Effect of endomorphin-2 on orofacial pain induced by orthodontic tooth movement in rats.

Endomorphin-2 demonstrates potent antinociceptive effects in various pain models. The objectives of the present study were to explore the role of endomorphin-2 in the modulation of orofacial pain induced by orthodontic tooth movement in rats. An orthodontic pain model was established in male Sprague-Dawley rats by ligating coiled springs to mimic orthodontic force (40 g). On days 0, 1, 3, 5, 7, and 14 following orthodontic tooth movement, bite force was recorded as a surrogate measure of orthodontic pain. Ipsilateral trigeminal ganglia, trigeminal nucleus caudalis, and periodontal tissues were harvested for immunostaining. Endomorphin-2, endomorphin-2 + naloxone (a non-selective opioid receptor antagonist), naloxone, and saline were injected into trigeminal ganglia and periodontal tissues to explore the role of endomorphin-2 on orthodontic pain. The results showed that following orthodontic tooth movement, endomorphin-2 expression levels in trigeminal ganglia were elevated on days 1, 3, 5, and 7. Orthodontic pain levels were increased on days 1, 3, and 5. The administration of endomorphin-2 into both trigeminal ganglia and periodontal tissues alleviated orthodontic pain. Moreover, the effects of endomorphin-2 could be blocked by naloxone completely in trigeminal ganglia but only partially in periodontal tissues. Therefore, endomorphin-2 plays an important role in the modulation of orthodontic pain both centrally and peripherally, probably through different pathways.