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Perineuronal net
Perineuronal nets (PNNs) are specialized extracellular matrix structures responsible for synaptic stabilization in the adult brain. PNNs are found around certain neuron cell bodies and proximal neurites in the central nervous system. PNNs play a critical role in the closure of the childhood critical period, and their digestion can cause restore critical period-like synaptic plasticity in the adult brain. They are largely negatively charged and composed of chondroitin sulfate proteoglycans, molecules that play a key role in development and plasticity during postnatal development and in the adult.
PNNs appear to be mainly present in the cortex, hippocampus, thalamus, brainstem, and the spinal cord. Studies of the rat brain have shown that the cortex contains high numbers of PNNs in the motor and primary sensory areas and relatively fewer in the association and limbic cortices.〔Galtrey, C. M., & Fawcett, J. W. (2007). The role of chondroitin sulfate proteoglycans in regeneration and plasticity in the central nervous system. Brain Research Reviews, 54(1), 1-18.〕 In the cortex, PNNs are associated mostly with inhibitory interneurons and are thought to be responsible for maintaining the excitatory/inhibitory balance in the adult brain.〔
== History ==
The existence of PNNs has been implicated by Golgi, Lugaro, Donaggio, Martinotti, Ramón y Cajal and Meyer. However, Ramón y Cajal credits Golgi with the discovery of PNNs because he was the first to draw attention to them and gave the first precise description in 1893. Moreover, Golgi brought interest to the subject due to his opinion that the PNN was not a neuronal structure but rather a "kind of corset of neurokeratin which impeded the spread of current from cell to cell". Despite debating the topic, Ramón y Cajal claimed that the perineuronal net was simply a staining artifact derived from the coagulation of extracellular fluids. Due to his influential opinion at the time, interest in the topic subsided.
Interest arose in the 1960s when several authors drew attention to the presence of periodic-acid-Schiff-positive (PAS-positive) material surrounding nerve cells. This PAS-positive material was suspected of being composed of negatively charged substances, such as chondroitin sulfate proteoglycans (CSPGs). However, the authors clung to the idea that the material was intricately connected to the blood–brain barrier and failed to see the similarities it had with the perineuronal net described by Golgi. Interest only rose again in the past few decades when it was discovered that PNNs constitute markers for physiologically mature neurons.〔Celio, M. R., Spreafico, R., De Biasi, S., & Vitellaro-Zuccarello, L. (1998). Perineuronal nets: past and present. Trends in Neurosciences, 21(12), 510-515.〕
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