Abstract
Large morphological sex differences in the vertebrate brain were initially identified in song control nuclei of oscines. Besides gross differences between volumes of nuclei in males and females, sex differences also concern the size and dendritic arborization of neurons and various neurochemical markers, such as the calcium-binding protein parvalbumin (PV). Perineuronal nets (PNN) of the extracellular matrix are aggregates of different compounds, mainly chondroitin sulfate proteoglycans, that surround subsets of neurons, often expressing PV. PNN develop in zebra finches song control nuclei around the end of the sensitive period for song learning and tutor deprivation, known to delay the end of the song learning sensitive period, decreases the numbers of PNN in HVC. We demonstrate here the existence in zebra finches of a major sex difference (males > females) affecting the number of PNN (especially those surrounding PV-positive cells) in HVC and to a smaller extent the robust nucleus of the arcopallium, RA, the two main nuclei controlling song production. These differences were not present in Area X and LMAN, the lateral magnocellular nucleus of the anterior nidopallium. A dense expression of material immunoreactive for chondroitin sulfate was also detected in several nuclei of the auditory and visual pathways. This material was often organized in perineuronal rings but quantification of these PNN did not reveal any sex difference with the exception that the percentage of PNN surrounding PV-ir cells in the dorsal lateral mesencephalic nucleus, MLd, was larger in females than in males, a sex difference in the opposite direction compared to what is seen in HVC and RA. These data confirm and extend previous studies demonstrating the sex difference affecting PNN in HVC-RA by showing that this sex difference is anatomically specific and does not concern visual or auditory pathways.
Highlights
Brain plasticity was initially considered only at the functional level and covered phenomena such as long-term potentiation or depression (LTP/LTD)[1]
We recently started investigating the possible sex difference in Perineuronal nets (PNN), PV and their association in the zebra finch brain. As these investigations were in progress, an article appeared indicating that PNN expression is higher in the HVC and RA of males as compared to females but that this sex difference was not present in nuclei of the anterior forebrain pathway, lateral magnocellular nucleus of the anterior nidopallium (LMAN) and Area X that is largely implicated in song learning and song stability [45]
Inspection at low magnification of sections with filters visualizing the red Alexa 546 associated with parvalbumin identified dense clusters of immunopositive cells that overlapped with the telencephalic song control nuclei (HVC, RA, Area X and LMAN) and with many other diencephalic and mesencephalic structures (Fig 1B and 1C)
Summary
Brain plasticity was initially considered only at the functional level and covered phenomena such as long-term potentiation or depression (LTP/LTD)[1]. We recently started investigating the possible sex difference in PNN, PV and their association in the zebra finch brain As these investigations were in progress, an article appeared indicating that PNN expression is higher in the HVC and RA of males as compared to females but that this sex difference was not present in nuclei of the anterior forebrain pathway, LMAN and Area X (or the corresponding part of the basal ganglia in females) that is largely implicated in song learning and song stability [45]. This question is relevant since previous work in other species showed that PNN regulate plasticity in multiple functional networks including those involved in vision, somatosensation, fear conditioning and spinal motor control [27,28]
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