Ion Channels And Neuronal Activity Research Paper

1. Activity-Dependent Increases In Neuronal Ca²⁺

Internal Ca²⁺ is maintained at extremely low concentrations creating an enormous driving force for its movement into the cytoplasm from internal stores or the extracellular space. Ca²⁺ levels are estimated to be at about 100 nM in neurons at their re sting hyperpolarized state, while extracellular Ca²⁺ is in the 2–3 mM range and 10 µM to 1 mM inside the endoplasmic reticulum (Ghosh and Greenberg 1996 , Mattson et al. 2000). The gating mechanisms for Ca²⁺ entry into the cell’s cytoplasm includes plasma membrane voltage-dependent Ca²⁺ channels (VDCC), plasma membrane neurotransmitter or ligand-activated channels, and Ca²⁺ release from internal stores. All of these mechanisms a re recruited by neuronal activity, they experience Ca²⁺ mediated feedback regulation, and they are modulated by neurotransmitter activation of G-proteins (Lee et al. 1999, Zhang et al. 1998, Zamponi and Snu tch 1998, Zuhlke et al. 1999). Each mechanism of Ca²⁺ entry is compartmentalized to produce localized increases in intracellular Ca²⁺ and site-specific activation of Ca²⁺ sensitive pathways (Magee et al. 1998, Neher 199 8, Yuste et al. 2000).

Adaptive thresholds for Ca²⁺ entry ensure its action as a second messenger will reflect high levels of neuronal activity. Voltage-gated and neurotransmitter gated channels interact during periods of neuronal activity to determine the degree of Ca²⁺ entry and thus the direction of Ca²⁺ -mediated adaptation. The classic sequence for initiating Ca²⁺ entry begins with presynaptic release of neurotransmitter acting postsynaptically to depolarize the cell beyond the threshold for triggering an action potential (Colquhoun and Sakmann 1998). The pioneering work of Hodgkin, Huxley, and Katz demonstrated action potentials were initiated by activating voltage-dependent Na⁺ channels and rapidly terminated by the combined action of Na+ channel inactivation and the opposing activation of voltage-dependent K channels (Hodgkin et al. 1952). Activation of VDCCs is facilitated by the rapid depolarization initiated by Na⁺ channel activation, but it is not an +absolute requirement. High- and low-threshold Ca²⁺ -mediated action potentials can be triggered by depolarization independent of Na⁺ channel activation (Magee et al. 1998, Huguenard 1996). The duration and peak depolarization of t he action potential, and consequently the extent of Ca²⁺ entry, is limited by activation of voltage-dependent K⁺ channels and intrinsic kinetics of Ca²⁺ channel inactivation.

Unique patterns of Ca²⁺ can be achieved by the selective expression of ion channels in different populations of nerve cells. For example, the expression of inwardly rectifying K⁺ (K_ir) channels raises the threshold for action potential generation by hyperpolarizing the resting membrane potential of striatal neurons, while the expression and interaction of low-threshold Ca²⁺ channels (T-type), Ca²⁺ -activated K⁺ channels (slo), and hyperpolarization activated K⁺ channels (I_H) contribute to regular, intrinsic firing in neurons of the substantianigra and thalamus (Huguenard 1996, Santoro et al. 2000, Wilson and Callaway 2000). Regional differences in ion channel distribution can also create microenvironments of Ca²⁺ control within the cell. Clustering of Na⁺, K⁺, and Ca²⁺ channels in dendrites create local sites for integrating synaptic input (Magee et al. 1998, Safronov 1 999), and direct association of P/Q and N-type Ca²⁺ channels with vesicle fusion proteins mediates activity-dependent release of neurotransmitter in synaptic terminals (Goda and Sudhof 1997, Neher 1998).

