Distribution of growth associated protein (B-50/GAP-43) and glial fibrillary acidic protein (GFAP) immunoreactivity in rat homotopic olfactory bulb transplants.

Abstract

Recent studies on olfactory bulb homotopic transplantation after partial or subtotal bulb ablation have shown that regenerated olfactory axon are able to form glomeruli-like structures either in the transplant or in the spared olfactory bulb. To investigate the maturation of olfactory axons and their terminal connections in the transplant and remnant olfactory bulb we performed immunohistochemistry utilizing as markers B-50/GAP43 for neurite outgrowth and glial fibrillary acidic protein (GFAP) for the glial response to the surgery. Radioactively prelabeled olfactory bulbs (E 18) were homotopically transplanted in unilaterally bulbectomized neonatal rats (P6). Two to four months after transplantation in the partially bulbectomized rats the laminar organization of the olfactory bulb remnant depended on the extent of lesion. The transplant was disorganized showing irregularly distributed glomeruli. In a few cases pseudo-laminar organization was observed resembling that of the normal olfactory bulb. In 2 month-old transplants, outgrowing axons had a newly formed glomeruli displayed prominent B-50/GAP43 immunoreactivity. Four months after the operation B-50/GAP43 immunoreactivity was still present in the regrowing axons and some glomeruli in the transplant were B-50/GAP43 positive, while other glomeruli revealed a patchy pattern. The same B-50/GAP43 immunostained patchy structure were present in remnants of the lesioned olfactory bulb. Distinct increase of GFAP activity was observed along the olfactory axons and in the glomeruli-like structures. The persisting B-50/GAP43 immunoreactivity in the glomeruli of the transplant and in the remnants of lesioned olfactory bulb suggests that maturation of the newly formed glomeruli was delayed in comparison to the intact control olfactory bulb. Furthermore, the decrease of B-50/GAP43 immunoreactivity and the patchy distribution may indicate that synaptogenesis occurred in the glomeruli of the transplant, in spite of the altered topography. Together with previous findings on reinnervation of the piriform cortex from projection neurons situated in olfactory bulb transplants, our immunohistochemical data support the notion that olfactory bulb transplantation may reestablish to some degree the neuronal circuits affected by the experimental surgery.