Neonatal oxytocin treatment modulates oxytocin receptor, atrial natriuretic peptide, nitric oxide synthase and estrogen receptor mRNAs expression in rat heart

Oxytocin (OT) has been implicated in reproductive functions, induction of maternal behavior as well as endocrine and neuroendocrine regulation of the cardiovascular system. Here we demonstrate that neonatal manipulation of OT can modulate the mRNAs expression for OT receptor (OTR), atrial natriuretic peptide (ANP), endothelial nitric oxide synthase (eNOS) and estrogen receptor alpha (ERalpha) in the heart. On the first day of postnatal life, female and male rats were randomly assigned to receive one of the following treatments: (a) 50microl i.p. injection of 7microg OT; (b) 0.7microg of OT antagonist (OTA); or (c) isotonic saline (SAL). Hearts were collected either on postnatal day 1 or day 21 (D1 or D21) and the mRNAs expression of OTR, ANP, inducible NOS (iNOS), eNOS, ERalpha and estrogen receptor beta (ERbeta) were compared by age, treatment, and sex utilizing real time PCR. OT treatment significantly increased heart OTR, ANP and eNOS mRNAs expression on D1 in both males and females, ERalpha increased only in females. While there were significant changes in the relative expression of all types of mRNA between D1 and D21, there were no significant treatment effects observed in D21 animals. OTA treatment significantly decreased basal ANP and eNOS mRNAs expression on D1 in both sexes. The results indicate that during the early postnatal period OT can have an immediate effect on the expression OTR, ANP, eNOS, and ERalpha mRNAs and that these effects are mitigated by D21. Also with the exception of ERalpha mRNA, the effects are the same in both sexes.     

Neonatal manipulation of oxytocin influences female reproductive behavior and success

During early neonatal development, oxytocin (OT) may influence the expression of adult behavior and physiology. Here we test the prediction that early postnatal exposure to OT or an oxytocin antagonist (OTA) can affect the subsequent expression of sexual receptivity and reproductive success of females. To test this hypothesis, female prairie voles (Microtus ochrogaster) received one of four treatments within 24 h of birth. Three groups received an intraperitoneal injection of OT, OTA, or isotonic saline. A fourth group was handled, but not injected. Around 75 days of age, females were paired with sexually experienced males for 72 h and sexual activity was recorded. Treatment had no effect on the probability of mating. Injection, regardless of treatment, reduced latency to mate compared with handled controls. OT and OTA treatment decreased mating bout frequency compared to saline and handled controls, while OTA treatment increased reproductive success, probability of successfully producing a litter. The results suggest that neonatally OT, both endogenous and exogenous, can affect the expression of adult female reproductive activity and that blocking the effects of endogenous OT during neonatal development can affect female reproductive success. Finally, the results suggest that a number of aspects of reproduction are regulated by OT during the postnatal period, but that the mechanism of action may differ depending upon the reproductive activity.     

Oxytocin maintains as well as initiates female sexual behavior: effects of a highly selective oxytocin antagonist

In previous studies, central administration of the oxytocin (OT) antagonist d(CH2)5[Tyr(Me)2, Thr4, Tyr-NH(9)2]OVT (OTA1) blocked receptive and proceptive components of female sexual behavior (FSB) and increased male-directed agonistic behavior when given before progesterone (P) treatment in estradiol-primed female rats but not when given shortly before behavioral testing 4-6 h after P. Because the considerable V(1a) antagonist potency of OTA1 may have contributed to these results, we tested the effects of the far more selective OT antagonist desGly-NH2, d(CH2)5[d-Tyr2, Thr4]OVT (OTA2). In ovariectomized, estradiol benzoate-primed (1 microg x 2 days sc) rats, icv infusion of OTA2 (1 microg) prior to P injection (250 microg sc) significantly suppressed lordosis and hops and darts and trended toward significantly increasing male-directed kicks during testing at 4 and 6 h. Infusion of OTA2 3 h and 40 min after P did not alter behavior at 4 and 6 h after P but significantly decreased lordosis as well as hops and darts and increased male-directed kicks 8-12 h after P. These results provide further evidence that central OT receptor activation shortly after P treatment contributes to the subsequent onset and early expression of FSB and demonstrate, for the first time, that OT receptor activation at later time points also contributes to maintaining FSB. The FSB-stimulating effect of central OT appears to persist for several hours.     

