Hypothalamus Of Ovariectomized Rats Research Articles

Estrogen receptor beta and G protein-coupled estrogen receptor 1 are involved in the acute estrogenic regulation of arginine-vasopressin immunoreactive levels in the supraoptic and paraventricular hypothalamic nuclei of female rats

The ovarian hormone 17β-estradiol is known to regulate the release, expression and immunoreactivity of arginine-vasopressin (AVP) in the supraoptic and paraventricular hypothalamic nuclei of rodents. Previous studies have shown that estrogen receptor α is involved in the effects of chronic estradiol administration on arginine-vasopressin immunoreactivity in the female rat hypothalamus. In this study we have examined the effect of an acute administration of estradiol or specific agonists for estrogen receptors α, β and G protein-coupled estrogen receptor 1 on the immunoreactivity of arginine-vasopressin in the hypothalamus of adult ovariectomized female rats. Acute estradiol administration resulted in a significant decrease in the number of arginine-vasopressin immunoreactive neurons in the supraoptic and paraventricular nuclei after 24 h. The effects of the specific estrogen receptors agonists suggest that the action of estradiol on arginine-vasopressin immunoreactivity is mediated in the supraoptic nucleus by G protein-coupled estrogen receptor 1 and in the paraventricular nucleus by both estrogen receptor β and G protein-coupled estrogen receptor 1. Thus, in contrast to previous studies on the effect of chronic estrogenic treatments, the present findings suggest that estrogen receptor β and G protein-coupled estrogen receptor 1 mediate the acute effects of estradiol on arginine-vasopressin immunoreactivity in the hypothalamus of ovariectomized rats.

Estradiol potentiates 8-OH-DPAT-induced sumoylation of 5-HT1A receptor: Characterization and subcellular distribution of sumoylated 5-HT1A receptors

Sumoylation is a recently described post-translational modification and only a few sumoylated neurotransmitter receptors are known. Through the present studies, we discovered that serotonin1A receptors (5-HT1A-Rs) can be sumoylated by SUMO1 (small-ubiquitin-related modifier 1) protein. The SUMO1-5-HT1A-R is ∼55kDa, is located in the membrane fraction, but not the cytosol, and is distributed in all of the brain regions expressing 5-HT1A-Rs examined. Acute stimulation of 5-HT1A-Rs significantly increased SUMO1-5-HT1A-R in rat hypothalamus. Pre-treatment with estradiol for 2 days, which causes a partial desensitization of 5-HT1A-R signaling, potentiated agonist-induced increases in SUMO1-5-HT1A-Rs in the hypothalamus of ovariectomized rats. Using discontinuous gradient centrifugation followed by digitonin treatment, we found that the majority of SUMO1-5-HT1A-Rs is co-localized with endoplasmic-reticulum and trans-Golgi-network markers. Although a small proportion of SUMO1-5-HT1A-Rs are located in the detergent resistant microdomain (DRM) that contain active G-protein coupled receptors, their distribution was different from that of the Gαz protein that couples to the receptors. These data suggest that the SUMO1-5-HT1A-Rs are an inactive form of 5-HT1A-Rs, a finding further supported by results showing minimal 5-HT1A-R agonist binding to SUMO1-5-HT1A-Rs. Furthermore, SUMO1-5-HT1A-Rs in the DRM were increased by treatment with a 5-HT1A-R agonist, 8-OH-DPAT ((+)8-hydroxy-2-dipropylaminotetralin). Together, these data suggest that sumoylation of 5-HT1A-Rs may be related to 5-HT1A-R trafficking and internalization, which may contribute to 5-HT1A-R desensitization. Since 5-HT1A-Rs play an important role in mood regulation, the present results significantly impact on the understanding of the pathogenesis of affective disorders and development of better therapeutic approaches for these diseases.

