Critical dependence of cAMP response element-binding protein phosphorylation on L-type calcium channels supports a selective response to EPSPs in preference to action potentials

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Ca2+ microdomains near plasma membrane Ca2+ channels: impact on cell function

In eukaryotic cells, a rise in cytoplasmic Ca(2+) can activate a plethora of responses that operate on time scales ranging from milliseconds to days. Inherent to the use of a promiscuous signal like Ca(2+) is the problem of specificity: how can Ca(2+) activate some responses but not others? We now know that the spatial profile of the Ca(2+) signal is important Ca(2+) does not simply rise uniformly throughout the cytoplasm upon stimulation but can reach very high levels locally, creating spatial gradients. The most fundamental local Ca(2+) signal is the Ca(2+) microdomain that develops rapidly near open plasmalemmal Ca(2+) channels like voltage-gated L-type (Cav1.2) and store-operated CRAC channels. Recent work has revealed that Ca(2+) microdomains arising from these channels are remarkably versatile in triggering a range of responses that differ enormously in both temporal and spatial profile. Here, I delineate basic features of Ca(2+) microdomains and then describe how these highly local signals are used by Ca(2+)-permeable channels to drive cellular responses.

Action potentials: to the nucleus and beyond

The neuronal nucleus is now widely accepted as playing a vital role in maintaining long-term changes in synaptic effectiveness. To act, however, the nucleus must be appropriately relayed with information regarding the latest round of synaptic plasticity. Several constraints of doing so in a neuron pertain to the often significant spatial distance of synapses from the nucleus and the number of synapses required for such a signal to reach functional levels in the nucleus. Largely based on the sensitivity of transcriptional responses to NMDA receptor antagonists, it has been postulated that the signals are physically relayed by biochemical messengers from the synapse to the nucleus. Alternatively, a second, less often considered but equally viable method of signal transduction may be initiated by action potentials generated proximal to the nucleus, wherefrom the signal can be relayed directly by calcium or indirectly by biochemical second messengers. We consider action potential-dependent signaling to the nucleus to have its own computational advantages over the synapse-to-nucleus signal for some functions. This minireview summarizes the logic and experimental support for these two modes of signaling and attempts to validate the action potential model as playing an important role in transcriptional regulation relating specifically to long-term synaptic plasticity.

Refinement of the retinogeniculate pathway

Much of our present understanding about the mechanisms contributing to the activity-dependent refinement of sensory connections comes from experiments done in the retinogeniculate pathway. In recent years the mouse has emerged as a model system of study. This review outlines the major changes in connectivity that occur in this species and a potential mechanism that can account for such remodelling. During early postnatal life when spontaneous activity of retinal ganglion cells sweeps across the retina in waves, retinal projections from the two eyes to the dorsal lateral geniculate nucleus (LGN) segregate to form non-overlapping eye-specific domains. There is a loss of binocular innervation, a pruning of excitatory inputs from a dozen or more to one or two, and the emergence of inhibitory circuitry. Many of these changes underlie the development of precise eye-specific visual maps and receptive field structure of LGN neurons. Retinal activity plays a major role both in the induction and maintenance of this refinement. The activity-dependent influx of Ca(2+) through L-type channels and associated activation of CREB signalling may underlie the pruning and stabilization of developing retinogeniculate connections.

Spinal cord injury causes plasticity in a subpopulation of lamina I GABAergic interneurons

Dysfunction of the spinal GABAergic system has been implicated in pain syndromes following spinal cord injury (SCI). Since lamina I is involved in nociceptive and thermal signaling, we characterized the effects of chronic SCI on the cellular properties of its GABAergic neurons fluorescently identified in spinal slices from GAD67-GFP transgenic mice. Whole cell recordings were obtained from the lumbar cord of 13- to 17-day-old mice, including those having had a thoracic segment (T8-11) removed 6-9 days prior to experiments. Following chronic SCI, the distribution, incidence, and firing classes of GFP+ cells remained similar to controls, and there were minimal changes in membrane properties in cells that responded to current injection with a single spike. In contrast, cells displaying tonic/initial burst firing had more depolarized membrane potentials, increased steady-state outward currents, and increased spike heights. Moreover, higher firing frequencies and spontaneous plateau potentials were much more prevalent after chronic SCI, and these changes occurred predominantly in cells displaying a tonic firing pattern. Persistent inward currents (PICs) were observed in a similar fraction of cells from spinal transects and may have contributed to these plateaus. Persistent Na+ and L-type Ca2+ channels likely contributed to the currents as both were identified pharmacologically. In conclusion, chronic SCI induces a plastic response in a subpopulation of lamina I GABAergic interneurons. Alterations are directed toward amplifying neuronal responsiveness. How these changes alter spinal sensory integration and whether they contribute to sensory dysfunction remains to be elucidated.

Dendritic excitability and synaptic plasticity

Most synaptic inputs are made onto the dendritic tree. Recent work has shown that dendrites play an active role in transforming synaptic input into neuronal output and in defining the relationships between active synapses. In this review, we discuss how these dendritic properties influence the rules governing the induction of synaptic plasticity. We argue that the location of synapses in the dendritic tree, and the type of dendritic excitability associated with each synapse, play decisive roles in determining the plastic properties of that synapse. Furthermore, since the electrical properties of the dendritic tree are not static, but can be altered by neuromodulators and by synaptic activity itself, we discuss how learning rules may be dynamically shaped by tuning dendritic function. We conclude by describing how this reciprocal relationship between plasticity of dendritic excitability and synaptic plasticity has changed our view of information processing and memory storage in neuronal networks.

Electrotonic signals along intracellular membranes may interconnect dendritic spines and nucleus

Synapses on dendritic spines of pyramidal neurons show a remarkable ability to induce phosphorylation of transcription factors at the nuclear level with a short latency, incompatible with a diffusion process from the dendritic spines to the nucleus. To account for these findings, we formulated a novel extension of the classical cable theory by considering the fact that the endoplasmic reticulum (ER) is an effective charge separator, forming an intrinsic compartment that extends from the spine to the nuclear membrane. We use realistic parameters to show that an electrotonic signal may be transmitted along the ER from the dendritic spines to the nucleus. We found that this type of signal transduction can additionally account for the remarkable ability of the cell nucleus to differentiate between depolarizing synaptic signals that originate from the dendritic spines and back-propagating action potentials. This study considers a novel computational role for dendritic spines, and sheds new light on how spines and ER may jointly create an additional level of processing within the single neuron.

Our study incorporates the fact that the endoplasmic reticulum (ER) forms a complete continuum from the spine head to the nuclear envelope and suggests that electrical current flow in a neuron may be better described by a cable-within-a-cable system, where synaptic current flows simultaneously in the medium between the cell membrane and the ER, and within the ER (the internal cable). Our paper provides a novel extension to the classical cable theory (namely, cable-within-cable theory) and presents several interesting predictions. We show that some of these predictions are supported by recent experiments, whereas the principal hypothesis may shed new light on some puzzling observations related to signaling from synapse-to-nucleus. Overall, we show that intracellular-level electrophysiology may introduce principles that appear counter-intuitive with views originating from conventional cellular-level electrophysiology.

