Inhibitory and facilitatory neural mechanisms involved in the regulation of lordosis behavior in female rats: Effects of dual cuts in the preoptic area and hypothalamus

Two types of cuts were performed at a 4–5 week interval in the same ovariectomized rats; horizontal half-circle cut located just above the anterior commissure (ARD) and half-dome cut located anterior to the ventromedial nucleus (AD). Behavioral tests were carried out following the pretreatment with estradiol benzoate for 3 days and progesterone on the fourth day. When females received AD first (Experiment 1) the mean LQ was significantly lower than that of controls without brain surgery. Then, the AD rats were subjected to ARD or ARD sham. At the second test, the mean LQ of AD-ARD rats increased to the control level, but the LQ of AD-ARD sham rats was still low. In the experiment 2, the order of the brain surgery was just reversed. In the first test, all ARD females showed high levels of lordosis and the mean LQ was higher than that of control. Then, these ARD females receiving AD or AD sham were subjected to the second test. The mean LQ of ARD-AD rats decreased to the level of the control, but the LQ of ARD-AD sham rats was still high. Thus, dorsal neural inputs to or through the preoptic area and hypothalamus may exert inhibitory influences on a lordosis mediating system and anterolateral outputs of the medial basal hypothalamus appear to be concerned with a lordosis facilitating mechanism. These two systems seem to have a mutual correlation in regulating lordosis response. However, ARD or AD could not completely reverse the suppressive effect of AD or the facilitatory effect of ARD in the animals with dual cuts. It is suggested that the dorsal extrahypothalamic inhibitory influence and the hypothalamic facilitatory influence can regulate the display of lordosis independently in female rats.     

Dissociation between the display of lordosis and soliciting behaviors in female rats with lesions of the dorsomedial pontine tegmentum

Lesions of the dorsomedial tegmentum (DMTL) between the midbrain pontine junction and the middle level of the pons effectively eliminate the induction by estrogen-progesterone of lordosis behavior in ovariectomized rats. However, soliciting behaviors such as ear wiggling and hopping were not inhibited by this type of lesion. The common damaged area in DMTL rats which failed to show lordosis was the medial periventricular gray. The lesions placed in the caudal pontine central gray were not effective in suppressing lordosis response. Lesions of the ventromedial tegmentum (VMTL) were also ineffective in suppressing lordosis. Most of the animals with the VMTL showed soliciting behaviors. In these rats, the incidence of lordosis and lordosis quotient (LQ) were comparable to those of sham operated rats. When bilateral lesions were placed in the lateral tegmentum region, the mean LQ and incidence of soliciting behavior were not significantly different from those of sham operated controls. These results suggest a clear dissociation of the regulatory mechanisms between the display of lordosis and soliciting behaviors at the pontine level.     

Recovery of lordotic activity by dorsal deafferentation of the preoptic area in male and androgenized female rats

Lordotic activity was examined in male and neonatally androgenized female rats following dorsal deafferentation of the preoptic area (POA). Female pups were injected with various doses (100, 250, 500, or 1000 micrograms) of testosterone propionate (TP) on day 3 postpartum. Ten weeks after birth, all animals were castrated, then half of the castrated males and females in each group were subjected to dorsal deafferentation of the POA (anterior roof deafferentation: ARD) by using an L-shaped Halász knife in order to transect the dorsal forebrain efferents which are thought to exert an inhibitory influence on the lordosis mediating mechanism. Animals were implanted subcutaneously with Silastic tubes containing estradiol-17 beta (E2). Observations of lordosis behavior were carried out 5, 10, and 15 days after implantation of E2. Three to six hours before each behavioral test, all rats were injected with 0.5 mg progesterone. Regardless of the dose of TP given neonatally, androgenized females, as well as males, showed low levels of lordotic behavior. In contrast, males with ARD and androgenized females with ARD displayed lordosis more frequently than males without ARD, and androgenized females without ARD. Lordotic activity in the androgenized females with ARD was negatively correlated with the dose of TP given neonatally. The ARD females injected with a large dose (1000 micrograms) of TP neonatally were significantly less receptive than those injected with lower doses of TP and ARD males. These results suggest that a large dose of neonatal TP may cause permanent changes in not only the neural substrates for lordosis inhibition affected by ARD but also other structures involved in lordosis facilitation.     

