Transneuronal effects of entorhinal lesions in the early postnatal period on synaptogenesis in the hippocampus of the rat

Fine structure of synapses on dendritic spines

Camillo Golgi's "Reazione Nera" led to the discovery of dendritic spines, small appendages originating from dendritic shafts. With the advent of electron microscopy (EM) they were identified as sites of synaptic contact. Later it was found that changes in synaptic strength were associated with changes in the shape of dendritic spines. While live-cell imaging was advantageous in monitoring the time course of such changes in spine structure, EM is still the best method for the simultaneous visualization of all cellular components, including actual synaptic contacts, at high resolution. Immunogold labeling for EM reveals the precise localization of molecules in relation to synaptic structures. Previous EM studies of spines and synapses were performed in tissue subjected to aldehyde fixation and dehydration in ethanol, which is associated with protein denaturation and tissue shrinkage. It has remained an issue to what extent fine structural details are preserved when subjecting the tissue to these procedures. In the present review, we report recent studies on the fine structure of spines and synapses using high-pressure freezing (HPF), which avoids protein denaturation by aldehydes and results in an excellent preservation of ultrastructural detail. In these studies, HPF was used to monitor subtle fine-structural changes in spine shape associated with chemically induced long-term potentiation (cLTP) at identified hippocampal mossy fiber synapses. Changes in spine shape result from reorganization of the actin cytoskeleton. We report that cLTP was associated with decreased immunogold labeling for phosphorylated cofilin (p-cofilin), an actin-depolymerizing protein. Phosphorylation of cofilin renders it unable to depolymerize F-actin, which stabilizes the actin cytoskeleton. Decreased levels of p-cofilin, in turn, suggest increased actin turnover, possibly underlying the changes in spine shape associated with cLTP. The findings reviewed here establish HPF as an appropriate method for studying the fine structure and molecular composition of synapses on dendritic spines.

Structural plasticity of hippocampal mossy fiber synapses as revealed by high-pressure freezing

Despite recent progress in fluorescence microscopy techniques, electron microscopy (EM) is still superior in the simultaneous analysis of all tissue components at high resolution. However, it is unclear to what extent conventional fixation for EM using aldehydes results in tissue alteration. Here we made an attempt to minimize tissue alteration by using rapid high-pressure freezing (HPF) of hippocampal slice cultures. We used this approach to monitor fine-structural changes at hippocampal mossy fiber synapses associated with chemically induced long-term potentiation (LTP). Synaptic plasticity in LTP has been known to involve structural changes at synapses including reorganization of the actin cytoskeleton and de novo formation of spines. While LTP-induced formation and growth of postsynaptic spines have been reported, little is known about associated structural changes in presynaptic boutons. Mossy fiber synapses are assumed to exhibit presynaptic LTP expression and are easily identified by EM. In slice cultures from wildtype mice, we found that chemical LTP increased the length of the presynaptic membrane of mossy fiber boutons, associated with a de novo formation of small spines and an increase in the number of active zones. Of note, these changes were not observed in slice cultures from Munc13-1 knockout mutants exhibiting defective vesicle priming. These findings show that activation of hippocampal mossy fibers induces pre- and postsynaptic structural changes at mossy fiber synapses that can be monitored by EM.

Fine structure of hippocampal mossy fiber synapses following rapid high-pressure freezing

Synapses of hippocampal neurons play important roles in learning and memory processes and are involved in aberrant hippocampal function in temporal lobe epilepsy. Major neuronal types in the hippocampus as well as their input and output synapses are well known, but it has remained an open question to what extent conventional electron microscopy (EM) has provided us with the real appearance of synaptic fine structure under in vivo conditions. There is reason to assume that conventional aldehyde fixation and dehydration lead to protein denaturation and tissue shrinkage, likely associated with the occurrence of artifacts. However, realistic fine-structural data of synapses are required for our understanding of the transmission process and for its simulation. Here, we used high-pressure freezing and cryosubstitution of hippocampal tissue that was not subjected to aldehyde fixation and dehydration in ethanol to monitor the fine structure of an identified synapse in the hippocampal CA3 region, that is, the synapse between granule cell axons, the mossy fibers, and the proximal dendrites of CA3 pyramidal neurons. Our results showed that high-pressure freezing nicely preserved ultrastructural detail of this particular synapse and allowed us to study rapid structural changes associated with synaptic plasticity.

A role for synaptopodin and the spine apparatus in hippocampal synaptic plasticity

Spines are considered sites of synaptic plasticity in the brain and are capable of remodeling their shape and size. A molecule thathas been implicated in spine plasticity is the actin-associated protein synaptopodin. This article will review a series of studies aimed at elucidating the role of synaptopodin in the rodent brain. First, the developmental expression of synaptopodin mRNA and protein were studied; secondly, the subcellular localization of synaptopodin in hippocampal principal neurons was analyzed using confocal microscopy as well as electron microscopy and immunogold labelling; and, finally, the functional role of synaptopodin was investigated using a synaptopodin-deficient mouse. The results of these studies are: (1) synaptopodin expression byhippocampal principal neurons develops during the first postnatal weeks and increases in parallel with the maturation of spines in the hippocampus. (2) Synaptopodin is sorted to the spine compartment, where it is tightly associated with the spine apparatus, an enigmatic organelle believed to be involved in calcium storage or local protein synthesis. (3) Synaptopodin-deficient mice generated by gene targeting are viable but lack the spine apparatus organelle. These mice show deficitsin synaptic plasticity as well as impaired learning and memory. Taken together, these data implicate synaptopodin and the spine apparatus in the regulation of synaptic plasticity in the hippocampus. Future studies will be aimed at finding the molecular link between synaptopodin, the spine apparatus organelle, and synaptic plasticity.

