Age-related changes in cytoplasmic estradiol receptor concentrations in microdissected brain nuclei: correlations with changes in steroid-induced sexual behavior

The purpose of this study was to; determine at what age changes in cytoplasmic estradiol receptors are evident in specific microdissected brain areas of the female rat; assess whether alterations parallel previous changes observed when large brain areas were used for determination of receptor concentrations; and assess whether changes in cytoplasmic estradiol receptors are correlated with changes in steroid-mediated physiological functions. To assess the effects of age on cytoplasmic estradiol receptor concentrations, we used virgin female Sprague-Dawley rats at 3-4 months, 7-8 months and 10-11 months of age. They were ovariectomized 7-14 days prior to use to allow maximal translocation of receptors to the cytoplasm. The animals were anesthetized and perfused with a 10% (v/v) solution of dimethylsulfoxide to protect the receptor proteins from the effects of freezing. Brains were removed and frozen. This procedure of freezing the brains caused a minimal (15-18%) loss in the number of receptors and no change in the dissociation constant. Consecutive 300 micron sections were sliced and the following nuclei and brain areas were microdissected: bed nucleus of the stria terminalis, suprachiasmatic-preoptic area, medial preoptic nucleus, periventricular preoptic nucleus, periventricular anterior hypothalamic area, paraventricular nucleus, dorsomedial nucleus, ventromedial nucleus, arcuate-median eminence, medial amygdala, and cortical amygdala. The pituitary gland was also removed and analyzed. The cytoplasmic fraction from a tissue pool from 3 animals was prepared and aliquots were incubated with [3H]estradiol at a final concentration of 1.5 nM in the presence or absence of 100-fold excess moxestrol. Receptor-bound [3H]estradiol was separated from free hormone by gel filtration.(ABSTRACT TRUNCATED AT 250 WORDS)

Estradiol-induced daily luteinizing hormone and prolactin surges in young and middle-aged rats: correlations with age-related changes in pituitary responsiveness and catecholamine turnover rates in microdissected brain areas

These studies assessed the effects of age on the ability of estradiol-17 beta (E2) to induce LH and PRL surges in ovariectomized young and middle-aged rats that previously had normal estrous cycles. We determined whether any changes in the timing or amplitude of these surges could be correlated with changes in pituitary responsiveness to GnRH or with changes in norepinephrine (NE) and dopamine (DA) turnover rates in microdissected brain regions involved in cyclic gonadotropin release. Young (3-4 months old) and middle-aged (9-12 months old) rats were ovariectomized. One week later (day 0), they received Silastic capsules containing E2 which produced physiological serum concentrations of E2. Groups of rats were bled sequentially via indwelling right atrial cannulae 1-4 days after capsule implantation (days 1-4). All young rats displayed maximal LH surges by day 2 and exhibited equivalent surges on days 3 and 4. Middle-aged rats required the presence of E2 for at least 3 days before a maximal positive feedback response was achieved. Even at these times the timing of the LH rise was delayed by 1 h and peak concentrations were lower in middle-aged rats. E2-induced PRL surges did not exhibit any age-related differences. Pituitary responsiveness to GnRH was tested by administering two injections of GnRH to pentobarbital-blocked young and middle-aged rats on days 2 and 4. Pituitary responsiveness to the first injection on day 2 was blunted in middle-aged rats; however, the LH response at all other times was normal. Catecholamine turnover rates were examined on days 2 and 4 by giving alpha-methyl-para-tyrosine at 1000 or 1500 h and killing rats 45 or 90 min later. Resting initial catecholamine concentrations were assessed in untreated rats killed at 1000 h or 1500 h. The medial preoptic nucleus, suprachiasmatic nucleus, and median eminence were microdissected and assayed for NE and DA by radioenzymatic assay. In young rats, NE turnover rates increased during the afternoon in all brain areas on both days. In contrast, in middle-aged rats, no increase in NE turnover rates was observed during the afternoon of day 2. By day 4, the delayed and attenuated LH surge was accompanied by increased turnover rates in the median eminence only; no change occurred in the suprachiasmatic nucleus or medial preoptic nucleus. No age-related differences were observed in DA turnover rates.(ABSTRACT TRUNCATED AT 400 WORDS)

