Injection of nerve growth factor (NGF) into the masseter muscle is not painful but does induce a localized, quick onset ( approximately 1h) and long-lasting mechanical sensitization in healthy human subjects. We tested the hypothesis that human NGF mechanically sensitizes masseter muscle nociceptors by increasing the sensitivity of peripheral N-methyl-d-aspartate (NMDA) receptors. Co-expression of the NR2B subunit of the NMDA receptor with P75 and TrkA NGF receptors by trigeminal ganglion neurons that innervate the masseter muscle was investigated immunohistochemically. Nociceptor activity was recorded extracellularly from the trigeminal ganglion of anaesthetized female rats. Nociceptor mechanical threshold was assessed before and every 30 min for 3h after injection of human NGF (25 microg/ml, 10 microl, n=12), and in subsequent experiments NGF with TrkA (n=12) or P75 (n=11) receptor antibodies. Glutamate (1M, 10 microl) was injected at the end of each experiment. Approximately 85% of NR2B positive masseter ganglion neurons co-expressed P75 or TrkA receptors, suggesting the potential for interaction. When compared with the vehicle control, it was found that injection of NGF into the masseter muscle did not evoke significant nociceptor discharge but did significantly reduce nociceptor mechanical threshold ( approximately 30%). There was no effect of NGF on glutamate-evoked nociceptor discharge or glutamate-induced mechanical sensitization. Additional experiments indicated that NGF-induced mechanical sensitization could be partially attenuated with TrkA receptor antibodies, but not P75 receptor antibodies. These findings indicate that human NGF-induced sensitization of masseter nociceptors results, in part, from the activation of TrkA receptors, but does not appear to be mediated through enhanced peripheral NMDA receptor activity.