Synaptic structural plasticity: role of synaptic shape

Lentinan has a beneficial effect on cognitive deficits induced by chronic Toxoplasma gondii infection in mice

BACKGROUND

Toxoplasma gondii (T. gondii) is increasingly considered a risk factor for neurodegenerative diseases. However, there is only limited information on the development of drugs for T. gondii infection. Lentinan from Lentinula edodes is a bioactive ingredient with the potential to enhance anti-infective immunity. The present study aimed to investigate the neuroprotective effect of lentinan on T. gondii-associated cognitive deficits in mice.

METHODS

A chronic T. gondii infection mouse model was established by administering 10 cysts of T. gondii by gavage. Lentinan was intraperitoneally administered 2 weeks before infection. Behavioral tests, RNA sequencing, immunofluorescence, transmission electron microscopy and Golgi-Cox staining were performed to assess the effect of lentinan on cognitive deficits and neuropathology in vivo. In vitro, the direct and indirect effects of lentinan on the proliferation of T. gondii tachyzoites were evaluated in the absence and presence of BV-2 cells, respectively.

RESULTS

Lentinan prevented T. gondii-induced cognitive deficits and altered the transcriptome profile of genes related to neuroinflammation, microglial activation, synaptic function, neural development and cognitive behavior in the hippocampus of infected mice. Moreover, lentinan reduced the infection-induced accumulation of microglia and downregulated the mRNA expression of proinflammatory cytokines. In addition, the neurite and synaptic ultrastructural damage in the hippocampal CA1 region due to infection was ameliorated by lentinan administration. Lentinan decreased the cyst burden in the brains of infected mice, which was correlated with behavioral performance. In line with this finding, lentinan could significantly inhibit the proliferation of T. gondii tachyzoites in the microglial cell line BV2, although lentinan had no direct inhibitory effect on parasite growth.

CONCLUSIONS

Lentinan prevents cognitive deficits via the improvement of neurite impairment and synaptic loss induced by T. gondii infection, which may be associated with decreased cyst burden in the brain. Overall, our findings indicate that lentinan can ameliorate T. gondii-related neurodegenerative diseases.

Role of Spirulina in Structural Remodeling of Synapse in Telencephalon of Chronic Unpredictable Stress Model of Zebrafish

Background

Stress can affect the morphology and synaptic organization of the telencephalon. These structural changes at the cellular level can lead to the development of various psychopathologies.

Purpose

Given that the telencephalon plays a major role in stress responses, the current study aimed to investigate the role of Spirulina platensis as a neuroprotectant supplement in the early life of zebrafish in averting the alteration of synapse morphology in the telencephalon caused by chronic unpredictable stress (CUS) in the later stage.

Methods

5dpf larvae were divided into two groups: one group was fed with a commercial fish diet and a second group with a 1% Spirulina-supplemented diet for 90 days. After 90 days, the adult zebrafish were exposed to CUS with different chronic stressors for 15 days. The synaptic plasticity was evaluated by morphometric analysis of synapse in telencephalon of zebrafish by transmission electron microscopy.

Results

The ultrastructural study demonstrated the protective role of Spirulina in the CUS model as no significant alterations in the length of the active zone, postsynaptic density, and synaptic cleft were observed as compared to the control group in the CUS model.

Conclusion

Thus, suggesting that the Spirulina supplementation can avert the remodeling effect of stress on synapse ultrastructure.

Effects of Exercise Training on the Phosphoproteomics of the Medial Prefrontal Cortex in Rats With Autism Spectrum Disorder Induced by Valproic Acid

BACKGROUND

The key neural pathological characteristics of autism spectrum disorder (ASD) include abnormal synaptic plasticity of the medial prefrontal cortex (mPFC). Exercise therapy is widely used to rehabilitate children with ASD, but its neurobiological mechanism is unclear.

METHODS

To clarify whether the structural and molecular plasticity of synapses in the mPFC are related to improvement in ASD behavioral deficits after continuous exercise rehabilitation training, we applied phosphoproteomic, behavioral, morphological, and molecular biological methods to investigate the impact of exercise on the phosphoprotein expression profile and synaptic structure of the mPFC in valproic acid (VPA)-induced ASD rats.

RESULTS

Exercise training differentially regulated the density, morphology, and ultrastructure of synapses in mPFC subregions in the VPA-induced ASD rats. In total, 1031 phosphopeptides were upregulated and 782 phosphopeptides were downregulated in the mPFC in the ASD group. After exercise training, 323 phosphopeptides were upregulated, and 1098 phosphopeptides were downregulated in the ASDE group. Interestingly, 101 upregulated and 33 downregulated phosphoproteins in the ASD group were reversed after exercise training, and these phosphoproteins were mostly involved in synapses. Consistent with the phosphoproteomics data, the total and phosphorylated levels of the proteins MARK1 and MYH10 were upregulated in the ASD group and reversed after exercise training.

CONCLUSIONS

The differential structural plasticity of synapses in mPFC subregions may be the basic neural architecture of ASD behavioral abnormalities. The phosphoproteins involved in mPFC synapses, such as MARK1 and MYH10, may play important roles in the exercise rehabilitation effect on ASD-induced behavioral deficits and synaptic structural plasticity, which requires further investigation.

Motor Cortex Plasticity During Functional Recovery Following Brain Damage

Although brain damage causes functional impairment, it is often followed by partial or total recovery of function. Recovery is believed to occur primarily because of brain plasticity. Both human and animal studies have significantly contributed to uncovering the neuronal basis of plasticity. Recent advances in brain imaging technology have enabled the investigation of plastic changes in living human brains. In addition, animal experiments have revealed detailed changes at the neural and genetic levels. In this review, plasticity in motor-related areas of the cerebral cortex, which is one of the most well-studied areas of the neocortex in terms of plasticity, is reviewed. In addition, the potential of technological interventions to enhance plasticity and promote functional recovery following brain damage is discussed. Novel neurorehabilitation technologies are expected to be established based on the emerging research on plasticity from the last several decades.

