticular nucleus (TRN), a central brain area that regulates cortical input from and attention to the sensory surround. Using TTA-A2 to limit bursting, we show that tonic spiking in one neuron of a pair results in long-term potentiation of electrical synapses. We use experiments and computational modeling to show that the magnitude of plasticity expressed alters the functionality of the synapse. Potentiation is expressed asymmetrically, indicating that regulation of connectivity depends on the direction of use. Furthermore, calcium pharmacology and imaging indicate that potentiation depends on calcium flux. We thus propose a calcium-based activity rule for bidirectional plasticity of electrical synapse strength. Because electrical synapses dominate intra-TRN connectivity, these synapses and their activity-dependent modifications are key dynamic regulators of thalamic attention circuitry. More broadly, we speculate that bidirectional modifications of electrical synapses may be a widespread and powerful principle for ongoing, dynamic reorganization of neuronal circuitry across the brain.NEW & NOTEWORTHY This work reveals a physiologically relevant form of activity pairing in coupled neurons that results in long-term potentiation of mammalian electrical synapses. These findings, in combination with previous work, allow the authors to propose a bidirectional calcium-based rule for plasticity of electrical synapses, similar to those demonstrated for chemical synapses. These new insights inform the field on how electrical synapse plasticity may modify the neural circuits that incorporate them.

Co-functionalization with phosphate and carboxylate on polydiacetylene for colorimetric detection of calcium ions in serum

In this study, co-functionalization with phosphate and carboxylate on polydiacetylene (PDA) was proposed to detect calcium ions in serum, inspired by biologically abundant phosphate-calcium ion and carboxylate-calcium ion binding. The cooperative interaction of calcium ions with phosphate and carboxylate in PDA induced the change of electronic properties in the backbone without aggregation of liposomes, accompanied by blue-to-purple color transition. The cooperative effect through the introduction of mixed ligands facilitated the selective detection of calcium ions over magnesium ions, which was a source of major interference in many calcium ion probes, and in the presence of major serum metal ions. The sensor system exhibited highly sensitive detection of calcium ions with an estimated limit of detection of 0.97 μM. In addition, the detection method was employed to determine the concentration of calcium ions in various serums.

Molecular and synaptic mechanisms regulating drug-associated memories: Towards a bidirectional treatment strategy

The successful treatment of substance use disorders is dependent on the establishment of a long-term abstinent state. Relapse can be suppressed by interfering with memories of drug use that are evoked by re-exposure to drug-associated contexts and cues. Two strategies for accomplishing this goal are either to prevent drug-memory reconsolidation or to induce the formation of a competing, extinction memory. However, clinical attempts to prolong abstinence by behavioral modification of drug-related memories have had limited success. One approach to improve behavioral treatment strategies is to identify the molecular mechanisms that regulate these memory processes and then use pharmacological tools as supplements to improve efficacy. Still, due to the involvement of several overlapping signaling cascades in both reconsolidation and extinction, it is difficult to specifically modify one of the two processes. For example, attempting to elicit extinction may instead initiate reconsolidation, resulting in the unintentional strengthening of drug-related memories. A better approach is to identify diverging components of the two processes, whereby a single medication would simultaneously weaken reconsolidation and enhance extinction. This review will provide an overview of the neural substrates that are involved in the regulation of drug-associated memories, and will discuss emerging approaches to pharmacologically weaken these memories, including recent efforts to precisely and bidirectionally target reconsolidation and extinction. Ultimately, pharmacologically-enhanced memory-based approaches have the potential to produce more informed relapse-prevention therapies.

Early-life seizures alter synaptic calcium-permeable AMPA receptor function and plasticity

Calcium (Ca²⁺)-mediated⁴ signaling pathways are critical to synaptic plasticity. In adults, the NMDA glutamate receptor (NMDAR) represents a major route for activity-dependent synaptic Ca²⁺ entry. However, during neonatal development, when synaptic plasticity is particularly high, many AMPA glutamate receptors (AMPARs) are also permeable to Ca²⁺ (CP-AMPAR) due to low GluA2 subunit expression, providing an additional route for activity- and glutamate-dependent Ca²⁺ influx and subsequent signaling. Therefore, altered hippocampal Ca²⁺ signaling may represent an age-specific pathogenic mechanism. We thus aimed to assess Ca²⁺ responses 48h after hypoxia-induced neonatal seizures (HS) in postnatal day (P)10 rats, a post-seizure time point at which we previously reported LTP attenuation. We found that Ca²⁺ responses were higher in brain slices from post-HS rats than in controls and that this increase was CP-AMPAR-dependent. To determine whether synaptic CP-AMPAR expression was also altered post-HS, we assessed the expression of GluA2 at hippocampal synapses and the expression of long-term depression (LTD), which has been linked to the presence of synaptic GluA2. Here we report a decrease 48h after HS in synaptic GluA2 expression at synapses and LTD in hippocampal CA1. Given the potentially critical role of AMPAR trafficking in disease progression, we aimed to establish whether post-seizure in vivo AMPAR antagonist treatment prevented the enhanced Ca²⁺ responses, changes in GluA2 synaptic expression, and diminished LTD. We found that NBQX treatment prevents all three of these post-seizure consequences, further supporting a critical role for AMPARs as an age-specific therapeutic target.

Loss of calbindin-D28K is associated with the full range of tangle pathology within basal forebrain cholinergic neurons in Alzheimer's disease

Basal forebrain cholinergic neurons (BFCN) are selectively vulnerable in Alzheimer's disease (AD). We have shown that most of the BFCN in the human brain contain the calcium-binding protein calbindin-D28K (CB), a large proportion lose their CB in the course of normal aging, and the BFCN which degenerate in AD lack CB. Here, we investigated the relationship between CB in the BFCN and the process of tangle formation in AD using antibodies to tau epitopes that appear early, intermediate or late in the process of tangle formation. Very small percentages (0%-3.7%) of CB-positive BFCN contained pretangles and/or tangles, and very small percentages (0%-5%) of the total BFCN pretangles and/or tangles were in CB-immunoreactive neurons. The number of CB-positive BFCN which contained tau immunoreactivity was highest for the early epitope and lower for intermediate epitopes. A late appearing epitope was absent from CB-positive BFCN. Age-related loss of CB appears to coincide with tangle formation in the BFCN and is associated with the full range of tau pathology, including late appearing epitopes.

Lipid rafts serve as signaling platforms for mGlu1 receptor-mediated calcium signaling in association with caveolin

Background

Group I metabotropic glutamate receptors (mGlu1/5 receptors) have important roles in synaptic activity in the central nervous system. They modulate neuronal excitability by mobilizing intracellular Ca²⁺ following receptor activation. Also, accumulating evidence has indicated the association of Ca²⁺ signaling with lipid rafts. Caveolin, an adaptor protein found in a specialized subset of lipid rafts, has been reported to promote the localization of membrane proteins to lipid rafts.

