The electrophysiological characters of hyerpolarization activated current (Ih) and its distribution in the cell bodies of rat dorsal root ganglion (DRG) neuron s with different diameters were studied by whole cell voltage and current cla mp methods. DRG neurons were classified into three group: small (diameter<30 μm )

median (30 μm ≤ d ≤ 40 μm ) and large (>40 μm) types. Ih was found as a slowly activating inward current evoked by hyperpolarizing voltage steps from a holding potential of -60 mV

and reversed at -32.34±2.56 mV(n=4). Ih could be specifically blocked by 2.0 mM Cs 2+ but not 1.0 mM Ba 2+

furthermore

100 μM cAMP applied into pipette solution could shift the activa tion curve to positive di rection (-91.4 mV vs -100.6 mV in control). Ih was observed in 11 of 49 small

17 of 22 median and 7 of 7 large diameter DRG neurons tested. The Density of Ih current in median and large cells was significantly higher than those in small o nes (11.88±1.59 pA/pF

5.05±0.39 pA/pF and 2.59±0.20 pA/pF

respectively). Ac tion potential at half amplitude (APD 1/2 ) varied in different types of DR G neurons. It was shown that large diameter cells had the shortest APD 1/2 than the other two types. These data suggested that Ih was mainly distributed in large and median diameter DRG neurons. Because some of Ih expressed neurons a re nociceptors

so it suggests that Ih may play a role in chronic pain.