The Structural Basis of Long-Term Potentiation in Hippocampal Synapses, Revealed by Electron Microscopy Imaging of Lanthanum-Induced Synaptic Vesicle Recycling

Hippocampal neurons in dissociated cell cultures were exposed to the trivalent cation lanthanum for short periods (15–30 min) and prepared for electron microscopy (EM), to evaluate the stimulatory effects of this cation on synaptic ultrastructure. Not only were characteristic ultrastructural changes of exaggerated synaptic vesicle turnover seen within the presynapses of these cultures—including synaptic vesicle depletion and proliferation of vesicle-recycling structures—but the overall architecture of a large proportion of the synapses in the cultures was dramatically altered, due to large postsynaptic “bulges” or herniations into the presynapses. Moreover, in most cases, these postsynaptic herniations or protrusions produced by lanthanum were seen by EM to distort or break or “perforate” the so-called postsynaptic densities (PSDs) that harbor receptors and recognition molecules essential for synaptic function. These dramatic EM observations lead us to postulate that such PSD breakages or “perforations” could very possibly create essential substrates or “tags” for synaptic growth, simply by creating fragmented free edges around the PSDs, into which new receptors and recognition molecules could be recruited more easily, and thus, they could represent the physical substrate for the important synaptic growth process known as “long-term potentiation” (LTP). All of this was created simply in hippocampal dissociated cell cultures, and simply by pushing synaptic vesicle recycling way beyond its normal limits with the trivalent cation lanthanum, but we argued in this report that such fundamental changes in synaptic architecture—given that they can occur at all—could also occur at the extremes of normal neuronal activity, which are presumed to lead to learning and memory.

Stimulation induces gradual increases in the thickness and curvature of postsynaptic density of hippocampal CA1 neurons in slice cultures

Activity can induce structural changes in glutamatergic excitatory synapses, including increase in thickness and curvature of the postsynaptic density (PSD); these structural changes can only be documented by electron microscopy. Here in organotypic hippocampal slice cultures where experimental conditions can be easily manipulated, increases in thickness and curvature of PSDs were noticeable within 30 s of stimulation and progressed with time up to 3 min. These structural changes were reversible upon returning the samples to control medium for 5-10 min. Thus, the postsynaptic density is a very dynamic structure that undergoes rapid reorganization of its components upon stimulation, and recovery upon cessation of stimulation. The gradual increase in thickness of PSD could result from a gradual translocation of some PSD proteins to the PSD, and the increase in curvature of the PSD is likely led by postsynaptic elements.

Chronic Unexpected Mild Stress Destroys Synaptic Plasticity of Neurons through a Glutamate Transporter, GLT-1, of Astrocytes in the Ischemic Stroke Rat

Background and Objective

Chronic unexpected mild stress (CUMS) destroys synaptic plasticity of hippocampal regenerated neurons that may be involved in the occurrence of poststroke depression. Astrocytes uptake glutamate at the synapse and provide metabolic support for neighboring neurons. Currently, we aim to investigate whether CUMS inhibits synaptic formation of regenerated neurons through a glutamate transporter, GLT-1, of astrocytes in the ischemic stroke rats.

Method

We exposed the ischemic stroke rats to ceftriaxone, during the CUMS intervention period to determine the effects of GLT-1 on glutamate circulation by immunofluorescence and mass spectrometry and its influences to synaptic plasticity by western blot and transmission electron microscopy.

Result

CUMS evidently reduced the level of astroglial GLT-1 in the hippocampus of the ischemic rats (p < 0.05), resulting in smaller amount of glutamate being transported into astrocytes surrounding synapses (p < 0.05), and then expression of synaptophysin was suppressed (p < 0.05) in hippocampal dentate gyrus. The ultrastructures of synapses in dentate gyrus were adversely influenced including decreased proportion of smile synapses, shortened thickness of postsynaptic density, reduced number of vesicles, and widened average distance of the synaptic cleft (all p < 0.05). Moreover, ceftriaxone can promote glutamate circulation and synaptic plasticity (all p < 0.05) by raising astroglial GLT-1 (p < 0.05) and then improve depressive behaviors of the CUMS-induced model rats (p < 0.05).

Conclusion

Our study shows that CUMS destroys synaptic plasticity of regenerated neurons in the hippocampus through a glutamate transporter, GLT-1, of astrocytes in the ischemic stroke rats. This may indicate one potential pathogenesis of poststroke depression.

L-type VDCCs participate in behavioral-LTP and memory retention

Although L-type voltage-dependent calcium channels (VDCCs) have been reported to display different even contrary actions on cognitive functions and long-term potentiation (LTP) formation, there is little information regarding the role of L-type VDCCs in behavioral LTP, a learning-induced LTP model, in the intact brain of freely behaving animals. Here we investigated the effects of verapamil, a non-selective blocker of L-type VDCCs, on behavioral LTP and cognitive functions. Population spikes (PS) were recorded by using electrophysiological methods to examine the role of verapamil in behavioral LTP in the hippocampal dentate gyrus (DG) region. Y-maze assay was used to evaluate the effects of verapamil on learning and memory. Electron microscope was used to observe the changes on synaptic ultrastructural morphology in hippocampal DG area. We found that intrahippocampal verapamil treatments had no significant changes on the PS amplitude during a 90min recordings period. However, intrahippocampal applications of verapamil, including pre- or post-training, reduced behavioral LTP magnitude and memory retention but did not prevent the induction of behavioral LTP and the acquisition of learning. The saline group with behaving trainings showed obvious increases in the number of smile synapses, the length of active zones and the thickness of postsynaptic density as compared to the baseline group, but verapamil with pre-training treatment almost returned these changes to the baseline levels except for the synaptic interface curvature. In conclusion, our results suggest that L-type VDCCs may only contribute to the magnitude of behavioral LTP and the memory maintenance with an activity-independent relationship. L-type VDCCs may be critical to new information long-term storage rather than acquisition in hippocampus.

