Diurnal variations in the photoreceptor synaptic terminals of the newt retina

Newt photoreceptor synaptic terminals undergo a variety of morphological changes over a 24-hr (LD 12:12) cycle. During the day, dense-cored synaptic vesicles were found to increase in number and accumulate near the synaptic lamellae; during the dark phase, the dense-cored vesicles decreased in number, while large clear vesicles and profiles of smooth endoplasmic reticulum increased in frequency. The most marked change in photoreceptor synaptic terminal morphology occurred after 10 hr of darkness, at 0730 hr. At this time, photoreceptor synaptic terminal cross-sectional area was found to increase dramatically. Morphometric analysis showed that the number of synaptic vesicles in these terminals remained constant throughout the day, as did the perimeter of photoreceptor terminal profiles. The observed increase in area of synaptic terminals at 0730 hr was found to be due to a decrease in the folding of the terminal plasma membrane. Qualitative observations showed endocytosis to be occurring at a rapid rate at this time as well; and since the number of synaptic vesicles and terminal perimeter did not change, exocytosis of synaptic vesicles was assumed to be occurring at an equally rapid rate. These findings support an extension to the hypothesis of Monaghan and Osborne (1975), suggesting that photoreceptor synaptic vesicles become "supercharged" with transmitter substance in the light.     

Cortical plasticity: It's all the range!

When rats learn a motor skill, synaptic potentials in the motor cortex are enhanced. A new study has revealed that this learning-induced enhancement limits further synaptic potentiation, but not synaptic depression. These findings support the view that activity-dependent synaptic plasticity is the brain's memory mechanism.     

Extracting Information from the Power Spectrum of Synaptic Noise

In cortical neurons, synaptic "noise" is caused by the nearly random release of thousands of synapses. Few methods are presently available to analyze synaptic noise and deduce properties of the underlying synaptic inputs. We focus here on the power spectral density (PSD) of several models of synaptic noise. We examine different classes of analytically solvable kinetic models for synaptic currents, such as the "delta kinetic models," which use Dirac delta functions to represent the activation of the ion channel. We first show that, for this class of kinetic models, one can obtain an analytic expression for the PSD of the total synaptic conductance and derive equivalent stochastic models with only a few variables. This yields a method for constraining models of synaptic currents by analyzing voltage-clamp recordings of synaptic noise. Second, we show that a similar approach can be followed for the PSD of the the membrane potential (Vm) through an effective-leak approximation. Third, we show that this approach is also valid for inputs distributed in dendrites. In this case, the frequency scaling of the Vm PSD is preserved, suggesting that this approach may be applied to intracellular recordings of real neurons. In conclusion, using simple mathematical tools, we show that Vm recordings can be used to constrain kinetic models of synaptic currents, as well as to estimate equivalent stochastic models. This approach, therefore, provides a direct link between intracellular recordings in vivo and the design of models consistent with the dynamics and spectral structure of synaptic noise.     

Synaptic potential in the motor giant axon of the crayfish

Some electrical properties of the synapses between central giant axons (presynaptic) and the motor giant axon (postsynaptic) of the crayfish abdominal nerve cord have been investigated. Postsynaptic potential change in response to presynaptic volleys contains two components: a spike potential and a synaptic potential of very long time course. Amplitude of the synaptic potential is graded according to the number of active presynaptic axons. Conductance increase in the synaptic membrane endures over most of the period of potential change, and it is this rather than the "electrical time constant" of the membrane that in large measure determines the form of the synaptic potential. Temporal summation of synaptic potential occurs during repetitive presynaptic stimulation, and after such stimulation the rate of decay of synaptic potential is greatly slowed.     

Studies of synaptic vesicle endocytosis in the nematode C. elegans

After synaptic vesicle exocytosis, synaptic vesicle proteins must be retrieved from the plasma membrane, sorted away from other membrane proteins, and reconstituted into a functional synaptic vesicle. The nematode Caenorhabditis elegans is an organism well suited for a genetic analysis of this process. In particular, three types of genetic studies have contributed to our understanding of synaptic vesicle endocytosis. First, screens for mutants defective in synaptic vesicle recycling have identified new proteins that function specifically in neurons. Second, RNA interference has been used to quickly confirm the roles of known proteins in endocytosis. Third, gene targeting techniques have elucidated the roles of genes thought to play modulatory or subtle roles in synaptic vesicle recycling. We describe a molecular model for synaptic vesicle recycling and discuss how protein disruption experiments in C. elegans have contributed to this model.     

