Sexual receptivity: Brain sites of estrogen action in female hamsters

In order to investigate the brain sites of estrogen action, ovariectomized hamsters were stereotaxically implanted with unilateral 27 gauge cannulae containing estradiol. Groups of females received implants into either the lateral septum, bed nucleus of the stria terminalis, preoptic area, anterior hypothalamus, ventromedial hypothalamus, arcuate nucleus, corticomedial amygdala or mesencephalic central gray. Another set of animals received implants containing cholesterol. One week later the animals were injected with progesterone and 4–5 hours later tested for sexual receptivity. The most receptivity and the most consistent response was seen in females with estradiol implants in the ventromedial hypothalamus. Only a few scattered animals in the other anatomical groups showed any receptivity. Only in animals with implants in the anterior hypothalamus was there any evidence of leakage of estrogen into peripheral circulation as measured by uterine weight. There was no response in females with cholesterol implants. Our results suggest that the ventromedial hypothalamus is the most sensitive brain area for the estrogenic induction of female sexual receptivity in hamsters.     

Localization of hypothalamic sites for the estrogen-priming of sexual receptivity in female hamsters

Ovariectomized female hamsters received small unilateral implants of estradiol at a variety of anterior-posterior levels of the medial preoptic area and hypothalamus. The results of an initial experiment using 27-ga. implants showed that females with estradiol implants in the ventromedial hypothalamus (VMN) or nearby anterior hypothalamus consistently showed higher levels of sexual receptivity than did females with implants farther rostral, in the preoptic area, or farther caudal, in the posterior hypothalamus. A second experiment used smaller, 28-ga. implants to compare directly the two areas at which implants were effective in the first experiment. The results confirm the findings of other recent studies of hamsters and rats by identifying the VMN as the most effective hypothalamic site for the estrogen priming of sexual receptivity.     

Dual progesterone action in diencephalon facilitates the induction of sexual receptivity in estrogen-primed golden hamsters

In order to identify and characterize the progesterone (P) sensitive neural system that regulates feminine sexual behavior, 28-gauge P-filled cannulae were implanted in the medial preoptic area (MPO), ventromedial hypothalamus (VMH), and central gray (CG) of ovariectomized estrogen-primed golden hamsters. Dual implants of P were placed either ipsilaterally or contralaterally in brain sites consisting of MPO-VMH, MPO-CG, or VMH-CG combinations. Tests for sexual receptivity commenced 44 hr after estrogen priming and consisted of a preimplantation test followed 4.5 to 5.5 hr later by a postimplantation test. In the preimplantation test, stimulus males were attacked when placed into the female's home cage which indicated that the subsequent display of sexual receptivity occurring in the postimplantation test was due to the action of P. Dual implants of P placed either ipsilaterally or contralaterally in MPO-VMH regions were significantly more effective in facilitating lordosis behavior than dual P implants placed in MPO-CG or VMH-CG regions. However, the duration of lordotic responsiveness produced by dual P implants in MPO and VMH regions appears to be shorter than the duration of lordosis typically observed in intact females on proestrus. Results suggest that MPO and VMH regions are sensitive to the lordosis facilitating actions of small dual implants of P.     

Facilitation of sexual receptivity by hypothalamic and midbrain implants of progesterone in female hamsters

In the first experiment, ovariectomized female hamsters were stereotaxically implanted with bilateral guide cannulae aimed at the medial preoptic area (POA), ventromedial hypothalamus (VMH), or ventral tegmentum (VTA). The following week these females were injected SC with 10 micrograms estradiol benzoate (EB) and then had 27-gauge cannulae containing crystalline progesterone inserted through the guide tubes. Sexual receptivity was observed in 3 of 11 animals with VMH implants of progesterone, in 2 of 10 with VTA progesterone, but in none with POA implants. In the second experiment, the amount of intracranial progesterone was increased by mechanically expelling a 1.5 micrograms progesterone pellet from the tip of each cannula insert. This treatment facilitated receptivity in 10 of 20 hamsters with VTA implants and in 9 of 32 VMH-implanted animals. This induction of receptivity required approximately 2 hr. Progesterone pellets in the POA, mammillary region, and lateral mesencephalon were generally ineffective. In hamsters, progesterone into either the VMH or the VTA is sufficient to facilitate receptivity, although neither site is highly sensitive to progesterone. These results differ from those in recent studies in rats and this difference may reflect important species differences in the control of lordosis.     