Nuclear estradiol and cytosol progestin receptor concentrations in the brain and the pituitary gland and sexual behavior in ovariectomized estradiol-treated middle-aged rats

We wished to determine whether the altered nuclear estradiol (E2) receptor concentrations in middle-aged rats can explain the diminished responsiveness to E2 observed in aging rats. Therefore, we measured receptor concentrations in various brain areas and the pituitary gland of young and middle-aged ovariectomized rats 2 and 4 days after implantation of Silastic capsules containing E2. To determine whether any observable changes had physiological consequences, we correlated age-dependent changes in E2 nuclear receptor concentrations with two E2-dependent parameters: cytosol progestin receptor levels in equivalent brain areas and pituitary gland and progesterone-facilitated reproductive behaviors. Young (3-4 months old) and middle-aged (10-12 months old) Sprague-Dawley rats were ovariectomized and received Silastic capsules containing E2 dissolved in oil 1 week later (day 0). Groups of rats were killed at 1200 h on either day 2 or day 4. Nuclear E2 and cytosol progestin receptor concentrations were assessed in a nuclear and cytoplasmic extract from the medial basal hypothalamus, preoptic area, amygdala, and pituitary gland. To test steroid-induced mating behavior, ovariectomized young and middle-aged rats were treated with E2-containing capsules for 2 or 4 days. At 0900 h progesterone (0.2 mg/kg BW) was injected sc and receptive and proceptive behaviors were observed when experienced males were introduced 4-6 h later. Two days after implantation of E2 capsules, middle-aged rats exhibited lower nuclear E2 receptor concentrations in the medial basal hypothalamus and preoptic area than young rats. By day 4, there were no significant age-related differences in any brain area or in the pituitary gland. Parallel age-related differences were observed in cytosol progestin receptor concentrations on day 2 but they were not evident by day 4. Similarly, middle-aged rats exhibited deficits in proceptive behavior, lordosis quotient, and lordosis quality score on day 2, but there were no differences compared to young rats on day 4. These data demonstrate that E2-induced nuclear E2 receptor concentrations are lower in selected areas of the brain of middle-aged rats. Such changes appear to be physiologically important because they are correlated with changes in E2-induced cytosol progestin receptor concentrations and steroid-induced behaviors. Furthermore, they may partially account for age-related differences in E2-induced LH surges on day 2.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in concentrations of estradiol nuclear receptors in the preoptic area, medial basal hypothalamus, amygdala, and pituitary gland of middle-aged and old cycling rats

This study determined whether the biochemical characteristics of the estradiol (E2) nuclear receptor located in three different brain areas and the pituitary gland (PIT) change in aging rats undergoing declining reproductive function. To measure maximal E2 nuclear receptor concentrations, groups of young (3-4 months old), middle-aged (8-11 months old), and old (16-18 months old) cycling rats were injected iv with E2 to translocate maximally cytosolic receptors into the nucleus. One hour later they were killed and nuclear extracts were prepared. Maximal E2 nuclear receptor concentrations and dissociation constants were assessed in the preoptic area (POA), medial basal hypothalamus (MBH), amygdala (AMYG), and PIT using an in vitro exchange assay and covariance analyses of double reciprocal plots (Lineweaver-Burk). Middle-aged cycling rats exhibited decreased E2 nuclear receptor concentrations in the POA, but no change in the MBH, AMYG, or PIT. Old rats exhibited decreased E2 nuclear receptor concentrations in the POA, MBH, and PIT and an unexpected increase in the AMYG. There was no change in the affinity of the receptor with age, although an apparent decrease in the pituitary E2 nuclear receptor of middle-aged rats was observed. This difference was not detected when saturation analyses were performed using varying dilutions of the pituitary nuclear extract. These results demonstrate that changes occur in maximal numbers of E2 nuclear receptors in the POA of middle-aged rats. As animals age further, the changes encompass a larger brain area and include the PIT. The data suggest that changes in E2 nuclear receptor concentrations may contribute to the age-related decline in reproductive function.

