Differential effects on gastrointestinal activity after intrathecal morphine, sufentanil and alfentanil in rats.
This paper reports on investigations of the effects on gastrointestinal (gi) transit of subcutaneous (sc) or intrathecal (i.t. ) administration of the opiates morphine, alfentanil and sufentanil. Prior sc of all 3 drugs produced a significant dose-dependent decrease in transit of a charcoal test meal. 
 Morphine i.t. to rats with catheters chronically implanted in the subarachnoid space did not decrease gi transit. This was in contrast to the effects of i.t. alfentanil and sufentanil, which caused marked dose-dependent slowing of the passage of the meal. Prior sc of the opiate antagonist nalaxone completely blocked the depression of gi transit caused by high doses of i.t. sufentanil. The different responses to i.t. morphine and sufentanil or alfentanil may be due to the different mu receptor affinities of the drugs to spinal receptors, or to the slower passage of morphine into the intravascular compartment and hence a slower effect on supraspinal structures. Alternatively the results may be interpreted as indicating the presence of multiple subtypes of mu receptors in spinal cord.