Differences in membrane structure between excitatory and inhibitory components of the reciprocal synapse in the olfactory bulb

The reciprocal dendro-dendritic synapse between granule and mitral or tufted dendrites in the external plexiform layer of the olfactory bulb consists of an excitatory mitral-to-granule synaptic contact and an adjacent inhibitory granule-to-mitral synaptic contact. The pre- and postsynaptic membranes of both synaptic contacts were identified in replicas of freeze-fractured external plexiform layer in rabbits, mice, and chinchillas. At the excitatory synaptic contact there is a prominent specialization in the postsynaptic memberane, represented by an aggregate of homogeneous particles associated with the external half of the membrane. In contrast, the postsynaptic membrane at the inhibitory granule-to-mitral synaptic contact lacks evident internal specializations, and the distribution of particles on both fracture faces resembles that at non-synaptic regions. Less marked differences in particle distribution characterized the cytoplasmic half of the presynaptic membranes. These differences probably reflect diversity in the nature or distribution of membrane proteins at excitatory and inhibitory synapses. Protuberances on the external half of the presynaptic membrane, possibly sites of vesicle interaction with the plasma membrane, surrounded but were not coextensive with both types of synaptic contact. A few gap junctions connected proximal dendrites of mitral or tufted cells with granule cell dendrites.     

Temporally distinct demands for classic cadherins in synapse formation and maturation

Classic cadherins are synaptic adhesion proteins that have been implicated in synapse formation and targeting. Brief inactivation of classic cadherin function in young neurons appears to abrogate synapse formation when examined acutely. It remains unknown if such abrogation is unique to young neurons, whether it occurs by stalling neuronal maturation or by directly interfering with the process of synapse assembly, or whether synapse targeting is altered. Here we asked if sustained pan-cadherin blockade would prevent or alter the progression of axonal and dendritic outgrowth, synaptogenesis, or the stereotypic distribution of excitatory and inhibitory synapses on cultured hippocampal neurons. While pre- and postsynaptic cadherins are required for synapse assembly in young neurons, we find that in neurons older than 10 days, classic cadherins are entirely dispensable for joining and aligning presynaptic vesicle clusters with molecular markers of the postsynaptic density. Furthermore, we find that the proportion and relative distributions of excitatory and inhibitory terminals on single neurons are not altered. However, synapses that form on neurons in which cadherin function is blocked are smaller; they exhibit decreased synaptic vesicle recycling and a decreased frequency of spontaneous EPSCs. Moreover, they fail to acquire resistance to F-actin depolymerization, a hallmark of mature, stable contacts. These data provide new evidence that cadherins are required to promote synapse stabilization and structural and functional maturation, but dispensable for the correct subcellular distribution of excitatory and inhibitory synapses.     

Thin-section and freeze-fracture studies of crayfish stretch receptor synapses including the reciprocal inhibitory synapse

The crayfish slow-adapting abdominal stretch receptor organ is innervated by three inhibitory and several excitatory axons. A previous study by Tisdale and Nakajima ('76) showed that under certain fixation conditions inhibitory and excitatory synapses can be distinguished on the basis of synaptic vesicle structure. Using this morphological criterion we describe six types of synapses in the receptor: (1) the inhibitory axo-dendritic synapse, (2) the excitatory neuromuscular synapse, (3) the inhibitory neuromuscular synapse, (4) the axo-axonic synapse which suggests presynaptic inhibition on the excitatory synapse, (5) the axo-axonic synapse which suggests presynaptic inhibition on the inhibitory synapse, (6) the reciprocal inhibitory axo-axonic synapse, which is a new type of synapse. The presence of these six types of synapse suggests that inhibitory and excitatory axons interact synaptically in a complicated manner, resulting in a delicate control of receptor function. In freeze fracture we have observed the presynaptic membrane structures of inhibitory and excitatory synapses. The active zone of the inhibitory synapse has ridges with loosely aggregated particles on the tops of the ridges and indentations (vesicle attachment sites) along their sides. The active zone of the excitatory neuromuscular synapse consists of bands of particle aggregates which are situated on slightly elevated membrane regions and surrounded by wide, relatively particle-free, flat membrane areas.     