Ventromedial hypothalamic lesions and cholinergic control of female sexual behavior

Ovariectomized female rats received bilateral electrolytic or sham lesions of the ventromedial nucleus of the hypothalamus (VMNH). They were tested for sexual receptivity in response to estrogen (EB) and progesterone (P) treatment. Following this they were treated with a small dose of EB and tested for their response to intraventricular microinjection of a muscarinic cholinergic agonist or a vehicle solution. Those animals with lesions involving almost the entire extent of the VMNH displayed an impaired response to EB and P and to the agonist. Lesions which spared 30-50% of the anterior portion of the VMNH did not result in an impairment to EB and P or to the agonist. It is suggested that the disruption of the cholinergic induction of receptivity by such lesions was due either to destruction of estrogen receptors located in cell bodies in the VMNH which send projections to extrahypothalamic cholinergic neurons or to estrogen- and cholinergic-responsive cells intrinsic to the VMNH which are critical for the expression of sexual receptivity.     

Parasagittal hypothalamic knife cuts and sexual receptivity in the female rat

In spayed female rats, bilateral parasagittal knife cuts lateral to the ventromedial nucleus of the hypothalamus eliminated or severely attenuated sexual receptiveness induced by injections of the ovarian hormones estrogen and progesterone. Apparently, axons leaving and/or entering the ventromedial region of the hypothalamus in the lateral plane are importantly involved in the display of sexual receptivity in the female rat.     

Ventromedial hypothalamic damage and sexual proceptivity in female rats

Sexual proceptivity is indicated by behaviors which reflect feminine initiation with respect to the occurrence of mating behavior. In this study we used ovariectomized female rats, and tested for sexual behavior in a relatively large arena with a sexually active male tethered in one corner. This testing situation gave the unrestrained female control over the occurrence of sexual interaction, and the frequency with which a female approaches a sexually active male provides a measure of sexual proceptivity. We find that administration of estrogen and progesterone to neurologically intact female rats induces sexual receptivity and increases the frequency with which they approach a sexually active male. Bilateral destruction of the ventromedial hypothalamus renders females non-receptive and virtually eliminates the tendency for females to approach males. Bilateral sagittal knife cuts which bracket the ventromedial region produce similar effects. Apparently, the ventromedial hypothalamus is importantly involved in the control of both receptive and proceptive components of feminine sexual behavior.     

Medial preoptic area oxytocin and female sexual receptivity

Intracerebroventricular (icv) administration of the nonapeptide oxytocin (OXT) increases sexual receptivity in female rats. The medial preoptic area (MPOA) appeared to be the most sensitive brain area to the facilitative effects of OXT. Bilateral infusions of 100 ng of OXT into the MPOA significantly elevated lordosis quotients in ovariectomized (OVX), estrogen-treated rats. This dose of OXT was ineffective when infused icv or into the ventromedial hypothalamus, mesencephalic central gray, or ventral tegmental area. A 500-ng dose of OXT significantly elevated lordosis responding when infused icv, corresponding with our previous findings. Mounting by males significantly increased immunoreactive levels of OXT and decreased the number of OXT immunostaining cells in the MPOA of sexually receptive rats pretreated with estrogen and progesterone. The MPOA is a primary site of the OXT facilitation of sexual receptivity where OXT may be released during mating.     

