Proteolytic conversion of oxytocin by brain synaptic membranes: role of aminopeptidases and endopeptidases.

The proteolytic conversion of oxytocin and vasopressin by purified rat brain synaptic membranes was studied at 37 degrees C and physiological pH 7.4. The formed peptide fragments were isolated by high performance liquid chromatography and characterized by amino acid analysis. When oxytocin was incubated with synaptic membranes, either C- or N-terminal fragments were found. The most abundant were [Cyt6]oxytocin(4-9), [Cyt6]oxytocin(3-9), [Cyt6]oxytocin(2-9), oxytocin(1-8) and oxytocin(1-7). In contrast, only C-terminal fragments, [Cyt6-Arg8]vasopressin(4-9), [Cyt6-Arg8]vasopressin(3-9) and [Cyt6-Arg8]vasopressin(2-9), were found by incubating [Arg8]vasopressin. The formation of C-terminal oxytocin and vasopressin fragments was inhibited by the aminopeptidase inhibitors amastatin and bestatin, while the formation of oxytocin(1-7) and (1-8) was inhibited by the divalent cations Hg(2+) and Zn(2+). The formation of oxytocin(1-7) was also partially prevented by the endopeptidase inhibitor phosphoramidon. The formation of both C- and N-terminal fragments was inhibited by o-phenanthroline. The results suggest that, while [Arg8]vasopressin is metabolized only by membrane-bound aminopeptidases, oxytocin is also metabolized by membrane-bound endopeptidases.     

Difference in susceptibility of arginine-vasopressin and oxytocin to aminopeptidase activity in brain synaptic membranes

Arginine-vasopressin and oxytocin, peptides which serve as putative precursors for neurotrophic fragments, were digested in the presence of the respective ¹⁴C-Tyr²- and ${}^{14}\text{C-GlyNH}{}_2{}^9\text{-labeled}$ nonapeptides with a purified synaptic membrane preparation of rat brain. In this preparation aminopeptidase activity predominates in the conversion of these peptides. The disappearance of intact peptide and the release of free ¹⁴C-Tyr and ¹⁴C-GlyNH₂ was followed simultaneously with time by HPLC. Oxytocin was about four times more resistant to proteolysis than arginine-vasopressin as measured by slower disappearance of intact oxytocin, and reflected by the slower release of ¹⁴C-Tyr, but not of ¹⁴C-GlyNH₂ from oxytocin. Comparison of degradation rates of structure analogues showed that peptides having Ile in position 3, as oxytocin, were more resistant than analogues having Phe in position 3, as arginine-vasopressin. The data demonstrate that arginine-vasopressin and oxytocin differ markedly in susceptibility to the aminopeptidase activity in brain synaptic membranes, and indicate that this difference resides primarily in the amino acid residue in position 3. It is suggested that the difference in susceptibility may affect the pattern of neurotrophic metabolites in brain.     

Proteolytic conversion of arginine-vasopressin and oxytocin by brain synaptic membranes. Characterization of formed peptides and mechanisms of proteolysis

This study concerned the fragmentation of the nonapeptides arginine-vasopressin (AVP-(1-9)) and oxytocin (OXT-(1-9)) by proteolytic enzymes present in a brain synaptic membrane preparation. The peptides formed during digestion of arginine-vasopressin and oxytocin were isolated by high pressure liquid chromatography and chemically characterized by amino acid composition, NH2-terminal amino acid residues, and the presence of 14C radioactivity in tyrosine-2 and glycinamide-9. The major peptide fragments of arginine-vasopressin were [Cyt6]-AVP-(2-9), [Cyt6]-AVP-(3-9), [less than Glu4, Cyt6]-AVP-(4-9), and a peptide having the AVP-(4-8) sequence. The characterized fragments of oxytocin were [Cyt6]-OXT-(2-9), [Cyt6]-OXT-(3-9), [Cyt6]-OXT-(4-9), [less than Glu4, Cyt6]-OXT-(4-9), and [Cyt6] OXT-(5-9). Employing differentially 14C-labeled arginine-vasopressin and oxytocin, the proteolysis of the two peptides into fragments was followed with time. The results showed the sequential formation of peptide fragments by proteolytic cleavage from the NH2 terminus onward, demonstrating the action of an aminopeptidase-like enzyme. Arginine-vasopressin was converted significantly more rapidly by the amino-peptidase activity than oxytocin. In contrast to known brain aminopeptidases, the synaptic membrane-associated activity cleaved the nonapeptides without prior reduction of the disulfide bridge. From the present data it is concluded that aminopeptidases predominate in the proteolytic mechanism by which brain synaptic membranes convert arginine-vasopressin and oxytocin. The role of the proteolytic events and the significance of formed peptide fragments is discussed in view of the concept that arginine-vasopressin and oxytocin are precursors for neuropeptides in brain.     

