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PubMed2001

Neurotensin gene expression increases during proestrus in the rostral medial preoptic nucleus: potential for direct communication with gonadotropin-releasing hormone neurons.

Neurotensin (NT)-containing neurons in the rostral portion of the medial preoptic nucleus (rMPN) of the brain may play a key role in regulating the pattern of secretion of GnRH, thereby influencing the reproductive cycle in females. The major goals of this study were to determine whether NT messenger RNA (mRNA) levels in the rMPN exhibit a unique pattern of expression in temporal association with the preovulatory LH surge and to assess whether NT neurons may communicate directly with GnRH neurons. We analyzed NT gene expression in rats using in situ hybridization over the day of proestrus and compared this with diestrous day 1. We also determined whether the high-affinity NT receptor (NT1) is expressed in GnRH neurons using dual-label in situ hybridization and whether this expression varies over the estrous cycle. We found that NT mRNA levels in the rMPN increase significantly on the day of proestrus, rising before the LH surge. No such change was detected on diestrous day 1, when the LH surge does not occur. Furthermore, we observed that a significant number of GnRH neurons coexpress NT1 mRNA and that the number of GnRH neurons expressing NT1 mRNA peaks on proestrus. Together with previous findings, our results suggest that increased expression of NT in the rMPN may directly stimulate GnRH neurons on proestrus, contributing to the LH surge. In addition, our results suggest that responsiveness of GnRH neurons to NT stimulation is enhanced on proestrus due to increased expression of NT receptors within GnRH neurons.

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↗
PubMed1995

LTP varies across the estrous cycle: enhanced synaptic plasticity in proestrus rats.

Previous studies have shown that the number of dendritic spines and synapses in hippocampal CA1 stratum radiatum decreases more than 30% between the proestrus (high estrogen) and estrus (low estrogen) phases of the rat estrous cycle [10,27]. In the present study, we investigated whether hippocampal synaptic plasticity, as measured by long-term potentiation (LTP), might also vary across the estrous cycle of the female rat. Male rats, and female rats at each phase of the estrous cycle were tested in either the morning or afternoon. There were no significant group differences in the pre-LTP I/O curves. However, females examined during the afternoon of proestrus, the phase during which prior studies indicate synapse number to be highest, demonstrated the greatest degree of potentiation. Diestrus, proestrus and estrus females tested in the morning demonstrated similar amounts of potentiation. There were also significant differences in post-LTP I/O curves between the afternoon proestrus females and males tested in the afternoon. These results suggest that gonadal hormones, interacting with the time of day, may regulate neural processes underlying learning and memory.

Analysis of Variance↗
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↗
PubMed2005

Spine-type densities of hippocampal CA1 neurons vary in proestrus and estrus rats.

Spines-mediated synaptic activity has been associated to learning ability. Dendritic spines from hippocampal CA1 pyramidal neurons of proestrus rats have been reported to be more numerous than in estrus animals, but some behavioral studies have reported a better performance during the estrus stage of the estrous cycle. Because spine shape has been shown to be strongly related to the post-synaptic processing of information, a quantitative morphological study related to the proportional density of each type of spine, was conducted in Golgi material of hippocampal CA1 pyramidal cells of proestrus and estrus rats. After three regular estrous cycles had been asserted, seven Sprague-Dawley female adult rats in proestrus and eight in estrus, were used. Mushroom-shaped spines from hippocampal pyramidal cells predominated in proestrous rats, being 15.1% more numerous in this stage than in estrus; while thin spines were the predominant type of spine in estrous animals, being 15.5% more abundant in estrus than in proestrus. The predominance of the mushroom-shaped or thin spines in the hippocampal CA1 pyramidal neurons during the respective stages of the rat estrous cycle, could be related to the organization of the hippocampal activity-dependent mnemonic information.

Animals↗
PubMed2002

Female Wistar rats tested during late proestrus or during pregnancy and ovariectomized rats tested after receiving progesterone or allopregnanolone displayed reduced conflict behavior.