D1 dopamine receptor activation of NFAT-mediated striatal gene expression

Exposure to drugs of abuse activates gene expression and protein synthesis that result in long-lasting adaptations in striatal signaling. Therefore, identification of the transcription factors that couple drug exposure to gene expression is of particular importance. Members of the nuclear factor of activated T-cells (NFATc) family of transcription factors have recently been implicated in shaping neuronal function throughout the rodent nervous system. Here we demonstrate that regulation of NFAT-mediated gene expression may also be a factor in drug-induced changes to striatal functioning. In cultured rat striatal neurons, stimulation of D1 dopamine receptors induces NFAT-dependent transcription through activation of L-type calcium channels. Additionally, the genes encoding inositol-1,4,5-trisphosphate receptor type 1 and glutamate receptor subunit 2 are regulated by striatal NFATc4 activity. Consistent with these in-vitro data, repeated exposure to cocaine triggers striatal NFATc4 nuclear translocation and the up-regulation of inositol-1,4,5-trisphosphate receptor type 1 and glutamate receptor subunit 2 gene expression in vivo, suggesting that cocaine-induced increases in gene expression may be partially mediated through activation of NFAT-dependent transcription. Collectively, these findings reveal a novel molecular pathway that may contribute to the enduring modifications in striatal functioning that occur following the administration of drugs of abuse.

Functional role of the secondary visual cortex in multisensory facilitation in rats

Recent studies reveal that multisensory convergence can occur in early sensory cortical areas. However, the behavioral importance of the multisensory integration in such early cortical areas is unknown. Here, we used c-Fos immunohistochemistry to explore neuronal populations specifically activated during the facilitation of reaction time induced by the temporally congruent audiovisual stimuli in rats. Our newly developed analytical method for c-Fos mapping revealed a pronounced up-regulation of c-Fos expression particularly in layer 4 of the lateral secondary visual area (V2L). A local injection of a GABA A receptor agonist, muscimol, into V2L completely suppressed the audiovisual facilitation of reaction time without affecting responses to unimodal stimuli. Such a selective suppression was not found following the injection of muscimol into the primary auditory and visual areas. To examine whether or not the rats might have shown the facilitated responses because of increment of stimulus intensity caused by the two modal stimuli, the behavioral facilitation induced by the high-intensity unimodal stimuli was tested by the injection of muscimol into V2L, which turned out not to affect the facilitation. These results suggest that V2L, an early visual area, is critically involved in the multisensory facilitation of reaction time induced by the combination of auditory and visual stimuli.

Evidence for an extended duration of GABA-mediated excitation in the developing male versus female hippocampus

Gamma-aminobutyric acid (GABA) is as an excitatory neurotransmitter during brain development. Activation of GABA(A) receptors in neonatal rat hippocampus results in chloride efflux and membrane depolarization sufficient to open voltage sensitive calcium channels. As development progresses, there is a decline in the magnitude of calcium influx subsequent to GABA(A) receptor activation and the number of cells that respond to GABA with excitation. By the second postnatal week in the rat, GABA action in the hippocampus is predominantly inhibitory. The functional consequences and endogenous regulation of developmental GABA-mediated excitation remains under-explored. Hippocampal neurons in the newborn male and female rat respond to GABA(A) receptor activation with increased intracellular calcium and are susceptible to GABA-mediated damage -- both being indicative of the excitatory nature of GABA. In the present study we observed that by postnatal day 7, only males are susceptible to GABA(A) agonist-induced damage and respond to GABA(A) agonist administration with elevated levels of intracellular calcium in cultured hippocampal neurons. By postnatal day 14, GABA(A) agonist administration was without effect on intracellular calcium in both males and females. The age-related sex difference in the impact of GABA(A) receptor activation correlates with a sex difference in chloride co-transporter expression. Males have elevated protein levels of pNKCC1 on PN0 and PN7, with no sex difference by PN14. In contrast, females displayed elevated levels of KCC2 on PN7. This converging evidence infers that sex affects the duration of GABA(A) receptor-mediated excitation during normal hippocampal development, and provides a mechanism by which the effect is mediated.

Cav1.2 calcium channels modulate the spiking pattern of hippocampal pyramidal cells

Ca(v)1.2 L-type calcium channels support hippocampal synaptic plasticity, likely by facilitating dendritic Ca2+ influx evoked by action potentials (AP) back-propagated from the soma. Ca2+ influx into hippocampal neurons during somatic APs is sufficient to activate signalling pathways associated with late phase LTP. Thus, mechanisms controlling AP firing of hippocampal neurons are of major functional relevance. We examined the excitability of CA1 pyramidal cells using somatic current-clamp recordings in brain slices from control type mice and mice with the Ca(v)1.2 gene inactivated in principal hippocampal neurons. Lack of the Ca(v)1.2 protein did not affect either affect basic characteristics, such as resting membrane potential and input resistance, or parameters of single action potentials (AP) induced by 5 ms depolarising current pulses. However, CA1 hippocampal neurons from control and mutant mice differed in their patterns of AP firing during 500 ms depolarising current pulses: threshold voltage for repetitive firing was shifted significantly by about 5 mV to more depolarised potentials in the mutant mice (p<0.01), and the latency until firing of the first AP was prolonged (73.2+/-6.6 ms versus 48.1+/- 7.8 ms in control; p<0.05). CA1 pyramidal cells from the mutant mice also showed a lowered initial spiking frequency within an AP train. In control cells, isradipine had matching effects, while BayK 8644 facilitated spiking. Our data demonstrate that Ca(v)1.2 channels are involved in regulating the intrinsic excitability of CA1 pyramidal neurons. This cellular mechanism may contribute to the known function of Ca(v)1.2 channels in supporting synaptic plasticity and memory.

A role for acetylcholine receptors in their own aggregation on muscle cells

Both neurotrophic factors and activity regulate synaptogenesis. At neuromuscular synapses, the neural factor agrin released from motor neuron terminals stimulates postsynaptic specialization by way of the muscle specific kinase MuSK. In addition, activity through acetylcholine receptors (AChRs) has been implicated in the stabilization of pre- and postsynaptic contacts on muscle at various stages of development. We show here that activation of AChRs with specific concentrations of nicotine is sufficient to induce AChR aggregation and that this induction requires the function of L-type calcium channels (L-CaChs). Furthermore, AChR function is required for agrin induced AChR aggregation in C2 muscle cells. The same concentrations of nicotine did not induce observable tyrosine phosphorylation on either MuSK or the AChR beta subunit, suggesting significant differences between the mechanisms of agrin and activity induced aggregation. The AChR/L-CaCh pathway provides a mechanism by which neuromuscular signal transmission can act in concert with the agrin-MuSK signaling cascade to regulate NMJ formation.

Critical role of type 2 ryanodine receptor in mediating activity-dependent neurogenesis from embryonic stem cells

Activity-induced neurogenesis via Ca(2+) entry may be important for establishing Hebbian neural network. However, it remains unclear whether intracellular Ca(2+) mobilization is required and which subtypes of Ca(2+) release channels expressed in Ca(2+) store organelles are involved in the activity-dependent neurogenesis. Here, we demonstrated that the activity of intracellular Ca(2+) signaling, expression of neuronal transcription factor NeuroD, and the rate of neurogenesis were significantly inhibited in neuronal cells derived from embryonic stem (ES) cells deficient in the Ca(2+) release channel type 2 ryanodine receptors (RyR2(-/-)). In wild-type (RyR2(+/+)) but not in RyR2(-/-) ES cells, activation of L-type Ca(2+) channels, GABA(A) receptors, or RyRs promoted neuronal differentiation, while inhibition of these channels/receptors had an opposite effect. Moreover, neuronal differentiation promoted by activation of GABA(A) receptors or L-type Ca(2+) channels in RyR2(+/+) cells was prevented by RyR inhibitors. No significant difference was detected in the expression level of GABA(A) receptors and L-type channels between neuronal cells derived from two types of ES cells. Thus, activity-induced Ca(2+) influx through L-type Ca(2+) channels alone is not sufficient in promoting neurogenesis. Instead, an intimate cooperation of L-type Ca(2+) channels with RyR2 is crucial for the activity-dependent neurogenesis induced by paracrine and/or autocrine GABA signaling.