Ultrastructural alterations in hippocampal mossy fiber synapses in schizophrenia: A postmortem morphometric study

Synapses formed between mossy fibers, the axons of hippocampal dentate granular cells, and the dendrites of CA3 pyramidal neurons are important links within the trisynaptic circuitry. Abnormalities in this circuitry are associated with the failure of schizophrenics to integrate affective experience with higher cognitive function, and with disturbances in memory and spatial learning processes. The abnormalities include reduced size and altered dendritic arborization of CA3 pyramidal neurons. In addition, decreased expression and binding activity of glutamate receptors have been reported, predominantly in the CA3 region of the hippocampus. These findings suggest that there are disturbed neuronal processes and connections in the hippocampus of schizophrenics. An electron microscope morphometric study of synaptic contacts between mossy fiber axon terminals (MFT) and branched dendritic spines of pyramidal neurons in stratum lucidum of the CA3 region of the hippocampus was performed in 10 normal controls and 9 age-matched chronic schizophrenics (postmortem delay 3-9 h). Schizophrenic cases with predominantly positive symptoms had a significantly reduced volume fraction of spines (-35%, P < 0.05), total number of invaginated spines (-47%, P < 0.01), and number of spines forming synapses (-32%, P < 0.05) per MFT compared with the control group. No effects of postmortem delay, age, duration of disease, or neuroleptic exposure were found. These data may reflect decreased efficacy of mossy fiber synapses in the CA3 hippocampal region in schizophrenics with predominantly positive symptoms. These data are in line with the neurodevelopmental hypothesis of schizophrenia.

Synaptopodin-deficient mice lack a spine apparatus and show deficits in synaptic plasticity

The spine apparatus is a cellular organelle that is present in many dendritic spines of excitatory neurons in the mammalian forebrain. Despite its discovery >40 years ago, the function of the spine apparatus is still unknown although calcium buffering functions as well as roles in synaptic plasticity have been proposed. We have recently shown that the 100-kDa protein synaptopodin is associated with the spine apparatus. Here, we now report that mice homozygous for a targeted deletion of the synaptopodin gene completely lack spine apparatuses. Interestingly, this absence of the spine apparatus is accompanied by a reduction in hippocampal long-term potentiation (LTP) in the CA1 region of the hippocampus and by an impairment of spatial learning in the radial arm maze test. This genetic analysis points to a role of the spine apparatus in synaptic plasticity.

Formation of dendritic spines in cultured striatal neurons depends on excitatory afferent activity

The role of afferent innervation in the formation of dendritic spines was studied in cultured rat striatum. The striatum is a unique structure in that it contains highly spiny GABAergic projection neurons, with no known local excitation. Grown alone in culture, striatal neurons did not express spontaneous network activity and were virtually devoid of dendritic spines. Adding GFP-expressing mouse cortical neurons to the striatal culture caused the appearance of spontaneous and evoked excitatory synaptic currents in the striatal neurons and a 10-fold increase in the density of spines on their dendrites. This effect was blocked by a continuous presence of TTX in the growth medium, while removal of the drug caused a rapid appearance of spines. Exposure to glutamate, or the presence of cortex-conditioned medium did not mimic the effect of cortical neurons on formation of spines in the striatal neurons. Also, the cortical innervation did not cause a selective enhancement of survival of specific subtypes of spiny striatal neurons. These experiments demonstrate that excitatory afferents are necessary for the formation of dendritic spines in striatal neurons.

Estrogen up-regulates estrogen receptor alpha and synaptophysin in slice cultures of rat hippocampus

Previous studies have shown that estrogen application increases the density of synaptic input and the number of spines on CA1 pyramidal neurons. Here, we have investigated whether Schaffer collaterals to CA1 pyramidal cells are involved in this estrogen-induced synaptogenesis on CA1 pyramidal neurons. To this end, we studied estrogen-induced expression of both estrogen receptor (ER) subtypes (ERalpha and ERbeta) together with the presynaptic marker synaptophysin in the rat hippocampus. In tissue sections as well as in slice cultures mRNA expression of ERalpha, ERbeta and synaptophysin was higher in CA3 than in CA1, and mRNA expression and immunoreactivity for both ER subtypes were found in both principal cells and interneurons. By using quantitative image analysis we found stronger nuclear immunoreactivity for ERalpha in CA3 than in CA1. In slice cultures, supplementation of the medium with 10(-8) M estradiol led to an increase of nuclear immunoreactivity for ERalpha, but not for ERbeta, which was accompanied by a dramatic up-regulation of synaptophysin immunoreactivity in stratum radiatum of CA1. Together these findings indicate that estrogen effects on hippocampal neurons are more pronounced in CA3 than in CA1 and that ER activation in CA3 neurons leads to an up-regulation of a presynaptic marker protein in the axons of these cells, the Schaffer collaterals. We conclude that estradiol-induced spine formation on CA1 pyramidal cells may be mediated presynaptically, very likely by activation of ERalpha in CA3 pyramidal cells, followed by an increase in Schaffer collateral synapses.

Dentate granule cells in reeler mutants and VLDLR and ApoER2 knockout mice

We have studied the organization and cellular differentiation of dentate granule cells and their axons, the mossy fibers, in reeler mutant mice lacking reelin and in mutants lacking the reelin receptors very low density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2). We show that granule cells in reeler mice do not form a densely packed granular layer, but are loosely distributed throughout the hilar region. Immunolabeling for calbindin and calretinin revealed that the sharp border between dentate granule cells and hilar mossy cells is completely lost in reeler mice. ApoER2/VLDLR double-knockout mice copy the reeler phenotype. Mice deficient only in VLDLR showed minor alterations of dentate organization; migration defects were more prominent in ApoER2 knockout mice. Tracing of the mossy fibers with Phaseolus vulgaris leukoagglutinin and calbindin immunolabeling revealed an irregular broad projection in reeler mice and ApoER2/VLDLR double knockouts, likely caused by the irregular wide distribution of granule cell somata. Mutants lacking only one of the lipoprotein receptors showed only minor changes in the mossy fiber projection. In all mutants, mossy fibers respected the CA3-CA1 border. Retrograde labeling with DiI showed that malpositioned granule cells also projected as normal to the CA3 region. These results indicate that ( 1 ) reelin signaling via ApoER2 and VLDLR is required for the normal positioning of dentate granule cells and (2) the reelin signaling pathway is not involved in pathfinding and target recognition of granule cell axons.