In a conflict test based on the rat's choice between an immediate punished reinforcer or a delayed nonpunished reinforcer, anxiolytic drugs increase the number of immediate punished reinforcers. In this study, two hypotheses were tested: first, during late proestrus or during midpregnancy, female rats will display an elevated amount of immediate punished reinforcers; second, ovariectomized rats will display an elevated amount of immediate punished reinforcers when they receive anxiolytic doses of neurosteroids. Thus, female rats (n = 15) were tested repeatedly during late proestrus, diestrus, and pregnancy in the aforementioned conflict task. They displayed an elevated amount of immediate punished reinforcers during late proestrus (P < .05) and during the 14th (P < .05) and 17th (P < .05) days of pregnancy compared to diestrus or 3rd, 7th, or 20th days of pregnancy. Likewise, ovariectomized rats (n = 90) displayed an elevated amount of immediate punished reinforcers compared to control rats only when they received anxiolytic doses of progesterone (1.0-2.0 mg/kg, P < .05) or allopregnanolone (1.0-2.0 mg/kg, P < .05). In conclusion, female rats displayed reduced conflict behavior during late proestrus and pregnancy, or after received anxiolytic doses of neurosteroids.

Animals↗
PubMed1998

GnRH-dependent up-regulation of nitric oxide synthase I level in pituitary gonadotrophs mediates cGMP elevation during rat proestrus.

We have recently provided evidence that the concentration and activity of the enzyme nitric oxide synthase type I (NOS I) was stimulated in gonadotrophs by GnRH, suggesting a role for the NOS I/NO pathway in the GnRH control of cell functions. To further establish the GnRH regulation of pituitary NOS I under physiological circumstances, we have examined the expression of NOS I during the 4-day estrus cycle in rats. Western blot analysis demonstrated that NOS I was present in the anterior pituitary at low levels during diestrus I (DI) and diestrus II, then was subject to a significant increase on the afternoon of proestrus to reach a maximal 3-fold increase between 19:00 and 20:00 h, after which NOS I decreased to return, during estrus, to levels within the range detected in diestrus. No such variation was apparent in the posterior pituitary lobe. NADPH-diaphorase histochemistry combined with immuno-identification of the cells revealed that active NOS I was expressed in both the gonadotrophs and the folliculo-stellate cells throughout the whole cycle but only the gonadotrophs showed an up-regulation during proestrus. A high temporal correlation was observed in the profiles of NOS I, pituitary cGMP and serum luteinizing hormone (LH) suggesting an implication of GnRH. Consistently, the administration of a potent GnRH antagonist to rats totally abolished the rise in pituitary NOS I and cGMP, in addition to suppressing, as expected, the surge in the secretion of LH. A role of NOS I as a mediator in the GnRH-induced augmentation in cGMP was further established in vitro by incubating anterior pituitaries in the presence of the NOS inhibitor, L-NMMA (1 mM). Altogether, these data demonstrate that the level and activity of NOS I is up-regulated in gonadotrophs during proestrus, in a manner consistent with a major implication of GnRH over a period during which its release from the hypothalamus, as well as gonadotroph responsiveness, are at maximum. This effect is accompanied by a NOS/NO-mediated rise in cGMP. In the absence of obvious effect on gonadotropin release, the roles of NO and cGMP in the regulation of gonadotroph functions, especially during proestrus, remain to be established.

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↗
PubMed1995

A relative depletion of luteinizing hormone-releasing hormone was observed in the median eminence of young but not middle-aged rats on the evening of proestrus.

