Ontogenetic Changes in Glial Fibrillary Acid Protein Phosphorylation, Glutamate Uptake and Glutamine Synthetase Activity in Olfactory Bulb of Rats

Study of the glial cytoarchitecture of the developing olfactory bulb of a shark using immunochemical markers of radial glia

During development of the olfactory bulb (OB), glial cells play key roles in axonal guiding/targeting, glomerular formation and synaptic plasticity. Studies in mammals have shown that radial glial cells and peripheral olfactory glia (olfactory ensheathing cells, OECs) are involved in the development of the OB. Most studies about the OB glia were carried out in mammals, but data are lacking in most non-mammalian vertebrates. In the present work, we studied the development of the OB glial system in the cartilaginous fish Scyliorhinus canicula (catshark) using antibodies against glial markers, such as glial fibrillary acidic protein (GFAP), brain lipid-binding protein (BLBP), and glutamine synthase (GS). These glial markers were expressed in cells with radial morphology lining the OB ventricle of embryos and this expression continues in ependymal cells (tanycytes) in early juveniles. Astrocyte-like cells were also observed in the granular layer and surrounding glomeruli. Numerous GS-positive cells were present in the primary olfactory pathway of embryos. In the developmental stages analysed, the olfactory nerve layer and the glomerular layer were the regions with higher GFAP, BLBP and GS immuno-reactivity. In addition, numerous BLBP-expressing cells (a marker of mammalian OECs) showing proliferative activity were present in the olfactory nerve layer. Our findings suggest that glial cells of peripheral and central origin coexist in the OB of catshark embryos and early juveniles. These results open the path for future studies about the differential roles of glial cells in the catshark OB during embryonic development and in adulthood.

Postnatal changes in expression of vesicular glutamate transporters in the main olfactory bulb of the rat

Olfactory information is initially processed through intricate synaptic interactions between glutamatergic projection neurons and GABAergic interneurons in the olfactory bulb. Although bulbar neurons and networks have been reported to develop even postnatally, much is yet unknown about the glutamatergic neuron development. To address this issue, we studied the postnatal ontogeny of vesicular glutamate transporters (VGLUT1 and VGLUT2) in the main olfactory bulb of rats, using in situ hybridization, immunohistochemistry, and their combination. In situ hybridization data showed that VGLUT1 mRNA is intensely expressed in differentiating mitral cells and smaller cells of the mitral cell layer (MCL) on postnatal day 1 (P1), and also at lower levels in small- and medium-sized cells, presumably tufted cell populations, of the external plexiform layer (EPL) from P5 onward. VGLUT2 mRNA was expressed in many MCL cell populations on P1, also in small- and medium-sized cells of the EPL at almost the same level as MCL cells between P5 and P7, and became apparently less intense in the MCL than in the EPL from P10 onward. The expression, unlike VGLUT1 mRNA, was also found in small-sized cells of the interglomerular region. In partial agreement with these data, immunohistochemical analyses demonstrated that subsets of mitral and EPL cells are stained for VGLUT1 or VGLUT2, with the former cells coexpressing both subtypes until P5. Moreover, a combined fluorescence in situ hybridization-immunohistochemical dual labeling of the P10 bulb revealed that neither VGLUT1 nor VGLUT2 mRNA is expressed in GABAergic or dopaminergic periglomerular cells, implying their expression in other periglomerular cell subclasses, external tufted cells and/or short-axon cells. Thus, the present study suggests that early in the postnatal development distinct glutamatergic bulbar neurons of rats express spatiotemporally either or both of the two VGLUT subtypes as a specific vesicular transport system, specifically contributing to glutamate-mediated neurobiological events.