Histochemical study of perineuronal nets in the retrosplenial cortex of adult rats

The retrosplenic cortex of rats, similar to many cortical or subcortical regions, is provided with special subsets of neurons that exhibited a fenestrated or reticular coat of condensed extracellular matrix on their soma, initial dendrites and proximal axon segment. This pericellular coating, currently termed "Perineuronal Nets", was detected on the surfaces of some neurons distributing throughout the cortical layers II-V. They presented direct interconnections with each other, and appeared in close association to the astroglial processes. In addition to their collagenous ligands, the perineuronal nets (PNs) were enriched with proteoglycans (PGs, sulfated glycoconjugates) and/or glycoproteins (GPs, unsulfated glycoconjugates with terminal N-acetylgalactosamine). Accordingly, the PNs were differentially identified as belonging to three categories, depending upon their organic nature or chemical composition. First, coats exclusively formed of PGs (stained with iron colloid); second, coats formed of GPs (labeled with plant lectins binding to terminal N-acetylgalactosamine); and third, complex coats formed of PG networks intermingled with glycoprotein molecules (double stained with iron colloid and lectin). Since differential distribution of protein containing substances (GPs and/or PGs) in the extracellular matrix contributes to functional terms, we suggest that these biochemical or morphological differences in the microenvironment of some retrosplenial neurons might reflect certain functional aspects concerned with processing of navigation or episodic memory.