Distribution of perineuronal nets in the adult rat neocortex.

Perineuronal nets (PNNs) are specialised extracellular matrix structures of the central nervous system that predominantly surround inhibitory interneurons. The development of PNNs is activity dependent and relies on sensory input to mature to an adult expression pattern, coinciding with the crysallization of synaptic circuitry following the closure of the developmental critical period. Our results of a neocortical characterisation demonstrate that the density of PNNs in the neocortex of the Long Evans rat was consistent across animals but varied as a function of the cortical region. Utilising a non-biased random effect model, it was shown that the retrosplenial granular cortex had the highest PNN density and the intermediate endopiriform cortex had the lowest. Given the importance of mystacial vibrissae to the behaviour of rodents, we further analysed the laminar distribution of PNNs within the primary somatosensory cortex (S1). Our results revealed that the layer IV, the 'barrel' region, contained the highest density of PNNs. Within S1, PNN density was consistent across all subregions. S1 had a significantly higher density of PNNs compared with the primary motor (M1) cortex, and PNN levels did not show lateralisation in either M1 or S1. Independent of cortical location, we observed a novel class of relatively large, brightly stained neurons ensheathed by PNNs, which were present exclusively along the layer VI-white matter border. A better understanding of PNNs and their distribution can assist in our understanding of how PNN manipulation can affect neurological conditions such as schizophrenia, epilepsy, stroke, central nervous system injuries, and degenerative processes.