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PubMed1991

Serum luteinizing hormone, prolactin, and thyrotropin and their pituitary subunit mRNA levels during proestrus in the Syrian hamster.

mRNA levels for alpha, luteinizing hormone beta (LH beta), and prolactin (Prl) were examined during the hamster estrous cycle, with sampling most frequent (1-hour intervals) on the afternoon of proestrus. These transcripts encode the peptide subunits for the pituitary hormones LH and Prl which are necessary for reproductive function. Serum hormone levels of LH and Prl, analyzed by 24-hour periodic regression, exhibited a 24-hour periodicity on proestrus characterized by a large surge peaking at about 18.00 h. Combining the data for non-proestrous days of the cycle disclosed a rhythm with similar timing for LH and Prl. Thyroid-stimulating hormone (TSH) and TSH beta RNA profiles during hamster proestrus are reported for the first time. Serum TSH exhibited a pronounced peak coincident with that of the other hormones on proestrus. Because of variations at other times on the day of proestrus, however, a 24-hour periodicity was not manifested by regressional analysis. Combined non-proestrous serum TSH data also revealed no consistently timed regressional 24-hour periodicity. During proestrus, pituitary mRNA values for alpha, LH beta, and Prl simultaneously exhibited a rise from the lowest to the highest of all proestrous values in the 3-5 h prior to the time of the pre-ovulatory peak of circulating hormone concentrations. RNA for TSH beta exhibited an earlier, broader peak on proestrus. Periodic regression indicated a significant 24-hour rhythm for alpha mRNA in data pooled from non-proestrous days (acrophase 05.00 h) and for TSH beta mRNA on proestrus (acrophase 04.54 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of Variance↗

PubMed1986

Ovarian steroid regulation of basal pulsatile luteinizing hormone release between the mornings of proestrus and estrus in the rat.

The aim of this study was to examine the regulation of basal pulsatile LH release by ovarian estradiol (E2) and progesterone (P) during the interval between the mornings of proestrus and estrus in the rat estrous cycle. Pulsatile LH release was studied in six groups of rats bled continuously through jugular venous cannulae between 0930-1230 h at a rate of 50 microliter whole blood/5 min: 1) bled on proestrus; 2) sham ovariectomy (OVX) at 0900-1000 h on proestrus and bled on estrus; 3-6) OVX at 0900-1000 h on proestrus, implanted with either empty or E2-, P-, or E2- plus P-containing Silastic capsules, and bled 24 h after OVX. In our colony, plasma E2 levels peaked at 1300 h, remained high through 1730 h, and then declined. Plasma P values increased between 1300 and 1730 h, peaked at 2000 h, and were rapidly declining by 2400 h. To reproduce the magnitude as well as the temporal pattern for these changes in plasma E2 and P levels, E2 capsules were inserted at the time of OVX on proestrus and removed at 1830 h. P capsules were inserted at 1400 h and removed at 2300 h. Groups of ovariectomized or sham-ovariectomized control animals had empty capsules implanted and removed at comparable times. Capsules producing basal E2 and P levels were not inserted after the removal of the original implant, since mean blood LH levels, pulse amplitude, and frequency were the same in rats sham ovariectomized or ovariectomized at 1830 h on proestrus and bled the next morning between 0930-1230 h. Mean blood LH levels decreased between the mornings of proestrus and estrus due to a reduction in LH pulse frequency as pulse amplitude remained stable. OVX at 0900-1000 h on proestrus increased mean blood LH levels 2.5-fold compared to values on estrus due to increases in both LH pulse frequency and amplitude. Restoration of physiological proestrous levels of only E2 returned LH pulse frequency to estrous values, but did not significantly affect LH pulse amplitude. P alone also had no significant effect on LH pulse amplitude, but slightly reduced pulse frequency, although, unlike E2, not to values seen on estrus. Replacing both E2 and P returned LH pulse amplitude to estrous levels and reduced LH pulse frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

Effect of recombinant inhibin on gonadotropin secretion during proestrus and estrus in the rat.

