Tyrosine hydroxylase expression in rat olfactory bulb transplants: an electron microscope study.

Juxtaglomerular (JG) neurons of rat olfactory bulb (OB) have been shown to express tyrosine hydroxylase (TH), the rate-limiting enzyme in the catecholamine synthesis pathway. These JG neurons act as inhibitory dopaminergic interneurons, modulating the incoming signal from the primary olfactory afferents. The JG neurons, comprised of periglomerular, external tufted, and short axon cells, stop expressing TH after lesions of the olfactory nerve or closure of the nares, both of which cause a loss of functional input. Upon reinnervation by a continuously regenerating olfactory nerve, these cells resume their expression of TH. In order to study deafferentation and subsequent reinnervation within this system, our laboratory utilizes a transplantation model. Sections from transplant (TX) OBs are reacted for TH using immunocytochemical localization protocols and studied by light- and especially electron microscopy (EM). Autoradiography of tritiated thymidine-labeled tissue was performed to confirm donor origin of the TX OBs. Although the architecture of the TX OB is somewhat disrupted and the TH-positive cells were not as uniform in their arrangement as they are in the normal OB, we found that the TH cells in the TH OB had a morphology similar to the JG cells observed in normal OB. These TH cells were also found to receive synaptic contacts with host olfactory nerve axons as well as make and receive contacts with the processes of donor neurons. These, synaptic contacts were formed within areas that resemble the glomeruli of normal olfactory bulb, suggesting that the inhibitory synaptic pathway is reestablished within the TX OB. These findings also suggested that host olfactory axons formed a functional contact with the TH cells, possibly inducing them to express this enzyme. This study implies that the TX OB retains a level of plasticity that enables it to recapitulate part of the interneuronal arrangement observed in the normal system.

Host primary olfactory axons make synaptic contacts in a transplanted olfactory bulb.

Previous light microscopic studies have shown that host olfactory neurons are able to grow into a transplanted fetal olfactory bulb, and behavioral studies have shown that animals with transplanted olfactory bulbs recover functional olfactory abilities. We examined the olfactory bulb transplant at the ultrastructural level to determine whether synaptic contacts are reestablished between host olfactory neurons and donor olfactory bulb. Mature rats that, as neonates, had received embryonic olfactory bulb transplants following olfactory bulb removal were studied. An antibody specific for olfactory marker protein was used to identify the primary olfactory neurons; it was bound by a gold-conjugated secondary antibody for visualization. To preserve the antigenicity of the olfactory marker protein for immunolabeling, Lowicryl K4M hydrophilic resin was used. Synaptic contacts were unmistakable between labeled axons of host olfactory neurons and unlabeled processes within glomerulus-like areas of the transplanted olfactory bulb. The surrounding neuropil contained other elements similar to those found in normal tissue, including synaptic contacts between unlabeled profiles. We clearly show that the transplanted olfactory bulb exhibits sufficient plasticity to form an array of normal synaptic contacts, including the contacts from host primary olfactory neurons.

Recovery of olfactory behavior. II. 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 establish connections with the olfactory peduncle and olfactory cortex. The focus of this experiment was to investigate the anatomical correlates of any behavioral recovery seen in rats that had one olfactory bulb removed along with an immediate transplant of a fetal olfactory bulb. Anatomical details, such as transplant organization and olfactory nerve repenetration patterns were analyzed using a variety of histological and immunohistochemical techniques. The rats in this experiment showed behavioral recovery of olfactory ability. The recovery rates observed in these animals were compared to two other groups of rats that this 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 normal levels of olfactory ability, they did start behavioral testing in a more behaviorally competent condition than rats with simple neonatal lesions. Anatomical analysis revealed that the transplanted olfactory bulb was heavily penetrated by incoming olfactory nerve fibers but olfactory nerve penetration was not limited to the transplanted olfactory bulb. The extra-bulbar host regions that were penetrated included the orbital frontal cortex and three olfaction-related areas; olfactory cortex, olfactory peduncle and the subependymal cell layer. The olfactory nerve penetration patterns observed beyond the transplant were essentially the same as those observed in rats with only neonatal lesions of the olfactory bulb. Thus, multiple pathways may have contributed to the recovery observed in the rats with olfactory bulb transplants.