Structure and plasticity of newly formed adult synapses: a morphometric study in the rat hippocampus

Neuropil pruning in Early-Course Schizophrenia: Immunological, Clinical, and Neurocognitive Correlates

INTRODUCTION

Neuropathological studies suggest neuropil reduction in schizophrenia. Altered synaptic pruning is proposed to underlie neuropil reduction. Underlying factors and clinical correlates of synaptic pruning are poorly understood. Using phosphorus magnetic resonance spectroscopy (31P MRS), it is feasible to assess membrane phospholipid (MPL) metabolites in the brain that specifically and sensitively reflect neuropil expansion (elevated MPL precursors) or contraction (elevated MPL catabolites).

METHODS

We examined MPL metabolites and their cognitive, clinical and immunologic correlates among 28 early-course schizophrenia individuals (illness duration 1.99±1.33 years; antipsychotic-naïve=18) and 21 controls. We acquired whole-brain multi-voxel 31P MRS data from 12 unique brain regions. Interleukin-6 and C-reactive protein (CRP) were assayed in the serum. Generalized linear mixed models examined case-control differences in MPL metabolites in these regions correcting for multiple testing. Partial correlations accounting for multiple tests examined the relationship of Interleukin-6 and CRP levels with MPL metabolite levels.

RESULTS

MPL catabolite levels were increased in the thalamus in schizophrenia compared to controls. Interleukin-6 and CRP levels did not show case-control differences. Interleukin-6 levels positively correlated with MPL catabolite levels in the thalamus after correcting for multiple tests. The left thalamus MPL catabolite levels correlated negatively with sustained attention (corrected p=0.039).

DISCUSSION

Elevated MPL catabolites in the thalamus suggest increased neuropil contraction that may be related to excessive synaptic pruning. The thalamic neuropil contraction is associated with Interleukin-6 levels suggesting central pathogenic mechanisms for the inflammatory mediators. Correlation of increased thalamic MPL catabolite levels with cognitive impairments suggests clinical correlates of neuropil contraction.

Ultrastructural synaptic changes associated with neurofibromatosis type 1: a quantitative analysis of hippocampal region CA1 in a Nf1(+/-) mouse model

Neurofibromatosis type 1 (NF1) is one of the most frequently diagnosed autosomal dominant inherited disorders resulting in neurological dysfunction, including an assortment of learning disabilities and cognitive deficits. To elucidate the neural mechanisms underlying the disorder, we employed a mouse model (Nf1(+/-) ) to conduct a quantitative analysis of ultrastructural changes associated with the NF1 disorder. Using both serial light and electron microscopy, we examined reconstructions of the CA1 region of the hippocampus, which is known to play a central role in many of the dysfunctions associated with NF1. In general, the morphology of synapses in both the Nf1(+/-) and wild-type groups of animals were similar. No differences were observed in synapse per neuron density, pre- and postsynaptic areas, or lengths. However, concave synapses were found to show a lower degree of curvature in the Nf1(+/-) mutant than in the wild type. These results indicate that the synaptic ultrastructure of Nf1(+/-) mice appears relatively normal with the exception of the degree of synaptic curvature in concave synapses, adding further support to the importance of synaptic curvature in synaptic plasticity, learning, and memory.

Inverse relationship between adult hippocampal cell proliferation and synaptic rewiring in the dentate gyrus

Adult neurogenesis is a key feature of the hippocampal dentate gyrus (DG). Neurogenesis is accompanied by synaptogenesis as new cells become integrated into the circuitry of the hippocampus. However, little is known to what extent the embedding of new neurons rewires the pre-existing network. Here we investigate synaptic rewiring in the DG of gerbils (Meriones unguiculatus) under different rates of adult cell proliferation caused by different rearing conditions as well as juvenile methamphetamine treatment. Surprisingly, we found that an increased cell proliferation reduced the amount of synaptic rewiring. To help explain this unexpected finding, we developed a novel model of dentate network formation incorporating neurogenesis and activity-dependent synapse formation and remodelling. In the model, we show that homeostasis of neuronal activity can account for the inverse relationship between cell proliferation and synaptic rewiring.

