Prelordotic behavior in the hamster: A hormonally modulated transition from aggression to sexual receptivity.

Analyzed the behavioral responses of 60 female golden hamsters to sexually experienced males as a function of the stage of the female's estrous cycle. Exogenous estradiol or estradiol followed by progesterone was given to ovariectomized Ss to determine the role of these hormones in regulation of cyclic changes in the female's response to the male. Ss were paired daily with sexually active males for 10 min, and behavioral interactions were recorded. Significantly more fighting occurred during early diestrus than later in the cycle. By 8 hrs prior to the onset of sexual receptivity, 86% of Ss exhibited a behavior resembling the onset of lordosis without immobilization. Following ovariectomy, fighting was at a high level. Estradiol replacement over 28 days resulted in a significant decrease in aggression and increase in display of the prelordotic response. Initially, treatment with progesterone following 7 days of estradiol caused lordosis display. After 24 hrs a significant increase in aggression, which continued as long as progesterone was present, was observed. Thus, estradiol causes the female to become tolerant of the male's approach, the female exhibiting prelordosis in response to the male's investigation. Estradiol and progesterone are necessary for normal sexual receptivity; however, after 24 hrs, estrogen/progesterone-treated females become agonistic to the male. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cholinergic influences on estrogen-dependent sexual behavior in female rats.

Examined the ability of cholinergic agents to influence hormone-dependent sexual behavior in Sherman rats. In Exp I, sexual behavior, indicated by the incidence of lordosis, was significantly increased in estrogen-treated Ss following bilateral infusion of a cholinergic receptor agonist, carbachol (.5 μg/cannula) into the medial preoptic area of the brain. Infusion of an artificial cerebrospinal fluid vehicle failed to facilitate lordosis. The incidence of lordosis was normally highest 15 min after carbachol infusion began to wane by 45 min, and had returned to control levels by 90 min. Centrally administered carbachol activated lordosis at lower levels of estrogen priming than did systemically administered progesterone. In Exp II, Ss brought into sexual receptivity by administration of estrogen and progesterone received preoptic infusions of an acetylcholine synthesis inhibitor, hemicholinium-3 (HC-3). Significant reductions in the incidence of lordosis were observed following bilateral infusion of HC-3 (1.25 μg/cannula). This inhibition of lordosis was prevented when carbachol (.5 μg/cannula) was infused along with HC-3. Results confirm the importance of cholinergic influences on sexual behavior in female rats. (20 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Use of computer interview data to test associations between risk factors and pregnancy outcomes

Estrogens are an important class of steroid hormones, involved in the development of brain, skeletal, and soft tissues. These hormones influence adult behaviors, endocrine state, and a host of other physiological functions. Given the recent cloning of a second estrogen receptor (ER) cDNA (the ER beta), work on alternate spliced forms of ER alpha, and the potential for membrane estrogen receptors, an animal with a null background for ER alpha function is invaluable for distinguishing biological responses of estrogens working via the ER alpha protein and those working via another ER protein. Data generated to date, and reviewed here, indicate that there are profound ramifications of the ER alpha disruption on behavior and neuroendocrine function. First, data on plasma levels of estradiol (E2), testosterone (T), and luteinizing hormone (LH) in wild-type (WT) versus ER alpha- mice confirm that ER alpha is essential in females for normal regulation of the hypothalamic-pituitary gonadal axis. Second, ovariectomized female ER alpha- mice do not display sexual receptivity when treated with a hormonal regime of estrogen and progesterone that induces receptivity in WT littermates. Finally, male sexual behaviors are disrupted in ER alpha- animals. Given decades of data on these topics our findings may seem self-evident. However, these data represent the most direct test currently possible of the specific role of the ER alpha protein on behavior and neuroendocrinology. The ER alpha- mouse can be used to ascertain the specific functions of ER alpha, to suggest functions for the other estrogen receptors, and to study indirect effects of ER alpha on behavior via actions on other receptors, neurotransmitters, and neuropeptides.     

