Activity-dependent accumulation of calcium in Purkinje cell dendritic spines

Preserving elemental content in adherent mammalian cells for analysis by synchrotron-based x-ray fluorescence microscopy

Trace metals play important roles in biological function, and x-ray fluorescence microscopy (XFM) provides a way to quantitatively image their distribution within cells. The faithfulness of these measurements is dependent on proper sample preparation. Using mouse embryonic fibroblast NIH/3T3 cells as an example, we compare various approaches to the preparation of adherent mammalian cells for XFM imaging under ambient temperature. Direct side-by-side comparison shows that plunge-freezing-based cryoimmobilization provides more faithful preservation than conventional chemical fixation for most biologically important elements including P, S, Cl, K, Fe, Cu, Zn and possibly Ca in adherent mammalian cells. Although cells rinsed with fresh media had a great deal of extracellular background signal for Cl and Ca, this approach maintained cells at the best possible physiological status before rapid freezing and it does not interfere with XFM analysis of other elements. If chemical fixation has to be chosen, the combination of 3% paraformaldehyde and 1.5 % glutaraldehyde preserves S, Fe, Cu and Zn better than either fixative alone. When chemically fixed cells were subjected to a variety of dehydration processes, air drying was proved to be more suitable than other drying methods such as graded ethanol dehydration and freeze drying. This first detailed comparison for x-ray fluorescence microscopy shows how detailed quantitative conclusions can be affected by the choice of cell preparation method.

Potentiation of convergent synaptic inputs onto pyramidal neurons in somatosensory cortex: dependence on brain wave frequencies and NMDA receptor subunit composition

N-methyl-d-aspartate receptors (NMDARs) at layer (L)1/primary whisker motor cortex synaptic inputs are distinct from thalamic/striatal (Str) synaptic inputs onto L5 pyramidal neurons in the rat somatosensory cortex. However, the consequences of differential expression of putative GluN3A-containing triheteromeric NMDARs at L1 inputs and GluN2A-containing diheteromeric NMDARs at Str inputs on plasticity of the underlying synapses at the respective inputs remain unknown. Here we demonstrate that L1, but not Str, synapses are potentiated following delta burst stimulation (dBS). This potentiation is blocked by d-serine and/or intracellular 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) suggesting that it is subunit-specific and dependent on elevations in intracellular Ca(2+). Interestingly, ifenprodil, the GluN2B-preferring antagonist, suppresses baseline L1 responses but does not prevent induction of dBS-evoked potentiation. Unlike L1, Str synapses are maximally potentiated following theta burst stimulation (tBS) and this potentiation is blocked with BAPTA and/or the GluN2A-preferring antagonist NVP-AAM077. We show further that while dBS is both necessary and sufficient to potentiate L1 synapses, tBS is most effective in potentiating Str synapses. Our data suggest distinct potentiating paradigms for the two convergent inputs onto pyramidal neurons in the somatosensory cortex and co-dependence of synaptic potentiation on brain wave-tuned frequencies of burst stimulation and subunit composition of underlying NMDARs. A model for predicting the likelihood of enhancing synaptic efficacy is proposed based on Ca(2+) influx through these receptors and integration of EPSPs at these inputs. Together, these findings raise the possibility of input-specific enhancements of synaptic efficacy in neurons as a function of the animal's behavioral state and/or arousal in vivo.

Physiology and pathology of calcium signaling in the brain

Calcium (Ca(2+)) plays fundamental and diversified roles in neuronal plasticity. As second messenger of many signaling pathways, Ca(2+) as been shown to regulate neuronal gene expression, energy production, membrane excitability, synaptogenesis, synaptic transmission, and other processes underlying learning and memory and cell survival. The flexibility of Ca(2+) signaling is achieved by modifying cytosolic Ca(2+) concentrations via regulated opening of plasma membrane and subcellular Ca(2+) sensitive channels. The spatiotemporal patterns of intracellular Ca(2+) signals, and the ultimate cellular biological outcome, are also dependent upon termination mechanism, such as Ca(2+) buffering, extracellular extrusion, and intra-organelle sequestration. Because of the central role played by Ca(2+) in neuronal physiology, it is not surprising that even modest impairments of Ca(2+) homeostasis result in profound functional alterations. Despite their heterogeneous etiology neurodegenerative disorders, as well as the healthy aging process, are all characterized by disruption of Ca(2+) homeostasis and signaling. In this review we provide an overview of the main types of neuronal Ca(2+) channels and their role in neuronal plasticity. We will also discuss the participation of Ca(2+) signaling in neuronal aging and degeneration.

Spinal cord injury, dendritic spine remodeling, and spinal memory mechanisms

Spinal cord injury (SCI) often results in the development of neuropathic pain, which can persist for months and years after injury. Although many aberrant changes to sensory processing contribute to the development of chronic pain, emerging evidence demonstrates that mechanisms similar to those underlying classical learning and memory can contribute to central sensitization, a phenomenon of amplified responsiveness to stimuli in nociceptive dorsal horn neurons. Notably, dendritic spines have emerged as major players in learning and memory, providing a structural substrate for how the nervous system modifies connections to form and store information. Until now, most information regarding dendritic spines has been obtained from studies in the brain. Recent experimental data in the spinal cord, however, demonstrate that Rac1-regulated dendritic spine remodeling occurs on second-order wide dynamic range neurons and accompanies neuropathic pain after SCI. Thus, SCI-induced synaptic potentiation engages a putative spinal memory mechanism. A compelling, novel possibility for pain research is that a synaptic model of long-term memory storage could explain the persistent nature of neuropathic pain. Such a conceptual bridge between pain and memory could guide the development of more effective strategies for treatment of chronic pain after injury to the nervous system.

Two types of independent bursting mechanisms in inspiratory neurons: an integrative model

The network of coupled neurons in the pre-Bötzinger complex (pBC) of the medulla generates a bursting rhythm, which underlies the inspiratory phase of respiration. In some of these neurons, bursting persists even when synaptic coupling in the network is blocked and respiratory rhythmic discharge stops. Bursting in inspiratory neurons has been extensively studied, and two classes of bursting neurons have been identified, with bursting mechanism depends on either persistent sodium current or changes in intracellular Ca(2+), respectively. Motivated by experimental evidence from these intrinsically bursting neurons, we present a two-compartment mathematical model of an isolated pBC neuron with two independent bursting mechanisms. Bursting in the somatic compartment is modeled via inactivation of a persistent sodium current, whereas bursting in the dendritic compartment relies on Ca(2+) oscillations, which are determined by the neuromodulatory tone. The model explains a number of conflicting experimental results and is able to generate a robust bursting rhythm, over a large range of parameters, with a frequency adjusted by neuromodulators.