Impaired auditory processing and altered structure of the endbulb of Held synapse in mice lacking the GluA3 subunit of AMPA receptors

AMPA glutamate receptor complexes with fast kinetics conferred by subunits like GluA3 and GluA4 are essential for temporal precision of synaptic transmission. The specific role of GluA3 in auditory processing and experience related changes in the auditory brainstem remain unknown. We investigated the role of the GluA3 in auditory processing by using wild type (WT) and GluA3 knockout (GluA3-KO) mice. We recorded auditory brainstem responses (ABR) to assess auditory function and used electron microscopy to evaluate the ultrastructure of the auditory nerve synapse on bushy cells (AN-BC synapse). Since labeling for GluA3 subunit increases on auditory nerve synapses within the cochlear nucleus in response to transient sound reduction, we investigated the role of GluA3 in experience-dependent changes in auditory processing. We induced transient sound reduction by plugging one ear and evaluated ABR threshold and peak amplitude recovery for up to 60 days after ear plug removal in WT and GluA3-KO mice. We found that the deletion of GluA3 leads to impaired auditory signaling that is reflected in decreased ABR peak amplitudes, an increased latency of peak 2, early onset hearing loss and reduced numbers and sizes of postsynaptic densities (PSDs) of AN-BC synapses. Additionally, the lack of GluA3 hampers ABR threshold recovery after transient ear plugging. We conclude that GluA3 is required for normal auditory signaling, normal ultrastructure of AN-BC synapses in the cochlear nucleus and normal experience-dependent changes in auditory processing after transient sound reduction.

Colocalization of synapse marker proteins evaluated by STED-microscopy reveals patterns of neuronal synapse distribution in vitro

BACKGROUND

Quantification of synapses and their morphological analysis are extensively used in network development and connectivity studies, drug screening and other areas of neuroscience. Thus, a number of quantitative approaches were introduced so far. However, most of the available methods are highly tailored to specific applications and have limitations for widespread use.

NEW METHOD

We present a new plugin for the open-source software ImageJ to provide a modifiable, high-throughput and easy to use method for synaptic puncta analysis. Our approach is based on colocalization of pre- and postsynaptic protein markers. Structurally completed glutamatergic and GABAergic synapses were identified by VGLUT1-PSD95 and VGAT-gephyrin colocalization, respectively. By combining conventional confocal microscopy with stimulated emission depletion (STED) imaging, we propose a method to quantify the number of scaffolding protein clusters, recruited to a single postsynaptic density.

RESULTS

In a proof-of-concept study, we reveal the differential distribution of glutamatergic and GABAergic synapse density with reference to perineuronal net (PNN) expression. Using super-resolution STED imaging, we demonstrate that postsynaptic puncta of completed synapses are composed of significantly more protein clusters, compared to uncompleted synapses.

COMPARISON WITH EXISTING METHODS

Our Synapse Counter plugin for ImageJ offers a rapid and unbiased research tool for a broad spectrum of neuroscientists. The proposed method of synaptic protein clusters quantification exploits super-resolution imaging to provide a comprehensive approach to the analysis of postsynaptic density composition.

CONCLUSIONS

Our results strongly substantiate the benefits of colocalization-based synapse detection.

Role of synaptic structural plasticity in impairments of spatial learning and memory induced by developmental lead exposure in Wistar rats

Lead (Pb) is found to impair cognitive function. Synaptic structural plasticity is considered to be the physiological basis of synaptic functional plasticity and has been recently found to play important roles in learning and memory. To study the effect of Pb on spatial learning and memory at different developmental stages, and its relationship with alterations of synaptic structural plasticity, postnatal rats were randomly divided into three groups: Control; Pre-weaning Pb (Parents were exposed to 2 mM PbCl₂ 3 weeks before mating until weaning of pups); Post-weaning Pb (Weaned pups were exposed to 2 mM PbCl₂ for 9 weeks). The spatial learning and memory of rats was measured by Morris water maze (MWM) on PND 85–90. Rat pups in Pre-weaning Pb and Post-weaning Pb groups performed significantly worse than those in Control group (p<0.05). However, there was no significant difference in the performance of MWM between the two Pb-exposure groups. Before MWM (PND 84), the number of neurons and synapses significantly decreased in Pre-weaning Pb group, but not in Post-weaning Pb group. After MWM (PND 91), the number of synapses in Pre-weaning Pb group increased significantly, but it was still less than that of Control group (p<0.05); the number of synapses in Post-weaning Pb group was also less than that of Control group (p<0.05), although the number of synapses has no differences between Post-weaning Pb and Control groups before MWM. In both Pre-weaning Pb and Post-weaning Pb groups, synaptic structural parameters such as thickness of postsynaptic density (PSD), length of synaptic active zone and synaptic curvature increased significantly while width of synaptic cleft decreased significantly compared to Control group (p<0.05). Our data demonstrated that both early and late developmental Pb exposure impaired spatial learning and memory as well as synaptic structural plasticity in Wistar rats.

Effects of hypoxic preconditioning on synaptic ultrastructure in mice

Hypoxic preconditioning (HPC) elicits resistance to more drastic subsequent insults, which potentially provide neuroprotective therapeutic strategy, but the underlying mechanisms remain to be fully elucidated. Here, we examined the effects of HPC on synaptic ultrastructure in olfactory bulb of mice. Mice underwent up to five cycles of repeated HPC treatments, and hypoxic tolerance was assessed with a standard gasp reflex assay. As expected, HPC induced an increase in tolerance time. To assess synaptic responses, Western blots were used to quantify protein levels of representative markers for glia, neuron, and synapse, and transmission electron microscopy was used to examine synaptic ultrastructure and mitochondrial density. HPC did not significantly alter the protein levels of astroglial marker (GFAP), neuron-specific markers (GAP43, Tuj-1, and OMP), synaptic number markers (synaptophysin and SNAP25) or the percentage of excitatory synapses versus inhibitory synapses. However, HPC significantly affected synaptic curvature and the percentage of synapses with presynaptic mitochondria, which showed concomitant change pattern. These findings demonstrate that HPC is associated with changes in synaptic ultrastructure.

