Spatially restricted actin-regulatory signaling contributes to synapse morphology.

The actin cytoskeleton in dendritic spines is organized into microdomains, but how signaling molecules that regulate actin are spatially governed is incompletely understood. Here we examine how the localization of the RacGEF kalirin-7, a well-characterized regulator of actin in spines, varies as a function of post-synaptic density area and spine volume. Using serial section electron microscopy, we find that extrasynaptic, but not synaptic, expression of kalirin-7 varies directly with synapse size and spine volume.

Synapse distribution suggests a two-stage model of dendritic integration in CA1 pyramidal neurons.

Competing models have been proposed to explain how neurons integrate the thousands of inputs distributed throughout their dendritic trees. In a simple global integration model, inputs from all locations sum in the axon. In a two-stage integration model, inputs contribute directly to dendritic spikes, and outputs from multiple branches sum in the axon.

Axospinous synaptic subtype-specific differences in structure, size, ionotropic receptor expression, and connectivity in apical dendritic regions of rat hippocampal CA1 pyramidal neurons.

The morphology of axospinous synapses and their parent spines varies widely. Additionally, many of these synapses are contacted by multiple synapse boutons (MSBs) and show substantial variability in receptor expression. The two major axospinous synaptic subtypes are perforated and nonperforated, but there are several subcategories within these two classes.

Synaptic strength and postsynaptically silent synapses through advanced aging in rat hippocampal CA1 pyramidal neurons.

Synaptic dysfunction is thought to contribute to age-related learning impairments. Detailed information regarding the presence of silent synapses and the strength of functional ones through advanced aging, however, is lacking. Here we used paired-pulse minimal stimulation techniques in CA1 stratum radiatum to determine whether the amplitude of spontaneous and evoked miniature excitatory postsynaptic currents (sEPSCs and eEPSCs, respectively) changes over the lifespan of rats in hippocampal CA1 pyramidal neurons, and whether silent synapses are present in adult and aged rats.

Distance-dependent differences in synapse number and AMPA receptor expression in hippocampal CA1 pyramidal neurons.

The ability of synapses throughout the dendritic tree to influence neuronal output is crucial for information processing in the brain. Synaptic potentials attenuate dramatically, however, as they propagate along dendrites toward the soma. To examine whether excitatory axospinous synapses on CA1 pyramidal neurons compensate for their distance from the soma to counteract such dendritic filtering, we evaluated axospinous synapse number and receptor expression in three progressively distal regions: proximal and distal stratum radiatum (SR), and stratum lacunosum-moleculare (SLM).

Reduction in size of perforated postsynaptic densities in hippocampal axospinous synapses and age-related spatial learning impairments.

A central problem in the neurobiology of normal aging is why learning is preserved in some aged individuals yet impaired in others. To investigate this issue, we examined whether age-related deficits in spatial learning are associated with a reduction in postsynaptic density (PSD) area in hippocampal excitatory synapses (i.e.

Aging, spatial learning, and total synapse number in the rat CA1 stratum radiatum.

The aim of this study was to determine whether spatial learning deficits in aged rats are associated with a loss of hippocampal synapses. The Morris water maze task was used to assess the spatial learning capacity of young and aged rats and to attribute aged animals to learning-impaired and learning-unimpaired groups. The number of axospinous synapses in the entire volume of the CA1 stratum radiatum was estimated with unbiased stereological techniques.

Synapses with a segmented, completely partitioned postsynaptic density express more AMPA receptors than other axospinous synaptic junctions.

Axospinous perforated synapses of one morphological subtype exhibit multiple transmission zones, each one being formed by an axon terminal protrusion apposing a postsynaptic density (PSD) segment and separated from others by complete spine partitions. Such segmented, completely partitioned (SCP) synapses have been implicated in synaptic plasticity and postulated to be exceptionally efficacious. The present study explored the validity of this supposition.

Differences in the expression of AMPA and NMDA receptors between axospinous perforated and nonperforated synapses are related to the configuration and size of postsynaptic densities.

Axospinous synapses are traditionally divided according to postsynaptic density (PSD) configuration into a perforated subtype characterized by a complex-shaped PSD and nonperforated subtype exhibiting a simple-shaped, disc-like PSD. It has been hypothesized that perforated synapses are especially important for synaptic plasticity because they have a higher efficacy of impulse transmission. The aim of the present study was to test this hypothesis.

Associative learning elicits the formation of multiple-synapse boutons.

The formation of new synapses has been suggested to underlie learning and memory. However, previous work from this laboratory has demonstrated that hippocampus-dependent associative learning does not induce a net gain in the total number of hippocampal synapses and, hence, a net synaptogenesis. The aim of the present work was to determine whether associative learning involves a specific synaptogenesis confined to the formation of multiple-synapse boutons (MSBs) that synapse with more than one dendritic spine.

Structural synaptic modifications associated with hippocampal LTP and behavioral learning.

An important problem in the neurobiology of memory is whether cellular mechanisms of learning and memory include the formation of new synapses or the remodeling of existing ones. To elucidate this problem, numerous studies have examined alterations in the number and structure of synapses following behavioral learning and hippocampal long-term potentiation (LTP), which is viewed as a synaptic model of memory. The data reported in the literature and obtained in this laboratory are analyzed here to evaluate what kind of structural modification is likely to account for synaptic plasticity associated with learning and memory.