Connections between the pontine central gray and the ventromedial hypothalamus are essential for lordosis in female rats

Lesions and knife cuts were used to study central gray (CG) and ventromedial hypothalamic (VMH) mediation of sexual receptivity in female rats. Lesions of the midbrain-pontine CG eliminated lordosis in female rats. Bilateral sagittal knife cuts that bracketed the rostral pontine CG also eliminated lordosis, and an experiment with the retrograde tracer Fluoro-Gold confirmed the effectiveness of these cuts in severing the lateral connections linking the VMH and the CG. Finally, females with a unilateral hypothalamic cut combined with a contralateral CG transection almost never showed lordosis. Each cut, at a different level for each side of the brain, transected axons linking the VMH and the CG. The demonstration that this combination eliminated lordosis provides new evidence that the lateral connections between the VMH and the CG are essential for the display of sexual receptivity in female rats.     

Effects of anterior roof deafferentation on lordosis behavior and estrogen receptors in various brain regions of female rats.

This study investigated whether the estrogen receptors (ER), located at different brain areas and anterior pituitary (AP), changed after anterior roof deafferentation (ARD), and on the effects of facilitating the lordosis reflex in female rats. Female rats were ovariectomized and implanted with estradiol capsules. ARD or sham operation was performed with a Halász knife. All animals were tested for lordosis both before and after surgery. One day after the last test they were sacrificed. Cytosol and nuclear ER in the AP, medial preoptic area (MPOA), basal medial hypothalamus (BMH), amygdala (AMYG), septum (SEP), hippocampus (HPC), and cortex (CTX) were measured using an in vitro exchange assay. Rats with ARD showed significantly higher mean levels of lordosis quotient than the control and the sham groups before ARD surgery. An increase of both cytosol and nuclear ER in the BMH area compared to the control was observed, whereas the ER, in the SEP was reduced. ER in other areas were not affected by ARD. Serum estradiol and progesterone levels were not altered by the operation. These data suggest that the dorsal inhibitory pathway from the extrahypothalamus to the preoptic area and hypothalamus may modulate the estrogen receptor and the display of lordosis in female rats. Change of ER level in the BMH area may influence the hormonal sensitivity of lordosis in female rats.     

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.     

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.     

Dissociation of hypothalamic effects on ultrasound production and copulation

Two studies were conducted to compare the brain mechanisms for copulation with those controlling other sexual behaviors, such as the ultrasonic calls that help hamsters and other rodents attract potential mates. Specifically, male and female golden hamsters were castrated and hormone-primed prior to being observed for rates of ultrasound production and levels of sex-typical copulatory behavior (mounts, intromissions and ejaculations in males; lordosis in females). Such tests were conducted before and after subjects received sham operations or bilateral lesions of the preoptic area (POA), anterior hypothalamus (AH) or ventromedial hypothalamus (VMN). The results confirmed previous work in showing the POA lesions decrease rates of intromission, ejaculation and ultrasound production, while VMH lesions decrease lordosis duration. More surprising was the tendency of VMN lesions to increase rates of ultrasonic calling by both males and females. For males, these effects identify differences between the neural circuits controlling copulatory and noncopulatory sexual behaviors. For females, they suggest a mechanism for the behavioral incompatibility of ultrasound production and lordosis. In particular, they raise the possibility that the suspension of ultrasonic calling that normally accompanies lordosis reflects an increase in VMN activity that simultaneously provokes lordosis and inhibits vocalization.     

Lesions of the mesencephalic central gray depress ultrasound production and lordosis by female hamsters

Lesions of the mesencephalic central gray (SGC) were used to examine the role of this structure in the control of ultrasound production and lordosis by female hamsters. Rates of ultrasound production by ovariectomized, hormone-primed, females were observed in separate tests with stimulus ultrasounds and males. The postoperative ultrasound rates of females with SGC lesions were compared with preoperative rates for the same females and with postoperative call rates of females with sham lesions. SGC lesions consistently caused decreases in rates of ultrasound production. These changes were most pronounced in tests with stimulus males, and in females with extensive damage to the caudal SGC. The females also were tested for lordosis using manual stimulation and stud males. The incidence of lordosis decreased significantly following SGC lesions, though some lesioned females continued to show lordosis of normal latency and duration. These results suggest that the SGC may help to integrate ultrasound production with other hormone-dependent reproductive behaviors.     

Role of the ventromedial nucleus of hypothalamus in the male-induced enhancement of lordosis in female rats.