Alterations of the synapse of the inner retinal layers after chronic intraocular pressure elevation in glaucoma animal model

BACKGROUND

Dendrites of retinal ganglion cells (RGCs) synapse with axon terminals of bipolar cells in the inner plexiform layer (IPL). Changes in RGC dendrites and synapses between bipolar cells in the inner retinal layer may critically alter the function of RGCs in glaucoma. Recently, synaptic plasticity has been observed in the adult central nervous system, including the outer retinal layers. However, few studies have focused on changes in the synapses between RGCs and bipolar cells in glaucoma. In the present study, we used a rat model of ocular hypertension induced by episcleral vein cauterization to investigate changes in synaptic structure and protein expression in the inner retinal layer at various time points after moderate intraocular pressure (IOP) elevation.

RESULTS

Synaptophysin, a presynaptic vesicle protein, increased throughout the IPL, outer plexiform layer, and outer nuclear layer after IOP elevation. Increased synaptophysin after IOP elevation was expressed in bipolar cells in the innermost IPL. The RGC marker, SMI-32, co-localized with synaptophysin in RGC dendrites and were significantly increased at 1 week and 4 weeks after IOP elevation. Both synaptophysin and postsynaptic vesicle protein, PSD-95, were increased after IOP elevation by western blot analysis. Ribbon synapses in the IPL were quantified and structurally evaluated in retinal sections by transmission electron microscopy. After IOP elevation the total number of ribbon synapses decreased. There were increases in synapse diameter and synaptic vesicle number and decreases in active zone length and the number of docked vesicles after IOP elevation.

CONCLUSIONS

Although the total number of synapses decreased as RGCs were lost after IOP elevation, there are attempts to increase synaptic vesicle proteins and immature synapse formation between RGCs and bipolar cells in the inner retinal layers after glaucoma induction.

Ultrastructural and electrophysiological analysis of Area X in the untutored and deafened Bengalese finch in relation to normally reared birds

Birdsong learning bears many similarities to human speech acquisition. Although the anterior forebrain pathway (AFP) is believed to be involved in birdsong learning, the underlying neural mechanisms are unclear. We produced two types of abnormal song learning: young birds untutored from adult "song tutors", or birds deafened by bilateral cochlear removal before the onset of sensory learning. We then studied how ultrastructure and electrophysiological activity changed in an AFP nucleus, Area X, among these birds at adulthood. Our results showed that, although the size of Area X did not change significantly, the numbers of synapses per unit area and compound synapses and the percent of concave synapses increased significantly in the untutored or deafened birds. The percent of perforated synapses or axo-spinous synapses decreased compared to the normally reared birds, suggesting a decreased efficiency of synaptic transmission in the untutored or deafened birds. We then identified several types of spontaneously firing cells in Area X. Cells with fast and slow firing rates did not show significant electrophysiological differences among the groups, but cells with moderate firing rates, most likely DLM-projecting neurons, fired at significantly lower rates in the untutored and deafened birds. In addition, cells firing irregularly were only found in the deafened birds. Thus, the decreased or irregular electrophysiological activity in the untutored or deafened birds, together with the corresponding ultrastructural findings, could be implicated in the abnormal song production in these two types of birds.

Changes in ultra-structures and electrophysiological properties in HVC of untutored and deafened Bengalese finches relation to normally reared birds: implications for song learning

Songbirds are increasingly used as an experimentally tractable system to study the neurobiological underpinnings of vocal learning. To gain additional insights into how birdsongs are learned, we compared the size of HVC, the high vocal center for song production, and its ultrastructural or electrophysiological properties between the normally reared Bengalese finches, and the untutored or deafened ones before the onset of sensory learning (around post-hatching day 20). Our results showed that HVC had more synapses and concave synaptic curvature, but fewer perforated synapse, in the untutored or deafened birds in comparison with those in the normally reared birds. Although there was no significant difference of the ratio of straight or compound synapses, there was an increasing tendency for the untutored and deafened birds to possess more straight and compound synapses. These data revealed that synapses in the isolated or deafened birds had lower synapse activity in relation to those with normal hearing. This was confirmed by our electrophysiological results to show significant decreases in the firing rates of spike or burst in the isolated or deafened birds in the three types of HVC neurons i.e., putative X-projecting neurons, RA-projecting neurons and interneurons. In addition, low firing frequency (<10Hz) occurred much more in the above three types of HVC neurons in the tutored or deafened birds than in the normally reared birds. These data suggest that all the three putative types of neurons in HVC might be involved in the activity of the production of adult normal songs.

Comparative study on song behavior, and ultra-structural, electrophysiological and immunoreactive properties in RA among deafened, untutored and normal-hearing Bengalese finches

To gain additional insight into how a birdsong is learned, we compared the songs of Bengalese finch males that were deafened early in development or raised without tutors to control finches that learned songs from adult models. Fewer note types and a more variable number of notes per bout were observed in untutored male songs, and no audible songs were detected in deafened males. We then investigated the ultrastructural, immunohistological, and electrophysiological correlates of the outcomes of song learning within the robust nucleus of the archopallium (RA), a forebrain nucleus for song production. In comparison to control birds, untutored and deafened birds had more synapses per unit volume, fewer vesicles per synapse, longer postsynaptic densities, and a lower proportion of perforated synapses, which suggest lower activity or decreased efficiency of synaptic transmission within the RA of the treated birds. For anesthetized birds, neurons within the RA of untutored and deafened males had lower spontaneous firing rates, fewer and shorter bursts, and higher coefficient of variation of the instantaneous firing rate than the normally reared males. Compared with controls, the untutored and deafened males had higher staining intensities within the RA of GABA and the GABA(A) receptor, less staining of tyrosine hydroxylase and no difference in the staining of NMDA receptors. Thus, both the ultrastructural and immunohistochemical results could explain for the stronger electrophysiological activities in normally reared birds. Because RA is involved in generating the motor commands, these data might account for the deficits in birds with abnormal song learning.

