Subthreshold membrane oscillations underlying integer multiples firing from injured sensory neurons

Integer multiples firing (IMF), a special temporal pattern of firing, was r ecently observed in spontaneous discharge from injured dorsal root ganglion (DRG) neurons. To investigate the mechanism underlying IMF, the injured DR G neurons of rat were recorded intracellularly. Of 64 recorded A-neurons di scharging spontaneously, eight fired spikes in the IMF pattern. Interspike interval (ISI) time series of IMF showed a structure of distinct bands on s catter map. Regular subthreshold membrane oscillations (SMOs) with relative ly stable amplitude and frequency were observed on all eight IMF neurons. I MF could be induced from the neurons in periodic firing by local applicatio n of tetrodotoxin (TTX), a Na+ channel antagonist. During this process, the amplitude of SMOs varied markedly. Some SMOs were below action potential t hreshold so that they did not trigger spikes. Nor did some SMOs, though the ir amplitude were obviously beyond the threshold measured from nearby spike s. The results indicate that regular SMOs existent in injured DRG A-neurons underlie IMF, and that TTX-induced transformation of firing pattern from p eriodic to integer multiples may occur in two ways: decreasing the amplitud e of SMOs and/or elevating action potential threshold. NeuroReport 12:1311- 1313 (C) 2001 Lippincott Williams & Wilkins.