A species difference in the slowing effect of intrathecal morphine on gastrointestinal transit

Intrathecal administration of morphine, levorphanol, bremazocine, ethylketocyclazocine or [D-Ser2,Leu5,Thr6]enkephalin to rats, at doses 10-50 times greater than that necessary to elicit analgesia in the tail flick test, had no marked effect on gastrointestinal transit as determined by the charcoal meal test. In contrast, intrathecal administration of various doses of morphine to mice significantly antagonized transit (A50 (that dose which inhibited transit to 50% of controls) = 14.7 (0.71-2.89) micrograms/mouse). These results suggest (1) a lack of involvement of opioid sensitive spinal structures in the control of gastrointestinal transit in rats, and (2) a species difference in the slowing effect of intrathecal morphine on gastrointestinal transit.