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.

Correlated calcium uptake and release by mitochondria and endoplasmic reticulum of CA3 hippocampal dendrites after afferent synaptic stimulation

Mitochondria and endoplasmic reticulum (ER) are important modulators of intracellular calcium signaling pathways, but the role of these organelles in shaping synaptic calcium transients in dendrites of pyramidal neurons remains speculative. We have measured directly the concentrations of total Ca (bound plus free) within intracellular compartments of proximal dendrites of CA3 hippocampal neurons at times after synaptic stimulation corresponding to the peak of the cytoplasmic free Ca2+ transient (1 sec), to just after its decay (30 sec), and to well after its return to prestimulus levels (180 sec). Electron probe microanalysis of cryosections from rapidly frozen slice cultures has revealed that afferent mossy fiber stimulation evokes large, rapid elevations in the concentration of total mitochondrial Ca ([Ca](mito)) in depolarized dendrites. A single tetanus (50 Hz/1 sec) elevated [Ca](mito) more than fivefold above characteristically low basal levels within 1 sec of stimulation and >10-fold by 30 sec after stimulation. This strong Ca accumulation was reversible, because [Ca](mito) had recovered by 180 sec after the tetanus. Ca sequestered within mitochondria was localized to small inclusions that were distributed heterogeneously within, and probably among, individual mitochondria. By 30 sec after stimulation an active subpopulation of ER cisterns had accumulated more Ca than had mitochondria despite a approximately 1 sec delay before the onset of accumulation. Active ER cisterns retained their Ca load much longer (>3 min) than mitochondria. The complementary time courses of mitochondrial versus ER Ca2+ uptake and release suggest that these organelles participate in a choreographed interplay, each shaping dendritic Ca2+ signals within characteristic regimes of cytosolic Ca2+ concentration and time.

Correlated calcium uptake and release by mitochondria and endoplasmic reticulum of CA3 hippocampal dendrites after afferent synaptic stimulation

Mitochondria and endoplasmic reticulum (ER) are important modulators of intracellular calcium signaling pathways, but the role of these organelles in shaping synaptic calcium transients in dendrites of pyramidal neurons remains speculative. We have measured directly the concentrations of total Ca (bound plus free) within intracellular compartments of proximal dendrites of CA3 hippocampal neurons at times after synaptic stimulation corresponding to the peak of the cytoplasmic free Ca2+ transient (1 sec), to just after its decay (30 sec), and to well after its return to prestimulus levels (180 sec). Electron probe microanalysis of cryosections from rapidly frozen slice cultures has revealed that afferent mossy fiber stimulation evokes large, rapid elevations in the concentration of total mitochondrial Ca ([Ca](mito)) in depolarized dendrites. A single tetanus (50 Hz/1 sec) elevated [Ca](mito) more than fivefold above characteristically low basal levels within 1 sec of stimulation and >10-fold by 30 sec after stimulation. This strong Ca accumulation was reversible, because [Ca](mito) had recovered by 180 sec after the tetanus. Ca sequestered within mitochondria was localized to small inclusions that were distributed heterogeneously within, and probably among, individual mitochondria. By 30 sec after stimulation an active subpopulation of ER cisterns had accumulated more Ca than had mitochondria despite a approximately 1 sec delay before the onset of accumulation. Active ER cisterns retained their Ca load much longer (>3 min) than mitochondria. The complementary time courses of mitochondrial versus ER Ca2+ uptake and release suggest that these organelles participate in a choreographed interplay, each shaping dendritic Ca2+ signals within characteristic regimes of cytosolic Ca2+ concentration and time.

Synaptic Contacts and the Transient Dendritic Spines of Developing Retinal Ganglion Cells

The dendrites of ganglion cells in the mammalian retina become extensively remodelled during synapse formation in the inner plexiform layer. In particular, after birth in the cat, many short spiny protrusions are lost from the dendrites of ganglion cells during the time when ribbon, presumably bipolar, synapses appear in the inner plexiform layer and when conventional, presumed amacrine, synapses increase significantly in number. It has therefore been postulated that these transient spines may be the initial or preferred substrates for competitive interactions between amacrine or bipolar cell terminals that subsequently result in the formation of appropriate synapses onto the ganglion cells. If so, the majority of synapses made onto developing ganglion cells should be found on these dendritic spines. To test this hypothesis, we determined the synaptic connectivity of identified ganglion cells in the postnatal cat retina during the period of peak spine loss and synapse formation. The dendritic trees of ganglion cells were intracellularly filled with Lucifer yellow that was subsequently photo-oxidized into an electron-dense product suitable for electron microscopy. In serial reconstructions of the dendrites of a postnatal day 11 (P11) alpha ganglion cell and a P14 beta ganglion cell, conventional and ribbon synapses were found predominantly on dendritic shafts. Only three out of a total of 341 dendritic spines from the two cells received direct presynaptic input, all of which were conventional synapses. Thus, our observations suggest that the transient dendritic spines are not the preferred postsynaptic sites as previously suspected. However, it is possible that these structures play a different role in synaptogenesis, such as mediating interactions between retinal neurons that may lead to cell - cell recognition, a necessary step prior to synapse formation at the appropriate target sites (Cooper and Smith, Soc. Neurosci. Abstr., 14, 893, 1988).

Structure and function of dendritic spines

Spines are neuronal protrusions, each of which receives input typically from one excitatory synapse. They contain neurotransmitter receptors, organelles, and signaling systems essential for synaptic function and plasticity. Numerous brain disorders are associated with abnormal dendritic spines. Spine formation, plasticity, and maintenance depend on synaptic activity and can be modulated by sensory experience. Studies of compartmentalization have shown that spines serve primarily as biochemical, rather than electrical, compartments. In particular, recent work has highlighted that spines are highly specialized compartments for rapid large-amplitude Ca(2+) signals underlying the induction of synaptic plasticity.

Total calcium ultrastructure: advances in excitable cells

This is a review which is written on the basis of a cell calcium lecture delivered on 22 July 2000 at the European Research Meeting 'Calcium as a molecule of cellular integration'.