Computer-assisted analysis was used to examine LHRH reaction product in the median eminence of young and middle-aged rats prior to and after the expected peak of the LH surge on proestrus. The area of LHRH reaction product was analyzed in 5 rostral-caudal levels (A-E) of the median eminence. The relative depletion of LHRH in the median eminence of young females on the evening compared to the afternoon of proestrus suggested LHRH neurosecretion in conjunction with the preovulatory LH surge. The pattern of depletion observed further suggested that LHRH release may occur preferentially from restricted regions of the median eminence or in a coordinated wave-like pattern. Four of the five levels of the median eminence exhibited a relative decrease in LHRH on the evening of proestrus in young females, and this time-related difference in LHRH reaction product was statistically significant in median eminence levels B and C. In contrast, little evidence of a relative depletion in LHRH reaction product from early to late proestrus was observed in the median eminence of aging animals. Moreover, the concentration of the densest LHRH reaction product appeared diminished in the median eminence of middle-aged compared to young females at the time points examined in the present study. The age-related differences observed in LHRH reaction product in the median eminence may contribute to the attenuated LH surge documented in middle-aged female rats. Whether these changes in LHRH immunoreactivity can be attributed to age-related alterations in afferents received by LHRH neuronal cell bodies or terminals or to intrinsic deficits in signalling mechanisms within LHRH neurons remains to be determined. Computer-assisted analysis of the immunocytochemical data enabled the assessment of relative changes in reaction product within specific elements of LHRH neurons in precise regions of the median eminence.

Aging↗
PubMed1984

Cell proliferation in the dog (beagle) ovary during proestrus and early estrus.

Cell proliferation in the ovaries of 5 Beagle dogs was studied by autoradiography after pulse labelling with (3H)-thymidine during different periods of proestrus and early estrus. Indices of labelling and of necrosis were determined for different follicle types grouped according to their stages of atresia. Cell proliferation became most obvious during early proestrus in antral and in Graafian follicles, and during the periovulatory period in preantral follicles. By contrast, low labelling indices appeared in all follicle types during the middle of proestrus. All follicle types demonstrated a continuous decrease in labelling from early to late atresia with the exception of the granulosal layer of antral follicles during the periovulatory period. While many necrotic granulosal cells were found in antral follicles during the first 6 days of proestrus, there were few during the periovulatory period. Even Graafian follicles displayed necrotic granulosal cells now and then. It may be concluded that (1) intact ovarian follicles can show subtle changes indicating the beginning of atresia, and (2) during the proliferative phase of a long estrous cycle cell replication in the ovary is characterized by two bursts at the beginning and at the end of this period.

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↗
PubMed1995

One day estrous cycle shortening induced by antiprogestagen RU486 administration in proestrus to 4-day cyclic rats.

Administration of 4 mg of the antiprogestagen RU486 to 4-day cyclic rats in proestrus, which blocks proestrous and diestrous progesterone actions, induced a one day shortening of the ovarian cycle and a reduction of the ovulation rate in the following cycle. These effects were not present when RU486 was administered in estrus or metestrus. RU486 injections either in proestrus or estrus increased the serum levels of LH and 17 beta-estradiol during metestrus. However, only rats injected with RU486 in proestrus presented a 24 hour advancement of the preovulatory surge of gonadotropins and a lack of the LH-inhibiting effect of exogenous estradiol. These results suggest that, in 4-day cyclic rats, the secretion of progesterone by the corpora lutea during diestrous phase retards the follicular development by lowering the serum concentrations of LH, whereas progesterone secretion by the preovulatory follicles in proestrus regulates the estrous cycle length by antagonizing the desensitization of the pituitary to the estrogen negative feedback on LH secretion.

Animals↗
PubMed1984

Anterior pituitary dopamine receptors during the rat estrous cycle. A detailed analysis of proestrus changes.

