| Summary: | 碩士 === 長庚大學 === 生物醫學研究所 === 99 === Abstract in English
Acute pain is a normal, adaptive, and physiological response to noxious stimuli. In contrast, inflammatory or neuropathic chronic pain is a pathological condition, and currently available drugs fail to treat chronic pain effectively, suggesting that other inflammatory mediators are important algogens. Recent studies clearly indicated that during chronic pain process resulting from subcutaneous inflammation or peripheral nerve injury, expression level of chemokine ligand 2 (CCL2) is upregulated in spinal dorsal horn during chronic pain. It has also been reported that injection of CCL2 into spinal cord in normal animal induced hyperalgesia and mechanical allodynia. These results suggest that CCL2 could induce hyperalgesia and spontaneous pain of chronic pain by enhancing nociceptive transmission in spinal dorsal horn. Therefore, in the present study we hypothesized that CCL2 enhances pain transmission in the spinal dorsal horn and causes pain hypersensitivity. To test this hypothesis, CCL2 was intrathecally administered to 2-month-old rats. Intrathecal injection of CCL2 induced heat hyperalgesia. Lamina II vertical neuron is believed to be glutamatergic excitatory interneurons and send an excitatory projection to lamina I nociceptive projection neurons.Whole-cell patch-clamp recordings using spinal cord slice showed that the amplitude of evoked excitatory postsynaptic currents (EPSCs) and slope of input-output curve for evoked EPSCs were increased in lamina II vertical neurons of CCL2-treated rats. Further, intrathecal administration of CCL2 increased the frequency of spontaneous miniature EPSCs (mEPSCs) without the amplitude of mEPSCs in lamina II vertical neurons. CCL2 also inhibited the spontaneous action potential-induced inhibitory postsynaptic currents (IPSCs) and decreased the frequency of spontaneous mIPSCs in lamina II vertical neurons without affecting the amplitude of mIPSCs. We hypothesized that CCL2 enhances excitatory transmission and decreases inhibitory transmission in lamina II vertical neurons by causing microglial activation, which is believed to play an important role in mediating pain hypersensitivity in spinal dorsal horn. In accordance with this hypothesis, intrathecal injection of CCL2 induced microglial activation in spinal dorsal horn visualized by Iba1-immunofluorescence staining. Pretreatment of minocycline, which is an inhibitor of microglial activation, completely inhibited CCL2-induced heat hyperalgesia. Pretreatment of minocycline also suppressed CCL2-induced enhancement of excitatory transmission and impairment of inhibitory transmission in lamina II vertical neurons. In conclusion, our results suggest that CCL2 causes heat hyperalgesia by inducing microglial activation, which strengthens glutamatergic transmission and impairs GABAergic transmission of lamina II nociceptive vertical neurons.
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