Axotomy results in major changes in BDNF expression by dorsal root ganglion cells: BDNF expression in large trkB and trkC cells, in pericellular baskets, and in projections to deep dorsal horn and dorsal column nuclei

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Abstract
Brain derived neurotrophic factor (BDNF) is normally expressed by a small number of predominantly trkA‐expressing dorsal root ganglion cells. Using immunocytochemistry and in situ hybridization, we have examined the effect of sciatic nerve section on the expression of BDNF in the adult rat. Following axotomy there was a long lasting (4‐week) increase in BDNF mRNA and protein in large‐diameter, trkB‐ and trkC‐expressing dorsal root ganglion cells. By 2 days postaxotomy, expression of BDNF mRNA had increased from 2% of trkB cells to 50%, and from 18% of trkC cells to 56%. In contrast, BDNF expression in most trkA cells was unchanged, although was increased in the small population of medium‐ and large‐sized trkA cells. Following axotomy, BDNF‐immunoreactive terminals appeared in the central axonal projections of large‐diameter cells, including the deep dorsal horn and gracile nucleus. Neuropeptide Y was also upregulated following axotomy and was coexpressed with BDNF in the cell bodies and central terminals of the large cells. Ultrastructural analysis in lamina IV of the spinal cord revealed that BDNF terminals in these central projections establish synaptic contacts. Immunoreactivity at 4 weeks was also observed in pericellular baskets that contained calcitonin gene‐related peptide (CGRP) and surrounded trkA‐ and trkB‐expressing cells in L4 and L5 lumbar ganglia. These baskets are likely to arise from local, highly immunoreactive, BDNF/CGRP/trkA‐expressing cells. Our results identify several novel targets for BDNF and imply that it acts locally in both autocrine and paracrine modes, as well as centrally in a synaptic mode, to modulate the response of somatosensory pathways in nerve injury.

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