Hypothalamic knife cuts dissociate maternal behaviors, sexual receptivity, and estrous cyclicity in female hamsters

Female hamsters were tested for their response to pups as virgins, then received sagittal cuts either lateral to the medial preoptic area-medial anterior hypothalamus (anterior cuts) or lateral to the medial anterior hypothalamus-ventromedial nucleus (posterior cuts). Postoperatively females were tested: (a) as virgins, for changes in pup retrieval and cannibalism, (b) for mating behavior, (c) for maternal care and cannibalism of their own pups, and (d) for object hoarding. Anterior cuts converted retrieving virgins to cannibalistic ones and reduced nest builing. All females with anterior cuts cannibalized their litters within a few days of parturition. Half of these females did retrieve pups in home cage tests during the first 3 days postpartum. However, unlike controls, when tested in a neutral arena 5–6 weeks later, no pup retrieval was seen, although most would hoard objects. In contrast, posterior cuts reduced pup retrieval and cannibalism in virgins, and disrupted object hoarding. These cuts did not reduce postpartum pup retrieval, but did reduce postpartum cannibalism. Anterior cuts did not reduce sexual receptivity; posterior cuts reduced sexual receptivity without eliminating ovulation. Anterior cuts are interpreted as having a somewhat selective effect of reducing maternal responsiveness by cutting the lateral connections of neurons in the medial anterior hypothalamus and adjacent medial preoptic area, while posterior cuts disrupt sexual receptivity by cutting lateral connections of the ventromedial hypothalamus which travel in the region of the supraoptic commissures     

Effects of intracerebral implants of steroid hormones on scent marking in the ovariectomized female gerbil (Meriones unguiculatus)

The effects of intracerebral implants of steroid hormones on scent marking in the female gerbil (Meriones unguiculatus) were studied in two experiments. In Experiment 1 various steroids were implanted alone or in combination into the preoptic and anterior hypothalamic area of ovariectomized females. Unilateral implants of testosterone + estrogen, estrogen, estrogen + progesterone, testosterone and testosterone + progesterone stimulated a significant level of marking when compared to controls. Experiment 2 utilized bilateral implants of estrogen dissolved in paraffin in order to explore the distribution hormone sensitive areas in the brain which might be important in the regulation of scent marking in the female gerbil. Pellets of estrogen-paraffin were implanted stereotaxically into either the anterior hypothalamus, preoptic area, septum, hippocampus, thalamus, amygdala or anterior olfactory nucleus of ovariectomized females. Total dosage of hormone implanted was 8.2–8.4 μg. A significant level of marking resulted in animals receiving implants into the anterior hypothalamus, preoptic area and septum when compared to controls. Marking appeared at about the same rate in each of these groups; however, the level of marking attained differed. By the last trial, anterior hypothalamic implanted animals were marking significantly more often than animals in either the preoptic or septum groups. Although there was no evidence of ieakage from the brain, the data suggested that some rapid diffusion of hormone, largely restricted to the brain, was taking place or that the three areas were differentially responsive to the hormone. The data do indicate that some localization of function does exist with respect to regulation of scent marking in the female.     

Forebrain sites of estradiol-17 beta action on sexual behavior and aggression in female Syrian hamsters.

The effects of intracranial implants of estradiol in the ventromedial hypothalamus (VMH), the anterior hypothalamus (AH), or the medial amygdala (AMG) on aggression, sexual behavior, and serum estradiol were examined in female Syrian hamsters. Estradiol implants in the VMH, followed by systemic progesterone, stimulated sexual behavior and inhibited aggression. Estradiol implants in other intracranial sites activated sexual behavior but did not reliably inhibit aggression. Intracranially implanted and systemically treated animals had equivalent peripheral estradiol concentrations at sacrifice. These results suggest that: (a) the VMH is an important neural site for estradiol actions on sexual and aggressive behavior, (b) the caudal AH and AMG also may be sites of estradiol action on sexual behavior, and (c) these intracranial implants may only be effective given systemic estradiol exposure or the concurrent stimulation of multiple brain areas.     

