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Neurotrophin receptor mRNA expression defines distinct populations of neurons in rat dorsal root ganglia

The biological actions of neurotrophins are mediated by specific neurotrophin receptor tyrosine kinases (Trks). A low‐affinity nerve growth factor (NGF) receptor, p75, appears to modulate sensitivity to neurotrophins in some neuronal populations. It has been recently demonstrated that genes encoding members of the Trk family are expressed in distinct patterns in the dorsal root ganglia DRG; Mu et al. [1993] (J. Neurosci. 13:4029–4041). However, the extent to which different neurotrophin receptor genes are coexpressed by individual DRG neurons is unknown. The question of coexpression is important since the expression of more than one member of the trk family by DRG neurons would suggest the potential for regulation by multiple neurotrophins. To address this question, a combination of isotopic and colorimetric in situ hybridization was performed on rat thoracic DRG using riboprobes specific for trk A, trkB, trkC, and p75. We show here that neurons that express trkA are largely distinct from those that express trkC, although there is a small subpopulation that expresses both of these genes. We also show that there is a distinct population of DRG neurons that expresses trkB and does not coexpress either trkA or trkC. P75 is expressed in almost all neurons that express trkA or trkB, but is coexpress in only 50% of trkC‐expressing neurons. Importantly, p75 is not expressed in DRG neurons independent of trk expression. Finally, a subpopulation of DRG neurons does not express any of the neurotrophin receptor mRNAs.