Progesterone receptor isoforms differentially regulate the expression of tryptophan and tyrosine hydroxylase and glutamic acid decarboxylase in the rat hypothalamus

Progesterone exerts a variety of actions in the brain through the interaction with its receptors (PR) which have two isoforms with different function and regulation: PR-A and PR-B. Progesterone may modulate neurotransmission by regulating the expression of neurotransmitters synthesizing enzymes or their receptors in several brain regions. The role of PR isoforms in this modulation is unknown. We explored the role of PR isoforms in the regulation of tryptophan (TPH) and tyrosine (TH) hydroxylase, and glutamic acid decarboxylase (GAD) expression in the hypothalamus of ovariectomized rats. Two weeks after ovariectomy, animals were subcutaneously injected with 5μg of estradiol benzoate (EB), and 40h later, progesterone (P) was intracerebroventricularly (ICV) injected. Each animal received two ICV injections of 1μg/μl (4nmol) of PR-B and total PR (PR-A+PR-B) sense or antisense (As) oligonucleotides (ODNs). First injection was made immediately before sc EB injection, and 24h later animals received the second one. Twenty-four hours after P administration, rats were euthanized and brains removed to measure the expression of PR-A and PR-B, TPH, TH and GAD by Western blot. We observed that sense ODNs modified neither PR isoforms nor enzymes expression in the hypothalamus, whereas PR A+B antisense (PR A+B As) clearly decreased the expression of both PR isoforms in this region. ICV administration of PR-B As only decreased PR-B isoform expression with no significant effects on PR-A expression. A differential protein expression of TPH, TH and GAD was observed after PR isoforms antisense administration. PR-B As administration decreased the expression of TPH (65% with respect to control). In contrast, PR A+B As and PR-B As administration increased (51.6% and 34.4%, respectively) TH expression. The administration of PR A+B As and PR-B As diminished GAD expression (33.4% and 41.6%, respectively). Our findings indicate that PR isoforms play a differential role in the regulation of the content of TPH, TH and GAD in the rat hypothalamus.

Effect of Ovariectomy on External Urethral Sphincter Activity in Anesthetized Female Rats

The postmenopausal hypoestrogen condition is associated with various lower urinary tract dysfunctions, including frequency, urgency, stress urinary incontinence and recurrent urinary infection. We determined whether hypoestrogen induced lower urinary tract dysfunction after ovariectomy is also associated with an alteration in external urethral sphincter activity. Bilateral ovariectomy was performed in female Sprague-Dawley® rats and sham operated rats served as controls. Transvesical cystometry and external urethral sphincter electromyogram activity were monitored 4, 6 and 12 weeks after sham operation or bilateral ovariectomy and at 6 weeks in bilaterally ovariectomized rats treated with estrogen. The micturition reflex was elicited in sham operated and bilaterally ovariectomized, urethane anesthetized animals. Post-void residual urine increased and voiding efficiency decreased in rats with 4 to 12 weeks of bilateral ovariectomy. The silent period of external urethral sphincter electromyogram activity was shortened significantly and progressively at increased times after bilateral ovariectomy. These effects were prevented by estradiol treatment. As evidenced by shortening of the external urethral sphincter electromyogram silent period in ovariectomized rats, the disruption of coordination between the external urethral sphincter and the detrusor muscle could decrease urine outflow and in turn voiding efficiency. Estrogen replacement reverses these changes, suggesting that the central pathways responsible for detrusor-sphincter coordination are modulated by gonadal hormones.

Progesterone receptor immunoreactivity in the brains of ovariectomized aged rats

We examined the induction of progesterone receptor-immunoreactive (PR-ir) cells by estrogen in the rat preoptic area and ventromedial hypothalamic nucleus. Ovariectomized young (3-month-old) and old (24-month-old) female rats were treated with estrogen or cholesterol for 4 days. Estrogen significantly increased PR-ir cells in the preoptic area and ventromedial hypothalamic nucleus in young rats. Cholesterol-treated old rats had very few PR-ir cells; estrogen treatment significantly increased the number of PR-ir cells in both the preoptic area and the ventromedial hypothalamic nucleus in old rats, although less than in young rats. Therefore, the ability of estrogen to induce PR immunoreactivity in the hypothalamus in ovariectomized rats is attenuated in old rats compared with young rats.

Estradiol increases catecholamine levels in the hypothalamus of ovariectomized rats during the dark-phase

