[Morphine tolerance and dependence liability in NRGC-destructed rats]

The nucleus reticularis gigantocellularis (NRGC), including the nucleus reticularis paragigantocellularis (NRPG), of male Sprague Dawley rats was destructed bilaterally by DC 0.5 mA for 40 sec (D-rats). Morphine analgesia estimated by the tail pinch method in 4 and 10 days morphine treated sham operated rats (S-rats), of which the test doses were 40 and 80 mg/kg, respectively, was equal to or smaller than the analgesia by 5 mg/kg in non-treated S-rats. Morphine analgesia at 5 mg/kg in non-treated D-rats was weaker than that in S-rats. Morphine analgesia in 4 and 10 days morphine treated D-rats, of which the test doses were 40 and 80 mg/kg, respectively, was stronger than the analgesia by 5 mg/kg but weaker than that by 20 mg/kg in non-treated D-rats. These results indicate development of morphine tolerance in S- and D-rats. Reverse of diurnal variation in body weight and disappearance of diurnal variation in body temperature and in plasma corticosterone concentration (Pcs) were observed similarly in S- and D-rats during morphine treatment. Body weight loss, increase in Pcs and plasma ACTH concentration, and increase in adrenal weight were elicited by morphine withdrawal in both morphine treated S- and D-rats. These signs during morphine treatment and after morphine abstinence indicate the development of morphine dependence in D-rats. These results suggest that the NRGC participates in development of morphine analgesia, but does not participate in development of morphine tolerance and dependence.