"Paradoxical" analgesia and aggravated morphine dependence induced by opioid antagonists

Chronic treatment with naloxone (Nx) or naltrexone (Ntx) induces paradoxical analgesia. In the present study, the effects of chronic treatment with opioid receptor antagonists, such as nor-binaltorphimine (nor-BNI) for kappa and naltrindole (NTI) for delta receptors, on analgesic response using the hot plate test and on morphine physical dependence in rats were examined. The hot plate latency was significantly increased by pretreatment with Nx (5 mg/kg, s.c.), nor-BNI (20 mg/kg, i.p.) or NTI (20 mg/kg, i.p.) for 5 days. After chronic pretreatment with these antagonists, the rats were treated with morphine-admixed food (0.5 mg/g of food) for 3 days. Chronic pretreatment with Nx and NTI significantly increased Nx precipitated body weight loss in morphine dependent rats, while chronic pretreatment with nor-BNI produced small increase. These results indicate that chronic treatment with nor-BNI or NTI as well as with Nx induces obviously paradoxical analgesia, and that chronic blockade of mu or delta may enhance the development of physical dependence on morphine.