Effect Of Enkephalinergic Neurons In Modulation Of Pain In Inhalation Anesthesia And Stress

One of the clinical symptoms that disturbs doctors is pain, whose mechanisms is not very clear now. Many studies indicated that a lot of structures in central nervous system including cortex, hippocampus, PAG, RVM, spinal cord, and various neurotransmitters played part of role in mediation of pain. However, the effect of enkephalinergic neurons in the medication of pain in anesthesia and stress is still unknown.Objective: To investigate the relationship between anesthesia and forced swimming induced analgesia and enkephalinergic neurons in the spinal cord or PAG in PPE-GFP transgenic mice.Methods:Experiment 1: After mice received inhalation or intrathecal administration of sevoflurane, formalin test was taken and FOS immunofluorescence in spinal cord was used as a marker to evaluate the antinociception. Double-labeling of immunofluorescence with GFP and FOS in spinal cord sections was analyzed to determine the status of the enkephalinergic neurons. Non-selective opioid receptor antagonist naloxone administrated intraperitoneally or intrathecally to identify the opioid system mediating sevoflurane antinociception.Experiment 2: Twelve male preproenkephalin-green fluorescent protein (PPE-GFP) transgenic mice were randomly divided into two groups: control and stress groups. Hot plate latency was measured using a hot plate analgesia instrument. Double-labeling of immunofluorescent staining was performed to observe the distribution and change in number of Fos-positive neurons and Fos/GFP double-labeled neurons in PAG.Results:Experiment 1: Effect of enkephalinergic neurons in spinal cord in antinociception in inhalation and intrathecal administration of sevoflurane1. Sevoflurane inhalation anesthesia both suppress the biphasic flinching behaviour and FOS expression in spinal cord induced by formalin injection. The different concentration of sevoflurane differs in the effect.2. Enkephalinergic interneurons were activated by inhalation of sevoflurane. The mean immunofluorescence intensity of spinal cord sections was increased by the treatment of sevoflurane inhalation. When effect of enkephalin in spinal cord was blocked by naloxone, the formalin score could be increased.3. Intrathecal injection of sevoflurane could decrease the score of formalin test, but naloxone could not reverse the antinociceptive effect of sevoflurane in the formalin test. However, intrathecal injection of sevoflurane caused neither change in immunofluorescence of double-labeled PPE-GFP and FOS, nor change in immunofluorescence density of PPE-GFP.Experiment 2: Relationship between forced swimming induced analgesia and enekpahlinergic neurons1. Hot plate latency increased to the peak value 10 min after the stress, then decreased at about 30 min, and decreased to the basic line level before stress at 60 min.2. Immunofluorescent staining revealed that Fos-positive neurons and Fos/GFP double-labeled neurons in PAG increased 2 h after the stress, which located mainly in ventrolateral PAG.Conclusion:1. Enkephalins in spinal cord released from enkephalinergic neurons play part of role in mediating antinociception of inhalation sevoflurane, which might be induced at supraspinal level through descending system, not induced directly by sevoflurane at spinal level.2. Forced swimming could induce analgesic effect in a short time, which may be related to activation of enkephalinergic neurons in ventrolateral PAG.