Intravenous 6-hydroxydopamine attenuates vasopressin and oxytocin secretion stimulated by hemorrhage and hypotension but not hyperosmolality in rats

The present study sought to determine whether chemical destruction of peripheral catecholaminergic fibers with 6-hydroxydopamine (6OHDA) attenuates vasopressin (VP) and oxytocin (OT) secretion stimulated by hemorrhage, hypotension, and hyperosmolality. Rats received 6OHDA (100 mg/kg iv) or vehicle (1 ml/kg iv) on days 1 and 7, and experiments were performed on day 8. Serial hemorrhage (4 samples of 2 ml per 300 g body wt at 10-min intervals) increased plasma VP and OT levels in both groups; however, the increase in plasma VP and OT levels was significantly attenuated in 6OHDA-treated vs. control rats despite a significantly lower mean arterial blood pressure. Similarly, the increase in plasma VP and OT levels in response to hypotension produced by the selective arteriolar vasodilator diazoxide was significantly attenuated in 6OHDA-treated rats. In marked contrast to hemorrhage and hypotension, hyperosmolality produced by an infusion of 1 M NaCl (2 ml/h iv) stimulated increases in plasma VP and OT levels that were not different between 6OHDA-treated and control rats. In a parallel set of experiments, intravenous 6OHDA treatment reduced dopamine--hydroxylase immunoreactivity in the posterior pituitary but had no substantial effect in the hypothalamic paraventricular and supraoptic nuclei. In each experiment, the pressor response to tyramine (250 microg/kg iv) was significantly attenuated in 6OHDA-treated rats, thereby confirming that 6OHDA treatment destroyed sympathetic catecholaminergic fibers. Collectively, these findings suggest that catecholaminergic fibers located outside the blood-brain barrier contribute to VP and OT secretion during hemorrhage and arterial hypotension.     

Central blockade of oxytocin receptors during mid-late gestation reduces amplitude of slow afterhyperpolarization in supraoptic oxytocin neurons

The neurohypophysial hormone oxytocin (OT), synthesized in magnocellular paraventricular (PVN) and supraoptic (SON) nuclei, is well known for its effects in lactation. Our previous studies showed that central OT receptor (OTR) binding is increased during gestation and that blockade of central OTRs, specifically during mid-late gestation, causes a delay in OT release during suckling and reduces weight gain in pups, suggesting decreased milk delivery. In the present study, we tested whether central OTR blockade during late gestation disrupts the gestation-related plasticity in intrinsic membrane properties. Whole cell current-clamp recordings were performed in OT neurons from pregnant rats (19-22 days in gestation) that were infused with an OTR antagonist (OTA) or artificial cerebrospinal fluid (aCSF) and from virgin rats infused with aCSF into the third ventricle via an osmotic minipump beginning on days 12-14 of gestation. The amplitudes of both Ca(2+)-dependent afterhyperpolarizations (AHPs), an apamin-sensitive medium AHP (mAHP) and an apamin-insensitive slow AHP (sAHP), were significantly increased during late gestation in control pregnant animals. However, the amplitude of the sAHP from pregnant rats treated with the OTA was significantly smaller than that of pregnant control rats and similar to that of virgins. These results indicate that the diminished efficiency in lactation due to OTR blockade may be partly a result of an altered sAHP that would shape OT bursting. These findings suggest that central actions of OT during late gestation are necessary for programming the plasticity of at least some of the intrinsic membrane properties in OT neurons during lactation.     

