Sexual Receptivity In Female Rats Research Articles

A Novel Membrane Estrogen Receptor Activated by STX Induces Female Sexual Receptivity through an Interaction with mGluR1a

Membrane initiated estradiol signaling has been shown to be vital for multiple physiological processes. Several receptors have been proposed to mediate the actions of estradiol at the membrane. Here, we examined the ability of STX, an agonist of a novel putative membrane estrogen receptor, to activate sexually receptive behavior in the female rat. Infusions of STX into the arcuate nucleus of the hypothalamus resulted in the activation and internalization of μ-opioid receptors in the medial preoptic nucleus, an action that is required for lordosis behavior. Indeed, STX was able to augment sexual receptivity in female rats given a sub-behavioral dose of estradiol. However, if the mGluR1a antagonist, LY367,385, was administered prior to STX, its circuit-activating effects, the internalization of μ-opioid receptors, were lost. This suggests that the receptor stimulated by STX activates rapid membrane-initiated signaling through an interaction with mGluR1a - an effect previously described for estrogen receptor-α at the membrane.

BOLD signal response to cocaine varies with sexual receptivity in female rats

Blood oxygen level dependent MRI was used to test whether cocaine-stimulated mesolimbic activity varied with sexual receptivity. Rats were randomly screened for lordotic responses and were then imaged for their responses to centrally administered cocaine. We observed that female rats expressing no lordosis showed a greater activation of mesolimbic and nigrostriatal structures than lordotic female rats. Our data suggest that the differential sensitivity to cocaine occurs not only as a result of hormonal changes of the estrous cycle, but also in association with changes in sexual receptivity.

Effect of acute stress on sexual behavior in female rats: Participation of the central angiotensinergic system

Stress might influence the reproductive behavior in females, and central angiotensin II (Ang II) is a peptide that plays a role in stress response and in the modulation of sexual behavior. The medial amygdala (MeA), an important structure that regulates this behavior, is strongly involved in stress response. The aim of the present study was to evaluate the effect of acute restraint stress on the night of proestrus on sexual receptivity in female rats and the participation of Ang II and MeA in this effect. Adult female Wistar rats with regular estrous cycles were utilized. The acute stress protocol utilized was the restraint stress for 15 min on the night of proestrus. The participation of Ang II was evaluated by injecting Ang II and Ang II receptor antagonists (losartan and PD12319) into the MeA. The lordosis quotient was recorded. The stress or the microinjection of Ang II into the MeA significantly reduced sexual behavior. The blockade of AT 1 or AT 2 receptors in the MeA prevented the effect of stress and the effect of Ang II microinjection into this nucleus on sexual receptivity. We concluded that acute restraint stress on the night of proestrus reduces sexual behavior in rats, and this effect is mediated by both AT 1 and AT 2 receptors in the MeA.

Protein Kinase C Signaling in the Hypothalamic Arcuate Nucleus Regulates Sexual Receptivity in Female Rats

Rapid membrane-mediated estradiol signaling regulating sexual receptivity requires the interaction of the estrogen receptor (ER)-alpha and the metabotropic glutamate receptor 1a (mGluR1a). A cell signaling antibody microarray revealed that estradiol activated 42 proteins in the arcuate nucleus of the hypothalamus (ARH). To begin an analysis of various signaling pathways, protein kinase A and protein kinase C (PKC)-theta, whose signaling pathways have been implicated in the estradiol regulation of sexual receptivity, were examined. In the ARH sample, the increase in phospho-protein kinase A could not be confirmed by Western blotting, in either cytosolic or membrane fractions. However, the increase in phosphorylated PKCtheta seen with the pathway array was verified by Western blotting. To study whether rapid estradiol activation of PKC regulates the ARH-medial preoptic nucleus pathway regulating lordosis, mu-opioid receptor (MOR) internalization and lordosis reflex were tested. Blocking PKC in ARH with 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]3-(1H-indol-3-yl) maleimide significantly attenuated estradiol-induced MOR internalization. Furthermore, disruption of PKC signaling within the ARH at the time of estradiol treatment significantly diminished the lordosis reflex. Moreover, blocking PKC prevented MOR internalization when the circuit was activated by the mGluR1a agonist, (RS)-3,5-dihydroxyphenylglycine. Activation of PKC with phorbol 12, 13-dibutyrate induced MOR internalization, indicating that PKC was a critical step for membrane ERalpha-initiated mGluR1a-mediated cell signaling and phorbol 12, 13-dibutyrate significantly facilitated the lordosis reflex. Together these findings indicate that rapid membrane ERalpha-mGluR1a interactions activate PKCtheta cell signaling, which regulates female sexual receptivity.