Results

In the present study, we investigated the role of lipid rafts on the mGlu1α receptor-mediated Ca²⁺ signaling in association with caveolin in hippocampal primary neurons and HEK293 cells. We show that the disruption of lipid rafts using methyl-β-cyclodextrin markedly decreased mGlu1α receptor-mediated Ca²⁺ transients and lipid rafts localization of the receptor. Furthermore, transfection of mGlu1α receptor with mutated caveolin-binding domain reduced localization of the receptor to lipid rafts. Also, application of a peptide blocker of mGlu1α receptor and caveolin binding reduced the Ca²⁺ signaling and the lipid rafts localization.

Conclusions

Taken together, these results suggest that the binding of mGlu1α receptor to caveolin is crucial for its lipid rafts localization and mGlu1α receptor-mediated Ca²⁺ transients.

Enhanced NMDA receptor-mediated intracellular calcium signaling in magnocellular neurosecretory neurons in heart failure rats

An enhanced glutamate excitatory function within the hypothalamic supraoptic and paraventricluar nuclei is known to contribute to increased neurosecretory and presympathetic neuronal activity, and hence, neurohumoral activation, during heart failure (HF). Still, the precise mechanisms underlying enhanced glutamate-driven neuronal activity in HF remain to be elucidated. Here, we performed simultaneous electrophysiology and fast confocal Ca²⁺ imaging to determine whether altered N-methyl-d-aspartate (NMDA) receptor-mediated changes in intracellular Ca²⁺ levels (NMDA-ΔCa²⁺) occurred in hypothalamic magnocellular neurosecretory cells (MNCs) in HF rats. We found that activation of NMDA receptors resulted in a larger ΔCa²⁺ in MNCs from HF when compared with sham rats. The enhanced NMDA-ΔCa²⁺ was neither dependent on the magnitude of the NMDA-mediated current (voltage clamp) nor on the degree of membrane depolarization or firing activity evoked by NMDA (current clamp). Differently from NMDA receptor activation, firing activity evoked by direct membrane depolarization resulted in similar changes in intracellular Ca²⁺ in sham and HF rats. Taken together, our results support a relatively selective alteration of intracellular Ca²⁺ homeostasis and signaling following activation of NMDA receptors in MNCs during HF. The downstream functional consequences of such altered ΔCa²⁺ signaling during HF are discussed.

Calcium homeostasis in aging neurons

The nervous system becomes increasingly vulnerable to insults and prone to dysfunction during aging. Age-related decline of neuronal function is manifested by the late onset of many neurodegenerative disorders, as well as by reduced signaling and processing capacity of individual neuron populations. Recent findings indicate that impairment of Ca(2+) homeostasis underlies the increased susceptibility of neurons to damage, associated with the aging process. However, the impact of aging on Ca(2+) homeostasis in neurons remains largely unknown. Here, we survey the molecular mechanisms that mediate neuronal Ca(2+) homeostasis and discuss the impact of aging on their efficacy. To address the question of how aging impinges on Ca(2+) homeostasis, we consider potential nodes through which mechanisms regulating Ca(2+) levels interface with molecular pathways known to influence the process of aging and senescent decline. Delineation of this crosstalk would facilitate the development of interventions aiming to fortify neurons against age-associated functional deterioration and death by augmenting Ca(2+) homeostasis.

The Inflammatory Chemokine CXCL10 Modulates Synaptic Plasticity and Neuronal Activity in the Hippocampus

Chemokines, a family member of cytokines, have been shown to play a major role in central nervous system inflammation. Among other chemokines, CXCR3 and its ligand CXCL10 are involved in the pathophysiology of several neuroinflammatory conditions. Most of these conditions are also associated with an increased incidence of seizure or epilepsy. Using age-matched wild-type (WT), as well as CXCR3-receptor-deficient (CXCR3-KO) mice, the present study aimed to investigate the effect of the chemokine CXCL10 and its receptor CXCR3 on synaptic plasticity as well as neuronal activities in hippocampal brain slices. Using field potential and intracellular recordings, the effect of exogenous CXCL10 on tetanus-induced long-term potentiation (LTP) as well as the neuronal spike activity was evaluated in hippocampal CA1 area. Exogenous application of CXCL10 enhanced LTP in WT mice, whereas it exerted no significant effect on CXCR3-KO mice. During intracellular recordings of spontaneous spike activity, exogenous application of CXCL10 significantly enhanced the amplitude, duration, and after-hyperpolarization of action potentials in slices obtained from WT mice compared to CXCR3-KO mice. In addition, CXCR3-KO mice exhibited a lower GABAA-mediated excitation in hippocampal CA1 neurons compared to WT mice. These data show that the inflammatory chemokine CXCL10, probably via its receptor CXCR3, modulates neuronal activity and synaptic plasticity in the hippocampus. CXCL10 may be involved in seizures observed during neuroinflammatory diseases such as meningitis and encephalitis.

Signaling in dendritic spines and spine microdomains

The specialized morphology of dendritic spines creates an isolated compartment that allows for localized biochemical signaling. Recent studies have revealed complexity in the function of the spine head as a signaling domain and indicate that (1) the spine is functionally subdivided into multiple independent microdomains and (2) not all biochemical signals are equally compartmentalized within the spine. Here we review these findings as well as the developments in fluorescence microscopy that are making possible direct monitoring of signaling within spines and, in the future, within sub-spine microdomains.

A colorimetric selective sensing probe for calcium ions with tunable dynamic ranges using cytidine triphosphate stabilized gold nanoparticles

Cytidine triphosphate (CTP) stabilized-Au nanoparticles (CTP-AuNPs) allow a quantitative assay of Ca(2+) down to a concentration of 10(-4) M with high selectivity towards Ca(2+) ions over various metal ions including Mg(2+) and the detection range of this probe also can be easily tuned by changing the concentration of CTP.

