Effects of different spinal morphine doses in rats.

BACKGROUND AND OBJECTIVES Spinal morphine promotes adequate pain relief, but is not free from side effects. This study aimed at investigating the effects of different spinal morphine doses. METHODS Five groups of seven rats were studied, 24 h after spinal catheter insertion via cisterna magna under anesthesia with muscular ketamine and xylazine. G1 received 10 microl saline solution; groups G2, G3, G4 and G5 received respectively 0.1; 0.3; 0.5 and 1 microg morphine in 10 microl saline solution. Animals were submitted to tail immersion test in hot water at M0 (prior to injection), and M15, M30, M60, M120 and M180 minutes after the injection. RESULTS Analgesic effects were observed in groups receiving morphine, in different times, as compared to control and to time before morphine injection. In G1 there was paws weakness in 4 animals. Agitation was observed at M15 in G2 and at M15 and M30 in G3. Mandible tremors were observed at moments M5, M15, M30 and M60 in G2; at M5 and M15 in G3, in M5 in G4, and in M5 in G5. Pruritus was observed at M5, M15, M30 and M60 in G2; at M5, M15, M30, M60 and M120 in G3; at M5, M15, M30, M60, and M120 in G4; and at M5, M15, M30, M60 and M120 in G5. Absence of diuresis was observed at M60 in G2; at M15, M30, M60 and M120 in G3; at M30 and M120 in G4, and at M30, and M60 in G5. Sedation was observed at M15, M30 and M60 in groups 2, 3 and 4; and at M15, M30, M60 and M120 in G5. Respiratory changes were observed at M15, M30 and M60 in G2; at M15, M30 and M60 in G3; at M15 and M30 in groups 4 and 5. CONCLUSIONS In this study all morphine doses below 1 microg have promoted short duration analgesia; all spinal morphine doses have produced side effects.