Regulation of female sexual behavior in the golden hamster: behavioral effects of mating and ovarian hormones

In the female hamster, sexual receptivity was abbreviated following copulation. Short-term effects of mating, lasting approximately 24-48 hr, were observed in females mated in either estradiol- or estradiol-and-progesterone-induced estrus. Long-term effects of copulation, of 9 days or more in duration, were apparent only in females chronically treated with both estradiol and progesterone, suggesting that progesterone exposure prolongs the inhibitory effects of mating. When progesterone stimulation was intermittent through the use of short-acting injection procedures, recovery from mating could be detected in 48 hr and was complete within 96 hr after copulation. The pattern of behavioral response to ovarian hormones in unmated females was also documented, replicating the "biphasic" effect of progesterone in this species. The inhibitory effects of mating and progesterone summate to produce consistent decrements in female sexual receptivity. It is postulated that short-term postcopulatory abbreviations in receptivity would reduce the vulnerability of the estrous female while long-term inhibitions, interacting with sustained progesterone stimulation, would reduce the probability of mating during pregnancy when hormone levels are elevated for a prolonged period.     

Temporal aspects of ventromedial hypothalamic progesterone action in the facilitation of estrous behavior in the female rat.

Examined the temporal parameters of progesterone (P) action in the ventromedial nucleus of the hypothalamus (VMN) in the facilitation of estrous behavior in estrogen-primed female Long-Evans rats stereotaxically outfitted with guide cannulae directed towards the VMN. Crystalline P was applied directly to the brain tissue via bilateral insert cannulae. Ss were ovariectomized and estrogen primed with 5% estradiol Silastic capsules. They received a counterbalanced series of 2 experimental tests: one involving a manipulation with a P-filled implant, and another with a blank implant. In Exp I, a significant increase in estrous responsiveness occurred only after 2 hr exposure of the VMN to P, whereas 4 hr were required for a full display of estrous behavior, including solicitation. In Exp II, P was lowered into the brain for either 1, 2, or 4 hr, and testing took place 4 hr after the lowering of the implant. It was found that 2 hr of P exposure was sufficient to facilitate full estrous responsiveness at 4 hr. In Exp III, it was revealed that the duration of estrous responsiveness was directly related to the time the P implant remained in the brain. In Exp IV, the time course of P retention in brain tissue, revealed by determination of –3H-progesterone levels in hypothalamus, agreed with the behavioral findings. Progesterone levels in the region of the VMN remained high while a P implant was in place, but declined rapidly after removal. A dual mechanistic hypothesis for P action in the facilitation of estrous behavior is presented. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dexamethasone: increased weights and decreased [3H] estradiol retention of uterus, vagina and pituitary in the ovariectomized rat

Ovariectomized rats given 100 mug dexamethasone per day for 5 days had significantly heavier dry weights for uterus, vagina and pituitary, indicating a growth promoting activity of dexamethasone on these tissues in which estrogen normally promotes growth changes. The dexamethasone treated animals also retained significantly less [3H]estradiol per mug dry weight of tissue for uterus, vagina and pituitary. When[3H]estradiol retention was examined in vitro for the nuclear fraction, a significant decrease in retention was found for uterus, vagina and pituitary but not for hypothalamus or cerebral cortex. The decreased ability to bind [3H]estradiol, shown by the estrogen target tissues of the dexamethasone-treated rats, along with the increased growth of the estrogen target tissues, demonstrates that these tissues were able to show trophic responses even when greater levels were one-third of normal. Dexamethasone-treated animals tested for sexual receptivity in the presence or absence of progesterone priming did not show induction of facilitation of sexual receptivity. However, estrogen plus progesterone injections induced sexual receptivity in the presence of dexamethasone. When dexamethasone was combined with a dosage of estrogen, which by itself did not induce sexual receptivity, there was a significant response with 6 to 10 animals showing a low level of receptivity. Thus, dexamethasone can apparently synergize with estrogen to facilitate sexual receptivity.     

Responses to copulatory stimulation in neonatally androgenized female rats.