The involvement of the ventromedial nucleus of the hypothalamus (VMH) in the mating-induced enhancement of lordosis in ovariectomized estrogen-primed rats was investigated. In the first experiment, females with bilateral VMH or sham lesions were primed with 2 micrograms estradiol benzoate, and 48 h later they were subjected to repeated-mating tests. The VMH-lesioned rats failed to exhibit lordosis during the tests; however, the sham-operated females exhibited a gradual increase in lordosis quotient (LQ) with repetitive matings. In the second experiment, ovariectomized females were bilaterally implanted with estradiol (E2) or cholesterol (C) in the VMH, 48 h prior to behavioral testing. Repeated-mating-induced elevation in LQ was observed in the females when they were bilaterally implanted with E2 in the VMH; C was ineffective. To exclude the possibility of the spread of E2 to areas adjacent to VMH, plasma-luteinizing hormone (LH) was measured. Elevation in the circulating LH levels following ovariectomy was not suppressed in the females following bilateral E2 implants in the VMH, suggesting that the effect of estrogen is localized within or immediately around the VMH. The results suggest that the integrity of the VMH is critical for the potentiation of lordosis behavior in ovariectomized estrogen-primed females by male-originating sensory cues, and that selective priming of the VMH with estrogen is sufficient for the male-induced enhancement of lordosis.     

Female and male sexual responses in female cats with ventromedial hypothalamic lesions

The effects of lesions in the ventromedial hypothalamus (VMH) on male and female sexual behavior in female cats were examined. Following preoperative testing for receptivity, proceptivity, and male mating behavior, 27 female cats received either lesions in the anterior or posterior portion of the VMH or sham lesions. Neither of the VMH lesion placements reduced proceptivity and receptivity scores. However, the female mating pattern was significantly altered in that although females in both lesion groups initially allowed a stud male to mount, they usually did not allow the male to intromit. As in rodents, the VMH in the cat appears to be an important neural area for the display of normal female sexual behavior. Low levels of male sexual behavior were seen in the three groups preoperatively, and no changes were observed in the levels of male behavior following lesion placement. Thus, because lesions that disrupted female behavior did not affect male sexual behavior, it appears that the neural areas controlling homotypical and heterotypical sexual behaviors are not necessarily neurally linked.     

Role of midbrain central gray in pain-induced defensive boxing of rats

Pain-induced defensive boxing behavior in the rat was impaired or abolished by lesions of the midbrain central gray at the level of the superior colliculus. Lesions which left part of this region intact along with its lateral and dorsal connections did not produce boxing deficits despite destruction of anterior, posterior, or ventral connections. On the basis of these data, consideration of the anatomy and physiology of the central gray, and comparison to data on affective defense in the cat, it was suggested that this region may function as a modal command system or a patterning mechanism independent of the forebrain for pain-induced defensive boxing in the rat and affective defense in the cat. The central gray was shown to be necessary for freezing and escape behaviors as well as pain-induced defensive boxing, which led to speculation about its role in committing the animal to one or another of these three alternative responses to pain or threat.     

Brain transections differentially alter lordosis and ear wiggling of 6-day-old rats

Six-day-old male and female rats display lordosis and ear wiggling in response to tactile stimulation of the flanks and rump, without priming by exogenous estrogen. The involvement of various brain regions in these behaviors, which resemble components of adult female sexual behavior, was examined by making acute transections along the neuraxis from the olfactory tract to the medulla in 6-day-old rats. Four to 5 hr after the transection procedure, pups were tested for lordosis and ear wiggling. Lordosis was severely reduced or eliminated in pups with cuts through the hindbrain or diencephalon (above the level of the mammillary bodies) but was relatively unaffected by cuts through the posterior hypothalamus and rostral tegmentum and by cuts rostral to the anterior hypothalamus. Ear wiggling was disrupted by transections throughout the hindbrain and was facilitated only in females by transections throughout the forebrain (anterior to the mammillary bodies). These data suggest that facilitation from the hypothalamus is required for lordosis in the infant rat and the forebrain inhibitory systems for ear wiggling are functional in female infants by 6 days of age. Similarities and differences between the neural control of lordosis and ear wiggling in infant and adult rats suggest that the infant sex-like behaviors may be precursors of adult female sexual behavior.     

