Inhibition of lordosis behavior in male and female rats by androgens and progesterone.

Several studies suggest that when manipulated experimentally in adulthood, the lordosis response to estrogen can be increased dramatically in male rats. Because adult-gonadectomized (Gx) animals were used in these studies, the lack of testicular hormones in adulthood may have been a factor. To examine this possibility, adult-Gx rats were implanted with blank (Bk)-, testosterone (T)-, 5alpha-dihydrotestosterone (DHT)-, or progesterone (P)-filled capsules, alone or in combination. We report a new finding, that a combined treatment of T plus P (T+P) at physiological doses for the male, but not T or P alone, reduced lordosis significantly in males, with and without estrogen priming. T+P did not inhibit lordosis in females, nor did this specific treatment affect open field, aggressive, and male copulatory behaviors. In confirming studies done with much higher doses, DHT reduced lordosis in both sexes. DHT and T+P also reduced lordosis in adrenalectomized/Gx males. Mechanisms responsible for the T+P inhibition of lordosis in males are not known, but they may include an upregulation of androgen receptors by P, and this possibility is discussed.     

Activation of mu-opioid receptors inhibits lordosis behavior in estrogen and progesterone-primed female rats

The present study investigated the effect of highly selective mu-opioid receptor (OR) agonists on lordosis behavior in ovariectomized rats treated with 3 microg of estradiol benzoate followed 48 h later by 200 microg of progesterone. Ventricular infusion of the endogenous mu-OR agonists endomorphin-1 and -2 suppressed receptive behavior in a time- and dose-dependent fashion. At 6 microg, both endomorphin-1 and -2 inhibited lordosis behavior within 30 min. However, while the effect of endomorphin-1 lasted 60 min, endomorphin-2 inhibition lasted up to 120 min after infusion. Pretreatment with naloxone (5 mg/kg sc) was able to block both endomorphin-1 and endomorphin-2 effects on lordosis. Site-specific infusions of endomorphin-1 or endomorphin-2 into the medial preoptic area (mPOA), the ventromedial nucleus of the hypothalamus (VMH), or into the mesencephalic central gray did not affect receptivity. In contrast, infusion of 1 mug of either compound into the medial septum/horizontal diagonal band of Broca inhibited lordosis in a pattern very similar to that seen after intraventricular infusions. Infusion of the potent synthetic mu-OR agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (0.08 microg) into the VMH and mPOA inhibited lordosis behavior for at least 60 min after infusion. The nonspecific opioid receptor antagonist naloxone was able to facilitate lordosis in partially receptive female rats when infused into the mPOA but not when infused into the VMH. The behavioral effects of the agonists and antagonist used in this study suggest that the endogenous mu-opioid system modulates estrogen and progesterone-induced lordosis behavior.     

Duration of estrogen stimulation and progesterone inhibition of maternal behavior in pregnancy-terminated rats

The duration of the effectiveness of estradiol benzoate (EB) on the latency to the onset of maternal behavior was measured in 16-day pregnant rats that were hysterectomized-ovariectomized (HO). Eight groups of HO animals were treated with either a single SC injection of 5 μg/kg of EB or oil at surgery and were initially presented with foster pups at either 24, 48, 72, or 96 hr postoperatively. Compared to their respective controls, EB-treated animals showed singificantly shorter latencies when testing began at 48 and 72 hr but not 24 or 96 hr. In the second experiment, 16-day HO rats were treated with 5 μg/kg of EB at surgery and either oil or 0.5 mg of progesterone at 0, 24, or 44 hr postoperatively. Additional groups received either progesterone or oil at surgery (instead of EB) and a second injection of oil 44 hr later. Testing began 48 hr following surgery for all groups, and the results showed that only the groups injected with EB alone or EB plus progesterone at 44 hr displayed short-latency maternal behavior. It was concluded that a significant reduction in the latency to the onset of maternal behavior can be obtained between 24 and 72 hr after EB treatment and that progesterone when injected concurrently or 24 hr later can inhibit the effectiveness of EB.     

