Differential expression of mRNAs for the NGF family of neurotrophic factors in the adult rat central olfactory system.

The cellular localization of mRNAs for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT3), in the rat central olfactory system was evaluated with in situ hybridization of 35S-labeled cRNA probes. In the main olfactory bulb, low levels of NGF and BDNF mRNA expression were detected. NGF mRNA was restricted to the glomerular region while BDNF mRNA was predominantly localized to the granule cell layer. No cellular hybridization to NT3 cRNA was seen. The accessory olfactory bulb did not express detectable levels of mRNA for any of the three related neurotrophic factors. Areas which receive olfactory bulb afferents expressed comparatively high levels of both NGF and BDNF mRNA. Cell labeling with cRNAs for NGF and BDNF occurred throughout the cellular layers of the anterior olfactory nucleus and in layers 2 and 3 of rostral piriform cortex. BDNF mRNA expression in these areas appeared more robust than that of NGF mRNA, while NT3 mRNA was not detectable. In contrast, tenia tecta exhibited dense labeling with the cRNAs for all three neurotrophic factors. The localization of NGF mRNA to primary target neurons of the olfactory nerve in the periglomerular region of the main olfactory bulb suggests that bulb cells may influence the ingrowth and continual turnover of olfactory sensory afferents. However, as there is a strong correlation between the distribution of neurotrophic factor mRNAs within rostral olfactory structures and the distribution of centrifugal cholinergic afferents, it is more likely that bulb-derived NGF, and possibly BDNF, act on the cholinergic neurons of the basal forebrain.