Network, cellular, and molecular mechanisms underlying long-term memory formation

The neural network stores information through activity-dependent synaptic plasticity that occurs in populations of neurons. Persistent forms of synaptic plasticity may account for long-term memory storage, and the most salient forms are the changes in the structure of synapses. The theory proposes that encoding should use a sparse code and evidence suggests that this can be achieved through offline reactivation or by sparse initial recruitment of the network units. This idea implies that in some cases the neurons that underwent structural synaptic plasticity might be a subpopulation of those originally recruited; However, it is not yet clear whether all the neurons recruited during acquisition are the ones that underwent persistent forms of synaptic plasticity and responsible for memory retrieval. To determine which neural units underlie long-term memory storage, we need to characterize which are the persistent forms of synaptic plasticity occurring in these neural ensembles and the best hints so far are the molecular signals underlying structural modifications of the synapses. Structural synaptic plasticity can be achieved by the activity of various signal transduction pathways, including the NMDA-CaMKII and ACh-MAPK. These pathways converge with the Rho family of GTPases and the consequent ERK 1/2 activation, which regulates multiple cellular functions such as protein translation, protein trafficking, and gene transcription. The most detailed explanation may come from models that allow us to determine the contribution of each piece of this fascinating puzzle that is the neuron and the neural network.

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.

Changes of synaptic ultrastructure in the guinea pig interpositus nuclei associate with response magnitude and timing after trace eyeblink conditioning

Learning-induced changes of synaptic ultrastructure have long been proposed as a mechanism that may contribute to support memory formation. Although recent studies have demonstrated that the interpositus nuclei (IN) play critical role in acquisition and retention of trace conditioned eyeblink responses (CRs), there is now limited evidence associating trace eyeblink conditioning with changes of synaptic ultrastructure in the IN. Here, we investigated this issue using a transmission electron microscope. Adult guinea pigs were randomly allocated to either a trace-paired, delay-paired, unpaired or exposure-only condition. The IN tissue was taken for morphological analysis 1h after the completion of the tenth training session. Serial section analysis of synaptic ultrastructure revealed that trace eyeblink conditioning induced increases in the thickness of excitatory PSD. Classification of the synapses into shape subtypes indicated that the increased thickness of excitatory PSD was mainly attributable to increase in the concave- and convex-shaped synapses. On the contrary, trace eyeblink conditioning resulted in decreases in the thickness of inhibitory PSD. Specifically, these significant changes of PSD thickness were limited to occur in the animals with good behavioral performance. Further analysis of correlations between the trace CR performance and synaptic ultrastructural modifications showed that the thickness of excitatory PSD within the IN correlated with the peak amplitude of trace CRs, whereas the thickness of inhibitory PSD correlated with the onset latency. The present findings suggest that trace eyeblink conditioning induces structural plasticity in the IN, which may play a crucial role in acquiring and executing adaptive eyeblink movements.

Prenatal exposure to perfluorooctanesulfonate in rat resulted in long-lasting changes of expression of synapsins and synaptophysin

Both animal and human studies have demonstrated that exposure to chemical pollutants during critical developmental period causes adverse consequences later in life. In uterus, perfluorooctanesulfonate (PFOS) exposure has been known to cause developmental neurotoxicity, such as increased motor activity, reduced habitation and impaired cognitive function. The possible mechanism of the impaired cognitive function induced by prenatal PFOS exposure was evaluated in this study. Pregnant Sprague Dawley (SD) rats were given 0.1, 0.6, and 2.0 mg kg(-1) birth weight (bw) d(-1) by gavage from gestation day (GD) 0 to GD20. Control received 0.5% Tween-20 vehicle (4 ml kg(-1) bw d(-1)). PFOS concentration in hippocampus of offspring was observed on postnatal day (PND) 0 and PND21. The ultrastructure of hippocampus and the gene expression of synaptic vesicle associated proteins in offspring hippocampus, which were important for the neurotransmitter release, were investigated. The transmission electron photomicrographs of the offspring hippocampus from PFOS-treated maternal groups showed the ultrastructure of synapses was negatively affected. The offspring from PFOS-treated maternal groups also differed significantly from controls with respect to the expression of synaptic vesicle associated proteins. The mRNA levels of synapsin1 (Syn1), synapsin2 (Syn2), and synaptophysin (Syp) were decreased in treated groups either on PND0 or on PND21. However, the mRNA level of synapsin3 (Syn3) decreased in 0.6- and 2.0-mg kg(-1) group on PND0, and showed no significant difference among control group and all treated groups on PND21. These results indicate that the impairment of cognitive function induced by PFOS may be attributed to the lower mRNA levels of synaptic vesicle associated proteins and the change of synaptic ultrastructure in hippocampus.

Alterations in synaptic curvature in the dentate gyrus following induction of long-term potentiation, long-term depression, and treatment with the N-methyl-D-aspartate receptor antagonist CPP

Alterations in curvature of the post synaptic density (PSD) and apposition zone (AZ), are believed to play an important role in determining synaptic efficacy. In the present study we have examined curvature of PSDs and AZs 24 h following homosynaptic long-term potentiation (LTP), and heterosynaptic long-term depression (LTD) in vivo, in awake adult rats. High frequency stimulation (HFS) applied to the medial perforant path to the dentate gyrus induced LTP while HFS stimulation of the lateral perforant path induced LTD in the middle molecular layer of the dentate gyrus (DG). Curvature changes were analysed in this area using three dimensional (3-D) reconstructions of electron microscope images of ultrathin serial sections. Very large and significant changes in 3-D measurements of AZ and PSD curvature occurred 24 h following both LTP and LTD, with a flattening of the normal concavity of mushroom spine heads and a change to convexity for thin spines. An N-methyl-D-aspartate (NMDA) receptor antagonist CPP (3-[(R)-2-Carboxypiperazin-4-yl]-propyl-1-phosphonic acid) blocked the changes in curvature of mushroom and thin spine PSDs and apposition zones, actually increasing the concavity of mushroom spines as the spine engulfed the presynaptic bouton. In order to establish whether these changes resulted from the effect of the NMDA antagonist or from its coincidence with synaptic activation during testing we examined the effects of CPP alone on PSD and apposition zone curvature. It was found that CPP alone also caused a small decrease in curvature of both PSD and apposition zone of mushroom and thin spines.