Inhibition of ovarian aromatase by prolactin in vivo

Ovarian aromatase activity was inhibited by prolactin treatment in both cycling rats and immature gonadotropin-primed animals. Cycling rats were injected s.c. with prolactin (4 mg/kg BW) beginning on diestrus 1. Aromatase activity in the largest follicles on diestrus 2 and on proestrus was reduced relative to controls. None of the prolactin-treated animals had ovulated when examined on the expected estrus morning. In other experiments, the immature pregnant mare's serum gonadotropin (PMSG)-primed rat was used as a model for the cycling rat. Microsomal aromatase activity but not the activity of the C17-21 lyase was reduced in ovaries of animals injected with prolactin [6 IU (200 micrograms)/rat] 48 h after PMSG administration and sacrificed 4 h later. Furthermore, aromatase activity was significantly increased when endogenous prolactin was inhibited by bromocryptine (CB-154). The effect of LH treatment, on the other hand, was to increase both aromatase and lyase activities. However, prolactin pretreatment did not consistently prevent the LH-induced increase in aromatase activity. The results suggest that inhibition of ovulation by prolactin is mediated, at least in part, by direct inhibition of aromatase activity.

Differences in the stress response of prolactin in young and aged female rats

The drawing of blood by orbital sinus puncture (OSP) under ether anesthesia is known to produce a marked increase in serum prolactin (PRL levels in young cycling female rats. The effect of this stressful procedure on PRL release was compared in young and aged female rats. Nonstressed PRL levels were obtained from blood drawn by decapitation. Whereas OSP with a one-minute ether exposure induced a marked increase in PRL levels in young rats on all days of the estrus cycle, older cycling female rats on the day of diestrus -1 and aged rats exhibiting prolonged diestrus (PD) showed virtually no increase above nonstressed levels. However, increasing the ether exposure time to five minutes did produce a rise in PRL levels. Old cycling female rats on the day of estrus and aged rats exhibiting constant estrus (CE) did show a PRL increase comparable to that seen in young animals. Ovariectomy (OVX) completely abolished the stress response seen in aged CE rats. The response, though markedly decreased, was still present in young ovariectomized rats. These experiments show that the stress-induced rise in PRL promoted by OSP under either anesthesia is markedly diminished in aged rats exhibiting a diestrus state. The attenuated response seen in these rats is believed due to factors characteristic of the diestrous state of aging.

Simultaneous measurements of estrogen nuclear (E2Rn) and progestin cytosol (PRc) receptors in the hypothalamus and pituitary gland of rats

The present methods used to measure estrogen nuclear (E2Rn) and progestin cytosol (PRc) receptors in the hypothalamus and pituitary gland require that separate assays be performed to determine the concentrations of each receptor. In the present studies we describe a method which simultaneously measures both E2Rn and PRc in hypothalamic and pituitary tissue. Tissue samples were homogenized in tris-EDTA-glycerol-dithiothreitol buffer and centrifuged at 1500 X g for 5 min. The supernatant was purified for the PRc assay while the nuclear pellet was extracted for the E2Rn exchange assay. For the PRc assay, the supernatant was centrifuged at 106,500 X g for 30 min and aliquots from the resultant supernatant then were incubated with 3H-R5020. For the E2Rn exchange assay, the original pellet was purified further by successively resuspending and centrifuging it through several sucrose solutions. Estrogen-receptor complexes were extracted from the chromatin pellet with a 0.4M KC1 solution and aliquots of the final supernatant were incubated with 3H-estradiol. In both assays, the samples were placed onto Sephadex LH20 columns and the receptor bound 3H-steroid was eluted directly into scintillation vials. Scatchard analyses revealed that these assays measure a single class of binding sites for E2Rn and PRc with dissociation constants (KD) and maximal number of binding sites (Bmax) similar to those previously reported using a separate assay for each receptor.