Selective localization of high concentrations of F-actin in subpopulations of dendritic spines in rat central nervous system: a three-dimensional electron microscopic study

Dendritic spines differ considerably in their size, shape, and internal organization between brain regions. We examined the actin cytoskeleton in dendritic spines in hippocampus (areas CA1, CA3, and dentate gyrus), neostriatum, and cerebellum at both light and electron microscopic levels by using a novel high-resolution photoconversion method based in the high affinity of phalloidin for filamentous (F)-actin. In all brain regions, labeling was strongest in the heads of dendritic spines, diminishing in the spine neck. The number of labeled spines varied by region. Compared with the cerebellar molecular layer and area CA3, where nearly every dendritic spine was labeled, less than half the spines were labeled in CA1, dentate gyrus, and neostriatum. Serial section reconstructions of spines in these areas indicated that phalloidin labeling was restricted to the largest and most morphologically diverse dendritic spines. The resolution of the photoconversion technique allowed us to examine the localization and organization of actin filaments in the spine. The most intense staining for actin was found in the postsynaptic density and associated with the spines internal membrane system. In mushroom-shaped spines, F-actin staining was particularly strong between the lamellae of the spine apparatus. Three-dimensional reconstruction of labeled spines by using electron tomography showed that the labeled dense material was in continuity with the postsynaptic density. These results highlight differences in the actin cytoskeleton between different spine populations and provide novel information on the organization of the actin cytoskeleton in vivo.

Microheterogeneity of calcium signalling in dendrites

Transient changes in the intracellular concentration of free Ca2+ ([Ca2+]i) originating from voltage- or ligand-gated influx and by ligand- or Ca2+-gated release from intracellular stores, trigger or modulate many fundamental neuronal processes, including neurotransmitter release and synaptic plasticity. Of the intracellular compartments involved in Ca2+ clearance, the endoplasmic reticulum (ER) has received the most attention because it expresses Ca2+ pumps and Ca2+ channels, thus endowing it with the potential to act as both an intracellular calcium sink and store. We review here our ongoing work on the role of calcium sequestration into, and release from, ER cisterns and the role that this plays in the generation and termination of free [Ca2+]i transients in dendrites of pyramidal neurons in hippocampal slices during and after synaptic activity. These studies have been approached by combining parallel microfluorometric measurements of free cytosolic [Ca2+]i transients with energy-dispersive X-ray microanalytical measurements of total Ca content within specific dendritic compartments at the electron microscopy level. Our observations support the emerging realization that specific subsets of dendritic ER cisterns provide spatial and temporal microheterogeneity of Ca2+ signalling, acting not only as a major intracellular Ca sink involved in active clearance mechanisms after voltage- and ligand-gated Ca2+ influx, but also as an intracellular Ca2+ source that can be mobilized by a signal cascade originating at activated synapses.

Correlated measurements of free and total intracellular calcium concentration in central nervous system neurons

Transient changes in the intracellular concentration of free calcium ([Ca2+])i) act as a trigger or modulator for a large number of important neuronal processes. Such transients can originate from voltage- or ligand-gated fluxes of Ca2+ into the cytoplasm from the extracellular space, or by ligand- or Ca2+(-)gated release from intracellular stores. Characterizing the sources and spatio-temporal patterns of [Ca2+]i transients is critical for understanding the role of different neuronal compartments in dendritic integration and synaptic plasticity. Optical imaging of fluorescent indicators sensitive to free Ca2+ is especially suited to studying such phenomena because this approach offers simultaneous monitoring of large regions of the dendritic tree in individual living central nervous system neurons. In contrast, energy-dispersive X-ray (EDX) microanalysis provides quantitative information on the amount and location of intracellular total, i.e., free plus bound, calcium (Ca) within specific subcellular dendritic compartments as a function of the activity state of the neuron. When optical measurements of [Ca2+]i transients and parallel EDX measurements of Ca content are used in tandem, and correlated simultaneously with electrophysiological measurements of neuronal activity, the combined information provides a relatively general picture of spatio-temporal neuronal total Ca fluctuations. To illustrate the kinds of information available with this approach, we review here results from our ongoing work aimed at evaluating the role of various Ca uptake, release, sequestration, and extrusion mechanisms in the generation and termination of [Ca2+]i transients in dendrites of pyramidal neurons in hippocampal slices during and after synaptic activity. Our observations support the long-standing speculation that the dendritic endoplasmic reticulum acts not only as an intracellular Ca2+ source that can be mobilized by a signal cascade originating at activated synapses, but also as a major intracellular Ca sink involved in active clearance mechanisms after voltage- and ligand-gated Ca2+ influx.

Relationships between striatin-containing neurons and cortical or thalamic afferent fibres in the rat striatum. An ultrastructural study by dual labelling

Striatin, a recently isolated rat brain calmodulin-binding protein belonging to the WD-repeat protein family, is thought to be part of a calcium signal transduction pathway presumably specific to excitatory synapses, at least in the striatum. This study was aimed to specify the cellular and subcellular localization of striatin, and to determine the possible synaptic relationships between the two main excitatory afferent pathways, arising from the cerebral cortex and the thalamus, and the striatin-containing elements, in the rat striatum. Anterograde tract-tracing by means of biotinylated dextran amine injection in the frontoparietal cerebral cortex or the parafascicular nucleus of the thalamus was combined with immunogold detection of striatin. Striatin-immunoreactivity was confined to the neuronal somatodendritic compartment, including spines. Whereas 90-95% of the striatal neurons were striatin-positive, only about 50% of the sections of dendritic spines engaged in asymmetrical synaptic contacts exhibited striatin labelling. Among the sections of striatin-immunopositive dendritic spines, the number of immunogold particles ranged from one to more than seven, indicating an heterogeneity of the spine labelling. Moreover, within each class of spines presenting at least two silver-gold particles, the distribution of the particles varied from a clear-cut alignment under the postsynaptic densities (24-33% of spines) to a location distant from the synaptic area. In the cell bodies and dendrites, striatin labelling was usually not associated with the cytoplasmic membrane nor with the postsynaptic densities. In the striatum ipsilateral to the tracer injections, only 34.8% of the synaptic contacts formed by corticostriatal afferents involved striatin-positive elements (slightly labelled dendritic spines), whereas 56.7% of the synaptic contacts formed by thalamostriatal boutons were made on striatin-positive targets (mostly dendrites). In both cases, striatin labelling was usually not associated with the postsynaptic density. Most of the immunoreactive dendritic spines were in contact with unidentified afferents. These data reveal that striatin is expressed in the vast majority of the cell bodies of striatal spiny neurons, but is heterogeneously distributed among the dendritic spines of those neurons. Data also indicate a preferential relationship between striatin-containing structures and afferents from the parafascicular thalamic nucleus with respect to the frontoparietal cerebral cortex. But, at the dendritic spine level, striatin may be involved in signal transduction mechanisms involving as yet unidentified excitatory afferents to striatal neurons.