Protein synthesis in the dendrite

In neurons, many proteins that are involved in the transduction of synaptic activity and the expression of neural plasticity are specifically localized at synapses. How these proteins are targeted is not clearly understood. One mechanism is synaptic protein synthesis. According to this idea, messenger RNA (mRNA) translation from the polyribosomes that are observed at the synaptic regions provides a local source of synaptic proteins. Although an increasing number of mRNA species has been detected in the dendrite, information about the synaptic synthesis of specific proteins in a physiological context is still limited. The physiological function of synaptic synthesis of specific proteins in synaptogenesis and neural plasticity expression remains to be shown. Experiments aimed at understanding the mechanisms and functions f synaptic protein synthesis might provide important information about the molecular nature of neural plasticity.     

Network Self-Organization Explains the Statistics and Dynamics of Synaptic Connection Strengths in Cortex

The information processing abilities of neural circuits arise from their synaptic connection patterns. Understanding the laws governing these connectivity patterns is essential for understanding brain function. The overall distribution of synaptic strengths of local excitatory connections in cortex and hippocampus is long-tailed, exhibiting a small number of synaptic connections of very large efficacy. At the same time, new synaptic connections are constantly being created and individual synaptic connection strengths show substantial fluctuations across time. It remains unclear through what mechanisms these properties of neural circuits arise and how they contribute to learning and memory. In this study we show that fundamental characteristics of excitatory synaptic connections in cortex and hippocampus can be explained as a consequence of self-organization in a recurrent network combining spike-timing-dependent plasticity (STDP), structural plasticity and different forms of homeostatic plasticity. In the network, associative synaptic plasticity in the form of STDP induces a rich-get-richer dynamics among synapses, while homeostatic mechanisms induce competition. Under distinctly different initial conditions, the ensuing self-organization produces long-tailed synaptic strength distributions matching experimental findings. We show that this self-organization can take place with a purely additive STDP mechanism and that multiplicative weight dynamics emerge as a consequence of network interactions. The observed patterns of fluctuation of synaptic strengths, including elimination and generation of synaptic connections and long-term persistence of strong connections, are consistent with the dynamics of dendritic spines found in rat hippocampus. Beyond this, the model predicts an approximately power-law scaling of the lifetimes of newly established synaptic connection strengths during development. Our results suggest that the combined action of multiple forms of neuronal plasticity plays an essential role in the formation and maintenance of cortical circuits.     

Dendritic enlightenment: using patterned two-photon uncaging to reveal the secrets of the brain's smallest dendrites

It has been a longstanding challenge for experimentalists to manipulate precisely the spatial and temporal patterns of synaptic input to the dendritic tree in order to mimic activity occurring in the intact brain and determine their importance for synaptic integration. In this issue of Neuron, Losonczy and Magee have used rapid multisite two-photon uncaging of glutamate to define patterns of synaptic input on a submillisecond and micron scale to investigate the rules for summation of synaptic inputs in the fine oblique dendrites of pyramidal neurons.     

A survey of the cytology and synaptic organization of the insular trigeminal-cuneatus lateralis nucleus in the rat, including an identification of spinal afferent inputs

The cytology and synaptic organization of the insular trigeminal-cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.     

Multiple roles for the active zone protein RIM1alpha in late stages of neurotransmitter release

The active zone protein RIM1alpha interacts with multiple active zone and synaptic vesicle proteins and is implicated in short- and long-term synaptic plasticity, but it is unclear how RIM1alpha's biochemical interactions translate into physiological functions. To address this question, we analyzed synaptic transmission in autaptic neurons cultured from RIM1alpha-/- mice. Deletion of RIM1alpha causes a large reduction in the readily releasable pool of vesicles, alters short-term plasticity, and changes the properties of evoked asynchronous release. Lack of RIM1alpha, however, had no effect on synapse formation, spontaneous release, overall Ca2+ sensitivity of release, or synaptic vesicle recycling. These results suggest that RIM1alpha modulates sequential steps in synaptic vesicle exocytosis through serial protein-protein interactions and that this modulation is the basis for RIM1alpha's role in synaptic plasticity.     