Failure of the hypothalamic noradrenergic system to function in adult androgen-sterilized rats

We have examined norepinephrine (NE) and dopamine (DA) initial concentrations, rate constants and turnover rates in microdissected regions of the hypothalamus and gonadotropin release in ovariectomized (OVX)estrogen-treated adult control and androgen-sterilized rats (ASR) (50 micrograms testosterone propionate at 5 days of age). When Silastic capsules containing estradiol (E2) were implanted one week after ovariectomy (Day 0) in control rats, afternoon luteinizing hormone (LH) surges occurred on Days 3 and 4. When 2 progesterone (P) silastic capsules were placed s.c. on Day 3 at 0900 h into E2-primed ovariectomized controls, the afternoon LH plasma surge concentrations were markedly amplified. In contrast, identical E2 or E2P treatment of adult ovariectomized ASR had no effect on plasma LH on either Days 3 or 4 after E2 capsule placement. Serum concentration differences of follicle-stimulating hormone (FSH), prolactin (Prl), E2 and P between control rats and ASR and the effects of E2 treatment on these hormone levels also were recorded. In E2-treated ASR, initial steady state concentrations of NE were significantly less than in controls in all brain regions examined. NE turnover rates increased significantly in E2-treated controls at 1500-1700 h compared to those at 1000-1200 h in median eminence (ME), medial preoptic nucleus (MPN), suprachiasmatic nucleus (SCN) but not in arcuate nucleus (AN). No increase in NE turnover rates occurred in MPN and ME and only slight increases in turnovers were observed in SCN and AN in E2-treated ASR during the afternoon. DA turnover rates declined between morning and afternoon in ME of control and ASR. In control rats, DA turnover rates increased during the afternoon in AN and MPN but remained unchanged in these same hypothalamic regions in ASR. A considerable amount of previous evidence suggests that NE may be the neural trigger which evokes the release of luteinizing hormone releasing hormone (LHRH) from ME axon terminals in normal proestrous or in E2-treated ovariectomized rats. The failure of NE to be released into the MPN and ME of ASR may account for the failure of estradiol to induce LH surges in these animals.

Alterations in the proestrous pattern of median eminence LHRH, serum LH, FSH, estradiol and progesterone concentrations in middle-aged rats

The purpose of the following study was to assess the changes in the proestrous hormone profile in middle-aged cycling rats to better understand the inter-relationship and possible interaction of these hormones during the transition to estrous acyclicity. Median eminence LHRH concentrations and serum LH, FSH, estradiol and progesterone concentrations were measured in young (3-4 months old) and middle-aged (8-10 months old) proestrous rats at 0900, 1200, 1500 and 1800h. The data demonstrate that (1) baseline hormone concentrations prior to the surge at 0900h are the same in middle-aged and young rats; (2) the proestrous gonadotropin surge is temporally delayed in middle-aged rats; (3) this delay is preceded by lower median eminence LHRH concentrations and serum estradiol concentrations at 1200h; (4) serum progesterone concentrations are lower in middle-aged rats during the preovulatory gonadotropin surge (at 1500 and 1800h) probably as a consequence of the delayed LH surge.

Negative feedback effects of progesterone correlated with changes in hypothalamic norepinephrine and dopamine turnover rates, median eminence luteinizing hormone-releasing hormone, and peripheral plasma gonadotropins

Progesterone (P) amplifies and advances gonadotropin surges when administered to estradiol (E2)-treated ovariectomized rats. While daily rhythmic LH surges continue to occur in E2-treated rats, they are extinguished in E2- and P (E2P)-treated animals 24 h after P treatment. We examined whether this negative feedback effect on P affect catecholamine activity within the median eminence, medial preoptic nucleus, arcuate nucleus, and suprachiasmatic nucleus and also the changes which occur in median eminence LHRH concentrations. Twenty-four hours after P exposure, LH and FSH surges in E2P-treated rats are extinguished, and the magnitude of the PRL surge is reduced. In E2-treated rats, there is an increase in medial preoptic nucleus, suprachiasmatic nucleus, and median eminence NE turnover rates from 1000--1200 to 1500--1700 h, but these changed in NE activity do not occur in E2-treated rats which received P 24 h earlier. Rather, such E2P-treated animals have markedly elevated medial preoptic nucleus, arcuate nucleus, and median eminence DA turnover rates during the period that plasma gonadotropin levels are suppressed. No differences in median eminence LHRH concentrations in E2- or E2P-treated rats were detected. P may exert its negative feedback action in suppressing LH, FSH, and PRL release by blocking activation of the hypothalamic noradrenergic system and by increasing dopaminergic activity within the tuberoinfundibular system.

