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

Toward Functional Restoration of the Central Nervous System: A Review of Translational Neuroscience Principles

Injury to the central nervous system (CNS) can leave patients with devastating neurological deficits that may permanently impair independence and diminish quality of life. Recent insights into how the CNS responds to injury and reacts to critically timed interventions are being translated into clinical applications that have the capacity to drastically improve outcomes for patients suffering from permanent neurological deficits due to spinal cord injury, stroke, or other CNS disorders. The translation of such knowledge into practical and impactful treatments involves the strategic collaboration between neurosurgeons, clinicians, therapists, scientists, and industry. Therefore, a common understanding of key neuroscientific principles is crucial. Conceptually, current approaches to CNS revitalization can be divided by scale into macroscopic (systems-circuitry) and microscopic (cellular-molecular). Here we review both emerging and well-established tenets that are being utilized to enhance CNS recovery on both levels, and we explore the role of neurosurgeons in developing therapies moving forward. Key principles include plasticity-driven functional recovery, cellular signaling mechanisms in axonal sprouting, critical timing for recovery after injury, and mechanisms of action underlying cellular replacement strategies. We then discuss integrative approaches aimed at synergizing interventions across scales, and we make recommendations for the basis of future clinical trial design. Ultimately, we argue that strategic modulation of microscopic cellular behavior within a macroscopic framework of functional circuitry re-establishment should provide the foundation for most neural restoration strategies, and the early involvement of neurosurgeons in the process will be crucial to successful clinical translation.

Rat hippocampal CA3 neuronal injury induced by limb ischemia/reperfusion: A possible restorative effect of alpha lipoic acid

Limb ischemia reperfusion (I/R) injury is associated with serious local and systemic effects. Reperfusion may augment tissue injury in excess of that produced by ischemia alone. The hippocampus has been reported to be vulnerable to I/R injury. Alpha lipoic acid (ALA) is an endogenous antioxidant with a powerful antioxidative, anti-inflammatory, and antiapoptotic properties. We studied the probable restorative effect of ALA on limb I/R-induced structural damage of rat hippocampus. Forty adult male albino rats were divided equally into four groups: group I (sham); group II (I/R-1 day) has undergone bilateral femoral arteries occlusion (3 h), then reperfusion for 1 day; group III (I/R-7 days) has undergone reperfusion for seven days; group IV (I/R-ALA) has undergone I/R as group III and received an intraperitoneal injection of ALA (100 mg/kg) for 7 days. I/R groups revealed degenerative changes in the pyramidal neuronal perikarya of CA3 field in the form of dark-stained cytoplasm, dilated RER cisternae, mitochondrial alterations, and dense bodies' accumulation. Their dendrites showed disorganized microtubules. Astrogliosis is featured by an increased number and increased immunoreactivity of astrocytes for glial fibrillary acid protein. Morphometric data revealed significant reduction of light neurons, surface area of neurons, and thickness of the CA3 layer. Most blood capillaries exhibited narrow lumen and irregular basal lamina. ALA ameliorated the neuronal damage. Pyramidal neurons revealed preservation of normal structure. Significant increase in the thickness of pyramidal layer in CA3 field and surface area and number of light neurons was observed but astrogliosis persisted. Limb I/R had a deleterious remote effect on the hippocampus aggravated with longer period of reperfusion. This work may encourage the use of ALA in the critical clinical settings with I/R injury.

Enhancing Nervous System Recovery through Neurobiologics, Neural Interface Training, and Neurorehabilitation

After an initial period of recovery, human neurological injury has long been thought to be static. In order to improve quality of life for those suffering from stroke, spinal cord injury, or traumatic brain injury, researchers have been working to restore the nervous system and reduce neurological deficits through a number of mechanisms. For example, neurobiologists have been identifying and manipulating components of the intra- and extracellular milieu to alter the regenerative potential of neurons, neuro-engineers have been producing brain-machine and neural interfaces that circumvent lesions to restore functionality, and neurorehabilitation experts have been developing new ways to revitalize the nervous system even in chronic disease. While each of these areas holds promise, their individual paths to clinical relevance remain difficult. Nonetheless, these methods are now able to synergistically enhance recovery of native motor function to levels which were previously believed to be impossible. Furthermore, such recovery can even persist after training, and for the first time there is evidence of functional axonal regrowth and rewiring in the central nervous system of animal models. To attain this type of regeneration, rehabilitation paradigms that pair cortically-based intent with activation of affected circuits and positive neurofeedback appear to be required-a phenomenon which raises new and far reaching questions about the underlying relationship between conscious action and neural repair. For this reason, we argue that multi-modal therapy will be necessary to facilitate a truly robust recovery, and that the success of investigational microscopic techniques may depend on their integration into macroscopic frameworks that include task-based neurorehabilitation. We further identify critical components of future neural repair strategies and explore the most updated knowledge, progress, and challenges in the fields of cellular neuronal repair, neural interfacing, and neurorehabilitation, all with the goal of better understanding neurological injury and how to improve recovery.

