Electrically coupled but chemically isolated synapses: Dendritic spines and calcium in a rule for synaptic modification

An influential model of learning assumes synaptic enhancement occurs when there is pre- and post-synaptic conjunction of neuronal activity, as proposed by Hebb (1949) and studied in the form of long-term potentiation (LTP). There is evidence that LTP has a post-synaptic locus of control and is triggered by an elevation of intracellular calcium ion concentration, [Ca2+]i. Since synapses which undergo LTP are usually situated on dendritic spines, three effects of spine morphology on this system should be considered: (i) synapses on spines are chemically isolated by the barrier to Ca2+ diffusion due to the spine neck dimensions; (ii) the resistance of the spine neck permits a given synaptic current to bring about greater depolarization (of the spine head membrane) than the same current into a dendrite; while (iii) the spine neck resistance does not significantly attenuate current flow (in the dendrite to spine direction) because of the relatively high impedance of the spine head, and this permits electrical coupling via the dendritic tree. The specificity of LTP to activated synapses on depolarized cells has recently been attributed to special properties of the receptor-linked channel specifically activated by N-methyl-D-aspartate (NMDA). This admits calcium and other ions only when there is both depolarization and receptor activation. However, consideration of point (ii) suggests that, for spines with high resistance necks, the current through a synapse on the spine head will cause sufficient depolarization to unblock the NMDA channel. Thus, the properties of the NMDA channel do not account for the requirement for conjunction of pre- and post-synaptic activity, if these channels are located on the spine head. This suggests that additional mechanisms are required to explain why it is necessary to depolarize the post-synaptic cell in order to induce LTP. As an alternative, it is postulated that there exist voltage-sensitive calcium channels (VSCCs) on the spine head membrane, of a type which require greater membrane depolarization for activation. To generate the greater depolarization required, both pre- and post-synaptic activation would be necessary. If so, the role of dendritic or somatically located NMDA channels may be to "prime" neurons for LTP by enchancing voltage-dependent responses. A corollary is that spine resistance may regulate the threshold number of synapses required to produce LTP. It is predicted that, on spines with very high neck resistance (say, greater than 600 M omega), synaptic current alone may produce sufficient depolarization to activate VSCCs.(ABSTRACT TRUNCATED AT 400 WORDS)     

Two-photon Na+ imaging in spines and fine dendrites of central neurons

Dendritic spines are assumed to be the smallest units of neuronal integration. Because of their miniature size, however, many of their functional properties are still unclear. New insights in spine physiology have been provided by two-photon laser-scanning microscopy which allows fluorescence imaging with high spatial resolution and minimal photodamage. For example, two-photon imaging has been employed successfully for the measurement of activity-induced calcium transients in individual spines. Here, we describe the first application of two-photon imaging to measure Na+ transients in spines and dendrites of CA1 pyramidal neurons in hippocampal slices. Whole-cell patch-clamped neurons were loaded with the Na(+)-indicator dye SBFI (sodium-binding benzofuran-isophthalate). In situ calibration of SBFI fluorescence with ionophores enabled the determination of the actual magnitude of the [Na+]i changes. We found that back-propagating action potentials (APs) evoked Na+ transients throughout the proximal part of the dendritic tree and adjacent spines. The action-potential-induced [Na+]i transients reached values of 4 mM for a train of 20 APs and monotonically decayed with a time constant of several seconds. These results represent the first demonstration of activity-induced Na+ accumulation in spines. Our results demonstrate that two-photon Na+ imaging represents a powerful tool for extending our knowledge on Na+ signaling in fine cellular subcompartments.     

Calcium transients in cerebellar Purkinje neurons evoked by intracellular stimulation.