The molecular diversity of ion channels present in neurons creates opportunity for exquisite control of Ca²⁺ influx through differences in gating mechanisms and kinetics of activation and inactivation (Catterall 1995, Ghosh and Greenberg 1995). Calcium channels are heteroligomeric complexes containing α₁, α₂δ and β subunits. Ca+ channels contain a single α subunit which serves as a pore and voltage sensor. Nine α subunit genes have been identified, corresponding to each type of VDCC channel previously categorized by pharmacology. α_1A encodes for ω-Aga IVA sensitive P-type, α_1B encodes for ω-Ctx GVIA sensitive N-type, α_1C, α_1D and α_1S all encode for dihydropyridine sensitive L-type and α_1G and _α1H encode for Ni^2+-sensitive T-type + calcium channels (Zamponi and Snutch 1998). K⁺ channels represent a much larger and more diverse collection of ion channels. At present, nine families of voltage-activated K+ channels are described (Kaczorowski and Garcia 1999). Many are derived from homologues of Drosophila channel genes Shaker, Shah, Shaw, and Shal, referred to as K_V1, K_V2, K_V3, and K_V4 in mammals. They usually form homotetrameric channels, but association with β- subunits can alter channel kinetics (Kaczorowski and Garcia 1999). A separate class of K⁺ channel fundamental in regulating neuronal excitability is Ca²⁺ – activated K⁺ channels. They are classified by single channel conductances of 2–25 pS (small or SK channel), 25–100 pS (intermediate or IK channel), and 100–300 pS (large or BK channel) (Kaczorowski and Garcia 1999). Ca^2+-activated K⁺ channels serve as an activity-dependent feedback mechanism for reducing neuron firing frequency and returning the membrane potential back to its resting state.

Excitatory synaptic input increases intracellular Ca²⁺ directly, by activating ionotropic and metabotropic glutamate receptors, and secondarily by driving the membrane potential beyond the voltage-dependent threshold for activating VDCCs. The cooperation of these mechanisms ensures robust Ca²⁺ signals in response to strong synaptic drive. Ionotropic glutamate receptors (AMPA, kainate, and NMDA receptors) are the postsynaptic mediators of fast excitatory synaptic transmission. Functional ionotropic glutamate receptors contain four or five structurally related subunits. The combination of subunits GluR1–4 make functional AMPA receptors, GluR5–7 and KA1–2 make kainate receptors, and NR1 and NR2A-D make NMDA receptors (Bigge 1999). Flip or flop splice variants of AMPA receptor subunits influence gating kinetics. AMPA receptors generally have low Ca²⁺ permeability, due to the Q*R site of GluR2 subunits. GluR2 is the most common subunit found in the brain, but examples of receptors without GluR2 exist and are implicated in Ca^2+-dependent changes in neuronal physiology (Conti and Weinberg 1999, Bigge 1999). NMDA receptors have greater glutamate affinity than AMPA receptors, slower activation and deactivation kinetics, and they are highly permeable to Ca²⁺. The Ca²⁺ conductance/Na⁺ conductance is > 5 for NMDA receptors, 2 for AMPA receptors lacking GluR2, and 0.1 for AMPA receptors containing GluR2 (Conti and Weinberg 1999). The combined action of greater Ca²⁺ permeability and slower channel inactivation results in dramatically more Ca²⁺ influx for NMDA receptors. NMDA receptors are made of NR1 and NR2 heterodimer complexes and generally composed of 4–5 subunits. NR1 is essential for channel function and NR2 confers the voltage-dependent Mg²⁺ block and glycine modulation of NMDA receptors (Conti and Weinberg 1999). The voltage-dependent Mg²⁺ -block acts as an activity sensor, allowing large increases in intracellular Ca²⁺ only when the depolarization threshold for releasing the Mg²⁺ block is exceeded. Kainate receptors, like AMPA receptors, lack voltage dependency. Compared to AMPA receptors, kainate receptors contribute little to the peak postsynaptic response, but their slower decay kinetics results in comparable charge transfer (Ferking and Nicoll 2000). Kainate receptors also mediate presynaptic inhibition of excitatory and inhibitory synapses in the hippocampus.

Slow excitatory synaptic events mediated by activation of metabotropic glutamate receptors (mGluRs) is another important mechanism for elevating intracellular Ca⁺². Group-I mGluRs stimulate phospholipase C via GTP-binding proteins. Phospholipase C cleaves phosphoinositol-(4,5)-bisphosphate (PIP₂) into diacylglycerol (DAG) and inositol 1,4,5 triphosphate (IP₃). DAG activates protein kinase C (PKC) and I(1,4,5)P₃ activates IP₃ receptors located on the surface of the smooth endoplasmic reticulum (Fagni et al. 2000, Mattson et al. 2000). IP₃ receptors trigger the release of Ca²⁺ stored in the endoplasmic reticulum at concentrations of 10–100 µM (Mattson et al. 2000). Other neurotransmitters use the same mechanism for releasing Ca²⁺ from intracellular stores, but glutamate is by far the most ubiquitous.

