Peripheral inflammation increases the capsaicin sensitivity of dorsal rootganglion neurons in a nerve growth factor-dependent manner

Inflammation results in a local increase in nerve growth factor production which potentially can modify the properties of nerve growth factor-responsi ve sensory neurons innervating the inflamed tissue. The sensitivity of prim ary sensory neurons to the neurotoxin capsaicin is regulated in vitro by ne rve growth factor and we have now investigated the effect of complete Freun d's adjuvant-induced inflammation on the capsaicin sensitivity of adult rat sensory neurons. Dorsal root ganglion neurons innervating inflamed tissue were identified in vivo by retrograde labelling with the dye Fast Blue. Neu ronal capsaicin sensitivity was measured in vitro with a quantitative cobal t-uptake densitometric technique, and was shown to increase significantly f ive days after inflammation. This increase in sensitivity was dependent on nerve growth factor as it could be inhibited by systemic treatment with ner ve growth factor neutralizing antibodies. The enhanced capsaicin sensitivity that results from Freund's adjuvant inje ction may contribute to inflammatory hyperalgesia. (C) 1999 IBRO. Published by Elsevier Science Ltd.