Effects of long-term sensory deprivation on asymmetric synapses in the whisker barrel field of the adult rat

Whisker trimming deprives the cortical barrel field from the patterned sensory input that derives from active touch but leaves passive tactile signals unaltered. We have studied in the rat barrel field, by stereological procedures, the effects of a sustained period of unilateral deprivation by whisker clipping during adolescence and early adulthood on (1) the surface density (SV) of asymmetric synapses, as determined from measuring the presynaptic membrane specializations, and (2) the numerical density of asymmetric synaptic profiles (NA), classified according to their postsynaptic target and their apparent curvature. Compared to control rats, the procedure did not change the overall volume of the region, the volume fraction occupied by each cortical layer, or the volume fraction occupied by unmyelinated axons and boutons. However, the deprived barrel cortex displayed an increase in SV in layers I and II, and an increase in NA in layer I and in the cortex as a whole, mainly due to an increase in profiles with a convex shape. Layer IV was the least affected by the deprivation. These results point to a net increase, rather than a decrease, of excitatory synapses in the deprived cortex, which could result from a deprivation-induced decrease in the rate of normal synapse loss. This effect occurs specifically in superficial layers, more involved in intracortical and cortico-cortical, rather than thalamo-cortical, processing.

Circulating insulin-like growth factor I and functional recovery from spinal cord injury under enriched housing conditions

Voluntary locomotor training as induced by enriched housing of rats stimulates recovery of locomotion after spinal cord injury (SCI). Generally it is thought that spinal neural networks of motor- and interneurons located in the ventral and intermediate laminae within the lumbar intumescence of the spinal cord, also referred to as central pattern generators (CPGs), are the 'producers of locomotion' and play a pivotal role in the amelioration of locomotor deficits after SCI. It has been suggested that locomotor training provides locomotor-specific sensory feedback into the CPGs, which stimulates remodeling of central nervous system pathways, including motor systems. Several molecules have been proposed to potentiate this process but the underlying mechanisms are not yet known. To understand these mechanisms, we studied the role of insulin-like growth factor (IGF) I in functional recovery from SCI under normal and enriched environment (EE) housing conditions. In a first experiment, we discovered that subcutaneous administration of IGF-I resulted in better locomotor recovery following SCI. In a second experiment, detailed analysis of the observed functional recovery induced by EE revealed full recovery of hindlimb coordination and stability of gait. This EE-dependent functional recovery was attenuated by alterations in the pre-synaptic bouton density within the ventral gray matter of the lumbar intumescence or CPG area. Neutralization of circulating IGF-I significantly blocked the effectiveness of EE housing on functional recovery and diminished the EE-induced alterations in pre-synaptic bouton density within the CPG area. These results support the use of IGF-I as a possible therapeutic aid in early rehabilitation after SCI.

Long-lasting synapse formation in cultured rat hippocampal neurons after repeated PKA activation

Recently, we reported that the repeated activation of cyclic-AMP-dependent protein kinase (PKA) in the rat hippocampus under tissue culture conditions induced the enhancement of excitatory postsynaptic potential (EPSP), which lasted more than 2 weeks and was accompanied by the formation of morphologically identifiable synapses. Here we examined whether an equivalent synapse formation is induced in dissociated cell cultures of rat hippocampal neurons. Brief (15-min) application of Sp-cAMPS (a membrane-permeable analog of cyclic AMP) induced an increase in the number of synaptic sites (identified by the apposition of immunocytochemically labeled pre- and postsynaptic structures). There were two types of increase: a short-lasting one that lasted less than 24 h after a single application of Sp-cAMPS, and a long-lasting one that lasted more than 2 weeks after repeated applications. The long-lasting increase in synaptic sites was dependent on the time and interval of application and was suppressed by Rp-cAMPS (a PKA inhibitor). The synapses were judged to be active based on the endocytosis of FM1-43, a fluorescent dye. Electron microscopy confirmed the increase in the number of synaptic ultrastructures. The present results show that the synaptogenesis induced by repeated PKA activation is reproducible in a neuronal network that is reconstituted under dissociated cell culture conditions. This experimental system, together with the synaptogenesis in the slice culture system described previously, serves as a good in vitro model for the analysis of the process of conversion from short-lasting plasticity (lasting for hours) into a long-lasting one (lasting for days-weeks).