1. Purkinje cells in thin slices from the guinea pig cerebellum were injected with fura-2 and high-speed sequences of fluorescence images from the cell body and entire dendritic tree were made while simultaneously recording somatic membrane potential during evoked and spontaneous electrical activity. The changes in fluorescence were interpreted in terms of changes in [Ca2+]i. 2. Individual calcium action potentials were usually accompanied by transient increases in [Ca2+]i all over the dendritic field. During evoked or spontaneous bursts of calcium spikes, [Ca2+]i increased more rapidly and to higher concentrations in fine dendrites than in thicker dendrites. At the end of a burst [Ca2+]i declined faster in thin dendrites than in thicker ones. These variations are most easily understood as deriving from the difference in surface-to-volume ratio of the two kinds of dendrites. 3. During bursts of calcium action potentials [Ca2+]i increases sometimes occurred only in individual dendritic branches, but always including the fine dendrites of that particular branch, showing that calcium action potentials can be regenerative in restrictive parts of the dendritic field without involving the soma or dendritic shaft. 4. Plateau or subthreshold potential changes evoked in the presence of tetrodotoxin (TTX) caused small, widespread increases in [Ca2+]i. The amplitude of these changes was much less than the increase corresponding to spike bursts. The distribution of these plateau Ca signals in thick and thin dendrites was similar to Ca spike-evoked signals, suggesting that the Ca conductances underlying these two potentials are the same or are distributed similarly in the dendrites. 5. Significant increases in [Ca2+]i in the soma were recorded during bursts of sodium-dependent action potentials in normal Ringer. Although much of this increase is due to calcium entry through calcium channels, some of this increase could be due to calcium entry through sodium channels.     

Plasticity of calcium channels in dendritic spines

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

Potassium and calcium concentrations in interstitial fluid of hippocampal formation during paroxysmal responses

The concentration of potassium ([K+]o) and of calcium ([Ca2+]o) in interstitial fluid of the hippocampal formation of rats anesthetized with urethan was recorded with double-barreled ion-selective microelectrodes. The ipsilateral angular bundle was stimulated with trains of repetitive pulses. [K+]o increased during angular bundle stimulation in both dendritic and cell body layers of the fascia dentata. When stimulation was frequent and intense enough to provoke intercurrent paroxysmal discharge (IPaD), [K+]o in the granule cell body layer rose much above the level it attained during previous, nonparoxysmal activation. No similar excess increase of [K+]o related to paroxysmal firing was observed in the dendritic layer. It is concluded that tonic paroxysmal discharge of the granule cells is associated with an outflow of K ions from the cell somata, but not the dendrites. Extracellular sustained potential (SP) shifts and responses of [K+]o associated with paroxysmal firing showed no consistent correlation in fascia dentata. It is concluded that paroxysmal SP shifts in fascia dentata (unlike in spinal cord and cerebral neocortex) are dominated by the extracellular currents generated by granule cells, not by neuroglia. In the postparoxysmal phase, however, a small residual SP shift was observed in both soma and dendrite layers, which had characteristics compatible with its being generated by glial cells. Responses of [Ca2+]o varied from rat to rat. During nonparoxysmal excitation [Ca2+]o increased, decreased, or remained unchanged. During paroxysmal firing [Ca2+]o always decreased in the granule cell body layer, but the magnitude of the response varied greatly. In the dendritic layer a similar but smaller decrease was observed in some but not all cases. Probable reasons for the unpredictability of the responses of [Ca2+]o are discussed. The responses of [Ca2+]o recorded in fascia dentata of urethan-anesthetized rats that have previously been kindled were not detectably different from those of control animals. Le?o's spreading depression (LD) was associated with large increase of [K+]o, decrease of [Ca2+ )o, and intense negative SP shift in both dendritic and cell body layers of fascia dentata, as well as in CA1 zone of hippocampus. It is concluded that LD in hippocampal formation is associated with more widespread depolarization of pyramidal and granule cells than in cerebral neocortex and cerebellar cortex where changes of [K+]o are limited to the more superficial layers.     

Low threshold calcium currents in rat cerebellar Purkinje cell dendritic spines are mediated by T-type calcium channels

The functional role of low voltage activated (LVA) calcium channels in the cerebellar Purkinje cell dendritic tree is not completely understood. Since the localization of these channels will influence their possible roles in dendritic integration and induction of plasticity, we set out to characterize the LVA calcium current in Purkinje cell dendrites in acute cerebellar slices of young rats. Using a combination of electrophysiological recordings and two-photon laser scanning microscopy, we show that LVA calcium current recorded at the soma can be correlated with voltage-dependent calcium transients in Purkinje cell dendritic spines. Blocking sodium and potassium conductances allowed us to isolate and characterize a fast inactivating inward current activated positive to −55 mV. Activation and steady-state inactivation kinetics, voltage-dependent deactivation kinetics, and pharmacological experiments (using ω-agatoxin-IVA, mibefradil and nickel) show that this current is carried by T-type calcium channels. Furthermore, the LVA calcium transient observed in the dendritic spines of the Purkinje cell is well correlated with the current recorded at the soma, suggesting that T-type calcium channels are the main component of the LVA calcium input in spines. The fast rising phase of the calcium transient in spines and the absence of delay between the onset in the spine and the parent dendrite show that T-type calcium channels are present both in spines and dendrites of the Purkinje cell.     

