Independent regulation of calcium revealed by imaging dendritic spines

Long-term potentiation and N-methyl-D-aspartate receptors: foundations of memory and neurologic disease?

Understanding the physiology of learning and memory is one of the great challenges of neuroscience. The discovery in recent years of long-term potentiation (LTP) of synaptic transmission and the elaboration of the mechanisms involved, in particular the NMDA receptor, offers the prospect not only of improving our understanding of normal memory storage and retrieval, but may also yield insights about various neurological and psychiatric clinical disorders. In this review, we begin by examining the different forms, properties, and methods of inducing LTP, followed by a description of molecular mechanisms thought to underlie the phenomenon. Molecular structure of the receptor is discussed, along with the roles of Ca2+ second messenger systems, synaptic morphology changes, and retrograde messengers in LTP. Finally, implications of the NMDA receptor and LTP in learning, memory, and certain clinical conditions such as epilepsy, Alzheimer's disease, and schizophrenia are discussed.

Differential localization of NMDA and AMPA receptor subunits in the lateral and basal nuclei of the amygdala: a light and electron microscopic study

Anatomical and physiological studies indicate that the amino acid L-glutamate is the excitatory transmitter in sensory afferent pathways to the amygdala and in intraamygdala circuits involving the lateral and basal nuclei. The regional, cellular, and subcellular immunocytochemical localizations of N-methyl-D-aspartate (NMDA) and L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), two major classes of glutamate receptors, were examined in these areas of the amygdala. A monoclonal antibody and a polyclonal antiserum directed against the R1 subunit of the NMDA receptor were used. Each immunoreagent produced distinct distributions of perikaryal and neuropilar staining. Dendritic immunoreactivity was localized primarily to asymmetric (excitatory) synaptic junctions, mostly on spines, consistent with the conventional view of the organization and function of NMDA receptors. Whereas the anti-NMDAR1 antiserum produced sparse presynaptic axon terminal labeling and extensive glial labeling, the anti-NMDAR1 antibody labeled considerably fewer glia and many more presynaptic axon terminals. Labeled presynaptic terminals formed asymmetric and symmetric synapses, suggesting presynaptic regulation of both excitatory and inhibitory transmission. Immunoreactivity for different subunits of the AMPA receptor (GluR1, GluR2/3, and GluR4) was uniquely distributed across neuronal populations, and some receptor subunits were specific to certain cell types. Immunoreactivity for GluR1 and Glu2/3 was predominantely localized to dendritic shafts and was more extensive than that of GluR4 due to heavy labeling of proximal portions of dendrites. The distribution of GluR4 immunoreactivity was similar to NMDAR1: GluR4 was seen in presynaptic terminals, glia, and dendrites and was primarily localized to spines. The presynaptic localization of GluR4 in the absence of GluR2 suggests glutamate-mediated modulation of presynaptic Ca++ concentrations. These data add to our understanding of the morphological basis of pre- and postsynaptic transmission mechanisms and synaptic plasticity in the amygdala.

Optical imaging of intracellular chloride in living brain slices

We developed an optical imaging technique to measure changes in intracellular levels of Cl- in neurons within the living brain slice. After rat brain slices were incubated with the permeant form of the Cl(-)-sensitive dye, 6-methoxy-N-ethylquinolinium chloride (MEQ), neurons could be imaged within the hippocampus, cerebral cortex and cerebellum using fluorescence microscopy. Both soma and dendrites were clearly visible in pyramidal neurons, interneurons, Purkinje cells and cerebellar granule cells. Increased intracellular levels of Cl- were produced by bath application of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). Within hippocampal pyramidal neurons and interneurons, GABA produced a concentration-dependent decrease in fluorescence (EC50 = 200 microM). The GABA response was mediated via the GABA receptor since it was blocked by picrotoxin and mimicked by the agonist, muscimol. Muscimol, which is not transported by the GABA re-uptake pump, was approximately 20-fold more potent than GABA. The method developed was also used to image intracellular Cl- levels with UV laser scanning confocal microscopy. Even greater resolution was obtained and deeper structures could be imaged in cerebral cortex and hippocampus. This is the first demonstration of optical imaging to measure intracellular Cl- dynamics in living brain slices using fluorescence microscopy and laser scanning confocal microscopy.

Dendritic spines for neuroprotection: a hypothesis

Ever since their first description in neurons, dendritic spines could be visualized only in fixed tissue, using high-power light and electron microscopy. Recent studies have been able to measure the free intracellular Ca2+ concentration ([Ca2+]i) in dendritic spines of live neurons, and the results suggest that the spine is an independent cellular Ca2+ compartment. Other recent observations have indicated that the density of spines on dendrites changes in a dynamic fashion depending on ongoing neuronal activity. Together, these findings have led to the proposal that the dendritic spine is not only a storage device for long-term memory but perhaps a means for isolating the cell from the harmful consequences of synaptically evoked surges in [Ca2+]i. In other words, the dendritic spine is a neuroprotectant. This hypothesis has specific testable implications, including relating cell activity to spine density.

Confocal imaging and local photolysis of caged compounds: dual probes of synaptic function

Chemical signals generated at synapses are highly limited in both spatial range and time course, so that experiments studying such signals must measure and manipulate them in both these dimensions. We describe an optical system that combines confocal laser scanning microscopy, to measure such signals, with focal photolysis of caged compounds. This system can elevate neurotransmitter and second messenger levels in femtoliter volumes of single dendrites within a millisecond. The method is readily combined with whole-cell patch-clamp measurements of electrical signals in brain slices. In cerebellar Purkinje cells, photolysis of caged IP3 causes spatially restricted intracellular release of Ca2+, and photolysis of a caged Ca2+ compound locally opens Ca(2+)-dependent K+ channels. Furthermore, localized photolysis of the caged neurotransmitter GABA transiently activates GABA receptors. The use of focal uncaging can yield new information about the spatial range of signaling actions at synapses.

Altered cell calcium regulation in synaptosomes and brain cells of the 30-month-old rat: prominent effects in hippocampus

A deficient regulation of neuronal cytosolic calcium levels has been suggested to play a role in the pathogenesis of neurodegeneration. However, evidence for an alteration in cytosolic calcium regulation in old age is at present controversial. The present work was aimed at studying whether changes in synaptosomal calcium homeostasis in 30-month-old rats are uniform throughout the brain or affect specific brain regions. A second question addressed in this work is whether the effect of ageing on calcium homeostasis is restricted to the nerve terminal or a more general process affecting also cell bodies. To study these questions cytosolic calcium regulation was studied in parallel in synaptosomes and a preparation of acutely dissociated brain cells obtained from different regions of 3- and 30-month-old rats. 45Ca2+ accumulation and distribution in mitochondria (assessed as FCCP-releasable 45Ca2+) was also studied. Mean [Ca2+]i obtained at rest and after high K+ depolarization were unchanged in cerebral cortex synaptosomes but increased in hippocampal synaptosomes at 30 months. Resting [Ca2+]i also increased with age in hippocampal, but not cerebral cortex cells, whereas the increase in [Ca2+]i obtained by depolarization was larger in both brain regions. Calcium compartmentation in mitochondria from hippocampal neurons incubated under high K+ conditions was also decreased with ageing. An altered calcium regulation in cell bodies and synaptic terminals in the hippocampus may be involved in the development of functional impairments in the hippocampal formation.