Five experiments evaluated the extent to which copulatory stimulation could ameliorate the anestrus and sterility exhibited by neonatally androgenized female Long-Evans rats (N?=?102). The age at which Ss began to exhibit persistent vaginal estrus and the degree of sexual receptivity exhibited under several testing paradigms were found to be inversely related to the dose of testosterone propionate (TP) injected neonatally. With increasing numbers of mounts received, both the number of androgenized Ss exhibiting sexual receptivity and the quality of the estrous behavior exhibited tended to increase. Ss injected with high doses of TP (500 μg) usually showed little or no receptive behavior even in the most extensive behavioral tests. However, under some testing conditions Ss receiving 50 μg of TP neonatally, while showing little or no receptivity during initial mounts, showed increased receptivity as behavioral tests were extended. Following matings that included 1–5 ejaculations, only control Ss were observed to become pregnant. However, when androgenized females cohabited with males for an extended period, Ss that had neonatally received .5 μg of TP, but not higher doses, did become pregnant. It is concluded that (1) the capacity of systems mediating reproductive physiology and behavior is facilitated by stimuli associated with males; and (2) mating is a characteristic of the female rat, which can be manipulated by injection of hormones during the neonatal period. (13 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

P300 subcomponents reflect different aspects of psychopathology in schizophrenia

With the results presented in this paper we devised an alternative method to precisely date the rat endometrium in relation to the estrous cycle. This is done by the exclusive use of scanning electron microscopy. Owing to its short estrous cycle (4 to 6 days, depending on age), the rat is ideally suited for the examination of cyclic changes occurring in the uterine epithelium. The cycle stage of rats predated by vaginal smear cytology was verified through the measurement of hormones relevant to the estrous cycle, i.e. estradiol-17-beta (E2), progesterone (P), luteinizing hormone (LH), and follicle stimulating hormone (FSH). Based on scanning electron images surface changes could thus be correlated to the cyclic variations of blood levels of sex hormones. The appearance of pseudoglands, the most prominent aspect during the cycle, is correlated with decreasing estrogen and rising progesterone levels. Pseudoglands are formed by apoptosis and necrosis of epithelial cells, and are most numerous during estrus. They had in previous studies been classified as genuine uterine glands.     

Estrogen-progesterone regulation of nest-building and incubation behavior in ovariectomized ring doves (Streptopelia risoria).

Determined the role of ovarian hormones in the induction of nest-building (tucking) and incubation behavior in female doves by systemic injections of estrogen, or progesterone, or estrogen combined with progesterone, or oil in 40 reproductively experienced, ovariectomized Ss. Combined estrogen and progesterone treatment was the most effective hormone regimen for eliciting both behavior patterns in females and also facilitated these behaviors in their 40 untreated mates. Differences in role of the gonadal progesterone in male and female doves are discussed. (23 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Concurrent inhibition of sexual behavior, but not brain [–3H]estradiol uptake, by progesterone in female rats.

In 7 experiments with ovariectomized female Sprague-Dawley rats, chronic injections of high doses of progesterone (5 mg) and low doses of estradiol benzoate (EB; 2 μg) resulted in less sexual behavior than did low doses of progesterone (.5 mg) and low doses of EB. In a typical procedure for inducing sexual behavior, EB and progesterone were given sequentially, separated by 42 hrs. High levels of progesterone (2.5 and 5 mg) administered concurrently with EB inhibited the induction of sexual receptivity. Increasing the dose of EB from 2 μg to 6 μg or 10 μg offset this inhibition. High doses inhibited the induction of sexual behavior, but the inhibition waned when progesterone was administered 48 hrs prior to EB. A single injection of progesterone (1 mg) that did not inhibit the induction of sexual behavior when administered concurrently with EB did inhibit lordosis when distributed into 5 injections (.2 mg) every 4 hrs. Results of 2 experiments in which progesterone did not inhibit the uptake or retention of [–3H]estradiol by brain cell nuclei suggest that the antiestrogenic action of progesterone in the CNS is not to interfere with the binding of estradiol. (37 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of mianserin, a new antidepressant, on the in vitro and in vivo uptake of monoamines

1 The effects of mianserin and of selected tricyclic antidepressants were compared in a number of monoamine uptake models. 2 The ability of mianserin to block the noradrenergic neurone membrane amine pump of rabbit brain stem slices was comparable to that of imipramine and amitriptyline and less than that of desipramine and nortriptyline. Both mianserin and desipramine were competitive inhibitors of noradrenaline uptake in vitro. The effect of mianserin on noradrenaline uptake in vivo was studied both peripherally and centrally. The ability of 6-hydroxydopamine to lower rat heart noradrenaline levels was found to be very sensitive to inhibition by tricyclic antidepressants. Mianserin was active in this model. However, its ability to block the 6-hydroxydopamine-induced fall in rat heart noradrenaline concentration was appreciably less than that of the tricyclics studied. 3 Mianserin, like tricyclic antidepressants, was essentially devoid of effect on dopamine uptake both in vitro and in vivo. 4 The ability of mianserin to inhibit [3H]-5-hydroxytryptamine uptake by rat hypothalamic synaptosomes was appreciably less than that of the tricyclic antidepressants studied. Mianserin was essentially devoid of effect on rat brain 5-hydroxytryptamine uptake in vivo. 5 It is concluded that in certain situations large doses of mianserin may block noradrenaline uptake in vivo. However, in no way does mianserin rival tricyclic antidepressants in blocking monoamine uptake in vivo. The clinical efficacy of mianserin cannot be attributed to inhibition of monoamine uptake.     

