Factors influencing the turnover of olfactory receptor neurons

Abstract

The olfactory system has a striking capacity for neuronal plasticity throughout life and the olfactory epithelium is able to generate both neuronal and non-neuronal cells in the normal adult animal as well as after experimental injury. In contrast to primary sensory neurons, olfactory axons are able to cross the border between the PNS and CNS in the olfactory bulb after transection in the adult animal. The knowledge about factors involved in neuronal regeneration in the primary olfactory system could presumably be of help to understand why neuronal regeneration in general does not occur in the central nervous system and how it could be stimulated. The present study aimed to shed light on the complex pattern of factors that contribute to neuronal turnover in the primary olfactory system.

To investigate if olfactory neurons die by necrosis or apoptosis after axotomy, the TUNEL-technique was used to detect DNA-fragmentation in vivo. By combining TUNEL-technique and immunohistochemistry the occurrence of apoptotic cell death of both immature and mature olfactory receptor neurons in the adult, uninjured rat olfactory epithelium could be demonstrated. The number of TUNEL-labeled neurons was significantly increased 1.5 days after transection of the olfactory axons. Since nerve growth factor withdrawal can induce apoptosis these data indicate that olfactory receptor neurons might be dependent on neurotrophins. To clarify if olfactory receptor neurons can bind and be influenced by neurotrophins we studied the expression of trkA, trkB and trkC mRNA and trkB protein in the primary olfactory system. The olfactory receptor neurons were found to express tyrosine kinase trkB receptors, while BDNF mRNA is expressed in their target cells in the olfactory bulb, indicating that BDNF might be a target derived factor involved in differentiation and/or survival of olfactory receptor neurons.

To gain insight in how the adhesion modulating molecule tenascin might influence neuronal regeneration, the expression patterns of tenascin-C, tenascin-X, tenascin-R and tenascin/J1 mRNAs were analyzed in the perinatal and adult rat spinal cord and olfactory system, as well as after lesions in the ventral funiculus of the spinal cord. Tenascin-C, -R and /J1 were differentially expressed during the perinatal period and similarities in the general tenascin expression patterns were seen in the areas studied. In the adult rat tenascin/J1 was the only extensively expressed tenascin. All tenascin forms were up regulated in the scar tissue after penetrating lesions in the ventral funiculus of the spinal cord. Thus tenascin expression in the adult primary olfactory pathway could not be correlated to tenascin expression in the scar tissue after spinal cord lesion.

VEGF is an angiogenetic factor with capability to increase vascular permeability. To increase the knowledge about the vascular supply in the primary olfactory system the distribution of VEGF and VEGF-receptors in the perinatal and adult rat olfactory epithelium and bulb was studied with immunohistochemistry and in situ hybridization. Differential expression of VEGF, VEGF receptor-1, -2 and neuropilin-1 was seen during perinatal development. Persistent expression of VEGF and VEGF receptor-1 were seen in the adult rat. To understand if VEGF in the adult rat act to modulate vascular permeability an additional study investigating to what extent the olfactory axons are exposed to blood-borne macromolecules was performed. The distribution of HRP and alpha-toxin from staphylococcus aureus in primary olfactory pathway was studied after intra vascular injections. The results showed that olfactory axons seem to be exposed to macromolecules in the blood. The morphology of the blood vessels in the same region was also studied by immunohistochemistry, bright field and electron microscopy.