Subtypes of dorsal root ganglion neurons based on different inward currents as measured by whole-cell voltage clamp

Electrophysiological and pharmacological properties distinguished subtypes of adult mammalian dorsal root ganglion neurons (DRGn) in monolayer dissociated cell culture. By analogy of action potential waveform and duration, neurons with short duration (SDn) and long duration (LDn) action potentials resembled functionally distinct subtypes of DRGn in intact ganglia. Patch clamp and conventional intracellular recording techniques were combined here to elucidate differences in the ionic basis of excitability of subtypes of DRGn in vitro. Both SDn and LDn were quiescent at the resting potential. Action potentials of SDn were brief (less than 2 msec), sensitive to tetrodotoxin (TTX, 5-10 nM), exhibited damped firing during long depolarizations, and did not respond to algesic agents applied by pressure ejection. Action potentials of LDn were 2-6 msec in duration, persisted in 30 microM TTX, and fired repetitively during depolarizing current pulses or exposure to algesic agents (e.g., capsaicin, histamine and bradykinin). Whole-cell recordings from freshly dissociated neurons revealed two inward sodium currents (INa; variable with changes in sodium but not calcium concentration in the superfusate) in various proportions: a rapidly activating and inactivating, TTX-sensitive current; and, a slower, TTX (30 microM)-resistant INa. Large neurons, presumable SDn, had predominantly TTX-sensitive current and little TTX-resistant current. The predominant inward current of small neurons, presumably LDn, was TTX-resistant with a smaller TTX-sensitive component. By analogy to findings from intact ganglia, these results suggest that fundamentally different ionic currents controlling excitability of subtypes of DRGn in vitro may contribute to functional differences between subtypes of neurons in situ.