Spinal synaptic mechanisms of pain relief by GABA transporter inhibitors

[Speaker] Misa Oyama:1,2
[Co-author] Shun Watanabe:1,2, Takashi Iwai:1,2, Mitsuo Tanabe:1,2
1:Laboratory of Pharmacology, School of Pharmaceutical Sciences, Kitasato University, Japan, 2:Medicinal Research Laboratories, School of Pharmacy, Kitasato University, Japan

 An excessive excitation in the superficial dorsal horn is considered to contribute to hyperalgesia and allodynia. Normalizing excitatory and inhibitory balance in the dorsal horn is a possible strategy to develop a novel therapeutic drug for chronic pain. To potentiate inhibitory influences, we have focused on GABAergic synaptic transmission, since pharmacological blockade of spinal GABA transporter subtypes 1 and 3 (GAT1 and GAT3) was reported to produce analgesic effects in animal models of neuropathic pain. In the present study, we investigated how GATs blockers affect excitatory and inhibitory synaptic transmission in the dorsal horn in slices prepared from adult mice by using whole-cell voltage-clamp technique.
 The spinal cord slice with an attached L4, L5 or L6 dorsal root was prepared from adult ddY strain male mice. Whole-cell recordings were made from dorsal horn neurons at a holding potential of -70 mV. Glutamatergic EPSCs or GABAergic IPSCs were evoked (eEPSCs or eIPSCs) by stimulating the neighboring area. Miniature EPSCs or IPSCs (mEPSCs or mIPSCs) were recorded in the presence of tetrodotoxin. To record monosynaptic A- or C- fiber-evoked EPSCs, the dorsal root was stimulated through a suction electrode.
 The GAT1 inhibitor NNC-711 suppressed the amplitude of eIPSCs and the frequency of mIPSCs. However, NNC-711 exhibited more potent inhibition of the frequency of mEPSCs potentially reflecting largely unitary synaptic events derived from spinal intrinsic interneurons, which was antagonized by the GABAB receptor antagonist CGP55845. Consistently, the analgesic effect of intrathecally injected NNC-711 in mice developing mechanical hypersensitivity after partial sciatic nerve ligation was completely abolished by intrathecal pretreatment of CGP55845. By contrast, the GAT3 inhibitor SNAP-5114 decreased the frequency of mIPSCs, reduced the amplitude of eEPSCs, and inhibited the amplitude of A- and C-fiber-evoked EPSCs. mEPSCs were not affected by SNAP.
 GAT1 inhibition appeared to produce GABAB receptor-mediated depression of excitatory neurotransmission derived mostly from spinal interneurons, while the GAT3 inhibition attenuated excitatory neurotransmission derived monosynaptically from primary afferent fibers. Together, inhibition of excitatory transmission rather than inhibitory transmission may contribute to the analgesic effects of GAT inhibitors.
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