Acetylcholinesterase from the motor nerve terminal accumulates on the synaptic basal lamina of the myofiber

Acetylcholinesterase (AChE) in skeletal muscle is concentrated at neuromuscular junctions, where it is found in the synaptic cleft between muscle and nerve, associated with the synaptic portion of the myofiber basal lamina. This raises the question of whether the synaptic enzyme is produced by muscle, nerve, or both. Studies on denervated and regenerating muscles have shown that myofibers can produce synaptic AChE, and that the motor nerve may play an indirect role, inducing myofibers to produce synaptic AChE. The aim of this study was to determine whether some of the AChE which is known to be made and transported by the motor nerve contributes directly to AChE in the synaptic cleft. Frog muscles were surgically damaged in a way that caused degeneration and permanent removal of all myofibers from their basal lamina sheaths. Concomitantly, AChE activity was irreversibly blocked. Motor axons remained intact, and their terminals persisted at almost all the synaptic sites on the basal lamina in the absence of myofibers. 1 mo after the operation, the innervated sheaths were stained for AChE activity. Despite the absence of myofibers, new AChE appeared in an arborized pattern, characteristic of neuromuscular junctions, and its reaction product was concentrated adjacent to the nerve terminals, obscuring synaptic basal lamina. AChE activity did not appear in the absence of nerve terminals. We concluded therefore, that the newly formed AChE at the synaptic sites had been produced by the persisting axon terminals, indicating that the motor nerve is capable of producing some of the synaptic AChE at neuromuscular junctions. The newly formed AChE remained adherent to basal lamina sheaths after degeneration of the terminals, and was solubilized by collagenase, indicating that the AChE provided by nerve had become incorporated into the basal lamina as at normal neuromuscular junctions.     

Neuromodulation and the functional dynamics of piriform cortex.

Acetylcholine and norepinephrine have a number of effects at the cellular level in the piriform cortex. Acetylcholine causes a depolarization of the membrane potential of pyramidal cells and interneurons, and suppresses the action potential frequency accommodation of pyramidal cells. Acetylcholine also has strong effects on synaptic transmission, suppressing both excitatory and inhibitory synaptic transmission. At the same time as it suppresses synaptic transmission, acetylcholine enhances synaptic modification, as demonstrated by experiments showing enhancement of long-term potentiation. Norepinephrine has similar effects. In this review, we discuss some of these different cellular effects and provide functional proposals for these individual effects in the context of the putative associative memory function of this structure.     

The effect of monoamine depleting drugs upon the synaptic bars in the inner ear of the bullfrog (Rana catesbeiana)

Summary Reserpine and guanethidine produce a highly significant reduction in electron density of the synaptic bars in the sensory cells of the bullfrog labyrinth. When amphetamine is administered simultaneously with guanethidine, the density of the synaptic bars is similar to those of untreated frogs. p-Chloramphetamine has no significant effect upon the electron density of synaptic bars. These observations are discussed in the light of what is known of the biological effects of these drugs, and are taken to indicate that the synaptic bars could be intracellular storage sites for a monoamine that mediates the synaptic contacts between the sensory cells and afferent nerve fibres. It is suggested that the monoamine involved is a catecholamine.Both of us thank Mrs. J. Birch and Miss J. Sutcliffe for their technical assistence. One of us (M. P. O.) was supported by a U.S. National Research Council Senior Research Associateship (1967–1968) during the earlier phase of this work, and we are both indebted to the Italian Consiglio Nazionale delle Ricerche for a grant (No. 69.01697.119.3) which financed the latter stages of this study.     

Proteomics in the study of hippocampal plasticity

Synaptic plasticity is the dynamic regulation of the strength of synaptic communication between nerve cells. It is central to neuronal development as well as experience-dependent remodeling of the adult nervous system as occurs during memory formation. Aberrant forms of synaptic plasticity also accompany a variety of neurological and psychiatric diseases, and unraveling the biological basis of synaptic plasticity has been a major goal in neurobiology research. The biochemical and structural mechanisms underlying different forms of synaptic plasticity are complex, involving multiple signaling cascades, reconfigurations of structural proteins and the trafficking of synaptic proteins. As such, proteomics should be a valuable tool in dissecting the molecular events underlying normal and disease-related forms of plasticity. In fact, progress in this area has been disappointingly slow. We discuss the particular challenges associated with proteomic interrogation of synaptic plasticity processes and outline ways in which we believe proteomics may advance the field over the next few years. We pay particular attention to technical advances being made in small sample proteomics and the advent of proteomic imaging in studying brain plasticity.     

