Tachykinins cause inward current through NK1 receptors in bullfrog sensory neurons

The effects of tachykinins on primary afferent neurons of bullfrog dorsal root ganglia (DRG) were examined by using whole-cell patch-clamp methods. Neurokinin A (NKA) caused inward current (I-NKA) in a concentration-dependent manner. Concentration-response curve showed that the EC(50) for NKA was 6 nM. The I-NKA showed strong tachyphylaxis, when NKA was continuously applied for more than 1 min. Substance P (SP) also produced inward current with potency similar to that of NKA. Neurokinin B (NKB) was less effective in producing the inward current. The order of agonist potency was NKA = SP >> NKB. Spantide ([D-Arg(1), D-Trp(7,9), Leu(11)]SP), a non-selective peptide antagonist at tachykinin receptors, reduced the tachykinin-induced current. CP-99,994, a selective non-peptide antagonist for neurokinin-1 (NK1) receptor, inhibited the inward currents produced by NKA and SP. The I-NKA was associated with decrease in K+ conductance. NKA suppressed both a voltage-dependent K+ current, the M-current (I-M), and a voltage-independent background K+ current, I-K(B). Intracellular dialysis with GTP gamma S (100 nM) or GDP beta S (100 mu M) depressed the I-NKA. Pre-treatment of DRG neurons with pertussis toxin (PTX) did not prevent the I-NKA. Depletion of intracellular ATP depressed the I-NKA. These results suggest that the tachykinin-induced inward current is mediated through the NK1 receptor which mainly couples to PTX-insensitive G-protein in bullfrog primary afferent neurons.