Serum estradiol concentration required to maintain body weight, attractivity, proceptivity, and receptivity in the ovariectomized female rat

Female hooded rats (230 to 260 g) were ovariectomized and given a subcutaneous implant of an estradiol-filled Silastic tube. The length of the tube was varied in order to produce a variety of serum estradiol levels. In the first experiment, animals were weighed over a 6-week period following surgery and then tested for sexual responsiveness to a male. The results demonstrated that ovariectomized females with an implant maintaining a serum estradiol concentration at about 15 pg/ml maintained body weight at the same level as that of intact females. A smaller implant gave rise to a higher weight gain and a larger implant to a lower weight gain. All implants resulted in a continuous state of receptivity. In a second experiment, ovariectomized females were implanted with smaller estradiol-filled implants in order to determine the threshold for maintaining proceptivity and receptivity. The results indicated that with a serum estradiol concentration below 15 pg/ml, the frequency of lordosis and of ear wiggling and darting decreased. Progesterone injections facilitated both proceptive and receptive behavior. In addition, following progesterone injections, the time required for a male to mount a female 10 times was decreased in females with low or no estradiol replacement. These results indicate that a constant concentration of estradiol at about the mean level present throughout the estrous cycle will result in normal body weight regulation and will maintain sexual behaviors that normally occur only during estrus. These results emphasize that Silastic implants of estradiol do not mimic normal endocrine function since, even at low levels, estradiol implants produce continuous receptivity.     

Failure of protein synthesis inhibition to block progesterone desensitization of lordosis in female rats

Progesterone's desensitization effect on lordosis has been shown to correlate with a decreased concentration of hypothalamic progestin receptors after progesterone injection. In a recent study, one group of investigators found that the protein synthesis inhibitor anisomycin appeared to block progesterone's desensitization effect. Despite decreased levels of cytoplasmic progestin receptors, progesterone + anisomycin-treated rats exhibited a high level of lordosis four hr after a second progesterone injection. Because this finding conflicts with a progestin receptor model of progesterone's desensitization effect, we investigated it further. In the first experiments, ovariectomized rats were injected with estradiol benzoate followed 24 hr later by either progesterone or vehicle. Anisomycin injected 3 hr after progesterone, at a dose that causes inhibition of hypothalamic protein synthesis for at least 4 hr, was without effect on progesterone desensitization a day later. In other experiments silastic implants containing estradiol were inserted into ovariectomized rats. Forty-five hr later, rats received progesterone or vehicle, followed by injections of anisomycin or saline. Rats receiving anisomycin + progesterone were still highly receptive at 30 hr while saline + progesterone controls were not. Furthermore, the results were similar 4 hr after a second injection of progesterone at 30 hr. In a related experiment, we confirmed that anisomycin delayed dramatically termination of the period of sexual receptivity. In this laboratory anisomycin does not seem to block progesterone's desensitization effect. However, with certain procedures anisomycin delays the termination of sexual receptivity. Thus it is important in investigations of the mechanism of progesterone's desensitization effect that animals be tested prior to the second progesterone injection to determine if they are actually responding to the progesterone.     

Involvement of the ventromedial and anterior hypothalamic nuclei in the hormonal induction of receptivity in the female rat

Bilateral lesions of the ventromedial hypothalamic area virtually eliminated the estrogen-induced display of sexual receptiveness in the female rat. Such lesions usually diminished sexual responsiveness to combined estrogen-progresterone stimulation, although not all lesions were equally effective in this regard. Damage to the anterior hypothalamic area was without apparent effect on the hormonally-induced display of receptivity, suggesting that previous observations to this effect are probably related to incicental damage to the ventromedial hypothalamic area incurred during placement of the lesions. This study complements other work indicating that the ventromedial hypothalamic area is relatively rich in estradiol concentrating cells and that estrogen implants to this area induce sexual receptivity in spayed female rats. Together, these studies affirm that the ventromedial hypothalamic area plays a critical role in the hormonal induction of receptivity in female rats.     

