Expression and coexpression of Trk receptors in subpopulations of adult primary sensory neurons projecting to identified peripheral targets.

To determine whether neurotrophins act on functionally distinct populations of adult sensory neurons, the distributions of mRNAs for TrkA and tyrosine kinase-containing isoforms of TrkB and TrkC were determined in rat DRG neurons projecting to different peripheral targets. Whereas trkA was expressed by a very high percentage of visceral afferents, trkC was expressed frequently only in muscle afferents. Among cutaneous afferents, the size distributions for trkA- and trkC-positive cells showed little overlap. The percentages and size distributions of cells labeled for the trks argue strongly that almost all trkB-expressing cells must also express trkA or trkC. These results indicate that NGF and NT-3 act on functionally distinct populations of adult sensory neurons and suggest that a sizeable number of small DRG neurons may not respond to neurotrophins via a known Trk in the adult rat.