An assessment of nalorphine's relative opiod efficacy within a cholecystokinin drug discrimination procedure

The partial agonist nalorphine appears to lie between naloxone (with little intrinsic activity) and morphine (with full intrinsic activity) on a continuum of relative efficacy (Smurthwaite & Riley, 1995). However, it is unclear whether nalorphine is more like naloxone or morphine. Using a drug discrimination procedure (Melton & Riley, 1993) that differentiated opioid antagonists and agonists on the basis of their ability to potentiate or block, respectively, CCK stimulus control, the present experiment attempted to assess nalorphine's relative efficacy. Nalorphine acted as an antagonist (i.e., potentiated CCK's stimulus properties) for some animals and as an agonist (i.e., blocked CCK stimulus control) for others, indicating that nalorphine may lie near the endogenous opiates on the continuum of relative efficacy. That is, in some animals nalorphine has less efficacy (i.e., acts as an opioid antagonist) and in others equal or greater efficacy than the endogenous opiates (i.e., acts as an opioid agonist).