Hippocampal mossy fibers induce assembly and clustering of PSD95-containing postsynaptic densities independent of glutamate receptor activation

Factors that regulate the formation, spatial patterning, and maturation of CNS synapses are poorly understood. We used organotypic hippocampal slice cultures derived from developing (P5-P7) rat to test whether synaptic activity regulates the development and organization of postsynaptic structures at mossy fiber (MF) giant synapses. Antibodies to a prominent postsynaptic density (PSD) scaffold protein, PSD95, identified large (>1 microm) and irregularly shaped PSD assemblies that codistributed with synapsin-I or metabotropic glutamate receptor 7b (mGluR7b) -immunolabeled MF terminals in area CA3. To investigate the spatial organization of synaptic PSDs on individual pyramidal cells, neurons in slice cultures were transfected with a vector encoding a GFP-PSD95 fusion protein. Confocal three-dimensional reconstructions revealed clusters of PSDs along proximal dendrites of transfected pyramidal neurons in area CA3, but not in CA1. Clusters averaged 7.6 microm in length (range, 2.2-29 microm) and contained up to 35 individual PSDs (mean, 8.3). PSD clusters failed to form when slices were cultured without MFs, indicating that MFs induce cluster assembly. Chronic blockade of N-methyl-D-apartate- and AMPA/kainate-type glutamate receptors did not disrupt MF targeting or de novo formation of PSD clusters with a normal distribution on target cells. Additionally, glutamate receptor blockers did not alter the ultrastructural development of MF giant synapses containing multiple puncta adherens-like junctions and asymmetric synaptic junctions at dendritic shaft and spine domains, respectively. The results indicate that MF axons can induce the assembly and clustering of PSD95-containing postsynaptic complexes, displaying a normal subcellular and tissue distribution, by mechanisms that are independent of ionotropic glutamate receptor activation.

Potential role of synaptopodin in spine motility by coupling actin to the spine apparatus

Dendritic spines are dynamic structures that rapidly remodel their shape and size. These morphological adaptations are regulated by changes in synaptic activity, and result from rearrangements of the postsynaptic cytoskeleton. A cytoskeletal molecule preferentially found in mature spines is the actin-associated protein synaptopodin. It is strongly expressed by spine-bearing neurons in the olfactory bulb, striatum, cerebral cortex, and hippocampus. In the hippocampus, principal cells express synaptopodin mRNA and sort the protein to the spine compartment. Within the spine microdomain, synaptopodin is preferentially located in the spine neck and is closely associated with the spine apparatus. On the basis of these data we hypothesize that synaptopodin could affect spine motility by bundling actin filaments in the spine neck. In addition, it could link the actin cytoskeleton of spines to intracellular calcium stores, i.e., the spine apparatus and the smooth endoplasmic reticulum.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.

Afferent innervation influences the development of dendritic branches and spines via both activity-dependent and non-activity-dependent mechanisms

The present investigation uses an in vitro co-culture system to study the role of afferent innervation in early development and differentiation of hippocampal neurons. Our experiments indicate that the formation of two key morphological features, dendritic branches and dendritic spines, is induced by afferent innervation. Hippocampal neurons develop multiple dendritic branches and spines only when extensively innervated by living axonal afferents. No morphological changes occurred when hippocampal neurons were plated on other cell surfaces such as fixed axons or astrocytes. Furthermore, afferents exerted their effect locally on individual dendrites that they contacted. When one portion of the dendritic arbor of a neuron was contacted by afferents and the other portion was not, morphological effects were restricted to the innervated dendrites. Innervation of some of the dendrites on a neuron did not produce global effects throughout the neuron. Afferent-induced dendritic branching is independent of activity, since branch induction was unaffected by chronic application of TTX or glutamate receptor blockers. In contrast, the formation of dendritic spines is influenced by activity. The number of developing spines was reduced when TTX or a cocktail of three glutamate receptor blockers was applied. Blockade of individual AMPA, NMDA, or metabotropic glutamate receptors did not affect the number of spines. These results, taken together, demonstrate that afferents can have a prominent influence on the development of postsynaptic target cells via both activity-dependent and non-activity-dependent mechanisms, indicating the presence of multiple signals. Accordingly, this suggests an important interplay between pre- and postsynaptic elements early in development.

Spine loss in experimental epilepsy: quantitative light and electron microscopic analysis of intracellularly stained CA3 pyramidal cells in hippocampal slice cultures

The sequence of neuronal alterations resulting from epileptic activity is poorly understood. In the hippocampus of some epileptic patients, there is a loss of certain neuronal types in the hilar region and in CA3. The neuronal alterations preceding this degeneration probably affect synaptic structures. Here we have estimated the number of dendritic spines, major postsynaptic elements of hippocampal neurons, in defined dendritic segments of identified (intracellularly stained) CA3 pyramidal neurons in "epileptic" slice cultures of hippocampus and in control cultures. Slice cultures were prepared from five- or six-day-old rat pups and maintained in vivo for 23 days before epileptic activity was induced by application of the convulsants bicuculline and picrotoxin for three days. Individual CA3 pyramidal neurons were then intracellularly injected with horseradish peroxidase, and the number of dendritic spines was counted in proximodistal dendritic segments by applying the Sholl method. In addition, the total dendritic length was measured and the branching index evaluated. The number of spines on CA3 pyramidal cell dendrites in the "epileptic" cultures was found to be decreased by 40%. This spine loss affected proximal and peripheral dendritic segments of the CA3 pyramidal neurons to a similar extent. No significant differences were observed between control and "epileptic" cultures in dendritic length or in the branching index. Quantitative electron microscopic analysis did not reveal differences between "epileptic" cultures and control cultures in the spine area of the labelled CA3 pyramidal cells, indicating that there was a real spine loss, not just a reduction in the size of the spines. We conclude that epileptic activity causes morphological alterations in defined postsynaptic compartments of hippocampal pyramidal cells surviving under these conditions.