Peripheral pulses of oxytocin increase partner preferences in female, but not male, prairie voles

Centrally administered oxytocin (OT) facilitates social behaviors including the partner preferences that characterize the monogamous social system of prairie voles. In contrast peripherally administered OT generally has been ineffective in influencing central processes including behavior. OT from the posterior pituitary gland is released in pulses into the peripheral circulation. We hypothesized that peripherally administered OT, if delivered in repeated injections mimicking these pulses, would influence behavior. Male and female prairie voles received three subcutaneous injections of OT, a single injection of OT, or isotonic saline. Animals then were placed with an adult member of the opposite sex, designated as a "partner," for a 1-h period of cohabitation, and subsequently tested for preference for the familiar partner versus a comparable stranger. Females treated with pulses of peripheral OT (1, 5, or 20 microg) displayed a significant preference for the partner compared to control females, while females receiving a lower dose of OT (0.1 microg) or a single injection (20 microg) did not. There was also a significant within-group effect as pulsed OT-treated females spent more time with the partner when compared to the stranger, while control females spent equal amounts of time with the partner and stranger. Peripheral pulses of OT were no longer effective in inducing partner preferences when females were pretreated with a selective OT receptor antagonist, administered either peripherally or centrally. In contrast to females, peripheral treatment with OT did not facilitate the formation of partner preferences in males.     

The effects of ochratoxin A on lipid peroxidation and antioxidant enzymes: a protective role of melatonin

Various studies indicate that the mycotoxin ochratoxin A (OTA) is a carcinogenic, genotoxic, teratogenic, immunotoxic, and nephrotoxic agent. In the present study, the activities of some enzymes in the serum and liver of rats with ochratoxicosis and the effects of melatonin on these enzymes were investigated. Rats were divided into three equal groups, each consisting of eight rats; control, OTA (289 microg/kg per day) and OTA + melatonin groups for this study. In the OTA treated group, the level of lipid peroxidation (LPO) and the activities of glutathione peroxidase (GSH-Px) were increased in the liver and serum in comparison with the control group. The activities of catalase (CAT) and superoxide dismutase (SOD) were significantly changed in the serum when compared with controls. Our results showed structural tissue damage in the liver in OTA-treated rats. Melatonin decreased the OTA-induced damage to support the antioxidant defense system and/or with free radical scavenger action.     

Central oxytocin modulates exercise-induced tachycardia

We have shown that vasopressinergic projections to dorsal brain stem are activated during exercise and facilitate exercise tachycardia in both trained (T) and sedentary (S) rats (Dufloth DL, Morris M, and Michelini LC. Am J Physiol Regulatory Integrative Comp Physiol 273: R1271-R1282, 1997). In the present study, we investigated whether oxytocinergic projections to the nucleus of the solitary tract (NTS)-dorsal motor nucleus of the vagus (DMV) complex (NTS/DMV) are involved in the differential heart rate (HR) response to exercise in T and S rats. Arterial pressure (AP) and HR responses to dynamic exercise (0.4-1.4 km/h) were compared in S and T pretreated with vehicle (saline), oxytocin (OT; 20 pmol/200 nl) or OT-receptor antagonist (OT(ant); 20 pmol/200 nl) into the NTS/DMV. OT content in specific brain regions and plasma were measured in separate S and T groups at rest and immediately after exercise. Exercise increased OT content in dorsal (4.5-fold) and ventral brain stem (2.7-fold) and spinal cord (3.4-fold) only in T rats. No significant changes were observed in neurosecretory regions or medial eminence and posterior pituitary, but plasma levels of T rats were reduced immediately after exercise. Blockade of NTS/DMV OT receptors did not change basal mean AP (MAP) and HR or the MAP response to exercise. However, OT(ant) potentiated exercise-induced tachycardia (average increase of 26%) only in the T group. Pretreatment with exogenous OT in the NTS/DMV blunted the tachycardic response both in S and T rats without changing the MAP response. Administration of OT-receptor antagonist or OT into the fourth cerebral ventricle had no effect on the cardiovascular response to dynamic exercise. Taken together, the results suggest that oxytocinergic projections to the NTS/DMV are stimulated when T rats exercise and that OT released at this level acts on OT receptors to restrain exercise-induced tachycardia.     