2. Activity-Dependent Control Of Neuronal Development

Ca²⁺ influx stimulates differentiation and directs the course of development in excitable cells (Spitzer et al. 1994). VDCCs, ionotropic glutamate receptors, an d mGluRs all participate in regulating intracellular Ca²⁺ at key steps in neuronal development. Glutamate activation of NMDA and AMPA receptors induces proliferation of progenitor cells during neurogenesis and their migration to appropriate cell layers (Haydar et al. 2000). Evidence also exists for neurogenesis mediated by Ca²⁺ entry through voltage-dependent L- type Ca²⁺ channels and Ca²⁺ waves mediated by release of Ca²⁺ from intracellular stores. In postmitotic cells, Ca²⁺ continues to direct neuronal differentiation by influencing ion channel expression, neurotransmitter phenotype, growth factor express ion, and neurite extension (Spitzer et al. 1994). Ca²⁺ influx through VDCCs (Land N-type) stimulate cell adhesion molecules (CAMs) (NCAM, cadherin, and L1) to promote neurite outgrowth via CAM-mediated cell to cell interactions and the binding of CAM to actin based cytoskeleton (Doherty and Walsh 1994). Ca²⁺ may initiate neurite extension by binding to calmodulin (CaM) and phosphorylating microtubule associate proteins (MAP2) through the activation of Ca²⁺/ CaM-dependent protein kinase II and IV (Kuhn et al. 1998). A parallel mechanism implicated in MAP phosphorylation is the mitogen activated protein kinase (MAPKs) pathway stimulated by mGluR activation (Quinlin and Halpain 1996). It is clear that Ca²⁺ -mediated remodeling is based upon a Ca²⁺ set point, since differences in responses exist depending on the Ca²⁺ level achieved and the neuronal population examined. Some neurons experience growth cone collapse with increase s in Ca²⁺ , due to severing of microfilaments by Ca²⁺-activated gelsolin (Neely and Nichols 1995). Support for this mechanism comes from the observation of downregulated VDCCs coincident with neurite extension and increases in neuronal activity producing decreases in CAM expression.

Protein synthesis is also critical to activity-dependent plasticity and growth, a process Ca²⁺ mediates through interacting steps of protein phosphorylation (Ghosh and Greenberg 1995). Activity-dependent increases in Ca²⁺ stimulate the expression of immediate early genes like c-fos and c-jun, as well as delayed ‘effector’ genes for neurotrophins and, in a mechanism of positive feedback, genes for VDCCs, NMDA receptors, and AMPA receptors (Ghosh and Greenberg 1995). The pattern of gene expression is dependent on the pattern of neuronal activity and Ca²⁺ accumulation. Intracellular pathways appear to act in parallel to alter gene expression, but they may respond differentially to changes in Ca²⁺ concentration. A final point of convergence for Ca²⁺ stimulated pathways is phosphorylation of cAMP responsive element binding protein (CREB), which activates RNA polymerase by forming a complex with CREB-binding protein (CRP). The independent path – ways proposed to trigger this mechanism include Ca²⁺ activation of Ras/mitogen activated protein kinase (MAPK) pathway, Ca²⁺ activation of adenlyate cyclase and subsequently protein kinase A (PKA) and Ca²⁺ binding to CaM, and activation of Ca²⁺ /CaM kinase II and IV (Ghosh and Greenberg 1995).