A Semi-Supervised Approach for Refining Transcriptional Signatures of Drug Response and Repositioning Predictions

We present a novel strategy to identify drug-repositioning opportunities. The starting point of our method is the generation of a signature summarising the consensual transcriptional response of multiple human cell lines to a compound of interest (namely the seed compound). This signature can be derived from data in existing databases, such as the connectivity-map, and it is used at first instance to query a network interlinking all the connectivity-map compounds, based on the similarity of their transcriptional responses. This provides a drug neighbourhood, composed of compounds predicted to share some effects with the seed one. The original signature is then refined by systematically reducing its overlap with the transcriptional responses induced by drugs in this neighbourhood that are known to share a secondary effect with the seed compound. Finally, the drug network is queried again with the resulting refined signatures and the whole process is carried on for a number of iterations. Drugs in the final refined neighbourhood are then predicted to exert the principal mode of action of the seed compound. We illustrate our approach using paclitaxel (a microtubule stabilising agent) as seed compound. Our method predicts that glipizide and splitomicin perturb microtubule function in human cells: a result that could not be obtained through standard signature matching methods. In agreement, we find that glipizide and splitomicin reduce interphase microtubule growth rates and transiently increase the percentage of mitotic cells-consistent with our prediction. Finally, we validated the refined signatures of paclitaxel response by mining a large drug screening dataset, showing that human cancer cell lines whose basal transcriptional profile is anti-correlated to them are significantly more sensitive to paclitaxel and docetaxel.

Age-dependent changes in intrinsic neuronal excitability in subiculum after status epilepticus

Kainic acid-induced status epilepticus (KA-SE) in mature rats results in the development of spontaneous recurrent seizures and a pattern of cell death resembling hippocampal sclerosis in patients with temporal lobe epilepsy. In contrast, KA-SE in young animals before postnatal day (P) 18 is less likely to cause cell death or epilepsy. To investigate whether changes in neuronal excitability occur in the subiculum after KA-SE, we examined the age-dependent effects of SE on the bursting neurons of subiculum, the major output region of the hippocampus. Patch-clamp recordings were used to monitor bursting in pyramidal neurons in the subiculum of rat hippocampal slices. Neurons were studied either one or 2-3 weeks following injection of KA or saline (control) in immature (P15) or more mature (P30) rats, which differ in their sensitivity to KA as well as the long-term sequelae of the KA-SE. A significantly greater proportion of subicular pyramidal neurons from P15 rats were strong-bursting neurons and showed increased frequency-dependent bursting compared to P30 animals. Frequency-dependent burst firing was enhanced in P30, but not in P15 rats following KA-SE. The enhancement of bursting induced by KA-SE in more mature rats suggests that the frequency-dependent limitation of repetitive burst firing, which normally occurs in the subiculum, is compromised following SE. These changes could facilitate the initiation of spontaneous recurrent seizures or their spread from the hippocampus to other parts of the brain.

L-Measure: a web-accessible tool for the analysis, comparison and search of digital reconstructions of neuronal morphologies

L-Measure (LM) is a freely available software tool for the quantitative characterization of neuronal morphology. LM computes a large number of neuroanatomical parameters from 3D digital reconstruction files starting from and combining a set of core metrics. After more than six years of development and use in the neuroscience community, LM enables the execution of commonly adopted analyses as well as of more advanced functions. This report illustrates several LM protocols: (i) extraction of basic morphological parameters, (ii) computation of frequency distributions, (iii) measurements from user-specified subregions of the neuronal arbors, (iv) statistical comparison between two groups of cells and (v) filtered selections and searches from collections of neurons based on any Boolean combination of the available morphometric measures. These functionalities are easily accessed and deployed through a user-friendly graphical interface and typically execute within few minutes on a set of approximately 20 neurons. The tool is available at http://krasnow.gmu.edu/cn3 for either online use on any Java-enabled browser and platform or download for local execution under Windows and Linux.

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.

Quantitative morphometry of hippocampal pyramidal cells: Differences between anatomical classes and reconstructing laboratories

The dendritic trees of hippocampal pyramidal cells play important roles in the establishment and regulation of network connectivity, synaptic plasticity, and firing dynamics. Several laboratories routinely reconstruct CA3 and CA1 dendrites to correlate their three-dimensional structure with biophysical, electrophysiological, and anatomical observables. To integrate and assess the consistency of the quantitative data available to the scientific community, we exhaustively analyzed 143 completely reconstructed neurons intracellularly filled and digitized in five different laboratories from 10 experimental conditions. Thirty morphometric parameters, including the most common neuroanatomical measurements, were extracted from all neurons. A consistent fraction of parameters (11 of 30) was significantly different between CA3 and CA1 cells. A considerably large number of parameters was also found that discriminated among neurons within the same morphological class, but reconstructed in different laboratories. These interlaboratory differences (8 of 30 parameters) far outweighed the differences between experimental conditions within a single lab, such as aging or preparation method (at most two significant parameters). The set of morphometrics separating anatomical regions and that separating reconstructing laboratories were almost entirely nonoverlapping. CA3 and CA1 neurons could be distinguished by global quantities such as branch order and Sholl distance. Differences among laboratories were largely due to local variables such as branch diameter and local bifurcation angles. Only one parameter (a ratio of branch diameters) separated both morphological classes and reconstructing laboratories. Compartmental simulations of electrophysiological activity showed that both differences between anatomical classes and reconstructing laboratories could dramatically affect the firing rate of these neurons under different experimental conditions.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Neonatal exposure to monosodium L-glutamate induces loss of neurons and cytoarchitectural alterations in hippocampal CA1 pyramidal neurons of adult rats