Activity-dependent calcium sequestration in dendrites of hippocampal neurons in brain slices

Synaptic activity-dependent changes in the spatio-temporal distribution of calcium ions regulate important neuronal functions such as dendritic integration and synaptic plasticity, but the processes that terminate the free Ca2+ transients associated with these changes remain unclear. We have characterized at the electron microscopic level the intracellular compartments involved in buffering free Ca2+ transients in dendritic cytoplasm of CA3 neurons by measuring the larger changes in the concentrations of total Ca that persist for several minutes after neuronal activity. Quantitative energy-dispersive x-ray microanalysis of cryosections from hippocampal slice cultures rapidly frozen 3 min after afferent synaptic activity identified a subset of dendritic endoplasmic reticulum (ER) as a high-capacity Ca2+ buffer. Calcium sequestration by cisterns of this subset of ER was graded, reversible, and dependent on a thapsigargin-sensitive Ca2+-ATPase. Sequestration was so robust that after repetitive high-frequency stimulation the Ca content of responsive ER cisterns increased as much as 20-fold. These results demonstrate that a subpopulation of ER is the major dendritic Ca sequestration compartment in the minutes after neuronal activity.

Activity-dependent calcium sequestration in dendrites of hippocampal neurons in brain slices

Synaptic activity-dependent changes in the spatio-temporal distribution of calcium ions regulate important neuronal functions such as dendritic integration and synaptic plasticity, but the processes that terminate the free Ca2+ transients associated with these changes remain unclear. We have characterized at the electron microscopic level the intracellular compartments involved in buffering free Ca2+ transients in dendritic cytoplasm of CA3 neurons by measuring the larger changes in the concentrations of total Ca that persist for several minutes after neuronal activity. Quantitative energy-dispersive x-ray microanalysis of cryosections from hippocampal slice cultures rapidly frozen 3 min after afferent synaptic activity identified a subset of dendritic endoplasmic reticulum (ER) as a high-capacity Ca2+ buffer. Calcium sequestration by cisterns of this subset of ER was graded, reversible, and dependent on a thapsigargin-sensitive Ca2+-ATPase. Sequestration was so robust that after repetitive high-frequency stimulation the Ca content of responsive ER cisterns increased as much as 20-fold. These results demonstrate that a subpopulation of ER is the major dendritic Ca sequestration compartment in the minutes after neuronal activity.

Slow-channel transgenic mice: a model of postsynaptic organellar degeneration at the neuromuscular junction

The slow-channel congenital myasthenic syndrome (SCCMS) is a dominantly inherited disorder of neuromuscular transmission characterized by delayed closure of the skeletal muscle acetylcholine receptor (AChR) ion channel and degeneration of the neuromuscular junction. The identification of a series of AChR subunit mutations in the SCCMS supports the hypothesis that the altered kinetics of the endplate currents in this disease are attributable to inherited abnormalities of the AChR. To investigate the role of these mutant AChR subunits in the development of the synaptic degeneration seen in the SCCMS, we have studied the properties of the AChR mutation, epsilonL269F, found in a family with SCCMS, using both in vitro and in vivo expression systems. The mutation causes a sixfold increase in the open time of AChRs expressed in vitro, similar to the phenotype of other reported mutants. Transgenic mice expressing this mutant develop a syndrome that is highly reminiscent of the SCCMS. Mice have fatigability of limb muscles, electrophysiological evidence of slow AChR ion channels, and defective neuromuscular transmission. Pathologically, the motor endplates show focal accumulation of calcium and striking ultrastructural changes, including enlargement and degeneration of the subsynaptic mitochondria and nuclei. These findings clearly demonstrate the role of this mutation in the spectrum of abnormalities associated with the SCCMS and point to the subsynaptic organelles as principal targets in this disease. These transgenic mice provide a useful model for the study of excitotoxic synaptic degeneration.

Slow-channel transgenic mice: a model of postsynaptic organellar degeneration at the neuromuscular junction

The slow-channel congenital myasthenic syndrome (SCCMS) is a dominantly inherited disorder of neuromuscular transmission characterized by delayed closure of the skeletal muscle acetylcholine receptor (AChR) ion channel and degeneration of the neuromuscular junction. The identification of a series of AChR subunit mutations in the SCCMS supports the hypothesis that the altered kinetics of the endplate currents in this disease are attributable to inherited abnormalities of the AChR. To investigate the role of these mutant AChR subunits in the development of the synaptic degeneration seen in the SCCMS, we have studied the properties of the AChR mutation, epsilonL269F, found in a family with SCCMS, using both in vitro and in vivo expression systems. The mutation causes a sixfold increase in the open time of AChRs expressed in vitro, similar to the phenotype of other reported mutants. Transgenic mice expressing this mutant develop a syndrome that is highly reminiscent of the SCCMS. Mice have fatigability of limb muscles, electrophysiological evidence of slow AChR ion channels, and defective neuromuscular transmission. Pathologically, the motor endplates show focal accumulation of calcium and striking ultrastructural changes, including enlargement and degeneration of the subsynaptic mitochondria and nuclei. These findings clearly demonstrate the role of this mutation in the spectrum of abnormalities associated with the SCCMS and point to the subsynaptic organelles as principal targets in this disease. These transgenic mice provide a useful model for the study of excitotoxic synaptic degeneration.

Perturbation of cellular calcium delays the secretion and alters the glycosylation of thyroglobulin in FRTL-5 cells

Treatment of FRTL-5 cells with a Ca2+ ionophore, A23187, or a specific inhibitor of the endoplasmic reticulum Ca2+ ATPases, thapsigargin, delayed thyroglobulin secretion. The secreted thyroglobulin showed an increased electrophoretic mobility and a reduced sensitivity to neuraminidase. Only thyroglobulin that was still in the endoplasmic reticulum was sensitive to the Ca(2+)-perturbant drugs as shown by experiments in which the drugs were added at different times during a chase. Analysis of the carbohydrate chains by BioGel P4 showed that thyroglobulin secreted in the presence of the Ca(2+)-perturbants displayed an increased ratio high mannose/complex chains.