Aroclor 1254, a developmental neurotoxicant, alters energy metabolism- and intracellular signaling-associated protein networks in rat cerebellum and hippocampus

The vast literature on the mode of action of polychlorinated biphenyls (PCBs) indicates that PCBs are a unique model for understanding the mechanisms of toxicity of environmental mixtures of persistent chemicals. PCBs have been shown to adversely affect psychomotor function and learning and memory in humans. Although the molecular mechanisms for PCB effects are unclear, several studies indicate that the disruption of Ca(2+)-mediated signal transduction plays significant roles in PCB-induced developmental neurotoxicity. Culminating events in signal transduction pathways include the regulation of gene and protein expression, which affects the growth and function of the nervous system. Our previous studies showed changes in gene expression related to signal transduction and neuronal growth. In this study, protein expression following developmental exposure to PCB is examined. Pregnant rats (Long Evans) were dosed with 0.0 or 6.0mg/kg/day of Aroclor-1254 from gestation day 6 through postnatal day (PND) 21, and the cerebellum and hippocampus from PND14 animals were analyzed to determine Aroclor 1254-induced differential protein expression. Two proteins were found to be differentially expressed in the cerebellum following PCB exposure while 18 proteins were differentially expressed in the hippocampus. These proteins are related to energy metabolism in mitochondria (ATP synthase, sub unit β (ATP5B), creatine kinase, and malate dehydrogenase), calcium signaling (voltage-dependent anion-selective channel protein 1 (VDAC1) and ryanodine receptor type II (RyR2)), and growth of the nervous system (dihydropyrimidinase-related protein 4 (DPYSL4), valosin-containing protein (VCP)). Results suggest that Aroclor 1254-like persistent chemicals may alter energy metabolism and intracellular signaling, which might result in developmental neurotoxicity.

TRPC channels and their implication in neurological diseases

Calcium is an essential intracellular messenger and serves critical cellular functions in both excitable and non-excitable cells. Most of the physiological functions in these cells are uniquely regulated by changes in cytosolic Ca2+ levels ([Ca2+](i)), which are achieved via various mechanisms. One of these mechanism(s) is activated by the release of Ca2+ from the endoplasmic reticulum (ER), followed by Ca2+ influx across the plasma membrane (PM). Activation of PM Ca2+ channel is essential for not only refilling of the ER Ca2+ stores, but is also critical for maintaining [Ca2+](i) that regulates biological functions, such as neurosecretion, sensation, long term potentiation, synaptic plasticity, gene regulation, as well as cellular growth and differentiation. Alterations in Ca2+ homeostasis have been suggested in the onset/progression of neurological diseases, such as Parkinson's, Alzheimer's, bipolar disorder, and Huntington's. Available data on transient receptor potential conical (TRPC) protein indicate that these proteins initiate Ca2+ entry pathways and are essential in maintaining cytosolic, ER, and mitochondrial Ca2+ levels. A number of biological functions have been assigned to these TRPC proteins. Silencing of TRPC1 and TRPC3 has been shown to inhibit neuronal proliferation and loss of TRPC1 is implicated in neurodegeneration. Thus, TRPC channels not only contribute towards normal physiological processes, but are also implicated in several human pathological conditions. Overall, it is suggested that these channels could be used as potential therapeutic targets for many of these neurological diseases. Thus, in this review we have focused on the functional implication of TRPC channels in neuronal cells along with the elucidation of their role in neurodegeneration.

Protein kinase C activators as synaptogenic and memory therapeutics

The last decade has witnessed a rapid progress in understanding of the molecular cascades that may underlie memory and memory disorders. Among the critical players, activity of protein kinase C (PKC) isoforms is essential for many types of learning and memory and their dysfunction, and is critical in memory disorders. PKC inhibition and functional deficits lead to an impairment of various types of learning and memory, consistent with the observations that neurotoxic amyloid inhibits PKC activity and that transgenic animal models with PKCbeta deficit exhibit impaired capacity in cognition. In addition, PKC isozymes play a regulatory role in amyloid production and accumulation. Restoration of the impaired PKC signal pathway pharmacologically results in an enhanced memory capacity and synaptic remodeling / repair and synaptogenesis, and, therefore, represents a potentially important strategy for the treatment of memory disorders, including Alzheimer's dementia. The PKC activators, especially those that are isozyme-specific, are a new class of drug candidates that may be developed as future memory therapeutics.

Calcium signaling in neurodegeneration

Calcium is a key signaling ion involved in many different intracellular and extracellular processes ranging from synaptic activity to cell-cell communication and adhesion. The exact definition at the molecular level of the versatility of this ion has made overwhelming progress in the past several years and has been extensively reviewed. In the brain, calcium is fundamental in the control of synaptic activity and memory formation, a process that leads to the activation of specific calcium-dependent signal transduction pathways and implicates key protein effectors, such as CaMKs, MAPK/ERKs, and CREB. Properly controlled homeostasis of calcium signaling not only supports normal brain physiology but also maintains neuronal integrity and long-term cell survival. Emerging knowledge indicates that calcium homeostasis is not only critical for cell physiology and health, but also, when deregulated, can lead to neurodegeneration via complex and diverse mechanisms involved in selective neuronal impairments and death. The identification of several modulators of calcium homeostasis, such as presenilins and CALHM1, as potential factors involved in the pathogenesis of Alzheimer's disease, provides strong support for a role of calcium in neurodegeneration. These observations represent an important step towards understanding the molecular mechanisms of calcium signaling disturbances observed in different brain diseases such as Alzheimer's, Parkinson's, and Huntington's diseases.

Advancing age alters the contribution of calcium release from smooth endoplasmic reticulum stores in superior cervical ganglion cells

In superior cervical ganglion (SCG) neurons calcium-induced calcium release (CICR), mediated by ryanodine receptors (RyRs), contributes to stimulation-evoked intracellular calcium ([Ca²⁺]_i) transients. Hypothesis: The contribution of CICR to electrical field stimulation (EFS)–evoked [Ca²⁺]_i transients in SCG cells declines with senescence and may be partially recovered in the presence of caffeine. We measured EFS-evoked [Ca²⁺]_i transients in isolated fura-2–loaded SCG cells from Fischer-344 rats aged 6, 12, and 24 months with either the RyR antagonist ryanodine to block the contribution of CICR to [Ca²⁺]_i transients or caffeine to sensitize CICR to EFS. EFS-evoked [Ca²⁺]_i transients increased from 6 to 12 months and declined at 24 months and ryanodine decreased [Ca²⁺]_i transients in SCG cells from 6- and 12-month-old animals only. Caffeine significantly increased EFS-evoked [Ca²⁺]_i transients in all age groups. These data suggest that CICR declines with senescence and residual CICR function may be reclaimed in senescent cells with caffeine.