AKAP79/150 anchoring of calcineurin controls neuronal L-type Ca2+ channel activity and nuclear signaling

Neuronal L-type calcium channels contribute to dendritic excitability and activity-dependent changes in gene expression that influence synaptic strength. Phosphorylation-mediated enhancement of L-type channels containing the CaV1.2 pore-forming subunit is promoted by A-kinase anchoring proteins (AKAPs) that target cAMP-dependent protein kinase (PKA) to the channel. Although PKA increases L-type channel activity in dendrites and dendritic spines, the mechanism of enhancement in neurons remains poorly understood. Here, we show that CaV1.2 interacts directly with AKAP79/150, which binds both PKA and the Ca2+/calmodulin-activated phosphatase calcineurin (CaN). Cotargeting of PKA and CaN by AKAP79/150 confers bidirectional regulation of L-type current amplitude in transfected HEK293 cells and hippocampal neurons. However, anchored CaN dominantly suppresses PKA enhancement of the channel. Additionally, activation of the transcription factor NFATc4 via local Ca2+ influx through L-type channels requires AKAP79/150, suggesting that this signaling complex promotes neuronal L channel signaling to the nucleus through NFATc4.

Ca2+ influx through both L- and N-type Ca2+ channels increases c-fos expression by electrical stimulation of sympathetic neurons

During direct membrane depolarization, Ca2+ influx primarily through L-type Ca2+ (L-) channels initiates activity-dependent gene transcription. This is surprising given that in most neurons a minority of the total Ca2+ current arises from L-channel activity. However, many studies have stimulated Ca2+ influx with unphysiological stimuli such as chronic membrane depolarization using high K+ medium. Few studies have tested whether other Ca2+ channels stimulate gene transcription in adult neurons as a consequence of direct electrical stimulation. Therefore, we evaluated the role of L- and N-type Ca2+ (N-) channel activity in regulating mRNA levels of c-fos, an activity-dependent transcription factor, in adult rat superior cervical ganglion (SCG) neurons as the majority of Ca2+ channels are N-type, while only a minority are L-type. Changes in c-fos mRNA levels were measured using semi-quantitative and single-cell RT-PCR. Phosphorylation of CREB (pCREB) and changes in c-Fos levels were visualized in dissociated cells by immunocytochemistry. Increases in pCREB, c-fos mRNA and c-Fos protein with either K+ or electrical depolarization required Ca2+ influx. These results support previous findings that elevated c-fos levels result from pCREB stimulating c-fos transcription. Elevation of pCREB, c-fos and c-Fos with K+ depolarization depended on L-channel activity. By contrast, antagonizing either channel at 10-Hz stimulation minimized these increases despite unequal numbers of the two channel types. Transition to exclusive L-channel involvement occurred with increasing frequency of stimulation (from 10 to 20 to 50 Hz). Our results demonstrate that N- and L-channel participation in regulating c-fos expression is encoded in the pattern of electrical stimulation.

Distribution of IP3-mediated calcium responses and their role in nuclear signalling in rat basolateral amygdala neurons

Metabotropic receptor activation is important for learning, memory and synaptic plasticity in the amygdala and other brain regions. Synaptic stimulation of metabotropic receptors in basolateral amygdala (BLA) projection neurons evokes a focal rise in free Ca(2+) in the dendrites that propagate as waves into the soma and nucleus. These Ca(2+) waves initiate in the proximal dendrites and show limited propagation centrifugally away from the soma. In other cell types, Ca(2+) waves have been shown to be mediated by either metabotropic glutamate receptor (mGluR) or muscarinic receptor (mAChR) activation. Here we show that mGluRs and mAChRs act cooperatively to release Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive intracellular Ca(2+) stores. Whereas action potentials (APs) alone were relatively ineffective in raising nuclear Ca(2+), their pairing with metabotropic receptor activation evoked an IP(3)-receptor-mediated Ca(2+)-induced Ca(2+) release, raising nuclear Ca(2+) into the micromolar range. Metabotropic-receptor-mediated Ca(2+)-store release was highly compartmentalized. When coupled with metabotropic receptor stimulation, large robust Ca(2+) rises and AP-induced amplification were observed in the soma, nucleus and sparsely spiny dendritic segments with metabotropic stimulation. In contrast, no significant amplification of the Ca(2+) transient was detected in spine-dense high-order dendritic segments. Ca(2+) rises evoked by photolytic uncaging of IP(3) showed the same distribution, suggesting that IP(3)-sensitive Ca(2+) stores are preferentially located in the soma and proximal dendrites. This distribution of metabotropic-mediated store release suggests that the neuromodulatory role of metabotropic receptor stimulation in BLA-dependent learning may result from enhanced nuclear signalling.

L-type calcium channels mediate acetylcholine receptor aggregation on cultured muscle

Agrin activation of muscle specific kinase (MuSK) initiates postsynaptic development on skeletal muscle that includes the aggregation of acetylcholine receptors (AChRs; Glass et al. [1996]: Cell 85: 513-523; Gautam et al. [1996]: Cell 85: 525-535). Although the agrin/MuSK signaling pathway remains largely unknown, changes in intracellular calcium levels are required for agrin-induced AChR aggregation (Megeath and Fallon [1998]: J Neurosci 18: 672-678). Here, we show that L-type calcium channels (L-CaChs) are required for full agrin-induced aggregation of AChRs and sufficient to induce agrin-independent AChR aggregation. Blockade of L-CaChs in muscle cultures inhibited agrin-induced AChR aggregation but not tyrosine phosphorylation of MuSK or AChR beta subunits. Activation of L-CaChs in the absence of agrin induced AChR aggregation but not tyrosine phosphorylation of MuSK or AChR beta subunits. Agrin responsiveness was significantly reduced in primary muscle cultures from the muscular dysgenesis mouse, a natural mutant, which does not express the L-CaCh. Our results establish a novel role for L-CaChs as important sources of the intracellular calcium necessary for the aggregation of AChRs.

Transcriptional profiling of depolarization-dependent phenotypic alterations in primary cultures of developing granule neurons

Rat cerebellar granule neurons cultured in medium supplemented with elevated KCl are extensively used as a model to examine the coupling between neural activity and Ca(2+)-dependent gene expression. Elevated (25 mM) KCl is believed to mimic endogenous neural activity because it promotes depolarization and Ca(+2)-dependent survival and some aspects of maturation. By comparison, at least half of the granule neurons grown in standard medium containing 5 mM KCl undergo apoptosis beginning approximately 4 days in vitro. However, accumulating evidence suggests that chronic depolarization induces phenotypic abnormalities whereas growth in chemically defined medium containing 5 mM KCl more closely resembles the constitutive phenotype. To examine this, oligonucleotide microarrays and RT-PCR of selected mRNAs were used to compare transcription profiles of cultures grown in 5 mM and 25 mM KCl. In some cases, N-methyl-D-aspartate (NMDA) which, like elevated KCl, promotes long-term survival was also tested. Robust changes in several gene groups were observed and indicated that growth in elevated KCl: induces expression of mRNAs that are not normally observed; represses expression of mRNAs that should be present; maintains expression of mRNAs that are markers of immature neurons. Supplementation of the growth medium with NMDA instead of elevated KCl produces similar abnormalities. Altogether, these data indicate that growth in 5 mM KCl more closely mimics survival and maturation of granule neurons in vivo and should therefore be adopted in future studies.

