Intrathecal eugenol administration alleviates neuropathic pain in male Sprague-Dawley rats

The main objective of this study was to determine the central effect of eugenol on neuropathic pain when injected intrathecally at the level of the lumbar spinal cord. In a preliminary study the penetrability of eugenol was evaluated in the CNS of rats. Blood, brain and spinal cord samples were collected at selected time points following eugenol administration and concentrations were determined by tandem liquid chromatography-mass spectrometry. Brain-to-plasma and spinal cord-to-plasma ratios (3.3 and 6.7, respectively) suggest that eugenol penetrates relatively well the CNS of rats, with a preferential distribution in the spinal cord. Following the induction of neuropathic pain in rats using the sciatic nerve ligation model, intrathecal injections of eugenol were done to evaluate the central effect of eugenol. Treatment with 50 μg of eugenol significantly decreased secondary mechanical allodynia after 15 min, 2 h and 4 h (p < 0.05; <0.005; <0.05, respectively) and improved thermal hyperalgesia after 2 h and 4 h (p < 0.001 and p < 0.05). The results support the hypothesis that eugenol may alleviate neuropathic pain, both allodynia and hyperalgesia, by acting centrally most probably at the level of the dorsal horn of the spinal cord where vanilloid receptors can be found.