Both oxytocin and vasopressin may influence alloparental behavior in male prairie voles

Neuropeptides, especially oxytocin (OT) and arginine vasopressin (AVP), have been implicated in several features of monogamy including alloparenting. The purpose of the present study was to examine the role of OT and AVP in alloparental behavior in reproductively na?ve male prairie voles. Males received intracerebroventricular (ICV) injections of artificial cerebrospinal fluid (aCSF), OT, an OT receptor antagonist (OTA), AVP, an AVP receptor antagonist (AVPA), or combinations of OTA and AVPA and were subsequently tested for parental behavior. Approximately 45 min after treatment, animals were tested for behavioral responses to stimulus pups. In a 10-min test, spontaneous alloparental behavior was high in control animals. OT and AVP did not significantly increase the number of males that showed parental behavior, although more subtle behavioral changes were observed. Combined treatment with AVPA and OTA (10 ng each) significantly reduced male parental behavior and increased attacks; following a lower dose (1 ng OTA/1 ng AVPA), males were less likely to display kyphosis and tended to be slower to approach pups than controls. Since treatment with only one antagonist did not interfere with the expression of alloparenting, these results suggest that access to either OT or AVP receptors may be sufficient for the expression of alloparenting.     

The influence of estradiol and progesterone on the concentrations of uterine oxytocin receptors and plasma PGFM in response to oxytocin in ovariectomized gilts

Peripubertal gilts (n = 25) were treated with corn oil (CO) or ovarian steroids, one month following an ovariectomy. The first day of treatment was assigned as the first day of the experiment. The gilts received: Group (Gr) I (n = 4)--CO (2 mL x day(-1) from 1st to 12th day), Gr II (n = 4) and Gr III (n = 4)--progesterone (P4; 10 to 100 mg x day(-1) from 1st to 12th day), Gr IV (n = 5)--estradiol benzoate (EB; 400 microg x day(-1) from 1st to 3rd day), Gr V (n = 4) and Gr VI (n = 4)--EB + P4 (EB 400 microg x day(-1) from 1st to 3rd day, 20 microg x day(-1) at 6th and 9th day, 50 microg at 12th day plus P4 10 to 100 mg from 4th to 15th day). All gilts were injected with oxytocin (OT; 20 IU; i.v.) on the following days of the experiment: 13th (Gr I and Gr II), 15th (Gr III and Gr IV), 16th (Gr V) and 18th (Gr VI). Concentrations of the PGF2alpha metabolite--PGFM were determined in blood samples, collected from 30 min before to 120 min after OT injection. Baseline PGFM concentrations (30 min before OT) differed among treatment groups and were the highest in Gr V and Gr VI (P < 0.01 vs. other groups). The magnitude of the PGFM response to OT increased only in four of the five gilts of Gr IV and in three of the four gilts of Gr VI, and it was higher (P = 0.009) in Gr VI than in Gr IV. In the remaining groups, PGFM concentrations did not increase above the baseline in response to OT. The day after OT injection, oxytocin receptors (OTR) were found in the uterine tissues of all animals studied. The lowest OTR concentrations were in Gr I--75.5 +/- 11.2 fmol x mg protein(-1) and the highest in Gr IV--712.9 +/- 86.7 fmol x mg protein(-1); (P < 0.05 vs. other groups). The values of K of OTR differed among groups (P < 0.001) and ranged from 1.62 +/- 0.44 nM in Gr I to 12. 08 +/- 1.9 nM in Gr VI. A positive correlation (r = 0.54; P < 0.01) between plasma E2 and uterine OTR concentrations was observed. In conclusion, E2 and P4 are involved in both PGF2 synthesis/secretion and OTR formation, however, full PGF response to OT does not develop before puberty. Estrogens are evident stimulators of uterine OTR synthesis ingilts.     

Myometrial oxytocin receptors and prostaglandin in the parturition process in the rat.