Decreased dopamine turnover in the median eminence in response to suckling in the lactating rat

The effects of suckling on the turnover of dopamine (DA) and norepinephrine (NE) were studied in terminal projection fields of the tuberoinfundibular (median eminence, ME), nigrostriatal (caudate nucleus, CN), incertohypothalamic (medial preoptic nucleus, MPN) and mesolimbic (nucleus accumbens, NA) dopaminergic neurons. Decreased dopamine turnover in the median eminence was found in suckled compared with nonsuckled rats at 10 days postpartum. This effect was specific as dopamine turnover in the CN, NA and MPN and norepinephrine turnover in the ME, NA and MPN were not affected by suckling. The suckling-induced prolactin response is markedly blunted in rats 20 days postpartum. In these rats, median eminence dopamine turnover did not decrease significantly in response to suckling. These results are consistent with the hypothesis that median eminence dopamine is a physiological prolactin inhibitory factor mediating suckling-induced prolactin release.

Catecholamine turnover rates in discrete hypothalamic areas and associated changes in median eminence luteinizing hormone-releasing hormone and serum gonadotropins on proestrus and diestrous day 1

We have correlated catecholamine [CA; i.e. norepinephrine (NE), dopamine (DA), and epinephrine (E)] turnover rates in discrete hypothalamic nuclei and in the median eminence (ME), with concentration changes in ME LHRH and serum LH, FSH, PRL, estradiol, and progesterone levels at various times during proestrus and diestrous day 1 in 4-day cyclic rats. CA concentrations were measured with a radioenzymatic assay at 0, 60, and 120 min after ip injection of 400 mg/kg alpha-methyl-p-tyrosine, and rate constants and turnover rates were calculated. In a separate assay NE, DA, and E were separated by two-dimensional thin layer chromatography, and concentrations and turnover rates of CAs were calculated. The microdissected hypothalamic nuclei examined for NE turnover rates included the medial preoptic nucleus (MPN), suprachiasmatic nucleus (SCN), arcuate nucleus (AN), and ME. DA turnover rates also were measured in the MPN, ME, and AN. ME LHRH and serum hormone concentrations were measured by RIA. Between 0900--1200 h, proestrous serum estradiol was elevated, but other serum hormones were basal, and CA turnover rates in the brain were low. However, ME LHRH concentrations increased significantly between 0900--1200 h on proestrus. Between 1200--1500 h, serum LH, FSH, PRL, and progesterone levels increased and ME LHRH levels declined significantly; during this time interval (1200--1400 h), a significant rise in ME NE and DA turnover rates occurred. Between 1500--1700 h on proestrus, while serum gonadotropins were still rising toward peak concentrations, increased ME NE turnover rates were maintained, but increased NE turnover rates also were evident in MPN, SCN, and AN. During this same time interval (1500--1700 h), a marked decline in ME and AN DA turnover rates occurred, although such rates remained unchanged within the MPN. There were no corresponding changes in MPN E turnover rates at any of the time intervals studied. The increased turnover rates of ME NE coupled with the concomitant decline in ME LHRH levels and the rise in plasma LH and FSH levels suggest that increased NE release may be important in initiating preovulatory LH and FSH surges. These changes in brain neurotransmitters and serum hormones are not the result of a diurnal rhythm, since corresponding changes in CA turnover rates or serum gonadotropins did not occur between 0900--1100 h and 1500--1700 h diestrous day 1.

