Opioid receptors in the mouse ileum

: A method is described for setting up a mouse ileum so as to obtain reproducible responses to electrical stimulation. The effects of the mu, kappa and delta opioid agonists on the electrically evoked response have been investigated. The results obtained show that whereas both kappa and delta receptor systems can exert marked modulatory effects on this response, the mu receptor system appears to be relatively unimportant. The selective mu agonist RX783006 has a very low agonist potency in this preparation and morphine behaves as a pure antagonist. A comparison of the Ke values of a number of antagonists at the kappa and delta systems in the mouse ileum with the kappa and delta systems in the mouse vas deferens show extremely good correlation. Because morphine shows no agonist activity in this preparation it has been possible to determine the Ke value of morphine at kappa (9.1 microM) and delta (15.6 microM) receptors. A comparison has been made between the relative proportions of opioid receptor subtypes found in the mouse ileum and those found in the mouse vas deferens and guinea-pig ileum.