Cyclic AMP mediates mu and delta, but not kappa opioid analgesia in the spinal cord of the rat.

Intrathecal (i.t.) injection of the membrane-permeable cyclic AMP (cAMP) analogue dibutyryl-cyclic AMP (Bt2cAMP) in 3 successive doses of 12.5, 12.5 and 25 ug in rats was very effective in reversing the antinociceptive effects produced by mu agonist morphine or delta agonist DPDPE, but not that by kappa agonist dynorphin A-(1-13). cAMP content of the spinal cord was significantly decreased by morphine, but not by dynorphin A-(1-13). The results imply that a decrease in spinal cAMP content may be important for the antinociceptive effect elicited by mu and delta, but not kappa opioid receptor agonists.