Nuclear receptor coactivators function in estrogen receptor- and progestin receptor-dependent aspects of sexual behavior in female rats

The ovarian hormones, estradiol (E) and progesterone (P) facilitate the expression of sexual behavior in female rats. E and P mediate many of these behavioral effects by binding to their respective intracellular receptors in specific brain regions. Nuclear receptor coactivators, including Steroid Receptor Coactivator-1 (SRC-1) and CREB Binding Protein (CBP), dramatically enhance ligand-dependent steroid receptor transcriptional activity in vitro. Previously, our lab has shown that SRC-1 and CBP modulate estrogen receptor (ER)-mediated induction of progestin receptor (PR) gene expression in the ventromedial nucleus of the hypothalamus (VMN) and hormone-dependent sexual receptivity in female rats. Female sexual behaviors can be activated by high doses of E alone in ovariectomized rats, and thus are believed to be ER-dependent. However, the full repertoire of female sexual behavior, in particular, proceptive behaviors such as hopping, darting and ear wiggling, are considered to be PR-dependent. In the present experiments, the function of SRC-1 and CBP in distinct ER- (Exp. 1) and PR- (Exp. 2) dependent aspects of female sexual behavior was investigated. In Exp. 1, infusion of antisense oligodeoxynucleotides to SRC-1 and CBP mRNA into the VMN decreased lordosis intensity in rats treated with E alone, suggesting that these coactivators modulate ER-mediated female sexual behavior. In Exp. 2, antisense to SRC-1 and CBP mRNA around the time of P administration reduced PR-dependent ear wiggling and hopping and darting. Taken together, these data suggest that SRC-1 and CBP modulate ER and PR action in brain and influence distinct aspects of hormone-dependent sexual behaviors. These findings support our previous studies and provide further evidence that SRC-1 and CBP function together to regulate ovarian hormone action in behaviorally-relevant brain regions.

Neonatal handling and reproductive function in female rats

Neonatal handling induces anovulatory estrous cycles and decreases sexual receptivity in female rats. The synchronous secretion of hormones from the gonads (estradiol (E2) and progesterone (P)), pituitary (luteinizing (LH) and follicle-stimulating (FSH) hormones) and hypothalamus (LH-releasing hormone (LHRH)) are essential for the reproductive functions in female rats. The present study aimed to describe the plasma levels of E2 and P throughout the estrous cycle and LH, FSH and prolactin (PRL) in the afternoon of the proestrus, and the LHRH content in the medial preoptic area (MPOA), median eminence (ME) and medial septal area (MSA) in the proestrus, in the neonatal handled rats. Wistar pup rats were handled for 1 min during the first 10 days after delivery (neonatal handled group) or left undisturbed (nonhandled group). When they reached adulthood, blood samples were collected through a jugular cannula and the MPOA, ME and MSA were microdissected. Plasma levels of the hormones and the content of LHRH were determined by RIA. The number of oocytes counted in the morning of the estrus day in the handled rats was significantly lower than in the nonhandled ones. Neonatal handling reduces E2 levels only on the proestrus day while P levels decreased in metestrus and estrus. Handled females also showed reduced plasma levels of LH, FSH and PRL in the afternoon of the proestrus. The LHRH content in the MPOA was significantly higher than in the nonhandled group. The reduced secretion of E2, LH, FSH and LHRH on the proestrus day may explain the anovulatory estrous cycle in neonatal handled rats. The reduced secretion of PRL in the proestrus may be related to the decreased sexual receptiveness in handled females. In conclusion, early-life environmental stimulation can induce long-lasting effects on the hypothalamus-pituitary-gonad axis.

