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 EC₅₀ 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-Arg¹, D-Trp^7,9, Leu¹¹]SP), 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 GTPγS (100 nM) or GDPβS (100 μM) 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.