Abstract

Inhibition of the electrically induced contractions of the guinea-pig ileum has been shown to be a reliable index to the relative potency of various narcotic analgesics. This property suggests that this preparation might be used as a model in attempts to elucidate the mechanism(s) by which morphine induces analgesia in the central nervous system. Since it has been demonstrated that some adenosine derivative may function as an endogenous inhibitory transmitter in the gut, the effects of adenosine, adenosine triphosphate (ATP) and morphine on the ileum were further characterized and compared. Morphine, adenosine and ATP produce a substantial inhibition of the isometric contractions induced by transmural field stimulation. The inhibition produced by each is antagonized by 2.5 times 10(-7) M tolazoline whereas that produced by ATP is potentiated by 4 times 10(-7) M 5-hydroxytryptamine. The inhibitory effects of morphine and ATP can also be markedly potentiated by two of the several phosphodiesterase inhibitors tested, Ro 20-1724 and dipyridamole. In addition, pretreatment of the ileum with either adenosine, ATP or morphine can produce a significant potentiation of the inhibitory effects of norepinephrine. The above suggests that cyclic adenosine 3',5'-monophosphate may play a role in mediating some of the inhibitory effects produced by exogenous adenosine, ATP and morphine. In addition, the similarities between the effects produced by these substances indicates that the biochemical pathways responsible for mediating the effects of each may share some common elements.