The mouse olfactory peduncle 4: Development of synapses, perineuronal nets, and capillaries.

Olfaction is critical for survival in neonatal mammals. However, little is known about the neural substrate for this ability as few studies of synaptic development in several olfactory processing regions have been reported. Odor information detected in the nasal cavity is first processed by the olfactory bulb and then sent via the lateral olfactory tract to a series of olfactory cortical areas. The first of these, the anterior olfactory nucleus pars principalis (AONpP), is a simple, two layered cortex with an outer plexiform and inner cell zone (Layers 1 and 2, respectively). Five sets of studies examined age-related changes in the AONpP. First, immunocytochemistry for glutamatergic (VGlut1 and VGlut2) and GABAergic (VGAT) synapses demonstrated that overall synaptic patterns remained uniform with age. The second set quantified synaptic development with electron microscopy and found different developmental patterns between Layers 1 and 2. As many of the interhemispheric connections in the olfactory system arise from AONpP, the third set examined the development of crossed projections using anterograde tracers and electron microscopy to explore the maturation of this pathway. A fourth study examined ontogenetic changes in immunostaining for the proteoglycans aggrecan and brevican, markers of mesh-like extracellular structures known as perineuronal nets whose maturation is associated with the end of early critical periods of synaptogenesis. A final study found no age-related changes in the density of vasculature in the peduncle from P5 to P30. This work is among the first to examine early postnatal changes in this initial cortical region of the olfactory system.

The mouse olfactory peduncle. 3. Development of neurons, glia, and centrifugal afferents.

The present series of studies was designed to provide a general overview of the development of the region connecting the olfactory bulb to the forebrain. The olfactory peduncle (OP) contains several structures involved in processing odor information with the anterior olfactory nucleus (cortex) being the largest and most studied. Results indicate that considerable growth occurs in the peduncle from postnatal day (P)10-P20, with reduced expansion from P20 to P30. No evidence was found for the addition of new projection or interneurons during the postnatal period. GABAergic cells decreased in both number and density after P10. Glial populations exhibited different patterns of development, with astrocytes declining in density from P10 to P30, and both oligodendrocytes and microglia increasing through the interval. Myelination in the anterior commissure emerged between P11 and P14. Dense cholinergic innervation was observed at P10 and remained relatively stable through P30, while considerable maturation of serotonergic innervation occurred through the period. Unilateral naris occlusion from P1 to P30 resulted in about a 30% reduction in the size of the ipsilateral peduncle but few changes were observed on the contralateral side. The ipsilateral peduncle also exhibited higher densities of GAD67-containing interneurons and cholinergic fibers suggesting a delay in normal developmental pruning. Lower densities of interneurons expressing CCK, somatostatin, and NPY and in myelin basic protein staining were also observed. Understanding variations in developmental trajectories within the OP may be an important tool for unraveling the functions of the region.