Duration of the preemptive analgesic effects of low- and high-frequency transcutaneous electrical nerve stimulation in rats with acute inflammatory pain.

Transcutaneous electrical nerve stimulation (TENS) activates various pathways to induce antinociceptive effects, based on the frequencies used. This study evaluates the preemptive analgesic effects and their duration of low- (LT: 4 Hz) and high-frequency TENS (HT: 100 Hz) using a rat model of acute inflammatory pain. Acute inflammation was induced by injecting 1% formalin into the hind paws of rats. LT or HT was applied for 30 min before formalin injection. Pain-related behaviors, such as licking, flinching, and lifting, were recorded for 60 min postinjection. Immunohistochemistry was used to assess the number of phosphorylated extracellular signal-regulated kinase (pERK)- and c-fos-positive cells in the spinal cord. Naloxone, a μ-opioid receptors (MORs) antagonist, and naltrindole, a δ-opioid receptors (DORs) antagonist, were administered before TENS application. Pain behavior duration and pERK- and c-fos-positive cell expression were then measured. LT and HT pretreatment significantly reduced both pain behaviors and the number of pERK- and c-fos-positive cells postformalin injection. Naloxone and naltrindole partially reversed the effects of LT and HT, respectively. Notably, HT's analgesic effect lasted up to 120 min whereas that of LT persisted for 90 min. LT and HT effectively exerted their preemptive analgesic effects on acute inflammatory pain by inhibiting pERK and c-fos expression in the spinal cord. HT presented a longer-lasting effect compared to LT. MOR and DOR activation may contribute to LT and HT's analgesic mechanisms, respectively.