L-type Ca2+ channels contribute to current-evoked spike firing and associated Ca2+ signals in cerebellar Purkinje neurons

The physiological properties of Purkinje neurons play a central role in their ability to regulate information transfer through the cerebellum. A number of ion channels contribute to Purkinje neuron physiology including an abundance of P-type Ca2+ channels, particularly in the dendritic region. Purkinje neurons also express L-type Ca2+ channels both during development and in the mature state. However, a role for L-type channels in Purkinje neuron physiology has yet to be fully defined. In the current study we used physiological recordings from cultured Purkinje neurons and the L-type Ca2+ channel agonist S-(-)-Bay K to assess a potential role for L-type Ca2+ channels in spike firing. Results show that Bay K alters current-evoked spike firing in young, immature Purkinje neurons without dendritic structure and in older, more mature Purkinje neurons with dendritic structure. Bay K also enhanced Ca2+ signals associated with the current-evoked spike firing. The effect of Bay K was more prominent in the young Purkinje neurons than in the older Purkinje neurons, suggesting that L-type Ca2+ channels may be more important in the Purkinje neuron physiology during the early stages of development rather than at mature stages. In the older Purkinje neurons, immunohistochemical studies using antibodies to L-type Ca2+ channels showed more intense immunolabeling in the somatic region than in the dendritic region. This result suggests that L-type Ca2+ channels may play a more important role in somatic physiology than dendritic physiology, whereas P-type channels may play a more important role in dendritic physiology.

Calcium-binding proteins in GABAergic calyciform synapses of the reticular nucleus

Large calyciform synapses in the rat reticular thalamic nucleus are characterized by the presence of gamma-aminobutyric acid. Presynaptic terminals are also loaded with calcium-binding proteins such as parvalbumin, calbindin, calretinin and calcineurin. The number of calyciform terminals containing gamma-aminobutyric acid and parvalbumin is 2005 in young adult rats; calbindin is present in 1,500, calretinin in 850 and calcineurin in 560 calyciform terminals. Developmental studies revealed that gamma-aminobutyric acid and calcium-binding proteins are virtually absent from calyciform terminals at birth but their occurrence increased considerably during postnatal life, suggesting increasing regulation of presynaptic calcium signaling during postnatal life. It is concluded that synaptic activity of large calyciform gamma-aminobutyric acid-containing synapses of the reticular thalamic nucleus is mediated, regulated or accompanied by calcium ions.

Supersensitive Ras activation in dendrites and spines revealed by two-photon fluorescence lifetime imaging

To understand the biochemical signals regulated by neural activity, it is necessary to measure protein-protein interactions and enzymatic activity in neuronal microcompartments such as axons, dendrites and their spines. We combined two-photon excitation laser scanning with fluorescence lifetime imaging to measure fluorescence resonance energy transfer at high resolutions in brain slices. We also developed sensitive fluorescent protein-based sensors for the activation of the small GTPase protein Ras with slow (FRas) and fast (FRas-F) kinetics. Using FRas-F, we found in CA1 hippocampal neurons that trains of back-propagating action potentials rapidly and reversibly activated Ras in dendrites and spines. The relationship between firing rate and Ras activation was highly nonlinear (Hill coefficient approximately 5). This steep dependence was caused by a highly cooperative interaction between calcium ions (Ca(2+)) and Ras activators. The Ras pathway therefore functions as a supersensitive threshold detector for neural activity and Ca(2+) concentration.

Selective elimination of glutamatergic synapses on striatopallidal neurons in Parkinson disease models

Parkinson disease is a common neurodegenerative disorder that leads to difficulty in effectively translating thought into action. Although it is known that dopaminergic neurons that innervate the striatum die in Parkinson disease, it is not clear how this loss leads to symptoms. Recent work has implicated striatopallidal medium spiny neurons (MSNs) in this process, but how and precisely why these neurons change is not clear. Using multiphoton imaging, we show that dopamine depletion leads to a rapid and profound loss of spines and glutamatergic synapses on striatopallidal MSNs but not on neighboring striatonigral MSNs. This loss of connectivity is triggered by a new mechanism-dysregulation of intraspine Cav1.3 L-type Ca(2+) channels. The disconnection of striatopallidal neurons from motor command structures is likely to be a key step in the emergence of pathological activity that is responsible for symptoms in Parkinson disease.

The interdependence and independence of amacrine cell dendrites: patch-clamp recordings and simulation studies on cultured GABAergic amacrine cells

Previously we reported that cultured rat GABAergic amacrine cells can evoke subthreshold graded depolarization and action potentials. Both types of electrical signals are thought to contribute to neurotransmitter release from their dendrites, because Ca(2+) channels in amacrine cells can be activated at a subthreshold level (around -50 mV). The aim of the present study is to describe the spatiotemporal pattern of the spread of these electrical signals in an amacrine cell, using a computer simulation study. The simulation is based on physiological data, obtained by dual whole-cell patch-clamp recordings on the soma and the dendrites of cultured rat GABAergic amacrine cells. We determined passive and active properties of amacrine cells from the physiological recordings. Then, using the NEURON simulator, we conducted computer simulations on a reconstructed model of amacrine cells. We show that graded potentials and action potentials spread through amacrine cells with distinct patterns, and discuss the electrical interrelationship among the dendrites of an amacrine cell. Subthreshold graded potentials applied to a distal dendrite were sufficiently localized, so that each dendrite could behave independently (dendritic independence). However, at a suprathreshold level, once action potentials were triggered, they propagated into every dendrite, exciting the entire cell (dendritic interdependence). We also showed that GABAergic inhibitory inputs on the dendrites suppress the dendritic interdependence of amacrine cells. These results suggest that an inhibitory amacrine cell can mediate both local and wide-field lateral inhibition, regulated by the spatiotemporal pattern of excitatory and inhibitory synaptic inputs on its dendrites.

Functional compensation by other voltage-gated Ca2+ channels in mouse basal forebrain neurons with Ca(V)2.1 mutations

Tottering (tg/tg) and leaner (tg(la)/tg(la)) mutant mice exhibit distinct mutations in the gene encoding the voltage-activated Ca(2+) channel alpha(1A) subunit (CACNA1A), the pore-forming subunit of the Ca(V)2.1 (P/Q type) Ca(2+) channels. These mice exhibit absence seizures and deficiencies in motor control and other functions. Previous work in cerebellar Purkinje neurons has shown that these mutations cause dramatic reductions in calcium channel function. Because Purkinje cell somata primarily express the Ca(V)2.1 channels, the general decrease in Ca(V)2.1 channel function is observed as a profound decrease in whole-cell current. In contrast to Purkinje cells, basal forebrain (BF) neurons express all of the Ca(2+) channel alpha(1) subunits, with Ca(V)2.1 contributing approximately 30% to the whole-cell current in wild-type (+/+) mice. Here, we show that whole-cell Ba(2+) current densities in BF neurons are not reduced in the mutant genotypes despite a reduction in the Ca(V)2.1 contribution. By blocking the different Ca(2+) channel subtypes with specific pharmacological agents, we found a significant increase in the proportion of Ca(V)1 Ca(2+) current in mutant phenotypes. There was no change in tissue mRNA expression of calcium channel subtypes Ca(V)2.1, Ca(V)2.2, Ca(V)1.2, Ca(V)1.3, and Ca(V)2.3 in the tottering and leaner mutant mice. These results suggest that Ca(V)1 channels may functionally upregulate to compensate for reduced Ca(V)2.1 function in the mutants without an increase in Ca(v)1 message. Single-cell reverse transcription polymerase chain reaction (RT-PCR) experiments in a subset of sampled neurons revealed that approximately 90% of the cells could be considered cholinergic based on choline acetyltransferase (ChAT) mRNA expression.

