Sensitization of rat sensory neurons by chronic exposure to forskolin or ‘inflammatory cocktail’ does not downregulate and requires continuous exposure

Abstract

Sensitization of sensory neurons is a critical component of hypersensitivity that occurs during chronic pain and inflammation. Questions remain, however, whether this sensitization depends on the continuous activation of signal transduction pathways in sensory neurons, whether the sensitization persists once the agent causing sensitization is removed, and whether downregulation occurs in the ability of these neurons to be sensitized. Because activation of the cAMP transduction cascade produces acute sensitization in rat sensory neurons, we examined whether continuous activation of the cAMP pathway augments bradykinin-stimulated release of immunoreactive substance P and immunoreactive calcitonin gene-related peptide from embryonic rat sensory neurons grown in culture. A 20 min exposure to 1 μM forskolin enhances bradykinin-stimulated release of both peptides. This ability of forskolin to sensitize sensory neurons persists even when the neurons are exposed to forskolin for 24 h or 7 days suggesting that there is no downregulation of the response. One hour after the removal of forskolin, however, the bradykinin-evoked release returned to control values. In a similar manner, the content of immunoreactive cAMP in the cultures is elevated in cells exposed to forskolin for 20 min, even after 24 h or 7 days of forskolin treatment, but returns to control levels after forskolin removal. When sensory neurons are treated with an inflammatory cocktail for 20 min, potassium-stimulated peptide release is significantly elevated, independent of whether the cells were pre-exposed to inflammatory mediators for 24 h. Potassium-stimulated release was not elevated 1 h after removal of the inflammatory cocktail. These data suggest that rat sensory neurons are capable of undergoing a long-term sensitization that does not downregulate, but requires the continual presence of a sensitizing agent.