The binding of [3H]-spiperone to partially purified rat anterior pituitary plasma membranes was quantified throughout the estrous cycle in relation with the serum prolactin (PRL) levels. Receptor affinity remained unchanged throughout the cycle (Kd:0.08-0.16 nM). The number of receptors was constantly high from diestrus I 10.30 h to proestrus 10.30 h, as long as serum PRL remained low. Between 10.30 and 17.30 h on proestrus, there was a rapid and marked decrease in receptor numbers (Bmax, from 180 +/- 50 to 48 +/- 10 fmol/mg protein: means +/- SEM of three independent determinations), which coincided with the preovulatory PRL surge. Subsequently, [3H]-spiperone binding gradually increased (from 48 +/- 10 to 110 +/- 21 fmol/mg protein, at 21.30 h), while PRL returned to basal levels. On the afternoon of estrus, the number of dopamine receptors was also negatively correlated with the increase in serum PRL. These results show that, while receptor affinity is constant, the number of dopamine binding sites changes significantly and rapidly on the afternoon of proestrus. A rapid decrease in receptor content is temporally correlated with the onset of the preovulatory PRL surge. Therefore, the number of [3H]-spiperone binding sites may be regulated during the estrous cycle. In addition, the proestrus decrease in the number of these receptors might be a decisive component in the initiation or maintenance of the preovulatory PRL surge in the rat.

Animals↗
PubMed1982

Hormonal secretion of cultured rat ovarian follicles isolated at various hours of proestrus.

Rat ovarian follicles were isolated at nine different times during proestrus and cultured for 3 d. During the period of cultivation, estrogen and progestagen secretion was measured. Other follicles were dissected out at the same time periods and analyzed directly for these hormones. The most intense hormone release was observed during the first 24 h in culture. The concentration of steroids secreted in vitro by follicles previously collected on the morning of proestrus was relatively low. The secreted estrogen concentration increased gradually from follicles previously isolated at later times during proestrus, with a maximum at 1800. The peak secretion of progestagens in culture was from follicles isolated at 2000. There was a marked decrease in secretion of both hormones from follicles isolated past their optimal times. The same secretion pattern of estrogens and progestagens was shown by follicles in vitro at 24 h as with similar follicles in vivo, according to their time of isolation. From these results it is evident that the normal rhythm of steroid secretion, predetermined at the time of follicle excision, was maintained during 24 h in organ culture. After a longer period in culture, the dynamics of hormone synthesis became altered, although follicles appeared morphologically normal up to 48 h.

Adult↗
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↗
PubMed1995

Glial fibrillary acidic protein mRNA increases at proestrus in the arcuate nucleus of mice.

Glial fibrillary acidic protein (GFAP) increases during proestrus in astrocytes of the hypothalamic arcuate nucleus (ARC). These changes are associated with altered astrocyte-neuron contacts and synaptic remodelling, during preparation for the preovulatory gonadotrophin surge. This study of young C57BL/6J mice showed transient elevations of GFAP mRNA on proestrus in the ARC by in situ hybridization. Basal GFAP mRNA was regained within 18 h. We hypothesize that changes in astrocytic GFAP on proestrus result from elevations of GFAP mRNA that are, in turn, driven by ovarian secretions of estradiol.

Animals↗
PubMed2003

Immunoprotection in proestrus females following trauma-hemorrhage: the pivotal role of estrogen receptors.

Immune responses in proestrus females are not altered after trauma-hemorrhage, whereas they are markedly depressed in males. Elevated levels of female sex steroids appear to be responsible for maintaining immune responses but it remains unknown, whether estrogen per se is responsible. To study this, proestrus female C3H/HeN mice were subjected to laparotomy (i.e., soft tissue trauma) and hemorrhagic shock (35+/-5 mmHg for 90 min, then resuscitated) or sham operation and received the estrogen receptor antagonist EM-800 or vehicle during resuscitation. Two hours following trauma-hemorrhage, splenocyte proliferation, IL-2, IL-3, IFN-gamma release, and splenic macrophage IL-6 release was maintained in vehicle-treated females. In EM-800-treated females, however, these immune parameters were significantly depressed. Following trauma-hemorrhage, Kupffer cell TNF-alpha release and circulating TNF-alpha were increased only in EM-800-treated females. These findings indicate that the ability of proestrus females to maintain immune function following trauma-hemorrhage is estrogen-dependent and mediated via estrogen receptors.

Animals↗