Synthesis and receptor binding of oxytocin analogs containing conformationally restricted amino acids

We report the solid-phase synthesis and receptor-binding properties of eleven oxytocin analogs (Mpa-Xxx-Ile-Gln-Asn-Cys-Sar-Arg-Gly-NH₂) containing non-coded amino acids in position 2: D-- and L--(2-indanyl)glycine, R,S-6-methoxy-2-aminotetralin-2-carboxylic acid, D- and L-pentafluorophenylalanine, D,L-2,4-dimethylphenylalanine, D,L-2,4,6-trimethylphenylalanine, R,R- and S,S-1,2,3,4-tetrahydro-1-methyl--carboline-3-carboxylic acid and R- and S-1,2,3,4-tetrahydro--carboline-3-carboxylic acid. Some of these amino acid analogs (2-indanylglycine and D-pentafluorophenylalanine) were earlier successfully applied for the synthesis of potent bradykinin antagonists [1,2]. Their receptor bindings were tested on isolated guinea-pig uterus, rat liver and rat kidney inner medulla plasma membranes. The extent of binding of the peptides to the oxytocin receptor was in several cases was even higher than that of the parent hormone (oxytocin). However, the real pharmacological value of these analogs can be evaluated only after in vivo measurements of their inhibition of uterine motor activity.     

Behaviorally active oxytocin fragments simultaneously attenuate heroin self-administration and tolerance in rats

Maintenance of intravenous heroin self-administration and the degree of tolerance to the analgesic effect of self-injected heroin were simultaneously measured in heroin-tolerant rats. Subcutaneous injection of oxytocin (OXT-(1-9)) and of its behaviorally active fragments desglycinamide9-oxytocin (OXT-(1-8)) and [pGlu4,Cyt6]-oxytocin-(4-8) (OXT-(4-8)) decreased the amount of heroin self-injected. The C-terminal tripeptide of oxytocin (prolyl-leucyl-glycinamide, PLG, OXT-(7-9)) and desglycinamide9-[Arg]8-vasopressin (AVP-(1-8] were ineffective in this respect. In spite of the lower amount of self-injected heroin after pretreatment with oxytocin fragments, no differences in the antinociceptive effect of self-injected heroin, as assessed by the lick response using a hot plate device, were observed after pretreatment with placebo and oxytocin fragments. These findings suggest that oxytocin and some of its behaviorally active fragments attenuate heroin tolerance and that this effect may result in a diminished heroin intake in tolerant animals self-injecting heroin.     

Circulating concentrations of oxytocin during the estrous cycle and early pregnancy in sheep

Plasma concentrations of oxytocin and progesterone have been measured by radioimmunoassay in jugular venous blood obtained daily from 5 sheep during 2 estrous cycles and in early pregnancy.Concentrations of oxytocin were relatively high (15–30 pg/ml) during the luteal phase of the cycle, but fell at estrus (to 1–17 pg/ml). A fall in oxytocin was also observed on day 15 of pregnancy, when, as expected, progesterone levels remained high. It is suggested that raised basal levels of oxytocin are unlikely to cause the increasing uterine release of prostaglandin F_2α which occurs at the end of the estrous cycle.     

Involvement of nigral oxytocin in locomotor activity: A behavioral,immunohistochemical and lesion study in male rats

