Activation of silent mechanoreceptive cat C and A delta sensory neurons and their substance P expression following peripheral inflammation

1. The effect of inflammation on the excitability and the level of substanc e P (XP) in cat mechanoreceptive C and A delta dorsal root ganglion (DRG) n eurons were studied in vivo using intracellular recording and immunocytoche mical techniques. 2. Following injections of carrageenan (Carg) into the cat hindpaw, the per centage of C neurons exhibiting spontaneous activity increased from 7.2 to 20.7% and the percentage of AS neurons increased from 6.9 to 18.6%. In cont rast to most cells from normal, cats, which fired regularly below 10 Hz, ma ny cells from Carg-treated cats fired at higher frequencies or in bursts. 3. Inflammation (Carg treatment) also depolarized membrane potentials, incr eased membrane input resistance, caused tile disappearance of inward rectif ying currents and lowered the mean current thresholds of tibial nerve-evoke d responses in DRG neurons. 4. With inflammation, the percentage of C or A delta neurons responding to lon threshold mechanoreceptive stimuli increased (C neurons: normal, 13%; i nflamed, 41%, A delta neurons: normal, 13 %; inflamed, 39 %), while the per centage of C or A delta neurons responding to high threshold mechanorecepti ve stimuli remained unchanged. 5. Some receptive field (RF)-responsive cells were injected with Lucifer Ye llow and their XP immunoreactivity was determined. Following Carg treatment , substantially higher percentages of RF-responsive cells were XP positive (C neurons: normal, 35.7%; inflamed, 60%; A delta neurons: normal, 18.2%; i nflamed, 66.7%). 6. These combined increases in the excitability of DRG neurons and SP-conta ining RF responsive neurons could lead to sensitization of sensory neurons, thus contributing to the development of hyperalgesia.