Structural and functional composition of the developing retinogeniculate pathway in the mouse

The advent of transgenic mice has made the developing retinogeniculate pathway a model system for targeting potential mechanisms that underlie the refinement of sensory connections. However, a detailed characterization of the form and function of this pathway is lacking. Here we use a variety of anatomical and electrophysiological techniques to delineate the structural and functional changes occurring in the lateral geniculate nucleus (LGN) of dorsal thalamus of the C57/BL6 mouse. During the first two postnatal weeks there is an age-related recession in the amount of terminal space occupied by retinal axons arising from the two eyes. During the first postnatal week, crossed and uncrossed axons show substantial overlap throughout most of the LGN. Between the first and second week retinal arbors show significant pruning, so that by the time of natural eye opening (P12–14) segregation is complete and retinal projections are organized into distinct eye-specific domains. During this time of rapid anatomical rearrangement, LGN cells could be readily distinguished using immunocytochemical markers that stain for NMDA receptors, GABA receptors, L-type Ca2+channels, and the neurofilament protein SMI-32. Moreover, the membrane properties and synaptic responses of developing LGN cells are remarkably stable and resemble those of mature neurons. However, there are some notable developmental changes in synaptic connectivity. At early ages, LGN cells are binocularly responsive and receive input from as many as 11 different retinal ganglion cells. Optic tract stimulation also evokes plateau-like depolarizations that are mediated by the activation of L-type Ca2+channels. As retinal inputs from the two eyes segregate into nonoverlapping territories, there is a loss of binocular responsiveness, a decrease in retinal convergence, and a reduction in the incidence of plateau potentials. These data serve as a working framework for the assessment of phenotypes of genetically altered strains as well as provide some insight as to the molecular mechanisms underlying the refinement of retinogeniculate connections.

Temporal integration of intracellular Ca2+ signaling networks in regulating gene expression by action potentials

Temporal aspects of intracellular calcium signaling are particularly important in activity-dependent regulation of gene expression in neurons. This review traces calcium-dependent intracellular signaling from the membrane to the nucleus in response to action-potential firing, and considers how specific genes are regulated by specific patterns of impulse firing. Modes of calcium influx, calcium-dependent protein kinases, transcription factors, individual genes, and genomic analysis are examined, with particular emphasis on the importance of temporal aspects of calcium dynamics in regulating these processes.

Neurobiology and treatment of anxiety: signal transduction and neural plasticity

The stress-dependence and chronic nature of anxiety disorders along with the anxiolytic effectiveness of antidepressant drugs suggests that neuronal plasticity may play a role in the pathophysiology of anxiety. Intracellular signaling pathways are known in many systems to be critical links in the cascades from surface signals to the molecular alterations that result in functional plasticity. Chronic antidepressant treatments can regulate intracellular signaling pathways and can induce molecular, cellular, and structural changes over time. These changes may be important to the anxiolytic effectiveness of these drugs. In addition, the signaling proteins implicated in the actions of chronic antidepressant action, such as cAMP response element binding protein (CREB), have also been implicated in conditioned fear and in anxiety. The cellular mechanisms underlying conditioned fear indicate roles for additional signaling pathways; however, less is known about such mechanisms in anxiety. The challenge to identify intracellular signaling pathways and related molecular and structural changes that are critical to the etiology and treatment of anxiety will further establish the importance of mechanisms of neuronal plasticity in functional outcome and improve treatment strategies.

Ca2+ and synaptic plasticity

The induction and maintenance of synaptic plasticity is well established to be a Ca2+-dependent process. The use of fluorescent imaging to monitor changes [Ca2+]i in neurones has revealed a diverse array of signaling patterns across the different compartments of the cell. The Ca2+ signals within these compartments are generated by voltage or ligand-gated Ca2+ influx, and release from intracellular stores. The changes in [Ca2+]i are directly linked to the activity of the neurone, thus a neurone's input and output is translated into a dynamic Ca2+ code. Despite considerable progress in measuring this code much still remains to be determined in order to understand how the code is interpreted by the Ca2+ sensors that underlie the induction of compartment-specific plastic changes.

Millisecond-timescale, genetically targeted optical control of neural activity

Temporally precise, noninvasive control of activity in well-defined neuronal populations is a long-sought goal of systems neuroscience. We adapted for this purpose the naturally occurring algal protein Channelrhodopsin-2, a rapidly gated light-sensitive cation channel, by using lentiviral gene delivery in combination with high-speed optical switching to photostimulate mammalian neurons. We demonstrate reliable, millisecond-timescale control of neuronal spiking, as well as control of excitatory and inhibitory synaptic transmission. This technology allows the use of light to alter neural processing at the level of single spikes and synaptic events, yielding a widely applicable tool for neuroscientists and biomedical engineers.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

N-type and L-type calcium channels mediate glycinergic synaptic inputs to retinal ganglion cells of tiger salamanders

Synaptically localized calcium channels shape the timecourse of synaptic release, are a prominent site for neuromodulation, and have been implicated in genetic disease. In retina, it is well established that L-type calcium channels play a major role in mediating release of glutamate from the photoreceptors and bipolar cells. However, little is known about which calcium channels are coupled to synaptic exocytosis of glycine, which is primarily released by amacrine cells. A recent report indicates that glycine release from spiking AII amacrine cells relies exclusively upon L-type calcium channels. To identify calcium channel types controlling neurotransmitter release from the population of glycinergic neurons that drive retinal ganglion cells, we recorded electrical and potassium evoked inhibitory synaptic currents (IPSCs) from these postsynaptic neurons in retinal slices from tiger salamanders. The L-channel antagonist nifedipine strongly inhibited release and FPL64176, an L-channel agonist, greatly enhanced it, indicating a significant role for L-channels. omega-Conotoxin MVIIC, an N/P/Q-channel antagonist, strongly inhibited release, indicating an important role for non-L channels. While the P/Q-channel blocker omega-Aga IVA produced only small effects, the N-channel blocker omega-conotoxin GVIA strongly inhibited release. Hence, N-type and L-type calcium channels appear to play major roles, overall, in mediating synaptic release of glycine onto retinal ganglion cells.

Calcitonin gene-related peptide regulates gene transcription in primary afferent neurons

Although primary afferent neurons express receptors for calcitonin gene-related peptide (CGRP), understanding of the cellular effects of these receptors is limited. We determined that CGRP receptors regulate gene transcription in primary afferent neurons through a cyclic AMP (cAMP)-dependent pathway. CGRP increased cAMP in neonatal dorsal root ganglion (DRG) neurons in a concentration-dependent manner that was blocked by the receptor antagonist CGRP(8-37). The response to CGRP also occurred in adult DRG cells. In contrast, CGRP did not alter the concentration of free intracellular calcium in neonatal or adult DRG neurons. Immunohistochemical data showed that one downstream effect of the cAMP signaling pathway was phosphorylation of cAMP response element binding (CREB) protein, suggesting that CGRP regulates gene expression. This interpretation was supported by evidence that CGRP increased CRE-dependent gene transcription in neurons transiently transfected with a CRE-luciferase DNA reporter construct. The effect of CGRP on gene transcription was inhibited by H89, myristoylated-protein kinase A inhibitor(14-22)-amide and U0126, indicating that protein kinase A and mitogen-activated protein kinase/extracellular receptor kinase kinase are enzymes that mediate effects of CGRP on gene transcription. Therefore, CGRP receptors may regulate expression of proteins by primary afferent neurons during development and in response to tissue-damaging stimuli.

L-Type Calcium Channels: The Low Down

L-type calcium channels couple membrane depolarization in neurons to numerous processes including gene expression, synaptic efficacy, and cell survival. To establish the contribution of L-type calcium channels to various signaling cascades, investigators have relied on their unique pharmacological sensitivity to dihydropyridines. The traditional view of dihydropyridine-sensitive L-type calcium channels is that they are high-voltage–activating and have slow activation kinetics. These properties limit the involvement of L-type calcium channels to neuronal functions triggered by strong and sustained depolarizations. This review highlights literature, both long-standing and recent, that points to significant functional diversity among L-type calcium channels expressed in neurons and other excitable cells. Past literature contains several reports of low-voltage–activated neuronal L-type calcium channels that parallel the unique properties of recently cloned CaV1.3 L-type channels. The fast kinetics and low activation thresholds of CaV1.3 channels stand in stark contrast to criteria currently used to describe L-type calcium channels. A more accurate view of neuronal L-type calcium channels encompasses a broad range of activation thresholds and recognizes their potential contribution to signaling cascades triggered by subthreshold depolarizations.