Parturition in rats is associated with an abrupt and marked increase in myometrial oxytocin (OT) receptor concentrations. In this study, we investigated the role of myometrial OT receptors in the initiation and the process of parturition. We produced chronic OT receptor blockade during the last 3 days of gestation by administration of a specific OT antagonist at 100 micrograms/day and 300 micrograms/day. We also suppressed OT receptor formation by inhibiting prostaglandin synthesis with naproxen sodium at 2 mg/day and 5 mg/day. We found that chronic blockade of OT receptors inhibited the uterotonic response to OT in Day 22 and Day 23 pregnant rats in a dose-dependent manner. OT antagonist treatment did not prolong the gestation period. However, the duration of parturition, fetal mortality, and the mortality incidence were increased in rats treated with the high dose of the OT antagonist compared to controls. Naproxen sodium at both dosage levels prolonged gestation by 24 h or longer, doubled the duration of parturition, and markedly increased fetal mortality and mortality incidence. Combined OT antagonist and naproxen treatment produced adverse outcomes similar to that produced by naproxen treatment alone. Myometrial OT receptor concentrations were markedly increased in all rats immediately postpartum, ranging from 210 to 425 fmol/mg protein compared to the 50 to 100 fmol/mg found in Day 21 and Day 22 pregnant rats. Correlation analyses between OT receptor concentrations and various parameters associated with gestation and parturition showed that there was a correlation between low OT receptor concentrations and long gestation period, prolonged parturition, and high fetal mortality rate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sex differences and developmental effects of oxytocin on aggression and social behavior in prairie voles (Microtus ochrogaster)

Various hormones, including sex steroids and neuropeptides, have been implicated in aggression. In this study we examined (1) sex differences in intrasexual aggression in na?ve prairie voles; (2) the effects of developmental manipulations of oxytocin on intrasexual aggression; and (3) changes in patterns of intrasexual aggression after brief exposure to an animal of the opposite sex. Within 24 h of birth, infants were randomly assigned to receive either an injection of oxytocin (OT) or oxytocin antagonist (OTA) or to one of two control (CTL) groups receiving either isotonic saline or handling without injection. As adults, animals were tested twice in a neutral arena; before (Test 1) and 24 h after (Test 2) a 4-h exposure to an animal of the opposite sex. In Test 1, CTL males were more likely to show aggressive and less likely to show social behavior than CTL females. No significant treatment differences were observed within either sex in Test 1. In Test 2, after brief exposure to a male, females treated with OT became more aggressive and less social than OTA or CTL females. Male aggressive behavior did not change after exposure to a female. An increase in aggression and decline in social behavior toward other females, seen here in OT-treated females, is typically observed only following several days of female-male cohabitation. These findings demonstrate a sex difference in intrasexual aggression and suggest that neonatal exposure to OT may facilitate the onset of the mate-guarding component of pair bonding in female prairie voles.     

Vasopressin and oxytocin release evoked by NaCl loads are selectively blunted by area postrema lesions

The present study investigated the effect of area postrema lesions (APX) on stimulated neurohypophysial secretion of vasopressin (VP) and oxytocin (OT) in conscious rats. Blunted increases in plasma levels of both pituitary hormones were observed when rats with APX were infused intravenously with 1 M NaCl solution (2 ml/h for 6 h). In contrast, plasma VP and OT increased normally in rats with APX when equivalent increases in plasma osmolality (but not plasma Na(+)) resulted from intravenous infusion of an equiosmotic solution of 1 M mannitol and 0.5 M NaCl. Furthermore, APX did not affect increases in plasma VP and OT stimulated by plasma volume deficits, nor did APX disrupt OT secretion stimulated by intravenous injection of cholecystokinin. These findings suggest that the area postrema plays an important role in mediating secretion of VP and OT in response to an NaCl load, but not in response to an equiosmotic load that does not cause substantial hypernatremia, and not in response to other stimuli of neurohypophysial hormone secretion. Together with previous reports, these results suggest that APX impairs Na(+) regulation in rats.     

Conjugated estradiol increases female sexual receptivity in response to oxytocin infused into the medial preoptic area and medial basal hypothalamus

