Diurnal changes in perineuronal nets and parvalbumin neurons in the rat medial prefrontal cortex

Abstract

Perineuronal nets (PNNs) surrounding fast-spiking, parvalbumin (PV) inhibitory interneurons are vital for providing excitatory:inhibitory balance within cortical circuits, and this balance is impaired in disorders such as schizophrenia, autism spectrum disorder, and substance use disorders. These disorders are also associated with altered diurnal rhythms, yet few studies have examined the diurnal rhythms of PNNs or PV cells. We measured the intensity and number of PV cells and PNNs labeled with Wisteria floribunda agglutinin (WFA) in the rat prelimbic medial prefrontal cortex (mPFC) at Zeitgeber times (ZT) ZT0, 6, 12, and 18. We also measured the oxidative stress marker 8-oxo-deoxyguanosine (8-oxo-dG). Relative to ZT0, the intensities of PNN and PV staining were increased in the dark (active) phase compared with the light (inactive) phase. The intensity of 8-oxo-dG was decreased from ZT0 at all time points (ZT6,12,18), in both PV cells and non-PV cells. To examine corresponding changes in inhibitory and excitatory inputs, we measured GAD 65/67 and vGlut1 puncta apposed to PV cells with and without PNNs. Relative to ZT6, there were more excitatory puncta on PV cells surrounded by PNNs at ZT18, but no changes in PV cells devoid of PNNs. No changes in inhibitory puncta were observed. Whole-cell slice recordings in fast-spiking (PV) cells with PNNs showed an increased ratio of -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor:N-methyl-D-aspartate receptor (AMPA:NMDA) at ZT18 vs. ZT6. The number of PV cells and co-labeled PV/PNN cells containing the transcription factor orthodenticle homeobox 2 (OTX2), which maintains PNNs, showed a strong trend toward an increase from ZT6 to ZT18. These diurnal fluctuations in PNNs and PV cells are expected to alter cortical excitatory:inhibitory balance and provide new insights into treatment approaches for diseases impacted by imbalances in sleep and circadian rhythms.