Regulation of dendritic spine density in cultured rat hippocampal neurons by steroid hormones

The effects of gonadal steroid hormones on dendritic spines were studied in hippocampal neurons that were dissociated and grown in culture for 2-3 weeks. Exposure to estradiol caused up to a twofold increase in dendritic spine density in these neurons. The effect of estradiol was stereospecific and blocked by the steroid antagonist tamoxifen. The estradiol-induced rise in spine density was blocked by the NMDA antagonist APV, but not by the AMPA/KA antagonist DNQX. The estradiol-induced rise in spine density was blocked by the serine/threonine kinase inhibitor H7, but not by the tyrosine kinase inhibitor genestein, and was partially mimicked by PMA, an activator of protein kinase C. Estradiol also caused an increase in the fluorescence intensity of synaptophysin-immunoreactive terminals, corresponding to presynaptic boutons. Finally, estradiol caused a rise in [Ca]i reactivity of the cultured neurons to topical application of glutamate. These studies are the first to examine receptor and second messenger regulation of dendritic spines, and they illustrate the viability of cultured neurons as a powerful test system to address issues related to the regulation of dendritic spine maturation.

Regulation of dendritic spine density in cultured rat hippocampal neurons by steroid hormones

The effects of gonadal steroid hormones on dendritic spines were studied in hippocampal neurons that were dissociated and grown in culture for 2-3 weeks. Exposure to estradiol caused up to a twofold increase in dendritic spine density in these neurons. The effect of estradiol was stereospecific and blocked by the steroid antagonist tamoxifen. The estradiol-induced rise in spine density was blocked by the NMDA antagonist APV, but not by the AMPA/KA antagonist DNQX. The estradiol-induced rise in spine density was blocked by the serine/threonine kinase inhibitor H7, but not by the tyrosine kinase inhibitor genestein, and was partially mimicked by PMA, an activator of protein kinase C. Estradiol also caused an increase in the fluorescence intensity of synaptophysin-immunoreactive terminals, corresponding to presynaptic boutons. Finally, estradiol caused a rise in [Ca]i reactivity of the cultured neurons to topical application of glutamate. These studies are the first to examine receptor and second messenger regulation of dendritic spines, and they illustrate the viability of cultured neurons as a powerful test system to address issues related to the regulation of dendritic spine maturation.

High resolution ultrastructural mapping of total calcium: electron spectroscopic imaging/electron energy loss spectroscopy analysis of a physically/chemically processed nerve-muscle preparation

We report on a procedure for tissue preparation that combines thoroughly controlled physical and chemical treatments: quick-freezing and freeze-drying followed by fixation with OsO4 vapors and embedding by direct resin infiltration. Specimens of frog cutaneous pectoris muscle thus prepared were analyzed for total calcium using electron spectroscopic imaging/electron energy loss spectroscopy (ESI/EELS) approach. The preservation of the ultrastructure was excellent, with positive K/Na ratios revealed in the fibers by x-ray microanalysis. Clear, high-resolution EELS/ESI calcium signals were recorded from the lumen of terminal cisternae of the sarcoplasmic reticulum but not from longitudinal cisternae, as expected from previous studies carried out with different techniques. In many mitochondria, calcium was below detection whereas in others it was appreciable although at variable level. Within the motor nerve terminals, synaptic vesicles as well as some cisternae of the smooth endoplasmic reticulum yielded positive signals at variance with mitochondria, that were most often below detection. Taken as a whole, the present study reveals the potential of our experimental approach to map with high spatial resolution the total calcium within individual intracellular organelles identified by their established ultrastructure, but only where the element is present at high levels.

Localization and quantification of endoplasmic reticulum Ca(2+)-ATPase isoform transcripts

The Ca(2+)-adenosinetriphosphatase pump of the sarcoplasmic or endoplasmic reticulum (SERCA) plays a critical role in Ca2+ signaling and homeostasis in all cells and is encoded by a family of homologous and alternatively spliced genes. To understand more clearly the role the different isoforms play in cell physiology, we have undertaken a quantitative and qualitative assessment of the tissue distribution of transcripts encoding each SERCA isoform. SERCA1 expression is restricted to fast-twitch striated muscles, SERCA2a to cardiac and slow-twitch striated muscles, whereas SERCA2b is ubiquitously expressed. SERCA3 is expressed most abundantly in large and small intestine, thymus, and cerebellum and at lower levels in spleen, lymph node, and lung. In situ hybridization analyses revealed SERCA3 transcripts in cells of the intestinal crypt, the thymic cortex, and Purkinje cells in cerebellum. In addition, SERCA3 was expressed abundantly in isolated rat spleen lymphocytes, in various murine lymphoid cell lines, and in primary cultured microvascular endothelial cells. This analysis demonstrates that SERCA3 is expressed selectively in cells in which Ca2+ signaling plays a critical and sensitive role in regulating physiological processes.

The cisternal organelle as a Ca(2+)-storing compartment associated with GABAergic synapses in the axon initial segment of hippocampal pyramidal neurones

The axon initial segment of cortical principal neurones contains an organelle consisting of two to four stacks of flat, membrane-delineated cisternae alternating with electron-dense, fibrillar material. These cisternal organelles are situated predominantly close to the synaptic junctions of GABAergic axo-axonic cell terminals. To examine the possibility that the cisternal organelle is involved in Ca2+ sequestration, we tested for the presence of Ca(2+)-ATPase in the cisternal organelles of pyramidal cell axons in the CA1 and CA3 regions of the hippocampus. Electron microscopic immunocytochemistry using antibodies to muscle sarcoplasmic reticulum ATPase revealed immunoreactivity associated with cisternal organelle membranes. The localisation of Ca(2+)-ATPase in cisternal organelles was also confirmed by enzyme cytochemistry, which produced reaction product in the lumen of the cisternae. These experiments provide evidence for the presence of a Ca2+ pump in the cisternal organelle membrane, which may play a role in the sequestration and release of Ca2+. Cisternal organelles are very closely aligned to the axolemma and the outermost cisternal membrane is connected to the plasma membrane by periodic electron-dense bridges as detected in electron micrographs. It is suggested that the interface acts as a voltage sensor, releasing Ca2+ from cisternal organelles upon depolarisation of the axon initial segment, in a manner similar to the sarcoplasmic reticulum of skeletal muscle. The increase in intra-axonal Ca2+ may regulate the GABAA receptors associated with the axo-axonic cell synapses, and could affect the excitability of pyramidal cells.