The effects of NR2 subunit-dependent NMDA receptor kinetics on synaptic transmission and CaMKII activation

N-Methyl-D-aspartic acid (NMDA) receptors are widely expressed in the brain and are critical for many forms of synaptic plasticity. Subtypes of the NMDA receptor NR2 subunit are differentially expressed during development; in the forebrain, the NR2B receptor is dominant early in development, and later both NR2A and NR2B are expressed. In heterologous expression systems, NR2A-containing receptors open more reliably and show much faster opening and closing kinetics than do NR2B-containing receptors. However, conflicting data, showing similar open probabilities, exist for receptors expressed in neurons. Similarly, studies of synaptic plasticity have produced divergent results, with some showing that only NR2A-containing receptors can drive long-term potentiation and others showing that either subtype is capable of driving potentiation. In order to address these conflicting results as well as open questions about the number and location of functional receptors in the synapse, we constructed a Monte Carlo model of glutamate release, diffusion, and binding to NMDA receptors and of receptor opening and closing as well as a model of the activation of calcium-calmodulin kinase II, an enzyme critical for induction of synaptic plasticity, by NMDA receptor-mediated calcium influx. Our results suggest that the conflicting data concerning receptor open probabilities can be resolved, with NR2A- and NR2B-containing receptors having very different opening probabilities. They also support the conclusion that receptors containing either subtype can drive long-term potentiation. We also are able to estimate the number of functional receptors at a synapse from experimental data. Finally, in our models, the opening of NR2B-containing receptors is highly dependent on the location of the receptor relative to the site of glutamate release whereas the opening of NR2A-containing receptors is not. These results help to clarify the previous findings and suggest future experiments to address open questions concerning NMDA receptor function.

Silent synapses and the emergence of a postsynaptic mechanism for LTP

Silent synapses abound in the young brain, representing an early step in the pathway of experience-dependent synaptic development. Discovered amidst the debate over whether long-term potentiation reflects a presynaptic or a postsynaptic modification, silent synapses--which in the hippocampal CA1 subfield are characterized by the presence of NMDA receptors but not AMPA receptors--have stirred some mechanistic controversy of their own. Out of this literature has emerged a model for synapse unsilencing that highlights the central role for postsynaptic AMPA-receptor trafficking in the expression of excitatory synaptic plasticity.

Neocortical plasticity deficits in fetal alcohol spectrum disorders: lessons from barrel and visual cortex

Fetal Alcohol Spectrum Disorder (FASD) is characterized by a constellation of behavioral and physiological abnormalities, including learning and sensory deficits. There is growing evidence that abnormalities of neuronal plasticity underlie these deficits. However, the cellular and molecular mechanisms by which prenatal alcohol exposure disrupts neuronal plasticity remain elusive. Recently, studies with the barrel and the visual cortex as models to study the effects of early alcohol exposure on neuronal plasticity shed light on this subject. In this Mini-Review, we discuss the effects of ethanol exposure during development on neuronal plasticity and suggest environmental and pharmacological approaches to ameliorate these problems.

Prenatal testosterone improves the spatial learning and memory by protein synthesis in different lobes of the brain in the male and female rat

The high density of the steroid hormone receptors in the structures of temporal lobe involved in learning and memory, such as the hippocampus, perirhinal cortex, entorhinal cortex and amigdaloid complex, shows that there must be a direct relationship between gonadal hormones and organizational effects of steroid hormones in those structures during development of the nervous system. The present study was undertaken in order to investigate the effect of testosterone administration during the third week of gestation on the spatial memory formation of the offspring rats and the level of soluble proteins in the temporal lobe and frontal lobe of brain, as evidence of important organizational effects of androgens during prenatal development in brain sexual dimorphism. Animals have received testosterone undecanoate on days 14, 15, 16 and 19, 20, 21 of gestation. Learning and memory tests were started 100 days after the testosterone treatment. At the end of the experiments, the temporal and frontal lobes of brain were removed for assessing the level of soluble proteins. Testosterone treatment significantly improved spontaneous alternations percentage of male offspring in Y-maze task comparative with female offspring and reference memory in radial 8 arm-maze task (decreasing in number of reference memory errors in both male and female offspring groups), suggesting effects of both short and long-term memory. Also, testosterone significantly increased the brain soluble protein level of treated female rats in 14–16 prenatal days compared with the control group as well as the brain soluble protein level of treated male rats. These results suggest that steroid hormones play an important role in the spatial learning and memory formation by means of protein synthesis in different lobes of the brain.

Cholinergic-mediated IP3-receptor activation induces long-lasting synaptic enhancement in CA1 pyramidal neurons

Cholinergic-glutamatergic interactions influence forms of synaptic plasticity that are thought to mediate memory and learning. We tested in vitro the induction of long-lasting synaptic enhancement at Schaffer collaterals by acetylcholine (ACh) at the apical dendrite of CA1 pyramidal neurons and in vivo by stimulation of cholinergic afferents. In vitro ACh induced a Ca2+ wave and synaptic enhancement mediated by insertion of AMPA receptors in spines. Activation of muscarinic ACh receptors (mAChRs) and Ca2+ release from inositol 1,4,5-trisphosphate (IP3)-sensitive stores were required for this synaptic enhancement that was insensitive to blockade of NMDA receptors and also triggered by IP3 uncaging. Activation of cholinergic afferents in vivo induced an analogous atropine-sensitive synaptic enhancement. We describe a novel form of synaptic enhancement (LTP(IP3)) that is induced in vitro and in vivo by activation of mAChRs. We conclude that Ca2+ released from postsynaptic endoplasmic reticulum stores is the critical event in the induction of this unique form of long-lasting synaptic enhancement.

Molecular changes in the intermediate medial mesopallium after a one trial avoidance learning in immature and mature chickens

Because brain maturation in chickens is protracted and occurs well after the major developmental period of synaptogenesis, chicken forebrain is suitable to investigate whether the molecular mechanisms underlying memory consolidation are different in immature and mature animals. We have used antibodies and western blotting to analyze subcellular fractions from the intermediate medial mesopallium region of 14-day and 8-week chicken forebrain prepared 0, 45, and 120 min after learning a discriminative taste avoidance task. At both ages learning induced changes in the phosphorylation of the glutamate receptor subunit 1 at Ser831, the levels of calcium-calmodulin stimulated/dependent protein kinase II and the phosphorylation of calcium-calmodulin stimulated/dependent protein kinase II at Thr286 were observed only in the fraction enriched in post-synaptic densities. The changes were of the same type at the two ages but occurred faster in mature animals. The changes in extracellular signal regulated kinase and phosphorylated-extracellular signal regulated kinase were more complex with different subcellular fractions showing different patterns of change at the two ages. These results imply that the molecular changes induced by learning a behavioral task are faster in mature than immature brain and may involve a different balance of intracellular signaling pathways.

Changes of the EPSP waveform regulate the temporal window for spike-timing-dependent plasticity

Using spike-timing-dependent plasticity (STDP) protocols that consist of pairing an EPSP and a postsynaptic backpropagating action potential (BAP), we investigated the contribution of the changes in EPSP waveform induced by the slow Ca2+-dependent K+-mediated afterhyperpolarization (sAHP) in the regulation of long-term potentiation (LTP). The "temporal window" between Schaffer collateral EPSPs and BAPs in CA1 pyramidal neurons required to induce LTP was narrowed by a reduction of the amplitude and decay time constant of the EPSP, which could be reversed with cyclothiazide. The EPSP changes were caused by the increased conductance induced by activation of the sAHP. Therefore, the EPSP waveform and its regulation by the sAHP are central in determining the duration of the temporal window for STDP, thus providing a possible dynamic regulatory mechanism for the encoding of cognitive processes.