Estrogens modulate critical homeostatic functions of the hypothalamus such as temperature regulation, sexual behavior and sleep with the most pronounced effects in rats occurring during the dark-phase. The neurochemical signals underlying estrogenic regulation of these hypothalamic functions have not been clearly identified, possibly due to the fact that previous studies have not explored the effects of estrogen treatments on neuronal signaling during the dark-phase. In the present study, ovariectomized rats received estradiol benzoate (5 μg/rat for 7 days, s.c.) and norepinephrine and dopamine levels were measured in the preoptic area of the hypothalamus across the light/dark cycle using in vivo microdialysis. Estradiol benzoate treatment increased extracellular norepinephrine and dopamine levels relative to vehicle treatment during the dark-phase. Increases in norepinephrine and dopamine were first detected by 30 min and 5.5 h after lights-off, respectively. Subsequent increases in norepinephrine and dopamine were also noted throughout the 9.5- h collection period. The effect of estradiol benzoate on catecholamine release did not correlate with increases in either tyrosine hydroxylase (TH) protein expression or activity levels in the anterior hypothalamus, although a marked decrease in TH activity correlated with a rise in extracellular norepinephrine at the beginning of the dark-phase. We conclude that subchronic estradiol benzoate treatment increases extracellular catecholamine levels in the preoptic area of the hypothalamus during the dark-phase without a concomitant increase in neurotransmitter biosynthesis. The estradiol benzoate-induced increases in norepinephrine and dopamine levels in the preoptic area during the dark-phase may play an important role in modulating critical hypothalamic functions.

Estrogen increases messenger RNA and immunoreactivity of aryl-hydrocarbon receptor nuclear translocator 2 in the rat mediobasal hypothalamus

We examined the effect of estrogen on the expression of aryl-hydrocarbon receptor (AhR) and two types of AhR nuclear translocator (Arnt1 and Arnt2) mRNAs in the hypothalamus of ovariectomized rats. Northern blotting demonstrated that, in the mediobasal hypothalamus, a subcutaneous injection of 20 μg estradiol benzoate (E 2) significantly increased the expression of Arnt2 mRNA, but induced no significant changes in the expression of AhR and Arnt1 mRNAs. The expression of Arnt2 mRNA was significantly increased at 4, 24, and 72 h after the injection. Immunocytochemical study revealed that the number of Arnt2 immunoreactive cells was also significantly increased at 72 h after the injection. Conversely, in the preoptic area, injection of E 2 did not cause significant changes in the expression of any of the three mRNAs. These observations suggest that estrogen regulates Arnt2 expression in the mediobasal hypothalamus and modulates the toxic action of dioxins in rats.

Injection of bicuculline elicits firing of luteinizing hormone releasing hormone pulse generator in muscimol-treated ovariectomized rats

We have already demonstrated that, although exogenous γ-aminobutyric acid A (GABA A) receptor agonist is capable of inhibiting the activity of luteinizing hormone releasing hormone (LHRH) pulse generator, the reduction in the endogenous GABA A receptor activity does not have a significant effect in ovariectomized rats, suggesting a minor role of inhibitory GABA neurons in the control of pulsatile release of LHRH. In this study, we further analyzed the role of the GABA A receptor system in the regulation of LHRH pulse generator activity observed by recording the multiunit activity (MUA) in the hypothalamus of ovariectomized rats. An abrupt increase (volley) in the MUA in the arcuate nucleus (ARC)-median eminence region (ME) was accompanied by the initiation of a luteinizing hormone (LH) pulse, determined by measuring LH concentrations in blood sampled simultaneously. The injection of 1–3 mg/kg muscimol (MUS), a GABA A receptor agonist, decreased the basal MUA and delayed the occurrence of the next MUA volley, but it did not change the LH pulse amplitude. I.v. injection of 5 mg/kg bicuculline (BIC), a GABA A receptor antagonist, during continuous infusion of MUA (5 mg/kg/h) which decreased the basal MUA and prevented the MUA volley from occurring, evoked MUA volleys and LH pulses whose amplitudes were similar to those found before the MUS infusion. Injection of the same dose of BIC during the infusion of saline increased the MUA transiently, but this increase was not accompanied by an LH pulse and afterwards there occurred MUA volleys at ordinary intervals accompanied by LH pulses with unchanged pulse amplitudes. These results suggest that the GABA A receptor activity nhibits the LHRH pulse generator activity and that when the GABAergic activity is extremely high, a reduction in the GABA A receptor activity is able to cause activation of the LHRH pulse generator in the ovariectomized condition.