Oxytocin is involved in the control of different behaviors, from sexual behavior and food consumption to empathy, social and affective behaviors. An imbalance of central oxytocinergic neurotransmission has been also associated with different mental pathologies, from depression, anxiety and anorexia/bulimia to schizophrenia, autism and drug dependence. This study shows that oxytocin may also play a role in the control of locomotor activity. Accordingly, intraperitoneal oxytocin (0.5–2000 μg/kg) reduced locomotor activity of adult male rats. This effect was abolished by d(CH₂)₅Tyr(Me)²-Orn⁸-vasotocin, an oxytocin receptor antagonist, given into the lateral ventricles at the dose of 2 μg/rat, which was ineffective on locomotor activity. Oxytocin (50–200 ng/site) also reduced and d(CH₂)₅Tyr(Me)²-Orn⁸-vasotocin (2 μg/site) increased locomotor activity when injected bilaterally into the substantia nigra, a key area in the control of locomotor activity. Conversely, the destruction of nigral neurons bearing oxytocin receptors by the recently characterized neurotoxin oxytocin-saporin injected into the substantia nigra, increased basal locomotor activity. Since oxytocin-saporin injected into the substantia nigra caused a marked reduction of neurons immunoreactive for tyrosine hydroxylase (e.g., nigrostriatal dopaminergic neurons) and for vesicular glutamate transporters VGluT1, VGluT2 and VGluT3 (e.g., glutamatergic neurons), but not for glutamic acid decarboxylase (e.g., GABAergic neurons), together these findings suggest that oxytocin influences locomotor activity by acting on receptors localized presynaptically in nigral glutamatergic nerve terminals (which control the activity of nigral GABAergic efferent neurons projecting to brain stem nuclei controlling locomotor activity), rather than on receptors localized in the cell bodies/dendrites of nigrostriatal dopaminergic neurons.     

Development of a sensitive enzymeimmunoassay for oxytocin determination in bovine plasma

A highly sensitive and specific second antibody enzymeimmunoassay (EIA) on microtiterplates for oxytocin determination in bovine plasma using the biotin–streptavidin amplification system was developed. Biotin was coupled to oxytocin and used to bridge between streptavidin-peroxidase and the immobilized oxytocin antiserum in the competitive assay. The assay was carried out directly in 200 μl of bovine plasma. Oxytocin standards prepared in hormone-free plasma were used. The sensitivity of the assay was 0.25 pg/well which corresponded to 1.25 pg/ml plasma; the 50% relative binding was seen at 2.8 pg/well. Plasma volumes for the assay ranging from 50 to 200 μl did not influence the shape of the oxytocin standard curve; however a distinct drop in the OD₄₅₀ was observed with higher plasma volumes. The oxytocin antiserum used in the assay showed no significant cross-reaction with other octapeptides tested. The assay was compared with a radioimmunoassay (RIA) procedure employing prior solvent extraction of plasma samples. The oxytocin concentrations assayed by EIA and RIA in plasma samples obtained from four cows before, during and after milking were highly correlated and very similar (r=0.97). Hence the assay developed offers an attractive alternative to the RIA since no prior laborious plasma extraction is needed. Further, the assay has the distinct advantage of being non-radioactive in nature.     

Synthesis and receptor binding of oxytocin analogs containing conformationally restricted amino acids

Summary We report the solid-phase synthesis and receptor-binding properties of eleven oxytocin analogs (Mpa-Xxx-Ile-Gln-Asn-Cys-Sar-Arg-Gly-NH₂) containing non-coded amino acids in position 2: D-α- and L-α-(2-indanyl)glycine, R,S-6-methoxy-2-aminotetralin-2-carboxylic acid, D- and L-pentafluorophenylalanine, D,L-2,4-dimethylphenylalanine, D,L-2,4,6-trimethylphenylalanine, R,R- and S,S-1,2,3,4-tetrahydro-1-methyl-β-carboline-3-carboxylic acid and R- and S-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid. Some of these amino acid analogs (2-indanylglycine and D-pentafluorophenylalanine) were earlier successfully applied for the synthesis of potent bradykinin antagonists [1, 2]. Their receptor bindings were tested on isolated guinea-pig uterus, rat liver and rat kidney inner medulla plasma membranes. The extent of binding of the peptides to the oxytocin receptor was in several cases was even higher than that of the parent hormone (oxytocin). However, the real pharmacological value of these analogs can be evaluated only afterin vivo measurements of their inhibition of uterine motor activity.     