Induction of progestin receptors in the mediobasal hypothalamus of gonadally intact male rats primed with estrogen in relation to display of lordosis behavior

The purpose of this study was to determine whether the effects of estrogen on lordosis behavior in the male rat were related to the number of progesterone (P) receptors in the mediobasal hypothalamus (MBH) and/or dependent on blood P concentration. Two groups of gonadally intact male rats were given five successive doses of 1.0 or 2.5 micrograms estradiol benzoate (EB) and tested for lordosis behavior with a male stimulus at the end of the treatment. One month later they were again injected with EB and sacrificed under the same temporal schedule, but they were not tested for lordosis so as to prevent any emotionally stressful effects of intermale cohabitation. The males given 2.5 micrograms EB more frequently displayed lordosis responses to male mounts than those receiving 1 microgram EB, with a parallel increase in the number of MBH P receptors. The total number of MBH P receptors also appeared to be higher in the animals that displayed lordosis responses (lordosis group) than in those which did not (no lordosis group). In contrast, the display of lordosis behavior was negatively correlated with blood P concentration. Comparing MBH P receptors and blood P values in the EB treated and in nonhormonally treated gonadally intact animals which had been selected for either ability or inability to spontaneously display lordosis behavior, we observed that (1) EB was capable of increasing the number of MBH P receptors in the male rat; and (2) in the absence of EB treatment blood P values were higher in the animals showing lordosis than in those which did not. These data are discussed with respect to observations made in castrated male rats and in ovariectomized females.     

Sexual behavior in male rats treated with estrogen in combination with dihydrotestosterone

Male rats castrated at 30 days of age were treated with estradiol benzoate (dose range: 0.05–50 μg EB for 26 days) and dihydrotestosterone (1 mg DHT for 36 days) as adults. The combined EB and DHT treatments resulted in display of male sexual behavior which did not differ from the behavior shown by intact untreated males or castrated, testosterone propionate (1 mg TP for 26 days) treated males. EB alone or DHT alone were relatively ineffective in activating male behavior in castrated males.     

Estrogen and progesterone dose-dependently reduce disruptive effects of restraint on lordosis behavior

Ovariectomized rats were hormonally primed with various doses of estradiol benzoate (EB; 0.5-10 microg) in combination with various doses of progesterone (2.5-500 microg) to induce sexual receptivity. Females were then subjected to 5 min restraint and the effect on lordosis behavior was monitored for the next 30 min. Such mild stress has been previously shown to transiently reduce lordosis behavior of ovariectomized females hormonally primed only with 10 microg EB. In the current study, doses of progesterone of 25 microg or more in combination with 10 microg EB reduced the effects of restraint. Also priming doses of EB from 4.0 to 10 microg in combination with 250 microg progesterone prevented the lordosis-inhibiting effects of restraint. These findings reinforce prior observations of the dose-dependency of both estrogen and progesterone in the facilitation of lordosis behavior and introduce the female's lordosis response to mild restraint as a potentially useful index of the female's response to stress.     

Endogenous estrogen, testosterone and progesterone levels in relation to breast cancer risk

Multiple lines of evidence support a central role of hormones in the etiology of breast cancer. In epidemiologic studies, considerable effort has focused on delineating the role of endogenous hormones in risk of breast cancer among postmenopausal women. Recently, substantial additional data has accrued from prospective studies where endogenous hormones are measured in study subjects prior to disease diagnosis. In this review, the epidemiologic evidence linking sex steroids—estrogens, testosterone, and progesterone, specifically—with subsequent risk of breast cancer in both premenopausal and postmenopausal women is summarized. Overall, a strong positive association between breast cancer risk and circulating levels of both estrogens and testosterone has now been well confirmed among postmenopausal women; women with hormone levels in the top 20% of the distribution (versus bottom 20%) have a two- to three-fold higher risk of breast cancer. Evidence among premenopausal women is more limited, though increased risk associated with higher levels of testosterone is consistent. However, both positive and null associations have been observed with estrogens and progesterone and clearly more evaluation is needed.     

Effects of estrone, estradiol and estriol combined with dihydrotestosterone on mounting and lordosis behavior in castrated male rats

Prepuberally castrated male rats were injected with estrone (1 or 5 μg), estradiol (1 or 5 μg) or estriol (1, 5, or 25 μg) either alone or in combination with dihydrotestosterone, (0.5 mg). Each of these steroids, when given alone, had no or only weak stimulatory effects on male sexual behavior. When combined with dihydrotestosterone all estrogens stimulated full copulatory behavior, the order of potency being estradiol, estrone, and estriol. Lordosis behavior in response to male mounting or manual stimulation was facilitated by all estrogens. All estrogens caused a slight weight increase of the seminal vesicles, ventral prostate and glans penis.     