Aclear division does not exist between the actions of Ca²⁺ and neurotrophins in driving activity-dependent change in neurons. Activity not only increases intracellular Ca²⁺, but it also causes the release of neurotrophins, and neurotrophins and Ca²⁺ produce similar responses in nerve cells (McAllister et al. 1999). Ca²⁺ and neurotrophins both trigger the Ras-MAPK pathway to change gene expression, neurotrophins by binding to membra ne-bound tyrosine kinase (trk) receptors, and Ca²⁺ by activating Ras directly (Heumann 1994). Neurotrophins have both directrapid and slow modulatory effects on ion channels involved in excitation (Benedikt and Poo 1996). Neurotrophins cause rapid depolarizations by activating Na⁺ channels linked to trk receptors and they modulate glutamate-activated channels and intrinsic voltage-activated channels to increase intracellular Ca²⁺ (Jarvis et al. 1997, Kaflitz et al. 1999). Neurotrophins also increase neuronal excitability and thus Ca²⁺ influx through acute inhibition of K_ir3.1, K_ir3.4, and K_ir2.1 (Wischmeyer et al. 1998). Neurotrophins also act presynaptically to increase neurotransmitter release (McAllister et al. 1999). Testament to the intimate relationship between neurotrophins and Ca²⁺, neurotrophins produce long-term changes in Ca²⁺ entry by increasing the expression of VDCCs, NMDA receptors, and AMP A receptors, and activity-dependent increases in Ca²⁺ produce a reciprocal increase in neurotrophin expression (McAllister et al. 1999).

The gearing of neurons toward excitation during development is clearly illustrated by the unique behavior displayed by neurotransmitter activated channels early in development. Embryonic synaptic AMPA receptors are highly Ca²⁺ permeable and mEPSC (minature excitatory postsynaptic current) amplitude can be more than twofold larger than at mature synapses (Rohrbough and Spitzer 1999). Postnatal neurons preferentially express flip variants of AMPA receptors, which increase the depolarization-mediated Ca²⁺ signal by slowing desensitization (Conti and Wienberg 1999). It should be noted, however, that many synapses show only functional NMDA receptors and only with activity to ‘silent’ AMPA receptors appear (Petralia et al. 1999). Early postnatal development also shows a shift in the molecular structure and physiological properties of NMDA receptors. NR2B subunits are preferentially expressed early in development and they show less voltage-dependent block by Mg²⁺ and slower kinetics, properties which lower the threshold for Ca²⁺ entry during synaptic excitation. With maturity, there is a shift to NR2A and NR2C subunits which increase the Mg²⁺ block and speed up the NMDA activated response. This postnatal disposition toward synaptic excitation is augmented by a developmental shift in the property of GABA_A receptor mediated inhibition (Ben-Ari et al. 1997). Rather than inhibiting action potential generation and VDCC activation, as is seen in adults, immature GABA_A synapses cause slow depolarizations triggering bursts of action potentials and Ca²⁺ influx through VDCCs (Ben-Ari et al. 1997). Later in development, the Cl^– gradient changes resulting in hyperpolarizing GABA_A responses and an inhibition of action potential generation and Ca²⁺ activation (Ben-Ari et al. 1997).

3. Activity And Synaptic Function

Ca²⁺ entering presynaptic terminals during action potentials stimulates the fusion of synaptic vesicles with the presynaptic membrane and the release of neurotransmitter into the synaptic cleft. P/Q and N- type Ca²⁺ channels are the primary VDCCs subserving this function in the central nervo us system (Goda and Sudhof 1997). Entrance of Ca²⁺ through individual channels is proposed to form microdomains, where Ca²⁺ levels may reach as high as 100 µM (Neher 1998). It is proposed that ‘core complex’ proteins of the synaptic fusion machinery maximize their proximity to these domains by binding syntaxin to Ca²⁺ channels (Go da and Sudhof 1997). Prior to and independent of Ca²⁺ influx, synaptic vesicles dock to the presynaptic membrane in active zones and undergo priming to make them ready for releasing neurotransmitter with Ca²⁺ Ca²⁺ influx (Goda and Sudhof 1997). The docking and priming steps are mediated by an interaction of a ‘core complex’ of NSF attachment protein receptors (SNAREs) located on the vesicle (v-SNARE) and attachment SNAREs located on the target membrane (t-SNARE). The core complex or SNARE is made up of syntaxin and synaptosome-associated membrane protein of 25 kDa (SNAP-25), which are localized in the synaptic plasma membrane, and the vesicular protein synaptobrevin (also known as vesicle associated membrane protein; VAMP) (Goda and Sudhof 1997). The process of vesicle fusion and neurotransmitter release is believed to be mediated by the interaction of the synaptotagmin Ca²⁺ sensor with the ‘core complex.’ Synaptotagmin I and II are the leading candidates as Ca²⁺ sensors, although other presynaptic proteins with Ca²⁺ C₂ -binding domains are proposed to serve the same function (Goda and Sudhof 1997). The mechanism for fusion is proposed to be a rapid electrostatic or conformational change in the fusion complex mediated by Ca²⁺ binding to synaptotagmin (Goda and Sudhof 1997). The vesicle collapses into the membrane and is retrieved away from the active zone by a clathrin-dependent mechanism involving the GTPase dynamin I (Cremona and De Camilli 1997).

