Dual allosteric modulation of opioid antinociceptive potency by α_2A-adrenoceptors

Abstract Opioid and α₂-adrenoceptor (AR) agonists are analgesic when administered in the spinal cord and show a clinically beneficial synergistic interaction when co-administered. However, α₂-AR antagonists can also inhibit opioid antinociception, suggesting a complex interaction between the two systems. The α_2A-AR subtype is necessary for spinal adrenergic analgesia and synergy with opioids for most agonist combinations. Therefore, we investigated whether spinal opioid antinociception and opioid-adrenergic synergy were under allosteric control of the α_2A-AR. Drugs were administered intrathecally in wild type (WT) and α_2A-knock-out (KO) mice and antinociception was measured using the hot water tail immersion or substance P behavioral assays. The α_2A-AR agonist clonidine was less effective in α_2A-KO mice in both assays. The absence of the α_2A-AR resulted in 10-70-fold increases in the antinociceptive potency of the opioid agonists morphine and DeltII. In contrast, neither morphine nor DeltII synergized with clonidine in α_2A-KO mice, indicating that the α_2AAR has both positive and negative modulatory effects on opioid antinociception. Depletion of descending adrenergic terminals with 6-OHDA resulted in a significant decrease in morphine efficacy in WT but not in α_2A-KO mice, suggesting that endogenous norepinephrine acts through the α_2A-AR to facilitate morphine antinociception. Based on these findings, we propose a model whereby ligand-occupied versus ligand-free α_2A-AR produce distinct patterns of modulation of opioid receptor activation. In this model, agonist-occupied α_2A-ARs potentiate opioid analgesia, while non-occupied α_2A-ARs inhibit opioid analgesia. Exploiting such interactions between the two receptors could lead to the development of better pharmacological treatments for pain management.