Stimulation of pyruvate dehydrogenase activity in adipocytes by oxytocin: Evidence for mediation of the insulin-like effect by endogenous hydrogen peroxide independent of glucose transport

Oxytocin, a nonapeptide posterior pituitary hormone, which is known to increase glucose oxidation in fat cells like insulin, is shown here to stimulate pyruvate dehydrogenase activity in these cells. The process appears to involve the activation of preexisting molecules since there was no change in the total enzyme content after full activation. The effect of oxytocin, as well as of insulin, appears to be mediated by endogenous H₂O₂ formation, as evident from (i) the enhanced [¹⁴C]formate oxidation and its greater inhibition by 3-amino-1,2,4-triazole in the hormone-treated cells than in the control. This is a measure of the catalase:H₂O₂ complex, and the dose dependence of this response is found to be identical with that of glucose oxidation via the hexose monophosphate shunt pathway and of pyruvate dehydrogenase activity; and (ii) treatment of the cells with low concentration of exogenous H₂O₂ causes the activation of pyruvate dehydrogenase to the extent which is comparable with the effect of the hormones. The ED₅₀ of oxytocin was 7 × 10^?9m, whereas the ED₅₀ of insulin was 5 × 10^?11m. The reduced, inactive (SH) derivatives of the hormones had the same dose-response relationship, but considerably lower effect (10 to 20% of the native molecules of the hormones), indicating the significant role of the disulfide bridge(s) in eliciting these metabolic responses. The stimulation of PDH by oxytocin or insulin is found to be essentially independent of medium glucose which, however, can sustain the response apparently by recycling the intracellular oxidation-reduction state. However, unlike insulin, oxytocin fails to stimulate the rapid uptake of 3-O-[³H]methyl-d-glucose in these cells. The data illustrate that the major metabolic actions of insulin, viz., glucose utilization and lipogenesis, are shared by another heterologous polypeptide hormone, e.g., oxytocin, through a common effector, H₂O₂. It is suggested that (i) oxytocin may play a limited surrogate role for insulin in these cells; and (ii) H₂O₂ production may be the general basis of oxytocin's action.     

Identification of a urea bioisostere of a triazole oxytocin antagonist

A series of azetidine ureas were investigated as potential bioisosteres of previously reported azetidinyltriazole oxytocin antagonists. Although potency was somewhat reduced in several close-in analogues, one compound, 9, was both a potent oxytocin antagonist and demonstrated significant selectivity over the closely related vasopressin V_1A receptor.     

Leydig cell receptors for luteinizing hormone releasing hormone and its agonists and their modulation by administration or deprivation of the releasing hormone

Leydig cells isolated from adult rat testes bound ¹²⁵I-labelled luteinizing hormone releasing hormone (LHRH) agonist with high affinity (K_A=1.2 × 10⁹M) and specificity. LHRH and the 3–9 and 4–9 fragments of LHRH agonist competed for binding sites with ¹²⁵I-LHRH agonist but with reduced affinities, whereas fragments of LHRH, and oxytocin and TRH were largely inactive. Somatostatin inhibited binding at high (10^?4M) concentrations but was inactive at 10^?6M and less. Pretreatment of rats for 7 days with 5 μg/day of LHRH agonist reduced binding of ¹²⁵I-LHRH agonist to Leydig cells $\text{in vitro}$ by 25%, whilst inhibition of endogenous LHRH by antibodies for 7 days caused a 40% decrease.     

Kukoamine A analogs with lipoxygenase inhibitory activity

Kukoamine A (KukA) is a spermine (SPM) conjugate with dihydrocaffeic acid (DHCA), with interesting biological activities. The four possible regioisomers of KukA, as well as a series of KukA analogs incorporating changes in either the SPM or the DHCA structural units, were evaluated for their antioxidant activity and their inhibitory activity on soybean lipoxygenase (LOX) and lipid peroxidation. The reducing properties of the compounds were evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging assay and found to be in the range 5–97.5%. KukA significantly inhibits LOX with IC₅₀ 9.5?μM. All tested analogs inhibited lipid peroxidation in the range of 11–100%. The most potent compounds KukA and its analog 3, in which the DHCA units had been replaced by O,O9-dimethylcaffeic acid units, were studied for their anti-inflammatory activity in vivo on rat paw edema induced by carrageenan and found to be of comparable activity to indomethacin. The results of the biological tests are discussed in terms of structural characteristics.     