Dopaminergic Regulation of Galanin Gene Expression in the Rat Anterior Pituitary Gland

Estrogen dramatically increases galanin mRNA and peptide levels in the rat anterior pituitary gland. We recently reported that galanin secretion in vitro from estrogen-exposed anterior pituitary cells is regulated by hypothalamic factors; dopamine and somatostatin inhibit galanin secretion, and thyrotrophin-releasing hormone stimulates galanin release. To determine whether galanin is regulated by a dopaminergic mechanism in vivo, we used ovariectomized Fischer 344 rats treated with 17ß-estradiol-containing or empty Silastic capsules. Rats were also administered bromocriptine, a dopamine receptor agonist, haloperidol, a dopamine receptor antagonist, or placebo for 2 weeks. Galanin peptide levels were measured in the anterior pituitary, neurointermediate lobe, medial basal hypothalamus, and plasma by radioimmunoassay. Plasma and pituitary prolactin levels were also determined. Bromocriptine decreased gaianin peptide levels in the anterior pituitary gland of ovariectomized rats by 30%, but had no effect on galanin in the neurointermediate lobe or medial basal hypothalamus. In contrast, haloperidol had no effect on galanin in the anterior pituitary or medial basal hypothalamus of ovariectomized rats, but decreased galanin peptide levels in the neurointermediate lobe. In the anterior pituitary gland of estrogen-treated rats, bromocriptine increased and haloperidol decreased both galanin and prolactin levels. Galanin mRNA levels were quantified in the anterior pituitary gland by solution hybridization. Bromocriptine increased galanin mRNA levels 3-fold in the anterior pituitary, whereas haloperidol had no effect. Galanin mRNA levels in the anterior pituitary were elevated 10-fold by estrogen. Bromocriptine reduced galanin mRNA levels in the pituitary by 50% in estrogen-treated rats, where again haloperidol had no effect. Estrogen increased plasma galanin levels 4-fold compared to ovariectomized rats and this effect was reduced 60% by bromocriptine and increased 20% by haloperidol. We conclude 1) galanin synthesis and release from the estrogen-exposed anterior pituitary gland is inhibited by a dopaminergic mechanism in vivo, 2) dopamine regulates galanin gene expression in the ovariectomized rat, 3) the changes in galanin peptide levels in the anterior pituitary of rats treated with estrogen and dopamine receptor ligands are primarily due to alterations in peptide secretion, and 4) galanin release from the neurointermediate lobe may also be regulated by a dopaminergic mechanism in vivo. These data, in conjunction with previous studies, provide evidence for the co-regulation of galanin and prolactin in estrogen-treated rats, and further discriminate between the regulation of galanin in the hypothalamus and pituitary gland.

Third ventricular injection of alpha-bungarotoxin decreases pulsatile luteinizing hormone secretion in the ovariectomized rat.

The injection of purified alpha-bungarotoxin (alpha-BTX), an antagonist of the muscle nicotinic cholinergic receptors, into the third ventricle (3rd V) of ovariectomized rats decreased the frequency of pulses of luteinizing hormone (LH) but had no significant effect on the amplitude or nadir of LH secretion. Approximately 24 h after the 3rd V injection of alpha-BTX, the pulsatile secretion rate of LH was increased and thus the alpha-BTX effect was reversible. The 3rd V injection of the two nicotinic cholinergic agents, cytisine (10 micrograms) and dihydro-beta-erythroidine, also decreased the frequency of pulses of LH. Cytisine and dihydro-beta-erythroidine inhibited the binding of (125I)alpha-BTX to rat hypothalamic synaptosomes (P2B) and also to frozen coronal sections of the hypothalamus as studied by film autoradiography. Injection of a lower dose of cytisine (1 micrograms) had no effect on LH secretion, but when administered with alpha-BTX into the 3rd V, cytisine significantly reduced the alpha-BTX-induced decreased LH secretion. The injection of cytisine with (125I)alpha-BTX into the 3rd V also decreased in vivo labeling of the (125I)alpha-BTX-binding sites in the hypothalamus of ovariectomized rats. The results suggest that alpha-BTX can inhibit the pulsatile secretion of LH in ovariectomized rats and this inhibition may be mediated by specific subclass of a CNS (central nervous system) nicotinic acetylcholine receptor.

Upregulation of imipramine binding and serotonin uptake by estradiol in female rat brain

This report describes the effect of chronic estradiol treatment on the serotonin transporter in the female rat brain. Both [3H]imipramine binding and [3H]serotonin uptake increased by 20-30% in the frontal cortex and hypothalamus of ovariectomized rats after 12 days of 17 beta-estradiol treatment. No differences were observed in the binding and uptake parameters as a function of the rats' estrous cycle or in untreated ovariectomized rats, as compared to controls. Estradiol in vitro, inhibits [3H]imipramine binding as well as serotonin uptake in rat brain and human platelets. Like serotonin, estradiol decreases the dissociation rate in vitro of [3H]imipramine from its binding site in a dose-dependent manner.

