Bridges of biomaterials promote nigrostriatal pathway regeneration.

Repair of central nervous system (CNS) lesions is difficulted by the lack of ability of central axons to regrow, and the blocking by the brain astrocytes to axonal entry. We hypothesized that by using bridges made of porous biomaterial and permissive olfactory ensheathing glia (OEG), we could provide a scaffold to permit restoration of white matter tracts. We implanted porous polycaprolactone (PCL) bridges between the substantia nigra and the striatum in rats, both with and without OEG. We compared the number of tyrosine-hydroxylase positive (TH+) fibers crossing the striatal-graft interface, and the astrocytic and microglial reaction around the grafts, between animals grafted with and without OEG. Although TH+ fibers were found inside the grafts made of PCL alone, there was a greater fiber density inside the graft and at the striatal-graft interface when OEG was cografted. Also, there was less astrocytic and microglial reaction in those animals. These results show that these PCL grafts are able to promote axonal growth along the nigrostriatal pathway, and that cografting of OEG markedly enhances axonal entry inside the grafts, growth within them, and re-entry of axons into the CNS. These results may have implications in the treatment of diseases such as Parkinson's and others associated with lesions of central white matter tracts. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 190-196, 2019.

Polysialic Acid Acute Depletion Induces Structural Plasticity in Interneurons and Impairs the Excitation/Inhibition Balance in Medial Prefrontal Cortex Organotypic Cultures.

The structure and function of the medial prefrontal cortex (mPFC) is affected in several neuropsychiatric disorders, including schizophrenia and major depression. Recent studies suggest that imbalances between excitatory and inhibitory activity (E/I) may be responsible for this cortical dysfunction and therefore, may underlie the core symptoms of these diseases. This E/I imbalance seems to be correlated with alterations in the plasticity of interneurons but there is still scarce information on the mechanisms that may link these phenomena. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) is a good candidate, because it modulates the neuronal plasticity of interneurons and its expression is altered in schizophrenia and major depression. To address this question, we have developed an in vitro model using mPFC organotypic cultures of transgenic mice displaying fluorescent spiny interneurons. After enzymatic depletion of PSA, the spine density of interneurons, the number of synaptic puncta surrounding pyramidal neuron somata and the E/I ratio were strongly affected. These results point to the polysialylation of NCAM as an important factor in the maintenance of E/I balance and the structural plasticity of interneurons. This may be particularly relevant for better understanding the etiology of schizophrenia and major depression.

Olfactory ensheathing glia enhances reentry of axons into the brain from peripheral nerve grafts bridging the substantia nigra with the striatum.

Reconstruction of lost axonal pathways in the central nervous system (CNS) is possible with the use of peripheral nerve grafts (PNG). However, these permit the entry of axons, while their reentry back into the CNS is compromised. Olfactory ensheathing glia (OEG) may permit this reentry of axons if cografted with PNG. We compared the number of tyrosine-hydroxylase positive (TH+) fibers reinnervating PNGs and crossing the graft-striatum interface in PNG placed between the substantia nigra and the striatum in rats receiving both PNG and OEGs and animals receiving PNG only. More TH fibers were seen inside the grafts when OEG was cografted. Although the number of fibers decreases along the graft's length, this effect is less severe when OEG is present. TH+ fibers are seen crossing the PNG-striatum interface in the OEG group. This is correlated with a higher synaptic density at the striatum near the graft when OEG is co-grafted. While these results must be replicated in animal models of Parkinsonism, their implications may apply both to the treatment of Parkinson's disease and to other pathologies, such as spinal cord lesions, where regeneration of long axonal pathways is necessary.

PSA-NCAM is Expressed in Immature, but not Recently Generated, Neurons in the Adult Cat Cerebral Cortex Layer II.

