Plasma corticosteroid levels in rats maintained on a long-acting naltrexone delivery system

A long-acting delivery system for naltrexone has been described, which blocked the antinociceptive action of 10 mg kg-1 s.c. dose of morphine in rats for a period of 2 to 3 months. Male Wistar rats implanted s.c. with such a delivery system showed highly significant depression of plasma corticosteroid levels (40.2% in one week and 22.4 to 27.2% in 3 months) as compared to placebo pellet-implanted animals. Morphine-dependent male rats implanted with 75 mg morphine pellets showed a small (17.5%) but significant increase in plasma corticosteroid levels as compared to the placebo controls 72 hr. after pellet implantation.