Effects of opioids on acetylcholine output of perfused human placental cotyleda

Vascular effects of PTHrP (1-34) and PTH (1-34) in the human fetal-placental circulation

The aim of this study was to examine the vasodilatory effects of parathyroid hormone-related protein (PTHrP) (1-34) and parathyroid hormone (PTH) (1-34) on the human fetal-placental circulation utilising an in vitro placental perfusion model. In all experiments, the vasculature of an isolated human placental cotyledon was pre-constricted with the thromboxane A2 mimetic U46619. A simple dose of PTHrP (1-34) or PTH (1-34) (1.7-300 nM) was then infused into the fetal-placental circulation of the cotyledon. In other experiments, cotyledons were repeatedly infused with PTHrP (1-34) or PTH (1-34) (51.3 nM). Vasodilatory responses were significantly reduced in response to repeated exposure to PTHrP (1-34) (P < 0.001), indicating that this peptide desensitizes the fetal-placental vasculature. PTHrP (1-34) and PTH (1-34) equipotently stimulated a significant vasodilation of the fetal-placental circulation (P < 0.0001). The PTHrP receptor antagonist [Asn10, Leu 11]PTHrP (7-34) (102 nM) was infused in U46619-constricted placentae in the presence and absence of PTHrP (1-34) (10.2 nM). The PTHrP antagonist alone had no significant effect in the fetal-placental circulation. The antagonist significantly attenuated the response to PTHrP (1-34) (P < 0.015). Based on the data obtained in this study it is suggested that locally produced PTHrP (1-34) may be involved in the regulation of normal human fetal-placental vascular tone in autocrine and/or paracrine fashion.

Endothelins-1, 2 and 3 are released in vitro from the human bilaterally perfused placenta

Immunoreactive-endothelin (ir-ET) has previously been detected in human fetal effluents from in vitro perfused placentae. To date however, because of a lack of radio-immunoassay specificity, the ET isoforms in fetal effluents had not been determined, nor had placental maternal effluents been examined for ETs. The aim of this study was to identify the isoforms of ET released into the maternal and fetal circulations of the human in vitro bilaterally perfused placenta. Both circulations of placentae, obtained after normal vaginal delivery, were perfused with a modified Krebs solution and maternal and fetal effluents from the start of the second hour of perfusion were collected, extracted on Scp-pak C18 cartridges, concentrated by vacuum evaporation and separated by reverse-phase HPLC separation. HPLC fractions were measured by ET-RIA and compared to known synthetic standards. Maternal and fetal effluents contained ET-1 (natural and oxidised ET-1), ET-2 and ET-3 (n = 5). Maternal and fetal release of ET-1 was 2.2 +/- 0.7 and 1.4 +/- 0.1 fmol/min/g wet weight of tissue respectively, ET-2 was 0.4 +/- 0.2 and 0.5 +/- 0.2 and fmol/min/g respectively, and ET-3 was 0.5 +/- 0.2 and 0.7 +/- 0.4 fmol/min/g respectively. There were no significant differences between the release of either ET-1, ET-2 or ET-3 in the maternal or fetal circulations. In conclusion, this study indicated that ET-1, ET-2 and ET-3 were all released into both the maternal and fetal effluents from the in vitro perfused human placenta.

Effects of methadone use during pregnancy on human placental opioid receptors

Human placenta was used to investigate the effects of chronic methadone use during pregnancy on villus tissue opioid receptors. Patients included in this investigation received 35-60 mg methadone per day. Methadone-exposed placenta villus tissue had no detectable opioid receptor binding sites measured by tritiated opioid agonists. In vitro release of acetylcholine and hCG from trophoblast tissue of methadone-exposed placentas was not modulated by opioids. Absence of opioid receptor binding sites and their two mediated responses in trophoblast tissue of placentas obtained from patients with documented chronic methadone use during pregnancy indicate that the receptors were down regulated or desensitized.

Properties and functions of human placental opioid system

Human placental villus tissue contains opioid receptors and peptides. Kappa opioid receptors (the only type present in this tissue) were purified with retention of their binding properties. The purified kappa receptor is a glycoprotein with an apparent molecular weight of 63,000. Two opioid receptor mediated functions were identified in trophoblast tissue, namely regulation of acetylcholine and hormonal (human chorionic gonadotrophin and human placental lactogen) release. Placental content of kappa receptors increases with gestational age. Term placental content of kappa receptors correlates with route of delivery (higher in those abdominally obtained). Opioid use and/or abuse during pregnancy affects placental receptor content at delivery, as well as its mediated functions. Opioid peptides identified in placental extracts were beta-endorphin, methionine enkephalin, leucine enkephalin and dynorphins 1-8 and 1-13. Dynorphin 1-8 seem to be the predominant opioid peptide present in placental villus tissue.

Opioids regulate the release of human chorionic gonadotropin hormone from trophoblast tissue

Opioid ligands were investigated for their effect on hCG release from trophoblast tissue obtained from term human placenta. Data obtained indicate that opiate agonists stimulate in vitro basal hCG release from trophoblast tissue. The potency of these opioid agonists correspond to their kappa receptor selectivity, i.e., the greater the selectivity the lower is the effective concentration causing maximum stimulation. Opioid antagonists inhibit the release of hCG due to their reversal of the stimulation caused by endogenous opioid peptides. Potency of the antagonists correspond also to their kappa receptor selectivity. Antagonists reverse the stimulation of hCG release caused by agonists indicating that the ligand's action is mediated by the placental kappa opioid receptors. The bell shaped response curves for agonists and antagonists suggest that opioids play a role in the regulation of hCG release from trophoblast tissue, but other mechanism(s) may also exist.