Rapid acquisition of dendritic spines by visual thalamic neurons after blockade of N-methyl-D-aspartate receptors

N-Methyl-D-aspartate (NMDA) receptors play an important role in the development of retinal axon arbors in the mammalian lateral geniculate nucleus (LGN). We investigated whether blockade of NMDA receptors in vivo or in vitro affects the dendritic development of LGN neurons during the period that retinogeniculate axons segregate into on-center and off-center sublaminae. Osmotic minipumps containing either the NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid (D-APV) or saline were implanted in ferret kits at postnatal day 14. After 1 week, LGN neurons were intracellularly injected with Lucifer yellow. Infusion of D-APV in vivo led to an increase in the number of branch points and in the density of dendritic spines compared with age-matched normal or saline-treated animals. To examine the time course of spine formation, crystals of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate were placed in the LGN in brain slices from 14- to 18-day-old ferrets. Labeled LGN cell dendrites were imaged on-line in living slices by confocal microscopy, with slices maintained either in normal perfusion medium or with the addition of D-APV or NMDA to the medium. Addition of D-APV in vitro at doses specific for blocking NMDA receptors led to a > 6-fold net increase in spine density compared with control or NMDA-treated slices. Spines appeared within a few hours of NMDA receptor blockade, indicating a rapid local response by LGN cells in the absence of NMDA receptor activation. Thus, activity-dependent structural changes in postsynaptic cells act together with changes in presynaptic arbors to shape projection patterns and specific retinogeniculate connections.

Proteolysis of spectrin by calpain accompanies theta-burst stimulation in cultured hippocampal slices

Tests were carried out to determine if repetitive bursts of afferent stimulation activate calpain, a calcium-dependent protease hypothesized to be involved in the production of long-term potentiation. Antibodies against a stable breakdown product that results from proteolysis of spectrin by calpain were used to identify sites of enzyme activation in cultured hippocampal slices. Slices in which theta-burst stimulation was applied to the Schaffer collateral fibers had pronounced accumulations of breakdown product that were restricted to field CA1, the zone innervated by the stimulated axons. Labelling occurred in the form of scattered puncta and was also present in dendritic processes. The extent of these effects was correlated (r = 0.73) with the amount of theta-burst stimulation delivered. Control slices or those receiving low frequency stimulation had variable, but uniformly lower, amounts of breakdown product and were clearly distinguishable from those given theta bursts. Statistical analyses using a six point rating scheme confirmed this point (P < 0.001). These results satisfy an essential prediction of the hypothesis that calpain plays an important role in the induction of long-term potentiation.

Neuropoiesis: proposal for a connectionistic neurobiology

Given current assumptions about the biology of neural organization, some connectionists believe that it may not be possible to accurately model the brain's neural architecture. We have identified five restrictive neurobiological dogmas that we believe have limited the exploration of more fundamental correlations between computational and biological neural networks. We postulate that: 1) the dendritic tree serves as a synapse storage device rather than a simple summation device; 2) connection strength between neurons depends on the number and location of synapses of similar weight, not on synapses of variable weights; 3) axonal sprouting occurs regularly in adult organisms; 4) the postsynaptic genome directly controls the presynaptic cell via mRNA, rather than indirectly by the expression of NCAMs, reverse neurotransmitters, etc.; 5) dendritic spines serve a trophic function by controlling development of new sprouts via a process we term retroduction. We entertain an alternative formulation of a computational neural element that is fully consistent with modern neuroscience research. We then show how our model neuron can learn under Hebbian conditions, and extend the model to explain non-Hebbian, one-trial learning. This work is significant because by stretching the theoretical boundaries of modern neuroscience, we show how connectionists can potentially create new, more biologically-based neural elements which, when, interconnected into networks, exhibit not only properties of existing backpropagation networks, but other physiological properties as well.

Long-term potentiation in the hippocampal CA1 region in the presence of N-methyl-D-aspartate receptor antagonists

The present study deals with the question of whether L-type voltage-gated calcium channels can support the induction of input specific long-term potentiation. Tetanus-induced potentiation of synaptic transmission was examined in the CA1 region of normal and disinhibited guinea-pig hippocampal slices. It was found that afferent tetanization in the presence of 50 microM of the N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid led to a prolonged input specific potentiation. This potentiation was found only in disinhibited slices, its induction required cooperativity, and it was associated with an increase in the early part of the field excitatory postsynaptic potential initial slope. It was not affected by the L-type voltage-gated calcium channel blocker nifedipine, but it was reduced when D(-)-2-amino-5-phosphonopentanoic acid was supplemented with other N-methyl-D-aspartate receptor antagonists. The present study also examined a potentiation that was not restricted to the activated synapses and that was not associated with an increase in the early part of the field excitatory postsynaptic potential initial slope. This potentiation was blocked by the L-type voltage-gated calcium channel antagonist nifedipine. It is concluded that calcium influx through L-type voltage-gated calcium channels participates in the generation of a prolonged potentiation, but not of the input specific N-methyl-D-aspartate receptor-dependent long-term potentiation. The results also suggest that high concentrations of D(-)-2-amino-5-phosphonopentanoic acid can be insufficient to fully prevent N-methyl-D-aspartate receptor activation by strong afferent tetanization in the disinhibited slice.

Imaging of calcium variations in living dendritic spines of cultured rat hippocampal neurons

1. Cultured rat hippocampal neurons were loaded with the Ca2+ indicator fura-2 through micropipettes and visualized with an inverted microscope equipped with a high power objective and a cooled CCD camera. The responses of dendritic spines and their parent dendrites to stimuli which evoke a rise of [Ca2+]i were monitored. 2. NMDA caused a rapid and transient rise in [Ca2+]i, which was more evident in the spine than in the parent dendrite. The recovery in both compartments had the same time course, and was dependent on normal [Na+]o. 3. Application of alpha-latrotoxin, which causes release of neurotransmitters from terminals, produced a rise of [Ca2+]i in the dendritic spines, more than in their parent dendrites. Prolonged exposure to the drug eliminated the spine/dendrite disparity. 4. The presence of voltage-gated calcium channels in dendritic spines is indicated by the enhanced calcium rise in spines rather than dendrites of cells depolarized by either intracellular current injection or by raising [K+]o. This rise was attenuated by nifedipine or verapamil, both L-type channel blockers. 5. It is suggested that the dendritic spine constitutes an independent calcium compartment that is closely linked to the parent dendrite.