Pharmacological and anatomical aspects of cholinergic activation of female sexual behavior

Forebrain infusion of cholinergic agonists activated the sexual response, lordosis, in ovariectomized female rats that had been primed with a low dose of estrogen. Carbachol, an agonist with both muscarinic and nicotinic properties, and oxotremorine, an agonist with a primarily muscarinic action, produced dose-related increases in the frequency of lordosis elicited by stimulus male rats. This facilitation of lordosis was prevented when females were pretreated systemically with atropine or scopolamine, two muscarinic receptor antagonists. These results indicate that the effect of carbachol and oxotremorine on lordosis is mediated by cholinergic muscarinic receptors. The location of these receptors within the brain has not been identified. Ventricular infusion of carbachol was as effective as infusion directly into the medial preoptic area (POA) and more effective than infusion directly into the ventromedial hypothalamus (VMH). Furthermore, when carbachol or oxotremorine was delivered to the POA through cannulae angled to avoid traversing the lateral ventricles, no facilitation of lordosis was observed. These data suggest that muscarinic receptors stimulated by central infusion of cholinergic agonists may not be located in either the POA or the VMH, two regions traditionally implicated in the regulation of lordosis.     

The differential descending projections from the anterior, central and posterior regions of the lateral hypothalamic area: an autoradiographic study

The descending projection sites of the anterior, central (or tuberal) and posterior regions of the lateral hypothalamic area were studied by anterograde axonal transport after local injection of tritiated amino acids. The results show that the neurons of the anterior regions project to the lateral mammillary nucleus, the ventral tegmental area, the midbrain central gray and the anterior parts of the dorsal raphe nucleus. The neurons of the central region project in the same structures and extend a projection into the dorsal tegmentum at the level of the pontine central gray, the midbrain and pontine reticular nuclei. In the ventral tegmentum region, the substantia nigra pars compacta, the interpeduncular nucleus and the anterior group of raphe nuclei were also found to be labelled. The neurons of the posterior region of the lateral hypothalamic area extend a projection to the level of the prepositus hypoglossi nucleus and to the nucleus of solitary tract. In the ventral tegmentum they project at the level of posterior group of the raphe nuclei and the inferior olivary complex.     

Midbrain central gray GABAA receptor activation enhances,and blockade reduces,sexual behavior in the female rat

Summary The inhibitory neurotransmitter, GABA, has been implicated in the control of lordosis behavior. Previous studies indicate that modulation of GABA_A transmission can have dual effects on lordosis, being facilitative in the ventromedial hypothalamus and inhibitory in the preoptic area. The midbrain central gray (MCG) is also known to be an important neural site for regulating lordosis as well as defensive and escape behaviors, and plays an integral role in the control of nociception. Because of the multitude of behaviors regulated at the level of the MCG, we utilized a two-chamber testing apparatus that allowed simultaneous measurement of the females' proceptive (hopping and darting), receptive and rejection behaviors, as well as an index of nociception and general motor activity. We found that microinfusion of the GABA_A antagonist, bicuculline, into the MCG of steroid-primed female rats resulted in a significant decrease in lordosis and proceptive behaviors at 5 min post-infusion. There was full recovery to pretest levels by 60 min. Furthermore, microinfusion of the GABA_A agonist, muscimol, to estrogen-treated females that displayed low levels of receptivity and high levels of rejection behavior during a pretest, resulted in a significant increase in lordosis responding and a decrease in rejection behaviors. Neither drug significantly affected time spent in the vicinity of the male, motor activity or vocalizations. It is concluded that the decrease in lordosis resulting from blockade of GABA transmission is not solely due to the induction of antagonistic behaviors since there was no increase in rejections after bicuculline administration. The current findings are consistent with the interpretation that GABA facilitates lordosis in the MCG via disinhibition. When the retrograde tracer, Fluoro-gold, was infused into the same cannula sites in the MCG as the GABA_A drugs it demonstrated the presence of strong projections from the ventromedial nucleus, zona incerta, medullary reticular formation and spinal cord. These projections to the MCG may be important for the integration of the diverse behaviors regulated at the level of the MCG and GABAergic transmission may play a role in this integration.     

