Perinueronal nets development correlates with developmental and adult neuroplasticity related to song learning in the songbird brain

erineuronal nets (PNN) are aggregations of extracellular matrix components surrounding the soma of specific neurons, mainly GABAergic interneurons expressing the calcium binding protein parvalbumin (PV+). In mammals, the development of PNN limits synaptogenesis around PV+ neurons and PNNs have been validated as a marker characterizing the end of critical periods for visual system development. In songbirds, song learning is limited to critical periods during ontogeny in closed-ended learners such as zebra finches and to specific phases of the annual cycle in open-ended learners such as canaries that are able to modify their song seasonally. Developmental and adult sensitive periods for song learning are associated with neuroplasticity in song control nuclei, including major morphological changes reflecting neurogenesis and synaptic reorganization The hormonal control of developmental and seasonal neuroplasticity is relatively well documented in songbirds but little is known about its possible regulation by PNN. Interspecific comparisons indicate however that more PNN are present in the song control nuclei of closed-ended compared to open-ended learners suggesting a relationship with song plasticity. To explore the expression of PNN throughout development, we used male and female zebra finch brains collected at different key ages in the song learning process (10, 20, 30, 40, 50, 60, 90, 120 days post-hatch, dph) and we quantified the expression of PNN and their colocalization around PV+ interneurons. The number of PNN and the % of PNN surrounding PV+ interneurons increased progressively during developmental song learning in 3 song control nuclei (HVC, RA and Area X). Females who never sing in this species had fewer PNN than males in HVC and RA, two song control nuclei involved in song production, and their number never increased with age so that they became different from males for all ages after 50 dph. Two separate experiments used adult male and female canaries treated with a subcutaneous Silastic implant filled with testosterone or left empty in control subjects to mimic what happens in the spring when the seasonal plasticity of the song ends and the song crystalizes. Testosterone significantly increased the number of PNN in these forebrain song control nuclei in both sexes. Together these data demonstrate that an increased expression of PNN in the songbird brain correlates with the end of sensitive periods for song plasticity and might thus limit further synaptic reorganization at the end of these periods of behavioral and neural plasticity.