Backpropagation of action potentials generated at ectopic axonal loci: hypothesis that axon terminals integrate local environmental signals

This review deals with the fascinating complexity of presynaptic axon terminals that are characterized by a high degree of functional distinctiveness. In vertebrate and invertebrate neurons, all-or-none APs can take off not only from the axon hillock, but also from ectopic axonal loci including terminals. Invertebrate neurons display EAPs, for instance alternating with somatic APs, during survival functions. In vertebrate, EAPs have been recorded in the peripheral and central nervous systems in time relationship with physiological or pathological neuronal activities. In motor or sensory axon, EAP generation may be the cause of motor dysfunctioning or sensory perceptions and pain respectively. Locomotion is associated with rhythmic depolarizations of the presynaptic axonal membrane of primary afferents, which are ridden by robust EAP bursts. In central axons lying within an epileptic tissue EAP discharges, coinciding with paroxysmal ECoG waves, get longer as somatic discharges get shorter during seizure progression. Once invaded by an orthodromic burst, an ectopic axonal locus can display an EAP after discharge. Such loci can also fire during hyperpolarization or the postinhibitory excitatory period of the parent somata, but not during their tonic excitation. Neurons are thus endowed with electrophysiological intrinsic properties making possible the alternate discharges of somatic APs and EAPs. In invertebrate and vertebrate neurons, ectopic axonal loci fire while the parent somata stop firing, further suggesting that axon terminal networks are unique and individual functional entities. The functional importance of EAPs in the nervous systems is, however, not yet well understood. Ectopically generated axonal APs propagate backwards and forwards along the axon, thus acting as a retrograde and anterograde signal. In invertebrate neurons, somatically and ectopically generated APs cannot have the same effect on the postsynaptic membrane. As suggested by studies related to the dorsal root reflex, EAPs may not only be implied in the presynaptic modulation of transmitter release but also contribute significantly during their backpropagation to a powerful control (collision process) of incoming volleys. From experimental data related to epileptiform activities it is proposed that EAPs, once orthodromically conducted, might potentiate synapses, initiate, spread or maintain epileptic cellular processes. For instance, paroxysmal discharges of EAPs would exert, like a booster-driver, a powerful synchronizing synaptic drive upon a large number of excitatory and inhibitory postsynaptic neurons. We have proposed that, once backpropagated, EAPs are likewise capable of initiating (and anticipating) threshold and low-threshold somatodendritic depolarizations. Interestingly, an antidromic EAP can modulate the excitability of the parent soma.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell-cell interactions define the innervation patterns of central leech neurons during development

In the last 20 years, the nervous system of the developing leech has been used to great advantage to study the processes by which neurons seek and finally innervate their targets. This review summarizes what is presently known about how neurons of the CNS interact with each other and with their targets during embryogenesis.

Morphological alterations in dendritic spines of rat hippocampal neurons exposed to N-methyl-D-aspartate

Individual cultured rat hippocampal neurons grown on glass coverslips were loaded with lucifer yellow (LY) and segments of their dendrites, containing dendritic spines, were exposed to the glutamate agonist N-methyl-D-aspartate (NMDA). Repeated pulse application of NMDA caused a reduction of spine length by 20%, within 1-2 h of exposure to the drug. No such changes were seen in spines exposed to NMDA in presence of the antagonist 2-aminophosphonovalerate or in spines on sister dendrites which were not exposed to NMDA. It is suggested that dendritic spines shrink following exposure to excessive synaptic stimulation.

Dendritic spines as basic functional units of neuronal integration

Most excitatory synaptic connections occur on dendritic spines. Calcium imaging experiments have suggested that spines constitute individual calcium compartments, but recent results have challenged this idea. Using two-photon microscopy to image fluorescence with high resolution in strongly scattering tissue, we measured calcium dynamics in spines from CA1 pyramidal neurons in slices of rat hippocampus. Subthreshold synaptic stimulation and spontaneous synaptic events produced calcium accumulations that were localized to isolated spines, showed stochastic failure, and were abolished by postsynaptic blockers. Single somatic spikes induced fast-peaking calcium accumulation in spines throughout the cell. Pairing of spikes with synaptic stimulation was frequently cooperative, that is, it resulted in supralinear calcium accumulations. We conclude: (1) calcium channels exist in spine heads; (2) action potentials invade the spines; (3) spines are individual calcium compartments; and (4) spines can individually detect the temporal coincidence of pre- and postsynaptic activity, and thus serve as basic functional units of neuronal integration.

Micromolar Ca2+ transients in dendritic spines of hippocampal pyramidal neurons in brain slice

The magnitude and dynamics of [Ca2+] changes in spines and dendrites of hippocampal CA1 pyramidal neurons have been characterized using a low affinity fluorescent indicator, mag-Fura 5, that is sensitive to Ca2+ in the micromolar range. During tetanic stimulation (1 s), we observed progressive [Ca2+] increases in distal CA1 spines to as much as 20-40 microM, both in organotypic slice culture and acute slice. Similar accumulations were reached during continuous depolarization (+10 mV, 1 s) when K+ channels had been blocked, but not with spike trains driven by postsynaptic current injection. The large [Ca2+] increases due to tetanic stimulation were blocked by APV, indicating that NMDA receptor-dependent influx was critical for the large responses. These findings have significant implications for low affinity Ca(2+)-dependent biochemical processes and show a new upper limit for [Ca2+] changes measured in these neurons during stimulation.

Imaging of calcium inside dendrites and spines: goals, issues and perspectives

The growing interest in dendritic spines in recent years originates from both the realization that the spine is likely to be the site where long-term plastic changes in synaptic properties take place, and that imaging methods are available which allow, for the first time, the study of these changes in the living dendritic spine. This report briefly summarizes methodological and biological issues associated with the study of dendritic spines in living tissue. The combined use of electrical and imaging methods for the study of dendritic spines certainly will contribute to a better understanding of synaptic integration and plasticity.

Ca2+ waves in PC12 neurites: a bidirectional, receptor-oriented form of Ca2+ signaling

Spatial and temporal aspects of Ca2+ signaling were investigated in PC12 cells differentiated with nerve growth factor, the well known nerve cell model. Activation of receptors coupled to polyphosphoinositide hydrolysis gave rise in a high proportion of the cells to Ca2+ waves propagating non decrementally and at constant speed (2-4 microns/s at 18 degrees C and approximately 10-fold faster at 37 degrees C) along the neurites. These waves relied entirely on the release of Ca2+ from intracellular stores since they could be generated even when the cells were incubated in Ca(2+)-free medium. In contrast, when the cells were depolarized with high K+ in Ca(2+)-containing medium, increases of cytosolic Ca2+ occurred in the neurites but failed to evolve into waves. Depending on the receptor agonist employed (bradykinin and carbachol versus ATP) the orientation of the waves could be opposite, from the neurite tip to the cell body or vice versa, suggesting different and specific distribution of the responsible surface receptors. Cytosolic Ca2+ imaging results, together with studies of inositol 1,4,5-trisphosphate generation in intact cells and inositol 1,4,5-trisphosphate-induced Ca2+ release from microsomes, revealed the sustaining process of the waves to be discharge of Ca2+ from the inositol 1,4,5-trisphosphate- (and not the ryanodine-) sensitive stores distributed along the neurites. The activation of the cognate receptor appears to result from the coordinate action of the second messenger and Ca2+. Because of their properties and orientation, the waves could participate in the control of not only conventional cell activities, but also excitability and differential processing of inputs, and thus of electrochemical computation in nerve cells.

