Dendritic calcium dynamics

A simple program for simulating the responses of neurons with arbitrarily structured and active dendritic trees

We describe a simple program to simulate neural responses. This program is based on the compartmental approach, in which all compartments of a neuron (i.e. axon, soma or dendrite) are represented by the same basic electrical structure. A parameter file is used to store the model parameters, including the nonlinear channel characteristics of each compartment. The model is then automatically configured according to the values specified in the parameter file. The computation of the conductance of each active channel over time is handled by a unique subroutine optimized according to the kinetics of each channel. The equations for arbitrarily structured trees are solved implicitly using a simple algorithm similar to that of Hines (Hines, M. (1984) Int. J. Biomed. Comput., 15:69-76). The output of the model uses PostScript format. The advantage of this program is that it is small in size, simple to use, efficient, and is platform independent.

Immunohistochemical study of calpain-mediated breakdown products to alpha-spectrin following controlled cortical impact injury in the rat

This study examined the effect of unilateral controlled cortical impact on the appearance of calpain-mediated alpha-spectrin breakdown products (BDPs) in the rat cortex and hippocampus at various times following injury. Coronal sections were taken from animals at 15 min, 1 h, 3 h, 6 h, and 24 h after injury and immunolabeled with an antibody that recognizes calpain-mediated BDPs to alpha-spectrin (Roberts-Lewis et al., 1994). Sections from a separate group of rats were also taken at the same times and stained with hematoxylin and eosin. Analyses of early time points (15 min, 1 h, 3 h, and 6 h following injury) revealed alpha-spectrin BDPs in structurally intact neuronal soma and dendrites in cortex ipsilateral to site of injury that was not present in tissue from sham-injured control rats. By 24 h after injury labeling was not restricted to clearly defined neuronal structures in ipsilateral cortex, although there was an increased extent of diffuse labeling. BDPs to alpha-spectrin in axons were not detected until 24 h after injury, in contrast to the more rapid accumulation of BDPs observed in neuronal soma and dendrites. The presence of BDPs to alpha-spectrin in the cortex at the site of impact, and in the rostral and contralateral cortex, coincided with morphopathology detected by hematoxylin and eosin. alpha-Spectrin BDPs were also observed in the hippocampus ipsilateral to the injury in the absence of overt cell death. This investigation provides further evidence that calpain is activated after controlled cortical impact and could contribute to necrosis at the site of injury. The appearance of calpain-mediated BDPs at sites distal to the contusion site and in the hippocampus also suggests that calpain activation may precede and/or occur in the absence of extensive morphopathological changes.

Mechanisms of calpain proteolysis following traumatic brain injury: implications for pathology and therapy: implications for pathology and therapy: a review and update

Much recent research has focused on the pathological significance of calcium accumulation in the central nervous system (CNS) following cerebral ischemia, spinal cord injury (SCI), and traumatic brain injury (TBI). Disturbances in neuronal calcium homeostasis may result in the activation of several calcium-sensitive enzymes, including lipases, kinases, phosphatases, and proteases. One potential pathogenic event in a number of acute CNS insults, including TBI, is the activation of the calpains, calcium-activated intracellular proteases. This article reviews new evidence indicating that overactivation of calpains plays a major role in the neurodegenerative cascade following TBI in vivo. Further, this article presents an overview from in vivo and in vitro models of CNS injuries suggesting that administration of calpain inhibitors during the initial 24-h period following injury can attenuate injury-induced derangements of neuronal structure and function. Lastly, this review addresses the potential contribution of other proteases to neuronal damage following TBI.

High endogenous calcium buffering in Purkinje cells from rat cerebellar slices

1. The ability of Purkinje cells to rapidly buffer depolarization-evoked intracellular calcium changes (delta [Ca2+]i) was estimated by titrating the endogenous buffer against incremental concentrations of the Ca(2+)-sensitive dye fura-2. 2. In cells from 15-day-old rats, pulse-evoked delta [Ca2+]i were stable during the loading with 0.5 mM fura-2 through the patch pipette. In cells from 6-day-old rats, delta [Ca2+]i decreased by approximately 50% during equivalent experiments. This decrease was not related to changes in Ca2+ influx, since the integral of the Ca2+ currents remained constant throughout the recording. 3. Experiments with high fura-2 concentrations (1.75-3.5 mM) were performed in order to obtain for each cell the curve relating delta [Ca2+]i to fura-2 concentration. From this relationship, values for the Ca2+ binding ratio (the ratio of buffer-bound Ca2+ changes over free Ca2+ changes) were calculated. 4. In Purkinje cells from 15-day-old rats, the Ca2+ binding ratio was approximately 2000, an order of magnitude larger than that of other neurones and neuroendocrine cells studied to date. This Ca2+ binding ratio was significantly smaller (approximately 900) in Purkinje cells from 6-day-old rats. 5. We propose that the large Ca2+ binding ratio of Purkinje cells is related to the presence of large concentrations of Ca2+ binding proteins and that these cells regulate their ability to handle Ca2+ loads during development through changes in the concentration of Ca2+ binding proteins.