This work investigated the ability of recombinant human (rh) inhibin A or the GnRH antagonist [Ac-D2Nal1, delta Cpa2, delta 3Pal3, Arg5 delta 5-(p-methoxyphenyl)5-oxo-2-aminopentanoic acid6, delta Ala10]GnRH to alter plasma immunoreactive LH and FSH levels in cycling female rats. In a first series of experiments, rh inhibin A (25 micrograms/kg) was injected iv between 0800 and 0830 h, or at 0800 and 1200 h, on proestrus, and plasma hormone levels were measured from 1200-1900 h. Both regimens of administration significantly (P less than or equal to 0.01) lowered mean plasma FSH levels but did not mask the primary FSH surge. This treatment also lowered the overall amount of LH released, although the difference between control and inhibin-treated rats only reached statistical significance (P less than or equal to 0.05) after two injections of the protein. Measurement of the number of tubal ova shed on the next estrus showed no difference between rats injected with the vehicle or inhibin at 0830 h. Finally, it was shown that administration of the GnRH antagonist (100 micrograms/kg) at 1200 h interfered with the primary surges of both LH and FSH. In a second series of experiments, rh inhibin A (25 micrograms/kg) was injected once at 2000 h on proestrus. In these animals, inhibin significantly (P less than or equal to 0.01) decreased mean plasma FSH levels between 2000 h on proestrus and 0500 h on estrus and totally suppressed the secondary FSH surge. LH secretion remained low in control animals, and no measurable changes were observed after inhibin treatment. Flushing of the ova 5 days later (i.e. during estrus of the subsequent cycle) showed no difference between control and inhibin-treated rats. Injection of the GnRH antagonist (100 micrograms/kg) at 2000 h on proestrus decreased the total amount of FSH secreted during late proestrus and early estrus. However, FSH secretion showed an increase at between 0100 and 0400 h of estrus despite blockade of GnRH receptors. These results indicate that administration of rh inhibin A interferes with both the primary and secondary FSH surges. In contrast to its inability to alter LH release by ovariectomized rats, inhibin also blunted LH secretion during the afternoon of proestrus. Whether these results represent differential effects of inhibin on LH secreted by intact and gonadectomized animals, or whether the documented changes in pituitary responsiveness to GnRH during proestrus are accompanied by an increased sensitivity to inhibin, needs further investigation.

Animals↗

PubMed1985

Ovarian steroid regulation of pulsatile luteinizing hormone release during the interval between the mornings of diestrus 2 and proestrus in the rat.

The object of this study was to determine the influence of ovarian steroids on pulsatile LH release in the interval between the mornings of diestrus 2 (D2) and proestrus in the rat. Four groups of rats were bled continuously for 3 h between 09.30-12.30 h at a rate of 75 microliters whole blood/6 min: bled on D2; sham ovariectomy (OVX) on D2 and bled on proestrus; OVX on D2, implanted with empty or oil-filled capsules, and bled 24 h later; and OVX on D2, implanted with estradiol (E2) capsules, and bled 24 h later. Between D2 and proestrus, plasma E2 levels increased from 13 +/- 1 to 42 +/- 9 pg/ml, and progesterone levels decreased from 27 +/- 3 to 13 +/- 2 ng/ml, the latter reflecting the decline of the corpus luteum early on D2. Between D2 and proestrus there was no change in mean blood LH levels, LH pulse amplitude, or pulse frequency. However OVX on D2 increased mean blood LH levels 2.5-fold over values on proestrus due to a 3.5-fold elevation in LH pulse amplitude and an 80% increase in pulse frequency. E2 levels fell in these rats to 8 +/- 1 pg/ml. Restoration of physiological proestrous levels of E2 (46 +/- 5 pg/ml) significantly reduced the increase in mean blood LH levels by lowering pulse frequency to proestrous values, and by causing a 50% reduction in pulse amplitude. However, LH pulse amplitude and therefore mean blood LH levels were still higher than values on proestrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1988

Evidence that a decrease in opioid tone on proestrus changes the episodic pattern of luteinizing hormone (LH) secretion: implications in the preovulatory LH hypersecretion.

