OVARIAL STEROIDS AND METHIONINE-ENKEPHALIN MODULATION OF ADRENERGIC TRANSMISSION IN RABBIT OVIDUCT

1. The effect of methionine-enkephalin on the [H-3]-noradrenaline ([H- 3]-NA) overflow and on contractions evoked by field electrical stimula tion (FES) and by exogenous NA were studied in vitro in the isthmic pa rt of oviduct of rabbits (untreated and treated with estradiol or prog esterone). 2. The evoked tritium overflow (which reflected [H-3]-NA ov erflow) was determined by liquid scintillation spectrometry. 3. Field electrical stimulation of 4 Hz (trains of 40 pulses, 0.3 msec) evoked guanethidine-sensitive contractions. 4. In all groups of animals methi onine-enkephalin dose dependently decreased FES-evoked contractions bu t not those evoked by exogenous NA. 5. The amount of tritium overflow evoked by 4 Hz stimulation (600 pulses, 1 msec) was significantly lowe r in tissues from estradiol treated (1.16 +/+ 0.19%) compared with tho se obtained in tissues from untreated (1.82 +/- 0.22%) and progesteron e treated (2.07 +/- 0.21%) rabbits. Methionine-enkephalin, 1 muM, decr eased the evoked tritium overflow in the isthmus from untreated rabbit s by 36.1 +/- 3.6%, in estradiol treated by 22.8 +/- 2.9% and in proge sterone treated by 52.3 +/- 4.5%. 6. The results suggest that the meth ionine-enkephaline effect on FES-evoked contractions could be due to a prejunctional effect on the adrenergic terminals and that there is a hormonal dependence of the opioid effect on [H-3]-NA overflow.