A novel calmodulin-binding protein, belonging to the WD-repeat family, is localized in dendrites of a subset of CNS neurons

A rat brain synaptosomal protein of 110,000 M(r) present in a fraction highly enriched in adenylyl cyclase activity was microsequenced (Castets, F., G. Baillat, S. Mirzoeva, K. Mabrouk, J. Garin, J. d'Alayer, and A. Monneron. 1994. Biochemistry. 33:5063-5069). Peptide sequences were used to clone a cDNA encoding a novel, 780-amino acid protein named striatin. Striatin is a member of the WD-repeat family (Neer, E.J., C.J. Schmidt, R. Nambudripad, and T.F. Smith. 1994. Nature (Lond.). 371:297-300), the first one known to bind calmodulin (CaM) in the presence of Ca++. Subcellular fractionation shows that striatin is a membrane-associated, Lubrol-soluble protein. As analyzed by Northern blots, in situ hybridization, and immunocytochemistry, striatin is localized in the central nervous system, where it is confined to a subset of neurons, many of which are associated with the motor system. In particular, striatin is conspicuous in the dorsal part of the striatum, as well as in motoneurons. Furthermore, striatin is essentially found in dendrites, but not in axons, and is most abundant in dendritic spines. We propose that striatin interacts, through its WD-repeat domain and in a CaM/Ca(++)-dependent manner, with one or several members of a surrounding cluster of molecules engaged in a Ca(++)-signaling pathway specific to excitatory synapses.

Elevation of intracellular calcium levels in neurons by nicotinic acetylcholine receptors

The recognition that intracellular free calcium serves as a ubiquitous intracellular signal has motivated efforts to elucidate mechanisms by which cells regulate calcium influx. One route of entry that may offer both spatial and temporal fine resolution for altering calcium levels is that provided by cation-permeable, ligand-gated ion channels. Biophysical measurements as well as calcium imaging techniques demonstrate that neuronal nicotinic acetylcholine receptors as a class have a high relative permeability to calcium; some subtypes equal or exceed all other known receptors in this respect. Activation of nicotinic receptors on neurons can produce substantial increases in intracellular calcium levels by direct passage of calcium through the receptor channel. When multiple classes of nicotinic receptors are expressed by the same neuron, each appears capable of increasing calcium in the cell but may differ with respect to location, temporal response, agonist sensitivity, or regulation in achieving it. As a result, nicotinic receptors must be considered strong candidates for signaling molecules through which neurons regulate a diverse array of cellular events.

Diminished microtubule-associated protein 2 (MAP2) immunoreactivity following cortical impact brain injury

This study employed Western blotting and qualitative immunohistochemistry to analyze the effects of cortical impact traumatic brain injury (TBI) on acute changes in MAP2 immunoreactivity in the rat cortex. We employed a lateral cortical impact injury device to induce severe TBI, which is associated with focal cortical contusion and neuronal death at the impact site. Three hours following TBI, Western blotting detected substantial MAP2 loss only in the cortex ipsilateral to the site of injury. Light microscopic studies of MAP2 revealed a prominent loss of MAP2 immunofluorescence in apical dendrites of pyramidal neurons within layers 3 and 5, as well as a loss of fine dendritic arborization within layer 1. These changes in MAP2 immunolabeling were associated with, but not exclusively restricted to, the presence of dark shrunken neurons labeled by hematoxylin and eosin staining, suggesting impending cell death. Alterations in MAP2 immunofluorescence were found both within and beyond areas of focal contusion and necrosis in the ipsilateral cortex. Thus, traumatic brain injury in rats can produce rapid and significant dendritic pathology within sites of contusion. However, immunohistochemical changes in MAP2 labeling outside of contused regions suggests that TBI-induced dendritic damage may not be exclusively associated with acute cell death.