The mossy fiber system of the hippocampal formation is decreased by chronic and postnatal but not by prenatal protein malnutrition in rats

We tested in 70-day-old Sprague-Dawley rats, whether malnutrition imposed during different periods of hippocampal development produced deleterious effects on the total reference volume of the mossy fiber system. Animals were treated under four nutritional conditions: (a) well nourished; (b) prenatal protein malnourished; (c) chronic protein malnourished and (d) postnatal protein malnourished. Timm's stained material was used in coronal hippocampal sections (40 microm) to estimate--using the Principle of Cavalieri--the total reference volume of the mossy fiber system in each experimental group. Our results show that chronic and postnatal protein malnourished, but not prenatal malnourished rats, decrease the mossy fiber system and the total reference volume of the mossy fiber system are selectively vulnerable to the type of dietary restriction. Thus, chronic and posnatal protein malnutrition produce deleterious effects, but only rats under prenatal protein malnutrition were able to reorganize synapses in this plexus. These findings raise the possibility that chronic malnutrition, as a long-term stressful factor, might be an important paradigm to test structural hippocampal changes that produce physiological and pathophysiological effects, or the possibility to recover its function for nutritional rehabilitation.

Changes in synaptic ultrastructure during reactive synaptogenesis in the rat dentate gyrus

Advances in stereology, combined with continuing relevance to aging, as well as recovery from disease and injury make the reexamination of reactive synaptogenesis (RS) overdue. Moreover, recent mathematical models have suggested novel aspects of morphology, such as compartmentalization, may have profound effects on synaptic transmission. Given these novel findings, their correlation with other models of synaptic plasticity, and their potential significance for behavioral function, the precise nature of these changes need to be explored through quantitative morphometry. Towards this goal, the synaptic morphology of the dentate gyrus was assessed via serial electron microscopy at 3, 6, 10, 15, and 30 days following unilateral entorhinal cortex lesions. Foremost, the results showed that degree of curvature is a plastic feature of synapses. During RS, concave synapses showed an immediate/long-lasting increase in curvature, suggesting their importance in the compensation response. Flat synapses showed unique changes in growth, having implications for development and activation following synaptogenesis. Moreover, changes in size and curvature showed a different dynamic depending on proximity from damage. In the directly denervated MML, synapses showed an increase in curvature proportionate to increases in size. In the neighboring IML, however, these changes were independent-increases in curvature far surpassed synaptic growth.

Comparative analyses of synaptic densities during reactive synaptogenesis in the rat dentate gyrus

Advancements in the field of synaptic plasticity have created the need for a reexamination of classic paradigms using new and more precise techniques. One prime candidate for such a reexamination is the process of reactive synaptogenesis (RS). Since the time course of RS was initially outlined in the 1970s and 1980s, advances in stereology have allowed for better characterization of synaptic ultrastructure. Thus, a reexamination was undertaken in the hippocampal dentate gyrus by assessing the densities and proportions of several synaptic subtypes in Long-Evans hooded rats at 3, 6, 10, 15 and 30 days following induction of unilateral lesions of the entorhinal cortex. Although initial synaptic loss in the denervated region was similar to previous reports, recovery during the first 30 days is not as dramatic as previously observed. Following lesioning, concave and perforated synapses retained pre-lesion density despite massive degeneration, underscoring their theoretical importance in plasticity and maintenance of neural function. Convex synapses showed opposite changes, having implications for excitation/inhibition imbalance following lesion induction. These complementary alterations in synaptic structures support ultrastructural changes as a means for compensation following synaptic loss. Nearby areas also seem to participate in this response, with a striking similarity to other models of plasticity, such as long-term potentiation.

