Perineuronal nets in the rodent suprachiasmatic nucleus

The circadian system maintains highly stable rhythms over the lifespan. The precision of the circadian clock emerges from the network organization of the master clock located in the suprachiasmatic nucleus (SCN). Perineuronal nets (PNNs), condensed extracellular matrix structures that are composed of chondroitin sulfate proteoglycans, help maintain synaptic connections in many brain regions after the end of critical developmental periods. We examine here if the SCN of mice, rats and hamsters contain PNNs. It was observed that mice and rats had little if any PNN staining in their SCN, while the SCN of hamsters had intense PNN staining. While PNNs were observed throughout the whole SCN, the staining was more intense in the Calbindin D28K and Arginine Vasopressin regions of the SCN. Their appearance over development lags when retinal innervation of the SCN would be expected, reaching adult-like levels around 20-25 days postnatal. Removal of PNNs from the adult SCN enhances phase shift responses to both phase delay and phase advancing light pulses. These findings indicate that PNNs may help regulate circadian properties of some, but not all, species.