Estradiol target sites immunoreactive for β-endorphin in the arcuate nucleus of rat and mouse hypothalamus

Combined autoradiography and immunocytochemistry was performed to examine estradiol-concentrating neurons of the arcuate nucleus in the hypothalamus of ovariectomized rats and mice for β-endorphin-like immunoreactivity. A high number of [ 3H]estradiol-labeled cells occurred in the arcuate nucleus of both species. Neurons immunoreactive for β-endorphin could be visualized throughout the arcuate nucleus as well as ventrolaterally outside of this nucleus. Colocalization of [ 3H]estradiol nuclear labeling and β-endorphin-like cytoplasmic immunoreactivity could be found scattered throughout the arcuate nucleus, most frequently in the ventromedial part, although most of the [ 3H]estradiol target neurons did not react with the antibodies. The results suggest that a subpopulation of β-endorphin neurons is directly addressed by estradiol and that the arcuate nucleus contains different groups of estradiol receptive neurosecretory cells.

Acute effects of estradiol on circulating luteinizing hormone and prolactin concentrations and on serotonin turnover in individual brain nuclei.

Two experiments tested whether serotonergic neurons participate in the acute feedback effects of estradiol on LH and PRL secretion. In the first study, ovariectomized female rats received oil vehicle or estradiol benzoate and were killed 3 h later. The turnover of serotonin [5-hydroxytryptamine (5-HT)] in microdissected brain nuclei was assessed from its accumulation after monoamine oxidase inhibition. Estradiol treatment produced a gradual rise in PRL and a decline in LH, and potentiated the accumulation of 5-HT in the medial preoptic, ventromedial, and cortical amygdaloid nuclei, suggesting increased turnover. Concentrations of the metabolite, 5-hydroxyindole acetic acid, were also elevated in these areas. 5-HT turnover was unaffected by estradiol in the interstitial nucleus of the stria terminalis, periventricular, dorsomedial, or lateral amygdaloid nuclei. The second experiment tested whether localized 5-HT depletions in these estradiol-activated nuclei blocked the acute effects of the steroid on LH or PRL. Stereotaxic microinjections of the 5-HT neurotoxin, 5,7, dihydroxytryptamine were made into the preoptic area, cortical amygdala, or medial basal hypothalamus in ovariectomized rats, pretreated with desmethylimipramine. Such treatment reduced 5-HT, without affecting catecholamines, in the targeted structures. Localized 5-HT depletion in the preoptic area and amygdala blunted the elevation of PRL, but not the decline of LH, after estradiol. 5-HT depletion in the medial basal hypothalamus elevated, whereas 5-HT depletion in the medial preoptic area lowered, resting levels of LH. These results suggest that: 1) estradiol may acutely increase PRL secretion by activating serotonergic projections to the medial preoptic and cortical amygdaloid nuclei, and 2) serotonergic neurons innervating the preoptic area stimulate LH release, whereas serotonergic systems in the medial basal hypothalamus inhibit LH release.

Changes in the characteristics of pulsatile LH secretion after estradiol implantation into the preoptic area and the basal hypothalamus in ovariectomized rats.

Intracranial actions of estradiol benzoate (EB) in modulating the characteristics of pulsatile LH secretion were examined in ovariectomized rats. A minute amount of crystalline EB was implanted into the medial basal hypothalamus (MBH) or the preoptic suprachiasmatic area (POSC) and blood samples were collected at 6-min intervals for radioimmunoassay of serum LH concentrations. In rats with EB implanted in the MBH, the LH pulse amplitude decreased while the frequency did not change for 3 h after implantation. In rats bearing EB implants in the POSC, in contrast, the LH pulse frequency decreased without a significant variation in the pulse amplitude. Control animals with empty cannulae inserted into the MBH or POSC exhibited unchanged amplitude and frequency of LH secretory pulses. These results indicate that estrogen can regulate the amplitude and frequency of pulsatile LH secretion via mechanisms independent of each other. It appears that MBH and/or the pituitary is a site of the action of estrogen in decreasing LH pulse amplitude, while the POSC is a site of the steroid action in decreasing the pulse frequency.