Aggressive behavior in female hamsters: The hormonal basis for fluctuations in female aggressiveness correlated with estrous state.

The frequency and sequencing of aggressive behaviors by naive female hamsters has been found to change during series of brief encounters, probably because of the lack of stable dominance relations. Such initial encounters seem most representative of interactions likely in free-ranging hamsters and have been emphasized in studies of the hormonal mediation of female aggression. The present 4 experiments, conducted with a total of 72 random-bred female hamsters, found that nonestrous females exhibited intense aggression toward conspecifics of either sex. Estrous females were not aggressive and spent much time in lordosis, indicative of sexual receptivity. While oil-injected adrenalectomized-ovariectomized females fought at high levels, comparable with intact nonestrous females, the combination of 17-β-estradiol benzoate and progesterone suppressed fighting completely. In contrast, replacement of estradiol, progesterone, or testosterone propionate individually had no consistent effect. Hypophysectomized females also fought at high levels, indicating that pituitary hormones are not required for vigorous aggression. Further, individual anterior pituitary hormones did not produce marked changes in fighting. Results emphasize the roles of estrogen and progesterone in synchronizing aggression with current reproductive state. (36 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of sex hormones on protein and collagen content of the temporomandibular joint disc of the rat

PURPOSE: The effect of sex hormones on the protein and collagen content of the temporomandibular joint (TMJ) disc of adult male and female rats. MATERIALS AND METHODS: One hundred forty-four Wistar rats were assigned to 14 groups of 12 each. Two groups, one female and one male, served as a control and received no treatment, and two other groups (one female and one male) received a sham gonadectomy and placebo hormone. The remaining 10 groups (five males and five females) received either orchiectomy or ovariectomy, followed by administration of estrogen, progesterone, combined estrogen and progesterone, or testosterone. The total protein and collagen content of the TMJ disc were determined using the calorimetric hydroxyproline method. RESULTS: The collagen content of TMJ discs of control males was statistically greater than the collagen content of the control female rats. This difference disappeared after ovariectomy of females and orchiectomy of males. Also, there was a general trend for a decrease in collagen and protein content to be produced by estrogen, progesterone, and by estrogen combined with progesterone in castrated male and female rats, and by orchiectomy of male rats. There was also a trend toward an increase in collagen and protein content after ovariectomy in female rats and administration of testosterone to castrated male and female rats. However, the only statistically significant effect of the drugs tested was that of estrogen combined with progesterone in ovariectomized female rats (a lowering effect on the total protein) and of estrogen alone in orchiectomized male rats (a lowering effect on the collagen content). CONCLUSION: Steroid sex hormones have an effect on the collagen and protein content of the TMJ disc of the rat as indicated by the difference in the values between control males and females and by the disappearance of this difference on castration of both male and female animals. This was also manifested by the significant effect of estradiol on collagen content of castrated males, by the effect of estrogen combined with progesterone on the protein content of castrated females.     

Influence of gonadal hormones and sexual behavior on ultrasonic vocalization in rats: I. Treatment of females.

