Acute physical dependence induced by continuous intravenous infusion of morphine or meperidine in the rat

An intravenous infusion method is described for rapidly producing physical dependence in rats. Rats were infused with morphine or meperidine for 24 or 48 h at constant rates and the development of physical dependence was assessed by body weight loss after naloxone challenge. Naloxone challenge induced body losses that were dependent upon magnitude, rate and duration of infusion. The steady-state concentrations of morphine (4 mg/kg/h) in serum and meperidine (6 mg/kg/h) in plasma were 4 and 2.5 microgram/ml, respectively. Morphine concentration in the brain in the steady-state (40 mg/kg/h) was 0.7 micrograms 1 g and in the serum was proportional to the infusion rate. Maximum body weight loss was significantly correlated with total amount of infused morphine, but not with the steady-state concentration of the drug in the serum. These results suggest that total doses of infused morphine, not steady-state concentrations, are critical in producing body weight loss.