Morphological alterations of hippocampal pyramidal neurons heterotopically transplanted into the somatosensory cortex of adult rats: a quantitative Golgi study

A characteristic feature of hippocampal organization is the laminated termination of extrinsic and intrinsic afferents. At present, it is not known to what extent these layer-specific fiber projections modulate the development and final shape of the dendritic arbor of hippocampal target neurons. In the present study, pieces of late embryonic (E18) rat hippocampus were transplanted heterotopically into a cavity in the somatosensory cortex of 6-8 week-old recipient rats. Here, the transplanted neurons differentiated and survived up to several months in the absence of their specific extrinsic afferents. Moreover, tracing of transplant connections with the carbocyanine dye DiI revealed only a limited projection between the transplant and the host neocortex. Golgi-impregnated transplants were used to analyze the postsynaptic structures (dendrites and spines) of hippocampal pyramidal cells quantitatively. Compared with controls, the transplanted pyramidal neurons showed a significant reduction of apical primary dendrites and basal dendritic branches, i.e. of peripheral dendritic portions that originate farther from the soma. In contrast, the number of basal primary dendrites originating directly from the perikaryon was enhanced. Spine density on the main apical dendritic shaft was significantly lower in all peripheral dendritic segments in transplanted neurons. We conclude from our results that the absence of layer-specific extrinsic afferents that normally terminate on peripheral parts of the dendritic arbor of hippocampal pyramidal neurons caused a reduction of these peripheral dendrites and spines. In contrast, the increase of dendrites and spines near the cell body might be induced by intrinsic fibers that normally terminate on these proximal dendritic portions and are known to sprout under transplant conditions.

Dendritic development of dentate granule cells in the absence of their specific extrinsic afferents

Dendrites and spines are postsynaptic structures that develop in association with presynaptic fibers. Recent studies have shown that granule cells of the fascia dentata survive in slice cultures and differentiate in a manner known from in situ studies. However, all extrinsic afferent fibers are absent under culture conditions. In the present study, we study whether dendrites and spines of granule cells in slice cultures differentiate normally, although they are not contacted by their normal layer-specific afferents. Slices of hippocampus were prepared from rat pups at the day of birth. After 5, 10, 15, and 20 days of incubation, granule cells in these cultures were Golgi impregnated. For comparison, perfusion-fixed hippocampal sections of 5-, 10-, 15-, and 20-day-old rats were impregnated the same way. Our results show that the total density of spines on granule cell dendrites in culture increased as in perfusion-fixed animals. However, after 20 days of incubation, the absolute number of dendritic spines on cultured neurons was reduced because of a reduction of peripheral dendrites. This reduction was accompanied by an increase in the number of stem dendrites originating from the perikaryon. The density of spines on these proximal dendrites was larger in cultured granule cells than in controls. Our results suggest that the lack of major extrinsic (entorhinal) afferents that normally terminate on peripheral granule cell dendrites causes retraction of these dendrites. At the same time, there is growth of proximal dendritic portions. Proximal dendrites are targets of associational fibers, which are known to sprout under these culture conditions.

Transneuronal changes in dendrites of GABAergic parvalbumin-containing neurons of the rat fascia dentata following entorhinal lesion

The perforant path fibers from the entorhinal cortex form synapses with both granule cells and GABAergic, parvalbumin-containing (PARV) nongranule cells. The authors recently reported a persistent reduction of PARV-positive dendrites in the termination zones of entorhinal fibers in the hippocampus proper and fascia dentata after lesion of the entorhinal cortex. In the present study the authors analyzed the effects of de-entorhination on the ultrastructure of postsynaptic PARV-positive dendrites in the molecular layer of the fascia dentata. PARV immunocytochemistry was performed 2, 8, 55, and 360 days after an ipsilateral entorhinal lesion and, for comparison, 10 days after an ipsilateral fimbria-fornix transection that disconnects the hippocampus from its septal and commissural afferents. Two days after entorhinal lesion, the authors observed swelling of the tissue close to the hippocampal fissure. Adjacent distal dendritic tips of PARV-positive dentate neurons appeared bloated and reduced in number. Reduction of PARV-positive dendrites in the former perforant path termination zone persisted 55 days after entorhinal lesion and could still observed after postlesional survival times for 1 year. Degenerating axon terminals were still present 55 days following lesion and PARV-positive dendrites exhibited abnormal invaginations. Fimbria transection did not result in similar dendritic changes in PARV-positive neurons. The results indicate a long-lasting process of reorganization in the molecular layer of the fascia dentata following entorhinal lesion and persisting changes in the morphology of PARV-immunoreactive dendrites. Entorhinal fibers seem to play a specific role for the maintenance of these dendrites, since similar changes did not occur following removal of septal and commissural fibers.

Are the fine-structural characteristics of mouse hippocampal mossy fiber synapses determined by the density of mossy fiber axons?

Heritable variation of mossy fiber synapses in hippocampal region CA3 was studied in the two inbred mouse strains C3H and CPB-K. Previous Timm studies had shown a larger mossy fiber projection in C3H mice. In contrast, synaptic boutons of CPB-K mice were larger in size and perimeter and were contacted by more dendritic spines than in C3H mice. These results point to an inverse relationship between the size of the mossy fiber projection and the number of spine synapses formed by an individual mossy fiber bouton. Thus, the fiber density of a projection may be crucial for the actual morphology of the synaptic contacts formed.