We have studied the LH secretion pattern evoked by diminution in the opioid tone produced by iv naloxone (NAL) infusion between 1100-1400 h on proestrus, the LH secretion pattern occurring spontaneously between 1430-1730 h on proestrus and the LH secretion pattern produced by exogenous LHRH administered either as a 10 ng/pulse at 20-, 30-, or 60-min intervals or infused continuously at a rate of 30 ng/h between 1200-1700 h in rats given pentobarbital at 1100 h on proestrus. Infusion of 0.5 ng NAL/h raised plasma NAL levels to 200-300 ng/ml and augmented LH secretion, as evident by increments in pulse amplitude and frequency discharge to one every 37.5 min from an average of one every 75 min in saline-infused control rats. A 4-fold increase in circulating NAL levels, produced by 2 mg/h NAL infusion, further augmented the frequency of LH episodes to 30-33 min and induced a surge-like LH secretion pattern which resembled that seen on the afternoon of proestrus. Further analysis of the secretory pattern of the preovulatory LH surge (n = 7) showed LH pulses of increased amplitude during the basal phase (n = 4), ascending phase (n = 2), and plateau and descending phases (n = 3); in two rats the LH rise was steep, and no LH pulses were identified. A LHRH pulse (10 ng/pulse) delivered at 20- or 30-min intervals or continuous infusion of LHRH at a rate of 30 ng/h produced LH surges, with peak levels reaching the range seen on the afternoon of proestrus. Further, despite the fact that 10 ng LHRH/pulse at 20-min intervals reproduced a proestrous-type LH surge, only 40% of the LHRH pulses were followed by identifiable LH pulses. Surprisingly, despite the observations that NAL evoked robust LH episodes, the basal pattern of FSH secretion in these rats was not altered. These findings show that a decrease in opioid tone on proestrus accelerates episodic LH discharge to the range that occurs after gonadectomy. A quantitative relationship between the degree of restraint on the opioid tone imposed by NAL and the magnitude of the LH response can be demonstrated. The evidence suggests that the preovulatory LH surge may occur in an episodic fashion and that it can be reproduced by LHRH delivered at a frequency rate of LH pulses seen in ovariectomized rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1975

Alterations in steroid and gonadotropin release resulting from surgical stress during the morning of proestrus in 5-day cyclic rats.

The purpose of this experiment was to determine whether surgical stress on the morning of proestrus would elicit an early release of gonadotropin from the pituitary. Animals exhibiting 5-day estrous cycles underwent bilateral sham-ovariectomy under ether anesthesia at 0800 h of proestrus. These animals had high levels of progesterone and estradiol following the surgery. These steroids were thought to be adrenal in origin, since animals adrenalectomized at 0800 h of proestrus had low progesterone levels and estradiol comparable to unoperated controls. Subsequently, the sham-operated animals showed high FSH but not LH values at 1300 h, prior to the normal critical period for gonadotropin release. By 1400, the LH surge had begun, and progesterone was again being released. Adrenalectomized and unoperated controls showed no increase in any steroid or gonadotropin measured before 1400 h. These findings suggest that stress-induced release of adrenal estradiol and progesterone, rather than some other consequence of the surgical procedure, during the morning of proestrus, can advance the onset of release of FSH, prior to LH. Ovariectomy at 0800 h proestrus led to a rapid and dramatic increase in FSH but not LH secretion by 4 h after surgery. By 6 h after ovariectomy FSH had increased to six times control values and LH had increased to twice control values. Estradiol remained at control values for 6 h following surgery but 20alpha-hydroxypreg-4-en-3-one (20alpha-OHP) dropped quickly to baseline values. It is possible that a reduction in circulating 20 alpha-OHP may be responsible for the increases in FSH prior to LH in this group, but the absence of other negative feedback factors from the ovary or adrenal may also be involved.