Neuronal production persists during adulthood in the dentate gyrus and the olfactory bulb, where substantial numbers of immature neurons can be found. These cells can also be found in the paleocortex layer II of adult rodents, but in this case most of them have been generated during embryogenesis. Recent reports have described the presence of similar cells, with a wider distribution, in the cerebral cortex of adult cats and primates and have suggested that they may develop into interneurons. The objective of this study is to verify this hypothesis and to explore the origin of these immature neurons in adult cats. We have analyzed their distribution using immunohistochemical analysis of the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) and their phenotype using markers of mature neurons and different interneuronal populations. Additionally, we have explored the origin of these cells administering 5'bromodeoxyuridine (5'BrdU) during adulthood. Immature neurons were widely dispersed in the cerebral cortex layers II and upper III, being specially abundant in the piriform and entorhinal cortices, in the ventral portions of the frontal and temporoparietal lobes, but relatively scarce in dorsal regions, such as the primary visual areas. Only a small fraction of PSA-NCAM expressing cells in layer II expressed the mature neuronal marker NeuN and virtually none of them expressed calcium binding proteins or neuropeptides. By contrast, most, if not all of these cells expressed the transcription factor Tbr-1, specifically expressed by pallium-derived principal neurons, but not CAMKII, a marker of mature excitatory neurons. Absence of PSA-NCAM/5'BrdU colocalization suggests that, as in rats, these cells were not generated during adulthood. Together, these results indicate that immature neurons in the adult cat cerebral cortex layer II are not recently generated and that they may differentiate into principal neurons.

The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) is expressed in a subpopulation of mature cortical interneurons characterized by reduced structural features and connectivity.

Principal neurons in the adult cerebral cortex undergo synaptic, dendritic, and spine remodeling in response to different stimuli, and several reports have demonstrated that the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) participates in these plastic processes. However, there is only limited information on the expression of this molecule on interneurons and on its role in the structural plasticity of these cells. We have found that PSA-NCAM is expressed in mature interneurons widely distributed in all the extension of the cerebral cortex and have excluded the expression of this molecule in most principal cells. Although PSA-NCAM expression is generally considered a marker of immature neurons, birth-dating analyses reveal that these interneurons do not have an adult or perinatal origin and that they are generated during embryonic development. PSA-NCAM expressing interneurons show reduced density of perisomatic and peridendritic puncta expressing different synaptic markers and receive less perisomatic synapses, when compared with interneurons lacking this molecule. Moreover, they have reduced dendritic arborization and spine density. These data indicate that PSA-NCAM expression is important for the connectivity of interneurons in the adult cerebral cortex and that its regulation may play an important role in the structural plasticity of inhibitory networks.

Injection of embryonic median ganglionic eminence cells or fibroblasts within the amygdala in rats kindled from the piriform cortex.

BACKGROUND: Intracerebral infusion of anticonvulsant agent secreting cells has proven to raise the threshold for seizure generation in epileptogenic areas. Median ganglionic eminence (MGE) is the main embryonic region where future GABAergic cells originate. Here we report the results of intraamygdaline grafting of MGE cells versus fibroblasts in a piriform cortex kindling model of epilepsy in the rat. MATERIAL AND METHODS: Rats were implanted with an electrode in the left piriform cortex and subjected to infusion at the left basolateral amygdala of cells obtained from the MGE of embryos or fibroblasts. Some of the donor cells were obtained from transgenic rats expressing the green fluorescent protein (GFP). Seizure and neurologic behavior were recorded, and inmunohistochemical and ultrastructural studies were carried out. RESULTS: Cells obtained from the embryonic MGE elevated both the afterdischarge and the seizure threshold progressively, being significant 3 weeks after their injection. On the contrary, fibroblasts injected into the amygdala raised the seizure thresholds the first week, the effect weaning during the following weeks. Fibroblasts and MGE cells were shown at the injected amygdala. No behavioral side effects were recorded in either experimental group. CONCLUSION: MGE cells implanted at the amygdala may control the focal component of temporolimbic seizures. This effect may be mediated by local release of GABA.

Improved technique for stereotactic placement of nerve grafts between two locations inside the rat brain.

Peripheral nerve grafts have shown the ability to facilitate central axonal growth and regenerate the adult central nervous system. However, the detailed description of a technique for atraumatic graft placement within the brain is lacking. We present a stereotactic procedure to implant a peripheral nerve graft within a rat's brain with minimal brain tissue damage. The procedure permits a correct graft placement joining two chosen points, and the survival and integration of the graft in the host tissue with a light glial reaction, with evidence of central axonal growth inside the graft, at least up to 8 weeks after its implantation.