Interaction of autophosphorylated Ca2+/calmodulin-dependent protein kinase II with neuronal cytoskeletal proteins. Characterization of binding to a 190-kDa postsynaptic density protein

Subcellular localization of Ca2+/calmodulin-dependent protein kinase II (CaMKII) by interaction with specific anchoring proteins may be an important mechanism contributing to the regulation of CaMKII. Proteins capable of binding CaMKII were identified by the use of a gel overlay assay with recombinant mouse CaMKII alpha (mCaMKII alpha) or Xenopus CaMKII beta (xCaMKII beta) 32P-autophosphorylated at Thr286/287 as a probe. Numerous [32P]CaMKII-binding proteins were identified in various whole rat tissue extracts, but binding was most prominent to forebrain proteins of 190 kDa (p190) and 140 kDa (p140). Fractionation of forebrain extracts localized p190 and p140 to a crude particulate/cytoskeletal fraction and isolated postsynaptic densities. [32P]m-CaMKII alpha-bound to p190 with an apparent Kd of 609 nM (subunit concentration) and a Bmax of 7.0 pmol of mCaMKII alpha subunit bound per mg of P2 protein, as measured using the overlay assay. Binding of 100 nM [32P]m-CaMKII alpha to p190 was competed by nonradioactive mCaMKII alpha autophosphorylated on Thr286 (EC50% = 200 nM), but to a much lesser extent by nonradioactive mCaMKII alpha autophosphorylated on Thr306 (EC50% > 2000 nM). In addition, nonphosphorylated mCaMKII alpha was a poor competitor for [32P]mCaMKII alpha binding to p190. The competition data indicate that Ca2+/CaM-dependent autophosphorylation at Thr286 promotes binding to p190, whereas, Ca2+/CaM-independent autophosphorylation at Thr306 does not enhance binding. Therefore, CaMKII may become localized to postsynaptic densities by p190 following its activation by an increase of dendritic Ca2+ concentration.

Calcium signaling in neurons: molecular mechanisms and cellular consequences

Neuronal activity can lead to marked increases in the concentration of cytosolic calcium, which then functions as a second messenger that mediates a wide range of cellular responses. Calcium binds to calmodulin and stimulates the activity of a variety of enzymes, including calcium-calmodulin kinases and calcium-sensitive adenylate cyclases. These enzymes transduce the calcium signal and effect short-term biological responses, such as the modification of synaptic proteins and long-lasting neuronal responses that require changes in gene expression. Recent studies of calcium signal-transduction mechanisms have revealed that, depending on the route of entry into a neuron, calcium differentially affects processes that are central to the development and plasticity of the nervous system, including activity-dependent cell survival, modulation of synaptic strength, and calcium-mediated cell death.

The alpha and gamma 1 isoforms of protein phosphatase 1 are highly and specifically concentrated in dendritic spines

Protein phosphatase 1 (PP1) is a highly conserved enzyme that has been implicated in diverse biological processes in the brain as well as in nonneuronal tissues. The present study used light and electron microscopic immunocytochemistry to characterize the distribution of two PP1 isoforms, PP1 alpha and PP1 gamma 1, in the rat neostriatum. Both isoforms are heterogeneously distributed in brain with the highest immunoreactivity being found in the neostriatum and hippocampal formation. Further, both isoforms are highly and specifically concentrated in dendritic spines. Weak immunoreactivity is present in dendrites, axons, and some axon terminals. Immunoreactivity for PP1 alpha is also present in the perikaryal cytoplasm and nuclei of most medium- and large-sized neostriatal neurons. The specific localization of PP1 in dendritic spines is consistent with a central role for this enzyme in signal transduction. The data support the concept that, in the course of evolution, spines developed as specialized signal transduction organelles enabling neurons to integrate diverse inputs from multiple afferent nerve terminals.

Measurements of exocytosis from single presynaptic nerve terminals reveal heterogeneous inhibition by Ca(2+)-channel blockers

The effect of various Ca(2+)-channel blockers on exocytosis has been studied at the level of single presynaptic terminals in rat hippocampal cell cultures. The fluorescence change of the styryl dye FM 1-43 has been used as a measure of exocytosis during electrical stimulation. omega-Conotoxin GVIA (2-10 microM) completely inhibited exocytosis in approximately 45% of the boutons in the field of view, while in approximately 55% exocytosis was inhibited incompletely (by 38%). This heterogeneity in response of presynaptic boutons was not seen with isradipine (5 microM) or omega-agatoxin IVA (80 nM), which inhibited exocytosis by 23% and 17%, respectively. However, it was observed with a combination of all three blockers. Pre- and postsynaptic events could be separated in single synapses by measuring FM1-43 release and NMDA-induced changes in the intracellular Ca2+ concentration independently.

Colocalization of calcium entry and exocytotic release sites in adrenal chromaffin cells

"Snapshot" images of localized Ca2+ influx into patch-clamped chromaffin cells were captured by using a recently developed pulsed-laser imaging system. Transient opening of voltage-sensitive Ca2+ channels gave rise to localized elevations of Ca2+ that had the appearance of either "hotspots" or partial rings found immediately beneath the plasma membrane. When the Ca2+ imaging technique was employed in conjunction with flame-etched carbon-fiber electrodes to spatially map the release sites of catecholamines, it was observed that the sites of Ca2+ entry and catecholamine release were colocalized. These results provide functional support for the idea that secretion occurs from "active zone"-like structures in neuroendocrine cells.

Multideterminant role of calcium in hippocampal synaptic plasticity

Hippocampal CA1 cells possess several varieties of long-lasting synaptic plasticity: two different forms of long-term potentiation (LTP) and at least one form of long-term depression (LTD). All forms of synaptic plasticity are induced by afferent activation, all involve Ca2+ influx, all can be blocked by Ca2+ chelators, and all activate Ca(2+)-dependent mechanisms. The question arises as how different physiological responses can be initiated by activation of the same second messenger. We consider two hypotheses which could account for these phenomena: voltage-dependent differences in cytosolic Ca2+ concentration acting upon Ca2+ substrates of differing Ca2+ affinities and compartmentalization of the Ca2+ and its substrates.

Three-dimensional organization of neuronal and glial processes: high voltage electron microscopy

Neurons and glia cells in the mammalian central nervous system have many complicated processes. They are too fine for light microscopic study and too complicated and widely spread for thin section electron microscopy. High-voltage electron microscopic (HVEM) stereo observation of thick Golgi preparation provides detailed 3-D images of their processes. Three-dimensional fine structures of astocytic processes in the neuropile and on the surface of neuronal somata, and those of the ruffed cell axon initial segment and thorny excrescences of CA3 pyramidal cell dendrites, are elucidated with the aid of HVEM stereoscopy of thick Golgi preparations. In addition, some results obtained by 3-D morphometrical analysis of dendritic spines using HVEM stereo images are shown. The examples presented here clearly show the usefulness of high-voltage electron microscope stereo observation of thick specimens for detailed morphological and morphometric study of the central nervous system.

Understanding the neostriatal microcircuitry: high-voltage electron microscopy

High voltage electron microscopy (HVEM) and HVEM tomography of selectively stained cell processes in the neostriatum have offered an alternative to serial thin section reconstruction for accurate 3-D visualization and measurement of axons, dendrites, and dendritic spines. Tissue preparation is simple and rapid, allowing examination of large numbers of specimens required for quantitation of neuronal morphology. The resolution of the images exceeds that available from any light microscopic technique and is appropriate for measurement of the finest axons and dendritic spine necks. HVEM tomography allows the direct measurement of dendritic surface area, required for computational modeling of synaptic integration.