20-alpha-Dihydroprogesterone↗

PubMed1983

Changes in beta-endorphin content in discrete areas of the hypothalamus throughout proestrus and diestrus of the rat.

The aim of the present study is to investigate changes in beta-endorphin content in the hypothalamus during different stages of the estrous cycle. Groups of 9 to 10 Sprague-Dawley rats were sacrificed every two hours on proestrus from 8.00 to 18.00 h and groups of 7 to 8 rats were sacrificed on diestrus at 8.00, 12.00, 14.00 and 18.00 h. Preoptic suprachiasmatic region, posterior hypothalamus, arcuate nucleus and median eminence were dissected and assayed for beta-endorphin. A significant increase in beta-endorphin content was detected in the arcuate nucleus during proestrus (9.00 h: 1.76 +/- .31; 14.00 h: 4.10 +/- .85 microgram/g tissue wet weight). Levels did not change during diestrus (1.18 +/- .06 microgram/g). The increase caused significant differences in beta-endorphin values between both days at 12.00, 14.00 and 18.00 h, while the concentrations at 8.00 h were similar. The opposite pattern was observed in the median eminence with significantly higher proestrous beta-endorphin levels at 8.00 h (11.24 +/- 3.1 vs 3.52 +/- .64 microgram/g) and nonsignificant differences for the rest of the day. No significant change in beta-endorphin concentration was seen in the preoptic suprachiasmatic region over the day of proestrus (1.35 +/- .09 microgram/g). Diestrous beta-endorphin concentrations in this region were higher during the morning (2.60 +/- .65 microgram/g) and lower at 18.00 h (0.94 +/- .12 microgram/g) when compared to proestrous values. This pattern was caused by a 50% increase in beta-endorphin during the afternoon of diestrus. No changes were observed in the posterior hypothalamus on either day with comparable levels of beta-endorphin except at 18.00 h, when values were significantly higher on proestrus (1.66 +/- .30 vs 0.83 +/- .06 microgram/g).

Animals↗

PubMed1987

Effect of calcitonin on the prolactin surge of proestrus.

The effect of calcitonin (CT) on the prolactin (PRL) surge of proestrus in rats was investigated under normal and perturbed lighting conditions. Salmon calcitonin (SCT) was injected i.p. on diestrus 2 or on proestrus, plasma PRL levels were measured by radioimmunoassay. SCT had no effect on the PRL surge under normal lighting conditions but it induced a small drop in PRL level measured on proestrus morning, 3 hours after CT injection. Animals submitted to perturbed light conditions had higher PRL levels than those kept under normal lighting. These data would indicate that for the female rats on proestrus the sensitivity to stress due to injection and blood sampling may be modulated by changing the photoperiod. SCT injection under these conditions may facilitate this destabilization in PRL level.

Animals↗

PubMed1978

The response of adenosine 3',5'-monophosphate to norepinephrine in the hypothalamus and pineal organ of female rats in proestrus or diestrus.

The response of cAMP to norepinephrine (NE) was measured in hypothalamic tissue and pineal glands of cycling female rats killed on the mornings of proestrus and diestrus (in 4-day cycles) or diestrus II (in 5-day cycles). The results from 4- or 5-day cycles were similar in all cases. The report of Weiss and Crayton that the response of adenylate cyclase in pineal homogenates to NE is reduced on proestrus was confirmed, and a similar but lesser reduction was also detected with intact pineal glands incubated in vitro. Chopped hypothalamic tissue incubated in vitro revealed a greater response of cAMP levels to NE on proestrus compared to diestrus in the anterior hypothalamic-preoptic area, but no significant changes in the middle or posterior hypothalamus. In an effort to reproduce the changes noted on proestrus, ovariectomized animals were injected with estradiol benzoate for 2 days, but no effects of the treatment on the cAMP response to NE were found. If the animals were also given reserpine, however, the estrogen was effective in reducing the response of pineal adenylate cyclase to NE. Possible explanations for these results and the potential significance of alterations in NE receptor systems for the control of gonadotropin release are discussed.