Invasion of the presynaptic terminal by multiple action potentials results in a short-term change in the probability of vesicle fusion and the release of neurotransmitter. Depending on the number and timing of the action potentials, and thus the degree and duration of Ca²⁺ increase, the processes of facilitation (time constant less than one second), augmentation (time constant of several seconds), and potentiation (time constant of a few minutes) are initiated (Zucker 1999). One mechanism proposed for mediating short-term synaptic enhancement is the binding of Ca²⁺ to synapsin I. Synapsin I is a Ca²⁺ -regulated ATPase which binds to actin and may facilitate vesicle transport via the actin matrix (Zucker 1999). Other mechanisms proposed to participate in short-term synaptic plasticity include facilitating actions of synaptotagmin and Ca²⁺ -dependent facilitation of presynaptic Ca²⁺ currents (Zucker 1999). An exciting new explanation for the longest form of short-term plasticity, PTP, is based on Ca²⁺ dumping by mitochondria following mitochondrial sequestration of Ca²⁺ during spike trains (Zucker 1999). New studies indicate another key role for Ca²⁺ in mediating short-term synaptic plasticity lies in a clathrin-independent form of rapid endocytosis of synaptic vesicles. The proposed mechanism suggests that Ca²⁺ bin ds to synaptophysin during repeated excitation and Ca²⁺ -stimulated synaptophysin binds to dynamin I to produce rapid endocytosis (Daley et al. 2000).

Large and sustained increases in intracellular Ca²⁺ on the postsynaptic side of the synapse is the primary event for inducing long-term forms of synaptic plasticity or long-term depression (LTD) and long-term potentiation (LTP). Critical to determining whether LT P or LTD occurs are the extent and kinetics of the Ca²⁺ signal and the molecular mechanism underlying the Ca²⁺ increase (Zucker 1999). These factors and other mechanisms vary considerably between brain areas. Based on cortical and cerebellar studies, a few general themes emerge. Ca²⁺ entry through NMDA receptors is a critical event in the induction of LTP and the Mg²⁺ block of these receptors sets a stimulus and depolarization threshold. Weaker stimuli, which activate VDCCs, often lead to the induction of LTD (Zucker 1999). LTD also requires the activation of group I mGluRs at many synapses and the subsequent activation of nitric oxide synthetase (Daniel et al. 1998). VDCCs can augment the NMDA Ca²⁺ signal if back-propagating action potentials precede activation of NMDA receptors (Zucker 1999). This receptor based scheme depends on the parameters of the Ca²⁺ signal, and thus examples of NMDA receptor- dependent LTD and mGluR-dependent LTP also exist (Nicoll et al. 1998).

The mechanisms used to induce synaptic plasticity in adult animals often follow the same pathways utilized for development and growth in the nervous system. Phosphorylation plays a critical role in LTP and it is triggered by Ca²⁺ influx ( Soderling and Derkach 2000). Ca²⁺ activation of Ca²⁺ / CaM kinase and Ca²⁺ activation of the Ras MAPK pathways play distinct roles in the maintenance phase of LTP. Autophosphorylation of Ca²⁺ /CaM kinase makes it Ca²⁺ independent and creates a mechanism for maintaining long-term phosphorylation. Active Ca²⁺/CaM kinase increases AMPA receptor responses and may be involved in the unmasking of silent receptors and the trafficking of new receptors to the extracellular membrane (Malinow et al. 2000, Soderling and Derkach 2000). Keeping Ca²⁺ /CaM kinase phosphorylated and active is opposed by the activity of protein phosphatase I. Herein lies another critical step in LTP: inhibiting protein phosphates. Phosphatase inhibition is mediated by Ca²⁺ activation of adenlyate cyclase, activation of PKA, and phosphorylation of inhibitor 1 (I1). I1 shuts off protein phosphatase I and Ca²⁺ /CaM kinase thus stays phosphorylated and active (Soderling and Derkach 2000). Another critical component of LTP expression is protein production. Ca²⁺ /CaM kinase is also implicated in gene expression as well as Ca²⁺-activation of Ras/MAPK and, just like observed developmentally, phosphorylation of CREB (Platenik et al. 2000). Another similarity to development is the clear involvement of neurotrophins in LT P expression. Neurotrophins directly modulate Ca²⁺ physiology and they activate the Ras MAPK pathway (McAllister et al. 1999, Klintsova and Greenough 1999).

