Interaction between CCK and opioids in the modulation of the rectocolonic inhibitory reflex in rats

The effects of cholecystokinin octapeptide (CCK-8) as well as the involvement of opioid system were evaluated in rectal distension (RD)-induced colonic motor inhibition in rats. Rats were surgically prepared with electrodes implanted on the proximal colon, and a catheter was implanted in lateral ventricle of the brain. RD was performed by inflation (0.0-1.6 ml) of a balloon rectally inserted. RD 1.6 ml of induced an inhibition of the colonic spike bursts (3.1 +/- 0.5 per 5 min vs. 8.1 +/- 0.4 before RD). Intracerebroventricular but not intravenous injection of CCK-8 and A-71623 (50 and 100 ng/kg) reduced the RD-induced colonic motor inhibition, whereas A-63387 was ineffective. PD-135,158 (10 micrograms/kg icv) suppressed the inhibitory reflex caused by RD. Devazepide (100 micrograms/kg icv) had no effect in this reflex function. Devazepide (1 microgram/kg), naloxone (0.1 mg/kg), and nor-binaltorphimine (nor-BNI; 10 mg/kg) reversed the blocking effect of CCK-8, whereas PD-135,158 (0.1 microgram/kg) and naltrindole (1 mg/kg) have no effect. In conclusion, CCK-8 acts on central alimentary cholecystokinin receptors to modulate the RD-induced inhibition of colonic motility through pathways involving activation of endogenous kappa-receptors.