Hypothalamic nuclear progestin receptors and the duration of sexual receptivity in ovariectomized and ovariectomized-hysterectomized rats

In order to determine if the enhancement of sexual behavior in hysterectomized rats is associated with an increased level of hypothalamic nuclear progestin receptors, ovariectomized (OVX) and ovariectomized-hysterectomized (OVHX) rats were injected with 2 micrograms estradiol benzoate. Twenty-four hr later, animals were injected with 0.5 mg progesterone and were tested for sexual receptivity every 4 hr. Hysterectomy had an overall facilitatory effect on lordosis and increased the duration of the period of sexual receptivity by about 4 hr. In a second experiment, similarly-treated animals were killed 4, 8, or 12 hr after progesterone injection, and hypothalamic nuclear progestin receptor levels were measured. In contrast to what has been reported for guinea pigs, nuclear progestin receptor levels decreased to baseline 8-12 hr before the termination of sexual receptivity. Nuclear progestin receptor concentrations were higher in OVHX rats than in OVX rats at 4 hr after progesterone injection, and there was a trend toward higher receptor concentrations in OVHX rats at 8 hr. These results demonstrate that hysterectomy-induced facilitation of sexual receptivity is associated with an increased level of hypothalamic nuclear progestin receptors. Furthermore, they suggest a fundamental difference in the regulation of nuclear progestin receptor retention between rats and guinea pigs.     

Effects of oestradiol 17B, progesterone and testosterone upon proceptivity and receptivity in ovariectomized common marmosets (Callithrix jacchus)

The sexual behaviour of eight pairs of marmosets was observed during blocks of 30 minute tests after the ovariectomized female partners were treated with physiological doses of testosterone, oestradiol and progesterone. The effects of an intra-vaginal progesterone treatment were also investigated. Pre-ovulatory levels of oestradiol significantly increased the females' proceptive and receptive tongue-flicking displays and reduced the percentage of mounts refused. Mid-luteal levels of progesterone, by comparison, virtually abolished proceptive and receptive tongue-flicking and significantly increased mount refusals. Testosterone treatment was without effect. Vaginal application of progesterone unlike subcutaneous implantation of the hormone did not significantly alter the females' sexual behaviour. These results indicate that central actions of oestradiol and progesterone upon proceptivity and receptivity may occur in this primate, although further research using intra-cranial implants of these steroids is required to test this hypothesis.     

Benextramine, a putative neuropeptide Y receptor antagonist, attenuates the termination of receptivity.

Sexual behavior in female rats is dependent on gonadal steroids. In ovariectomized rats, progesterone treatment typically exerts a biphasic effect on copulatory behavior induced by prior treatment with estradiol. Thus, there is an initial augmentation of the facilitative effect of estradiol occurring 4-10 h after progesterone. This is followed by a profound inhibitory effect, essentially terminating receptivity. We hypothesized that the receptivity terminating effect of progesterone could be due to increased synthesis and release of neuropeptide Y in relevant brain regions. Rats were tested for female sexual behavior 4 h after progesterone (52 h postestradiol). Immediately following this test, benextramine was administered (0, 3, or 15 mg/kg, IP). Subsequently, behavioral tests were administered 24, 48, 72, and 96 h postbenextramine. Benextramine treatment attenuated the inhibitory effects of progesterone on receptivity (lordosis quotients and percent of responding animals) without affecting either proceptive or rejection behaviors. These data suggest that blockade of NPY (and alpha-adrenergic) receptors is associated with selective enhancements of specific components of sexual behavior in female rats. Specifically, blockade of NPY receptors by benextramine is associated with continued receptivity.     