Calbindin D-28K and parvalbumin expression in embryonic chick hippocampus is enhanced by prenatal auditory stimulation

Calcium-binding proteins (CaBPs) buffer excess of cytosolic Ca(2+), which accompanies neuronal activity following external stimuli. Prenatal auditory stimulation by species-specific sound and music influences early maturation of the auditory pathway and the behavioral responses in chicks. In this study, we determined the volume, total number of neurons, proportion of calbindin D-28K and parvalbumin-positive neurons along with their levels of expression in the developing chick hippocampus following prenatal auditory stimulation. Fertilized eggs of domestic chicks were exposed to sounds of either species-specific calls or sitar music at 65 dB for 15 min/h round the clock from embryonic day (E) 10 until hatching. Hippocampi of developmental stages (E12, E16 and E20) were examined. With an increase in embryonic age during normal development, the hippocampus showed an increase in its volume, total number of neurons as well as in the neuron proportions and levels of expression of calbindin D-28K and parvalbumin. A significant increase of volume at E20 was noted only in the music-stimulated group compared to that of their age-matched control (p<0.05). On the other hand, both auditory-stimulated groups showed a significant increase in the proportion of immunopositive neurons and the levels of expression of calbindin D-28K and parvalbumin as compared to the control at all developmental stages studied (p<0.003). The increase in proportions of CaBP neurons during development and in the sound-enriched groups suggests an activity-dependent increase in Ca(2+) influx. The enhanced expression of CaBPs may help in cell survival by preventing excitotoxic death of neurons during development and may also be involved in long-term potentiation.

Persistent signalling and changes in presynaptic function in long-term potentiation

Long-term potentiation (LTP) is an example of a persistent change in synaptic function in the mammalian brain, thought to be essential for learning and memory. At the synapse between hippocampal CA3 and CA1 neurons LTP is induced by a Ca2+ influx through glutamate receptors of the NMDA (N-methyl-D-aspartate) type (see Collingridge et al 1992, this volume). How does a rise in [Ca2+]i lead to enhancement of synaptic function? We have tested the popular hypothesis that Ca2+ acts via a Ca(2+)-dependent protein kinase. We found that long-lasting synaptic enhancement was prevented by prior intracellular injection of potent and selective inhibitory peptide blockers of either protein kinase C (PKC) or Ca2+/calmodulin-dependent protein kinase II (CaMKII), such as PKC(19-31) or CaMKII(273-302), but not by control peptides. Evidently, activity of both PKC and CaMKII is somehow necessary for the postsynaptic induction of LTP. To determine if these kinases are also involved in the expression of LTP, we impaled cells with microelectrodes containing protein kinase inhibitors after LTP had already been induced. Strikingly, established LTP was not suppressed by a combination of PKC and CaMKII blocking peptides, or by intracellular postsynaptic H-7. However, established LTP remained sensitive to bath application of H-7. Thus, the persistent signal may be a persistent kinase, but if so, the kinase cannot be accessed within the postsynaptic cell. Evidence for a presynaptic locus of expression comes from our studies of quantal synaptic transmission under whole-cell voltage clamp. We find changes in synaptic variability expected to result from enhanced presynaptic transmitter release, but little or no increase in quantal size. Furthermore, miniature synaptic currents in hippocampal cultures are increased in frequency but not amplitude as a result of a glutamate-driven postsynaptic induction. The combination of postsynaptic induction and presynaptic expression necessitates a retrograde signal from the postsynaptic cell to the presynaptic terminal.

Pharmacological Characterization of a Novel, Potent Adenosine A1 and A2A Receptor Dual Antagonist, 5-[5-Amino-3-(4-fluorophenyl)pyrazin-2-yl]-1-isopropylpyridine-2(1H)-one (ASP5854), in Models of Parkinson's Disease and Cognition

Central adenosine A(2A) receptor is a promising target for drugs to treat Parkinson's disease (PD), and the central blockade of adenosine A(1) receptor improves cognitive function. In the present study, we investigated the effect of a novel adenosine A(1) and A(2A) dual antagonist, 5-[5-amino-3-(4-fluorophenyl) pyrazin-2-yl]-1-isopropylpyridine-2(1H)-one (ASP5854), in animal models of PD and cognition. The binding affinities of ASP5854 for human A(1) and A(2A) receptors were 9.03 and 1.76 nM, respectively, with higher specificity and no species differences. ASP5854 also showed antagonistic action on A(1) and A(2A) agonist-induced increases of intracellular Ca(2+) concentration. ASP5854 ameliorated A(2A) agonist 2-[p-(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680)- and haloperidol-induced catalepsy in mice, with the minimum effective doses of 0.32 and 0.1 mg/kg, respectively, and it also improved haloperidol-induced catalepsy in rats at doses higher than 0.1 mg/kg. In unilateral 6-hydroxydopamine-lesioned rats, ASP5854 significantly potentiated l-dihydroxyphenylalanine (L-DOPA)-induced rotational behavior at doses higher than 0.032 mg/kg. ASP5854 also significantly restored the striatal dopamine content reduced by 1-metyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment in mice at doses higher than 0.1 mg/kg. Furthermore, in the rat passive avoidance test, ASP5854 significantly reversed the scopolamine-induced memory deficits, whereas the specific adenosine A(2A) antagonist 8-((E)-2-(3,4-dimethoxyphenyl)ethenyl)-1,3-diethyl-7-methyl-3,7-dihydro-1H-purine-2,6-dione (KW-6002; istradefylline) did not. Scopolamine- or 5H-dibenzo[a,d]cyclohepten-5,10-imine (dizocilpine maleate) (MK-801)-induced impairment of spontaneous alternation in the mouse Y-maze test was ameliorated by ASP5854, whereas KW-6002 did not exert improvement at therapeutically relevant dosages. These results demonstrate that the novel, selective, and orally active dual adenosine A(1) and A(2A) receptors antagonist ASP5854 improves motor impairments, is neuroprotective via A(2A) antagonism, and also enhances cognitive function through A(1) antagonism.