Raloxifene effects upon the neuronal system controlling sexual receptivity in female rats

Selective estrogen receptor modulators (SERMs) constitute a new family of drugs with growing interest in the management of estrogen-associated pathology. Raloxifene is a SERM that mimics estrogen action on bone and blood lipid concentration but whether it acts as estrogen in the central nervous system remains to be fully established. In the present communication, we aimed at evaluating the estrogenic/antiestrogenic effects of raloxifene upon organization and activation of sexual receptivity, an estrogen-dependent event, in female rats. To this end, the effects of raloxifene, administered during the neonatal period, were compared with those of estrogen in terms of disruption of sexual receptivity in estrogen–progesterone-primed ovariectomized (OVX) female rats. In addition, the ability of raloxifene to induce sexual receptivity in progesterone-primed OVX females was analyzed. Similarly, the effects of the combined administration of estrogen and raloxifene were studied. Our results suggest that raloxifene does not act as estrogen upon the organization of the neuronal system involved in the control of sexual receptivity in female rats and exerted an antiestrogenic action in adult OVX estrogen-primed female rats.

Sensory Cues Mediating Mating-Induced Potentiation of Sexual Receptivity in Female Rats

Repeated mating of estradiol-primed female rats increases sexual receptivity. Two studies were conducted to determine the contribution of vaginal–cervical stimulation (VCS) to this increase. In the first study, female rats were repeatedly mated for 165 min. The vaginas of half of the females were covered with tape (masked) to prevent intromissions by the males. The remaining females were unmasked. Only females receiving intromissions (unmasked) showed a significant increase in sexual receptivity during repeated mating, suggesting that VCS from intromissions is necessary for repeated mating to increase sexual receptivity. In the second experiment, female rats received either experimentally administered VCS or control scapular stimulation administered with a plastic probe 1 h prior to testing for sexual receptivity. VCS applied in this manner significantly increased sexual receptivity. Together, these findings suggest that VCS from intromissions is one of the primary factors responsible for increases in sexual receptivity following repeated mating.

From the Cover: Progesterone receptor and dopamine receptors are required in Delta 9-tetrahydrocannabinol modulation of sexual receptivity in female rats

Ovarian steroids, estrogen and progesterone, influence the sensitivity of certain neural processes to cannabinoid treatment by modulation of brain dopaminergic activity. We examined the effects of the active ingredient of cannabis, Delta(9)-tetrahydrocannabinol (THC), on sexual behavior in female rats and its influence on steroid hormone receptors and neurotransmitters in the facilitation of sexual receptivity. Our results revealed that the facilitatory effect of THC was inhibited by antagonists to both progesterone and dopamine D(1) receptors. To test further the idea that progesterone receptors (PR) and/or dopamine receptors (D(1)R) in the hypothalamus are required for THC-facilitated sexual behavior in rodents, antisense and sense oligonucleotides to PR and D(1)R were administered intracerebroventricularly (ICV) into the third cerebral ventricle of ovariectomized, estradiol benzoate-primed rats. Progesterone- and THC-facilitated sexual behavior was inhibited in animals treated with antisense oligonucleotides to PR or to D(1)R. Antagonists to cannabinoid receptor-1 subtype (CB(1)), but not to cannabinoid receptor-2 subtype (CB(2)) inhibited progesterone- and dopamine-facilitated sexual receptivity in female rats. Our studies indicate that THC acts on the CB(1) cannabinoid receptor to initiate a signal transduction response that requires both membrane dopamine and intracellular progesterone receptors for effective induction of sexual behavior.

Progesterone receptor and dopamine receptors are required in Delta 9-tetrahydrocannabinol modulation of sexual receptivity in female rats.

Ovarian steroids, estrogen and progesterone, influence the sensitivity of certain neural processes to cannabinoid treatment by modulation of brain dopaminergic activity. We examined the effects of the active ingredient of cannabis, Delta(9)-tetrahydrocannabinol (THC), on sexual behavior in female rats and its influence on steroid hormone receptors and neurotransmitters in the facilitation of sexual receptivity. Our results revealed that the facilitatory effect of THC was inhibited by antagonists to both progesterone and dopamine D(1) receptors. To test further the idea that progesterone receptors (PR) and/or dopamine receptors (D(1)R) in the hypothalamus are required for THC-facilitated sexual behavior in rodents, antisense and sense oligonucleotides to PR and D(1)R were administered intracerebroventricularly (ICV) into the third cerebral ventricle of ovariectomized, estradiol benzoate-primed rats. Progesterone- and THC-facilitated sexual behavior was inhibited in animals treated with antisense oligonucleotides to PR or to D(1)R. Antagonists to cannabinoid receptor-1 subtype (CB(1)), but not to cannabinoid receptor-2 subtype (CB(2)) inhibited progesterone- and dopamine-facilitated sexual receptivity in female rats. Our studies indicate that THC acts on the CB(1) cannabinoid receptor to initiate a signal transduction response that requires both membrane dopamine and intracellular progesterone receptors for effective induction of sexual behavior.

