Neurokinin A depolarizes neurons of bullfrog dorsal root ganglia by suppressing K+ conductances.

The effect of neurokinin A (NKA) on neurons of bullfrog dorsal root ganglia (DRG) in primary culture was examined by using whole-cell patch-clamp methods. Application of NKA (1 microM) depolarized the DRG neurons, resulting in spontaneous firing of action potentials. Under voltage-clamp condition, NKA (3 nM-1 microM) caused an inward current (INKA) associated with decreased membrane conductance. The INKA reversed its polarity at the equilibrium potential for K+. The INKA was blocked by extracellular Ba2+ (1 mM) but not by nominally 0 mM Ca2+, tetraethylammonium (40 mM), 4-aminopyridine (2 mM) or apamin (50 nM). Intracellular Cs+ blocked the INKA. NKA depressed a voltage-dependent non-inactivating K+ current, the M-current (IM), at potentials more positive than -55mV. NKA reduced the maximum M-conductance (GM) without changing the kinetics of M-channels. NKA also depressed a voltage- and time-independent background K+ current, IK(B). It is concluded that the INKA is produced by suppression of both IM and IK(B) in bullfrog primary afferent neurons.