Adenylyl Cyclases↗

PubMed1982

Increase of beta-endorphin concentrations in the plasma and pituitary neuro-intermediate lobe of the rat on the afternoon of proestrus.

Beta-endorphin (beta-EP) concentrations in the plasma and the anterior and neuro-intermediate lobes of the pituitary (AP and NIL) were quantitated by radioimmunoassay (RIA) and gel filtration chromatography in female rats at 1000, 1400, and 1900 h on the day of proestrus and diestrus day-1. There were no significant changes in beta-EP in the plasma, AP, or NIL on diestrus day-1. On proestrus, beta-EP in the plasma and NIL, but not the AP, increased significantly from 1000 to 1400 h and returned to basal levels by 1900 h. The time course of this increase of beta-EP in the NIL and plasma is consistent with the temporal sequence of the prolactin and gonadotropin surges on the afternoon of proestrus, suggesting that beta-EP in the NIL may be involved in the regulation of these neuroendocrine events.

Animals↗

PubMed1976

Quantitative changes in the metabolism of 20alpha-hydroxy-4-pregnen-3-one by rat hypothalamus and pituitary during proestrus.

The in vitro conversion of 20alpha-hydroxy-4-pregnen-3-one (20alpha-DHP) by medial basal hypothalamus and anterior pituitary was investigated throughout the day of proestrus in the 4-day cyclic rat. Reverse isotopic dilution analysis was utilized to quantitate the substrate remaining and three metabolic products: 20alpha-hydroxy-5alpha-pregnan-3-one, 5alpha-pregnane-3alpha,20alpha-diol and progesterone. Serum levels of 20alpha-DHP, progesterone, LH and FSH were measured by radioimmunoassay. Conversion of 20alpha-DHP to its 5alpha-reduced metabolites (20alpha-hydroxy-5alpha-pregnan-3-one and 5alpha-pregnane-3alpha,20alpha-diol) by the pituitary was constant throughout proestrus except for a significant decrease at 1600 h, near the end of the critical period. Although 5alpha-reduction of 20alpha-DHP by the hypothalamus fluctuated, it was relatively high at 1600 h and was lowest at 1400 h. Small amounts of progesterone (less than2%) were formed but there was not variation with time. The decrease in pituitary enzymic activity coincided with the time when serum levels of LH, FSH and progesterone were increasing but not with later times when the elevated serum levels were maintained. Thus, there may be endogenous regulation of 5alpha-reductase and 3alpha-hydroxysteroid dehydrogenase activity in rat pituitary and perhaps hypothalamus on the afternoon of proestrus. The regulation and subsequent effects of quantitative changes in 5alpha-reduction of 20alpha-DHP by pituitary and hypothalamus remain to be elucidated.

20-alpha-Dihydroprogesterone↗

PubMed1986

Catecholamines in sow graafian follicles at proestrus and at diestrus.

Endogenous dopamine, noradrenaline, and adrenaline were detected in the sow graafian follicular wall and in the follicular fluid. Noradrenaline represented the highest level and adrenaline the lowest. Dopamine and noradrenaline concentrations found in the follicular fluid were lower at early proestrus than at mid-diestrus, whereas adrenaline levels in the fluid did not differ at either stage of the estrous cycle. The sow follicular wall contained less dopamine, noradrenaline and adrenaline at early proestrus than at mid-diestrus. Concomitantly, a decrement of [3H]-dopamine and [3H]-noradrenaline uptake, and of dopamine-beta-hydroxylase activity was detected at early proestrus compared to levels detected at mid-diestrus. The findings in sow graafian follicles show the existence of relationships between hormonal status, dopamine, noradrenaline and adrenaline endogenous levels and uptake, and dopamine-beta-hydroxylase activity. Possible links between estradiol, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) levels during the pig estrous cycle and ovarian catecholamines are discussed, as is a plausible involvement of these neurotransmitters in the contractile activity of the theca layer and the processes of follicular rupture and ovulation.