Confocal imaging of dendritic Ca2+ transients in hippocampal brain slices during simultaneous current- and voltage-clamp recording

Changes in the intracellular Ca2+ concentration ([Ca2+]i) within CA1 hippocampal pyramidal neurons were imaged using confocal laser scanning microscopy in conjunction with Ca(2+)-sensitive fluorescent indicators. The imaging was performed in thick hippocampal brain slices while simultaneously measuring or controlling electrical activity with sharp microelectrodes or whole-cell patch-clamp electrodes. The combination of imaging and electrophysiology was essential for interpreting the changes in [Ca2+]i. We compared the increases in [Ca2+]i produced by either of two methods--direct depolarization of the cell via the somatic electrode or high-frequency stimulations of synaptic inputs. The increases in [Ca2+]i in the soma and proximal dendrites caused by both methods were of comparable magnitude and they always decayed within seconds in healthy cells. However, the spatial patterns of distal Ca2+ increases were different. Separate sets of synaptic inputs to the same cell resulted in different spatial patterns of [Ca2+]i transients. We isolated and observed what appeared to be a voltage-independent component of the synaptically mediated [Ca2+]i transients. This work demonstrates that the combination of neurophysiology and simultaneous confocal microscopy is well suited for visualizing and analyzing [Ca2+]i changes within highly localized regions of neurons in thick brain slices. The approach should allow further analysis of the relative contribution of voltage- and agonist-dependent influences on [Ca2+]i within neurons throughout the CNS and it raises the possibility of routinely relating subcellular [Ca2+]i changes to structural and functional modifications.

Mechanism of action and persistence of neuroprotection by cell-permeant Ca2+ chelators

Cell-permeant Ca2+ chelators such as 1,2-bis-(2-aminophenoxy)ethane- N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM) have been reported to protect neurons in experimental focal cerebral ischemia. However, their in vivo actions are uncertain, and their protective efficacy is proven only in brief cerebral ischemia paradigms. Here we examine their mechanism of action in vitro and duration of efficacy in vivo. Electrophysiological studies were made in CA1 neurons in rat hippocampal slices. When superfused with BAPTA-AM (30-50 microM), CA1 somatic field potential recordings showed attenuation of the population spike amplitude, and intracellular recordings showed reduced excitatory postsynaptic potentials, indicating inhibition of excitatory synaptic transmission. Also, Ca(2+)-dependent accommodation and post-spike-train hyperpolarizations were reduced, indicating Ca2+ chelation hear the internal cell membrane surface. To determine whether Ca2+ chelators reduce the size of cerebral infarction rather than simply delaying its evolution, we studied the effects of BAPTA-AM treatment on infarction size 24 h after permanent middle cerebral artery occlusion. Fischer rats (n = 8 per group) were pretreated with saline, BAPTA-AM (20 mg/kg), or MK-801 (0.5 mg/kg). Infarction volumes in animals treated with BAPTA-AM were reduced by 50.5% compared with controls (p = 0.018), whereas animals treated with MK-801 experienced a statistically insignificant infarct volume reduction (26%; p = 0.27). These data show a persistence of neuroprotection by the Ca2+ chelator at 24 h and indicate that it may act by attenuating synaptic transmission and subplasma membrane Ca2+ excess.

Ultrastructural plasticity of the dentate gyrus granule cells following recurrent limbic seizures: II. Alterations in somatic synapses

Hilus lesion-induced recurrent limbic seizures cause a dramatic increase in the numbers of somatic spines on dentate gyrus granule cells in the adult rat. Somatic spines are maximally increased 3 h after the initiation of seizures at which time many of these spines form synapses. The present quantitative electron microscopic study assessed the numbers and types of synapses present on the granule cell perikarya and somatic spines of control and experimental seizure rats with the goal of determining if newly elaborated somatic spines arise at the site of pre-existing synapses or are associated with new innervation. Experimental rats were sacrificed 5 h after hilar lesion placement (or 3 h after seizure onset). In both control and hilus-lesioned (HL) rats, 15-20% of the somatic spines could be seen to form synaptic contacts within a single plane of section; these synapses were almost exclusively of the asymmetric type. With the increased incidence of spines in experimental-seizure rats, there was a 6.25-fold greater number of spine synapses in HL versus control rats. There was, in addition, a 60% decrease in the number of asymmetric synapses occurring directly on the granule cell perikarya but no change in the total (spine plus somatic) number of asymmetric synapses. Although few asymmetric synapses were associated with spines in control tissue, 60-70% of asymmetric synapses were associated with spines in experimental-seizure tissue. In addition, in hilus lesion rats symmetric somatic synapses were increased by 20% on cells in deep stratum granulosum resulting in a dissolution of the superficial-to-deep innervation gradient present in the untreated rat. These findings support the conclusion that spines induced by seizure activity form at the site of pre-existing asymmetric synapses on the granule cells and demonstrate that brief seizure episodes can rapidly induce marked changes in innervation patterns in the adult brain.

Anatomical and functional imaging of neurons using 2-photon laser scanning microscopy

Light scattering by brain tissue and phototoxicity are major obstacles to the use of high-resolution optical imaging and photo-activation ('uncaging') of bioactive compounds from inactive ('caged') precursors in intact and semi-intact nervous systems. Optical methods based on 2-photon excitation promise to reduce these obstacles (Denk, 1994; Denk et al., 1990, 1994). Here we show a range of imaging modes based on 2-photon laser scanning microscopy (TPLSM) as applicable to problems in neuroscience. Fluorescence images were taken of neurons labeled with ion-sensitive and voltage-sensitive dyes in invertebrate ganglia, mammalian brain slices, and from the intact mammalian brain. Scanning photochemical images with whole-cell current detection (Denk, 1994) show how the distribution of neurotransmitter receptors on the surface of specific cells can be mapped. All images show strong optical sectioning and usable images can be obtained at depths greater than 100 microns below the surface of the preparation.

Dendritic Ca2+ accumulations and metabotropic glutamate receptor activation associated with an N-methyl-D-aspartate receptor-independent long-term potentiation in hippocampal CA1 neurons

Bathing hippocampal slices in the potassium channel blocker tetraethylammonium (TEA), while stimulating the Schaffer collaterals at a low frequency, induces Ca(2+)-dependent, N-methyl-D-aspartate (NMDA) receptor-independent long-term potentiation of synaptic transmission (LTPk) in CA1 neurons. We have combined ratio imaging of fura-2 and mag-fura-5 in hippocampal CA1 neurons with intracellular and field recordings to evaluate postsynaptic Ca2+ changes that occur in the induction of LTPk. Test stimuli were applied at 0.05 Hz to stratum radiatum in the presence of the NMDA receptor antagonists D,L-2-amino-5-phosphonovaleric acid (100 microM) or MK-801 (10 microM). During TEA exposure (15-25 mM; 10 min), cells fired prolonged action potentials both spontaneously and in response to test stimuli resulting in transient, micromolar Ca2+ accumulations in both somata and dendrites. The initial EPSP slope, measured 60 min after TEA wash-out, was potentiated to approximately 200% of control. The Ca2+ channel blocker nimodipine (10 microM) greatly reduced Ca2+ transients in both magnitude and duration and prevented LTPk induction. Pretreatment of slices with compounds that block metabotropic glutamate receptor (mGluR)-stimulated phosphoinositide hydrolysis, L-2-amino-3-phosphonopropionic acid (L-AP3, 50-200 microM) or L-aspartate-beta-hydroxamate (50-100 microM), as well as protein kinase C (PKC) inhibitors (sphingosine, 20 microM; RO-31-8220, 0.2 microM; or calphostin C, 2 microM) also blocked LTPk. Ca2+ transients were unaffected by L-AP3 or RO-31-8220. These findings suggest that Ca2+ influx through voltage-gated channels and co-activation of PKC by mGluRs are both necessary for induction of LTPk. Activation of mGluRs must also occur in NMDA receptor-dependent induction paradigms, but is possibly of lesser importance owing to the much greater gating of Ca2+ directly into the dendritic spines.

