RECOVERY OF OLFACTORY BEHAVIOR .2. NEONATAL OLFACTORY-BULB TRANSPLANTS ENHANCE THE RATE OF BEHAVIORAL RECOVERY

Previous experiments in this laboratory have shown that transplants of a fetal olfactory bulb into a neonatal rat are viable and that they e stablish connections with the olfactory peduncle and olfactory cortex. The focus of this experiment was to investigate the anatomical correl ates of any behavioral recovery seen in rats that had one olfactory bu lb removed along with an immediate transplant of a fetal olfactory bul b. Anatomical details, such as transplant organization and olfactory n erve repenetration patterns were analyzed using a variety of histologi cal and immunohistochemical techniques. The rats in this experiment sh owed behavioral recovery of olfactory ability. The recovery rates obse rved in these animals were compared to two other groups of rats that t his laboratory has shown to be behaviorally competent: normal rats and rats with neonatal ablations of the olfactory bulb but no transplant. Although the animals with transplants did not recover to completely n ormal levels of olfactory ability, they did start behavioral testing i n a more behaviorally competent condition than rats with simple neonat al lesions. Anatomical analysis revealed that the transplanted olfacto ry bulb was heavily penetrated by incoming olfactory nerve fibers but olfactory nerve penetration was not limited to the transplanted olfact ory bulb. The extra-bulbar host regions that were penetrated included the orbital frontal cortex and three olfaction-related areas; olfactor y cortex, olfactory peduncle and the subependymal cell layer. The olfa ctory nerve penetration patterns observed beyond the transplant were e ssentially the same as those observed in rats with only neonatal lesio ns of the olfactory bulb. Thus, multiple pathways may have contributed to the recovery observed in the rats with olfactory bulb transplants.