[Spinal analgesic mechanism of minocycline in formalin-induced inflammatory pain]

Objective: To investigate the spinal analgesic mechanism of minocycline in formalin-induced inflammatory pain. Methods: Behavioral test: Male Sprague-Dawley rats(3-5-week old) were randomly assigned into four groups: control, model, vehicle-controlled and minocycline group. Ten percent neutral formalin was injected subcutaneously into the right hind paw dorsum of the rats in model, vehicle-controlled and minocycline group. Normal saline was injected subcutaneously into the right hind paw dorsum of the rats in control group. Before 1 h of formalin injection, the rats in vehicle-controlled and minocycline group received intraperitoneal injection of saline and minocycline, respectively. Licking and lifting time was observed as the behavior results of inflammatory pain. Electrophysiologic experiment: In vitro spinal cord parasagittal slices were prepared from the same rats as above. The effect of minocycline on spontaneous inhibitory postsynaptic currents(sIPSCs) of substantia gelatinosa(SG) neurons was observed using whole-cell patch-clamp technique. Results: Compared with the control group, the licking and lifting time in the model group was significantly increased. Compared with the vehicle-controlled group, the licking and lifting time in the minocycline group was significantly decreased. Minocycline significantly increased the frequency(t=9.32, P<0.05)but not the amplitude(t=1.54, P>0.05) of sIPSCs of SG neurons, the frequency of sIPSCs of control and minocycline group were (2.5±0.3)Hz and (5.2±0.6)Hz, respectively. When calcium was removed from the extracellular solution, the frequency before and after minocycline perfusion were (0.9±0.1)Hz and (0.9±0.1)Hz, respectively, the amplitude before and after minocycline perfusion were (18.2±0.7)pA and (18.5± 0.6)pA, respectively, the difference of frequency or amplitude was not statistically significant(t=0.32, 0.82, all P>0.05). However, minocycline still increased the frequency of sIPSCs when glutamate receptor antagonists 6-Cyano-7-nitroquinoxaline-2, 3-dione(CNQX) and D-(-)-2-Amino-5-phosphonopentanoic acid(APV) were included in extracellular solution(t=13.51, P<0.05), the frequency of sIPSCs were (2.0±0.1)Hz and (4.3±0.4)Hz, respectively. Minocycline still increased the frequency of IPSCs when voltage-gated sodium channel blocker tetrodotoxin(TTX) were included in extracellular solution(t=8.67, P<0.05), the frequency of IPSCs were (2.2±0.2)Hz and (5.2±0.5)Hz. Conclusion: Minocycline can attenuate formalin-induced inflammatory pain which may be associated with its increase in the inhibitory synaptic transmission of SG neurons.