Acute fasting decreases sexual receptivity and neural estrogen receptor-alpha in female rats

Acute food deprivation or chronic food restriction suppresses reproduction in female mammals. Although a link between undernutrition and ovarian function is well established in rats, a similar link with reproductive behavior in this species is yet to be described. Therefore, we compared the display of estrous behaviors induced by exogenous steroid hormone treatment in ovariectomized fed and fasted rats. In addition, estrogen receptor-alpha immunoreactivity (ERIR) was measured in fed and fasted animals to determine whether changes in behavior were associated with changes in the number of detectable ERIR-containing cells in several brain regions. Fasting for 74 h decreased lordosis quotients (LQ) and lordosis ratings (LR) in ovariectomized, steroid-primed rats. The number of detectable ERIR cells decreased after a 74-h fast in the mid-region of the arcuate (ARC), paraventricular (PVN) and ventromedial nuclei of the hypothalamus (VMH) and the ventral bed nucleus of the stria terminalis (BST) but did not change in a number of other areas examined. Taken together, these data demonstrate that, similar to the effect on the reproductive-endocrine axis, food deprivation for 74 h suppresses steroid-induced display of lordosis in adult, female rats. Furthermore, this suppression in sexual receptivity is associated with a decrease in ERIR in a number of areas, including the VMH, a region of the hypothalamus known to be critical for the display of reproductive behaviors in female rats.     

Mesencephalic mechanisms for integration of female reproductive behavior in the rat.

Placement of bilateral electrolytic lesions in the mesencephalic central gray (CG) of estrogen-primed ovariectomized female rats produced an immediate decline in performance of the lordosis reflex. Lesions that destroyed the dorsal half of the CG and the adjacent subtectal region were effective. The decrease in individual animals in terms of the lordosis reflex score ranged from 20 to 100% of the prelesion performance. Such lesions abolished the facilitation of lordosis by electrical stimulation of the ventromedial nucleus of the hypothalamus. Similar abrupt losses of lordosis followed bilateral lesions of either a) the area between CG and the cuneiform nucleus of the mesencephalic reticular formation (NCf); or b) the ventrolateral quadrant of the NCf. The difference between these two lesions was that the effect of the latter could be overridden by electrical stimulation of the CG, whereas that of the former could not. We conclude that the CG is an important supraspinal component of the circuit for lordosis behavior, constituting a link between ascending somatosensory and descending motor systems for lordosis. It probably facilitates lordosis when activated by behaviorally relevant peripheral somatosensory and/or ventromedial hypothalamic inputs.     

Effects of medial hypothalamic lesions on the lordosis response and other behaviors in remale golden hamsters

The lordosis response in ovariectomized, hormone-primed females was measured in repeated weekly tests before and after bilateral lesions of various medial hypothalamic regions. Changes in body weight, frequency of vaginal marking, and agonistic behavior were also recorded. Forty-two females were divided into five groups based on lesion location: (1) medial preoptic (MPO), (2) medial anterior hypothalamus (MAH), (3) dorsomedial nucleus-dorsal premammillary nucleus (DM-DPM), (4) dorsal ventromedial nucleus (Dorsal VMN), (5) medial basal hypothalamus (MBH). Unoperated control females were tested in an identical manner. Only lesions in the region of the ventromedial nucleus eliminated lordosis. Lesions which disrupted lordosis also increased agonistic behavior in male-female pairs. The frequency of vaginal marking was reduced by lesions in the MPO, MAH, and Dorsal VMN groups. Rapid weight increases and obesity were also seen following certain medial hypothalamic lesions, however no significant correlations were found between degree of weight change and degree of lordosis deficit. A quantitative analysis of VMN damage revealed a relatively low, but statistically significant positive correlation between degree of VMN damage and lordosis deficits. There were no significant correlations between degree of VMN damage and weight change.     

Ibotenic acid-induced lesions of the medial zona incerta decrease lordosis behavior in the female rat

To examine the role of the medial zona incerta (mZI) in female sexual behavior, ovariectomized estrogen- and progesterone-treated female rats were tested for sexual receptivity after bilateral injections of the selective neurotoxin ibotenic acid (3 micrograms/0.3 microliter) directly into the mZI. These injections produced a significant attenuation of lordosis behavior in highly receptive females when compared with saline-injected controls. This decrease in sexual receptivity was also reflected in a significant increase of rejections of male mount attempts. However, these lesions did not abolish the display of lordosis behavior. In addition, the frequency of hopping and darting was decreased in ibotenic acid-injected females when compared with controls. Consistent with previous studies, these lesions also produced a transient impairment of drinking behavior (hypodipsia) typical of rats with large electrolytic lesions of the mZI. This study demonstrates that mZI neurons play a role in mediating sexual receptivity in the female rat. Collectively, these results suggest that in addition to the projection from the ventromedial nucleus of the hypothalamus to the midbrain central gray, the functional integrity of the mZI is of crucial importance for the expression of sexual receptivity in the female rat.