Biosynthesis, in vivo, of gonadotropin-releasing hormone in the hypothalamus of normal and ovariectomized female rats

Normal female and ovariectomized rats were infused into the 3rd ventricle with [ 3H]glycine or [ 3H]alanine. Some rats were pretreated with cycloheximide. Hypothalamus, anterior pituitary and fragments of cortex were excised, homogenized, extracted and treated with specific antiserum to GnRH, bound to Sepharose. The radioactivity of immuno-adsorbed products was counted either immediately or after extraction and thin-layer chromatography by using two different solvent systems. With the two systems, the location of the immuno-adsorbed radio-activity always coincided with the spot of synthetic GnRH. Our results show that [ 3H]glycine was incorporated, as a function of time, into GnRH isolated from rat hypothalami. The amount of radioactivity incorporated into hypothalami from diestrous-I rats was similar to that of ovariectomized rats and twice as high as in late proestrous rats. Only minute amounts of radioactivity were incorporated into anterior pituitary of normal and castrated rats. [ 3H] Alanine was not incorporated into the immuno-adsorbed product. Cycloheximide inhibited incorporation of [ 3H]glycine into the immuno-adsorbed product to the same extent as its incorporation into the total protein from the hypothalamus. Our experimental results support the hypothesis of ribosomal mechanisms being involved in the biosynthesis of GnRH. They also suggest that the rate of accumulation of newly synthesized labeled GnRH is of the same order in the hypothalamus of ovariectomized rats as in diestrous-I rats.

Ultrastructure of neurons in the ventromedial nucleus or the hypothalamus in ovariectomized rats with or without estrogen treatment

The fine structure of the ventrolateral and dorsomedial subdivisions of the ventromedial nucleus (VMN) of the hypothalamus was examined in ovariectomized/control and ovariectomized/estrogen-treated rats to compare neurons of these areas to other neurons (specifically the ventrolateral thalamus), and to determine the effects of estrogen on these cells. The neurons of the VMN contain a large nucleus with a prominent nucleolus, rough endoplasmic reticulum (RER), polysomes, a Golgi complex, coated, uncoated and dense-cored vesicles, lysosome-like bodies, inclusion bodies, multivesicular bodies whorl bodies and myelin figures. Similar organelles were present in the neurons of the ventrolateral thalamus, although polysomes wee more prominent, and the cells lacked dense-cored vesicles in the perikarya. Differences in the cells of the VMN between ovariectomized/control and ovariectomized/estrogen-treated rats included a more conspicuous stacking of the RER and greater number of dense-cored vesicles in the estrogen-treated group in both the ventrolateral and dorsomedial subdivisions. In both areas the differences were statistically significant, although more marked in the ventrolateral subdivision. In both VMN subdivisions, the increased stacking of the RER could be correlated with the greater number of dense-cored vesicles and may reflect increased biosynthesis of a secretory product.

Modulation by Thyroid Hormones of Cytoplasmic Estrogen Receptor Concentrations in Reproductive Tissues of the Rat

The influence of induced variations in circulating levels of thyroid hormones upon the estrogen receptor content of rat uterine, anterior pituitary and hypothalamic cytosol fractions has been investigated. Sucrose gradient centrifugation analyses showed no observable effects of thyroidectomy or acute induced hyperthyroidism upon receptor levels in uterus hypothalamus of ovariectomized rats, but an apparent positive thyroidal influence upon anterior pituitary receptor content. Qualitatively, the receptor complex sedimentation coefficients were unaltered. Binding assays showed that thyroid hormone administration to the ovariectomized rat resulted in a marked increase in pituitary receptor content, whereas the same treatment had no significant effect on the pituitary receptor level of the intact animal. Conversely, a marked fall in uterine receptor content was found in thyroxine-treated intact, but not castrate, animals. The changes in measurable receptor levels could not be attributed to competition for estrogen binding sites by thyroxine or triiodothyronine. Pituitary receptor content of ovariectomized rats was studied following administration of varying levels of thyroxine; 5 mug/day significantly depressed receptor levels, while enhancement of binding capacity was occasioned by levels of thyroxine ewual to or greather than 25 mug/day. The hypothalamus was completely unaffected by similar treatment and the uterus responded with decreased receptor level only at the highest dosage used. Thyroxine replacement reversed the decrease in anterior pituitary receptor concentration detectable following thyroidectomy. The effect of the thyroid hormones on the pituitary receptor content was not a result of changes in the capability of the tissue to respond to estrogen-induced synthesis of receptor.