Age-dependent changes in Ca2+ homeostasis in peripheral neurones: implications for changes in function

Calcium ions represent universal second messengers within neuronal cells integrating multiple cellular functions, such as release of neurotransmitters, gene expression, proliferation, excitability, and regulation of cell death or apoptotic pathways. The magnitude, duration and shape of stimulation-evoked intracellular calcium ([Ca2+]i) transients are determined by a complex interplay of mechanisms that modulate stimulation-evoked rises in [Ca2+]i that occur with normal neuronal function. Disruption of any of these mechanisms may have implications for the function and health of peripheral neurones during the aging process. This review focuses on the impact of advancing age on the overall function of peripheral adrenergic neurones and how these changes in function may be linked to age-related changes in modulation of [Ca2+]i regulation. The data in this review suggest that normal aging in peripheral autonomic neurones is a subtle process and does not always result in dramatic deterioration in their function. We present studies that support the idea that in order to maintain cell viability peripheral neurones are able to compensate for an age-related decline in the function of at least one of the neuronal calcium-buffering systems, smooth endoplasmic reticulum calcium ATPases, by increased function of other calcium-buffering systems, namely, the mitochondria and plasmalemma calcium extrusion. Increased mitochondrial calcium uptake may represent a 'weak point' in cellular compensation as this over time may contribute to cell death. In addition, we present more recent studies on [Ca2+]i regulation in the form of the modulation of release of calcium from smooth endoplasmic reticulum calcium stores. These studies suggest that the contribution of the release of calcium from smooth endoplasmic reticulum calcium stores is altered with age through a combination of altered ryanodine receptor levels and modulation of these receptors by neuronal nitric oxide containing neurones.

Injury-induced alterations in CNS electrophysiology

Mild to moderate cases of traumatic brain injury (TBI) are very common, but are not always associated with the overt pathophysiogical changes seen following severe trauma. While neuronal death has been considered to be a major factor, the pervasive memory, cognitive and motor function deficits suffered by many mild TBI patients do not always correlate with cell loss. Therefore, we assert that functional impairment may result from alterations in surviving neurons. Current research has begun to explore CNS synaptic circuits after traumatic injury. Here we review significant findings made using in vivo and in vitro models of TBI that provide mechanistic insight into injury-induced alterations in synaptic electrophysiology. In the hippocampus, research now suggests that TBI regionally alters the delicate balance between excitatory and inhibitory neurotransmission in surviving neurons, disrupting the normal functioning of synaptic circuits. In another approach, a simplified model of neuronal stretch injury in vitro, has been used to directly explore how injury impacts the physiology and cell biology of neurons in the absence of alterations in blood flow, blood brain barrier integrity, or oxygenation associated with in vivo models of brain injury. This chapter discusses how these two models alter excitatory and inhibitory synaptic transmission at the receptor, cellular and circuit levels and how these alterations contribute to cognitive impairment and a reduction in seizure threshold associated with human concussive brain injury.

Neuronal localization of Ca(v)1.2 L-type calcium channels in the rat basolateral amygdala

L-type high-voltage-activated calcium channels are involved in the conduction and integration of electrical activity and associative long-term potentiation in the basolateral amygdalar complex (BLC). However, little is known about the neuronal localization of these channels in this brain region. We used immunohistochemical techniques to determine which cell types in the BLC express the Ca(v)1.2 subtype of L-type calcium channels. Immunofluorescence experiments demonstrated that Ca(v)1.2 calcium channels were mainly found in somata and dendrites of pyramidal neurons that exhibited immunoreactivity for calcium/calmodulin-dependent protein kinase II (CaMK). Only a few parvalbumin-positive and calretinin-positive interneurons exhibited Ca(v)1.2 immunoreactivity. The presence of high levels of Ca(v)1.2 immunoreactivity in BLC pyramidal cells is consistent with physiological findings showing that calcium entry through L-type calcium channels in pyramidal cell dendrites in the lateral amygdala is required for associative LTP and the conversion of synaptic events into long-term emotional memory.

Ovariectomy-induced disruption of long-term synaptic depression in the hippocampal CA1 region in vivo is attenuated with chronic estrogen replacement

Endogenous cyclical changes in the levels of estrogen can have marked effects on hippocampal synaptic plasticity. In two experiments, we examined the effect of chronic estrogen loss and replacement following ovariectomy on the induction of bidirectional changes in synaptic plasticity in the CA1 region in vivo. In Experiment 1, ovariectomy carried out either 5 days or 5 weeks before testing impaired the induction of long-term depression (LTD) and but not long-term potentiation (LTP). In Experiment 2, chronic estrogen replacement (0.2 ml of 10 microg injection of 17beta-estradiol every 48 h) over the course of 5 weeks enhanced the magnitude of paired-pulse-induced LTD in the CA1 region but had no effect on the induction of LTP. The results demonstrate that acute and chronic estrogen deprivation disrupted dynamic synaptic plasticity processes in the hippocampal CA1 region and that this disruption was ameliorated by chronic estrogen replacement. The findings are discussed with reference to: (1) the contribution of Ca(2+) regulated synaptic signalling pathways in the CA1 region to estradiol modulation of LTP and LTD and (2) the potential functional significance of ovariectomy-induced changes in synaptic plasticity for learning and memory processes.

Calcium dynamics in dendritic spines and spine motility

A dendritic spine is an intracellular compartment in synapses of central neurons. The role of the fast twitching of spines, brought about by a transient rise of internal calcium concentration above that of the parent dendrite, has been hitherto unclear. We propose an explanation of the cause and effect of the twitching and its role in the functioning of the spine as a fast calcium compartment. Our molecular model postulates that rapid spine motility is due to the concerted contraction of calcium-binding proteins. The contraction induces a stream of cytoplasmic fluid in the direction of the dendritic shaft, thus speeding up the time course of spine calcium dynamics, relative to pure diffusion. Simulations indicate that chemical reaction rate theory at the molecular level can explain spine motility. They reveal two time periods in calcium dynamics, as measured recently by other researchers. It appears that rapid motility in dendritic spines increases the efficiency of calcium conduction to the dendrite and speeds up the emptying of the spine. This could play a major role in the induction of synaptic plasticity. A prediction of the model is that alteration of spine motility will modify the time course of calcium in the dendritic spine and could be tested experimentally.