New roles for astrocytes: regulation of synaptic transmission

Abstract Although glia often envelop synapses, they have traditionally been viewed as passive participants in synaptic function. Recent evidence has demonstrated, however, that there is a dynamic two-way communication between glia and neurons at the synapse. Neurotransmitters released from presynaptic neurons evoke Ca2+ concentration increases in adjacent glia. Activated glia, in turn, release transmitters, including glutamate and ATP. These gliotransmitters feed back onto the presynaptic terminal either to enhance or to depress further release of neurotransmitter. Transmitters released from glia can also directly stimulate postsynaptic neurons, producing either excitatory or inhibitory responses. Based on these new findings, glia should be considered an active partner at the synapse, dynamically regulating synaptic transmission.     

Differences in membrane structure between excitatory and inhibitory synapses in the cerebellar cortex

Several synapses of known physiological action are antomically segregated in the cerebellar cortex and are readily identified in freeze-fracture preparations. Excitatory synapses, such as the parallel fiber-to-Purkinje spine synapse, climbing fiber-to-Purkinje spine synapse, and mossy or climbing fiber-to-granule cell dendrite synapse, were characterized by small aggregates of large particles on the cytoplasmic half of the presynaptic membrane, by a distinctly widened synaptic cleft, and by a large aggregate of particles on the external half of the postsynaptic membrane. Inhibitory synapses, such as the stellate cell axon-to-Purkinje dendrite synapse and the basket cell axon-to-Purkinje soma synapse, had no comparable specialization of either the pre- and postsynaptic membrane. The striking contrast in membrane structure at excitatory and inhibitory synaptic contacts presumably reflects differences in either the composition or organization of membrane proteins integral to synaptic function. Puncta adhaerentia between granule cell dendrites in cerebellar glomeruli were characterized by particles aggregated on the external half of both apposed membranes and were further differentiated from synaptic contacts by the smaller size of the particles. Protuberances on the external half of the presynaptic membrane were either small and coextensive with the synaptic contact or were larger and surrounded it; it is suggested that the small protuberances are synaptic vesicle sites whereas the large ones are coated vesicle sites.     

Changes in synaptic morphology associated with presynaptic and postsynaptic activity: an in vitro study of the electrosensory organ of the thornback ray

The influence of synaptic activity on synaptic structure was studied by selectively stimulating the presynaptic or postsynaptic membranes of ribbon synapses in an in vitro preparation, and examining the ultrastructure of synapses with conventional electron microscopic methods. Functionally significant changes in synaptic morphology were observed after direct depolarization of the presynaptic membrane or incubation with the neurotransmitter glutamate to depolarize the postsynaptic membrane. After depolarizing the presynaptic membrane for 30 seconds, the depth of the postsynaptic trough was reduced, and other morphological changes correlated with decreased sensitivity and spontaneous activity were evident. Depolarizing the postsynaptic membrane by incubating synapses with the neurotransmitter glutamate, produced opposite effects. These results suggest that synapses can undergo functionally significant morphological changes in response to certain patterns of activity. The mechanism for these changes might include synaptic vesicle recycling processes, changes in ion concentration, or cytoskeletal alterations in the presynaptic, postsynaptic, or support cells. These mechanisms could operate in association with long-term changes in synaptic efficacy or account for some physiological phenomena such as synaptic fatigue or accommodation.     

Electrical synapse formation disrupts calcium-dependent exocytosis, but not vesicle mobilization

Electrical coupling exists prior to the onset of chemical connectivity at many developing and regenerating synapses. At cholinergic synapses in vitro, trophic factors facilitated the formation of electrical synapses and interfered with functional neurotransmitter release in response to photolytic elevations of intracellular calcium. In contrast, neurons lacking trophic factor induction and electrical coupling possessed flash-evoked transmitter release. Changes in cytosolic calcium and postsynaptic responsiveness to acetylcholine were not affected by electrical coupling. These data indicate that transient electrical synapse formation delayed chemical synaptic transmission by imposing a functional block between the accumulation of presynaptic calcium and synchronized, vesicular release. Despite the inability to release neurotransmitter, neurons that had possessed strong electrical coupling recruited secretory vesicles to sites of synaptic contact. These results suggest that the mechanism by which neurotransmission is disrupted during electrical synapse formation is downstream of both calcium influx and synaptic vesicle mobilization. Therefore, electrical synaptogenesis may inhibit synaptic vesicles from acquiring a readily releasable state. We hypothesize that gap junctions might negatively interact with exocytotic processes, thereby diminishing chemical neurotransmission.     