Animals↗

PubMed1975

Role of brain monoamines in release of gonadotropin before proestrus in the cyclic rat.

To determine whether brain monoaminergic neurons are involved in the release of gonadotropins responsible for estrogen increases before proestrus, various inhibitors and precursors of monoamine biosynthesis were administered subcutaneously or intracranially to the 3rd ventricle at 10.00 or 20.00 on the day before proestrus, the 2nd day of diestrus (DII) in 4-day cycling rats. The inhibitors used were alpha-methyl-p-tyrosine (alpha-MPT) and bis-(4-methyl-1-homopiperazinyl-thiocarbonyl)-disulfide (FLA-63). The effects of these drugs on changes in vaginal cytology, ovulation, uterine weight of uterine intraluminal fluid, and on serum concentrations of LH and FSH were evaluated in selected experiments. (1) Administration of alpha-MPT (150 mg/kg s.c.), an inhibitor of tyrosine hydroxylase, at 10.00 on DII reduced weights of uterus and intraluminal fluid on the day of expected proestrus (P), prevented vaginal cornification on estrus (E), blocked ovulation in all 10 rats, and induced prolonged diestrus. (2) Administration of FLA-63 (10 mg/kg s.c.), an inhibitor of dopamine-beta-hydroxylase, at 10.00 on DII reduced weights of uterus and intraluminal fluid on P, blocked ovulation for a few days but did not prevent vaginal cornification at the expected time of E. (3) Administration of alpha-MPT (200 mg/kg) or FLA-63 (15 mg/kg) at 20.00 on DII blocked ovulation in all of 8 and 7 rats, respectively, but these treatments did not block vaginal cornification at the expected time in any animal. (4) Administration of L-DOPA (100 mg/kg) or dihydroxy-phenylserine (DOPS, 200 mg/kg) with alpha-MPT (200 mg/kg) at 20.00 on DII reversed the blocking effect of alpha-MPT on ovulation in 3 out of 6 and 3 out of 5 rats, respectively. (5) Direct application of crystalline alpha-MPT or FLA-63 (about 3-5 mug) to the 3rd ventricle at 20.00 on DII also blocked ovulation in all of 7 and 5 rats, respectively. (6) Both systemic and intraventricular injections of alpha-MPT at 10.00 on DII reduced serum LH on P but not serum FSH. FLA-63 by intraventricular injection also reduced serum LH but not serum FSH. (7) Injection of 17beta-estradiol (40 mug s.c.) with alpha-MPT or FLA-63 partially removed the ovulatory blockade induced by conditions 1, 2, 3 and 5. Therefore, norepinephrine seems to be an important neurotransmitter in the release of gonadotropin responsible for estrogen secretion before P, but dopamine may also be involved during its early stage as represented by 10.00 on DII.

Animals↗

PubMed1984

Median eminence serotonin involved in the proestrus gonadotropin release.

