Procaine excites nociceptors in cultures from dorsal root ganglion of the rat

Abstract

Procaine, a classical local anesthetic, produces, at low concentration (2–200 μM), excitation in a distinct population of small sensory neurons isolated from newborn rats (2D) and cultured for 1–5 days. The excitation or inward current (>50 pA) induced by procaine was observed in 59 out of 78 neurons. Nearly all these procaine-sensitive neurons (56 of 59) were also sensitive to capsaicin while 8 procaine-insensitive neurons responded to capsaicin (1 μM). In procaine-sensitive neurons tested for responsiveness to noxious heat, a 10 s temperature ramp from 24 to 48°C induced an inward current of 413±47 pA (SEM, n=27) and this current was enhanced, in the presence of procaine, about 3-fold (2.8±0.4, SEM, n=27). The responses to procaine were concentration dependent and underwent pronounced tachyphylaxis after repeated applications. The voltage-current relationship exhibited outward rectification and the apparent reversal at 25±4.2 mV (SEM, n=9) suggesting that the current is carried by cations including Ca 2+. This procaine effect may offer an explanation for toxic consequence of the clinical use of local anesthetics.