Progestin Signals through DARPP

Female sexual receptivity in mice and rats is mediated by the progestin receptors. Mani et al . found that antisense nucleotides to DARPP-32, a dopamine- and cAMP-regulated protein, as well as DARPP-32 knockout mice, demonstrated decreased sexual receptivity in response to stimulation of progestin receptors or dopamine receptors. The data suggest that the dopamine and progestin pathways converge at the level of DARPP-32 to regulate sexual receptivity. Mani, S.K., Feinberg, A.A., O'Callaghan, J.P., Snyder, G.L., Allen, P.B., Dash, P.K., Moore, A.N., Mitchell, A.J., Bibb, J., Greengard, P., and O'Malley, B.W. (2000) Requirement for DARPP-32 in progesterone-facilitated sexual receptivity in female rats and mice. Science 287 : 1053-1056. [Abstract] [Full Text]

Progestin Signals through DARPP

Female sexual receptivity in mice and rats is mediated by the progestin receptors. Mani et al. found that antisense nucleotides to DARPP-32, a dopamine- and cAMP-regulated protein, as well as DARPP-32 knockout mice, demonstrated decreased sexual receptivity in response to stimulation of progestin receptors or dopamine receptors. The data suggest that the dopamine and progestin pathways converge at the level of DARPP-32 to regulate sexual receptivity. Mani, S.K., Feinberg, A.A., O'Callaghan, J.P., Snyder, G.L., Allen, P.B., Dash, P.K., Moore, A.N., Mitchell, A.J., Bibb, J., Greengard, P., and O'Malley, B.W. (2000) Requirement for DARPP-32 in progesterone-facilitated sexual receptivity in female rats and mice. Science 287: 1053-1056.[Abstract] [Full Text]

Requirement for DARPP-32 in progesterone-facilitated sexual receptivity in female rats and mice.

DARPP-32, a dopamine- and adenosine 3',5'-monophosphate (cAMP)-regulated phosphoprotein (32 kilodaltons in size), is an obligate intermediate in progesterone (P)-facilitated sexual receptivity in female rats and mice. The facilitative effect of P on sexual receptivity in female rats was blocked by antisense oligonucleotides to DARPP-32. Homozygous mice carrying a null mutation for the DARPP-32 gene exhibited minimal levels of P-facilitated sexual receptivity when compared to their wild-type littermates. P significantly increased hypothalamic cAMP levels and cAMP-dependent protein kinase activity. These increases were not inhibited by a D1 subclass dopamine receptor antagonist. P also enhanced phosphorylation of DARPP-32 on threonine 34 in the hypothalamus of mice. DARPP-32 activation is thus an obligatory step in progestin receptor regulation of sexual receptivity in rats and mice.

Inhibitory effect of progesterone on sexual receptivity in female rats: a temporal relationship to estrogen administration.

The inhibitory effect of progesterone (P) injected at various times on female sexual behavior was investigated in estradiol benzoate (EB) treated ovariectomized rats. Four behavioral tests were carried out at two-week intervals. All females received 5 micrograms/kg b.w. EB and 0.5 mg P 44 hr after the EB. In the P-control group, an additional 5 mg P was administered at the same time as the injection of EB in four tests. Instead of P, oil was given concurrently with EB in the Oil control group. In the experimental groups, female rats were treated with 5 mg P from 1 to 40 hr before (PB group) or after (PA group) the EB-injection. A sexual behavioral test was started 4 hr after 0.5 mg P. The results show that low levels of lordosis and soliciting behavior were observed in the P group, compared to the Oil-control group. In the PB groups, lordosis quotient (LQ) was low when P was given from 1 to 24 hr before EB. Moreover, animals in which P was given 27-40 hr before EB showed lower LQ than Oil-control animals, but higher LQ than rats in the P-control group. In the PA groups, when P was administered from 1 to 24 hr after EB, low levels of lordosis response were observed, whereas animals which received P 27-40 hr after EB showed a high score of LQ, being comparable to that in the Oil control. These results suggest that the period of 24 hr before and after EB injection is a critical period for inhibitory action of P on female sexual behavior in female rats.