Sequential changes in the synaptic structural profile following long-term potentiation in the rat dentate gyrus: III. Long-term maintenance phase

LTP has been associated with changes in synaptic morphology but the nature of these changes over the time course of the enhanced electrophysiological response has not been fully determined. The current research involved an examination of synaptic structure in the rat hippocampus during the long-term maintenance phase of LTP. Synapses were examined in the middle third of the molecular layer (MML) of the rat dentate gyrus following repeated high frequency tetanization of the perforant path. Synapses from both the ipsilateral inner third of the dentate molecular layer (IML), which was not directly stimulated during the induction of LTP, as well as implanted, nonstimulated animals, served as controls. LTP was induced over a 4-h period, and the animals were sacrificed 5 days after the final stimulation of the LTP group. Ultrastructural quantification included the total number of synapses per neuron, synaptic curvature, the presence of synaptic perforations, and the maximum length of the synapses. No overall changes in the number of synapses per neuron, shape, or synaptic perforations were observed. There was, however, a significant increase in the length of synapses in the directly stimulated LTP tissue. This increase in synaptic length was particularly evident in the concave-shaped synapses which were also more perforated. These results, together with previous findings, describe a sequence of changes in synaptic morphology that accompany LTP in a structure that is associated with learning and memory.

Sequential changes in the synaptic structural profile following long-term potentiation in the rat dentate gyrus. II. Induction/early maintenance phase

Long-term potentiation (LTP), one of the most compelling models of learning and memory, has been associated with changes in synaptic morphology. In this study, LTP was induced and animals were sacrificed 1 h after the stimulation of the LTP group (induction / early maintenance phase). Synapses in the directly stimulated middle third of the dentate gyrus molecular layer (MML) were examined while synapses from the inner third of the dentate molecular layer (IML) of the LTP animals and both the MML and the IML of implanted animals served as controls. The total number of synapses per neuron, synaptic curvature, the presence of synaptic perforations, and the maximum length of the synaptic contact and active zone were examined. No overall change in the number of synapses per neuron was observed in the LTP tissue. LTP was associated with a significant increase in the proportion of perforated and irregular-shaped synapses compared to controls. The increase in perforated synapses was particularly apparent in the proportion of concave perforated synapses. Nonperforated concave synapses were found to be significantly larger in potentiated tissue. The total synaptic length per neuron of synapses in a concave configuration was also significantly higher following potentiation. These results suggest that the specific structural profile associated with 1-h post-LTP induction, which differed from the profile observed at 24 h post-induction, may represent a unique early phase of synaptic remodeling in a series of changes observed during LTP induction, maintenance, and decay.

Neuronal degeneration and reorganization: a mutual principle in pathological and in healthy interactions of limbic and prefrontal circuits

Based on developmental principles and insights from animal research about neuroplasticity in cell assemblies, this article is to propose a view of plasticity that promotes a link between hippocampal and prefrontal structure and function. Both the mitotic activity (counting of BrdU-labeled cells) in hippocampal dentatus and the maturation of dopamine fibres (quantitative immunochemistry of mesoprefrontal projection) in the prefrontal cortex proved to be a measurable combination for investigating the complex chain of events that relate activity dependent neuroplasticity to normal as well as to pathological maturational processes. With our animal model we demonstrate that both rearing conditions and neuroactive substances can effectively interfere with developmental plasticity and induce a malfunctional adaptation of prefrontal structures and neurotransmitter systems (dopamine, GABA). In the hippocampal dentatus, where ontogenetic plasticity proved to be preserved by continued neuro- and synaptogenesis, serious damage can be internalized without simultaneous disruption of neural dynamics offering an approach to reverse dysfunctional reorganization in the prefrontal cortex.