Glutamatergic post-synaptic receptors are closely related to the known excitotoxic effects of high doses of L-glutamate. Several behavioral abnormalities, glial reaction, and an increase of expression of the NMDA receptor sub-units have been observed in the rat hippocampus after early monosodium glutamate exposure. Thus, a quantitative morphological study was carried out to determine the effects of early exposure to monosodium glutamate on post-synaptic structures that mediate glutamate excitatory neurotransmission in the hippocampal CA1 field. Four milligrams per gram body weight of monosodium glutamate was subcutaneously injected into neonatal Wistar rats, at 1, 3, 5, and 7 days. Cell loss and several cytoarchitectonic parameters were evaluated in pyramidal cells from the hippocampal CA1 field in the treated rats at 60 days of age. An untreated group of rats were used as controls. Cell number in the hippocampus of experimental rats was 11.5% less than that in control animals. In addition, both dendritic arborization and dendritic spine density were adversely affected, and thin and mushroom-shaped spines became proportionally more numerous, while the opposite occurred to stubby spines. These results strongly suggest the occurrence of cell death and also show some cytoarchitectural modifications in the surviving neurons. These could lead to functional alterations in the hippocampal integrative activity, due to an early cytoexcitotoxic effect of monosodium glutamate.

Alterations of hippocampal GAbaergic system contribute to development of spontaneous recurrent seizures in the rat lithium-pilocarpine model of temporal lobe epilepsy

Reorganization of excitatory and inhibitory circuits in the hippocampal formation following seizure-induced neuronal loss has been proposed to underlie the development of chronic seizures in temporal lobe epilepsy (TLE). Here, we investigated whether specific morphological alterations of the GABAergic system can be related to the onset of spontaneous recurrent seizures (SRS) in the rat lithium-pilocarpine model of TLE. Immunohistochemical staining for markers of interneurons and their projections, including parvalbumin (PV), calretinin (CR), calbindin (CB), glutamic acid decarboxylase (GAD), and type 1 GABA transporter (GAT1), was performed in brain sections of rats treated with lithium-pilocarpine and sacrificed after 24 h, during the silent phase (6 and 12 days), or after the onset of SRS (10-18 days after treatment). Semiquantitative analysis revealed a selective loss of interneurons in the stratum oriens of CA1, associated with a reduction of GAT1 staining in the stratum radiatum and stratum oriens. In contrast, interneurons in CA3 were largely preserved, although GAT1 staining was also reduced. These changes occurred within 6 days after treatment and were therefore insufficient to cause SRS. In the dentate gyrus, extensive cell loss occurred in the hilus. The pericellular innervation of granule cells by PV-positive axons was markedly reduced, although the loss of PV-interneurons was only partial. Most strikingly, the density of GABAergic axons, positive for both GAD and GAT1, was dramatically increased in the inner molecular layer. This change emerged during the silent period, but was most marked in animals with SRS. Finally, supernumerary CB-positive neurons were detected in the hilus, selectively in rats with SRS. These findings suggest that alterations of GABAergic circuits occur early after lithium-pilocarpine-induced status epilepticus and contribute to epileptogenesis. In particular, the reorganization of GABAergic axons in the dentate gyrus might contribute to synchronize hyperexcitability induced by the interneuron loss during the silent period, leading to the onset of chronic seizures.

Functional recovery of respiratory behavior during axonal regeneration in snails (Lymnaea stagnalis) is experience dependent

This study investigated the role of experience in recovery of pulmonary respiration during axonal regeneration in Lymnaea stagnalis. Pulmonary respiration occurs when snails break the water surface and open the lung orifice, the pneumostome. It was shown that axotomy of all the axons innervating the pneumostome and surrounding area prevents the occurrence of lung respiration in 69% of snails. In the remaining 31%, lung respiration persisted, indicating that peripheral components alone are capable of initiating pneumostome openings and closures. Five weeks postsurgery, all snails with previous nerve crushes showed opening of the pneumostome with normal latency after breaking the water surface. However, prevention of pulmonary respiration during the recovery period dramatically changed the recovered behavior. Thus, experience in pulmonary respiration during axonal regeneration plays a role in the recovery of this behavior.