Neonatal dihydrotestosterone and estrogen stimulation: Effects on sexual behavior of male rats

Male rats castrated on the second day after birth (Day 2) were, for the next 10 days, given daily injections of one of five steroids or steroid combinations: 200 μg of testosterone propionate (TP); 200 μg of dihydrotestosterone propionate (DHTP); 5 μg of estradiol benzoate (EB); 5 μg of estradiol benzoate plus 200 μg of dihydrotestosterone propionate; oil vehicle (VH). Control male rats castrated on Day 90 received a sham castration and oil vehicle in the neonatal period. All animals were given TP in adulthood and tested for male sexual behavior. There was no difference in mounting activity among the subjects. Day 2 DHTP subjects displayed intromissions but were incapable of ejaculating. The more frequent display of intromissions by Day 2 DHTP animals in comparison to Day 2 VH animals could be solely due to their larger and more highly developed penes. On the other hand, the ejaculatory failure of the Day 2 DHTP subjects was attributed to some deficiency in central neural processes controlling ejaculatory mechanisms rather than inadequate penile development. Equivocal results were obtained with the Day 2 EB and Day 2 EB-DHTP animals in that only a few animals in both groups showed an ejaculatory pattern.     

Female lordosis pattern in the male rat induced by estrogen and progesterone: effect of interruption of the dorsal inputs to the preoptic area and hypothalamus.

Lordosis behavior was very rare in castrated male rats which had been pretreated with 50 mug estradiol benzoate (EB) for successive 2 days and 1 mg progesterone (P) 6-8 hr prior to testing on the third day. Only one out of 8 rats displayed lordosis in response to mounts by the sexually matured males. However, the occurrence of lordosis behavior was markedly increased in similarly treated castrated males in which the dorsal afferents to the preoptic area (POA) and hypothalamus were removed by the surgical cut. Twelve out of 19 rats of the group showed lordosis response. The incidence was less frequent in rats receiving sham deafferentation. These results suggest that the dorsal inputs to the POA and hypothalamus may exert a tonic inhibitory influence on the lordosis mediating system in the male rat.     

Effects of voluntary and forced exercise on lordosis behavior in female rats

We measured sexual behavior (lordosis) in ovariectomized, steroid-treated, exercising female rats. When estradiol-treated animals exercised voluntarily, lordosis was directly related to the amount of exercise. Forced exercise (swimming) following treatment with estradiol 17 beta and progesterone significantly increased lordosis scores but only after animals swam for 2.5 hr per day. Lordosis behavior returned to baseline levels 7 weeks after forced exercise was stopped. Circulating levels of estrogen and progesterone measured immediately after behavioral testing did not differ between animals forced to exercise and their controls. The mechanisms responsible for the increase in lordosis behavior following exercise are unclear. It is possible that a change in body composition, a change in neural sensitivity to gonadal steroids, and/or changes in pituitary or adrenal function contribute to the increased lordosis behavior in exercised animals.     

Hormone induced lordosis and its relation to masculine sexual activity in male rats

Intact and castrated male rats were injected with a single dose of estrogen (75 μg), followed by progesterone (1 mg) or by oil. Intact males showed higher lordosis quotients as compared to castrated males. Progesterone facilitated lordosis behavior in castrated as well as in intact males. An estrogen-progesterone treated group of sexually inactive male rats and of sexually exhausted males showed lower levels of lordosis as compared to the intact sexually active males.     

Antecedent short-term central nervous system administration of estrogen and progesterone alters counterregulatory responses to hypoglycemia in conscious male rats

The aim of this study was to test the hypothesis that antecedent short-term administration of estradiol or progesterone into the central nervous system (CNS) reduces levels of neuroendocrine counterregulatory hormones during subsequent hypoglycemia. Conscious unrestrained male Sprague-Dawley rats were studied during randomized 2-day experiments. Day 1 consisted of an 8-h lateral ventricle infusion of estradiol (1 mug/mul; n = 9), progesterone (1 mug/mul; n = 9), or saline (0.2 mul/min; n = 10). On day 2, a 2-h hyperinsulinemic (30 pmol.kg(-1).min(-1)) hypoglycemic (2.9 +/- 0.2 mM) clamp was performed on all rats. Central administration of estradiol on day 1 resulted in significantly lower plasma epinephrine levels during hypoglycemia compared with saline, whereas central administration of progesterone resulted in increased levels of plasma norepinephrine and decreased levels of corticosterone both at baseline and during hypoglycemia. Glucagon responses during hypoglycemia were unaffected by prior administration of estradiol or progesterone. Endogenous glucose production following day 1 estradiol was significantly lower during day 2 hypoglycemia, and consequently, the glucose infusion rate to maintain the glycemia was significantly greater after estradiol administration compared with saline. These data suggest that 1) CNS administration of both female reproductive hormones can have rapid effects in modulating levels of counterregulatory hormones during subsequent hypoglycemia in conscious male rats, 2) forebrain administration of reproductive hormones can significantly reduce pituitary adrenal and sympathetic nervous system drive during hypoglycemia, 3) reproductive steroid hormones produce differential effects on sympathetic nervous system activity during hypoglycemia, and 4) reduction of epinephrine resulted in significantly blunted metabolic counterregulatory responses during hypoglycemia.     