Polychlorinated biphenyl mixtures (Aroclors) induce apoptosis via Bcl-2, Bax and caspase-3 proteins in neuronal cell cultures

Polychlorinated biphenyls (PCBs) are a group of persistent and widely dispersed environmental pollutants, some of which may be neurotoxic. In the present study, we have investigated the effect of PCB commercial mixtures (Aroclors) on neuronal cell cultures by assessing cell viability and apoptotic cell death. We have combined morphological and biochemical techniques to establish the relevance of apoptosis in neuronal cell death induced by Aroclors. Treatment with both Aroclor 1248 and Aroclor 1260 caused the loss of cell viability and accelerated apoptosis both in a concentration- and time-dependent manner. However, the extent of apoptosis resulted greater for Aroclor 1248 than for Aroclor 1260. This is correlated with the loss of cell viability since Aroclor 1248 is more cytotoxic. The apoptosis induced by Aroclors involves the increase of caspase-3 activity. To correlate the caspase-3 activity with respect to changes in protein processing, caspase-3 precursor protein (procaspase-3) was evaluated by Western blot analysis. Also, Bcl-2 and Bax protein were assessed in order to elucidate the cell death machinery induced in cortical neuronal cell cultures by Aroclor 1248. The results indicate that the increase in Aroclor-induced apoptosis correlates with a reduction in the expression of antiapoptotic Bcl-2 and an increase in the expression of proapoptotic Bax. These results suggest that, with our experimental conditions, Aroclors induce apoptosis in primary cultures of cortical neurons via proteins of the Bcl-2 and caspase families.

Ischemia-induced increase in long-term potentiation is warded off by specific calpain inhibitor PD150606

In the present study, the effect of specific, membrane-permeable calpain inhibitor, PD150606, was analysed on synaptic efficacy in in vitro brain slices experiments after ischemic insult of rats in vivo, and on cell viability in a glutamate excitotoxicity test in mouse cell culture. Bilateral common carotid artery ligation (BCCL) for 24 h markedly increased calpain activity and enhanced LTP induction in rat hippocampus, although the CA1 layer significantly shrank. The enhancement of LTP could be diminished by short-term application of PD150606 (40 microM) into the perfusion solution. Intracerebroventricular administration of PD150606 (100 microM) parallel with ischemic insult prevented LTP and effectively inhibited hippocampal calpain activity. Intracerebroventricularly applied PD150606 inhibited the CA1 layer shrinkage after common carotid ligation. High level of exogenous glutamate caused marked decrease of cell viability in mouse cerebellar granule cell cultures, which could be partly warded off by 20 microM PD150606. Our data witness that calpain action is intricately involved in the regulation of synaptic efficacy.

Acute exposure to CXC chemokine ligand 10, but not its chronic astroglial production, alters synaptic plasticity in mouse hippocampal slices

Brain levels of CXC chemokine ligand 10 (CXCL10) are elevated in a number of neuropathological conditions. To determine its impact on neuronal function, we measured synaptic transmission and plasticity in hippocampal slices prepared from transgenic (TG) mice with chronic astroglial production of CXCL10. We also tested the acute effect of recombinant CXCL10 applied to slices from normal C57Bl/6J mice, CXCL10 TG mice and CXCR3 knock out (KO) mice. Chronic production of CXCL10 did not alter synaptic plasticity. By contrast, exogenous CXCL10 (10 ng/ml) significantly inhibited long-term potentiation (LTP) in slices from normal C57Bl/6J mice and CXCL10 TG. The effect was probably receptor-mediated because CXCL10-induced inhibition of LTP was not observed in CXCR3 KO mice. Our findings suggest that acute exposure to CXCL10 alters synaptic plasticity via CXCR3 in mouse hippocampus.

Lesion-induced enhancement of LTP in rat visual cortex is mediated by NMDA receptors containing the NR2B subunit

There is emerging evidence that injury of the cerebral cortex is followed by processes of enhanced neuroplasticity. In the present study, we investigate the functional properties of NMDA receptors (NMDARs) in the surround of focal lesions with recordings of extracellular field potentials (FPs) in acute slices of rat visual cortex at survival times of 2-6 days. FPs were recorded in cortical layer III lateral to the lesion, while long-term potentiation (LTP) was induced by theta-burst stimulation (TBS) in layer IV. The predominantly AMPA receptor-mediated FPs displayed a significantly enhanced LTP in the surround of the lesion at distances of 2-3.2 mm. The LTP was completely blocked by the NMDAR antagonist D-AP5. Ifenprodil, an antagonist of NMDARs containing the NR2B subunit, only slightly affected the LTP in slices from sham-operated animals, but significantly reduced the LTP in slices from lesioned rats. We quantitatively analysed the proportion of NMDARs containing the NR2B subunit after lesions by applying ifenprodil to pharmacologically isolated NMDAR-FPs. The NR2B antagonist reduced the NMDAR-FPs significantly more strongly at distances of 2.0-3.2 mm from the border of the lesion. This indicates that the early phase of increased synaptic long-term plasticity in the surround of cortical lesions is accompanied by an up-regulation of NMDARs containing the NR2B subunit.

The amygdaloid complex: anatomy and physiology

A converging body of literature over the last 50 years has implicated the amygdala in assigning emotional significance or value to sensory information. In particular, the amygdala has been shown to be an essential component of the circuitry underlying fear-related responses. Disorders in the processing of fear-related information are likely to be the underlying cause of some anxiety disorders in humans such as posttraumatic stress. The amygdaloid complex is a group of more than 10 nuclei that are located in the midtemporal lobe. These nuclei can be distinguished both on cytoarchitectonic and connectional grounds. Anatomical tract tracing studies have shown that these nuclei have extensive intranuclear and internuclear connections. The afferent and efferent connections of the amygdala have also been mapped in detail, showing that the amygdaloid complex has extensive connections with cortical and subcortical regions. Analysis of fear conditioning in rats has suggested that long-term synaptic plasticity of inputs to the amygdala underlies the acquisition and perhaps storage of the fear memory. In agreement with this proposal, synaptic plasticity has been demonstrated at synapses in the amygdala in both in vitro and in vivo studies. In this review, we examine the anatomical and physiological substrates proposed to underlie amygdala function.

Active decay of composite excitatory postsynaptic potentials in hippocampal slices from young rats

NMDA receptor dependent synaptic plasticity was examined in hippocampal slices using a novel pharmacological pairing procedure. Field excitatory postsynaptic potentials (EPSPs) were recorded from the CA1 area of slices maintained in a low Mg(2+) solution using a stimulus rate of 0.1-0.2 Hz. The NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) was initially included in the perfusion solution to establish baseline recording of isolated AMPA EPSPs. Washing out AP5 led to the expression of composite EPSPs, containing both AMPA and NMDA receptor mediated components. Following an initial, transient potentiation of the AMPA component, the composite responses gradually decayed for several hours, involving AMPA and NMDA components to a similar extent. This decay was input specific and could be terminated at any stage by reapplication of AP5. Subsequent long-term potentiation (LTP) reversed the effect to an extent inversely related to the degree of depression. Experiments to test the interaction with long-term depression (LTD) revealed a significant but incomplete overlap between the two depression processes. In conclusion, pairing synaptic activation at test stimulus frequency with pharmacological unblocking of NMDA receptors allows for expression of composite EPSPs that decay substantially, due to an active mechanism. The underlying process appears to be at least partly distinct from those involved in homosynaptic LTP and LTD.