Postnatal development of axosomatic synapses in the rat visual cortex: morphogenesis and quantitative evaluation

Postnatal development of axosomatic synapses was studied in the rat visual cortex in order to obtain experimental data that may explain how the unequal distribution of asymmetric and symmetric synapses evolves on the soma of cortical neurons. Three types of synaptic junctions were identified: asymmetric or type 1 synapses, with postsynaptic densities greater than or equal to 20 nm, symmetric type 2 synapses, and symmetric synapses with an intermediate structure. The third synapse type had a structure similar to that of type 1 synapses, although the postsynaptic densities were thinner than 20 nm. Type 1 synapses developed in three phases. In phase 1, the first postnatal week, there were many free postsynaptic thickenings and immature synapses whereby a higher degree of postsynaptic differentiation was visible in comparison to the presynaptic elements. During the following 10 days, phase 2, type 1 synapses containing thin postsynaptic densities and intermediate synapses temporarily increased in number. Intermediate synapses are interpreted as precursors of type 1 synapses that have relatively immature postsynaptic elements. Toward the end of synaptogenesis, phase 3, the free postsynaptic thickenings reappeared while type 1 synapses containing well developed postsynaptic elements prevailed. Throughout the whole postnatal period, the numerical density of axosomatic type 1 synapses remained very low and the ratio of asymmetric to symmetric synapses at the neuronal somata was inversely proportional to that at the dendrites. Also, there was a significant decrease in the numerical density of type 1 synapses between postnatal days (P) 17 and 30. Data normalized according to cortical growth suggest that this is probably due to a decrease in the number of axosomatic type 1 synapses. This corresponds to the observation that in layers III and V a few type 1 synapses were found on pyramid-like cells up to P10 which then disappeared in later stages. Axosomatic type 2 synapses appear to be formed by two different presynaptic processes. The first presynaptic type contains flocculent material with glycogen granules and resembles axonal growth cones. These junctions contain multiple adhesion patches, intermediate junctions, one or more active zones, narrow synaptic clefts, and small pleomorphic vesicles. All of these are structural features of adult type 2 synapses. The growth-cone-like presynaptic elements disappeared after about 3 weeks. The second presynaptic type is smaller in size and also forms contacts with a structure similar to adult type 2 synapses.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electron microscopy of immunoreactivity patterns for glutamate and γ-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (subnucleus caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter γ-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA⁺ vesicle-containing dendrites. These Glu⁺ terminals are also postsynaptic to GABA⁺ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA⁺ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms. © 1996 Wiley-Liss, Inc.     

Differential depression at excitatory and inhibitory synapses in visual cortex

The function of cortical circuits depends critically on the balance between excitation and inhibition. This balance reflects not only the relative numbers of excitatory and inhibitory synapses but also their relative strengths. Recent studies of excitatory synapses in visual and somatosensory cortices have emphasized that synaptic strength is not a fixed quantity but is a dynamic variable that reflects recent presynaptic activity. Here, we compare the dynamics of synaptic transmission at excitatory and inhibitory synapses onto visual cortical pyramidal neurons. We find that inhibitory synapses show less overall depression than excitatory synapses and that the kinetics of recovery from depression also differ between the two classes of synapse. When excitatory and inhibitory synapses are stimulated concurrently, this differential depression produces a time- and frequency-dependent shift in the reversal potential of the composite postsynaptic current. These results indicate that the balance between excitation and inhibition can change dynamically as a function of activity.     

Astrocyte transforming growth factor beta 1 promotes inhibitory synapse formation via CaM kinase II signaling

The balance between excitatory and inhibitory synaptic inputs is critical for the control of brain function. Astrocytes play important role in the development and maintenance of neuronal circuitry. Whereas astrocytes‐derived molecules involved in excitatory synapses are recognized, molecules and molecular mechanisms underlying astrocyte‐induced inhibitory synapses remain unknown. Here, we identified transforming growth factor beta 1 (TGF‐β1), derived from human and murine astrocytes, as regulator of inhibitory synapse in vitro and in vivo. Conditioned media derived from human and murine astrocytes induce inhibitory synapse formation in cerebral cortex neurons, an event inhibited by pharmacologic and genetic manipulation of the TGF‐β pathway. TGF‐β1‐induction of inhibitory synapse depends on glutamatergic activity and activation of CaM kinase II, which thus induces localization and cluster formation of the synaptic adhesion protein, Neuroligin 2, in inhibitory postsynaptic terminals. Additionally, intraventricular injection of TGF‐β1 enhanced inhibitory synapse number in the cerebral cortex. Our results identify TGF‐β1/CaMKII pathway as a novel molecular mechanism underlying astrocyte control of inhibitory synapse formation. We propose here that the balance between excitatory and inhibitory inputs might be provided by astrocyte signals, at least partly achieved via TGF‐β1 downstream pathways. Our work contributes to the understanding of the GABAergic synapse formation and may be of relevance to further the current knowledge on the mechanisms underlying the development of various neurological disorders, which commonly involve impairment of inhibitory synapse transmission. GLIA 2014;62:1917–1931     

Distribution and structure of identified tonic and phasic synapses between L-neurones in the locust ocellar tract.