Spiny nonpyramidal neurons in the CA3 region of the rat hippocampus are glutamate-like immunoreactive and receive convergent mossy fiber input

There is increasing evidence that the various types of hippocampal nonpyramidal neurons control the principal cells in different ways. In the present study a type of spiny nonpyramidal cell in stratum lucidum of rat hippocampal region CA3 was studied by Golgi impregnation. Three Golgi-impregnated and gold-toned neurons of this type were further analyzed by electron microscopy and postembedding immunocytochemistry. The dendrites of these bipolar neurons seemed to be restricted to stratum lucidum and ran parallel with the mossy fibers that terminate in this layer. A characteristic feature of this neuron is the presence of long, thin spines on both cell body and dendrites. Although these dendrites were exposed to a large number of mossy fibers, no thorny excrescences were formed which are characteristic postsynaptic elements of CA3 pyramidal neurons for synaptic contact with the mossy fibers. Semithin sections of Golgi-impregnated and gold-toned stratum lucidum cells displayed immunoreactivity of the cell body region for glutamate but not for GABA. A fine-structural analysis of gold-toned sections revealed a large cell body with numerous cytoplasmic organelles and an indented nucleus. Numerous asymmetric synapses were found on dendritic shafts as well as on the long, thin somatic and dendritic spines. Usually, several presynaptic boutons contacted a single spine. The majority of these asymmetric spine synapses were probably of mossy fiber origin, although no giant mossy fiber synapses were formed. The long spines were contacted by much smaller en passant synapses of preterminal axons. In contrast, giant mossy fiber boutons were found presynaptic to dendritic shafts and cell bodies of these cells. Our morphological analysis of a glutamate-immunoreactive, GABA-negative type of nonpyramidal neuron that receives convergent mossy fiber input suggests that the impulse flow within the "trisynaptic pathway" is more complex than previously assumed.

Modulation by GABA of neuroplasticity in the central and peripheral nervous system

Apart from being a prominent (inhibitory) neurotransmitter that is widely distributed in the central and peripheral nervous system, gamma-aminobutyric acid (GABA) has turned out to exert trophic actions. In this manner GABA may modulate the neuroplastic capacity of neurons and neuron-like cells under various conditions in situ and in vitro. In the superior cervical ganglion (SCG) of adult rat, GABA induces the formation of free postsynaptic-like densities on the dendrites of principal neurons and enables implanted foreign (cholinergic) nerves to establish functional synaptic contacts, even while preexisting connections of the preganglionic axons persist. Apart from postsynaptic effects, GABA inhibits acetylcholine release from preganglionic nerve terminals and changes, at least transiently, the neurochemical markers of cholinergic innervation (acetylcholinesterase and nicotinic receptors). In murine neuroblastoma cells in vitro, GABA induces electron microscopic changes, which are similar in principle to those seen in the SCG. Both neuroplastic effects of GABA, in situ and in vitro, could be mimicked by sodium bromide, a hyperpolarizing agent. In addition, evidence is available that GABA via A- and/or B-receptors may exert direct trophic actions. The regulation of both types of trophic actions (direct, receptor-mediated vs. indirect, bioelectric activity dependent) is discussed.

Three-dimensional analysis of the structure and composition of CA3 branched dendritic spines and their synaptic relationships with mossy fiber boutons in the rat hippocampus

This paper is the third in a series to quantify differences in the composition of subcellular organelles and three-dimensional structure of dendritic spines that could contribute to their specific biological properties. Proximal apical dendritic spines of the CA3 pyramidal cells receiving synaptic input from mossy fiber (MF) boutons in the adult rat hippocampus were evaluated in three sets of serial electron micrographs. These CA3 spines are unusual in that they have from 1 to 16 branches emerging from a single dendritic origin. The branched spines usually contain subcellular organelles that are rarely found in adult spines of other brain regions including ribosomes, multivesicular bodies (MVB), mitochondria, and microtubules. MVBs occur most often in the spine heads that also contain smooth endoplasmic reticulum, and ribosomes occur most often in spines that have spinules, which are small nonsynaptic protuberances emerging from the spine head. Most of the branched spines are surrounded by a single MF bouton, which establishes synapses with multiple spine heads. The postsynaptic densities (PSDs) occupy about 10-15% of the spine head membrane, a value that is consistent with spines from other brain regions, with spines of different geometries, and with immature spines. Individual MF boutons usually synapse with several different branched spines, all of which originate from the same parent dendrite. Larger branched spines and MF boutons are more likely to synapse with multiple MF boutons and spines, respectively, than smaller spines and boutons. Complete three-dimensional reconstructions of representative spines with 1, 6, or 12 heads were measured to obtain the volumes, total surface areas, and PSD surface areas. Overall, these dimensions were larger for the complete branched spines than for unbranched or branched spines in other brain regions. However, individual branches were of comparable size to the large mushroom spines in hippocampal area CA1 and in the visual cortex, though the CA3 branches were more irregular in shape. The diameters of each spine branch were measured along the cytoplasmic path from the PSD to the origin with the dendrite, and the lengths of branch segments over which the diameters remained approximately uniform were computed for subsequent use in biophysical models. No constrictions in the segments of the branched spines were thin enough to reduce charge transfer along their lengths.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal development of mossy cells in the human dentate gyrus: a light microscopic Golgi study

Mossy cells in the human dentate gyrus of adults and children of different ages were impregnated using the rapid-Golgi method. In every case the cause of death was verified by autopsy and the brains were used when neither the history of the patient nor autopsy revealed brain-related disease. Mossy cells in the human share common light microscopic features with the same cell type in rats and monkeys. Their most characteristic feature is the extremely large and complex excrescences on their proximal dendrites. Distal dendrites display pedunculate spines. Mossy cells have a few somal spines. The axon of mossy cells originates from the cell body and gives rise to several collaterals in the hilar region. The axons could be followed for several hundred microns, but in only one case did an axon collateral enter the granule cell layer of the adult dentate gyrus. In the newborn child, mossy cells display immature somal and dendritic features. The soma frequently bear spines. The dendrites are varicose and terminate in presumed growth cones. Both proximal and distal portions of the dendrites bear a few pedunculate spines and long-irregular filopodia. A few small excrescences are present on the proximal dendrites. The first large, complex excrescences on the proximal dendrites of mossy cells appeared in the 7-month-old child. Both somata and dendrites display adult-like characteristics in mossy cells from a 5-year-old child. However, not all mossy cells are alike and some dendrites still display long filopodia. The axons of immature mossy cells were similar to adults. The present results indicate that connections between granule cells and hilar mossy cells of the human dentate gyrus develop through an extended postnatal period of time that may last until the fifth year.

