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