The ultrastructures and distributions of the discrete anatomical synapses which constitute two distinct types of output connections made by individual ocellar L-neurons, L1-3, are described. Outputs to neurones L4-5 are excitatory and transmit tonically, whereas reciprocal connections among the three L1-3 neurones are inhibitory and incapable of transmission for longer than a few milliseconds. The tonically transmitting synapses are located in the lateral ocellar tract and are made between the axons of L1-3, which do not receive inputs, and short branches of L4-5, which make no outputs. Each excitatory connection is composed of a few hundred discrete anatomical synapses, each characterised by a bar-shaped presynaptic density which is 0.15-1.5 microns in length and associated with a large number of round synaptic vesicles. Two postsynaptic profiles are apposed to each presynaptic density. Associated with tonic synapses are abundant invaginations of the presynaptic membrane. Synapses of the reciprocal, inhibitory, phasic connections occur in the protocerebral arbors of L1-3, among numerous output synapses of these neurones. Each phasic connection is composed of a few tens of discrete anatomical synapses. Each bar-shaped presynaptic density is associated with two postsynaptic profiles, and is 0.1-1.0 microns long. Compared with the tonic, excitatory connection, there are fewer vesicles and fewer invaginations of the presynaptic membrane associated with each synapse.     

Synaptic potentiation induces increased glial coverage of excitatory synapses in CA1 hippocampus

Patterns of activity that induce synaptic plasticity at excitatory synapses, such as long‐term potentiation, result in structural remodeling of the postsynaptic spine, comprising an enlargement of the spine head and reorganization of the postsynaptic density (PSD). Furthermore, spine synapses represent complex functional units in which interaction between the presynaptic varicosity and the postsynaptic spine is also modulated by surrounding astroglial processes. To investigate how activity patterns could affect the morphological interplay between these three partners, we used an electron microscopic (EM) approach and 3D reconstructions of excitatory synapses to study the activity‐related morphological changes underlying induction of synaptic potentiation by theta burst stimulation or brief oxygen/glucose deprivation episodes in hippocampal organotypic slice cultures. EM analyses demonstrated that the typical glia‐synapse organization described in in vivo rat hippocampus is perfectly preserved and comparable in organotypic slice cultures. Three‐dimensional reconstructions of synapses, classified as simple or complex depending upon PSD organization, showed significant changes following induction of synaptic potentiation using both protocols. The spine head volume and the area of the PSD significantly enlarged 30 min and 1 h after stimulation, particularly in large synapses with complex PSD, an effect that was associated with a concomitant enlargement of presynaptic terminals. Furthermore, synaptic activity induced a pronounced increase of the glial coverage of both pre‐ and postsynaptic structures, these changes being prevented by application of the NMDA receptor antagonist D‐2‐amino‐5‐phosphonopentanoic acid. These data reveal dynamic, activity‐dependent interactions between glial processes and pre‐ and postsynaptic partners and suggest that glia can participate in activity‐induced structural synapse remodeling. © 2009 Wiley‐Liss, Inc.     

Activity-dependent alteration of the morphology of a hippocampal giant synapse

Activity-dependent synaptic plasticity is a fundamental cellular process for learning and memory. While electrophysiological plasticity has been intensively studied, morphological plasticity is less clearly understood. This study investigated the effect of presynaptic stimulation on the morphology of a giant mossy fiber-CA3 pyramidal cell synapse, and found that the mossy fiber bouton altered its morphology with an increase in the number of segments. This activity-dependent alteration in morphology required the activation of glutamate receptors and an increase in postsynaptic calcium concentration. In addition, the intercellular retrograde messengers nitric oxide and arachidonic acid were necessary. Simultaneous recordings demonstrated that the morphological complexity of the presynaptic bouton and the amplitude of excitatory postsynaptic currents were well correlated. Thus, a single mossy fiber synapse has the potential for activity-dependent morphological plasticity at the presynaptic bouton, which may be important for learning and memory.     