Effects of testosterone on the synaptology of the medial preoptic nucleus of male Japanese quail

The medial preoptic nucleus (POM) of male Japanese quail is a sexually dimorphic testosterone-dependent structure that plays a key role in the activation of male sexual behavior. Both the total volume of the nucleus and the size of the dorsolateral neurons are decreased in gonadectomized males. Immunocytochemical studies have revealed a complex pattern of innervation: immunopositive fibers for several neuropeptides and neurotransmitters have been detected in the POM; some of them (e.g. vasotocin-immunoreactive fibers) are sexually dimorphic and testosterone-dependent To understand the anatomical bases of these testosterone-dependent neurochemical changes, we performed an ultrastructural study of the POM neuropil in intact sexually mature, gonadectomized, or testosterone-treated gonadectomized males. A complex synaptic organization of the POM neuropil was observed in intact male quail reflecting the heterogeneity of the neurotransmitters and neuropeptides present in this nucleus. Changes in this organization were observed after the endocrine manipulations. The number of axosomatic synapses per cell body decreased after gonadectomy and was restored to the level observed in the intact group after the administration of testosterone. By contrast, no significant change was observed in the density of axodendritic and axospinal synapses after hormonal manipulations which suggests that the total number of synapses in the nucleus should be affected by testosterone (constant density in a changing total volume). The cross-sectional area of synaptic boutons was also decreased by castration and restored to intact level by testosterone. The action of testosterone on the activation of male copulatory behavior in gonadectomized birds is hence paralleled by an extensive rearrangement of neuropil in the POM.

Age-changes of brain synapses and synaptic plasticity in response to an enriched environment

Numerical synaptic density and synaptic vesicle density in rat frontal cortex were examined by electron microscopy as a function of age. The density of axospinous synapses, a major population of synapses, was found to peak at age 1 month, and to gradually decrease with aging. The synaptic vesicle density in axospinous synapses was shown to rapidly increase to a peak during the first 3 weeks and then decrease to the adult level, which remained unchanged in senescence. The time course of synaptic changes in aging is presented in this study. In a previous report (Saito et al. [1994] J. Neurosci. Res. 39:57-62), we showed that enriched rearing conditions restored the age-related decrease of synaptophysin contents. This might be due to increased numerical synaptic density or enhanced packing density of synaptic vesicles in synapses. The results of the present study support the latter explanation; that is, synaptic vesicle contents were increased without changes in synaptic density. Synaptic plasticity induced by environmental stimulation is shown to relate with synaptic strengthening, but not with the formation of new synapses.

Sequential changes in the synaptic structural profile following long-term potentiation in the rat dentate gyrus: I. The intermediate maintenance phase

Changes in synaptic structure have been reported following the induction of long-term potentiation (LTP). The structure of synapses during the intermediate maintenance of LTP has yet to be fully characterized in chronically implanted freely moving animals. The present study examined synapses in the middle third of the molecular layer (MML) of the rat dentate gyrus following repeated high frequency tetanization of the perforant path. Synapses from both 1) the ipsilateral inner third of the dentate molecular layer (IML), which was not directly stimulated during the induction of LTP, as well as 2) implanted, nonstimulated animals, served as controls. LTP was induced over a 4-h period, and the animals were sacrificed 24 h after the final stimulation of the LTP group. Ultrastructural quantification included the total number of synapses, synaptic curvature, the presence of synaptic perforations, and the maximum length of the synaptic contact. Although LTP was not associated with an overall increase in synaptic number, there was a significant increase in the proportion of presynaptically concave-shaped synapses. Further, the concave synapses in the LTP tissue were found to be significantly smaller than control concave synapses. There was also a significant increase in the number of perforated concave synapses which exceeded the overall increase in concave synapses, and occurred despite the lack of a general increase in perforated synapses. It was concluded that this specific structural profile, observed at 24 h postinduction, may help support the potentiated response observed at this stage of LTP maintenance.

