Is cholecystokinin octapeptide (CCK-8) a candidate for endogenous anti-opioid substrates?

Cholecystokinin octapeptide (CCK-8), given intracerebroventricularly (icv) or intrathecally (ith) at the dose range of 0.25-4.0 ng, dose-dependently antagonised the effect of morphine analgesia and electroacupuncture analgesia (EAA) in the rat. That CCK-8 antiserum was capable of reversing the tolerance to EAA and changing the non-responders of EAA into responders suggest CCK-8 to be the endogenous anti-opioid substrate and that blocking the effect of CCK-8 may prove to be a powerful way of augmenting the effect of morphine analgesia and EA analgesia.