Synaptic plasticity along the sleep-wake cycle: implications for epilepsy

Activity-dependent changes in synaptic efficacy (i.e., synaptic plasticity) can alter the way neurons communicate and process information as a result of experience. Synaptic plasticity mechanisms involve both molecular and structural modifications that affect synaptic functioning, either enhancing or depressing neuronal transmission. They include redistribution of postsynaptic receptors, activation of intracellular signaling cascades, and formation/retraction of dendritic spines, among others. During the sleep-wake cycle, as the result of particular neurochemical and neuronal firing modes, distinct oscillatory patterns organize the activity of neuronal populations, modulating synaptic plasticity. Such modulation, for example, has been shown in the visual cortex following sleep deprivation and in the ability to induce hippocampal long-term potentiation during sleep. In epilepsy, synchronized behavioral states tend to contribute to the initiation of paroxystic discharges and are considered more epileptogenic than desynchronized states. Here, we review some of the current understandings of synaptic plasticity changes in wake and sleep states and how sleep may affect epileptic seizures.

Ischemia-induced modifications in hippocampal CA1 stratum radiatum excitatory synapses

Relatively mild ischemic episode can initiate a chain of events resulting in delayed cell death and significant lesions in the affected brain regions. We studied early synaptic modifications after brief ischemia modeled in rats by transient vessels' occlusion in vivo or oxygen-glucose deprivation in vitro and resulting in delayed death of hippocampal CA1 pyramidal cells. Electron microscopic analysis of excitatory spine synapses in CA1 stratum radiatum revealed a rapid increase of the postsynaptic density (PSD) thickness and length, as well as formation of concave synapses with perforated PSD during the first 24 h after ischemic episode, followed at the long term by degeneration of 80% of synaptic contacts. In presynaptic terminals, ischemia induced a depletion of synaptic vesicles and changes in their spatial arrangement: they became more distant from active zones and had larger intervesicle spacing compared to controls. These rapid structural synaptic changes could be implicated in the mechanisms of cell death or adaptive plasticity. Comparison of the in vivo and in vitro model systems used in the study demonstrated a general similarity of these early morphological changes, confirming the validity of the in vitro model for studying synaptic structural plasticity.

Behaviorally-induced ultrastructural plasticity in the hippocampal region after cerebral ischemia

Behavioral training has been shown to induce synaptic plasticity in both intact and injured animals. Because of the possibility that the adaptive changes after ischemic damage may make the brain more malleable to behavioral training, we examined the effects of complex environment (EC) housing and exercise (EX) after global cerebral ischemia on synaptic structural alterations. Forty-two adult male Wistar rats were included in the study and assigned to either ischemia or sham group. Following ischemic or sham surgery, rats were randomized to either EC, EX, or social condition (SC, paired housing) group. CA1 was processed for electron microscopy and unbiased stereological techniques were used to evaluate plasticity. Significantly decreased neuron density was seen in anterior and medial CA1 in ischemic animals regardless of behavioral training. Neuron density in anterior CA1 was 31% less than the medial area. Synaptogenesis was influenced by cerebral ischemia and behavioral training in that all ischemic groups and sham EC animals showed greater number of synapses per neuron compared to the sham EX and SC groups. Analysis of synapse configuration showed that the synaptogenesis in ischemia EX and SC rats was formed mainly by synapses with single synaptic boutons, whereas in the ischemia EC and sham EC rats synaptogenesis was formed mainly by synapses with multiple synaptic boutons. Furthermore, housing of sham and ischemia rats in EC resulted in increased number of synapses with perforated postsynaptic density. Together, these data suggest that behavioral experience in EC after insult may be able to enhance synaptic plasticity.

Quantitative study of aluminum-induced changes in synaptic ultrastructure in rats

Some evidence suggests that aluminum is one of the risk factors in Alzheimer's disease (AD); however, there is no study on the relationship between the synaptic configuration changes and aluminum uptake. In the present study, 40 rats were fed with water solution aluminum for 3 months and the effects of aluminum on the Gray's type I synapses in hippocampus and frontal cortex of rats were quantitatively investigated by electron microscopy. The effects of aluminum on the ability to learn and memorize were tested by a Morris water maze. Length of synaptic active zone, width of synaptic cleft, curvature of synapse, and thickness of postsynaptic density (PSD) were measured in the interface of synapses by the IBM-PC microimage processing system. The flat synapse, positive and negative curvature synapses, as well as the perforated synapse were classified and counted according to the type of the interfacial structure of a synapse. The amounts of aluminum deposited in brain were measured with an atomic absorption spectrophotometer. Our results show that the time and the distance taken by the rats to find the water maze increased after the rats were fed aluminum for 3 months. As compared with control, the synapses in aluminum-induced rats exhibited the following significant changes: decreased thickness of PSD (P < 0.01), increased width of synaptic cleft (P < 0.05), increased numbers of flat synapse, decreased numbers of positive curvature synapse and perforated synapse, and significantly increased aluminum deposits in hippocampus and frontal cortex (P < 0.01). Our study indicates that aluminum can decrease the ability of rats to learn and memorize and induce their synaptic configuration changes. These changes may be related to synaptic efficacy and may be one of the mechanisms for AL to induce AD.