Sequential events of degeneration and synaptic remodelling in the viper optic tectum following retinal ablation. A degeneration, radioautographic and immunocytochemical study

The ultrastructural changes taking place in the retino-recipient layers of the viper optic tectum were examined between 5 and 122 days after retinal ablation. The initial degeneration of retinotectal terminals proceeds at widely different rates and is characterized by a marked degree of polymorphism in which a number of different patterns can be discerned. In the final stages of degeneration, either both the degenerating bouton and the distal portion of the postsynaptic element are engulfed by reactive glia, or, more frequently, only the degenerating terminal is eliminated and the postsynaptic differentiation remains. The free postsynaptic differentiations are reoccupied predominantly by boutons containing pleiomorphic vesicles and which are for the most part gamma-aminobutyric acid (GABA)ergic, thus forming heterologous synapses; less frequently these sites are occupied by boutons of the ipsilateral visual contingent to form homologous synapses. These two processes, both of which depend on terminal axonal sprouting, take place within the first 3 postoperative months. They are followed by a decrease in the number of heterologous synapses and a concurrent increase in the number of homologous synapses newly formed by optic boutons generated by collateral preterminal sprouting of ipsilateral retinotectal fibres. The data suggest that partial deafferentation of the optic tectum induces a transitory GABAergic innervation of free postsynaptic sites prior to the restoration of new retinal synaptic contacts.

The mossy cells of the fascia dentata: a comparative study of their fine structure and synaptic connections in rodents and primates

In this study the fine structure and synaptic connections of mossy cells in the rat and monkey fascia dentata were analyzed. In order to study commissural connections of identified mossy cells in the rat, hilar neurons were retrogradely labeled by horseradish peroxidase (HRP) or Fast Blue (FB) injections into the contralateral hippocampus. Vibratome sections containing retrogradely HRP-labeled hilar neurons were Golgi-impregnated and gold-toned. Hilar commissural neurons identified by contralateral FB injection were intracellularly labeled with Lucifer Yellow (LY). Lucifer Yellow staining was made electron-dense by photoconversion thereby allowing for an electron microscopic analysis of the retrogradely labeled and intracellularly stained neurons. With these two different approaches, we succeeded in identifying rat mossy cells projecting to the contralateral hippocampus. Mossy cells in the fascia dentata of primates (Papio anubis, Macaca mulatta, Saimiri sciureus) were, like mossy cells of rats, either Golgi-impregnated and gold-toned or intracellularly injected with LY. No major differences were found between mossy cells of rats and monkeys. The mossy cell dendrites originated from the two sides of an ovoid cell body and were mainly oriented parallel to the granule cell layer. In contrast to the rat, dendrites of mossy cells in the primate did not respect the granule cell layer and penetrated frequently into the molecular layer. The occurrence of excrescences on proximal dendrites was a characteristic feature of all mossy cells. These large spines were more complex in the primate than in the rat. In both rats and primates they formed numerous asymmetric synapses with large boutons of mossy fibers. Peripheral dendrites were covered with small, simple spines. Interestingly, these peripheral dendrites lacking excrescences also established asymmetric synapses with mossy fiber boutons as well as asymmetric and symmetric contacts with smaller terminals of unknown origin. These findings indicate that in both rats and primates the thorny excrescences are not the only target of the mossy terminals. While the proximal portions of the mossy cell dendrites appear to be exclusively contacted by the granule cells, a larger number of neuron types may converge on the distal dendrites. The axons of mossy cells, in both rats and primates, although incompletely stained with the present methods, were seen to ramify in the hilar region. Our results demonstrate that, despite minor species differences, the mossy cells of the fascia dentata represent a cell type that is preserved in phylogenetically distant species.

Synaptic organization of intracellularly stained CA3 pyramidal neurons in slice cultures of rat hippocampus

Pyramidal cells of regio inferior in slice cultures of the rat hippocampus were impaled and intracellularly stained with horseradish peroxidase. A correlated light- and electron-microscopic analysis was then performed to study the properties of these neurons under culture conditions with particular emphasis on input synapses onto these cells. Like pyramidal cells in situ, CA3 pyramidal neurons in slice cultures had a triangular cell body with an apical stem dendrite emerging from it. Several basal dendrites and the axon arose from the basal pole of the cell body. The peripheral thin branches of both apical and basal dendrites were covered with small spines, whereas proximal thick dendritic segments and portions of the cell body exhibited large spines or excrescences. The axon gave off numerous fine varicose collaterals which projected to stratum radiatum of CA1 (Schaffer collaterals), to the alveus and to the hilar region. In one case a collateral could be followed to stratum moleculare of the fascia dentata. Electron-microscopic analysis of the injected pyramidal neurons revealed that their cell bodies, dendritic shafts and spines formed synaptic contacts with presynaptic terminals. Mossy fiber endings were identified by their large size and their numerous clear synaptic vesicles with some dense-core vesicles intermingled, and were observed to form synaptic contacts on the large spines or excrescences. Since extrinsic afferents degenerate in slice cultures, the numerous synaptic boutons on the identified pyramidal neurons probably arise from axons of intrinsic neurons that have sprouted in response to deafferentation. This assumption is supported by the finding that collaterals of the injected neurons formed abundant synaptic contacts on dendritic shafts and spines of other cells. These results suggest that, although pyramidal cells under culture conditions retain a remarkable number of their normal characteristics, considerable synaptic reorganization does take place.

Fine structure and synaptic connections of identified neurons in the rat fascia dentata

A survey is given of the synaptic connections of identified neurons in the rat fascia dentata based on our own Golgi/electron microscopic and light and electron microscopic immunocytochemical findings as well as on results obtained from the literature. The report largely deals with the dominating cell type in the region, the dentate granule cell. Of the various types of hilar cells, the GABAergic neurons, particularly the inhibitory basket cells, are taken into account. Differences in fine structure between granule cells and basket cells as well as mutual synaptic connections between these two types of dentate neurons are elaborated. This survey may provide a basis for further neurophysiological and pharmacological studies on these cells.

