Background and objective: Isoflurane has been used as an inhaled anaesthetic for nearly 30 years. Isoflurane inhalation during anaesthesia also produces an anti-nociceptive effect. Whether this occurs at the spinal or supraspinal level remains unknown. With a novel type of liquid isoflurane, the present study examined the effects of intrathecal isoflurane on the nociceptive response and Fos expression in the rat spinal cord.
Methods: Thirty-six rats were randomly assigned to three groups as follows: group A (n = 6), intrathecal physiological saline 50 μl kg⁻¹; and group B and C (n = 6 each), intrathecal isoflurane at doses of 25 μl kg⁻¹ or 50 μl kg⁻¹, respectively. Noxious thermal (Hargreaves test), mechanical (von Frey test) and chemical (formalin 5%, 50 μl) stimuli were applied to a hind paw after intrathecal isoflurane injection to study its anti-nociceptive effect. In addition, the expression of Fos protein and c-fos mRNA in the spinal dorsal horns was detected by immunohistochemistry and real-time reverse transcriptase PCR, respectively.
Results: Compared with the physiological saline control, intrathecal isoflurane significantly suppressed spontaneous paw flinches in rats induced by formalin injection and paw withdrawal induced by thermal and mechanical stimuli in a dose-dependent manner. Immunohistochemistry and real-time reverse transcriptase PCR revealed that isoflurane administration inhibited formalin injection-induced c-fos expression in the spinal cord.
Conclusions: These data suggest that isoflurane can exert anti-nociceptive effects at the spinal level by preventing neuronal activation.