Effects of an estrogen antagonist,MER-25, on mounting behavior and lordosis behavior in the female rat

Four daily injections of 20 mg ethamoxytriphetol, MER-25, to intact female rats with regular 4-day estrous cycles inhibited lordosis behavior, but had no inhibitory effect on mounting behavior. Ten mg/day of MER-25 for 9 days partially antagonized the stimulatory effect of 2 μg/day of estradiol benzoate on lordosis behavior in ovariectomized female rats, but had no inhibitory effect upon mounting behavior. MER-25 (10 mg/day for 9 days) stimulated the display of mounting behavior in ovariectomized female rats. No effects of MER-25 treatment (10 mg for 10 days) comparable to those of testosterone propionate (10, 50, or 250 μg for 10 days) on testicular, seminal vesicle, or ventral prostate weights of intact male rats or on seminal vesicle or ventral prostate weights of castrated male rats were observed. The results show that MER-25 acts differently upon various estrogen sensitive behaviors in the female rat.     

Sexual behavior and penile reflexes of neonatally castrated male rats treated in infancy with estrogen and dihydrotestosterone

Male rats castrated neonatally and treated with a combination of 0.5 μg estradiol benzoate (EB) plus 50μg dihydrotestosterone propionate (DHTP) for the next 14 days displayed normal sexual behavior when injected with testosterone propionate (TP) in adulthood. Neither EB nor DHTP alone had this developmental effect inasmuch as only 20–25% of the neonatal castrates treated with just 0.1, 0.5, or 10 μg EB, or 50 μg DHTP, displayed ejaculatory responses. The periodic application of mildly painful electric shock, which has been previously shown to markedly facilitate ejaculatory responding in normal male rats, failed to improve sexual performance in these latter subjects. This was true even of the castrates treated neonatally with DHTP which frequently intromitted. Castrates treated with EB or DHTP alone neonatally were subjected to spinal transection (after testing of sexual behavior) for examination of penile reflexes. Those treated with DHTP showed normal reflexes, characterized by numerous erections and flips, indicating the presumably nonaromatizable DHTP has developmental effects on penile reflexes similar to those of testosterone. Subjects treated with EB, including four animals that had ejaculated at least once, displayed very few, if any, erections on reflex tests and no flips. These results show that sometimes intromissive and ejaculatory patterns can occur even though the animal appears to have little or no capacity for penile reflexes.     

The effects of estrogen and progesterone on female rat proceptive behavior

The relative importance of estrogen (EB) and progesterone (P) in stimulating proceptivity in ovariectomized female rats was studied. Proceptive behavior was measured quantitatively, providing a clear measure of response to experimental manipulation. When rats were tested biweekly after daily treatment with 0.4 μg/100 g body wt EB for 4 days, they showed maximal lordosis but low levels of proceptive behavior by the second test. Additional EB (3.0 μg/100 g body wt daily) failed to stimulate additional proceptivity. A graded increase in proceptive behavior resulted from administration of increasing doses of P (50, 100, 500 μg and 1.0 mg) to animals receiving EB priming as described above. The level of “soliciting” was significantly higher than EB-only-treated rats when 500 μg or 1.0 mg P was given. Ovariectomized, adrenalectomized rats, primed with 2.5 μg/100 g body wt EB daily for 7 days and tested on Day 8, were significantly less proceptive than ovariectomized, sham-adrenalectomized rats with the same hormone treatment. Four hours after injection of 1.0 mg P, there was no difference in proceptive or receptive behavior between sham- and adrenalectomized rats. It was concluded that if an EB dose is sufficient to induce maximal receptivity, additional estrogen does not stimulate proceptivity; unlike previous studies, the present data are not consistent with a global effect of ovarian steroids on both components of female behavior. Progesterone is more effective than estrogen in stimulating proceptive behavior, although proceptivity is not absolutely dependent on progesterone for expression. Proceptivity in EB-only-treated rats appears to be facilitated by adrenal P.