Type III sodium channel mRNA is expressed in embryonic but not adult spinal sensory neurons, and is reexpressed following axotomy.

1. In situ hybridization with subtype-specific probes was used to ask whether there is a change in the types of sodium channels that are expressed in dorsal root ganglion (DRG) neurons after axotomy. 2. Types I and II sodium channel mRNA are expressed at moderate-to-high levels in control DRG neurons of adult rat, but type III sodium channel mRNA is not detectable. 3. When adult rat DRG neurons are examined by in situ hybridization 7-9 days following axotomy, type III sodium channel mRNA is expressed at moderate-to-high levels, in addition to types I and II mRNA that are present at relatively high levels. 4. To determine whether the expression of type III sodium channel mRNA following axotomy represents up-regulation of a gene that had been expressed at earlier developmental stages, we also studied DRG neurons from embryonic (E17) rats. In these embryonic DRG neurons, type I sodium channel mRNA is expressed at low levels, type II mRNA at high levels, and type III at high levels. 5. These results demonstrate altered expression of sodium channel mRNA in DRG neurons following axotomy, and suggest that in at least some DRG neurons, there is a de-differentiation after axotomy that includes a reversion to an embryonic mode of sodium channel expression. Different channel characteristics, as well as an altered spatial distribution of sodium channels, may contribute to the electrophysiological changes that are observed in axotomized neurons.