In Vivo Availability of Pro-Resolving Lipid Mediators in Oxazolone Induced Dermal Inflammation in the Mouse

The activation and infiltration of polymorphonuclear neutrophils (PMN) are critical key steps in inflammation. PMN-mediated inflammation is limited by anti-inflammatory and pro-resolving mechanisms, including specialized pro-resolving lipid mediators (SPM). We examined the effects of 15-epi-LXA₄ on inflammation and the biosynthesis of pro-inflammatory mediators, such as prostaglandins, leukotriene B₄ and various hydroxyeicosatetraenoic acids and SPM, in an oxazolone (OXA)-induced hypersensitivity model for dermal inflammation. 15-epi-LXA₄ (100 μM, 5 μL subcutaneously injected) significantly (P < 0.05) reduced inflammation in skin, 24 hours after the OXA challenge, as compared to skin treated with vehicle. No significant influence on the biosynthesis of prostaglandins or leukotriene B₄ was observed, whereas the level of 15S-hydroxy-eicosatetraenoic acid was significantly (P < 0.05) lower in the skin areas treated with 15-epi-LXA₄. In spite of the use of a fully validated analytical procedure, no SPM were detected in the biological samples. To investigate the reason for the lack of analytical signal, we tried to mimic the production of SPM (lipoxins, resolvins, maresin and protectin) by injecting them subcutaneously into the skin of mice and studying the in vivo availability and distribution of the compounds. All analytes showed very little lateral distribution in skin tissue and their levels were markedly decreased (> 95%) 2 hours after injection. However, docosahexaenoic acid derivatives were biologically more stable than SPM derived from arachidonic acid or eicosapentaenoic acid.     

Ex vivo influence of carbetocin on equine myometrial muscles and comparison with oxytocin

To determine the intercyclic effect of oxytocin and carbetocin on equine myometrial tissue, the effect of the drugs was evaluated through pharmacokinetic and pharmacodynamic studies. The complete pharmacokinetic profile for oxytocin was unknown and had to be established. To do so, 25 IU of oxytocin were administered intravenously to six cycling mares and blood samples were collected before and 2, 4, 8, and 15 min after administration. The half-life of oxytocin was determined to be 5.89 min, the clearance rate 11.67 L/min, mean residence time (MRT) 7.78 min. The effective plasma concentration was estimated to be 0.25 ng/mL. This was similar to the concentration achieved for the organ bath study where the concentration that produced 50% of the maximum effect (EC₅₀) was calculated at 0.45 ng/mL. To determine the intercyclic effect of oxytocin and carbetocin uterine myometrial samples were collected from slaughtered mares in estrus, diestrus, and anestrus. The samples were mounted in organ baths and exposed to four ascending, cumulative doses of oxytocin and carbetocin. Area under the curve and amplitude, maximum response (E_max), and concentration that produced 50% of the maximum effect were studied for each agonist and statistically evaluated. The effect of oxytocin on equine myometrial tissue was higher during diestrus, and surprisingly anestrus, than during estrus, whereas the effect of carbetocin was the same independent of the stage of estrous cycle. A significant difference was found for estrous and anestrous samples when oxytocin was used but not when carbetocin was used.     

Sensitivity of the Bothrops jararaca snake uterus to oxytocin and acetylcholine

The influence of hormones on the uterine smooth muscle sensitivity has been demonstrated in mammals; however in nonmammalian species much remains to be clarified. This study investigated the sensitivity of the snake (Bothrops jararaca) uterus to oxytocin and acetylcholine. The snakes were divided into three experimental groups: snakes with uterine segments weighing ≤20.00 mg (A), snakes with uterine segments weighing between 20.01 and 35.00 mg (B) and snakes with uterine segments weighing ≥35.01 mg (C). The histomorphometric analysis of the uterus showed an increase in the smooth muscle layer thickness in groups B and C, when compared with group A, suggesting different hormonal conditions of the animals. Cumulative concentration-effect curves to oxytocin and acetylcholine were obtained in uteri of the three experimental groups and the pD₂ values determined. The sensitivity of the snake uterus to oxytocin increased in groups B and C (concentration-effect curves shifted to the left and pD₂ values increased) when compared with group A. The concentration-effect curves to acetylcholine were biphasic and shifted to the left, suggesting two binding sites (low and high affinity binding sites) in snake uteri of groups B and C. These results suggest that sex steroids may modulate the sensitivity of the snake uterus to oxytocin and acetylcholine.     