Impairment of GABAA receptor function by N-methyl-D-aspartate-mediated calcium influx in isolated CA1 pyramidal cells

Mechanisms of regulation of GABAA receptor function by intracellular calcium ([Ca2+]i) were examined in cell somata and apical dendrites of pyramidal cells, acutely dissociated from the CA1 hippocampal subfield of adult guinea-pigs. GABAA receptor-mediated currents were measured by whole-cell clamp recordings. N-methyl-D-aspartate receptor-mediated currents were used as conditioning source of calcium influx. Peak amplitudes of somatic GABAA whole-cell currents were reduced to about 15% of control values when net inward charge accumulation by N-methyl-D-aspartate currents reached 1.85 nC. A similar decline of GABAA currents was observed in dendritic recordings. The N-methyl-D-aspartate-mediated reduction of somatic and dendritic GABAA currents was accompanied by a well correlated decrease in peak and chord conductances. Pharmacological blockade of N-methyl-D-aspartate currents by 2-amino-5-phosphonopentanoic acid prevented the N-methyl-D-aspartate-mediated suppression of GABAA responses. The N-methyl-D-aspartate effect was mediated by the calcium component of N-methyl-D-aspartate receptor-mediated currents as demonstrated by a lack of effect in the absence of extracellular calcium and faster N-methyl-D-aspartate-mediated suppression of GABAA responses in lower intracellular 1,2-bis(2-aminophenoxy)ethane-N,N,N',N"-tetra-acetate. N-methyl-D-aspartate-mediated suppression of GABAA currents was significantly less expressed when intracellular ATP was replaced by its analog adenosine 5'-O-(3-thiotriphosphate) and when the specific phosphatase 2B inhibitor cypermethrin was added intracellularly. The reduction of GABAA responses persisted after cessation of N-methyl-D-aspartate-mediated calcium influx, indicating a long-term action of N-methyl-D-aspartate on GABAA responses. Voltage-activated calcium currents did not affect GABAA responses under the experimental conditions applied. In conclusion, the data presented show that calcium influxes through N-methyl-D-aspartate receptor channels result in long-term suppression of GABAA receptor function in CA1 pyramidal cells. Intracellular mechanisms of N-methyl-D-aspartate-mediated reduction of GABAA conductances involve activation of phosphatase 2B and consecutive dephosphorylation of the GABAA receptor or a closely associated GABAA receptor-regulating enzyme. Possible mechanisms of such a distinct N-methyl-D-aspartate-dependent calcium signalling pathway in the dephosphorylation-dependent suppression or GABAA receptor function are discussed.

Spatial changes in calcium signaling during the establishment of neuronal polarity and synaptogenesis

Calcium imaging techniques were used to obtain a clear although indirect evidence about the distribution of functional glutamate receptors of NMDA and non-NMDA type in cultured hippocampal neurons during establishment of polarity and synaptogenesis. Glutamate receptors were expressed and were already functional as early as one day after plating. At this stage NMDA and non-NMDA receptors were distributed in all plasmalemmal areas. During the establishment of neuronal polarity, responses to either types of glutamate receptors became restricted to the soma and dendrites. Compartmentalization of glutamate receptors occurred at stages of development when synaptic vesicles were already fully segregated to the axon. Formation of synapses was accompanied by a further redistribution of receptors, which segregated to synapse-enriched portions of dendrites. Receptor compartmentalization and dendritic redistribution as well as accumulation of synaptic vesicles at synaptic sites occurred also in neurons cultured in the presence of either the sodium channel blocker tetrodotoxin or glutamate receptor antagonists. These results indicate that signals generated by neuronal electrical activity or receptor activation are not involved in the establishment of neuronal polarity and synaptogenesis.

Local communication within dendritic spines: models of second messenger diffusion in granule cell spines of the mammalian olfactory bulb

Dendritic spines are generally believed to play a role in modulating synaptically induced electrical events. In addition, they may also confine second messengers and thus topologically limit the distance over which second messenger cascades may be functionally significant. In order to address this possibility, computer simulations of transient second messenger concentration changes were performed. The results show the importance of spine morphology and binding and extrusion mechanisms in controlling second messenger transients. In the presence of intrinsic cytoplasmic binding sites and kinetic rates similar to that expected for calcium, second messengers were confined to the spine head. In the absence of binding/extrusion mechanisms, the size and time course of the input transient to the spine head influenced the second messenger transients that might be seen at the base of the spine neck and in other spines. Large and/or sustained increases in second messenger concentration in the spine head were communicated to the spine base and to other spine heads. The results emphasize the importance of a knowledge of breakdown pathways, concentrations and kinetics of binding sites, and extrusion mechanisms for understanding the dynamics of local chemical changes for dendritic spine function.

Pulsed laser imaging of rapid Ca2+ gradients in excitable cells

Excitable cells are thought to respond to action potentials by forming short lived and highly localized Ca2+ gradients near sites of Ca2+ entry or near the site of Ca2+ release by intracellular stores. However, conventional imaging techniques lack the spatial and temporal resolution to capture these gradients. Here we demonstrate the use of pulsed-laser microscopy to measure Ca2+ gradients with submicron spatial resolution and millisecond time resolution in two preparations where the Ca2+ signal is thought to be fast and highly localized: adrenal chromaffin cells, where the entry of Ca2+ through voltage dependent Ca2+ channels triggers exocytotic fusion; and skeletal muscle fibers, where intracellular Ca2+ release from the sarcoplasmic reticulum initiates contraction. In chromaffin cells, Ca2+ gradients developed over 10-100 ms and were initially restricted to discrete submembrane domains, or hot spots, before developing into complete rings of elevated Ca2+ concentration. In frog skeletal muscle large, short-lived (approximately 6 ms) Ca2+ gradients were observed within individual sarcomeres following induction of action potentials. The pulsed laser imaging approach permits, for the first time, the capture and critical examination of rapid Ca2+ signaling events such as those underlying excitation-secretion and excitation-contraction coupling.