Impairment of long-term potentiation and spatial memory in leptin receptor-deficient rodents

Leptin is well known to be involved in the control of feeding, reproduction and neuroendocrine functions through its action on the hypothalamus. However, leptin receptors are found in brain regions other than the hypothalamus (including the hippocampus and cerebral cortex) suggesting extrahypothalamic functions. We investigated hippocampal long-term potentiation (LTP) and long-term depression (LTD), and the spatial-memory function in two leptin receptor-deficient rodents (Zucker rats and db/db mice). In brain slices, the CA1 hippocampal region of both strains showed impairments of LTP and LTD; leptin (10(-12) M) did not improve these impairments in either strain. These strains also showed lower basal levels of Ca(2+)/calmodulin-dependent protein kinase II activity in the CA1 region than the respective controls, and the levels did not respond to tetanic stimulation. These strains also showed impaired spatial memory in the Morris water-maze test (i.e. longer swim-path lengths during training sessions and less frequent crossings of the platform's original location in the probe test. From these results we suggest that the leptin receptor-deficient animals show impaired LTP in CA1 and poor spatial memory due, at least in part, to a deficiency in leptin receptors in the hippocampus.

Neuronal populations of rat cerebral cortex and hippocampus expressed a higher density of L-type Ca 2+ channel than corresponding cerebral vessels

Dihydropyridine (DHP)-type Ca2+ antagonists block primarily L-type Ca2+ channels and are used in the therapy of hypertension. They were also proposed for the treatment of several central nervous system disorders. In brain, these compounds bind both neuronal and vascular Ca2+ channels, but no studies have evaluated comparatively their density at neuronal and vascular level. This study has analyzed the pharmacological profile and the anatomical localization of L-type Ca2+ channels in rat frontal cortex, hippocampus and in forebrain pial and intracerebral arteries by radioligand binding assay and high resolution light microscope autoradiography. The DHP derivative [3H]nicardipine was used as a radioligand. Binding of [3H]nicardipine was consistent with the labeling of L-type Ca2+ channels. In frontal cortex, the highest density of binding sites was found in nerve cell body region, followed by the neuropil and the wall of intracerebral arteries. In hippocampus, the density of binding sites was higher in the nerve cell body region than in the neuropil of CA1, CA3, and CA4 subfields. In the dentate gyrus, a higher density of silver grains was developed in neuropil than in nerve cell body of granule neurons. With the exception of dentate gyrus, neuronal binding sites were more expressed than vascular binding sites in the hippocampus. In pial arteries [3H]nicardipine binding density decreased concomitant with the reduction of vessel diameter, whereas in intracerebral arteries [3H]nicardipine binding density displayed an opposite pattern. The above findings indicate that in brain the density of neuronal L-type Ca2+ channels was significantly higher than that of vascular ones. This may account for more pronounced neuronal than vascular effects after pharmacological manipulation of cerebral Ca2+ channels.

The Relative Contribution of NMDA Receptor Channels in the Expression of Long-term Potentiation in the Hippocampal CA1 Region

Long-term potentiation (LTP) was studied in the hippocampal CA1 region of guinea-pigs using a solution containing 0.1 mM magnesium and 10 microM of the non-N-methyl-d-aspartate (non-NMDA) antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), leaving an NMDA-mediated field excitatory postsynaptic potential (EPSP). Brief high-frequency afferent tetanization induced a substantial synapse-specific potentiation of the NMDA EPSP with a time course closely resembling that previously described for LTP of the non-NMDA-mediated EPSP. This NMDA EPSP potentiation was occluded by prior induction of LTP in normal solution. Using a solution containing 0.1 mM magnesium and 1 microM CNQX, the EPSP was composed of both a non-NMDA- and an NMDA-mediated component which could be measured separately and in parallel. Manipulations that cause increased transmitter release, such as phorbol ester application and changes in stimulation frequency, enhanced the two measures nearly equally. Afferent tetanization induced an increase of both EPSP components, with a similar time course, the NMDA component showing a relative increase of about one-third of that of the non-NMDA one. These results suggest that, to the extent that LTP is based on an increased release of transmitter, the mechanism exhibits features distinct from those underlying other forms of enhanced release.

Quisqualate Metabotropic Receptors Modulate NMDA Currents and Facilitate Induction of Long-Term Potentiation Through Protein Kinase C

Using intracellular and extracellular recordings in rat hippocampal slices, we have investigated the interactions between the quisqualate metabotropic receptor (QP) and currents mediated by N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA). We found that trans-(t)-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD) and 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) potentiated NMDA but not AMPA-mediated currents. Intracellular injections of selective protein kinase C inhibitors prevented the up-regulation of the NMDA response. The physiological consequence of the up-regulation by ACPD of the NMDA response on the threshold of long-term potentiation induction was tested. We found that a subthreshold train of electrical stimulation that produced short-term potentiation generated long-term potentiation when coupled with ACPD application, an effect which was not produced by AMPA or NMDA. This effect was blocked by an inhibitor of protein kinase C. These results demonstrate for the first time that one subtype of glutamate receptor (QP) can regulate another subtype of glutamate receptor (NMDA) through the activation of protein kinase C. Our results also suggest that the NMDA receptor is regulated by protein kinase C, and that the intracellular level of protein kinase C may determine the threshold for induction of long-term potentiation.

Differential calmodulin gene expression in the rodent brain

Apparently redundant members of the calmodulin (CaM) gene family encode for the same amino acid sequence. CaM, a ubiquitous cytoplasmic calcium ion receptor, regulates the function of a variety of target molecules even in a single cell. Maintenance of the fidelity of the active CaM-target interactions in different compartments of the cell requires a rather complex control of the total cellular CaM pool comprising multiple levels of regulatory circuits. Among these mechanisms, it has long been proposed that a multigene family maximizes the regulatory potentials at the level of the gene expression. CaM genes are expressed at a particularly profound level in the mammalian central nervous system (CNS), especially in the highly polarized neurons. Thus, in the search for clear evidence of the suggested differential expression of the CaM genes, much of the research has been focused on the elements of the CNS. This review aims to give a comprehensive survey on the current understanding of this field at the level of the regulation of CaM mRNA transcription and distribution in the rodent brain. The results indicate that the CaM genes are indeed expressed in a gene-specific manner in the developing and adult brain under physiological conditions. To establish local CaM pools in distant intracellular compartments (dendrites and glial processes), local protein synthesis from differentially targeted mRNAs is also employed. Moreover, the CaM genes are controlled in a unique, gene-specific fashion when responding to certain external stimuli. Additionally, putative regulatory elements have been identified on the CaM genes and mRNAs.