4. Ca²⁺ In Aging And Degeneration

The role of Ca²⁺ in development and plasticity requires flawless control over its intracellular concentration. Aging in the brain, and in many other tissues, occurs in part through a disruption in the ability to maintain Ca²⁺ homeostasis (Thibault et al. 1998). A system level theory for explaining aging poses that chronic exposure to glucocorticoid stress hormones creates a disruption of Ca²⁺ homeostasis (Thibault et al. 1998). In the hippocampus, stress hormones and/or aging increases the expression of L-type Ca²⁺ channels (Thibault et al. 1998). The increased Ca²⁺ signal suppresses hippocampal neuron activity by stimulating Ca²⁺ -activated K+ channels and, consequently, the hippocampus becomes less effective in its role of terminating the stress response. Circulating levels of stress hormones therefore progressively increase over an animal’s lifetime and consequently the deleterious effects of Ca²⁺ overload also become progressively worse. Elevated Ca²⁺ in aged neurons is proposed to create dysfunctional plasticity and to reduce the threshold for Ca²⁺ -mediated cell death.

Disruption of Ca²⁺ homeostasis is also proposed to be a critical event in apoptosis and necrosis produced in many age-related neurodegenerative disorders (Mattson 1998, Mattson et al. 2000, Thibault et al. 1998). Periods of excessive excitation, as might occur during stroke or in response to β-amyloid stimulation, cause abnormally high levels of Ca + ions to enter the cell. The candid ate mechanisms for the pathological increase in Ca²⁺ influx include VDCCs, NMDA receptor gated channels, and mGluR activation and downstream release of Ca²⁺ from internal stores. The consequent Ca²⁺ overload stimulates Ca²⁺ -sensitive proteases and it plays a significant role in the generation of reactive oxygen species (ROS) (Ma ttson et al. 2000). High levels of cytoplasmic Ca²⁺ cause mitochondrial Ca²⁺ loading which stimulates mitochondrial ROS production and release of apoptotic factors (Mattson 19 98).

The flood of Ca²⁺ triggers abnormal activation of calpain and free radicals activate caspases, both of which are viewed to create excessive proteolysis. In addition, Ca²⁺ sequestration by mitochondria during excessive excitation reduces the production of ATP with a critical cost of shutting down ATPases responsible for maintaining the cell’s membrane potential (Albin and Greenamyre 1992). Plasma membrane Na+ K+-ATPases require huge amounts of energy and their reduced activity leads to cellular depolarization , activation of VDCCs, and the removal of Mg + voltage-dependent block from NMDA gated channels.

Ca²⁺ also triggers cytoplasmic production of ROS through the activation of Ca²⁺ -dependent enzymes. Ca²⁺ -activated proteases convert xanthine dehydrogenase into xanthine oxidase which produces superoxide anions and H O . High levels of cytoplasmic Ca²⁺ triggered by NMDA receptor activation also activate nitric oxide synthetase (NOS). NO produced by the activation of NOS is a free radical, as well as a ubiquitous mediator of biological processes (Dawson and Dawson 1998). NO turns into a much more potent oxidant, however, after it reacts with the superoxide anion under conditions of oxidative stress to produce highly reactive peroxynitrite. The ROS generated by these mechanisms cause lipid, protein, and nucleic acid oxidation and ultimately cell death.