Cellular distribution of calbindin D28K mRNAs in the adult mouse brain

We have determined the cellular distribution of calbindin D28K mRNAs throughout the mouse brain by in situ hybridization. While these studies identified neuronal populations similar to those previously identified in rat brain by immunohistochemistry, some discrepancies exist. These may derive from species differences or from the immunological cross-reactivity of calbindin D28K antiserum with other proteins. We note an intriguing association between the distribution of neurons containing calbindin D28K mRNA and those reported by others to contain the inositol 1,4,5-triphosphate (InsP3) receptor.

Water distributions of hydrated biological specimens by valence electron energy loss spectroscopy

A technique has been developed for measuring the water distribution in thin frozen hydrated biological specimens by means of electron energy loss spectroscopy (EELS). The method depends on the quantification of subtle changes in the valence electron excitation spectrum as a function of composition. It involves determining the single-scattering intensities, calculating oscillator strengths and applying a multiple-least-squares fitting procedure to reference spectra for water and the organic constituents. The direct EELS approach has an important advantage over other indirect methods that are based on X-ray generation or elastic scattering measurements since these are applied to freeze-dried specimens where differential shrinkage between compartments may produce errors. Precision and accuracy of the EELS method have been tested on cryosectioned solution of bovine serum albumin; data have also been obtained from cryosections of rapidly frozen erythrocytes. Results suggest that a precision of better than +/- 5% (s.d.) is attainable from a single measurement and the accuracy may be as high as +/- 2% if repeated measurements are made. The lateral spatial resolution of the water determinations is limited by radiation damage to approximately 100 nm which is of the same order as the specimen thickness.

Measurement of low calcium concentrations in cryosectioned cells by parallel-EELS mapping

Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope provides a high sensitivity for microanalysis of certain important biological elements such as calcium whose physiological concentrations in cells are rather low. Application of parallel-EELS mapping to the analysis of freeze-dried cryosections of rapidly frozen tissue provides a means of detecting small amounts of calcium in structures with diameter approximately 50 nm. Detector pattern noise due to channel gain variations can be reduced by acquiring difference spectra at each pixel. By segmenting nitrogen maps that reflect the structure through the protein distribution it is possible to sum spectra from specific compartments. These are then processed by fitting reference spectra for the Ca L23-edge and the carbon background. It has been found that useful data can be collected at 100 keV beam energy from freeze-dried cryosections of cerebellar cortex cut to nominal thickness of 100 nm. The analysis results in a sensitivity of +/- 0.4 mmol Ca/kg dry weight with a total acquisition time of 400 s, a significant improvement over that achievable with energy-dispersive X-ray spectroscopy.

Regulation of cytosolic free calcium concentration by intrasynaptic mitochondria

By the use of digitonin permeabilized presynaptic nerve terminals (synaptosomes), we have found that intrasynaptic mitochondria, when studied "in situ," i.e., surrounded by their cytosolic environment, are able to buffer calcium in a range of calcium concentrations close to those usually present in the cytosol of resting synaptosomes. Adenine nucleotides and polyamines, which are usually lost during isolation of mitochondria, greatly improve the calcium-sequestering activity of mitochondria in permeabilized synaptosomes. The hypothesis that the mitochondria contributes to calcium homeostasis at low resting cytosolic free calcium concentration ([Ca2+]i) in synaptosomes has been tested; it has been found that in fact this is the case. Intrasynaptic mitochondria actively accumulates calcium at [Ca2+]i around 10(-7) M, and this activity is necessary for the regulation of [Ca2+]i. When compared with other membrane-limited calcium pools, it was found that depending on external concentration the calcium pool mobilized from mitochondria is similar or even greater than the IP3- or caffeine-sensitive calcium pools. In summary, the results presented argue in favor of a more prominent role of mitochondria in regulating [Ca2+]i in presynaptic nerve terminals, a role that should be reconsidered for other cellular types in light of the present evidence.

Independent regulation of calcium revealed by imaging dendritic spines

The dendritic spine is a basic structural unit of neuronal organization. It is assumed to be a primary locus of synaptic plasticity, and to undergo long-term morphological and functional changes, at least some of which are regulated by intracellular calcium concentrations. It is known that physiological stimuli can cause marked increases in intracellular calcium levels in hippocampal dendritic shafts, but it is completely unknown to what extent such changes in the dendrites would also be seen by calcium-sensing structures within spines. Will calcium levels in all spines change in parallel with the dendrite or will there be a heterogeneous response? This study, through direct visualization and measurement of intracellular calcium concentrations in individual living spines, demonstrates that experimentally evoked changes in calcium concentrations in the dendritic shaft ([Ca2+]d).

Organization of mitochondria in olfactory bulb granule cell dendritic spines

In contrast to dendritic spines with only postsynaptic functions, the spines of olfactory bulb granule cells subserve both pre- and postsynaptic roles. In single sections these spines were previously seen to contain mitochondria, most likely needed to provide energy for presynaptic functions, but their frequency and distribution were unknown. In order to understand the organization of mitochondria in these specialized dendritic appendages, we have studied the geometry and cytoplasmic organization of granule cell spines with computer-assisted reconstructions of serial electron micrographs. The spine heads were seen to be elliptical in shape with a single pair of reciprocal synapses on the concave face apposed to the mitral/tufted cell dendrite. Mitochondria were found localized in the spine neck as well as the spine head and often extended between the two compartments. Based on their variable distribution it seems reasonable to suggest that these mitochondria are motile and move in and out of spine compartments from the parent dendrite. Spine apparatus was apparent in most of the spines as membrane bound cisterns of smooth endoplasmic reticulum located close to mitochondria. The possible role of spine apparatus in facilitating the movement of mitochondria in the necks and heads of granule cell spines in the absence of microtubules is discussed.

