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

Imaging Synaptic Density: The Next Holy Grail of Neuroscience?

The brain is the central and most complex organ in the nervous system, comprising billions of neurons that constantly communicate through trillions of connections called synapses. Despite being formed mainly during prenatal and early postnatal development, synapses are continually refined and eliminated throughout life via complicated and hitherto incompletely understood mechanisms. Failure to correctly regulate the numbers and distribution of synapses has been associated with many neurological and psychiatric disorders, including autism, epilepsy, Alzheimer’s disease, and schizophrenia. Therefore, measurements of brain synaptic density, as well as early detection of synaptic dysfunction, are essential for understanding normal and abnormal brain development. To date, multiple synaptic density markers have been proposed and investigated in experimental models of brain disorders. The majority of the gold standard methodologies (e.g., electron microscopy or immunohistochemistry) visualize synapses or measure changes in pre- and postsynaptic proteins ex vivo. However, the invasive nature of these classic methodologies precludes their use in living organisms. The recent development of positron emission tomography (PET) tracers [such as (¹⁸F)UCB-H or (¹¹C)UCB-J] that bind to a putative synaptic density marker, the synaptic vesicle 2A (SV2A) protein, is heralding a likely paradigm shift in detecting synaptic alterations in patients. Despite their limited specificity, novel, non-invasive magnetic resonance (MR)-based methods also show promise in inferring synaptic information by linking to glutamate neurotransmission. Although promising, all these methods entail various advantages and limitations that must be addressed before becoming part of routine clinical practice. In this review, we summarize and discuss current ex vivo and in vivo methods of quantifying synaptic density, including an evaluation of their reliability and experimental utility. We conclude with a critical assessment of challenges that need to be overcome before successfully employing synaptic density biomarkers as diagnostic and/or prognostic tools in the study of neurological and neuropsychiatric disorders.

The synapse in traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability worldwide and is a risk factor for dementia later in life. Research into the pathophysiology of TBI has focused on the impact of injury on the neuron. However, recent advances have shown that TBI has a major impact on synapse structure and function through a combination of the immediate mechanical insult and the ensuing secondary injury processes, leading to synapse loss. In this review, we highlight the role of the synapse in TBI pathophysiology with a focus on the confluence of multiple secondary injury processes including excitotoxicity, inflammation and oxidative stress. The primary insult triggers a cascade of events in each of these secondary processes and we discuss the complex interplay that occurs at the synapse. We also examine how the synapse is impacted by traumatic axonal injury and the role it may play in the spread of tau after TBI. We propose that astrocytes play a crucial role by mediating both synapse loss and recovery. Finally, we highlight recent developments in the field including synapse molecular imaging, fluid biomarkers and therapeutics. In particular, we discuss advances in our understanding of synapse diversity and suggest that the new technology of synaptome mapping may prove useful in identifying synapses that are vulnerable or resistant to TBI.

EphB3 signaling propagates synaptic dysfunction in the traumatic injured brain

Traumatic brain injury (TBI), ranging from mild concussion to severe penetrating wounds, can involve brain regions that contain damaged or lost synapses in the absence of neuronal death. These affected regions significantly contribute to sensory, motor and/or cognitive deficits. Thus, studying the mechanisms responsible for synaptic instability and dysfunction is important for protecting the nervous system from the consequences of progressive TBI. Our controlled cortical impact (CCI) injury produces ~20% loss of synapses and mild changes in synaptic protein levels in the CA3-CA1 hippocampus without neuronal losses. These synaptic changes are associated with functional deficits, indicated by >50% loss in synaptic plasticity and impaired learning behavior. We show that the receptor tyrosine kinase EphB3 participates in CCI injury-induced synaptic damage, where EphB3(-/-) mice show preserved long-term potentiation and hippocampal-dependent learning behavior as compared with wild type (WT) injured mice. Improved synaptic function in the absence of EphB3 results from attenuation in CCI injury-induced synaptic losses and reduced d-serine levels compared with WT injured mice. Together, these findings suggest that EphB3 signaling plays a deleterious role in synaptic stability and plasticity after TBI.