Influence of age on aluminum-induced neurobehavioral effects and morphological changes in rat brain

Aluminum (Al) is potentially toxic for mammals. In contrast to well documented Al neurotoxicity, neurobehavioral studies of Al in rodents have generally not produced robust or consistent results. In the present study, 16 young male (21 days old) and 16 old male (18 months) rats were exposed to 0 (control group) and 100 mg/kg/day of Al administered as Al nitrate nonahydrate in drinking water concurrently with citric acid (356 mg/kg/day) for a period of 100 days. At the end of the exposure period, rats were evaluated for motor activity in an open-field apparatus and learning in a passive avoidance test. After behavioral testing, rats were sacrificed and brain samples were collected to determine Al concentrations and to study synapses in the left CA1 fields of hippocampal formation. While no significant effects of Al exposure between groups could be detected on behavior, the total number of synapses decreased with age and Al exposure. In turn, the percentage of perforated synapses significantly increased in old Al-loaded rats.

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.

Protracted postnatal development of inhibitory synaptic transmission in rat hippocampal area CA1 neurons

In the CNS, inhibitory synaptic function undergoes profound transformation during early postnatal development. This is due to variations in the subunit composition of subsynaptic GABA(A) receptors (GABA(A)Rs) at differing developmental stages as well as other factors. These include changes in the driving force for chloride-mediated conductances as well as the quantity and/or cleft lifetime of released neurotransmitter. The present study was undertaken to investigate the nature and time course of developmental maturation of GABAergic synaptic function in hippocampal CA1 pyramidal neurons. In neonatal [postnatal day (P) 1-7] and immature (P8-14) CA1 neurons, miniature inhibitory postsynaptic currents (mIPSCs) were significantly larger, were less frequent, and had slower kinetics compared with mIPSCs recorded in more mature neurons. Adult mIPSC kinetics were achieved by the third postnatal week in CA1 neurons. However, despite this apparent maturation of mIPSC kinetics, significant differences in modulation of mIPSCs by allosteric agonists in adolescent (P15-21) neurons were still evident. Diazepam (1-300 nM) and zolpidem (200 nM) increased the amplitude of mIPSCs in adolescent but not adult neurons. Both drugs increased mIPSC decay times equally at both ages. These differential agonist effects on mIPSC amplitude suggest that in adolescent CA1 neurons, inhibitory synapses operate differently than adult synapses and function as if subsynaptic receptors are not fully occupied by quantal release of GABA. Rapid agonist application experiments on perisomatic patches pulled from adolescent neurons provided additional support for this hypothesis. In GABA(A)R currents recorded in these patches, benzodiazepine amplitude augmentation effects were evident only when nonsaturating GABA concentrations were applied. Furthermore nonstationary noise analysis of mIPSCs in P15-21 neurons revealed that zolpidem-induced mIPSC augmentation was not due to an increase in single-channel conductance of subsynaptic GABA(A)Rs but rather to an increase in the number of open channels responding to a single GABA quantum, further supporting the hypothesis that synaptic receptors may not be saturated during synaptic function in adolescent neurons. These data demonstrate that inhibitory synaptic transmission undergoes a markedly protracted postnatal maturation in rat CA1 pyramidal neurons. In the first two postnatal weeks, mIPSCs are large in amplitude, are slow, and occur infrequently. By the third postnatal week, mIPSCs have matured kinetically but retain distinct responses to modulatory drugs, possibly reflecting continued immaturity in synaptic structure and function persisting through adolescence.

Sex steroids and the brain: lessons from animal studies

Gonadal steroid hormones have multiple effects throughout development on steroid responsive tissues in the brain. The belief that the cellular morphology of the adult brain cannot be modulated or that the synaptic connectivity is "hard-wired" is being rapidly refuted by abundant and growing evidence. Indeed, the brain is capable of undergoing many morphological changes throughout life and gonadal steroids play an important role in many of these processes. Gonadal steroids are implicated in the development of sexually dimorphic structures in the brain, in the control of physiological behaviors and functions and the brain's response to physiological or harmful substances. The effect of sex steroids on neuroprotection and neuroregeneration is an important and expanding area of investigation. Astroglia are targets for estrogen and testosterone and are apparently involved in the actions of sex steroids on the central nervous system. Sex hormones induce changes in the expression of glial fibrillary acidic protein, the growth of astrocytic processes and the extent to which neuronal membranes are covered by astroglial processes. These changes are linked to modifications in the number of synaptic inputs to neurons and suggest that astrocytes may participate in the genesis of gonadal steroid-induced sex differences in synaptic connectivity and synaptic plasticity in the adult brain. Astrocytes and tanycytes may also participate in the cellular effects of sex steroids by releasing neuroactive substances and by regulating the local accumulation of specific growth factors, such as insulin-like growth factor-I, that are involved in estrogen-induced synaptic plasticity and estrogen-mediated neuroendocrine control. Astroglia may also be involved in the regenerative and neuroprotective effects of sex steroids since astroglial activation after brain injury or after peripheral nerve axotomy is regulated by sex hormones.

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.

Intramembranous structure of synaptic membranes with special reference to spinules in the rat sensorimotor cortex

The intramembranous structure of the synaptic contact zone at presynaptic and postsynaptic membranes in the rat sensorimotor cortex was examined by means of the freeze-etching technique. In axospinous synapses, the synaptic contact zone is characterized by perforated and nonperforated aggregates of intramembranous particles at the extracellular half or E-face of the postsynaptic membrane. On some perforated synaptic contact zones, both synaptic membranes are marked by so called spinules. These invaginations of the postsynaptic membrane and the parallel presynaptic membrane into the axon terminal are situated at the particle free zones among the postsynaptic E-face intramembranous particle aggregates or in close proximity to it. Intramembranous characteristics of the spinules at both freeze-etched faces of presynaptic and postsynaptic membranes and their density of perforated axospinous synapses were analysed. The results are discussed in terms of plasticity at the synaptic contact zone of the axospinous synapses of the sensorimotor cortex in the rat.

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.

Asynchronous changes in size, number, and shape of synapses in the developing superior colliculus

The ultra-structural development of synapses in retino-receptive layers of the opossum superior colliculus was studied by the ethanolic phosphotungstic acid (E-PTA) method. There was a tendency for a slight reduction in the diameter of synaptic disks, a rise and fall of numerical densities and, except for an ephemeral period, a general increase in the proportion of "frown" among curve synapses. The lack of strict synchrony and the occurrence of different patterns of changes suggest that multiple factors contribute to synaptic maturation.