Lesion-induced transneuronal plasticity in the adult rat hippocampus

The process of reactive synaptogenesis has been demonstrated in several areas of the central nervous system, including the hippocampal dentate gyrus. After a complete unilateral entorhinal lesion, approximately 85% of the input to the outer two-thirds of the ipsilateral dentate molecular layer is lost. Bilateral fluctuations in synaptic density within non-denervated zones of the dentate molecular layer predict further alterations in neural circuitry at sites located transneuronally to the denervated dentate granule cells. Using quantitative electron microscopy, our study demonstrates a complete cycle of synapse loss and reacquisition within the ipsilateral but not contralateral CA4/hilus region of the hippocampal formation. This area is one of the terminal fields for the dentate granule cell mossy fiber axons. In addition the granule cell mossy fiber axons sprout during the postlesion time course and form a significantly increased number of new mossy fiber terminals within the ipsilateral and contralateral CA4/hilus area. Our results indicate that responses to brain injury may no longer be confined to a local denervated site, but probably include polyneuronal circuitry loops, which may encompass one or more areas of the central nervous system. Previous difficulties in providing a close behavioral or functional correlation to localized structural events may be explained by a more global brain response to an injury.

Long-term effects of parallel fiber loss in the cerebellar cortex of the adult and weanling rat

Short- and long-term effects of parallel fiber deafferentation of adult and weanling cerebellar cortex were investigated following parasagittal transections of the lateral cerebellar hemisphere. Short-term electron microscopic examination revealed that parallel fibers undergo rapid electron-dense degeneration within 5 days of axotomy. These axons were the only neuronal elements immediately affected by the lesion. The continued maintenance of Purkinje cell terminal branchlets and stellate cell dendrites is dependent upon the presence of an adequate parallel fiber milieu. Morphological evidence is provided which suggests that Purkinje cell dendritic spines may be phagocytically removed by Bergmann glial cells following parallel fiber loss. Although a marked decrease was reported in the number of spines projecting from terminal branchlets following deafferentation of both adult and weanling rats, these data suggest that some spines are capable of increasing their length. The elongation of these spines may represent a form of dendritic plasticity. No evidence was found to suggest that deafferentated terminal branchlets are receptive to forming heterologous synaptic contacts. The primary response to parallel fiber deafferentation for both the adult and weanling cerebellum therefore appears to be transneuronal degeneration.

Mossy fibres form synapses with identified pyramidal basket cells in the CA3 region of the guinea-pig hippocampus: a combined Golgi-electron microscope study

Mossy fibres, i.e. the axons of dentate granule cells, terminate with characteristic giant boutons on large spines or excrescences of the pyramidal cells in regio inferior of the hippocampus. In addition to pyramidal cells there are several types of non-pyramidal neuron which extend their dendrites into the termination zone of mossy fibres. By using the combined Golgi-electron microscope technique mossy fibre terminals were found, which established asymmetric synaptic contacts with both spines of pyramidal cells and smooth dendrites of identified (Golgi-stained) pyramidal basket cells in the CA3 region of the guinea-pig hippocampus. The observed synaptic connection with pyramidal basket cells suggests an involvement of the mossy fibre system in feed-forward inhibition of the hippocampal pyramidal neurons.

Long-term effect of mossy fiber degeneration in the rat

Mossy fiber-deafferentated rats (20) were permitted to survive from 34 to to 120 days and subsequently examined following Golgi-Cox preparation or after processing for electron microscopy. The primary response to mossy fiber deafferentation was transneuronal degeneration of the granule cell system. Morphological evidence is provided that suggests that the mossy fiber varicosity plays an important role in the fragmentation and removal of the granule cell digitiform dendrite. Computer-assisted image analysis of Golgi-impregnated Purkinje cells indicated significant losses in both smooth branch and spiny branchlet numbers following loss of the mossy fiber input. Ultrastructural examination revealed that a secondary transneuronal degeneration occurred within the dendritic arborization of both Purkinje cells and molecular layer interneurons. Although an overall reduction in the number of dendritic spines occurred along the terminal branchlets following mossy fiber deafferentation, several of the existing spines underwent marked changes in length, with some elongating to more than twice their size. By increasing the length of their spines, denervated Purkinje cells may acquire new synaptic contacts.

Lesion-induced mossy fibers to the molecular layer of the rat fascia dentata: identification of postsynaptic granule cells by the Golgi-EM technique

The axons of the dentate granule cells, the hippocampal mossy fibers, sprout "backward" into the dentate molecular layer when this is heavily denervated. Using the combined Golgi-electron microscopy (EM) technique we now demonstrate that these aberrant supragranular mossy fibers at least in part terminate on granule cell dendrites. Sprouting of mossy fibers into the dentate molecular layer was induced in adult rats by simultaneous surgical removal of the commissural and entorhinal afferents to the fascia dentata. After at least 7 weeks survival, the presence of mossy fiber terminals in the inner part of the dentate molecular layer was demonstrated by light microscopy. In the electron microscope the mossy fiber terminals were identified by their unique structural characteristics, namely, the unusually large size of the terminals, the dense packing of clear synaptic vesicles with a few dense core vesicles intermingled, the presence of asymmetric synaptic contacts with spines and desmosome-like contacts with dendritic shafts, and the continuity with a thin unmyelinated preterminal axon. Golgi-stained granule cells were first identified in the light microscope, and then, after deimpregnation, the same cells were examined in the electron microscope. In ultrathin, serial sections lesion-induced mossy fiber terminals were found in synaptic contact with spines on proximal dendritic segments of such identified Golgi-impregnated granule cells. From this we conclude that the aberrant, supragranular mossy fibers can innervate dendrites of the parent cell group, the dentate granule cells. The results, moreover, provide an example of reactive synaptogenesis where both the sprouted afferents and its postsynaptic element have been identified.

Transneuronal vestibular afferent influence on the nodular molecular layer synaptogenesis

The effect of vestibular afferent deprivation on the synaptogenesis of the nodular molecular layer has been studied quantitatively. No detectable effect on the time sequence of the development of the molecular layer and the external granular layer was found. Only around hatching a significantly reduced synaptic profile density was found in otocyst-deprived chickens on both halves of the nodulus. This effect can most easily be explained by the assumption of an anterograde transient transneuronal influence of vestibular afferents on the ability of parallel fibers to form synapses.