LHRH-induced LH and FSH responses in the aged female rat

LHRH-induced LH and FSH concentrations were measured in 8 to 12-month-old rats which had just begun to exhibit constant vaginal estrus (CE) or prolonged vaginal diestrus (PD) and in young rats on the day of proestrus (PE). All animals were anesthetized with sodium pentobarbital (31 mg/kg). They received (1) i.v. infusion of LHRH (Beckman 100 ng/ml at a rate of 0.5 ml/hr) for 3 hrs or (2) 2 i.v. injections of LHRH (50 ng/100 g body wt) spaced 120 min apart. LHRH stimulation resulted in a comparable gonadotropin response in young rats and old CE rats; however, old PD rats exhibited a markedly attenuated LH and FSH response to LHRH infusion. These data suggest that pituitary responsiveness to LHRH does not change in 8 to 12-month-old CE rats and therefore does not play a major role in the transition to acyclicity. In contrast, pituitary responsiveness is decreased in old PD rats early during the aging process and therefore may be an important factor in the transition to their acyclic vaginal smear pattern.

Regional distribution of luteinizing hormone-releasing hormone (LH-RH) in rat brain determined by microdissection and radioimmunoassay

We have re-examined the distribution of LH-RH in rat brain and in particular those rostral hypothalamic areas reported to contain perikarya by immunocytological techniques. Adult male rats were decapitated and 20 discrete brain structures micro-dissected. Tissue from single rats was homogenized in 50 microliter 0.1 N HCl and 40 microliters of this acid extract was radioimmunoassayed for LH-RH using the R-42 antibody. Low but detectable concentrations which ranged from0.2 to 1.0 pg/micrograms protein were found in the interstitial nucleus of the stria terminalis (ventral aspect), medial preoptic nucleus (dorsal and ventral aspects), lateral preoptic nucleus, anterior hypothalamic nucleus, paraventricular nucleus, ventromedial nucleus (medial and lateral aspects) and the dorsomedial nucleus. Slightly higher concentrations (1.0--4.0 pg/micrograms protein) were measured in a rostral, medial basal corridor made up of the suprachiasmatic preoptic nucleus, the suprachiasmatic nucleus and the retrochiasmatic area. LH-RH was not detected (i.e. less than 0.8 pg/area) in the bed nucleus of the diagonal band of Broca, caudate nucleus, lateral septal nucleus or the cingulate cortex.

Effect of ovariectomy on plasma LH, FSH, estradiol, and progesterone and medial basal hypothalamic LHRH concentrations old and young rats

Resting plasma concentrations of LH, FSH, estradiol, and progesterone and medial basal hypothalamic concentrations of LHRH (MBH-LHRH) were measured by RIA in 8- to 12-month-old female rats which had begun to exhibit constant estrous (CE) or prolonged diestrous (PD) vaginal smear patterns and compared to young cycling rats on proestrus, estrus, or diestrus. In addition, we examined the effect of ovariectomy on these hormonal profiles. Old CE rats have normal plasma LH, FSH and progesterone concentrations, but exhibit elevated estradiol levels and decreased MBH-LHRH concentrations compared to young cycling rats on the day of estrus. Ovariectomy results in an attenuated rise in plasma LH and FSH and a much lesser decrease in MBH-LHRH when compared to young rats, despite comparable steroid changes. Old PD rats have normal LH and FSH levels,but have elevated estradiol and progesterone concentrations and decreased MBH-LHRH levels when compared to young rats on the day of diestrus. Ovariectomy causes a normal decrease in MBH-LHRH; however, the increased gonadotropin levels are significantly less than seen in young controls.

Evidence that the pituitary gland is the site of inhibitory action of porcine follicular fluid upon FSH secretion in the rat

This study has examined the effects of porcine follicular fluid (PFF) on the adenohypophyseal secretion of LH and FSH following LHRH treatment. The animal model selected for study the the proestrous rat in which preovulatory LH and FSH surges were blocked with phenobarbital. To elicit the release of gonadotropins from this preparation, two different i.v. LHRH infusion protocols were used. The first elicited a proestrous-like LH and FSH surge while the second infusion schedule evoked the predominant release of FSH with only minor changes in LH. PFF suppressed LHRH-induced FSH secretion in both preparations but had only negligible effects on LH secretion. Interestingly, the suppressive effects of PFF were variable and depended upon the mode in which LHRH was presented.