The ovarian steroid estradiol (E) has been found to increase both receptor affinity and release of the neuropeptide oxytocin (OT) in plasma membrane preparations. Therefore, we hypothesized that E conjugated to bovine serum albumin at position 6 (E-6-BSA) would increase behavioral responsiveness to OT. Preliminary results showed that 200 ng/microl of E-6-BSA increased sexual receptivity slightly, but not significantly. Therefore, this dose was used as a subthreshold dose to test whether it would increase sexual responsiveness when infused in combination with 100 ng/microl OT. After recovery from cannula implantation surgery animals were injected with 0.5 microg E benzoate daily for 3 days before testing. On the fourth day, after a baseline preinfusion test rats were infused bilaterally with E-6-BSA alone or with OT, OT with BSA, or conjugated progesterone, luteinizing hormone-releasing hormone equimolar to OT alone, or with E-6-BSA or conjugated progesterone alone. When infused into either the medial preoptic area-anterior hypothalamus or the medial basal hypothalamus the combination of OT and E-6-BSA significantly increased sexual receptivity over receptivity after artificial cerebrospinal fluid control infusions. Neither bilateral infusions of OT in combination with conjugated progesterone nor E-6-BSA in combination with luteinizing hormone-releasing hormone enhanced sexual receptivity. Results presented here strongly support the conclusion that some of the effects that E has in sensitizing brain systems to the facilitating effects of OT occur at the membrane level in the medial preoptic area-anterior hypothalamus and medial basal hypothalamus.     

Adolescent exposure to oxytocin,but not the selective oxytocin receptor agonist TGOT,increases social behavior and plasma oxytocin in adulthood

There are indications that exposing adolescent rodents to oxytocin (OT) may have positive “trait-changing” effects resulting in increased sociability and decreased anxiety that last well beyond acute drug exposure and into adulthood. Such findings may have relevance to the utility of OT in producing sustained beneficial effects in human psychiatric conditions. The present study further examined these effects using an intermittent regime of OT exposure in adolescence, and using Long Evans rats, that are generally more sensitive to the acute prosocial effects of OT. As OT has substantial affinity for the vasopressin V1a receptor (V1aR) in addition to the oxytocin receptor (OTR), we examined whether a more selective peptidergic OTR agonist – [Thr4, Gly7]-oxytocin (TGOT) – would have similar lasting effects on behavior. Male Long Evans rats received OT or TGOT (0.5–1 mg/kg, intraperitoneal), once every three days, for a total of 10 doses during adolescence (postnatal day (PND) 28–55). Social and anxiety-related behaviors were assessed during acute administration as well as later in adulthood (from PND 70 onwards). OT produced greater acute behavioral effects than TGOT, including an inhibition of social play and reduced rearing, most likely reflecting primary sedative effects. In adulthood, OT but not TGOT pretreated rats displayed lasting increases in social interaction, accompanied by an enduring increase in plasma OT. These findings confirm lasting behavioral and neuroendocrine effects of adolescent OT exposure. However, the absence of such effects with TGOT suggests possible involvement of the V1aR as well as the OTR in this example of developmental neuroplasticity.     

Endogenous oxytocin is necessary for preferential Fos expression to male odors in the bed nucleus of the stria terminalis in female Syrian hamsters

Successful reproduction in mammals depends on proceptive or solicitational behaviors that enhance the probability of encountering potential mates. In female Syrian hamsters, one such behavior is vaginal scent marking. Recent evidence suggests that the neuropeptide oxytocin (OT) may be critical for regulating this behavior. Blockade of OT receptors in the bed nucleus of the stria terminalis (BNST) or the medial preoptic area (MPOA) decreases vaginal marking responses to male odors; lesion data suggest that BNST, rather than MPOA, mediates this effect. However, how OT interacts with sexual odor processing to drive preferential solicitation is not known. To address this issue, intact female Syrian hamsters were exposed to male or female odors and their brains processed for immunohistochemistry for Fos, a marker of recent neuronal activation, and OT. Additional females were injected intracerebroventricularly (ICV) with an oxytocin receptor antagonist (OTA) or vehicle, and then tested for vaginal marking and Fos responses to sexual odors. Colocalization of OT and Fos in the paraventricular nucleus of the hypothalamus was unchanged following exposure to male odors, but decreased following exposure to female odors. Following injections of OTA, Fos expression to male odors was decreased in BNST, but not in MPOA or the medial amygdala (MA). Fos expression in BNST may be functionally relevant for vaginal marking, given that there was a positive correlation between Fos expression and vaginal marking for BNST, but not MPOA or MA. Together, these data suggest that OT facilitation of neuronal activity in BNST underlies the facilitative effects of OT on solicitational responses to male odors.     