Input from the amygdala to the rat nucleus accumbens: its relationship with tyrosine hydroxylase immunoreactivity and identified neurons

Both tyrosine hydroxylase-positive fibres from the mesolimbic dopamine system and amygdala projection fibres from the basolateral nucleus are known to terminate heavily in the nucleus accumbens. Caudal amygdala fibres travelling dorsally via the stria terminalis project densely to the nucleus accumbens shell, especially in the dopamine rich septal hook. The amygdala has been associated with the recognition of emotionally relevant stimuli while the mesolimbic dopamine system is implicated with reward mechanisms. There is behavioural and electrophysiological evidence that the amygdala input to the nucleus accumbens is modulated by the mesolimbic dopamine input, but it is not known how these pathways interact anatomically within the nucleus accumbens. Using a variety of neuroanatomical techniques including anterograde and retrograde tracing, immunocytochemistry and intracellular filling, we have demonstrated convergence of these inputs on to medium-sized spiny neurons. The terminals of the basolateral amygdala projection make asymmetrical synapses predominantly on the heads of spines which also receive on their necks or adjacent dendrites, symmetrical synaptic input from the mesolimbic dopamine system. Some of these neurons have also been identified as projection neurons, possibly to the ventral pallidum. We have shown a synaptic level how dopamine is positioned to modulate excitatory limbic input in the nucleus accumbens.

Glutamate-mediated astrocyte-neuron signalling

Neurotransmitter released from neurons is known to signal to neighbouring neurons and glia. Here we demonstrate an additional signalling pathway in which glutamate is released from astrocytes and causes an NMDA (N-methyl-D-aspartate) receptor-mediated increase in neuronal calcium. Internal calcium was elevated and glutamate release stimulated by application of the neuroligand bradykinin to cultured astrocytes. Elevation of astrocyte internal calcium was also sufficient to induce glutamate release. To determine whether this released glutamate signals to neurons, we studied astrocyte-neuron co-cultures. Bradykinin significantly increased calcium levels in neurons co-cultured with astrocytes, but not in solitary neurons. The glutamate receptor antagonists D-2-amino-5-phosphonopentanoic acid and D-glutamylglycine prevented bradykinin-induced neuronal calcium elevation. When single astrocytes were directly stimulated to increase internal calcium and release glutamate, calcium levels of adjacent neurons were increased; this increase could be blocked by D-glutamylglycine. Thus, astrocytes regulate neuronal calcium levels through the calcium-dependent release of glutamate.

Postsynaptic regulation of the development and long-term plasticity of Aplysia sensorimotor synapses in cell culture

The monosynaptic component of the neuronal circuit that mediates the withdrawal reflex of Aplysia californica can be reconstituted in dissociated cell culture. Study of these in vitro monosynaptic connections has yielded insights into the basic cellular mechanisms of synaptogenesis and long-term synaptic plasticity. One such insight has been that the development of the presynaptic sensory neurons is strongly regulated by the postsynaptic motor neuron. Sensory neurons which have been cocultured with a target motor neuron have more elaborate structures--characterized by neurites with more branches and varicosities--than do sensory neurons grown alone in culture or sensory neurons that have been cocultured with an inappropriate target cell. Another way in which the motor neuron regulates the development of sensory neurons is apparent when sensorimotor cocultures with two presynaptic cells are examined. In such cocultures the outgrowth from the different presynaptic cells is obviously segregated on the processes of the postsynaptic cell. By contrast, when two sensory neurons are placed into cell culture without a motor neuron, their processes readily grow together. In addition to regulating the in vitro development of sensory neurons, the motor neuron also regulates learning-related changes in the structure of sensory neurons. Application of the endogenous facilitatory transmitter serotonin (5-HT) causes long-term facilitation of in vitro sensorimotor synapses due in part to growth of new presynaptic varicosities. But 5-HT applied to sensory neurons alone in culture does not produce structural changes in these cells. More recently it has been found that sensorimotor synapses in cell culture can exhibit long-term potentiation (LTP). Like LTP of some hippocampal synapses, LTP of in vitro Aplysia synapses is regulated by the voltage of the postsynaptic cell. Pairing high-frequency stimulation of sensory neurons with strong hyperpolarization of the motor neuron blocks the induction of LTP. Moreover, LTP of sensorimotor synapses can be induced in Hebbian fashion by pairing weak presynaptic stimulation with strong postsynaptic depolarization. These findings implicate a Hebbian mechanism in classical conditioning in Aplysia. They also indicate that Hebbian LTP is a phylogenetically ancient form of synaptic plasticity.

Dendritic calcium dynamics

Further characterization of the biochemical components that contribute to calcium handling, together with advances in optical imaging of ion concentration, are providing quantitative information on the dynamics of calcium in the dendrites of neurons in tissue culture, brain slices and in vivo. It has recently been demonstrated that strong spatial gradients and transient calcium elevations result from the geometry and membrane properties of dendrites. These studies are adding to our understanding of calcium's role in synaptic plasticity and in shaping the electrophysiological properties of neurons.

Chronic intrastriatal quinolinic acid produces reversible changes in perikaryal calbindin and parvalbumin immunoreactivity

We recently reported the use of a chronic dialytic delivery system for intrastriatal administration of quinolinic acid in the rat. This system produces neurodegeneration with some characteristics similar to post mortem brain tissue from Huntington's disease patients, including reduced cytochrome oxidase staining, a decreased number of Nissl-stained neurons, and relative sparing of striatal NADPH-diaphorase containing neurons. The present findings show that chronic dialytic delivery of quinolinic acid also produces a Huntington's disease-like pattern of reduced calbindin and parvalbumin perikaryal immunoreactivity that is reversed in rats allowed four to eight weeks' recovery after cessation of quinolinic acid. Furthermore, cytochrome oxidase staining and the number of Nissl-stained cells were unchanged in the region of transient calbindin and parvalbumin immunoreactive perikaryal staining alterations. These results suggest that changes in calbindin and parvalbumin perikaryal immunoreactivity provide a relatively sensitive measure of quinolinic acid induced neurotoxicity. The reversible nature of reduced perikaryal immunoreactivity suggests a premorbid state of neurotoxicity, possibly marked by cellular redistribution of calbindin and parvalbumin.

Calcium transients in dendrites of neocortical neurons evoked by single subthreshold excitatory postsynaptic potentials via low-voltage-activated calcium channels

Simultaneous recordings of membrane voltage and concentration of intracellular Ca2+ ([Ca2+]i) were made in apical dendrites of layer 5 pyramidal cells of rat neocortex after filling dendrites with the fluorescent Ca2+ indicator Calcium Green-1. Subthreshold excitatory postsynaptic potentials (EP-SPs), mediated by the activation of glutamate receptor channels, caused a brief increase in dendritic [Ca2+]i. This rise in dendritic [Ca2+]i was mediated by the opening of low-voltage-activated Ca2+ channels in the dendritic membrane. The results provide direct evidence that dendrites do not function as passive cables even at low-frequency synaptic activity; rather, a single subthreshold EPSP changes the dendritic membrane conductance by opening Ca2+ channels and generating a [Ca2+]i transient that may propagate towards the soma. The activation of these Ca2+ channels at a low-voltage threshold is likely to influence the way in which dendritic EPSPs contribute to the electrical activity of the neuron.