Neonatal sensory nerve injury-induced synaptic plasticity in the trigeminal principal sensory nucleus

Sensory deprivation studies in neonatal mammals, such as monocular eye closure, whisker trimming, and chemical blockade of the olfactory epithelium have revealed the importance of sensory inputs in brain wiring during distinct critical periods. But very few studies have paid attention to the effects of neonatal peripheral sensory nerve damage on synaptic wiring of the central nervous system (CNS) circuits. Peripheral somatosensory nerves differ from other special sensory afferents in that they are more prone to crush or severance because of their locations in the body. Unlike the visual and auditory afferents, these nerves show regenerative capabilities after damage. Uniquely, damage to a somatosensory peripheral nerve does not only block activity incoming from the sensory receptors but also mediates injury-induced neuro- and glial chemical signals to the brain through the uninjured central axons of the primary sensory neurons. These chemical signals can have both far more and longer lasting effects than sensory blockade alone. Here we review studies which focus on the consequences of neonatal peripheral sensory nerve damage in the principal sensory nucleus of the brainstem trigeminal complex.

Neonatal infraorbital nerve crush-induced CNS synaptic plasticity and functional recovery

Infraorbital nerve (ION) transection in neonatal rats leads to disruption of whisker-specific neural patterns (barrelettes), conversion of functional synapses into silent synapses, and reactive gliosis in the brain stem trigeminal principal nucleus (PrV). Here we tested the hypothesis that neonatal peripheral nerve crush injuries permit better functional recovery of associated central nervous system (CNS) synaptic circuitry compared with nerve transection. We developed an in vitro whisker pad-trigeminal ganglion (TG)-brain stem preparation in neonatal rats and tested functional recovery in the PrV following ION crush. Intracellular recordings revealed that 68% of TG cells innervate the whisker pad. We used the proportion of whisker pad-innervating TG cells as an index of ION function. The ION function was blocked by ∼64%, immediately after mechanical crush, then it recovered beginning after 3 days postinjury and was complete by 7 days. We used this reversible nerve-injury model to study peripheral nerve injury-induced CNS synaptic plasticity. In the PrV, the incidence of silent synapses increased to ∼3.5 times of control value by 2-3 days postinjury and decreased to control levels by 5-7 days postinjury. Peripheral nerve injury-induced reaction of astrocytes and microglia in the PrV was also reversible. Neonatal ION crush disrupted barrelette formation, and functional recovery was not accompanied by de novo barrelette formation, most likely due to occurrence of recovery postcritical period (P3) for pattern formation. Our results suggest that nerve crush is more permissive for successful regeneration and reconnection (collectively referred to as "recovery" here) of the sensory inputs between the periphery and the brain stem.

Structural plasticity in the dentate gyrus- revisiting a classic injury model

The adult brain is in a continuous state of remodeling. This is nowhere more true than in the dentate gyrus, where competing forces such as neurodegeneration and neurogenesis dynamically modify neuronal connectivity, and can occur simultaneously. This plasticity of the adult nervous system is particularly important in the context of traumatic brain injury or deafferentation. In this review, we summarize a classic injury model, lesioning of the perforant path, which removes the main extrahippocampal input to the dentate gyrus. Early studies revealed that in response to deafferentation, axons of remaining fiber systems and dendrites of mature granule cells undergo lamina-specific changes, providing one of the first examples of structural plasticity in the adult brain. Given the increasing role of adult-generated new neurons in the function of the dentate gyrus, we also compare the response of newborn and mature granule cells following lesioning of the perforant path. These studies provide insights not only to plasticity in the dentate gyrus, but also to the response of neural circuits to brain injury.

