en Pharmacology Pharmacology Original Article Original Article Background.&nbsp; Many mechanisms are involved in pain transmission in the spinal cord.&nbsp; Therefore, combination of drugs acting on different kinds of mechanisms might be useful for analgesia.&nbsp; We investigated the interaction between&gamma;-aminobutyric acid （GABA）_B receptor agonist, baclofen, and N-methyl-D-aspartate （NMDA） receptor antagonist, AP-5, or&alpha;-amino-3-hydroxy-5-methylisoxazole-4-propionic acid （AMPA） receptor antagonist, YM-872, on analgesic effects in acute thermal and formalin induced pain models of rats.<br> Methods<em>.</em>&nbsp; Male Sprague-Dawley rats implanted with lumbar intrathecal catheters were given intrathecal baclofen, AP-5, YM872 or combination of baclofen and AP-5 or YM872, then tail flick test or formalin test was performed.&nbsp; Fifty % effective doses of combinations were obtained and isobolographic analysis was done.<br> Results.&nbsp; The combination of baclofen and AP-5 or YM872 showed dose dependent increases in tail flick latency and decreases in flinching response in the formalin test.&nbsp; In the tail flick test, ED50s of the combination of baclofen + AP-5 or YM872 were close to the additive points.&nbsp; In both phases of the formalin test, ED50s of the combination of baclofen + AP-5 or YM872 were significantly lower than additive points.&nbsp;<br> Conclusions<em>.</em>&nbsp; For acute thermal pain, both AP-5 and YM872 had similar additive analgesic effects with baclofen.&nbsp; For chemical induced acute pain, both AP-5 and YM872 had similar synergistic analgesic effects with baclofen, but for facilitated pain, YM872 had stronger synergistic analgesic effects with baclofen than AP-5. Analgesia, GABAB receptor, NMDA receptor, AMPA receptor, Spinal cord 0 0 http://ibbj.org/browse.php?a_code=A-10-353-2&slc_lang=en&sid=1 Tomoki Nishiyama nishit-tky@umin.ac.jp 10031947532846003738 10031947532846003738 Yes Maruyama Memorial General Hospital