Intracellular Ca2+ stores in chicken Purkinje neurons: differential distribution of the low affinity-high capacity Ca2+ binding protein, calsequestrin, of Ca2+ ATPase and of the ER lumenal protein, Bip

To identify intracellular Ca2+ stores, we have mapped (by cryosection immunofluorescence and immunogold labeling) the distribution in the chicken cerebellar cortex of an essential component, the main low affinity-high capacity Ca2+ binding protein which in this tissue has been recently shown undistinguishable from muscle calsequestrin (Volpe, P., B. H. Alderson-Lang, L. Madeddu, E. Damiani, J. H. Collins, and A. Margreth. 1990. Neuron. 5:713-721). Appreciable levels of the protein were found exclusively within Purkinje neurons, distributed to the cell body, the axon, and the elaborate dendritic tree, with little labeling, however, of dendritic spines. At the EM level the protein displayed a dual localization: within the ER (rough- and smooth-surfaced cisternae, including the cisternal stacks recently shown [in the rat] to be highly enriched in receptors for inositol 1,4,5-triphosphate) and, over 10-fold more concentrated, within a population of moderately dense, membrane-bound small vacuoles and tubules, identified as calciosomes. These latter structures were widely distributed both in the cell body (approximately 1% of the cross-sectional area, particularly concentrated near the Golgi complex) and in the dendrites, up to the entrance of the spines. The distribution of calsequestrin was compared to those of another putative component of the Ca2+ stores, the membrane pump Ca2+ ATPase, and of the ER resident lumenal protein, Bip. Ca2+ ATPase was expressed by both calciosomes and regular ER cisternae, but excluded from cisternal stacks; Bip was abundant within the ER lumena (cisternae and stacks) and very low within calciosomes (average calsequestrin/Bip immunolabeling ratios were approximately 0.5 and 36.5 in the two types of structure, respectively). These results suggest that ER cisternal stacks do not represent independent Ca2+ stores, but operate coordinately with the adjacent, lumenally continuous ER cisternae. The ER and calciosomes could serve as rapidly exchanging Ca2+ stores, characterized however by different properties, in particular, by the greater Ca2+ accumulation potential of calciosomes. Hypotheses of calciosome biogenesis (directly from the ER or via the Golgi complex) are discussed.

Analysis of directly frozen macromolecules and tissues in the field-emission STEM

A VG Microscopes HB501 field-emission high-resolution scanning transmission electron microscope (STEM) was used to image and analyse rapidly frozen, isolated macromolecules and small organelles in tissue cryosections. Dark-field images were obtained from frozen-hydrated microtubules demonstrating that sufficient contrast is available to reveal structural information. The samples were subsequently freeze-dried in the STEM and low-dose (approximately 10(3) e/nm2) dark-field mass maps were recorded with single electron sensitivity. Elemental analysis of individual macromolecules was achievable at high dose using parallel-detection electron energy-loss spectroscopy, albeit with some structural degradation. Detection of copper (320 atoms) in di-decameric haemocyanin molecules was easily within the limits of sensitivity. Elemental analysis of hydrated cryosections is limited by radiation damage to a resolution of approximately 1 micron2. For freeze-dried sections, however, the high probe current and stable cold stage of the HB501 STEM allow energy-dispersive X-ray (EDX) microanalysis of low elemental concentrations in highly localized subcellular volumes. EDX spectra from cryosections of cerebellar cortex show that a 100-s analysis time is sufficient to quantify the calcium content of 400-nm2 regions within Purkinje cell dendrites with an uncertainty of +/- 2 mmol/kg dry weight, equivalent to +/- 12 atoms.

The inositol 1,4,5,-trisphosphate receptor in cerebellar Purkinje cells: quantitative immunogold labeling reveals concentration in an ER subcompartment

The Ca2+ mobilization effect of inositol 1,4,5-trisphosphate, the second messenger generated via receptor-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, is mediated by binding to intracellular receptors, which are expressed in high concentration in cerebellar Purkinje cells. Partially conflicting previous reports localized the receptor to various subcellular structures: elements of ER, both rough and smooth-surfaced, the nuclear envelope, and even the plasma membrane. We have now reinvestigated the problem quantitatively by using cryosections of rat cerebellar tissue immunolabeled with polyclonal monospecific antibodies against the inositol 1,4,5-trisphosphate receptor. By immunofluorescence the receptor was detected only in Purkinje cells, whereas the other cells of the cerebellar cortex remained negative. In immunogold-decorated ultrathin cryosections of the Purkinje cell body, the receptor was concentrated in cisternal stacks (piles of up to 12 parallel cisternae separated by regularly spaced bridges, located both in the deep cytoplasm and beneath the plasma membrane; average density, greater than 5 particles/micron of membrane profile); in cisternal singlets and doublets adjacent to the plasma membrane (average density, approximately 2.5 particles/micron); and in other apparently smooth-surfaced vesicular and tubular profiles. Additional smooth-surfaced elements were unlabeled. Perinuclear and rough-surfaced ER cisternae were labeled much less by themselves (approximately 0.5 particles/micron, two- to threefold the background), but were often in direct membrane continuity with heavily labeled, smooth-surfaced tubules and cisternal stacks. Finally, mitochondria, Golgi cisternae, multivesicular bodies, and the plasma membrane were unlabeled. In dendrites, approximately half of the nonmitochondrial, membrane-bound structures (cisternae, tubules, and vesicles), as well as small cisternal stacks, were labeled. Dendritic spines always contained immunolabeled cisternae and vesicles. The dendritic plasma membrane, of both shaft and spines, was consistently unlabeled. These results identify a large, smooth-surfaced ER subcompartment that appears equipped to play a key role in the control of Ca2+ homeostasis: in particular, in the generation of [Ca2+]i transients triggered by activation of specific receptors, such as the quisqualate-preferring trans(+/-)-1-amino-1,3-cyclopentamedicarboxylic acid glutamatergic receptors, which are largely expressed by Purkinje cells.

Arborisation pattern and postsynaptic targets of physiologically identified thalamocortical afferents in striate cortex of the macaque monkey

The monosynaptic targets of different functional types of geniculocortical axons were compared in the primary visual cortex of monkeys. Single thalamocortical axons were recorded extracellularly in the white matter by using horseradish-peroxidase-filled pipettes. Their receptive fields were mapped and classified as corresponding to those of parvi- or magnocellular neurons in the lateral geniculate nucleus. The axons were then impaled and injected intraaxonally with horseradish peroxidase. Two magnocellular (MA) and two parvicellular (PA) axons were successfully recovered and reconstructed in three dimensions. The two MA axons arborised mainly in layer 4C alpha, as did the two PA axons in layer 4C beta. Few collaterals formed varicosities in layer 6. Both MA axons had two large, elongated clumps of bouton (approx. 300-500 x 600-1,200 microns each) and a small clump. One PA axon had two clumps (each with a core appr. 200 microns in diameter); the other had only one (appr. 150-200 microns in axon had 1,380; one MA axon had 3,200 boutons; and those of the more extensive MA axon were not counted. The distribution of postsynaptic targets as well as the number of synapses per bouton has been established for a sample of 150 PA boutons and 173 MA boutons from serial ultrathin sections. The MA axons made on average 2.1 synapses per bouton compared to 1.79 for one PA axon and 2.6 for the other. The sample of boutons taken from the two physiological types of axons contacted similar proportions of dendritic spines (52-68%), shafts (33-47%), and somata (0-3%). The postsynaptic elements were further characterized by immunostaining for GABA. All postsynaptic perikarya and some of the dendrites (4.5-9.5% of all targets) were positive for the amino acid. Near the thalamic synapse GABA-negative dendritic shafts frequently contained lamellar bodies, an organelle identical in structure to spine apparatus. Dendritic shafts and spines postsynaptic to the thalamocortical boutons frequently received an adjacent synapse from GABA-immunoreactive boutons. The similarity between the magno-and parvicellular axons in their targeting of postsynaptic elements, including the GABAergic neurons, suggests that the structural basis of the physiological differences between 4C alpha and 4C beta neurons should be sought in other aspects of the circuitry of layer 4C, such as local cortical circuits, or in the far greater horizontal extent of the thalamocortical and GABAergic axons in layer 4C alpha compared to those in the beta subdivision.

