Central administration of the neuropeptide neurotensin (NT) was shown to induce antinociceptive responses both spinally and supraspinally. Although NTS2 receptors play an important role in modulating the activity of spinal neurons, we have recently implicated NTS1 receptors in NT's analgesic effects in acute spinal pain paradigms. The current experiments were thus designed to examine the antinociceptive effects of intrathecal administration of NTS1 agonists in formalin-induced tonic pain in rats. We first established, using immunoblotting and immunohistochemical approaches, that NTS1 receptors were present in small- and medium-sized dorsal root ganglion cells and localized in the superficial layers of the dorsal horn of the spinal cord. We then examined the effects of intrathecal injection of NT (1-15 microg/kg) or NTS1 preferring agonists on the nocifensive response to intraplantar formalin. Both NTS1-agonists, PD149163 (10-120 microg/kg) and NT69L (1-100 microg/kg), dose-dependently attenuated the formalin-induced behaviors. Accordingly, NTS1 agonists markedly suppressed pain-evoked c-fos expression in the superficial, nucleus proprius and neck regions of the spinal dorsal horn. The concomitant administration of PD149163 with the NTS1 antagonist SR48692 (3 microg/kg) significantly reversed PD149163-induced antinociception, confirming the implication of NTS1 in tonic pain. In contrast, NT69L's analgesic effects were partly abolished by co-administration of SR48692, indicating that NT69L-induced effects may also be exerted through interaction with NTS2. These results demonstrate that NTS1 receptors play a key role in the mediation of the analgesic effects of NT in persistent pain and suggest that NTS1-selective agonists may represent a new line of analgesic compounds.