Activity-induced elevations of intracellular calcium concentration in pyramidal and nonpyramidal cells of the CA3 region of rat hippocampal slice cultures.

1. Depolarization-induced elevations of intracellular calcium concentration ([Ca2+]i) were examined in slice-cultured hippocampal pyramidal and nonpyramidal cells of the CA3 region by combined intracellular and multisite fura-2 recording techniques. 2. In pyramidal cells, spiking activity induced by depolarizing current pulses (200-800 ms) induced transient elevations of somatic as well as of proximal dendritic [Ca2+]i. The calcium signals from the proximal dendrites were larger in amplitude and decayed much faster than those from the soma. Depolarization of presumed interneurons induced comparable somatic and dendritic calcium transients, which decayed faster than those observed in pyramidal cell somata. 3. The calcium transients of pyramidal cells, but not those of nonpyramidal cells, were associated with a slow afterhyperpolarization (sAHP), whose time course was correlated with that of the somatic calcium signal. We conclude that the lack of a sAHP in non-pyramidal cells cannot be explained by the absence of an efficient rise in [Ca2+]i but rather by the absence of the potassium conductance underlying the sAHP in pyramidal cells.     

The integrative properties of spiny distal dendrites.

The dendritic spines of many central neurons are generally thought to modulate the ability of individual synaptic conductances to depolarize the dendritic shaft. A compartmental analysis using typical spine dimensions shows that spine neck resistances are probably far too low to support such a function, because low conductance synapses act as time-varying current sources. However, the collective presence of all spines on a dendrite significantly modifies the electrical properties of the branch in ways which have previously been overlooked. In particular, they lower its input impedance and length constant, reducing the amplitude of the unitary excitatory postsynaptic potential as well as the strength of spatial summation. This enables a dendrite to integrate large numbers of synaptic inputs while occupying minimal volume. In this way, dendritic spines are analogous to axonal myelin, which also alters transcellular impedance in order to maximize neurite function and minimize volume. Unlike membrane resistance changes, spines have little effect on the membrane time-constant so they maintain a long window for temporal summation. Though spine shape and neck resistance do not significantly affect dendritic potentials, spine area does. Therefore, while changes in spine morphology probably do not directly potentiate the strength of individual synapses, changes in spine density can regulate the synaptic excitability of an entire dendrite. The shortened length-constant of the spiny dendrite requires excitable membranes to be located in distal dendrites. These, in turn, eliminate many of the electrotonic nonlinearities associated with summation in long, thin processes, and make all distal synapses equipotent. The short length-constant also enhances the sensitivity of dendritic spikes to local impedance changes while decreasing the sensitivity to distant impedance changes. This would enable a neuron to effectively use inhibitory synapses or branch points to regulate propagation through its spiny dendritic tree. A model neuron is developed in which dendritic spines, excitable membranes, and dendritic branching combine to form a two-stage filter, which serves as a synaptic input coincidence detector with adjustable gain. Gain is regulated by potassium conductances which modulate branch point safety factor. The model is consistent with the notion of functional independence of distal dendrites and demonstrates that certain aspects of dendritic spiking which have previously been thought to require membrane hot-spots can also result from geometrical properties. It is suggested that the activation of spiny neurons may depend as much on the density as on the number of active synapses, and that spiny neurons may tend to have discrete output states whereas nonspiny neurons may be more continuous.     

Spread of dendritic excitation in layer 2/3 pyramidal neurons in rat barrel cortex in vivo

In layer 2/3 pyramidal neurons of barrel cortex in vivo, calcium ion concentration ([Ca2+]) transients in apical dendrites evoked by sodium action potentials are limited to regions close to the soma. To study the mechanisms underlying this restricted pattern of calcium influx, we combined two-photon imaging of dendritic [Ca2+] dynamics with dendritic membrane potential measurements. We found that sodium action potentials attenuated and broadened rapidly with distance from the soma. However, dendrites of layer 2/3 cells were electrically excitable, and direct current injections could evoke large [Ca2+] transients. The restricted pattern of dendritic [Ca2+] transients is therefore due to a failure of sodium action-potential propagation into dendrites. Also, stimulating subcortical activating systems by tail pinch can enhance dendritic [Ca2+] influx induced by a sensory stimulus by increasing cellular excitability, consistent with the importance of these systems in plasticity and learning.     

