Dopamine and the depressant action of morphine on stimulated guinea-pig ileum.

1 Morphine reduces the amplitude of the contractions induced by electrical stimulation, in the myenteric plexus-longitudinal muscle preparation of guinea-pig ileum. Dopamine and apomorphine have the same effect but at much higher concentrations. 2 Dopamine, at concentrations lower than those which would normally be inhibitory, partially reverses the depressant effect of morphine. 3 Pre-treatment of guinea-pigs with 6-hydroxydopamine results in a slight supersensitivity of innervated longitudinal muscle preparations to dopamine and has no effect on morphine activity. 4 Naloxone antagonizes the depressant effect of morphine but not that of dopamine or apomorphine. 5 The response of theileum preparation to morphine is not affected by phentolamine or propranolol; the effect of dopamine, however, is abolished by alpha-adrenoceptor blockade.