Ca2+ signalling in postsynaptic dendrites and spines of mammalian neurons in brain slice

Postsynaptic Ca2+ changes are involved in control of cellular excitability and induction of synaptic long-term changes. We monitored Ca2+ changes in dendrites and spines during synaptic and direct stimulation using high resolution microfluorometry of fura-2 injected into CA3 pyramidal neurons in guinea pig hippocampal slice. When driven by current injection from an intracellular electrode or with synaptic stimulation, postsynaptic Ca2+ accumulations were highest in the proximal dendrites with a pronounced fall-off towards the soma and some fall-off towards more distal dendrites. Muscarinic activation by low concentrations of carbachol strongly increased intradendritic Ca2+ accumulation during directly-evoked repetitive firing. This enhancement occurred in large part because muscarinic activation suppressed the normal Ca(2+)-dependent activation of K-channels that mediates adaptation of firing. Repetitive firing of cholinergic fibers in the slice reproduced the effects of carbachol. Inhibition of acetylcholine-esterase activity by eserine enhanced the effects of repetitive stimulation of chlolinergic fibers. All effects were reversible and were blocked by the muscarinic antagonist atropine. Ca2+ accumulations in postsynaptic spines might be the basis of specificity of synaptic plasticity. With selective stimulation of few associative/comissural fibers, Ca2+ accumulated in single postsynaptic spines but not in the parent dendrite. With strong stimulation, dendrite levels also increased but spine levels were considerably higher. The NMDA-receptor antagonist AP-5 blocked Ca(2+)-peaks in spines, but left Ca2+ changes in dendrite shafts largely unaffected. Sustained steep Ca2+ gradients between single spines and the parent dendrite, often lasting several minutes, developed with repeated stimulation. Our results demonstrate a spine entity that can act independent from the dendrite with respect to Ca(2+)-dependent processes. Muscarinic augmentation of dendritic Ca2+ levels might reduce diffusional loss of Ca2+ from hot spines into the parent dendrite, thus supporting cooperativity and associativity of synaptic plasticity.

Map of the synapses formed with the dendrites of spiny stellate neurons of cat visual cortex

The synaptic input of six spiny stellate neurons in sublamina 4A of cat area 17 was assessed by electron microscopy. The neurons were physiologically characterized and filled with horseradish peroxidase in vivo. After processing the neurons were reconstructed at the light microscopic level using computer-assisted methods and analyzed quantitatively. The extensive branching of the dendritic tree about 50 microns from the soma meant that the distal branches constituted five times the length of proximal dendrite. Proximal and distal portions of a single dendrite from each neuron were examined in series of ultrathin sections (1,456 sections) in the electron microscope. The majority (79%) of the 263 synapses examined were asymmetric; the remainder (21%) were symmetric. Symmetric synapses formed 35% of synapses sampled on proximal dendrites and were usually located on the shaft. They formed only 4% of synapses sampled on distal dendrites. Spines accounted for less than half of the total asymmetric synapses (45%); the remainder were on shafts. Symmetric synapses formed with four of 92 spines. Nine spines formed no synapses. Spiny stellate neurons in cat visual cortex appear to differ considerably from pyramidal neurons in having a significant asymmetric (excitatory) synaptic input to the dendritic shaft.

Calcium-dependent postsynaptic exocytosis: a possible mechanism for activity-dependent synaptic modulation

Elevation of cytosolic Ca2+ level in the postsynaptic cell is critical for the induction of many forms of activity-dependent synaptic modulation. Based on our recent evidence that in muscle cells and fibroblasts constitutive exocytosis is increased by elevating cytosolic Ca2+ levels, we hypothesize that Ca(2+)-dependent exocytosis at the postsynaptic site may provide a mechanism for a localized, activity-dependent synaptic modulation.

Confocal laser scanning microscopy reveals voltage-gated calcium signals within hippocampal dendritic spines

The induction of long-term potentiation (LTP) is generally assumed to be triggered by Ca2+ entry into dendritic spines via NMDA receptor-gated channels. A previous computational model proposed that spines serve several functions in this process. First, they compartmentalize and amplify increase in [Ca2+]i. Second, they augment the nonlinear relationship between synaptic strength and the probability or magnitude of LTP induction. Third, they isolate the metabolic machinery responsible for LTP induction from increases in [Ca2+]i produced by voltage-gated Ca2+ channels in the dendritic shaft. Here we examine this last prediction of the model using methods that combine confocal microscopy with simultaneous neurophysiological recordings in hippocampal brain slices. Either of two Ca(2+)-sensitive dyes were injected into CA1 pyramidal neurons. Direct depolarization of the neurons via the somatic electrode produced clear increases in Ca2+ signals within the dendritic spines, a result that was not predicted by the previous spine model. Our new spine model suggests that some of this signal could theoretically result from Ca(2+)-bound dye diffusing from the dendritic shaft into the spine. Dye diffusion alone cannot, however, explain the numerous cases in which the Ca2+ signal in the spine was considerably larger than that in the adjacent dendritic shaft. The latter observations raise the possibility of voltage-gated Ca2+ entry directly into the spine or else perhaps via Ca(2+)-dependent Ca2+ release. The new spine model accommodates these observations as well as several other recent experimental results.

5-hydroxydopamine-labeled dopaminergic axons: three-dimensional reconstructions of axons, synapses and postsynaptic targets in rat neostriatum

Previous studies employing 5-hydroxydopamine to identify nigrostriatal dopaminergic axons and their synapses found that labeled axons made few synapses or that asymmetric contacts predominated. In contrast, recent studies using tyrosine hydroxylase or dopamine antibody techniques indicate that presumed dopaminergic axons form small symmetric contacts. We re-examined 5-hydroxydopamine-labeled material from the rat neostriatum using serial three-dimensional reconstruction techniques to characterize the morphology of labeled axons, synapses and postsynaptic targets. This ultrastructural analysis revealed a class of heavily labeled axons that are small (0.06-1.5 microns in diameter) and lack large varicosities. These axons form small (0.011-0.09 microns 2), en passant, symmetric synapses, mainly onto dendritic spines and spiny dendritic shafts and, in some cases, onto aspiny dendritic segments near branch points. The sites of these synapses along the axon appeared unrelated to the locations of axonal enlargements, suggesting that counting varicosities may not be an accurate indication of the extent of dopaminergic innervation in the neostriatum. The characteristics of these 5-hydroxydopamine-labeled elements correspond in all respects to axons and synapses identified as dopaminergic by immunohistochemistry in previous studies. In tissue in which all labeled and unlabeled synapses were classified, approximately 9% of all synapses were identified as dopaminergic by this type of label. Three-dimensional reconstructions provided additional insight concerning the interaction of dopaminergic afferents with postsynaptic striatal targets and their relation to other afferents to these neurons. They reveal that a short, unbranched dopaminergic axonal segment can make multiple synapses onto dendritic spines, shafts and branch points of one or more dendrites. In addition, one dendrite can receive contacts from several labeled axons. Dopamine synapses onto spines are always associated with unlabeled, asymmetric synapses onto the same spine. Synapses of various morphologies with a distinctly different, lighter form of labeling were much rarer, and may represent other aminergic afferents to the neostriatum. The presence of this second form of label in earlier 5-hydroxydopamine studies may have contributed to the long-standing controversy over the appearance of dopaminergic synapses examined by different techniques. Our results help to resolve this controversy and confirm that the nigrostriatal projection makes small symmetric synapses with a variety of striatal targets.