Changes in ovarian oxytocin secretion as an indicator of corpus luteum response to prostaglandin F(2alpha) treatment in cattle

Exogenous prostaglandin F(2alpha) (PGF(2alpha)) rapidly increases ovarian oxytocin (OT) release and decreases progesterone (P4) secretion in cattle. Hence, the measurement of OT secretion (the area under the curve and the height of the peak) after different doses of Oestrophan - PGF(2alpha) analogue (aPGF(2alpha)) on Days 12 and 18 of the estrous cycle (estrus = day 0), could be a suitable indicator of corpus luteum (CL) sensitivity to PGF(2alpha) treatment. Mature heifers (n = 36) were used in this study. Blood samples were collected from the jugular vein for the estimation of OT, P4 and 13, 14-dihydro-15-keto-prostaglandin F(2alpha) (PGFM). In Experiment 1, different doses of aPGF(2alpha) (400, 300, 200 and 100 microg) given on Day 12 of the estrous cycle (n = 8) shortened (P < 0.05) the cycle duration (15.2 +/- 0.6 d) compared with that of the control (21.7 +/- 0.4 d). Successive heifers were also treated on Day 12 with 200 (n = 2), 100 (n = 2), 75 (n = 2) or 50 microg aPGF(2alpha) (n = 2). Only the 50 microg aPGF(2alpha) dose did not cause CL regression, although it increased OT concentrations to levels comparable to those observed during spontaneous luteolysis (50 to 70 pg/ml). In Experiment 2, on Day 18 of the cycle heifers (n = 8) were treated with 50, 40, 30 and 20 microg aPGF(2alpha). There was a dose-dependent effect of aPGF(2alpha) on OT secretion on Day 18 of the estrous cycle (r = 0.77; P < 0.05). In Experiment 3, an injection of 500 microg aPGF(2alpha) on Day 12 (n = 4) and 50 microg aPGF(2alpha) on Day 18 (n = 4) caused a similar (P > 0.05) increase in the OT concentration (288.5 +/- 23.0 and 261.5 +/- 34.7 pg/ml, respectively). Thus the effect of the same dose of aPGF(2alpha) (50 microg) on OT secretion was different on Days 12 and 18 of the cycle. To evoke similar OT secretion on Days 12 and 18 the dose of aPGF(2alpha) on Day 18 could be reduced 10-fold, confirming that CL sensitivity to PGF(2alpha) appears to increase in the late luteal phase.     

Secretion of oxytocin in pregnant and parturient cows: corpus luteum may contribute to plasma oxytocin at term

Plasma oxytocin (OT) concentrations were determined in 14 late-pregnant and parturient Angus-Hereford cows. Jugular and utero-ovarian veins were cannulated for simultaneous withdrawal of blood samples. Samples were collected at 10-min intervals for 6 h once weekly beginning 60-14 days before the date of expected delivery (group 1), or daily 3-7 days before the due date (group 2). In a third group, samples were collected at 15-min intervals every other day for 12 h beginning 1 wk before calving. Basal levels of OT were low, the overall mean for both veins was 0.46 +/- 0.03 microU/ml until a week before parturition, and then increased to 0.77 +/- 0.1 microU/ml (P < 0.02). Spurts of OT occurred intermittently on all days. Interpeak intervals averaged 71.0 +/- 10.7 min until Day -14, and from Day -14 to Day -1 the intervals were 44.0 +/- 5.3 min (P < 0.05). From Day -60 to Day -25 the amplitudes of OT peaks were low and similar in both veins (mean 1.37 +/- 0.1 microU/ml). From Day -14 to Day -1 the peak amplitudes were 3.6 +/- 0.4 microU/ml on average (P < 0.02). During the last 2 wk the utero-ovarian peak of OT was frequently higher than the peripheral peak. In addition, a number of spurts were observed in the utero-ovarian vein only (solo peaks). On the day of parturition during the first stage of labor, peak amplitudes had increased to 7.3 +/- 2.0 microU/ml, and the interpeak intervals had become shorter than before labor (mean 25.1 +/- 2.6 min). A large surge of OT initiated the expulsive stage of labor. Basal levels rose to 43.1 +/- 16 microU/ml and 38.7 +/- 12.6 microU/ml, and peak levels to 77.4 +/- 19.1 microU/ml and 91.6 +/- 21 microU/ml in the jugular and utero-ovarian veins, respectively. Interpeak intervals had decreased to 17.2 +/- 3.3 min (P < 0.05). Oxytocin levels remained high after delivery of the calf until the placenta was expelled. The posterior pituitary was the source of circulating OT during most of gestation and labor, but the solo peaks observed during late gestation in the utero-ovarian vein were probably of luteal origin or possibly of caruncular origin, because near term, both tissues express OT mRNA. Fetal posterior pituitary is another possible source for these peaks. Our conclusions are that during bovine pregnancy, low amplitude spurts of OT are secreted intermittently; near term, both the frequency and peak amplitude of the spurts increase; and during labor, a dramatic increase in plasma OT precedes the expulsion of the calf. The main source of OT is the posterior pituitary, but near term, a utero-ovarian source secretes additional OT into the systemic circulation.     

