MELATONIN IS INVOLVED IN CHOLECYSTOKININ-INDUCED CHANGES OF ILEAL MOTILITY IN RATS

The aim of this study was to determine, in the rat, the interaction between melatonin and cholecystokinin in the regulation of the ileal interdigestive motility. This was analyzed by the chronic electromyography technique. Ileal motility was defined by the presence of intermittent spike bursts corresponding to the contractile activity of the organe. In control rats, these spike bursts were organized in cyclic myoelectrical complexes. Each complex is characterized by two successive spiking activity phases: the irregular phase (ISA) followed by the regular phase (RSA). Pinealectomy suppressed the RSA phase so ileal motility was constituted only by the ISA phase. When melatonin (1 mg/kg i.v.) was injected into pinealectomized rats, RSA phases were immediately and definitively restored. RSA phases were also re-established when the ''alimentary'' type of cholecystokinin receptors (CCKA) were blocked by selective antagonists such as L364,718 or SR27897 (1 mg/kg i.v.). The latter had better brain accessibility than L364,718. Unlike the effects of melatonin, the effect of these antagonists was neither immediate (the latency is longer for L364,718 than for SR27897) nor definitive. In control rats, cholecystokinin (5 mu g/kg i.v.) induced a characteristic long-lasting (29 +/- 2 min) excitomotor effect on the ileum. This effect was suppressed in pinealectomized rats and was restored after melatonin treatment. These results suggest that, via the central nervous system, melatonin is involved in the modulation of cholecystokinin action on ileal motility.