Changes in dendritic structure and function following hippocampal lesions: correlations with developmental events?

Recovery after nervous system lesions may lead to partial re-institution of developmental schemes and processes. Here we review several of these proposed schemes, with the conclusion that though some processes may involve re-expression of embryonic phenotypes, there are many processes invoked during recovery from lesions that do not mirror developmental phenomena. The inability to fully revert to embryonic schemes because of adult phenotype may partially account for the decreased recovery observed in adults compared to that noted after lesions during development.

Free postsynaptic densities in the hippocampus of the female rat

The number of synapses in the adult, female hippocampal CA1 region fluctuates naturally across the estrous cycle in an ovarian steroid-dependent manner. This phasic variation in synapse number occurs without identifiable degenerating synapses. Ultrastructural correlates of the dynamic aspect of this synapse loss and synapse formation thus remain undescribed. During early development, one hallmark of synaptogenesis is the presence of free postsynaptic densities (PSDs). Here we report that the incidence of free PSDs in CA1 fluctuates across the rat estrous cycle. The number of free PSDs is greatest on the afternoon of proestrus and is significantly decreased on the afternoon of estrus, 24 h later. We hypothesize that these free PSDs reflect synapse turnover in the adult CA1 region.

Differential spine loss and regrowth of striatal neurons following multiple forms of deafferentation: a Golgi study

Golgi-Cox method and morphometric analyses were used to study the plasticity of striatal medium spiny I neurons in 6-month-old C57BL/6N mice after unilateral or bilateral lesion of the cerebral cortex or combined lesions of the ipsilateral cerebral cortex and intralaminar thalamus. In adult mouse, unilateral lesions of the cerebral cortex did not result in a net gain or loss of linear dendritic length in a randomly selected population of striatal medium spiny I neurons. In addition, there was a well-defined time course of striatal spine loss and replacement occurring after a unilateral cortical lesion. By day 3 postlesion the average 20-microm dendritic segment had lost 30% of the unlesioned control spine value, reached its nadir, lost 45.5%, at 10 days postlesion, and recovered to 80% of unlesioned control levels by 20 days postlesion. The recovery of spines was blocked by a secondary lesion on the contralateral cortex but not on the ipsilateral intralaminar thalamus. These data suggest that striatal medium spiny I neurons of adult mice have a remarkable capacity for plasticity and reactive synaptogenesis following a decortication. The recovery of spine density is primarily induced by axonal sprouting of survival homologous afferent fibers from the contralateral cortex.

Effect of chronic administration of NMDA antagonists on synaptic development

The current research assessed the role of the N-methyl-D-aspartate (NMDA) receptor in developmental synaptic plasticity. This was accomplished by quantitative analysis of synaptic number and morphology following pharmacological manipulation of NMDA receptor activity using either the competitive antagonist 2-amino-5-phosphonovaleric acid (APV) or the noncompetitive antagonist phencyclidine (PCP). In the first group, 15-day-old male Long-Evans rats were implanted with osmotic minipumps, which administered 50 mM APV or vehicle at a rate of 0.5 microliter per h into the subjects' occipital cortex for 14 days. At age 30 days (P30), the rats were sacrificed and their occipital neocortices were examined. A second group of rats was given subcutaneous injections of 10 mg/kg PCP or vehicle once daily beginning on P5 for a period of 15 days, and was sacrificed on P20. To determine the effects following withdrawal from long-term NMDA antagonism, a third group of animals was given the same PCP injection routine until P20, but was sacrificed on P21, P26, P36, and P56. Developmental administration of APV was associated with a decreased molecular layer depth and estimated total number of synapses. Similarly, PCP induced a reduction in brain weight, molecular layer depth, and estimated total number of synapses. Withdrawal from NMDA antagonism was initially associated with similar results, i.e., reduced brain weight, cortex depth, synaptic density, and estimated total number of synapses, along with an increase in synaptic length. By P36, however, there was a transitory rebound associated with increased molecular layer depth and estimated total number of synapses. These results support the suggestion that NMDA receptor activation is integral to naturally occurring developmental synaptogenesis, and underscore the importance of NMDA receptor involvement in the process of synaptic plasticity.