Dynamic synapse: A new concept of neural representation and computation

Presynaptic mechanisms influencing the probability of neurotransmitter release from an axon terminal, such as facilitation, augmentation, and presynaptic feedback inhibition, are fundamental features of biological neurons and are cardinal physiological properties of synaptic connections in the hippocampus. The consequence of these presynaptic mechanisms is that the probability of release becomes a function of the temporal pattern of action potential occurrence, and hence, the strength of a given synapse varies upon the arrival of each action potential invading the terminal region. From the perspective of neural information processing, the capability of dynamically tuning the synaptic strength as a function of the level of neuronal activation gives rise to a significant representational and processing power of temporal spike patterns at the synaptic level. Furthermore, there is an exponential growth in such computational power when the specific dynamics of presynaptic mechanisms varies quantitatively across axon terminals of a single neuron, a recently established characteristic of hippocampal synapses. During learning, alterations in the presynaptic mechanisms lead to different pattern transformation functions, whereas changes in the postsynaptic mechanisms determine how the synaptic signals are to be combined. We demonstrate the computational capability of dynamic synapses by performing speech recognition from unprocessed, noisy raw waveforms of words spoken by multiple speakers with a simple neural network consisting of a small number of neurons connected with synapses incorporating dynamically determined probability of release. The dynamics included in the model are consistent with available experimental data on hippocampal neurons in that parameter values were chosen so as to be consistent with time constants of facilitative and inhibitory processes governing the dynamics of hippocampal synaptic transmission studied using nonlinear systems analytic procedures. © 1997 Wiley-Liss, Inc.     

Neuroligins and neurexins: linking cell adhesion, synapse formation and cognitive function

Cell adhesion represents the most direct way of coordinating synaptic connectivity in the brain. Recent evidence highlights the importance of a trans-synaptic interaction between postsynaptic neuroligins and presynaptic neurexins. These transmembrane molecules bind each other extracellularly to promote adhesion between dendrites and axons. This signals the recruitment of presynaptic and postsynaptic molecules to form a functional synapse. Remarkably, neuroligins alone can induce the formation of fully functional presynaptic terminals in contacting axons. Conversely, neurexins alone can induce postsynaptic differentiation and clustering of receptors in dendrites. Therefore, the neuroligin-neurexin interaction has the unique ability to act as a bi-directional trigger of synapse formation. Here, we review several recent studies that offer clues as to how these proteins form synapses and how they might function in the brain to establish and modify neuronal network properties and cognition.     

A model synapse that incorporates the properties of short- and long-term synaptic plasticity.

We propose a general computer model of a synapse, which incorporates mechanisms responsible for the realization of both short- and long-term synaptic plasticity-the two forms of experimentally observed plasticity that seem to be very significant for the performance of neuronal networks. The model consists of a presynaptic part based on the earlier 'double barrier synapse' model, and a postsynaptic compartment which is connected to the presynaptic terminal via a feedback, the sign and magnitude of which depend on postsynaptic Ca(2+) concentration. The feedback increases or decreases the amount of neurotransmitter which is in a ready for release state. The model adequately reproduced the phenomena of short- and long-term plasticity observed experimentally in hippocampal slices for CA3-CA1 synapses. The proposed model may be used in the investigation of certain real synapses to estimate their physiological parameters, and in the construction of realistic neuronal networks.     

Advancements in the study of synaptic plasticity and mitochondrial autophagy relationship

Synapses serve as the points of communication between neurons, consisting primarily of three components: the presynaptic membrane, synaptic cleft, and postsynaptic membrane. They transmit signals through the release and reception of neurotransmitters. Synaptic plasticity, the ability of synapses to undergo structural and functional changes, is influenced by proteins such as growth-associated proteins, synaptic vesicle proteins, postsynaptic density proteins, and neurotrophic growth factors. Furthermore, maintaining synaptic plasticity consumes more than half of the brain's energy, with a significant portion of this energy originating from ATP generated through mitochondrial energy metabolism. Consequently, the quantity, distribution, transport, and function of mitochondria impact the stability of brain energy metabolism, thereby participating in the regulation of fundamental processes in synaptic plasticity, including neuronal differentiation, neurite outgrowth, synapse formation, and neurotransmitter release. This article provides a comprehensive overview of the proteins associated with presynaptic plasticity, postsynaptic plasticity, and common factors between the two, as well as the relationship between mitochondrial energy metabolism and synaptic plasticity.