TYROSINE-HYDROXYLASE EXPRESSION IN RAT OLFACTORY-BULB TRANSPLANTS - AN ELECTRON-MICROSCOPE STUDY

Juxtaglomerular (JG) neurons of rat olfactory bulb (OB) have been show n to express tyrosine hydroxylase (TH), the rate-limiting enzyme in th e catecholamine synthesis pathway. These JG neurons act as inhibitory dopaminergic interneurons, modulating the incoming signal from the pri mary olfactory afferents. The JG neurons, comprised of periglomerular, external tufted, and short axon cells, stop expressing TH after lesio ns of the olfactory nerve or closure of the nares, both of which cause a loss of functional input. Upon reinnervation by a continuously rege nerating olfactory nerve, these cells resume their expression of TH. I n order to study deafferentation and subsequent reinnervation within t his system, our laboratory utilizes a transplantation model. Sections from transplant (TX) OBs are reacted for TH using immunocytochemical l ocalization protocols and studied by light- and especially electron mi croscopy (EM). Autoradiography of tritiated thymidine-labeled tissue w as performed to confirm donor origin of the TX OBs. Although the archi tecture of the TX OB is somewhat disrupted and the TH-positive cells w ere not as uniform in their arrangement as they are in the normal OB, we found that the TH cells in the TX OB had a morphology similar to th e JG cells observed in normal OB. These TH cells were also found to re ceive synaptic contacts with host olfactory nerve axons as well as mak e and receive contacts with the processes of donor neurons. These syna ptic 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, p ossibly inducing them to express this enzyme. This study implies that the TX OB retains a level of plasticity that enables it to recapitulat e part of the interneuronal arrangement observed in the normal system. (C) 1996 Wiley-Liss, Inc.