“…Several studies have shown that environmental enrichment either during early life (Carstens et al, 2016 ; Stamenkovic et al, 2017 ; O’Connor et al, 2019 ) or adulthood (Foscarin et al, 2011 ; Slaker et al, 2016 ) alters PNNs, as does a high-fat diet during adolescence (Reichelt et al, 2019 , 2021 ) and adulthood (Dingess et al, 2018 , 2020 ). Two studies have shown circadian/diurnal changes, with higher numbers or intensity of PNNs in the dark phase in rodents (Pantazopoulos et al, 2020 ; Harkness et al, 2021 ). Given that PNNs are altered in many ways throughout central nervous system development, and their maturation is brain region-specific and generally coincides with the end of critical periods of plasticity (for a recent review see Carulli and Verhaagen, 2021 ), it is not surprising that numerous studies have also demonstrated changes in PNNs or their composition during aging (Tanaka and Mizoguchi, 2009 ; Karetko-Sysa et al, 2014 ; Brewton et al, 2016 ; Foscarin et al, 2017 ; Richard et al, 2018 ; Ueno et al, 2019 ; Mafi et al, 2020 ).…”