sentence: 5 However , we determined that the length of duration of single square pulses of the small intestine of the rat and mouse was slightly longer but similar to that described in the guinea pig.30 ,31 In the systems described , morphine significantly inhibits electrically induced contractions in the mouse and rat intestine at IC values higher than for the guinea pig which is in concordance with other opioid studies.3 ,5,32 This follows from the profound effect that morphine and other opiates have on the opioid receptor system by delaying gastric emptying and small intestinal transit in these rodents.2 ,4,9,33,34 The rat is a common laboratory animal used in gut physiology studies and was included with the comparison of mouse and guinea pig in this study to determine cross-species sensitivities that may give an insight into the level of peptide antagonists that might be required in subsequent anti-morphine or other bioactive gut trials.32 ,35,36 In the electrically driven mouse model , for the opioid agonists tested , we have determined that the Δ - selective agonists DADLE and metenkephalin , and the κ - selective agonist dynorphin A were only slightly more potent than the μ - selective agonist DAMGO .