Bibliography:

1. Albin R L, Greenamyre J T 1992 Alternative excitotoxic hypotheses. Neurology 42: 733–8
2. Ben-Ari Y, Khazipov R, Leinekugel X, Caillard O, Gaiarsa J L 1997 GABAA, NMDA and AMPA receptors: A developmentally regulated ‘menage a trois’. Trends in Neurosciences 20(11): 523–9
3. Benedikt B, Poo M-m 1996 Fast actions of neurotrophic factors. Current Opinion in Neurobiology 6: 324–30
4. Bigge C F 1999 Ionotropic glutamate receptors. Current Opinion in Chemical Biology 3: 441–7
5. Catterall W A 1995 Structure and function of voltage-gated ion channels. Annual Reviews in Biochemistry 64: 493–531
6. Colquhoun D, Sakmann B 1998 From muscle endplate to brain synapses: A short history of synapses and agonist-activated ion channels. Neuron 20: 381–7
7. Conti F, Weinberg R J 1999 Shaping excitation at glutamatergic synapses. Trends in Neurosciences 22: 451–8
8. Cremona O, De Camilli P D 1997 Synaptic vesicle endocytosis. Current Opinion in Neurobiology 7: 323–30
9. Daniel H, Levenes C, Crepel F 1998 Cellular mechanisms of cerebellar LTD. Trends in Neurosciences 21(9): 401–7
10. Daley C, Sugimori M, Moreira J E, Ziff E B, Llinas R 2000 Synaptophysin regulates clathrin-independent endocytosis of synaptic vesicles. PNAS 97: 6120–5
11. Dawson V L, Dawson T M 1998 Nitric oxide in neuro-degeneration. Progress in Brain Research 118: 215–29
12. Doherty P, Walsh F S 1994 Signal transduction events underlying neurite outgrowth stimulated by cell adhesion molecules. Current Opinion in Neurobiology 4: 49–55
13. Fagni L, Chavis P, Ango F, Bockaert J 2000 Complex interactions between mGluRs, intracellular Ca²⁺ stores and ion channels in neurons. TINS 23: 80–8
14. Ferking M, Nicoll R A 2000 Synaptic kainate receptors. Current Opinion in Neurobiology 10: 342–51
15. Ghosh A, Greenberg M E 1995 Calcium signaling in neurons: Molecular mechanisms and cellular consequences. Science 268: 239–46
16. Goda Y, Sudhof T C 1997 Calcium regulation of neurotransmitter release: Reliably unreliable? Current Opinion in Cell Biology 9: 513–18
17. Haydar T F, Wang R, Schwartz M L, Rakic P 2000 Differential modulation of proliferation in the neocortical ventricular and subventricular zones. Journal of Neuroscience 20: 5764–74
18. Heumann R 1994 Neurotrophin signalling. Current Opinion in Neurobiology 4: 668–79
19. Hodgkin A L, Huxley A F, Katz B 1952 Measurements of current–voltage relations in the membrane of the giant axon of Loligo. Journal of Physiology (London) 116: 424–48
20. Huguenard J R 1996 Low-threshold calcium currents in central nervous system neurons. Annual Reviews in Physiology 58: 329–48
21. Jarvis C R, Xiong Z G, Plant J R, Churchill D, Lu W Y, MacVicar B A, MacDonald J F 1997 Neurotrophin modulation of NMDA receptors in cultured murine and isolated rat neurons. Journal of Neurophysiology 78(5): 2363–71
22. Kaczorowski G J, Garcia M L 1999 Pharmacology of voltagegated and calcium-activated potassium channels. Current Opinion in Chemical Biology 3: 448–58
23. Kaflitz K W, Rose C R, Thoenen H, Konnerth A 1999 Neurotrophin-evoked rapid excitation through TrkB receptors. Nature 401: 918–21
24. Klintsova A Y, Greenough W T 1999 Synpatic plasticity in cortical systems. Current Opinion in Neurobiology 9: 203–8
25. Kuhn T B, Williams C V, Dou P, Kater S B 1998 Laminin directs growth cone navigation via two temporally and functionally distinct calcium signals. Journal of Neuroscience 18(1): 184–94
26. Lee A, Wong S T, Gallagher D, Li B, Storm D R, Scheuer T, Catterall W A 1999 Ca²⁺ calmodulin binds to and modulates P Q calcium channels. Nature 399: 155–9
27. Magee J, Hoffman D, Colbert C, Johnston D 1998 Electrical and calcium signaling in dendrites of hippocampal pyramidal neurons. Annual Review of Physiology 60: 327–46
28. Malinow R, Mainen Z F, Hayashi Y 2000 LTP mechanisms: From silence to four-lane traffi Current Opinion in Neurobiology 10: 352–7
