Increase in spontaneous action potentials and sensitivity in response to norepinephrine in dorsal root ganglion neurons of adjuvant inflamed rats.

To gain an understanding of the cellular mechanisms of hyperalgesia and spontaneous pain in adjuvant-induced chronic inflammation, we investigated the effects of nerve growth factor (NGF), which is known to increase in inflamed tissues and to cause hyperalgesia, on the spontaneous activities and norepinephrine-induced excitation of dorsal root ganglion (DRG) neurons. Intracellular recordings were obtained from freshly dissociated and cultured DRG neurons (<30 microm) from intact and adjuvant inflamed (AI) rats. Of more than 100 freshly dissociated DRG neurons from the intact rats, none produced spontaneous action potentials, whereas 23% of the neurons from the AI rats did. Spontaneous activities were induced in 34% neurons from intact rats when cultivated for one day with NGF. No neurons from the intact rats responded to norepinephrine (NE), irrespective of whether they were freshly dissociated or cultured with NGF. In contrast, 11% of neurons from the AI rats, both freshly dissociated and cultured without NGF, had a small depolarization in response to NE. The present results suggest that, in AI rats NGF plays an important role in inducing spontaneous activities in DRG neurons, but not in inducing sensitivity to NE.