Ultrasonic vocalizations were measured when male Long-Evans rats were placed with ovariectomized females that had experienced various hormonal and behavioral treatment. In Exp I, 18 males were tested with females in each of the following conditions: nonestrous (OVX), estrogen treated (E), estrogen and progesterone treated (EP), and estrogen and progesterone treated and given 2 intromissions from a stud male prior to testing (EPI). Control conditions included clean cage (CL) and cage soiled by an estrous female (SOI). The treatments differed in effect on rate and maintenance of vocalization, in the order of greatest to least: EP, E, EPI, and OVX (equal), SOI, and CL. In tests in which males produced a high rate of vocalization, some males with short intromission latencies shifted from the normal 50-kHz pulse to a 22-kHz pulse. In Exp II, the effect of the female's vocalization and movement on the rate of and latency to vocalization was measured. 21 males were presented with each of the following stimulus conditions: estrous female with red light (EP), estrous female without red light (EP dark), estrous anesthetized female (EP anes), and nonestrous anesthetized female (OVX anes). Effects on vocalization of various treatments were in descending order: EP and EP-dark (equal), EP anes, and OVX anes. Data suggest that the 50-kHz vocalizations constitute a graded response influenced by the female's hormonal and sexual condition. (16 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

"Estrogen-Progesterone Regulation of Nest-Building and Incubation Behavior in Ovariectomized Ring Doves (Streptopelia risoria)": Erratum.

Reports an error in the original article by Mei-Fang Cheng and Rae Silver (Journal of Comparative & Physiological Psychology, 1975[Jan], Vol 88[1], 256-263). The address for the request for reprints should have read "Institute of Animal Behavior, Rutgers University, Newark, New Jersey 07102." (The following abstract of this article originally appeared in record 1975-08949-001.) Determined the role of ovarian hormones in the induction of nest-building (tucking) and incubation behavior in female doves by systemic injections of estrogen, or progesterone, or estrogen combined with progesterone, or oil in 40 reproductively experienced, ovariectomized Ss. Combined estrogen and progesterone treatment was the most effective hormone regimen for eliciting both behavior patterns in females and also facilitated these behaviors in their 40 untreated mates. Differences in role of the gonadal progesterone in male and female doves are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Evidence that a factor besides progesterone, prolactin, or plasma-estradiol-binding protein inhibits estrogen-induced sexual receptivity in pregnant rats.

Reexamined in 4 experiments the assumption that progesterone is responsible for the inhibition of estrogen-induced receptive behavior in Wistar hooded rats. Daily administration of estradiol benzoate (EB) stimulated significantly less lordotic behavior during the 2nd half of pregnancy than in ovariectomized Ss that received sc progesterone implants, pituitary grafts that raised plasma prolactin, or both treatments combined. Following an initial facilitation of receptivity, Ss with progesterone implants showed only moderate reductions in lordosis quotients over 3 test days. The capacity of Ss' plasma to bind estradiol was found to increase significantly during the 2nd half of pregnancy. However, daily administration of a synthetic estrogen, R 2858, which is not bound by plasma protein, was no more effective than EB in stimulating receptive behavior. Administration of EB also stimulated significantly lower levels of sexual behavior in pregnant Ss than in Ss in which pseudopregnancy had been prolonged by previous hysterectomy or induction of uterine decidualization. These findings suggest that some endocrine factor other than progesterone, prolactin, or estradiol-binding protein is primarily responsible for the potent suppression of behavioral responsiveness to estrogen that occurs in pregnant rats. It is suggested that 5-alpha-reduced androgens may cause these behavioral effects. (53 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Fluctuations in relative levels of choline acetyltransferase mRNA in different regions of the rat basal forebrain across the estrous cycle: effects of estrogen and progesterone

Quantitative in situ hybridization techniques were used to compare relative cellular levels of choline acetyltransferase (ChAT) mRNA in different regions of the female rat basal forebrain at different stages of the estrous cycle and at different time points after the administration of physiological levels of estrogen and progesterone. Significant fluctuations in relative levels of ChAT mRNA were detected during the course of the estrous cycle. In the medial septum (MS) and striatum, the highest levels of ChAT mRNA were detected on diestrus 1. Fluctuations in the nucleus basalis magnocellularis (NBM) were highly variable, with the highest levels detected on diestrus 2. In ovariectomized animals, significant increases in ChAT mRNA were detected in the MS, NBM, and striatum within 1-3 d after a single administration of estradiol. In addition, the effects of estradiol on ChAT mRNA expression in the NBM and striatum were significantly enhanced by the subsequent administration of progesterone. The magnitude and timing of the effects of steroid replacement were consistent with the magnitude and time course of the fluctuations detected during the course of the estrous cycle. These data demonstrate that estrogen and progesterone can increase basal forebrain levels of ChAT mRNA significantly in specific regions of the rat basal forebrain, that the magnitude and time course of the effects vary between different subpopulations of cholinergic neurons, and that the effects are associated with changes in the functioning of specific basal forebrain cholinergic neurons across the estrous cycle.     

