PERINEURONAL NETS PROVIDE A POLYANIONIC, GLIA-ASSOCIATED FORM OF MICROENVIRONMENT AROUND CERTAIN NEURONS IN MANY PARTS OF THE RAT-BRAIN

The nature and function of previously described perineuronal nets are still obscure. In the present study their polyanionic components were demonstrated in the rat brain using colloidal iron hydroxide (CIH) sta ining. In subcortical regions, such as the red nucleus, cerebellar, an d vestibular nuclei, most neurons were ensheathed by CIH-binding mater ial. In the cerebral cortex perineuronal nets were seen around numerou s nonpyramidal neurons. Biotinylated hyaluronectin revealed that hyalu ronan occurs in perineuronal nets. Two plant lectins Wisteria floribu nda agglutinin (WFA) and Vicia villosa agglutinin (VVA)! with affinity for N-acetylgalactosamine visualized perineuronal nets similar to tho se rich in anionic components. Glutamic acid decarboxylase (GAD)-immun oreactive synaptic boutons were shown to occupy numerous meshes of per ineuronal VVA-positive nets. Electron microscopically, VVA binding sit es were scattered throughout perisynaptic profiles, but accumulated at membranes and in the extracellular space except not in synaptic cleft s. To investigate the spatial relationship between glial cell processe s and perineuronal nets, two astrocytic markers (S100-protein and glut amine synthetase) were visualized at the light and electron microscopi c level. Two methods to detect microglia by the use of Griffonia simpl icifolia agglutinin (GSA I-B4) and the monoclonal antibody, OX-42, wer e also applied. Labelled structures forming perineuronal nets were obs erved with both astrocytic, but not with microglial, markers. It is co ncluded that perineuronal nets are composed of a specialized type of g lia-associated extracellular matrix rich in polyanionic groups and N-a cetylgalactosamine. The net-like appearance is due to perisynaptic arr angement of the astrocytic processes and these extracellular component s. Similar to the ensheathment of nodes of Ranvier, perineuronal nets may provide a special ion buffering capacity required around various, perhaps highly active, types of neurons.