Infusions of Diazepam and Allopregnanolone into the Midbrain Central Gray Facilitate Open-Field Behavior and Sexual Receptivity in Female Rats

We have previously shown that blocking GABAA receptors with bicuculline in the dorsal and lateral quadrants of the rostral two-thirds of the midbrain central gray (MCG) in rats reduces the lordosis quotient (LQ) and proceptive behaviors without increasing the defensive index (McCarthy, Pfaff, and Schwartz-Giblin, 1991b), indicating that tonic release of endogenous GABA in the MCG has a primary facilitatory effect on reproductive behaviors. The present study tests whether two modulators of the GABAA-benzodiazepine-receptor complex, diazepam and the progesterone metabolite, allopregnanolone, facilitate female receptivity as well as open-field behavior. Ovariectomized estradiol benzoate (EB)-treated rats were bilaterally infused in the MCG with vehicle, diazepam, or allopregnanolone and tested 20 min after diazepam and 12 min after allopregnanolone. Animals were tested once each for both receptive or open-field behavior with testing sessions being at least 1 week apart in a crossover design between vehicle and one of the two drugs. Both diazepam and allopregnanolone significantly increased the lordosis quotient in a dose-dependent manner, the lowest effective dose tested being 100 and 250 ng, respectively. In the open-field test, infusion of diazepam (100 ng) in EB-treated rats significantly increased the ratio of central squares entered to total squares entered (C/T) without a change in total activity. Rats without EB elicited comparable C/T ratios when tested with five times as much diazepam. Diazepam infusions at more dorsal sites in the midbrain and in overlying cortex were without effect on C/T ratios in EB-treated rats. Allopregnanolone (500 ng) infusions in MCG of EB-treated rats did not alter C/T ratios compared to animals without EB. Our results are consistent with diazepam and allopregnanolone acting through the GABAA receptors of the MCG to enhance female sexual receptivity and with estradiol potentiating the effect of diazepam in the MCG to increase openfield behavior.

The facilitatory effect of N-methyl-D-aspartate on sexual receptivity in female rats through GnRH release.

The purpose of this study was to examine whether N-methyl-D-aspartate affects the sexual receptivity of female rats. Monosodium L-glutamate was used as a neurotoxin to induce hypogonadal status. Matured normal and monosodium L-glutamate-treated rats were ovariectomized and implanted subcutaneously with estradiol capsules. One week later, lordosis responsiveness was observed before and 10 min after N-methyl-D-aspartate (40 mg/kg of BW, ip) administration. The results showed that N-methyl-D-aspartate caused a remarkable increase of lordosis quotient in control rats but not in monosodium L-glutamate-treated rats. Moreover, the possible action site of N-methyl-D-aspartate in the enhancement of receptivity was evaluated by the post-castrational LH rise, pituitary LH release in response to GnRH, and N-methyl-D-aspartate-evoked GnRH releasability. The results revealed that: (a) serum levels of LH in monosodium L-glutamate-treated rats were lower (p < 0.01) than those of control rats after ovariectomy; (b) there was no significant difference of pituitary LH release responsiveness to GnRH test between two groups; and (c) N-methyl-D-aspartate-evoked LH release in monosodium L-glutamate-treated rats was similar to that in the control rats. In conclusion, N-methyl-D-aspartate may facilitate the sexual receptivity through stimulating GnRH release. The failure of N-methyl-D-aspartate in enhancing receptivity in monosodium L-glutamate-treated rats is probably due to the cellular damage by monosodium L-glutamate on specific areas responsible for lordosis.

