Effects of intracisternal injection of interleukin-6 on nociceptive jaw opening reflex and orofacial formalin test in freely moving rats

The present study was performed to investigate the effects of central cytokines on the modulation of nociception in the orofacial area. To achieve this purpose, a nociceptive jaw opening reflex and an orofacial formalin test were monitored before and after intracisternal administration of interleukin-6 (IL-6) in freely moving rats. In the nociceptive jaw opening reflex, the digastric electromyogram (dEMG) was not significantly changed after intracisternal injection of 200pg and 2ng IL-6. However, 20ng IL-6 suppressed dEMG to 74±7% of the control values. In the inflammatory orofacial formalin test, intracisternal injection of 200pg and 2ng IL-6 did not change the number of noxious behavioral responses produced by formalin injection. However, 20ng IL-6 injected intracisternally significantly increased the number of noxious behavioral responses produced by formalin. The hyperalgesic action of intracisternal IL-6 in the orofacial formalin test was blocked by pretreatment with interleukin-1 (IL-1) receptor antagonist. These results suggest that IL-6 injected intracisternally modulates the transmission of nociceptive information in the orofacial area. However, the hypo/hyper-algesic response of central cytokines seems to depend on the orofacial pain model. The hyperalgesic response of central IL-6 seems to be mediated by the IL-1 receptor.