VARIATION IN I(H), I(IR), AND I(LEAK) BETWEEN ACUTELY ISOLATED ADULT-RAT DORSAL-ROOT GANGLION NEURONS OF DIFFERENT SIZE

1. The distribution of I-H,I-IR, and I-LEAK was studied in different diameter rat dorsal root ganglion( DRG) neuron cell bodies(neurons). DRG neurons were studied in three diameter ranges: small (19-27 mu m), medium(33-37 mu m), and large(44-54 mu m). I-H was defined as a slowly activating inward current evoked by hyperpolarizing voltage steps from a holding potential (HP) of -60 mV, and blocked by 1 mM Cs2+ but not 1 mM Ba2+. Inward rectifier current (I-IR) was defined as a rapidly activating current evoked by hyperpolarizations from HP -60 mV, which rectified inwardly around the reversal potential for potassium (E(K)), and was completely blocked by 100 mu M Ba2+. I-LEAK was defined as an outward resting current at HP -60 mV, which did not rectify and was blocked by 100 mu M Ba2+ but not by 2 mM Cs+. 2. I-H was observed in 23 of 23 large, 11 of 12 medium, and in 9 of 20 small diameter DRG neurons tested. Peak I-H normalized to membrane surface area was significantly greater in large than in medium or small diameter DRG neurons expressing I-H. All neurons exhibiting I-H under voltage clamp conditions had short duration action potentials and exhibited time-dependent rectification under current clamp conditions, properties similar to A-type DRG neurons. The 11 small diameter neurons not expressing I-H had long duration action potentials and did not exhibit time-dependent rectification, properties similar to C-type DRG neurons. 3. I-IR was detected in 18 of 22 medium diameter neurons tested. When modeled using a form of the Boltzmann equation, I = [G(IN)(V-E(IN))] + (G(IR,MAX)(V-E(K))/{1 + exp[(V-V-1/2)/k]}), G(IR,MAX) averaged 13.3 +/- 0.8 (SE) nS in these 18 neurons. V-1/2 averaged -96.4 +/- 2.3 (SE) mV and k averaged 17.4 +/- 1.1 (SE) mV. Inward rectification was infrequently observed in large diameter neurons (4 out of 30) or small diameter neurons ( 3 out of 20). 4. I-LEAK was detected in 15 of 28 large diameter neurons by a 0.7 +/- 0.12 (SE) nA inward shift in holding current(HP -60 mV), after superfusion with 1 mM Ba2+. The inward shift in holding current was associated with a 58 +/- 4.4% (SE) block of chord conductance (fast current) at a test potential of -110 mV. The Ba2+ sensitive fast current in 14 of 15 neurons expressing I-LEAK had a linear current-voltage (I-V) relationship throughout a series of hyperpolarizing voltage steps. The reversal potential of the Ba2+ sensitive current in these neurons averaged -78 +/- 1.4 (SE) mV (n = 10). Ba2+-sensitive I-LEAK was less frequently expressed in medium diameter neurons (2 of 12) or small diameter neurons (2 of 8). 5. In conclusion, the expression of I-H, I-IR, and I-LEAK in different diameter DRG cells varied significantly. Sensory modality is likely to vary among the three diameter groups studied. Thus the modulation of individual ion currents could result in selective filtering of different types of sensory information. The variations in ion current expression observed in acutely isolated DRG neurons can explain the differences in input resistance and time-dependent rectification observed between A- and C-type DRG neurons recorded from intracellularily in intact ganglia.