Granule neurons in cerebellum express distinct splice variants of the inositol trisphosphate receptor that are modulated by calcium

Primary cultures of granule cells (GC) from rat cerebellar cortex were used to determine whether bioelectric activity, via a Ca(2+)/calmodulin-dependent kinase (CaMK) signaling cascade, modulates expression and exon selection in the inositol trisphosphate receptor type 1 (IP(3)R1). IP(3)R1 contains or lacks three exons (S1, S2, and S3) that are regulated in a regionally and temporally specific manner. The neuronal, or long, form of IP(3)R1 is distinguished from peripheral tissues by inclusion of the S2 exon. Although previous studies indicated that IP(3)R1 are undetectable in the cerebellar granular layer in vivo, receptor protein and mRNA are induced in cultured GC grown in medium supplemented with 25 mM KCl or NMDA, two trophic agents that promote long-term survival, compared with GC grown in 5 mM KCl. IP(3)R1 induction in response to 25 mM KCl or NMDA is attenuated by coaddition of voltage-sensitive calcium channel or NMDA receptor antagonists, respectively. Actinomycin D, CaMK, and calcineurin antagonists likewise suppress induction. Unlike the major variants of IP(3)R1 in Purkinje neurons, which lack S1 and S3, GC grown with trophic agents express mRNA containing these exons. Both neuronal types contain S2. Evidence obtained using mutant mice with Purkinje cell lesions, laser-microdissected GC neurons from slices, and explant cultures indicates that GC predominantly express the S1-containing variant of IP(3)R1 in vivo.

Effects of pyrethroids on voltage-sensitive calcium channels: a critical evaluation of strengths, weaknesses, data needs, and relationship to assessment of cumulative neurotoxicity

The Food Quality Protection Act of 1996 requires that the U.S. Environmental Protection Agency conduct cumulative risk assessments for classes of pesticides that have a common mode or mechanism of action. For the pyrethroid insecticides, disruption of voltage-sensitive sodium channel function is generally accepted as the mechanism underlying acute neurotoxicity. However, data exist which suggest that voltage-sensitive calcium (Ca(2+)) channels (VSCC) may also be important targets of pyrethroid action. VSCC are important to neuronal function during development and for neurotransmitter release, gene expression, and electrical excitability in the nervous system. Disruption of these and other processes mediated by VSCC can result in neurotoxicity. If effects on VSCC are demonstrated to contribute to pyrethroid neurotoxicity, then such effects will have to be considered when making decisions regarding cumulative risk of exposure to this class of compounds. This document provides a critical review of the data related to the hypothesis that VSCC are important targets of pyrethroid effects. Data supporting effects of pyrethroids on VSCC have been generated by several different laboratories using different techniques and biological preparations. Thus, the many reports of effects on VSCC provide evidence that pyrethroids may interact with VSCC. However, evidence to support a role of VSCC in pyrethroid neurotoxicity is based entirely on in vitro observations, and numerous limitations exist in these data, including: (1) lack of defined concentration-response relationships, with some effects observed only at relatively high concentrations, (2) the use of indirect measures of VSCC function, (3) data from nonmammalian species, (4) data from studies that have not been peer-reviewed, (5) the need for replication of some effects, and (6) inconsistent or contradictory results from different laboratories/preparations. Thus, at the present time, it is premature to conclude that effects on VSCC play an important role in the acute neurotoxicity of pyrethroid insecticides in mammals. To demonstrate that VSCC are important targets of pyrethroid neurotoxicity in mammals, in vivo studies supporting a role for pyrethroid effects on VSCC are needed. Additional support could be provided by demonstration of direct effects of pyrethroid compounds on mammalian neuronal VSCC in vitro, including demonstration that concentration-response relationships are similar, or greater, in sensitivity to effects of pyrethroids on voltage-sensitive sodium channels. If such effects were to be demonstrated, the rationale for considering VSCC as targets of pyrethroid compounds when assessing cumulative risk would be strengthened. However, at the present time, the data available neither support nor refute conclusively the hypothesis that effects on VSCC are important to the acute neurotoxicity of pyrethroids.

Weak synaptic activity induces ongoing signaling to the nucleus that is enhanced by BDNF and suppressed by low-levels of nicotine

The developing nervous system adapts to a wide array of stimuli, in part, by evoking activity-dependent mechanisms that signal to the nucleus and induce long-term modifications in neuronal function. It is well established that one such stimulus is strong synaptic activity. Our interest, however, is whether weak activity generated at developing synapses also signals to the nucleus and if so, can these signals be modulated by extrinsic factors. Using cultured hippocampal neurons and a highly sensitive readout of CRE-mediated gene expression, we demonstrate that weak synaptic transmission, including non-evoked, spontaneous transmitter release, induces ongoing gene expression. These weak synaptic stimuli, acting through NMDA receptors, signal to the nucleus through a MAPK pathway, without a significant contribution of L-type Ca2+ channels. In addition, we show that BDNF, a molecule that has clear effects on synaptic plasticity, enhances this CRE-dependent gene expression by acting upstream of NMDA receptors. On the other hand, low levels of nicotine, which also effects synaptic plasticity, suppress ongoing CRE-mediated gene expression indirectly by acting on GABAergic neurons; this indirect action on gene expression suggests an alternative mechanism for how nicotine produces long-lasting changes.

Inhibition of α1E Ca2+ Channels by Carbonic Anhydrase Inhibitors

We examined if a range of carbonic anhydrase inhibitors (CAIs) interacted with the high-voltage activated voltage-sensitive calcium channels (VSCCs) encoded by the human alpha(1E) subunit. Whole-cell recordings were made from HEK293 cells stably expressing human alpha(1E)beta(3)-mediated calcium channels. SNX-482 (an alpha(1E) inhibitor) blocked alpha(1E)-mediated VSCCs with an IC(50) close to 10 nM. The anticonvulsant CAI ethoxyzolamide also inhibited these currents, with an IC(50) close to 1 microM, and produced an accompanying 20-mV hyperpolarizing shift in the steady-state inactivation profile. Other structurally diverse CAIs (e.g., acetazolamide and benzolamide) produced approximately 30 - 40% inhibition of alpha(1E)beta(3)-mediated Ca(2+) currents at 10 microM. Topiramate (10 microM), an anticonvulsant with CAI activity, inhibited these currents by 68 +/- 7%. This off-target activity of CAIs at VSCCs may contribute to some of the effects they produce both in vitro and in vivo.

Glucagon-like peptide 1 modulates calcium responses to glutamate and membrane depolarization in hippocampal neurons

Glucagon-like peptide 1 (GLP-1) activates receptors coupled to cAMP production and calcium influx in pancreatic cells, resulting in enhanced glucose sensitivity and insulin secretion. Despite evidence that the GLP-1 receptor is present and active in neurons, little is known of the roles of GLP-1 in neuronal physiology. As GLP-1 modulates calcium homeostasis in pancreatic beta cells, and because calcium plays important roles in neuronal plasticity and neurodegenerative processes, we examined the effects of GLP-1 on calcium regulation in cultured rat hippocampal neurons. When neurons were pre-treated with GLP-1, calcium responses to glutamate and membrane depolarization were attenuated. Whole-cell patch clamp analyses showed that glutamate-induced currents and currents through voltage-dependent calcium channels were significantly decreased in neurons pre-treated with GLP-1. Pre-treatment of neurons with GLP-1 significantly decreased their vulnerability to death induced by glutamate. Acute application of GLP-1 resulted in a transient elevation of intracellular calcium levels, consistent with the established effects of GLP-1 on cAMP production and activation of cAMP response element-binding protein. Collectively, our findings suggest that, by modulating calcium responses to glutamate and membrane depolarization, GLP-1 may play important roles in regulating neuronal plasticity and cell survival.

