Postnatal development of the microstructure of cortical GABAergic synapses and perineuronal nets requires sensory input

GABAergic integration of transient and persistent neurons in the developing mouse somatosensory cortex

GABA is an essential element in the function of neocortical circuits. The origin, migration and mechanisms of synaptogenesis of GABAergic neurons have been intensively studied. However, little information is available when GABAergic synapses are formed within the different cortical layers, neuronal cell types and subcellular compartments. To quantify the distribution of GABAergic synapses in the immature somatosensory mouse cortex, GABAergic synapses were identified by spatially coincident immunoprofiles for the pre- and postsynaptic markers vGAT and gephyrin at postnatal days (P)0-12. Between P0-5, GABAergic synapses are mainly restricted to the marginal zone, while at later developmental stages a more homogenous distribution is obtained. Cajal-Retzius neurons represent a major target of GABAergic synapses in the marginal zone with a homogeneous synapse distribution along the dendrite. The number of GABAergic synapses per pyramidal neuron increases substantially between P0 and P12, with a stable density and distribution in basal dendrites. In contrast, along apical dendrites synapses accumulate to more proximal positions after P8. Overall, the results of this study demonstrate that early GABAergic synaptogenesis is characterized by a consistent increase in the density of synapses with first a stringent overrepresentation in the marginal zone and a delayed establishment of perisomatic synapses in pyramidal neurons.

Perineuronal Net Microscopy: From Brain Pathology to Artificial Intelligence

Perineuronal nets (PNN) are a special highly structured type of extracellular matrix encapsulating synapses on large populations of CNS neurons. PNN undergo structural changes in schizophrenia, epilepsy, Alzheimer's disease, stroke, post-traumatic conditions, and some other brain disorders. The functional role of the PNN microstructure in brain pathologies has remained largely unstudied until recently. Here, we review recent research implicating PNN microstructural changes in schizophrenia and other disorders. We further concentrate on high-resolution studies of the PNN mesh units surrounding synaptic boutons to elucidate fine structural details behind the mutual functional regulation between the ECM and the synaptic terminal. We also review some updates regarding PNN as a potential pharmacological target. Artificial intelligence (AI)-based methods are now arriving as a new tool that may have the potential to grasp the brain's complexity through a wide range of organization levels-from synaptic molecular events to large scale tissue rearrangements and the whole-brain connectome function. This scope matches exactly the complex role of PNN in brain physiology and pathology processes, and the first AI-assisted PNN microscopy studies have been reported. To that end, we report here on a machine learning-assisted tool for PNN mesh contour tracing.