Structural reorganization of the dentate gyrus following entorhinal denervation: species differences between rat and mouse

Deafferentation of the dentate gyrus by unilateral entorhinal cortex lesion or unilateral perforant pathway transection is a classical model to study the response of the central nervous system (CNS) to denervation. This model has been extensively characterized in the rat to clarify mechanisms underlying denervation-induced gliosis, transneuronal degeneration of denervated neurons, and collateral sprouting of surviving axons. As a result, candidate molecules have been identified which could regulate these changes, but a causal link between these molecules and the postlesional changes has not yet been demonstrated. To this end, mutant mice are currently studied by many groups. A tacit assumption is that data from the rat can be generalized to the mouse, and fundamental species differences in hippocampal architecture and the fiber systems involved in sprouting are often ignored. In this review, we will (1) provide an overview of some of the basics and technical aspects of the entorhinal denervation model, (2) identify anatomical species differences between rats and mice and will point out their relevance for the axonal reorganization process, (3) describe glial and local inflammatory changes, (4) consider transneuronal changes of denervated dentate neurons and the potential role of reactive glia in this context, and (5) summarize the differences in the reorganization of the dentate gyrus between the two species. Finally, we will discuss the use of the entorhinal denervation model in mutant mice.

Plasticity of synaptopodin and the spine apparatus organelle in the rat fascia dentata following entorhinal cortex lesion

Synaptopodin is an actin-associated molecule essential for the formation of a spine apparatus in telencephalic spines. To study whether synaptopodin and the spine apparatus organelle are regulated under conditions of lesion-induced plasticity, synaptopodin and the spine apparatus were analyzed in granule cells of the rat fascia dentata following entorhinal denervation. Confocal microscopy was employed to quantify layer-specific changes in synaptopodin-immunoreactive puncta densities. Electron microscopy was used to quantify layer-specific changes in spine apparatus organelles. Within the denervated middle and outer molecular layers, the layers of deafferentation-induced spine loss, synaptogenesis, and spinogenesis, the density of synaptopodin puncta and the number of spine apparatuses decreased by 4 days postlesion and slowly recovered in parallel with spinogenesis by 180 days postlesion. Within the nondenervated inner molecular layer, the zone without deafferentation-induced spine loss, a rapid loss of synaptopodin puncta and spine apparatuses was also observed. In this layer, spine apparatus densities recovered by 14 days postlesion, in parallel with plastic remodeling at the synaptic level and the postlesional recovery of granule cell activity. These data demonstrate layer-specific changes in the distribution of synaptopodin and the spine apparatus organelle following partial denervation of granule cells: in the layer of spine loss, spine apparatus densities follow spine densities; in the layer of spine maintenance, however, spine apparatus densities appear to be regulated by other signals.

Modeling behavioral recovery following lesion induction in the rat dentate gyrus

Unilateral entorhinal lesions have enjoyed immense popularity as a model of recovery from damage. In part, the popularity has been supported the laminar organization of the hippocampal formation, which allows for the dissection of the contribution of individual afferent pathways to the recovery process. The commissural/associational pathway is of particular interest, since electrophysiological and gross anatomical data, although limited, have correlated sprouting in this pathway with behavioral recovery. Unfortunately, information relating recovery to synaptic structure is lacking. Addressing this issue, two analyses were conducted. Initially, a quantitative review of the literature reporting behavioral recovery following this type of lesion was conducted using meta-analytic techniques. Using this detailed information across decades of research, multiple linear regression analysis was conducted to address whether the morphological correlates of recovery could predict behavioral recovery. This resulted in an equation relating morphology and recovery that stood up well to several diagnostic tests. Moreover, this model suggests that synapse structure (in particular, synapse size and curvature, as well as terminal compartmentalization and the density of multi-synaptic terminals) holds a greater potential to predict behavioral recovery than increases in synapse number, which is typically seen as the optimal anatomical measure of recovery. This initial attempt to identify, quantify, and validate a model of lesion recovery is an important initial step in understanding how synaptic morphology may help mediate recovery of function.