Ca2+ buffering and action potential-evoked Ca2+ signaling in dendrites of pyramidal neurons

The effect of the fluorescent Ca2+ indicator dye Fura-2 on Ca2+ dynamics was studied in proximal apical dendrites of neocortical layer V and hippocampal CA1 pyramidal neurons in rat brain slices using somatic whole-cell recording and a charge-coupled device camera. A single action potential evoked a transient increase of intradendritic calcium concentration ([Ca2+]i) that was reduced in size and prolonged when the Fura-2 concentration was increased from 20 to 250 microM. Extrapolation to zero Fura-2 concentration suggests that "physiological" transients at 37 degrees C have large amplitudes (150-300 nM) and fast decays (time constant < 100 ms). Assuming a homogeneous compartment model for the dendrite, 0.5-1% of the total Ca2+ entering during an action potential was estimated to remain free. Washout of cytoplasmic Ca2+ buffers was not detectable, suggesting that they are relatively immobile. During trains of action potentials, [Ca2+]i increased and rapidly reached a steady state (time constant < 200 ms), fluctuating around a plateau level which depended linearly on the action potential frequency. Thus, the mean dendritic [Ca2+]i encodes the action potential frequency during physiological patterns of electrical activity and may regulate Ca(2+)-dependent dendritic functions in an activity-dependent way.

Mapping miniature synaptic currents to single synapses using calcium imaging reveals heterogeneity in postsynaptic output

The amplitudes and kinetics of miniature excitatory synaptic currents (MESCs) in mammalian central neurons vary widely. It is unclear whether this variability occurs at each synapse or arises from differences among a heterogeneous population of synapses. Furthermore, it is not known how variability in these currents would affect their associated postsynaptic Ca2+ transients. To address these questions, we conducted simultaneous Ca2+ imaging and patch-clamp recordings from cultured cortical neurons and mapped individual MESCs to identified synapses displaying coincident dendritic miniature synaptic Ca2+ transients (MSCTs). Measurements of MSCTs at dendritic sites that displayed multiple events revealed that MSCT amplitude varied considerably at each site. Simultaneous measurement of MESCs and MSCTs at these sites indicated that variability in coincident synaptic currents contributes to the differences in Ca2+ transient amplitude. The ability of single synapses to exhibit variable output may enable them to engage intracellular signaling pathways at different levels of intracellular Ca2+.

Neuronal Ca2+ stores: activation and function

The intracellular concentration of free Ca2+ ([Ca2+]i) displays complex fluctuations in response to a variety of stimuli, and acts as a pluripotent signal for many neuronal functions. It is well established that various 'metabotropic' neurotransmitter receptors can mediate the mobilization of Ca2+ stores via actions of inositol-polyphosphate second messengers, and more recent evidence suggests that 'ionotropic' receptor-mediated Ca2+ signals in neurones might also involve release of Ca2+ from intracellular stores. These two mechanisms of release of Ca2+ enable considerable temporal and spatial complexity of increases in the [Ca2+]i via multiple interactions at the level of intracellular-receptor activation. The complexity of Ca2+ signalling that is elicited via these interconnecting pathways might underlie mechanisms that are central to information transfer and integration within neuronal compartments.

Spatial synaptic integration in Purkinje cell dendrites

Synaptic integration occurs within a framework of synaptic connections, and cell type-specific, intrinsic and transmitter-gated ion channels. These components are differentially distributed over the somato-dendritic membrane. Recent results from Purkinje cells and pyramidal cells exemplify some of these mechanisms of spatial synaptic integration. This paper focusses on the cerebellar Purkinje cell. In these neurons, the amplitude and distribution of single climbing fibre and parallel fibre EPSP-evoked Ca2+ influx were regulated by the transient outward, IA-like current in the distal (spiny) dendrites. The synaptically evoked Ca2+ influx was graded from a local response involving only a few terminal spiny dendrites to a propagated Ca2+ spike. The climbing fibre-evoked Ca2+ influx in the spiny dendrites was finely graded by parallel fibre-induced depolarization. Climbing fibre and parallel fibre-evoked Ca2+ influx elicited a short lasting afterhyperpolarization that affected subsequent dendritic Ca2+ influx. In addition, inhibitory synaptic input controlled dendritic Ca2+ influx. Interaction between information from different sources along the dendrites is thus controlled by intrinsic potassium conductances and IPSPs. Different electrophysiological properties are found in the cerebellar neurons. Thus, Golgi cells, stellate cells and granule cells seem to integrate on a shorter intrinsic timescale than do Purkinje cells, the output neuron of the cerebellar cortex. The specific mechanisms by which different types of presynaptic neurons specifically innervate a given dendritic compartment remain to be elucidated, but recent results provide some experimental evidence of a differential distribution of cell adhesion molecules between the axonal and the somato-dendritic membrane, suggesting one mechanism contributing to the ordered distribution of synapses during synaptogenesis.

Dendritic function. Where does it all begin?

Dendrites from a variety of neurons support action potentials, but the role of dendritic electrogenesis in spike initiation remains unclear.