Decoding of synaptic voltage waveforms by specific classes of recombinant high-threshold Ca(2+) channels

Studies suggest that the preferential role of L-type voltage-sensitive Ca(2+) channels (VSCCs) in coupling strong synaptic stimulation to transcription is due to their selective activation of local chemical events. However, it is possible that selective activation of the L-type channel by specific voltage waveforms also makes a contribution. To address this issue we have examined the response of specific Ca(2+) channel types to simulated complex voltage waveforms resembling those encountered during synaptic plasticity (gamma and theta firing frequency). L-, P/Q- and N-type VSCCs (alpha1C, alpha1A, alpha1B/beta1B/alpha2delta, respectively) were all similarly activated by brief action potential (AP) waveforms or sustained step depolarization. When complex waveforms containing large excitatory postsynaptic potentials (EPSPs), APs and spike accommodation were applied under voltage clamp we found that the integrated L-type VSCC current was approximately three times larger than that produced by the P/Q- or N-type Ca(2+) channels (gamma frequency 1 s stimulation). For P/Q- or N-type channels the complex waveforms led to a smaller current than that expected from the response to a simple 1 s step depolarization to 0 or +20 mV. EPSPs present in the waveforms favoured the inactivation of P/Q- and N-type channels. In contrast, activation of the L-type channel was dependent on both EPSP- and AP-mediated depolarization. Expression of P/Q-type channels with reduced voltage-dependent inactivation (alpha1A/beta2A/alpha2delta) or the use of hyperpolarized intervals between AP stimuli greatly increased their response to complex voltage stimuli. We propose that in response to complex synaptic voltage waveforms P/Q- and N-type channels can undergo selective voltage-dependent inactivation leading to a Ca(2+) current mediated predominantly by L-type channels.

Brain-derived neurotrophic factor activation of NFAT (nuclear factor of activated T-cells)-dependent transcription: a role for the transcription factor NFATc4 in neurotrophin-mediated gene expression

A member of the neurotrophin family, brain-derived neurotrophic factor (BDNF) regulates neuronal survival and differentiation during development. Within the adult brain, BDNF is also important in neuronal adaptive processes, such as the activity-dependent plasticity that underlies learning and memory. These long-term changes in synaptic strength are mediated through alterations in gene expression. However, many of the mechanisms by which BDNF is linked to transcriptional and translational regulation remain unknown. Recently, the transcription factor NFATc4 (nuclear factor of activated T-cells isoform 4) was discovered in neurons, where it is believed to play an important role in long-term changes in neuronal function. Interestingly, NFATc4 is particularly sensitive to the second messenger systems activated by BDNF. Thus, we hypothesized that NFAT-dependent transcription may be an important mediator of BDNF-induced plasticity. In cultured rat CA3-CA1 hippocampal neurons, BDNF activated NFAT-dependent transcription via TrkB receptors. Inhibition of calcineurin blocked BDNF-induced nuclear translocation of NFATc4, thus preventing transcription. Further, phospholipase C was a critical signaling intermediate between BDNF activation of TrkB and the initiation of NFAT-dependent transcription. Both inositol 1,4,5-triphosphate (IP3)-mediated release of calcium from intracellular stores and activation of protein kinase C were required for BDNF-induced NFAT-dependent transcription. Finally, increased expression of IP3 receptor 1 and BDNF after neuronal exposure to BDNF was linked to NFAT-dependent transcription. These results suggest that NFATc4 plays a crucial role in neurotrophin-mediated synaptic plasticity.

Plasticity of calcium channels in dendritic spines

Voltage-sensitive Ca2+ channels (VSCCs) constitute a major source of calcium ions in dendritic spines, but their function is unknown. Here we show that R-type VSCCs in spines of rat CA1 pyramidal neurons are depressed for at least 30 min after brief trains of back-propagating action potentials. Populations of channels in single spines are depressed stochastically and synchronously, independent of channels in the parent dendrite and other spines, implying that depression is the result of signaling restricted to individual spines. Induction of VSCC depression blocks theta-burst-induced long-term potentiation (LTP), indicating that postsynaptic action potentials can modulate synaptic plasticity by tuning VSCCs. Induction of depression requires [Ca2+] elevations and activation of L-type VSCCs, which activate Ca2+/calmodulin-dependent kinase II (CaMKII) and a cyclic adenosine monophosphate (cAMP)-dependent pathway. Given that L-type VSCCs do not contribute measurably to Ca2+ influx in spines, they must activate downstream effectors either directly through voltage-dependent conformational changes or via [Ca2+] microdomains.

Calcium channel and NMDA receptor activities differentially regulate nuclear C/EBPbeta levels to control neuronal survival

Insulin-like growth factor-1 (IGF-1) promotes the survival of cerebellar granule neurons by enhancing calcium influx through L-type calcium channels, whereas NMDA receptor-mediated calcium influx can lead to excitotoxic death. Here we demonstrate that L and NMDA receptor channel activities differentially regulate the transcription factor C/EBPbeta to control neuronal survival. Specifically, we show that L channel-dependent calcium influx results in increased CaMKIV activity, which acts to decrease nuclear C/EBPbeta levels. Conversely, NMDA receptor-mediated influx rapidly elevates nuclear C/EBPbeta and induces excitotoxic death via activation of the calcium-dependent phosphatase, calcineurin. Moderate levels of AMPA receptor activity stimulate L channels to improve survival, whereas higher levels stimulate NMDA receptors and reduce neuronal survival, suggesting differential synaptic effects. Finally, N-type calcium channel activity reduces survival, potentially by increasing glutamate release. Together, these results show that the L-type calcium channel-dependent survival and NMDA receptor death pathways converge to regulate nuclear C/EBPbeta levels, which appears to be pivotal in these mechanisms.

Signaling from synapse to nucleus: the logic behind the mechanisms

Signaling from synapse to nucleus is vital for activity-dependent control of neuronal gene expression and represents a sophisticated form of neural computation. The nature of specific signal initiators, nuclear translocators and effectors has become increasingly clear, and supports the idea that the nucleus is able to make sense of a surprising amount of fast synaptic information through intricate biochemical mechanisms. Information transfer to the nucleus can be conveyed by physical translocation of messengers at various stages within the multiple signal transduction cascades that are set in motion by a Ca(2+) rise near the surface membrane. The key role of synapse-to-nucleus signaling in circadian rhythms, long-term memory, and neuronal survival sheds light on the logical underpinning of these signaling mechanisms.

Interactions with PDZ proteins are required for L-type calcium channels to activate cAMP response element-binding protein-dependent gene expression

After brief periods of heightened stimulation, calcium entry through L-type calcium channels leads to activation of the transcription factor cAMP response element-binding protein (CREB) and CRE-dependent transcription. Many of the details surrounding the mechanism by which L-type calcium channels are privileged in signaling to CREB, to the exclusion of other calcium entry pathways, has remained unclear. We hypothesized that the PDZ interaction sequence contained within the last four amino acids of the calcium channel alpha1C (Ca(V)1.2) subunit [Val-Ser-Asn-Leu (VSNL)] is critical for L-type calcium channels (LTCs) to interact with the signaling machinery that triggers activity-dependent gene expression. To disrupt this interaction, hippocampal CA3-CA1 pyramidal neurons were transfected with DNA encoding for enhanced green fluorescent protein tethered to VSNL (EGFP-VSNL). EGFP-VSNL significantly attenuated L-type calcium channel-induced CREB phosphorylation and CRE-dependent transcription, although somatic calcium concentrations after stimulation remained unchanged. The effect of EGFP-VSNL was specific to the actions of L-type calcium channels, because CREB signaling after NMDA receptor stimulation remained intact. The importance of the PDZ interaction sequence was verified using dihydropyridine (DHP)-insensitive alpha1C subunits. Neurons transfected with alpha1C lacking the terminal five amino acids (DHP-LTCnoPDZ) exhibited attenuated CREB responses in comparison with cells expressing the full-length subunit (DHP-LTC). Collectively, these data suggest that localized calcium responses, regulated by interactions with PDZ domain proteins, are necessary for L-type calcium channels to effectively activate CREB and CRE-mediated gene expression.