Protein components of a rat brain synaptic junctional protein preparation

Antisera against a rat brain synaptic protein preparation, the postsynaptic density (PSD) fraction, were used to isolate cDNA clones by expression screening of a rat brain cDNA library. About one fifth of more than 200 analyzed cDNAs encoding potential synapse-associated proteins were previously unknown. Identifiable proteins include, among others, components of the pre- and postsynaptic cytoskeleton, synaptic vesicle proteins and several protein kinases and kinase substrates. This demonstrates that both pre- and postsynaptic elements purify with the PSD fraction.

Intramembranous structure of the postsynaptic membrane in the rat sensorimotor cortex

Intramembranous structure of the postsynaptic membrane of the axodendritic synapses in the rat sensorimotor cortex was examined by means of freeze-etching technique. Perforated and non-perforated aggregates of particles were found at the extracellular half (E-face) of the postsynaptic membrane. To study correlation between membrane structure and synaptic plasticity we compared size and particle packing density in both type aggregates of particles at the E-face of the postsynaptic dendritic spine membrane. The results are discussed in terms of plasticity on the synaptic contact zone (SCZ) at the postsynaptic membrane of the excitatory axospinous synapses.

Activity-related features of synapse morphology: a study of endbulbs of held

The myelinated fibers of the auditory nerve can be divided into two separate populations on the basis of sensitivity to sound, average levels of spike activity, and central branching patterns. The synaptic endings of these populations were investigated for the presence of structural specializations that might correlate with levels of neural activity. We applied intracellular recording and staining methods in cats to analyze directly the relationship between spike activity and the structure of synapses using endbulbs of Held, the large synaptic endings in the anteroventral cochlear nucleus. Endbulbs from fibers having low or high levels of activity were examined and compared using light and electron microscopic methods. All endbulbs exhibited relatively large but incomplete coverage by one-to-several lamellae of glial processes. Endbulbs of high activity fibers were large and contained larger mitochondria than endbulbs of low activity fibers. Furthermore, the synapses of high activity endbulbs were on average smaller but more numerous, possessed greater numbers of associated synaptic vesicles, and exhibited greater curvature of their postsynaptic densities. These structural features are hypothesized to reflect specializations that optimize synaptic transmission.

An analysis of contemporary morphological concepts of synaptic remodelling in the CNS: perforated synapses revisited

Perforated synapses refer to a synaptic type found in the central nervous system. They are characterized by their large size and by a discontinuity of the postsynaptic density when viewed in transverse sections, and by a doughnut or horseshoe shape when viewed in en face views. Of recent morphological studies, one approach has followed their characteristics throughout development and maturity, while others have concentrated on their probable roles in activities including kindling, long-term potentiation, spatial working memory, differential rearing, and the functioning of neuroleptics. An assessment is made of the hypotheses and models that have proved determinative in the emergence of perforated synapses as being significant in synaptic plasticity. Their distribution and frequency are summarized, with emphasis on the importance of unbiased stereological procedures in their analysis. Using three-dimensional approaches various subtypes are recognized. Of these, a complex or fragmented subtype appears of especial significance in synaptic plasticity. Ideas regarding the life-cycle of perforated synapses are examined. The view that they originate from conventional, non-perforated synapses, enlarge, and subsequently split to give rise to a new generation of non-perforated synapses, is critically assessed. According to an alternative model, perforated and non-perforated synapses constitute separate populations from early in their development, each representing complementary forms of synaptic plasticity. An attempt is also made to discover whether synaptic studies on the human brain in normal aging and in Alzheimer's disease throw light on the role of perforated synapses in synaptic plasticity. The loss of synapses in Alzheimer's disease may include a loss of perforated synapses - of particular relevance for an understanding of certain neuropathological conditions.

Synaptic reorganisation in the rat striatum after dopaminergic deafferentation: an ultrastructural study using glutamate decarboxylase immunocytochemistry

The ultrastructure of GABAergic and non-GABAergic synapses in the adult rat neostriatum was examined 6-8 months after unilateral removal of the nigrostriatal dopaminergic pathway by 6-hydroxydopamine injection into the medial forebrain bundle. GABAergic profiles were identified by preembedding glutamate decarboxylase (GAD) immunocytochemistry performed on parasagittal vibratome sections. In three representative fields of the striatum, the nature and number of boutons and their postsynaptic partners were determined and the differences between the striata ipsi- and contralateral to the lesion analyzed. The percentage of GAD-immunoreactive boutons was increased from 23% on the intact side to 28% on the lesioned side. In addition, the GABAergic boutons underwent significantly more multiple contacts with several independent postsynaptic profiles, preferentially with dendritic spines. This could reflect a lesion-induced sprouting of local GABAergic axon terminals. On the other hand, although the vast majority of GABAergic boutons underwent synaptic contacts with dendrites (77% vs. 80%), the number of boutons per dendrite or per dendritic circumference remained unchanged. Thus, the higher frequency of GABAergic boutons may simply reflect the loss of the dopaminergic nerve endings, without a heterosynaptic replacement by GABAergic boutons. The deafferentation also induced structural changes of the postsynaptic profiles. Some dendritic spines had a shortened neck; others were completely integrated in the dendrite which now contained a spine apparatus and was contacted by boutons with the features of axospinous synapses. The spine retraction resulted in a quantitative decrease in the number of spines. Analysis of the synaptic curvature revealed that only spines with a flat contact zone were lost. Concurrently, the number of dendrites was increased, of the GAD-containing in particular, suggesting that the denrites of GABAergic interneurons tend to elongate and/or arborize. Taken together, the results of the present study show that the dopaminergic denervation caused a remodeling of the postsynaptic neurons. The relative increase of the number of GABAergic boutons and their synaptic contacts suggests that an altered wiring of the intrinsic GABAergic system contributes to the changes in the striatal output activity.

Gonadal hormones as promoters of structural synaptic plasticity: cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.