Estradiol increases spine density and NMDA-dependent Ca2+ transients in spines of CA1 pyramidal neurons from hippocampal slices

To investigate the physiological consequences of the increase in spine density induced by estradiol in pyramidal neurons of the hippocampus, we performed simultaneous whole cell recordings and Ca2+ imaging in CA1 neuron spines and dendrites in hippocampal slices. Four- to eight-days in vitro slice cultures were exposed to 17beta-estradiol (EST) for an additional 4- to 8-day period, and spine density was assessed by confocal microscopy of DiI-labeled CA1 pyramidal neurons. Spine density was doubled in both apical and basal dendrites of the CA1 region in EST-treated slices; consistently, a reduction in cell input resistance was observed in EST-treated CA1 neurons. Double immunofluorescence staining of presynaptic (synaptophysin) and postsynaptic (alpha-subunit of CaMKII) proteins showed an increase in synaptic density after EST treatment. The slopes of the input/output curves of both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) postsynaptic currents were steeper in EST-treated CA1 neurons, consistent with the observed increase in synapse density. To characterize NMDA-dependent synaptic currents and dendritic Ca2+ transients during Schaffer collaterals stimulation, neurons were maintained at +40 mV in the presence of nimodipine, picrotoxin, and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). No differences in resting spine or dendritic Ca2+ levels were observed between control and EST-treated CA1 neurons. Intracellular Ca2+ transients during afferent stimulation exhibited a faster slope and reached higher levels in spines than in adjacent dendrites. Peak Ca2+ levels were larger in both spines and dendrites of EST-treated CA1 neurons. Ca2+ gradients between spine heads and dendrites during afferent stimulation were also larger in EST-treated neurons. Both spine and dendritic Ca2+ transients during afferent stimulation were reversibly blocked by D, L-2-amino-5-phosphonovaleric acid (D,L-APV). The increase in spine density and the enhanced NMDA-dependent Ca2+ signals in spines and dendrites induced by EST may underlie a threshold reduction for induction of NMDA-dependent synaptic plasticity in the hippocampus.     

Rapid plasticity of dendritic spine: hints to possible functions?

Contrary to a century-old belief that dendritic spines are stable storage sites of long term memory, the emerging picture from a recent flurry of exciting observations using novel high resolution imaging methods of living cells in culture is that of a dynamic structure, which undergoes fast morphological changes over periods of hours and even minutes. Concurrently, the nature of stimuli which cause formation or collapse of dendritic spines has changed from a mysterious Hebbian-governed plasticity producing stimulus to the more trivial activation of the synapse by strong/weak stimulation. The molecular mechanisms underlying spine plasticity are beginning to emerge; the role of presynaptic and/or postsynaptic activity, genetic, central or local factors in the formation and retraction of spines are currently being analyzed. A common mechanism for both, formation/elongation and pruning/retraction of spines, involving changes in intracellular calcium concentration ([Ca(2+)](i)), is emerging. It appears that [Ca(2+)](i) is related to changes in spines in a bell shape form: lack of synaptic activity causes transient outgrowth of filopodia but eventual elimination of spines, a moderate rise in [Ca(2+)](i) causes elongation of existing spines and formation of new ones, while a massive increase in [Ca(2+)](i) such as that seen in seizure activity, causes fast shrinkage and eventual collapse of spines. Nuclear signals (e.g. CREB), activated by an increase in [Ca(2+)](i), are involved in the central regulation of spine formation, while spine shrinkage and elongation are probably triggered by local [Ca(2+)](i) changes. This hypothesis provides a parsimonious explanation for conflicting reports on activity-dependent changes in dendritic spine morphology. Still, the many differences between cultured neurons, with which most of current studies are conducted, and the neuron in the real brain, require a cautious extrapolation of current assumptions on the regulation of spine formation.     

Mobilization of calcium by the brief application of oxytocin and prostaglandin E2 in single cultured human myometrial cells.

