Rapid sodium channel augmentation in response to inflammation induced by complete Freund's adjuvant

The mechanisms by which inflammation induces a chronic pain state are poorly understood. Following the induction of many painful conditions, an increase in the spontaneous firing rate of neurons is often observed in peripheral sensory ganglia. Since ion channels are essential mediators of spike generation and impulse conduction, it is reasonable to postulate that local changes in ion channel expression might underlie the changes in membrane excitability. Such alterations may serve to enhance the efficiency by which painful stimuli are transduced and then conducted to the central nervous system. In these studies, we employed immunocytochemical methods to investigate the changes in sodium channel expression in dorsal root ganglia of rats following a subcutaneous injection of complete Freund's adjuvant, an inducer of chronic inflammation. We find that sodium channel immunoreactivity within primary sensory neurons is dramatically increased within 24 h of the complete Freund's adjuvant injection. These changes persist in small neurons for at least 2 months and roughly parallel the time course of behaviorally measured changes in pain thresholds. Thus, the regulation of sodium channel synthesis may play a role in the generation and maintenance of the hyperesthetic state seen in chronic inflammation.