Expression of alpha(2)-adrenergic receptors in rat primary afferent neurones after peripheral nerve injury or inflammation

1. Immunocytochemistry with polyclonal antibodies directed against specific fragments of intracellular loops of alpha(2A)- and alpha(2C)-adrenergic re ceptors (alpha(2A)-AR, alpha(2C)-AR) was used to explore the possibility th at expression of these receptors in dorsal root ganglion (DRG) neurones of rat alters as a result of peripheral nerve injury or localized inflammation . 2. Small numbers of neurones with positive alpha(2A)-AR immunoreactivity (a lpha(2A)-AR-IR) were detected in DRG from normal animals or contra,lateral to nerve lesions. In contrast, after complete or partial sciatic nerve tran section the numbers of ipsilateral L-4 and L-5 DRG somata expressing alpha( 2A)-AR-IR sharply increased (>5-fold). There was no discernible change in t he number of DRG neurones exhibiting a,A-AR-IR innervating a region in asso ciation with localized chemically induced inflammation. 3. After nerve injury, double labelling with Fluoro-Gold, a marker of retro grade transport from transected fibres, or by immunoreactivity for c-jun pr otein, an indicator of injury and regeneration, suggested that many of the neurones expressing alpha(2A)-AR-IR were uninjured by the sciatic lesions. 4. In general the largest proportionate increase in numbers of neurones lab elled by alpha(2A)-AR-IR after nerve lesions appeared in the medium-large d iameter range (31-40 mu m), a group principally composed of cell bodies of low threshold mechanoreceptors. The number of small diameter DRG: neurones labelled by alpha(2A)-AR-IR, a category likely to include somata of nocicep tors, also increased but proportionately less. 5. Relatively few DRG neurones exhibited alpha(2C)-AR-IR; this population d id not appear to change after either nerve lesions or inflammation. 6. These observations are considered in relation to effects of nerve injury on excitation of primary afferent neurones by sympathetic activity or adre nergic agents, sympathetically related neuropathy and reports of sprouting of sympathetic fibres in DRG.