Functional recovery after olfactory bulbectomy in rats: effect of embryonal brain grafts

Regeneration of olfactory receptor neurones after bulbectomy can lead to formation of extrabulbar synapses, the functional significance of which remains controversial. Adult hooded rats (n = 27) were bilaterally bulbectomized under pentobarbital anaesthesia. Small pieces of brain tissues (1-2 mm3; OB: olfactory bulb; St: corpus striatum) were obtained from embryos of the same strain and placed into the bulbectomy-produced cavity in contact with the exposed brain surface. Smell was tested at 2- to 3-week intervals from 7 weeks to 7 months after the operation. The latency to find hidden food gradually improved and attained the intact control level in bulbectomized rats without grafts, but remained poor in the OB and St transplanted groups. Seven to ten months after transplantation, spontaneous unit activity and unit reactions to amyl acetate vapours were examined with a carbon fibre microelectrode. Unit responses in the transplants resembled those in the normal OB, but were less pronounced. Morphological examination of the transplant and of its connections with the olfactory receptor neurones and with the host brain suggested that functional recovery was mediated by the connections of the olfactory axons with the remnants of the OB, with the anterior olfactory nucleus and/or with the frontal cortex. The adverse effect of OB and St transplants was probably due to their poor connectivity with the host brain which prevented the regenerating olfactory axons from reaching higher olfactory centres.