Numerous studies have suggested that serotonin (5-HT) is involved in the regulation of anterior pituitary hormone release. In the present study, the 5-HT concentrations of the median eminence and anterior pituitary lobe were measured during the estrous cycle and lactation in order to correlate changes in 5-HT levels with changes in serum luteinizing hormone, follicle-stimulating hormone, and prolactin. On the day of proestrus, median eminence 5-HT concentrations declined significantly between 14.00 and 16.30 h at the beginning of the gonadotropin and prolactin surges. No changes in 5-HT concentrations were found between the morning and afternoon on other days of the cycle. In the anterior pituitary, the levels of 5-HT did not change during the estrous cycle. 5-HT turnover rates were also estimated in the median eminence on proestrus and diestrus 1. The median eminence 5-HT synthesis rate increased in the afternoon of proestrus at 16.30 h. 5-HT was also measured in the anterior pituitary and the median eminence of lactating rats in four experimental situations: mothers with their litter until decapitation, mothers separated from their pups 4 h earlier, and mothers separated from their pups 4 h earlier, after which the pups were allowed to suckle for 5 or 30 min. In spite of the acute changes in circulating prolactin, 5-HT levels in the median eminence were not affected in any situation studied. These results suggest that 5-HT in the median eminence is involved in the control of gonadotropin release. The data further suggest that 5-HT does not act directly on the anterior pituitary to modulate gonadotropin or prolactin release.

Animals↗

PubMed1986

Interaction between estradiol and a nonsteroidal factor in porcine follicular fluid in regulating LH pulse amplitude between the mornings of diestrus 2 and proestrus in the rat.

The object of this study was to examine the effect of porcine follicular fluid (PFF) alone or in combination with estradiol (E2) on pulsatile LH release during the interval between the mornings of diestrus 2 (D2) and proestrus in the rat. Steroids were removed from PFF by charcoal extraction. Preliminary studies indicated that 1 ml PFF given intraperitoneally suppressed FSH secretion for up to 15 h, with an onset of action between 3 and 4 h and maximal suppression between 6 and 9 h. In subsequent experiments, six groups of animals were bled continuously for 3 h between 07.30 and 10.30 h at a rate of 50 microliter whole blood/5 min: group 1 was bled on D2; group 2 was sham ovariectomized on D2 (08.30-09.30 h), immediately implanted with an empty capsule, given saline at 12.00 and 24.00 h, and bled on proestrous AM; groups 3-6 were ovariectomized on D2, implanted with an empty or E2 capsule, given 1 ml saline or PFF at 12.00 and 24.00 h, and bled 24 h following ovariectomy (OVX). Between D2 and proestrus plasma E2 levels increased, and there was no change in any parameter of pulsatile LH release. However, OVX on D2 reduced plasma E2 levels and increased mean blood LH levels above proestrous values due to increases in LH pulse amplitude and frequency. Restoration of physiological proestrous levels of E2 reduced the increase in mean blood LH levels, by lowering pulse frequency to proestrous values and by greatly reducing pulse amplitude. However, LH pulse amplitude and mean blood LH levels were still higher than values on proestrus. PFF alone produced no alteration in any parameter of pulsatile LH release compared with saline-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1988

Role of oxytocin on prolactin secretion during proestrus and in different physiological or pharmacological paradigms.

The present study was designed to evaluate the possible physiological role of oxytocin (OXY) on PRL release by examining the effect of administration of potent pharmacological antagonists of OXY on the stimulation of PRL secretion observed in vitro from anterior pituitary (AP) cells in response to OXY administration or in a number of in vivo paradigms. OXY caused a dose-related increase in PRL release from dispersed AP cells and short term AP cell cultures which was blocked by administration of the OXY antagonists [1-deaminopenicillamine, 2-O-methyltyrosine, 8-ornithine]vasotocin (dPOMeOVT) or [1-(beta-mercapto-beta,beta-cyclopentamethylene propanoic acid)2-O-methyltyrosine, 8-ornithine]vasotocin (MPOMeOVT), respectively. The antagonists were given in vivo in a dose that completely blocked suckling-induced milk let-down for up to 90 min. Injection of the antagonists did not alter the 5-hydroxytryptophan-induced increase in plasma PRL or the increase associated with acute ether stress or acute suckling stimuli, suggesting that OXY is not a major component involved in the neuroendocrine mechanisms responsible for those particular increases. On the other hand, iv administration of dPOMeOVT or MPOMeOVT prevented the increase in plasma PRL normally observed on the afternoon of proestrus in the cycling female rat. The characteristic surge of LH was also blocked by high doses of these antagonists. These data demonstrate that PRL secretion undergoes a differential regulation, in that OXY appears to play a major role in regulating the increase in plasma PRL observed on the afternoon of proestrus, but apparently provides little, if any, contribution toward the neuroendocrine regulation of the increases in PRL associated with 5-hydroxytryptophan administration, acute ether stress stimulus, or acute suckling stimulus. The data also suggest that OXY receptors located in the AP that are involved in the OXY-induced increase in PRL release may be similar to those OXY receptors located in mammary and uterine tissue, since specific biological effects of OXY in those tissues are effectively blocked by the OXY antagonists dPOMeOVT and MPOMeOVT. A possible role of OXY neurons in the neural mechanisms triggering the LH surge during proestrus is also suggested.