The biological activities of interleukin-1

Interleukin-1 (IL-1) refers to a hormone-like polypeptide that mediates a broad spectrum of activities in host defence as well as a variety of disease processes. Originally described as a substance produced by activated macrophages, IL-1 is now recognized as a polypeptide produced by many other cell types. Two distinct genes have been identified that code for two structurally related forms of the molecule, termed IL-1 alpha and beta. IL-1 is the primary mediator of the acute phase response and is responsible for many of the changes associated with the onset of infection. It is involved in the immune response to antigenic challenge. IL-1 induces fever and has profound endocrinologic, neurologic, metabolic and hematologic effects. Both forms of IL-1 bind to a common receptor that has been identified on a variety of cell types including lymphocytes, hepatocytes, endothelial cells and fibroblasts. Many of the activities of IL-1 are mediated by the induction of other cytokines like IL-2, IL-6, interferons, tumor necrosis factor, and colony-stimulating factors. In animals IL-1 protects against the effects of radiation, it enhances natural resistance of infection, and it stimulates bone marrow recovery after myelosuppression. These studies suggest that IL-1 may be used as a therapeutic agent.

Interleukin-1 and the response to injury

Traumatic injury is the leading cause of morbidity and mortality in Americans less than 45 years old. People surviving the initial insult undergo metabolic, hemodynamic and immunologic changes which contribute to both early and late complications. Though necessary for normal immunologic response and for wound healing, pathologic alterations of IL-1 synthesis, degradation, and binding to receptors on both a local and systemic level could lead to these changes. Manipulation of IL-1-mediated effects might be of therapeutic utility in the management of trauma in the future.

Structural alterations of dendritic spines induced by neural degeneration of their presynaptic afferents

Morphological parameters were compared for dendritic spines of spiny stellate neurons in layer IV of the barrel region of mouse somatosensory cortex, which synapse with degenerated thalamocortical afferents (TC spines) and with intact, unidentified axon terminals (UI spines). Spiny stellate neurons were labeled for light and electron microscopic identification by Golgi impregnation and gold toning. Dendritic spines were examined in series of thin sections, and TC spines were ultrastructurally detectable because of the degeneration-induced characteristic appearance of the TC axon terminals. Results show that the means of the width of the spine head and of the length of the spine stalk were significantly higher in TC spines than in UI spines by about 11 and 25%, respectively. The variability of these two morphological parameters was significantly lower for TC spines. The mean of the spine stalk width at the narrowest cross section of the stalk was about 0.12 microns, with no significant difference observed between the two spine groups. No specific relationship was found in either the TC or the UI groups of spines between the length of the spine stalk and the width of the spine stalk at its narrowest profile. As structural features typifying transneuronal degeneration were not observed along the dendritic spines examined, it is speculated that the morphological differences encountered between the TC and UI spines may result, at least in part, from the degeneration-induced synaptic inactivity of the TC axospinous synapses, rather than exclusively from any direct effects of the degeneration process.

Membrane and cytoplasmic structure at synaptic junctions in the mammalian central nervous system

Application of rapid freezing, freeze substitution fixation, and freeze fracture techniques to the study of synaptic junctions in the mammalian central nervous system has revealed new aspects of synaptic structure that are consistent with and partially explicate advances in synaptic biochemistry and physiology. In the axoplasm adjacent to the presynaptic active zone, synaptic vesicles are linked to large spectrin-like filamentous proteins by shorter proteins that resemble synapsin I in morphology. This mesh of presynaptic filamentous proteins serves to concentrate synaptic vesicles in the vicinity of the active zone. The affinity with which the vesicles are bound by the mesh is probably modulated by the extent of phosphorylation at specific sites on the constituent filamentous proteins, and changes in the binding affinity result in changes in transmitter release. The structural organization of the postsynaptic density in Purkinje cell dendritic spines consists of very fine strands with adherent, heterogeneous globular proteins. Some of these globular proteins probably correspond to protein kinases and their substrates. The postsynaptic density, positioned at the site of the maximal depolarization caused by synaptic currents, apparently serves as a supporting framework for a variety of proteins, which respond to and transduce postsynaptic depolarization. At least two classes of filamentous protein fill the cytoplasm of spines with a complex mesh, which presumably contributes to maintenance of the spine shape. Membrane bound cisterns are a ubiquitous feature of Purkinje cell dendritic spines. Studies of rapidly frozen tissue with electron probe microanalysis and elemental imaging reveal that these cisterns take up and sequester calcium, which is derived from the extracellular space, and which probably enters the spine as part of the synaptic current.

Calcium and neuronal function

Calcium is unique among metals because its ions have a very large concentration gradient across the plasma membrane of all cells, from 10(-3) M Ca2+ outside, to 10(-7) M Ca2+ inside. This gradient is maintained by the use of metabolic energy through ion pumping, and its existence allows cells to use transient increases in the intracellular Ca2+ concentration as signals, which regulate cell function. In neurones these Ca signals are initiated by electrical activity (action potentials) which open voltage-dependent Ca channels in the plasma membrane, allowing Ca to enter the cell. Intracellular Ca signals can also be produced by transmitters at synapses, which open Ca channels, either directly, or indirectly by causing local depolarization and the opening of voltage-dependent Ca channels. The main effects of Ca signals on neurones are to alter their electrical activity, by modifying the opening and closing of Na and K channels, and to stimulate the release of transmitter substance. Ca has a host of other effects, such as the regulation of metabolic activity, the regulation of cell growth, and the long-term modification of synaptic efficiency, and it is even implicated in the destruction of neurones.