Intravenous administration of progesterone and the onset of receptivity in female hamsters

Progesterone-facilitated receptivity is believed to be a genomically mediated event. However, in contrast to estrogen, progesterone's effects occur rapidly. This experiment was designed to examine the temporal onset of receptivity in hamsters following intravenous (IV) administration of progesterone. Ovariectomized female hamsters were tested for their responsiveness to 10 micrograms estradiol benzoate (EB) plus 0, 50, or 200 micrograms progesterone, administered subcutaneously (SC). The hamsters were tested for sexual receptivity at 0.5, 1, 1.5, 2, 3 and 4 h. Three days later they were fitted with jugular catheters. Two days after catheter implantation they received 10 micrograms EB SC; 48 h later they were tested for receptivity and then received an IV injection of 0, 50, or 200 micrograms progesterone in 0.2 ml propylene glycol. Following the IV P injection the animals were again tested at 0.5, 1, 1.5, 2, 3 and 4 h. Progesterone administration (either SC or IV) resulted in an increase in total lordosis duration (TLD) over time. TLD had significantly increased from the pre-progesterone levels by 2 h after 50 micrograms progesterone IV. In contrast, 50 micrograms progesterone SC did not cause an increase in TLD until 3 h postinjection. Moreover, in comparison to SC administration, the effects of IV progesterone were short-lived; TLD was significantly decreased by 4 h after injection. Following 200 micrograms progesterone, the latency to respond was shorter than with 50 micrograms progesterone (1.5 vs. 2 h). There was no difference in the onset of receptivity as a function of route of administration at the 200 micrograms dose.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of mesencephalic tegmentum in regulating female rat sexual behaviors

Feminine sexual behaviors were tested in estradiol benzoate (EB) and progesterone (P) primed ovariectomized rats following four types of radiofrequency lesions in the midbrain tegmentum. The dorsomedial lesion (DML) which destroyed the ventromedial central gray including the dorsal raphe nucleus and adjacent area induced high sexual receptivity in the females primed with low dose (0.5 micrograms) of EB-P. All females with DML exhibited lordosis and ear wiggling, the mean lordosis quotient (LQ) being significantly higher than that of castrated controls or sham operated rats. Sexual receptivity in females with ventromedial tegmental lesion was not significantly different from those of the control and sham groups. In contrast to the medially lesioned groups, the mean LQ was low in the animals with bilateral lateral tegmental lesions even when the dose of EB was increased to 2 micrograms which was sufficient to induce high sexual receptivity in castrated and sham operated control females. In the animals with dorsolateral tegmental lesions (DLL), a much more severe loss of lordosis was seen than in those with ventrolateral tegmental lesions (VLL). None of the DLL females displayed sexual behavior throughout the present experiments. These results lead us to conclude that the midbrain dorsomedial tegmental area (ventral central gray and the adjacent area) is concerned with female sexual behavior inhibiting system, whereas the lateral tegmental area may be involved in the facilitatory system.     

Behavioral insensitivity to progesterone during lactation in female rats

Lactating female rats failed to display sexual receptivity after receiving 50 μg of estradiol benzoate followed by 1 mg of progesterone. Lactating rats appear to be insensitive to progesterone, based on several experiments. In ovariectomized control rats receiving moderate estrogen priming (1 μg EB for 3 days), progesterone greatly facilitated sexual receptivity; similarly estrogen-primed lactating females showed no responsiveness to progesterone injections, even at a high dose of progesterone (10 mg). Consistent with this reduced behavioral responsiveness to progesterone, lactating females had significantly reduced nuclear progestin receptor levels after an injection of 1 mg progesterone compared to ovariectomized controls. On the other hand, both ovariectomized controls and lactating rats responded with high levels of receptivity to 3 days of priming with 10 μg of estradiol benzoate (without progesterone). Lactating females treated for 3 days with a moderate dose (1 μg) of estradiol benzoate showed slightly reduced receptivity compared to ovariectomized controls; this result could reflect a reduced sensitivity to estrogen but is more likely related to the somewhat lower serum levels of estradiol and consequently lower nuclear estrogen receptors in lactating females compared to ovariectomized controls. The possibility of reduced sensitivity to estrogen leading to a reduced sensitivity to progesterone cannot be eliminated (since animals respond to progesterone only after estrogen priming); however, the reported results favor the idea that lactating females are primarily refractory to progesterone and do not have a generalized insensitivity to estrogen.