A-type K+ current dominates somatic excitability of delayed firing neurons in rat substantia gelatinosa

Substantia gelatinosa neurons display three main types of intrinsic firing behavior: tonic, adapting, and delayed onset. Here, voltage‐gated currents expressed by delayed firing neurons were studied in nucleated patches obtained in spinal cord slices of 3–5 weeks‐old rats. Inward Na⁺ current was negligible under these conditions and was usually occluded by superposition of much larger outward currents. Two kinds of outward currents were found, an A‐type (K_A) and delayed rectifier (K_DR) potassium currents. K_A activated rapidly (<1.5 ms at >?20 mV) and operated at subthreshold membrane potentials; voltages of steady‐state half‐maximal activation and inactivation were ?38.7 and ?87.2 mV, respectively. Inactivation was biexponential with a dominant fast component (～90%, time constant ～8 ms). K_DR activated more slowly (<8 ms at >?20 mV), half‐maximal activation was ?23.6 mV, and decayed mono‐exponentially with a time constant 70–110 ms. Maximal amplitudes of K_A were almost 10‐times larger than those of K_DR, their respective densities were 8.5 and 0.97 μS μm^?2. Tetraethylammonium, 5 mM, blocked K_DR but not K_A, whereas both currents were depressed by 5 mM 4‐aminopyridine. In current‐clamp recordings, 4‐action potential but not tetraethylammonium abolished firing delay suggesting the causative role of K_A. Thus, the predominance of fast K_A over other somatic currents is a distinctive feature of delayed firing neurons among all other types of substantia gelatinosa neurons and likely explains the appearance of their typical firing delay. Synapse, 2011. © 2010 Wiley‐Liss, Inc.