Subtype-selective reconstitution of synaptic transmission in sympathetic ganglion neurons by expression of exogenous calcium channels

Fast cholinergic neurotransmission between superior cervical ganglion neurons (SCGNs) in cell culture is initiated by N-type Ca(2+) currents through Ca(v)2.2 channels. To test the ability of different Ca(2+)-channel subtypes to initiate synaptic transmission in these cells, SCGNs were injected with cDNAs encoding Ca(v)1.2 channels, which conduct L-type currents, Ca(v)2.1 channels, which conduct P/Q-type Ca(2+) currents, and Ca(v)2.3 channels, which conduct R-type Ca(2+) currents. Exogenously expressed Ca(v)2.1 channels were localized in nerve terminals, as assessed by immunocytochemistry with subtype-specific antibodies, and these channels effectively initiated synaptic transmission. Injection with cDNA encoding Ca(v)2.3 channels yielded a lower level of presynaptic labeling and synaptic transmission, whereas injection with cDNA encoding Ca(v)1.2 channels resulted in no presynaptic labeling and no synaptic transmission. Our results show that exogenously expressed Ca(2+) channels can mediate synaptic transmission in SCGNs and that the specificity of reconstitution of neurotransmission (Ca(v)2.1 > Ca(v)2.3 >> Ca(v)1.2) follows the same order as in neurons in vivo. The specificity of reconstitution of neurotransmission parallels the specificity of trafficking of these Ca(v) channels to nerve terminals.

Molecular aspects of glutamate dysregulation: implications for schizophrenia and its treatment

The glutamate system is involved in many aspects of neuronal synaptic strength and function during development and throughout life. Synapse formation in early brain development, synapse maintenance, and synaptic plasticity are all influenced by the glutamate system. The number of neurons and the number of their connections are determined by the activity of the glutamate system and its receptors. Malfunctions of the glutamate system affect neuroplasticity and can cause neuronal toxicity. In schizophrenia, many glutamate-regulated processes seem to be perturbed. Abnormal neuronal development, abnormal synaptic plasticity, and neurodegeneration have been proposed to be causal or contributing factors in schizophrenia. Interestingly, it seems that the glutamate system is dysregulated and that N-methyl-D-aspartate receptors operate at reduced activity. Here we discuss how the molecular aspects of glutamate malfunction can explain some of the neuropathology observed in schizophrenia, and how the available treatment intervenes through the glutamate system.

Identification of calcium-dependent and -independent signaling pathways involved in polychlorinated biphenyl-induced cyclic AMP-responsive element-binding protein phosphorylation in developing cortical neurons

Cyclic AMP (cAMP)-responsive element-binding protein (CREB) is a transcription factor important in developing nervous system cells and is activated by a variety of signaling molecules. Aroclor 1254 (A1254), a polychlorinated biphenyl mixture, perturbs Ca(2+) homeostasis and increases CREB phosphorylation in rat neonatal cortical cell cultures in a time- and concentration-dependent manner. The present experiments determined that the cell type responding to A1254 with Ca(2+) increases and phosphorylated CREB (phospho-CREB) was predominantly of neuronal morphology and microtubule-associated protein (MAP2)-positive phenotype. Similarly, glutamate (100 microM) increased phospho-CREB immunoreactivity selectively in MAP2-immunopositive cells. Using Western blotting and immunocytochemical techniques, we identified key signal transduction pathways operative in phosphorylating CREB in cortical cell cultures and examined their participation in 3 ppm A1254-induced CREB activation. Cortical cultures treated with glutamate, forskolin or the phorbol ester phorbol 12-myristate 13-acetate exhibited robust increases in phospho-CREB. Tetrodotoxin (1 microM) completely inhibited CREB phosphorylation by A1254, suggesting that synaptic activity is involved in A1254-induced CREB activation. Buffering [Ca(2+)](i) with bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester in the absence of extracellular Ca(2+) partially inhibited A1254-induced CREB phosphorylation. Inhibition of mitogen-activated protein kinase (10 microM U0126) or protein kinase C (PKC; bisindoylmaleimide, 5 microM) activation did not inhibit A1254-induced CREB phosphorylation. By contrast, inhibition of protein kinase A (PKA) with 100 microM PKA inhibitor peptide, PKI, blocked A1254-induced CREB phosphorylation. Thus, we examined whether A1254 activates PKA by increasing cAMP; 10 microM forskolin, but not A1254, elevated intracellular cAMP levels. These results indicate that in neocortical cells in culture, CREB phosphorylation occurs via Ca(2+)-, PKA-, and PKC-dependent pathways. Furthermore, A1254-induced CREB phosphorylation occurs predominantly in neurons, is dependent on synaptic activity and mediated by Ca(2+)- and PKA-dependent pathways.

Hydrogen peroxide modulation of synaptic plasticity

Unlike the proposed role of reactive oxygen species in neurodegeneration, acute effects of reactive oxygen on synaptic plasticity are poorly understood. Using rat hippocampal slices, we found that exposure to a high concentration (0.5-5 mm) of H(2)O(2) reduces EPSPs in both potentiated and nonpotentiated synapses. Exposure of the slices to 20 microm H(2)O(2) did not affect expression of preestablished long-term potentiation (LTP) but prevented induction of new LTP and enhanced long-term depression (LTD). Surprisingly, 1 microm H(2)O(2) caused a twofold increase in LTP compared with controls, and it further enhanced NMDA-independent LTP. A low concentration of H(2)O(2) also suppressed LTD. Nifedipine, an L-type calcium channel blocker, did not affect control LTP but blocked effects of both 1 and 20 microm H(2)O(2). Calcineurin inhibitors [FK506 (FR900506) and cyclosporin A but not rapamycin] acted similarly and also restored LTP in the presence of 20 microm H(2)O(2). These results suggest that H(2)O(2) alters NMDA-independent, voltage-gated calcium channel-mediated LTP by activating calcineurin.

Taurine-induced synaptic potentiation and the late phase of long-term potentiation are related mechanistically

The application of taurine (2-aminoethanesulfonic acid) induces a long-lasting increase of synaptic efficacy and axon excitability (LLP-TAU) in rat hippocampal CA1 area. After taurine withdrawal, LLP-TAU lasted at least 3 h. This fact prompted us to assess whether the mechanisms involved in the maintenance of this particular potentiation were similar to those implicated in the late phase of long-term potentiation (L-LTP). In the presence of KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase, taurine perfusion (10 mM, 30 min) did not affect the induction of LLP-TAU. However, LLP-TAU maintenance was completely suppressed by KT5720, an inhibitor of the cAMP-dependent protein kinase (PKA). Moreover, the late phase of LLP-TAU was blocked by inhibiting protein synthesis with anisomycin. In addition, taurine perfusion increased the phosphorylation of cAMP response element-binding protein (CREB), although did not affect cAMP levels. These features of LLP-TAU do not appear to be caused by the activation of D1/D5 dopamine receptors, as taurine also induced synaptic potentiation in the presence of SCH23390, an antagonist of this type of receptors. Finally, the late phase of both L-LTP and LLP-TAU occluded mutually. These results suggest that taurine triggers the sequence of some of the molecular events involved in the induction of L-LTP.