Intracellular calcium ([Ca2+]i) mobilization was studied in single cultured human myometrial cells in response to the agonists oxytocin and prostaglandin E2 (PGE2) using the fluorescent dye Fura-2. Oxytocin and PGE2 applications were associated with an increase in [Ca2+]i, although there was a marked intercell variation in the amplitude of the agonist-induced response. Removal of extracellular calcium ([Ca2+]o) reduced the oxytocin-induced rise and abolished the PGE2-induced rise in [Ca2+]i, thereby demonstrating that oxytocin but not PGE2 can mobilize intracellular stores of calcium. In nominally calcium-free medium, [Ca2+]i was not increased by PGE2 but subsequent application of oxytocin increased [Ca2+]i, thereby demonstrating that, within a single cell, calcium stores were mobilized by oxytocin and not PGE2. The intracellular calcium stores were completely depleted by a single application of oxytocin and not replenished in the absence of [Ca2+]o. Perfusion with calcium-containing medium for 100 s enabled store refilling. Cell depolarization by 140 mM-K+ caused a transient increase followed by a sustained elevation of [Ca2+]i on which were superimposed small fluctuations. Oxytocin caused an influx of calcium in cells depolarized by K+. This was more marked than that obtained with PGE2.     

Dendritic spines disappear with chilling but proliferate excessively upon rewarming of mature hippocampus

More dendritic spine synapses occur on mature neurons in hippocampal slices by 2 h of incubation in vitro, than in perfusion-fixed hippocampus. What conditions initiate this spinogenesis and how rapidly do the spines begin to proliferate on mature neurons? To address these questions, CA1 field of the hippocampus neurons expressing green fluorescent protein in living slices from mature mice were imaged with two-photon microscopy. Spines disappeared and dendrites were varicose immediately after slice preparation in ice-cold artificial cerebrospinal fluid (ACSF). Electron microscopy (EM) revealed disrupted dendritic cytoplasm, enlarged or free-floating postsynaptic densities, and excessive axonal endocytosis. Upon warming dendritic varicosities shrank and spines rapidly reappeared within a few minutes illustrating the remarkable resilience of mature hippocampal neurons in slices. When membrane impermeant sucrose was substituted for NaCl in ACSF dendrites remained spiny at ice-cold temperatures and EM revealed less disruption. Nevertheless, spine number and length increased within 30 min in warm ACSF even when the extracellular calcium concentration was zero and synaptic transmission was blocked. When slices were first recovered for several hours and then chilled in 6 degrees C ACSF many spines disappeared and the dendrites became varicose. Upon re-warming varicosities shrank and spines reemerged in the same position from which they disappeared. In addition, new spines formed and spines were longer suggesting that chilling, not the initial injury from slicing, caused the spines to disappear while re-warming triggered the spine proliferation on mature neurons. The new spines might be a substrate for neuronal recovery of function, when neurons have been chilled or exposed to other traumatic conditions that disrupt ionic homeostasis.     

Cortical area and species differences in dendritic spine morphology

Dendritic spines receive most excitatory inputs in the neocortex and are morphologically very diverse. Recent evidence has demonstrated linear relationships between the size and length of dendritic spines and important features of its synaptic junction and time constants for calcium compartmentalisation. Therefore, the morphologies of dendritic spines can be directly interpreted functionally. We sought to explore whether there were potential differences in spine morphologies between areas and species that could reflect potential functional differences. For this purpose, we reconstructed and measured thousands of dendritic spines from basal dendrites of layer III pyramidal neurons from mouse temporal and occipital cortex and from human temporal cortex. We find systematic differences in spine densities, spine head size and spine neck length among areas and species. Human spines are systematically larger and longer and exist at higher densities than those in mouse cortex. Also, mouse temporal spines are larger than mouse occipital spines. We do not encounter any correlations between the size of the spine head and its neck length. Our data suggests that the average synaptic input is modulated according to cortical area and differs among species. We discuss the implications of these findings for common algorithms of cortical processing.     

