Myenteric plexus-longitudinal muscle strip preparations from guinea-pig incubated in Krebs solutions at room temperature and bubbled with carbogen gas did not show any functional alteration during 24 h. 2. Moreover, preparations incubated in Krebs solutions did not show any change on the inhibition response produced by liberation of opiaceous material caused by 10 Hz tetanizing stimulation, nor in sensitivity to morphine. 3. When these preparations were incubated in 2.5 X 10(-7) M morphine solutions in Krebs in the same conditions than the preparations incubated without morphine a greater inhibitory response after the 10 Hz tetanizing shock could be seen, as well as a greater sensitivity to morphine, being both phenomena correlatable with the incubation time. Significant statistical differences could be seen for incubation times longer than one hour. From these results we conclude that prolonged in vitro exposition of the myenteric plexus-longitudinal muscle preparation to morphine produced a sensitization of the opiate receptor.
