Effect of prolyl-leucyl-glycinamide and alpha-melanocyte-stimulating hormone on levorphanol-induced analgesia, tolerance and dependence.

Prolyl-leucyl-glycinamide (PLG) at a low dose (10 ng/mouse) administered by an intracerebroventricular (i.c.v.) injection did not affect levorphanol analgesia, but PLG at higher doses (10 and 100 micrograms/mouse) and alpha-melanocyte-stimulating hormone (alpha-MSH) (10 ng/mouse) antagonized levorphanol analgesia. Development of levorphanol tolerance was facilitated by 10 ng/mouse of PLG, unaffected by 10 micrograms/mouse of PLG, but antagonized by 100 micrograms/mouse of PLG and 10 ng/mouse of alpha-MSH. The effect of PLG on levorphanol dependence was assessed by changes in body weight and temperature during naloxone-induced withdrawal. PLG (10 ng/mouse) facilitated the development of levorphanol dependence, but 10 micrograms/mouse of PLG had no effect. PLG (100 micrograms/mouse) antagonized development of levorphanol dependence. PLG at doses of 10 and 100 micrograms/mouse precipitated withdrawal in levorphanol-dependent mice. alpha-MSH (10 ng/mouse) antagonized development of levorphanol dependence as evidenced by an increase in the ED50 of naloxone required to induce withdrawal jumping. These results indicate that PLG and alpha-MSH affected levorphanol-induced analgesia, tolerance and dependence in a qualitatively similar manner to their effect on morphine-induced analgesia, tolerance and dependence.