Molecular mechanisms of dendritic spine development and remodeling

Dendritic spines are small protrusions that cover the surface of dendrites and bear the postsynaptic component of excitatory synapses. Having an enlarged head connected to the dendrite by a narrow neck, dendritic spines provide a postsynaptic biochemical compartment that separates the synaptic space from the dendritic shaft and allows each spine to function as a partially independent unit. Spines develop around the time of synaptogenesis and are dynamic structures that continue to undergo remodeling over time. Changes in spine morphology and density influence the properties of neural circuits. Our knowledge of the structure and function of dendritic spines has progressed significantly since their discovery over a century ago, but many uncertainties still remain. For example, several different models have been put forth outlining the sequence of events that lead to the genesis of a spine. Although spines are small and apparently simple organelles with a cytoskeleton mainly composed of actin filaments, regulation of their morphology and physiology appears to be quite sophisticated. A multitude of molecules have been implicated in dendritic spine development and remodeling, suggesting that intricate networks of interconnected signaling pathways converge to regulate actin dynamics in spines. This complexity is not surprising, given the likely importance of dendritic spines in higher brain functions. In this review, we discuss the molecules that are currently known to mediate the exquisite sensitivity of spines to perturbations in their environment and we outline how these molecules interface with each other to mediate cascades of signals flowing from the spine surface to the actin cytoskeleton.     

Evidence for P2Y-type ATP receptors on the serosal membrane of frog skin epithelium

The present study presents the first evidence for P2Y-type adenosine 5'-triphosphate (ATP) receptors on the basolateral membranes of frog skin epithelial cells. Cytosolic calcium ([Ca2+]i) was measured with fura-2 and Calcium-Green-1 using epifluorescence microscopy and confocal laser scanning microscopy respectively. In the presence of Ca2+ in the solutions ATP increased [Ca2+]i. The increase in [Ca2+]i was due to the agonist activity of ATP and not to the activity of the potential products of ATP metabolism, i.e. adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) or adenosine, as shown by a comparison of the magnitude of the increases in [Ca2+]i caused by the various compounds. The rise in [Ca2+]i was predominantly monophasic at low ATP concentrations (below 100 microM). At higher concentrations the initial spike was followed by a plateau phase. In the absence of Ca2+ in the extracellular solution ATP caused Ca2+ release from intracellular stores. This could be inhibited by pre-treatment of the tissue with 1 microM thapsigargin, an inhibitor of the endoplasmic reticulum calcium ATPase. The nucleotide uridine 5'-triphosphate (UTP) had similar effects on [Ca2+]i although the plateau level of the [Ca2+]i response was higher with this P2Y agonist. Confocal laser scanning microscopy showed that all cell layers of the epithelium responded to ATP. Our data indicates that serosal ATP acts on serosal P2Y-type receptors in frog skin epithelium. This is the first evidence of a phospholipase C-coupled receptor in this tissue.     

Selective induction of LTP and LTD by postsynaptic [Ca2+]i elevation

Long-term potentiation (LTP) and long-term depression (LTD), two prominent forms of synaptic plasticity at glutamatergic afferents to CA1 hippocampal pyramidal cells, are both triggered by the elevation of postsynaptic intracellular calcium concentration ([Ca2+]i). To understand how one signaling molecule can be responsible for triggering two opposing forms of synaptic modulation, different postsynaptic [Ca2+]i elevation patterns were generated by a new caged calcium compound nitrophenyl-ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid in CA1 pyramidal cells. We found that specific patterns of [Ca2+]i elevation selectively activate LTP or LTD. In particular, only LTP was triggered by a brief increase of [Ca2+]i with relatively high magnitude, which mimics the [Ca2+]i rise during electrical stimulation typically used to induce LTP. In contrast, a prolonged modest rise of [Ca2+]i reliably induced LTD. An important implication of the results is that both the amplitude and the duration of an intracellular chemical signal can carry significant biological information.     

Physiological variability in brain-derived neurotrophic factor expression predicts dendritic spine density in the mouse dentate gyrus

Dendritic spines are the predominant sites of excitatory neurotransmission in the adult brain, and brain-derived neurotrophic factor (BDNF) is a well-characterized determinant of dendritic spine number and morphology. The relationship between BDNF expression and dendritic spine number is particularly evident in the hippocampus, where environmental conditions that enhance hippocampal BDNF levels also promote local increases in dendritic spine density. However, the relationship between physiological variability in hippocampal BDNF expression and spine number has yet to be assessed. To determine whether natural variability in BDNF expression is associated with hippocampal dendritic spine number, correlations between BDNF protein levels and dendritic spine density among Golgi-impregnated neurons in the hippocampal dentate gyrus and CA1 subfields were assessed in adult male C57Bl/6J mice. In the dentate gyrus, but not in the apical oblique dendrites of CA1 pyramidal cells, BDNF protein expression was significantly correlated with dendritic spine density. This observation suggests that there may be a subregionally specific relationship between hippocampal BDNF expression and the density of spines.