Ultrastructure of mossy fiber endings in in vitro hippocampal slices

0.2 to 0.4 mm thick slices of guinea pig hippocampus were studied morphologically after varying periods of incubation at 36 degrees C in Krebs-Ringer solution. Prior to fixation, the slices were tested for the presence of synaptically driven discharges of CA 3 neurons following mossy fiber (mf) stimulation because tissue preservation was satisfactory only in slices in which electrical responses were obtained. The fine structure of the MF layer in slices was compared with the ultrastructure of this region in hippocampal tissue fixed by transcardial perfusion or immersion of the tissue in the fixative. In the central part of the slices many intact neuronal structures of the mf layer could be seen even after 4 h of incubation. In the outer parts of the slices, neurons were swollen and vacuolated. These alterations were not observed in hippocampal tissue fixed by transcardial perfusion or by immersion. In all parts of the slices dark neurons and processes were found. Since dark neurons were also numerous in tissue blocks immersed in the fixative but were rare in perfused material, these changes were obviously caused by damage to unfixed tissue and fixation by immersion.

Bouton renewal patterns in rat hindlimb cortex after thoracic dorsal funicular lesions

The transneuronal effect of bilateral, dorsal funicular lesions (T 12) on the frequency of boutons on cells in layer IV of hindlimb cortex was studied. Adult rats were utilized 1, 2, 3, 7, 14, 30, 45, 60, 90, or 120 days postoperative (DPO), and tissue was processed for the light microscopic visualization of silver-impregnated boutons (Rasmussen method). Bouton counts were taken on soma, or along 5- and 10-micrometers segments or proximal dendrite branching from soma. The soma diameter also was measured on those neurons chosen for bouton counts on the circumference of the soma. Statistically significant, increased afferentation on soma and proximal dendrite occurred during the first postlesion week relative to longer survival times; bouton counts on the proximal dendrite showed a trend (not statistically significant) toward increases above normal. These data mirror similar, consistent increases in bouton counts reported in thalamic ventroposterolateral nucleus of these same cases. At 14 DPO, bouton counts on the soma decreased below normal (P less than 0.005) and, except at 60 DPO, remained so through 120 DPO (P less than 0.025). Bouton counts on the proximal dendrite also decreased below normal at 14 DPO (P less than 0.005), thereafter exhibiting either periodic (along 5-micrometers) or extended (along 10-micrometers) periods in significant decreases from normal. Correlations in lesioned cases between the number of boutons on the soma and either bouton counts on proximal dendrite or soma diameter were positive and statistically significant (P less than 0.005 in all correlations). Possible anterograde (via the dorsal column-medial lemniscal system) and/or retrograde (via the corticospinal tract) transneuronal mediation of these effects in hindlimb cortex is discussed.

Synaptogenesis in the intermediate gray region of the lumbar spinal cord in the postnatal rat

Mid-thoracic spinal cord transection produces dramatically different behavioral results depending upon a rat's age at the time of surgery. The present study was initiated to determine whether the synaptic development in the gray matter of the normal, developing spinal cord differs before and after the period when maximal behavioral recovery occurs. The L6 segments from 10 groups of animals, 0--30 days of age, taken at 3 day intervals (4 animals/group) were studied by light microscopy. Areal measurements of the gray matter were made using an integrating x-y tablet interfaced to a computer. Cell size, cell density and area of neuropil were evaluated in the lateral portions of the intermediate gray matter, laminae VI and VII. Electron microscopic analyses of synaptogenesis were performed on material from the same region in animals 3, 12, 15, 21 and 30 days old using similar morphometric methods while taking note of vesicle, junctional, and mitochondrial morphology. A 60% increase in area of neuropil paralleled a linear increase, of comparable magnitude, in area of the gray matter until 15 days of age when both curves reached plateau. Neuronal perikaryal size remained constant (congruent to 200 sq. microns in plane of nucleolus) throughout development and so could not have contributed to the increase in area of gray matter. Areal measurements of the size and counts of the number of vesicle containing profiles demonstrated a 50% increase in density of axon terminals between 3 and 12 days of age and a steady decline thereafter. The size of vesicle-containing profiles in laminae VI and VII remained constant at a small value (congruent to 0.35 sq microns) until 12 days of age, showed rapid growth to 0.54 sq. microns between 12 and 15 days of age, followed by a more moderate increase in sectional area after 15 days. These results suggest that during the period when recovery of function follows spinal injury, synaptogenesis in the intermediate gray region of the lumbar spinal cord proceeds rapidly, while at stages when little recovery of function follows spinal transection, synaptogenesis is essentially complete.

Induction and maintenance of free postsynaptic membrane thickenings in the adult superior cervical ganglion

The superior cervical ganglion (SCG) of adult rats was exposed to GABA, either by long lasting microapplication (implantation of glass bulbs for 1-24 days) or in short term experiments (external application up to 6 h). Autoradiography showed that [3H] GABA accumulated selectively in satellite cells. The GABA produced the following effects: (1) Specialized membrane thickenings--similar in fine structural appearance to those seen as postsynaptic membrane thickenings at Gray type I synapses--were formed at the extrasynaptic dendritic surface of principal ganglion cells. (2) Morphometry revealed that the surface to volume ratio of dendrites increased significantly corresponding to an enlargement of their extrasynaptic surface as a result of the formation of spine-like projections. (3) Electrophysiology confirmed that, at least after short term application, the action potentials induced by preganglionic stimulation were heavily suppressed. These results suggest that, in the course of depressed ganglionic activity, so-called free postsynaptic membrane thickenings are generated and maintained in the SCG of adult rats even in the absence of significant axonal degeneration. The discussion focuses on two points: (1) possible similarities between the conditions of neurons after denervation and under the influence of GABA; (2) a possible role of GABA and other substances with inhibitory action in synaptogenesis.