Background: N-methyl-D-aspartate (NDMA) antagonists have minimal effects on acute nociception but block facilitated states of processing. In contrast, the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) antagonists decrease acute noxious responses. Morphine (a mu-opioid agonist) can also decrease acute nociceptive processing. The authors hypothesized that the interaction between morphine and AMPA receptor antagonists would be synergistic, whereas morphine and NMDA antagonists show no such interaction in acute nociception.
Methods: Sprague-Dawley rats (weight, 250-300 g) were implanted with chronic lumbar intrathecal catheters and were assigned to receive one of several doses of morphine--ACEA 1021 (NMDA glycine site antagonist), ACEA 2085 (AMPA antagonist), AP-5 (NMDA antagonist), saline or vehicle--and were tested for their effect on the response latency using a 52.5 degrees C hot plate. The combinations of morphine and other agents also were tested.
Results: Intrathecal morphine (ED50:2 microg/95% confidence interval, 1-4 microg) and ACEA 2085 (6 ng/2-15 ng), but not AP-5 or ACEA 1021, yielded a dose-dependent increase in the thermal escape latency. A systematic isobolographic analysis was carried out between intrathecal morphine and ACEA 2085 using the ED50 dose ratio of 357:1. A potent synergy was observed with decreased side effects. Morphine dose-response curves were carried out for morphine and fixed doses of ACEA 1021 (12 microg) or AP-5 (10 microg). No synergistic interactions were noted.
Conclusions: Spinal mu-receptor activation and AMPA receptor antagonism showed a synergistic antinociception in response to an acute thermal stimulus. NMDA or NMDA glycine site antagonism had no effect alone nor did they display synergy with morphine. These results suggest an important direction for development of acute pain strategies may focus on the AMPA receptor.