Cyclic nucleotide concentrations in relation to renal growth and hypertrophy

In rats no consistent change in the concentration of cyclic GMP or cyclic AMP concentration was found in the renal cortex between 2 hours and seven days after unilateral nephrectomy. In regenerating liver tissue, between 2 hours and seven days after removal of one-third of the liver, there were no consistent changes in cyclic GMP concentrations, but cyclic AMP concentrations were higher than in controls. During postnatal growth, no consistent changes occurred in the cyclic GMP concentration of the spleen, the testes, the kidney cortex, the renal papilla, the liver or the ventricle between two and sixty days after birth. Cyclic AMP concentration on the other hand, in all these tissues with the exception of the spleen, was depressed between the twenty-first and fortieth day after birth, i.e., at a period of rapid growth. In the spleen, the concentration of cyclic AMP increased continuously from the second to the fifth day after birth. During renal parenchymal hyperplasia induced by a large intravenous dose of folic acid two days before sacrifice, the concentration of cyclic GMP in renal cortical tissue increased consistently. A model is proposed to explain the different patterns of changes in the cyclic nucleotide concentrations found.

Effect of ovariectomy on serum prolactin concentrations in old and young rats

Changes in endogenous prolactin levels in rats with age and after ovariectomy varied according to the reproductive state, emphasizing the different mechanisms underlying infertility in rats in constant oestrus and prolonged dioestrus.

Cytoplasmic oestrogen receptor complex of female ovine pituitary: changes associated with the reproductive state and oestradiol treatment

Binding of oestrone and oestradiol to a pituitary cytosol receptor was studied in ovariectomized sheep during the breeding season, during anoestrus and after oestradiol treatment during anoestrus. All sheep were ovariectomized 2 weeks before removal of the anterior pituitary. Oestrogen treatment consisted of the subcutaneous implantation of Silastic capsules containing oestradiol-17beta 3 weeks before ovariectomy. Association constants (KA) for oestrone and oestradiol and the concentration of binding sites for oestradiol varied with changes in reproductive state. Oestradiol bound more tightly and the concentration of available binding sites was lower during the breeding season than during anoestrus. The affinity of oestrone binding was less during the breeding season than during anoestrus, whilst the concentration of binding sites for this steroid did not change with changes in reproductive state. Treatment of anoestrous sheeep with oestradiol increased the KA for oestradiol and decreased the concentration of binding sites. However, no effect of oestradiol treatment was observed with regard to the KA or concentration of binding sites for oestrone. The results suggest that either the pituitary oestrogen cytoplasmic receptor undergoes a conformational change during the breeding season causing oestradiol to be more tightly bound or the onset of the breeding season is accompanied by the appearance of a new receptor which binds to oestradiol with greater affinity. Oestradiol treatment of anoestrous sheep mimics this change.

The estrogen cytosol receptor of female ovine pituitary

The biochemical parameters of estrone and estradiol binding to the cytosol fraction of ovine anterior pituitary were investigated. When increasing amounts of [3H]estrone or [3H]estradiol were incubated with the 105,000 g fraction from the pituitary, both hormones bound to a receptor with the same apparent KD (mean +/- S.E., estrone = 1.40 +/- 0.30 X 10(-10) M, estradiol = 1.03 +/- 0.11 X 10(-10)M) and the same concentration of binding sites (estrone = 3.22 +/- 0.58 X 10(-14) moles/mg protein, estradiol = 3.92 +/- 0.19 X 10(-14)). No conversion of [3H]estrone to [3H]estradiol under the experimental conditions used could be demonstrated. The receptor-estrogen complex exhibited identical sedimentation coefficients (7-8 S) with either hormone. The receptor was specific only for estrogens; neither 500-fold excess of testosterone nor progesterone affected binding. Competitive inhibition using increasing amounts of non-radioactive estrone or estradiol with [3H]estrone or [3H]estradiol resulted in parallel displacement of the radioactive hormone. These results strongly suggest that both hormones bind to the same pituitary cytosol receptor.