Localization of the inositol 1,4,5-trisphosphate receptor in synaptic terminals in the vertebrate retina.

Inositol 1,4,5-trisphosphate (InsP3) mobilizes internal Ca2+ in cells by binding to a receptor protein, which has recently been purified and molecularly cloned. To clarify those neuronal functions that are regulated by InsP3, we have localized this InsP3 receptor protein immunocytochemically in the retina, a neural tissue of well-defined structure and function. Positive staining in neurons is confined almost exclusively to the synaptic layers. Using dissociated retinal neurons, we have further localized the receptor to presynaptic terminals of photoreceptors and bipolar cells, as well as the synaptic processes of amacrine cells. The specific association of InsP3 receptors with synaptic terminals suggests a role for InsP3 in synaptic modulation, especially with respect to transmitter release.     

Correlation of glutamate binding activity with glutamate-binding protein immunoreactivity in the brain of control and alcohol-treated rats

Specific antibodies raised against a glutamate binding protein purified from bovine brain were used to trace the immunoreactivity of this protein in rat brain subcellular fractions. In the subcellular fractions obtained from whole brain homogenates, the synaptic membranes had the highest immunochemical reactivity towards the anti-glutamate-binding protein antibodies. The combination of measurements of glutamate binding activity and glutamate-binding protein immunoreactivity indicated that in brain synaptic membranes from control animals the highest activity in these two measures was associated with a synaptic plasma membrane subfraction that was enriched with synaptic junctions. In animals treated with ethanol for 14 days, there was a significant increase in the density of synaptic membrane glutamate binding sites. This increase in glutamate binding capacity was correlated with a greater than two-fold increase in the glutamate binding activity and binding protein immunoreactivity of the light synaptic membrane subfraction, a subfraction which does not contain many recognizable synaptic junctions. Acute administration of ethanol to rats produced a moderate but non-significant decrease in glutamate binding capacity of synaptic membranes. The increase in the number of glutamate binding protein subunits in brain plasma membranes may be an adaptive response of central nervous system neurons to the acute effects of ethanol on glutamate synaptic transmission.     

Localization of the glycine transporter GLYT1 in glutamatergic synaptic vesicles

We have previously shown the presence of the glycine transporter GLYT1 in glutamatergic terminals of the rat brain. In this study we present immunohistochemical and biochemical evidence indicating that GLYT1 is expressed not only at the plasma membrane of glutamatergic neurons, but also at synaptic vesicles. Confocal microscopy, immunoblots analysis of a highly purified synaptic vesicle fraction and immunoisolation of synaptic vesicles with anti-synaptophysin antibodies strongly suggested the presence of GLYT1 in synaptic vesicles. Moreover, direct observation with the electron microscope of purified vesicles immunoreacted with anti-GLYT1 and colloidal gold demonstrated that about 40% of the small vesicles of the purified vesicle fraction contained GLYT1. Double labeling for GLYT1 and synaptophysin of this vesicular fraction revealed that more of ninety percent of them were synaptic vesicles. Moreover, a significant part of the GLYT1 containing vesicles (86%) also contained the vesicular glutamate transporter vGLUT1, suggesting a functional role of GLYT1 in a subpopulation of glutamatergic vesicles.     

Exploring the Ca2+-dependent synaptic dynamics in vibro-dissociated cells

Dynamic alteration of the synaptic strength is one of the most important processes occurring in the nervous system. Combination of electrophysiology, confocal imaging and molecular biology led to significant advances in this research field. Yet, a progress in this area, in particular in studies of changes in the quantal behavior of central synapses and impact of glial cells on individual synapses, is hampered by technical difficulties of resolving small quantal synaptic currents. In this paper we will show how the technique of non-enzymatic vibro-dissociation, which enables to isolate living neurons avoiding artifacts of cell culture and preserving functional synapse, can be used to obtain a valuable information on fine details and mechanisms of synaptic plasticity. In particular, we will describe our recent results on Ca²⁺-dependent modulation of the postsynaptic AMPA and NMDA receptors in the individual synaptic boutons.