Patch-clamp techniques and Ca 2+ imaging were used to examine the interaction between neurokinins (NK) and the capsaicin (CAPS)-evoked transient receptor potential vanilloid receptor 1 (TRPV1) responses in rat dorsal root ganglia neurons. Substance P (SP; 0.2-0.5 μM) prevented the reduction of Ca 2+ transients (tachyphylaxis) evoked by repeated brief applications of CAPS (0.5 μM). Currents elicited by CAPS were increased in amplitude and desensitized more slowly after administration of SP or a selective NK 2 agonist, [Ala 8 ]-neurokinin A (4-10) (NKA). Neither an NK 1 -selective agonist, [Sar 9 , Met 11 ]-SP, nor an NK 3 -selective agonist, [MePhe 7 ]-NKB, altered the CAPS currents. The effects of SP on CAPS currents were inhibited by a selective NK 2 antagonist, MEN 10,376, but were unaffected by the NK 3 antagonist, SB 235,375. Phorbol 12,13-dibutyrate (PDBu), an activator of protein kinase C (PKC), also increased the amplitude and slowed the desensitization of CAPS responses. Phosphatase inhibitors, decamethrin and α-naphthyl acid phosphate (NAcPh), also enhanced the currents and slowed desensitization of CAPS currents. Facilitatory effects of SP, NKA and PDBu were reversed by bisindolylmaleimide, a PKC inhibitor, and gradually decreased in magnitude when the agents were administered at increasing intervals after CAPS application. The decrease was partially prevented by prior application of NAcPh. These data suggest that activation of NK 2 receptors in afferent neurons leads to PKC-induced phosphorylation of TRPV1, resulting in sensitization of CAPS-evoked currents and slower desensitization. Thus, activation of NK 2 autoreceptors by NKs released from the peripheral afferent terminals or by mast cells during inflammatory responses may be a mechanism that sensitizes TRPV1 channels and enhances afferent excitability.