29. Mattson M P 1998 Free radicals, calcium, and the synaptic plasticity-cell death continuum: Emerging roles of the transcription factor NF B. International Review of Neurobiology 42: 103–68
30. Mattson M P, LaFerla F M, Chan S L, Leissring M A, Shepel P N, Geiger J D 2000 Calcium signaling in the ER: Its role in neuronal plasticity and neurodegenerative disorders. Trends in Neurosciences 23: 222–9
31. McAllister A K, Katz L C, Lo D C 1999 Neurotrophins and synaptic plasticity. Annual Review of Neuroscience 22: 295–318
32. Neely M D, Nichols J G 1995 Electrical activity, growth cone motility and the cytoskeleton. Journal of Experimental Biology 198: 1433–46
33. Neher E 1998 Vesicle pools and Ca²⁺ microdomains: New tools for understanding their roles in neurotransmitter release. Neuron 20: 389–99
34. Nicoll R A, Oliet S H, Malenka R C 1998 NMDA receptordependent and metabotropic glutamate receptor-dependent forms of long-term depression coexist in CA1 hippocampal pyramidal cells. Neurobiology of Learning & Memory 70(1–2): 62–72
35. Quinlan E M, Halpain S 1996 Emergence of activity-dependent, bidirectional control of microtubule-associated protein MAP2 phosphorylation during postnatal development. Journal of Neuroscience 16(23): 7627–37
36. Petralia R S, Esteban J A, Wang Y X, Partridge J G, Zhao H M, Wenthold R J, Malinow R 1999 Selective acquisition of AMPA receptors over postnatal development suggests a molecular basis for silent synapses. Nature Neuroscience 2(1): 31–6
37. Platenik J, Kuramoto N, Yoneda Y 2000 Molecular mechanisms associated with long-term consolidation of the NMDA signals. Life Sciences 67(4): 335–64
38. Rohrbough J, Spitzer N C 1999 Ca (2+)-permeable AMPA receptors and spontaneous presynaptic transmitter release at developing excitatory spinal synapses. Journal of Neuroscience 19(19): 8528–41
39. Safronov B V 1999 Spatial distribution of Na⁺ and K⁺ channels in spinal dorsal horn neurones: Role of the soma, axon and dendrites in spike generation. Progress in Neurobiology 59(3): 217–41
40. Santoro B, Chen S, Luthi A, Pavlidis P, Shumyatsky G P, Tibbs G R, Siegelbaum S A 2000 Molecular and functional heterogeneity of hyperpolarization-activated pacemaker channels in the mouse CNS. Journal of Neuroscience 20(14): 5264–75
41. Soderling T R, Derkach V A 2000 Postsynaptic protein phosphorylation and LTP. Trends in Neurosciences 23: 75–80
42. Spitzer N C, Gu X, Olson E 1994 Action potentials, calcium transients and the control of differentiation of excitable cells. Current Opinion in Neurobiology 4: 70–7
43. Thibault O, Porter N M, Chen K C, Blalock E M, Kaminker P G, Clodfelter G V, Brewer L D, Landfield P W 1998 Calcium dysregulation in neuronal aging and Alzheimer’s disease: History and new directions. Cell Calcium 24(5–6): 417–33
44. Wilson C J, Callaway J C 2000 Coupled oscillator model of the dopaminergic neuron of the substantia nigra. Journal of Neurophysiology 83(5): 3084–100
45. Wischmeyer E, Doring F, Karschin A 1998 Acute suppression of inwardly rectifying K_ir1 channels by direct tyrosine kinase phosphorylation. Journal of Biological Chemistry 273(51): 34063–8
46. Yuste R, Majewska A, Holthoff K 2000 From form to function: calcium compartmentalization in dendritic spines. Nature Neuroscience 3(7): 653–9
47. Zamponi G W, Snutch T P 1998 Modulation of voltagedependent calcium channels by G proteins. Current Opinion in Neurobiology 8: 351–6
48. Zhang S, Ehlers M D, Bernhardt J P, Su C-T, Huganir R L 1998 Calmodulin mediates calcium-dependent inactivation of Nmethyl-D-aspartate receptors. Neuron 21: 443–53
49. Zucker R S 1999 Calcium and activity-dependent synaptic plasticity. Current Opinion in Neurobiology 9: 305–13
50. Zuhlke R D, Pitt G S, Deisseroth K, Tsien R W, Reuter H 1999 Calmodulin supports both inactivation and facilitation of Ltype calcium channels. Nature 399: 159–62