Postnatal development of methionine-enkephalin modulation of cholinergic transmission in cat ileum

The age-dependent effects of methionine-enkephalin (Met-enk) on contractions and [3H]-acetylcholine ([3H]-ACh) release evoked by electrical field stimulation (EFS) were studied in vitro on cat terminal ileum of young (2-month old) and adult cats. Radioactivity was measured by liquid scintillation spectrometry and the effect of Met-enk was evaluated by the FrS2/FrS1 ratio. Scopolamine, 1 microM, abolished the EFS-induced contractions. Met-enk dose dependently (0.01-1 microM) decreased the EFS-induced contractions. The inhibitory effect of Met-enk on the evoked contractions was more pronounced in adult cats compared to that in young cats. In adult animals Met-enk, 1 microM, almost completely inhibited the evoked contractions (94.6 +/- 2.6% of inhibition) while in the young cats its inhibitory effect was less pronounced (52.1 +/- 6.8%). Met-enk decreased the EFS-evoked release of [3H]-ACh by 44.6 +/- 4.6% in adult cats and by 21.6 +/- 1.6% in young cats. The data suggest opioid modulation of cholinergic transmission in cat terminal ileum at neuronal level occurring as early as the postnatal period and increasing with the ontogenesis.