Synaptogenesis in the Hippocampal CA1 Field following Traumatic Brain Injury

Traumatic brain injury (TBI) results in both acute and chronic disruption of cognitive ability that may be mediated through a disruption of hippocampal circuitry. Experimental models of TBI have demonstrated that cortical contusion injuries can result in the loss of specific neurons in the CA3 subfield of the ipsilateral hippocampus, resulting in partial loss of afferents to the CA1 subfield. Numerous studies have documented the ability of the central nervous system to compensate for deafferentation by initiating a plasticity response capable of restoring lost synaptic contacts. The present study was designed to examine the time course of loss and replacement of synaptic contacts in stratum radiatum dendritic field of CA1. Young adult rats were subjected to a lateral cortical contusion injury and assayed for total synaptic numbers using unbiased stereology coupled with transmission electron microscopy. Injured animals demonstrated a 60% loss of synapses in CA1 at 2 days post-injury, followed by a reinnervation process that was apparent as early as 10 days post-injury. By 60 days post-injury, total synaptic numbers had approached pre-injury levels but were still significantly lower. Some animals were behaviorally tested for spatial memory in a Morris Water Maze at 15 and 30 days post-injury. While there was some improvement in spatial memory, injured animals continued to demonstrate a significant deficit in acquisition. These results show that the hippocampus ipsilateral to the cortical contusion is capable of a significant plasticity response but that synapse replacement in this area does not necessarily result in significant improvement in spatial learning.

Ultrastructural correlates of vesicular docking in the rat dentate gyrus

To determine the extent to which CA1 synapses are typical of those found in other regions of the hippocampal formation, we have carried out a quantitative analysis of synapses in the middle molecular layer of the rat dentate gyrus, reconstructed from serial electron microscopy, and have compared these data with previous observations from CA1. In general, the morphology of synapses in areas CA1 and the dentate agree, other than an increased density of multisynaptic boutons. Thus, it seems that either area may form an equally effective model for the function of individual synapses in the hippocampal formation. In addition, the current study examines presynaptic curvature, which recent mathematical models have suggested may have profound effects on synaptic transmission. When synapses of distinct curvature profiles (i.e., presynaptically concave, convex, and flat) are examined, the average characteristics of these three synapse populations are distinct. In general, concave synapses have a greater number of morphologically docked vesicles, and thus, likely a greater probability of release. This, however, seems to be accounted for by the fact that these synapses are larger--the spatial density of docked vesicles remains identical across these curvature profiles. This study provides crucial data for further modeling of individual synapse function.

Changes in number of synapses and mitochondria in presynaptic terminals in the dentate gyrus following cerebral ischemia and rehabilitation training

Damage to the adult brain can result in adaptive plasticity in regions adjacent to the site of the principal insult and that the plastic changes may be modulated by post-injury rehabilitation training. In this study, we examined the effects of rehabilitation training on synaptic morphology in the dentate gyrus following transient global cerebral ischemia and the metabolic correlates of the ultrastructural changes. Forty 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 complex environment housing (EC), exercise (EX), or social condition (SC, paired housing) group. Electron microscopy and unbiased stereological methods were used to evaluate synaptic plasticity and the number and size of mitochondria in synaptic axon terminals. Increased number of granule neurons was seen in all ischemic groups and in the sham EC rats. Changes in the number of synapses per neuron in the outer and inner molecular layers of the dentate gyrus parallel those seen in granule neurons. Similarly, ischemia and behavioral experience in EC independently increased the number of synaptic mitochondria in presynaptic terminals in both the outer and inner molecular layers; however, no significant changes were seen in mitochondrial size. These data suggest a link between behavioral training and synaptic plasticity in the region adjacent to the injury and that the likely metabolic correlate of this synaptic plasticity is increased number of mitochondria at synaptic axon terminals.

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.