Effect of ovarian secretions on female behavioral potentiality in the rat.

Measured receptivity in female Sprague-Dawley rats ovariectomized at 5 ages, in neonatally gonadectomized females and males implanted with ovaries, and in neonatally castrated males injected with estradiol benzoate (EB) or oil. Mean receptivity, darting, and lordosis scores were higher during the 1st 4-5 mating tests in females and males having ovaries prepubertally. In amounts greater than .01 mg., EB inhibited female behavior. Even after ovariectomy, body weight was lightest in males and females having ovaries for 60 days. Progesterone and EB decreased weight gain faster in Ss gonadectomized prepubertally. Results indicated that physiological amounts of ovarian hormones, while not necessary for development of female potentiality, permanently influence it by modifying rate of utilization of estrogen circulating during adulthood. (24 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Differential effects of courtship and mating on receptivity and brain metabolism in female red-sided garter snakes (Thamnophis sirtalis pareitalis).

In the female red-sided garter snake (Thamnophis sirtalis parietalis), the loss of receptivity following intromission during mating can be prevented by injection of a local anesthetic (tetracaine) in the cloacal region prior to courtship and mating. Females that were courted and then mated had significantly higher uptake of radio-labeled [1?C]2-fluoro-2-deoxyglucose (2-DG) in the preoptic area (25%) and significantly lower uptake in the ventromedial hypothalamus (-20%) compared with females that were courted but not mated. Tetracaine-treated females had accumulation patterns similar to courted but unmated females and to females exposed only to other females. These results suggest that in the female red-sided garter snake, sensory input from the cloaca during mating alters patterns of metabolism in those brain areas most often associated with female sexual behavior. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of estrogen and progesterone on the expression of hepatic and extrahepatic sterol 27-hydroxylase in baboons (Papio sp)

Sterol 27-hydroxylase plays an important role in cholesterol metabolism in hepatic and extrahepatic tissues. To determine whether female sex steroid hormones influence its expression, we measured plasma and hepatic 27-hydroxycholesterol, hepatic mRNA levels, activity of sterol 27-hydroxylase, and adrenal mRNA levels of this enzyme in baboons (n = 6 per group) treated with placebo, estrogen, estrogen + progesterone, and progesterone. We also measured hepatic cholesterol concentration and hepatic acyl coenzyme A:cholesterol acyltransferase (ACAT) activity to determine their relationship with hepatic sterol 27-hydroxylase activity. Plasma 27-hydroxycholesterol concentration was increased by estrogen and estrogen + progesterone and was negatively correlated with plasma (P = .090) and LDL (P = .026) cholesterol concentrations. Similarly, hepatic sterol 27-hydroxylase activity was increased by estrogen and estrogen + progesterone and was negatively correlated with plasma (P = .056) and LDL (P = .052) cholesterol concentrations but was positively correlated with hepatic and plasma 27-hydroxycholesterol concentrations (P < .001). Hepatic ACAT activity was increased by progesterone (P < .004) and was positively correlated with plasma (P = .002) and LDL (P = .009) cholesterol concentrations but was negatively correlated with hepatic sterol 27-hydroxylase activity (P = .035). Hepatic and adrenal gland mRNA levels for sterol 27-hydroxylase were increased by estrogen alone or in combination with progesterone (P < .05). Hepatic sterol 27-hydroxylase activity was positively correlated with hepatic mRNA levels (P < .001), an observation suggesting that estrogen increases the activity of sterol 27-hydroxylase by increasing its synthesis. Hepatic cholesterol concentration was not influenced by the hormone treatment. These observations suggest that estrogen alone or in combination with progesterone increases the synthesis of sterol 27-hydroxylase in hepatic and extrahepatic tissues, and the increased activity of hepatic sterol 27-hydroxylase resulting from the increased synthesis is associated with a hypolipidemic effect on plasma LDL levels. Furthermore, progesterone alone increases the hepatic ACAT activity, but given in combination with estrogen progesterone does not have the same effect on hepatic ACAT activity. The effect of estrogen on hepatic ACAT activity may be mediated by sterol 27-hydroxylase and its effect on cholesterol metabolism (decreased cholesterol synthesis and increased output of cholesterol in the bile) in liver.