MESOTOCIN RECEPTORS DURING PREGNANCY, PARTURITION AND LACTATION IN THE TAMMAR WALLABY

Mestocin receptor concentrations in membrane preparations from reprodu ctive tissues of the tammar Macropus eugenii throughout gestation and lactation were assessed using [H-3]-oxytocin as the ligand. There was a single binding site which bound both mesotocin and oxytocin with hig h and similar affinities. Mesotocin receptor concentrations in the myo metrium were low (708 +/- 199 fmol mg(-1) protein) in early and middle gestation but increased significantly on day 23 of pregnancy of the 2 6-day gestation period to 1921 +/- 552 fmol mg(-1) protein. Myometrial receptors reached a peak of 2483 +/- 575 fmol mg(-1) protein on days 25 and 26 of gestation, but returned to basal levels about an hour aft er birth, Receptor concentrations in the contralateral non-gravid uter us were much lower (605 +/- 75 fmol mg-l) and did not significantly in crease throughout the period of gestation but dropped one day before b irth, Mesotocin receptors were undetectable in the endometrium, the yo lk sac placenta and the lateral, median and anterior vagina of all ani mals tested. In the lactating mammary gland after birth mesotocin rece ptors were initially high (588 +/- 38 fmol mg-l) but decreased after 2 00 days and by late lactation were 224 +/- 55 fmol mg(-1) protein on d ay 240, close to the time of weaning. Mesotocin receptors in the ipsil ateral non-lactating gland were also high in early lactation (430 +/- 153 fmol mg-l) and declined in late lactation (62 +/- 20 fmol mg-l). T he changing concentrations of mesotocin receptors in pregnancy and lac tation demonstrate that they are specifically regulated in tammar repr oductive tissues. The increase in mesotocin receptors in gravid, but n on-gravid myometrium three days before birth may make the uterus respo nsive to the surge of mesotocin at. birth, Since this rise is unilater al and only occurs in the gravid myometrium it must be due to local ef fects from the ipsilateral ovary or the fete-placental unit. Likewise, the down-regulation of mesotocin receptors in the contralateral, non- gravid myometrium may be due to its proximity to the developing follic le. The changing concentrations in the lactating and the adjacent, non -lactating mammary gland also reflect a differential regulation of mes otocin receptors. probably mediated via the sucking stimulus. Thus, lo cal influences appear to be of primary importance in the regulation of mesotocin receptors during reproduction in this marsupial. (C) 1998 E lsevier Science B.V.