ELECTRON-MICROSCOPY OF COLOCALIZATION OF GABA AND TYROSINE-HYDROXYLASE EXPRESSION IN RAT OLFACTORY-BULB TRANSPLANTS

Jurtaglomerular (JG) neurons of rat olfactory bulb (OB) are a subset o f inhibitory interneurons within the OB, acting via lateral inhibition to modulate the afferent input of the primary olfactory nerve. The JG neurons, composed of periglomerular, external tufted, and short axon cells, have been found to express various neurotransmitters, including gamma-amino butyric acid (GABA) and dopamine. A specific set of neuro ns within the periglomerular population have also been shown to coexpr ess these neurotransmitters. Deafferentation or functional odor depriv ation of the normal OB causes a loss of tyrosine hydroxylase (TH) (the rate limiting enzyme in the dopamine synthesis pathway) expression wi thin the JG cell population, but appears to have no effect on GABA-lev els. Our laboratory has developed a transplantation model to further s tudy the effects of deafferentation and subsequent reinnervation withi n this system. Sections from transplant (TX) OBs were reacted for GABA and TH using immuno-cytochemical localization protocols and studied b y electron microscopy. Numerous neuronal populations were found to be either TH or GABA positive in this study, with a specific subpopulatio n showing colocalization of both. Although the architecture of the TX OB is somewhat disrupted and the TH- and GABA-positive cells were not as uniform in their arrangement as they are in the normal OB, we found that these cells in the TX OB were morphologically similar to the JG; cells of normal OB. Positively labeled profiles were also found to re ceive and form numerous synaptic contacts with both host olfactory ner ve axons as well as with the processes of donor neurons. These synapti c contacts were within areas that resemble the glomeruli of normal OB, suggesting that lateral inhibition may occur within the TX OB as it d oes in the normal. The coexpression of GABA and TH within specific neu rons also indicates that a unique population of JG neurons that occur in normal OB are also found within this transplanted system as well. ( C) 1997 Academic Press.