Noradrenergic innervation to the VMN or MPN is not necessary for lordosis

The purpose of the present study was to determine the importance of noradrenergic neurons terminating in the ventromedial nucleus (VMN) and medial preoptic nucleus (MPN) of the hypothalamus for lordosis behavior in ovariectomized, estrogen/progesterone-treated female rats. Seven days following bilateral injections of the noradrenergic neurotoxin 5-amino-2,4-dihydroxy-α-methylphenylethylamine (5-ADMP) into the ventral noradrenergic bundle (VNAB), norepinephrine (NE) concentrations (ng/mg protein) were reduced to 30–35% of control in the VMN and MPN. 5-ADMP-induced lesions of the VNAB also reduced lordosis quotients in these animals, and this effect was reversed by intracerebral ventricular administration of the α 1-adrenergic receptor agonist phenylephrine. These results indicate that neurotoxin-induced disruption of noradrenergic neurons in the VNAB is associated with a deficit in sexual receptivity in female rats. To determine if the reduction in sexual receptivity following 5-ADMP-induced lesions of the VNAB resulted from loss of noradrenergic neuronal projections specifically to the VMN or MPN, lordosis quotients were determined in ovariectomized, estrogen/progesterone-treated rats in which noradrenergic terminals in these hypothalamic nuclei were selectively lesioned. Injection of 5-ADMP directly into either the VMN or MPN reduced NE concentrations to 17% of control in these hypothalamic nuclei, but failed to alter lordosis. Furthermore, injection of phenylephrine into either the VMN or MPN of VNAB-lesioned rats failed to reinstate lordosis to the levels comparable to sham-lesioned controls. Taken together, these results indicate that noradrenergic neurons terminating in either the VMN or MPN are not essential for gonadal steroid induction of sexual receptivity in ovariectomized female rats.

A selective oxytocin antagonist attenuates progesterone facilitation of female sexual behavior.

Although previous studies have demonstrated that exogenous administration of oxytocin (OT) enhances sexual receptivity in female rats, there is no compelling evidence that endogenous OT has a physiological role in the regulation of female sexual behavior. In the current studies we centrally administered d(CH2)5[Tyr(Me)2Thr4,Tyr-NH2(9)]ornithine vasotocin (or OTA), a selective OT receptor antagonist, to block endogenous OT in ovariectomized females primed with different levels of gonadal steroids. After OTA administration (100-1000 ng), females primed with estradiol benzoate (EB; 1 microgram) and progesterone (P; 250 micrograms) showed reductions in both receptive and proceptive behaviors. These effects of OTA were also evident, though less striking, in females primed with higher doses of EB (10 micrograms) and P (250 micrograms), but significant OTA effects were absent in females primed with EB (10 micrograms) alone. Thus, OTA appeared to attenuate P's facilitation of sexual behavior. Surprisingly, these behavioral effects of OTA administration were not apparent immediately, but emerged only when OTA was given with P 4-6 h before behavioral testing. To determine if these delayed, but lasting, behavioral effects were associated with OTA occupancy of the OT receptor, we measured OT receptor binding ex vivo using receptor autoradiography. Six hours after intracerebroventricular administration of OTA (1000 ng), OT receptor binding was reduced at least 75% in the ventromedial nucleus of the hypothalamus relative to control levels of binding. Thus, those OT receptors previously implicated in the regulation of sexual receptivity appear to be significantly blocked throughout the period of OTA's behavioral effects. Together, these studies lend support to the hypothesis that endogenous OT has a physiological role in the regulation of female sexual behavior.

Role of the Vomeronasal Organ in the Male-Induced Enhancement of Sexual Receptivity in Female Rats

The role of the vomeronasal organ (VNO) in the male-induced enhancement of sexual receptivity in ovariectomized estrogen-primed rats was investigated. Removal of the VNO significantly reduced the enhancement of sexual receptivity following mating, as compared with the sham-operated controls. The sham-operated females exhibited a surge of luteinizing hormone (LH) following mating; however, LH release induced by pairing with males was less manifested in the VNO-removed females. This suggests that in female rats, VNO removal impairs the male-induced release of LH-releasing hormone (LHRH). Since LHRH enhances sexual receptivity in ovariectomized estrogen-primed rats, the present results suggest that the increase in sexual receptivity in female rats following mating probably results from a VNO-mediated LHRH release.

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.