The conformation of oxytocin in dimethylsulfoxide as revealed by carbon-13 spin-lattice relaxation times

Information was obtained on rates of overall molecular reorientation and segmental motion of amino acid sidechains of oxytocin in dimethylsulfoxide by determination of spin-lattice relaxation times (T₁) at 25 MHz for carbon-13 in natural abundance in the hormone. The T₁ values of the α-carbons of amino acid residues located in the 20-membered ring of oxytocin are all about 50 msec. The overall correlation time for the hormone backbone was estimated to be 8.8 × 10^?10 sec. The sidechains of Tyr, Ile and Gln undergo segmental motion with respect to the backbone of the ring. The T₁ value of the α-carbon of the Leu residue is greater than for any α-carbon in the ring, indicating an increased mobility of the backbone of the C-terminal acyclic peptide as compared to the ring. The β- and γ-carbons of the Pro residue undergo an exo-endo interconversion with regard to the plane formed by α-carbon, δ-carbon and N atom of the Pro pyrollidine ring. These data are discussed in light of results from other experimental and theoretical studies, including carbon-13 spin-lattice relaxation times for oxytocin in aqueous solution.     

Stimulation of rat pituitary phospholipid methyltransferase by vasopressin but not oxytocin

Rat pituitary extracts contain two methyltransferases that catalyze stepwise methylation of phosphatidyl-ethanolamine to phosphatidylcholine using S-adenosylmethionine as the methyl donor. The activities of both of these enzymes were stimulated by 40 μM lysine or arginine vasopresin but not oxytocin, arginine vasotocin and Pro-Leu-Gly NH₂. The concentration of lysine-vasopressin required for the half-maximal stimulation of phospholipid methylation was 27 μM. A comparison of the chemical structure of different peptides with their ability to stimulate phospholipid methylation suggests that the stimulatory activity resides in the covalent ring structure (pressinoic acid) of the vasopressin molecule.     

Effect of oxytocin on transepithelial transport of water and Na+ in distinct ventral regions of frog skin (Rana catesbeiana)

Thoracic, abdominal, and pelvic fragments of ventral skin of Rana catesbeiana were analysed regarding the effect of oxytocin on: (1) transepithelial water transport; (2) short-circuit current; (3) skin conductance and electrical potential difference; (4) Na⁺ conductance and electrical potential difference; (4) Na⁺ conductance, the electromotive force of Na⁺ transport mechanism, and shunt conductance; (5) short-circuit current responses to fast Na⁺ by K⁺ replacement in the outer compartment, and (6) epithelial microstructure. Unstimulated water and Na⁺ permeabilities were low along the ventral skin. Hydrosmotic and natriferic responses to oxytocin increased from thorax to pelvis. Unstimulated Na⁺ conductance was greater in pelvis than in abdomen, the other electrical parameters being essentially similar in both skin fragments. Contribution of shunt conductance to total skin conductance was higher in abdominal than in pelvic skin. Oxytocin-induced increases of total skin conductance, Na⁺ conductance, and shunt conductance in pelvis were significantly larger than in abdomen. An oscillatory behaviour of the short-circuit current was observed only in oxytocin-treated pelvic skins. Decrease of epithelial thickness and increase of mitochondria-rich cell number were observed from thorax to pelvis. Oxytocin-induced increases of interspaces were more conspicuous in pelvis and abdomen than in thorax.Abbreviations E _Na electromotive force of sodium transport mechansim - G _KCI skin conductance with external KCI Ringer - G _Na sodium conductance (series conductance) - G _shunt shunt pathway conductance - G _total total skin conductance - J _v water flux (in units of volume per area per time) - MRC mitochondria-rich cells - PD potential difference across skin - R _shunt resistance of the shunt pathway - SCC short-circuit current     

Effect of progesterone administration on the release of uterine prostaglandin F2α and ovarian oxytocin in the goat

The effects of intramuscular progesterone administration (20 mg·day⁻¹) on plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F_2α (PGFM-pulmonary metabolite of prostaglandin F_2α) and oxytocin were examined in seventeen goats after either bilateral ovariectomy, hysterectomy or during days 12–16 of the estrous cycle. Daily mean values of PGFM in animals treated with progesterone after ovariectomy were significantly greater (P<0.001) than in their corresponding controls on the last two treatment days (10 and 11); concentrations of oxytocin, however, remained at or near the limits of assay sensitivity. In hysterectomized goats PGFM concentrations remained extremely low and oxytocin release appeared steady rather than pulsatile. In the intact animals, undergoing luteolysis, daily mean concentrations of both PGFM and oxytocin were significantly greater (P<0.01) in progesterone-treated goats than in their oil-treated controls; furthermore, in the progesterone-treated goats, increases in PGFM concentrations, observed after the peaks of progesterone, were either coincident with or prior to pulses of oxytocin. These results demonstrate that uterine PGF_2α stimulates the pulsatile release of oxytocin from the ovary during luteolysis in the goat.