Neocortical damage alters synaptic responses of neostriatal neurons in vitro

The present experiments were designed to investigate the physiological impact of a partial decortication upon neostriatal synaptic responses using intracellular recording techniques in the in vitro brain slice preparation. In the intact rat, the locally evoked neostriatal synaptic response is primarily mediated by excitatory amino acid receptor activation. Following neocortex damage, the contributions of both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor activation were significantly diminished, although responses remained robust in amplitude and duration. Components of the locally evoked synaptic response mediated by activation of GABAA receptors were relatively unchanged, while presynaptic inhibition mediated by activation of GABAB receptors was markedly reduced. Furthermore, the normally minimal acetylcholine contribution to the synaptic response was significantly increased after neocortical damage. This enhanced cholinergic role in the generation of the synaptic response appeared to be mediated primarily by activation of nicotinic receptors. Thus, neocortical damage leads to novel physiological relationships between intrinsic neostriatal cholinergic interneurons and the GABAergic projection neurons. One possibility is that cholinergic interneurons have the potential for substituting for the loss of excitation created by the absence of neocortical glutamatergic input.

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. At this time point, synaptic responses were still significantly enhanced relative to baseline, although the extent of their potentiation was lower than 1 hour after the last high-frequency stimulation. Stimulated, but not potentiated, rats served as controls. Using the stereological double disector method, estimates of the number of different morphological types of synapses per postsynaptic neuron were obtained. The number of asymmetrical axodendritic synapses increased (by 28%) during LTP maintenance, whereas the number of other synaptic types was not significantly altered. Our previous work demonstrated that the induction of LTP is followed by a selective increase in the number of axospinous perforated synapses with multiple, completely partitioned, transmission zones. Thus, the induction and maintenance phases of LTP are characterized by different structural synaptic alterations. These alterations may be related to each other as indicated by another finding of the present study regarding the existence of perforated synapses that appear to be transitional between axospinous and axodendritic junctions. This suggests a model of structural synaptic plasticity associated with LTP in which some axospinous perforated synapses increase in numbers shortly after the induction of LTP and are then converted into axodendritic ones during LTP maintenance.

The remote astroglial response (RAR): a holistic approach for evaluating the effects of lesions of the central nervous system

The right dorsal lateral geniculate nucleus was stereotaxically destroyed in adult albino rats. After 3 to 150 days of survival the visual cortices from both hemispheres were processed for semithin histology, electron microscopy, GFAP immunohistochemistry and immunoblotting. In visual cortices with histologically disclosed degeneration of the geniculo-cortical tract, a hypertrophy of astrocytes without change in their total numbers was seen from postoperative day 3. From day 7, a rise in GFAP immunoreactivity was observed, reaching its peak between days 11-14, after which a decrease occurred. Observations were confirmed by computer-assisted image analysis of immunohistochemical preparations. Using the immunoblot technique, relative GFAP levels were found to change in a fashion similar to immunohistochemical findings. This showed that synaptic degeneration triggered an up-regulation of GFAP synthesis in the perisynaptic astrocyte processes as a second, cytoskeletal phase of the astrocyte reaction. The phenomenon is denoted as the remote astroglial response (RAR) and is thought to be a marker of synapse removal during plastic changes either related to function or induced by lesions. An extrapolation is made to the possible significance of whole-brain GFAP levels in assessing the effects of focal CNS lesions.