Animals↗

PubMed1991

Regulation of ovarian steroid biosynthesis by estrogen during proestrus in the rat.

Studies were performed to test the hypothesis that the rapid decline in estradiol (E2) levels on proestrus before ovulation was due to a reduction in androgen substrate for aromatase, and that this decline in androgen was regulated by an estrogen receptor-mediated mechanism. Aromatase activity, concentrations of E2, androstenedione (A), testosterone (T), and progesterone (P4) in follicular, corpora lutea, and ovarian homogenates as well as peripheral E2, A, P4, and LH were measured in cycling rats from 1400-2000 h on proestrus. These parameters were also recorded after the expected E2 surge in animals treated at 1900 h on diestrous day 2 with the antiestrogen keoxifene (20 mg/kg), with or without an ovulatory dose of PMSG at 1600 h on proestrus. In a second experiment, P-450-17 alpha-hydroxylase/C17,20-lyase (P-450(17 alpha] activity was measured in a group of control rats at 1500, 1700, and 1900 h. Aromatase activity remained unchanged, even though serum and ovarian E2 levels were reduced from peak values at 1500 h to basal values at 1800 h (P less than 0.01). Peripheral A as well as ovarian androgens (specifically follicular but not luteal) A and T were also reduced over this time period (P less than 0.01). Although total ovarian P4 remained unchanged, follicular levels rose from 1400-2000 h (P less than 0.01). These reductions in androgens and E2 levels coincided with a marked reduction in follicular P-450(17 alpha) activity. Treatment with keoxifene with or without PMSG prevented the fall in peripheral E2 and A and the increase in peripheral P4 seen in controls. Ovarian and follicular E2, A, and to a lesser extent T were also remained at values similar to those during the E2 surge. Follicular P4 was reduced by both treatments. Neither treatment had any effect on aromatase. These results indicate that the fall in peripheral and ovarian E2 levels before ovulation was due to a decline in aromatizable androgen, through an inhibition of follicular P-450(17 alpha) enzyme activity, which appears to be mediated by an estrogen receptor-regulated mechanism.

Analysis of Variance↗

PubMed1990

Suppression of the proestrus prolactin surge in the rat estrous cycle by urethane anesthesia.

Urethane-anesthetized male rats have been used for the analysis of prolactin (PRL)-releasing substances on PRL secretion. However, there are only a few reports investigating the effect of urethane anesthesia on PRL secretion in female rats. In this study, we intended to examine the effects of urethane anesthesia on PRL secretion during proestrus in the rat. Proestrus PRL surge was completely blocked when urethane was administered to rats prior to the critical period of proestrus both at doses of 1.0 g/kg and 1.5 g/kg. Additionally, urethane, at a dose of 1.5 g/kg, was also effective in blocking spontaneous ovulation. An experiment examining pituitary PRL concentration at 1800 h confirmed that urethane (1.0 g/kg) anesthesia prevents the PRL surge from the pituitary. Similarly, urethane anesthesia blocked the LH surge from the pituitary, but LH levels in the urethane-treated group were higher than those in the pentobarbital-treated group.

Anesthesia↗