Effect of long-term GHRH and somatostatin administration on GH release and body weight in prepubertal female rats

In order to find a chronic GHRH administration capable of stimulating growth rate without depleting pituitary GH content, prepubertal female rats were subcutaneously (sc) treated with GHRH (1-29)-NH2 and somatostatin (SS). In experiment 1, the rats received sc injections of GHRH and cyclic natural SS for 19 days. In the second study, female rats were continuously treated during 21 days with GHRH, using a slow release pellet, alone or combined with one daily injection of long acting SS (octreotide). In experiment 1, body weight was significantly increased when GHRH was administered at the highest daily dosage (1200 microg/day), accompanied by an slight increment in pituitary GH content. Hypothalamic SS concentrations decreased when GHRH or SS were administered alone whereas the combined treatment with both peptides did not modify this parameter, which suggests the existence of a balance between the chronic actions of both peptides on hypothalamus. In experiment 2, the continuous infusion of GHRH increased plasma GH levels and tended to enhance pituitary GH content. Nevertheless, GHRH effect was not effective enough to increase body weight. By adding one daily injection of SS both GHRH effects on the pituitary gland were abolished. Our study indicates that female rats retain responsiveness to chronic GHRH and SS treatments at both pituitary and hypothalamic levels.     

Time course of induction of oxytocin messenger ribonucleic acid levels in the hypothalamic paraventricular nucleus of ovariectomized rats following gonadal steroid administration

The nonapeptide oxytocin (OT) is important for uterine contractility at parturition, milk ejection during lactation, and the induction of maternal behavior. OT messenger ribonucleic acid (mRNA) levels increase in the paraventricular and supraoptic nuclei (PVN and SON) of late pregnant and lactating rats and are modulated by the steroid milieu that accompanies these states. Specifically, sequential exposure to estradiol (E₂) and progesterone (P) followed by P withdrawal 48 hrs prior to sacrifice increases PVN, and to a lesser but significant degree, SON OT mRNA. To better define the time course of induction of OT mRNA levels following P withdrawal, ovariectomized Sprague-Dawley rats were treated with empty or steroid-filled capsules. On day 1, animals received an E₂-filled or empty capsule, followed by P-filled or empty capsules on day 3. On day 14, P-filled or empty capsules were removed and animals were sacrificed 24, 36, or 48 hrs later. The hypothalamic PVN were analyzed for OT mRNA by in situ hybridization histochemistry. Significant differences in PVN OT mRNA were found among the groups (P < 0.0001, Kruskal-Wallis). Animals in the 48 hr (P = 0.007) and 36 hr (P = 0.005), but not the 24 hr, steroid-treated groups had significantly increased OT mRNA relative to their respective sham-treated cohorts (Mann-Whitney U test). The relative abundance of PVN OT mRNA differed among the steroid-treated groups (Kruskal-Wallis, P < 0.0003), with highest levels at 48 hr. We conclude that increases in PVN OT mRNA occur by 36 hrs, and are highest at 48 hrs, after P withdrawal in the E₂-primed rat. Future studies will determine if OT-mediated changes in behavior or physiology that surround parturition are related to these changes in OT mRNA.