Remodeling of hippocampal synapses after hippocampus-dependent associative learning.

The aim of this study was to determine whether hippocampus-dependent associative learning involves changes in the number and/or structure of hippocampal synapses. A behavioral paradigm of trace eyeblink conditioning was used. Young adult rabbits were given daily 80 trial sessions to a criterion of 80% conditioned responses in a session.

Unbiased stereological estimation of the total number of synapses in a brain region.

Modern stereological methods have been used to make unbiased estimates of the total number of synapses in the striatum radiatum of the hippocampal CA1 region of five rabbits. The approach used involved a two stage analysis and is generally applicable to all parts of the nervous system. During the first stage of the analysis, the reference volume was estimated by point counting, at the light microscope level, according to the Cavalieri principle.

Synapse restructuring associated with the maintenance phase of hippocampal long-term potentiation.

Synapses in the middle molecular layer of the rat dentate gyrus were analyzed by electron microscopy during the maintenance phase of long-term potentiation (LTP). LTP was induced by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. The dentate gyrus was examined electron microscopically 13 days following the fourth stimulation.

Structural synaptic correlate of long-term potentiation: formation of axospinous synapses with multiple, completely partitioned transmission zones.

Synapses were analyzed in the middle molecular layer (MML) and inner molecular layer (IML) of the rat dentate gyrus following the induction of long-term potentiation (LTP) by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. Potentiated animals were sacrificed 1 hour after the fourth high frequency stimulation. Stimulated but not potentiated and implanted but not stimulated animals served as controls.

Perforated axospinous synapses with multiple, completely partitioned transmission zones: probable structural intermediates in synaptic plasticity.

Analysis of axospinous synapses in the rat dentate gyrus, using three-dimensional reconstructions from electron micrographs of serial sections, revealed a novel synaptic subtype. Synapses of this subtype exhibit partitions that emanate from the postsynaptic spine head and invaginate the presynaptic axon terminal, dividing its portion contracted by the spine into distinct protrusions. Such complete spine partitions provide barriers between two to four discrete transmission zones, each one being formed by a separate axon terminal protrusion and delineated by a separate segment of the postsynaptic density (PSD).

Structural synaptic plasticity associated with the induction of long-term potentiation is preserved in the dentate gyrus of aged rats.

Changes in synaptic numbers were examined in the hippocampal dentate gyrus of aged (28 months old) rats following the induction of long-term potentiation (LTP) by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days. Potentiated animals were sacrificed 1 hour after the fourth stimulation. Stimulated but not potentiated and implanted but not stimulated rats of the same chronological age served as controls.

Age-related loss of axospinous synapses formed by two afferent systems in the rat dentate gyrus as revealed by the unbiased stereological dissector technique.

Previous attempts to elucidate whether a loss of hippocampal synapses occurs during aging provided conflicting results, possibly due to the unavailability, at the time, of unbiased methods for synapse quantitation. This study was designed to reexamine the issue by means of modern technical procedures that provide unbiased estimates of synaptic numbers. Groups of 14 young adult (5 months old) and 14 aged (28 months old) male Fischer-344 rats were compared.

Increase in the number of axospinous synapses with segmented postsynaptic densities following hippocampal kindling.

Kindling results from intermittent electrical stimulation of a local brain region and leads to a virtually permanent augmentation of synaptic responsiveness in the stimulated circuit. It has been hypothesized that an increase in the number of synapses may represent a structural basis for the enduring expression of synaptic plasticity following kindling, but such an alteration has not been demonstrated unequivocally. The present report provides evidence that hippocampal kindling is indeed accompanied by an increase in synaptic numbers.

Induction of long-term potentiation is associated with an increase in the number of axospinous synapses with segmented postsynaptic densities.

Long-term potentiation (LTP) is characterized by a long-lasting enhancement of synaptic efficacy which may be due to an increase in synaptic numbers. The present study was designed to verify the validity of this suggestion using recently developed unbiased methods for synapse quantitation. LTP was elicited in young adult rats by high-frequency stimulation of the medial perforant path carried out on each of 4 consecutive days.

The brain's record of experience: kindling-induced enlargement of the active zone in hippocampal perforated synapses.

Kindling is a consequence of intermittent electrical stimulation of a local forebrain area leading to a durable augmentation of synaptic responsiveness in the stimulated circuit. The basis for this functional change is unknown, but there is evidence suggesting that it entails a structural modification of synapses. The present report demonstrates that hippocampal kindling induces a selective enlargement of active zones in perforated axospinous synapses formed by stimulated axons.

Increase in the relative proportion of perforated axospinous synapses following hippocampal kindling is specific for the synaptic field of stimulated axons.

A comparative analysis of axospinous synapses was performed in the middle (MML) and inner (IML) molecular layer of the hippocampal dentate gyrus of rats kindled via medial perforant path stimulation and sacrificed 4 weeks after reaching a criterion of 5 generalized seizures. The MML was a directly stimulated structure, while the IML was not. Both are immediately adjacent synaptic fields likely to be equally susceptible to any generalized effects of convulsions and hypoxia.