Different doses of estradiol benzoate induce conditioned place preference after paced mating

The ovarian hormones estrogen and progesterone are required for the complete display of sexual behavior in female rats. Paced mating produces a reward state in intact cycling and ovariectomized (OVX), hormonally primed females as evaluated by the conditioned place preference (CPP) paradigm. Most of the studies that have evaluated CPP induced by paced mating in OVX females have used relatively high doses of estradiol benzoate (EB). In the present study we determined if different doses of EB, combined with progesterone (P), could induce CPP after paced mating. For this purpose OVX female rats were divided in five groups that received one of different doses of estradiol benzoate (5, 2.5, 1.25 or 0.625 μg estradiol + 0.5 mg of progesterone) before being allowed to pace the sexual interaction and conditioned in a CPP paradigm. We found that the lowest dose of EB used (0.625 μg) significantly reduced the lordosis quotient and the lordosis coefficient. Even though these females paced the sexual interaction, they didn't change its original preference, suggesting that sexual interaction did not induce a positive affective, reward state. Females allowed to pace the sexual interaction with higher doses of EB developed CPP after paced mating. These results indicate that a threshold of estradiol is required for paced mating to induce CPP.     

Sensitization of sexual behavior in ovariectomized rats by chronic estradiol treatment

The ovariectomized (OVX) rat treated with estradiol benzoate (EB) is used to elucidate neuroendocrine mechanisms of sexual behavior. Chronic behavioral and pharmacological manipulations can be confounded by rising baselines, since females are behaviorally more sensitive to repeated EB injections. The literature lacks a systematic examination of chronic effects of EB administered alone to the sexually experienced OVX rat. Long–Evans rats were repeatedly treated (8 tests) with s.c. injections of 2, 5, or 10 μg EB at different time intervals (4 or 8 days). Female sexual behaviors as well as receipt of mounts, intromissions and ejaculations from the male were observed in the unilevel 4-hole pacing chamber. The effects of adrenalectomy (ADX) and strain (Long–Evans vs. Wistar) were also assessed. Long–Evans OVX rats treated with 5 μg EB every 8 days showed persistently low levels of sexual behavior. Sensitization was most robust following 10 μg EB at 4-day intervals. Very few sexual behaviors were ever induced by 2 μg EB. ADX did not affect the development of behavioral sensitization by 10 μg EB. Therefore, to achieve a low steady state of sexual behaviors in sexually experienced Long-Evans OVX rats 5 μg of EB administered every 8 days is optimal, whereas a persistently high level of sexual behaviors is induced with 10 μg EB administered every 4 days. OVX Wistar rats are behaviorally more sensitive to EB. Behavioral sensitization to EB may serve as a mechanism to optimize reproductive success.     

Glutamate release in the ventromedial hypothalamus of the female rat during copulation: Modulation by estradiol

Binding of glutamate or its ionotropic receptor agonists in the ventromedial hypothalamus (VMH) of female rats inhibits both appetitive and consummatory aspects of sexual behavior. Because vaginocervical stimulation activates glutamate neurons in the VMH, and administration of estradiol benzoate (EB) and progesterone (P) delays this effect, the present study examined the effects of hormonal priming on glutamate release within the VMH of female rats paired with sexually vigorous males. Ovariectomized, sexually experienced rats were implanted with guide cannula aimed at the ventrolateral VMH, through which microdialysis probes were inserted prior to testing. Females were assigned randomly to one of three hormone treatment conditions: EB + P, EB alone, or the oil vehicle. Testing was conducted over 5 h, including a 120-min period of habituation to the testing chamber, a 60-min period of baseline sample collection, and a 120-min period during which a sexually vigorous male was introduced into the testing chamber. Dialysates were collected every 20 min during the test and were analyzed for glutamate using HPLC. Females primed with oil had large and significant increases in glutamate release from baseline once the male was introduced to the chamber. Treatment with EB alone decreased glutamate release in response to male cues. Although treatment with EB + P did not differ significantly from EB alone, the degree of reduced glutamate release was less than with EB alone. These results indicate that priming with EB reduces glutamate transmission in the VMH in response to male cues. Taken together with our previous findings, estradiol blunts the activation of glutamate neurons in the VMH thus allowing female rats to copulate.     

Neonatal estradiol exposure to female rats changes GABAA receptor expression and function,and spatial learning during adulthood

Exposure of female rats to estradiol during the perinatal period has profound effects on GABAergic neurotransmission that are crucial to establish sexually dimorphic brain characteristics. We previously showed that neonatal β-estradiol 3-benzoate (EB) treatment decreases brain concentrations of the neurosteroid allopregnanolone, a potent positive modulator of extrasynaptic GABA_A receptors (GABA_AR). We thus evaluated whether neonatal EB treatment affects GABA_AR expression and function in the hippocampus of adult female rats. Neonatal EB administration increased the expression of extrasynaptic α4/δ subunit-containing GABA_ARs and the modulatory action of THIP on tonic currents mediated by these receptors. The same treatment decreased the expression of synaptic α1/α4/γ2 subunit-containing receptors, as well as phasic currents. These effects of neonatal EB treatment are not related to ambient allopregnanolone concentrations per se, given that vehicle-treated rats in diestrus, which have opposite neurosteroid levels than EB-treated rats, show similar changes in GABA_ARs. Rather, these changes may represent a compensatory mechanism to counteract the long-term reduction in allopregnanolone concentrations, induced by neonatal EB. Given that both α4/δ receptors and allopregnanolone are involved in memory consolidation, we evaluated whether neonatal EB treatment alters performance in the Morris water maze test during adulthood. Neonatal EB treatment decreased the latency and the cumulative search error to reach the platform, as well as thigmotaxis, suggesting improved learning, and also enhanced memory performance during the probe trial. These enduring changes in GABA_AR plasticity may be relevant for the regulation of neuronal excitability in the hippocampus and for the etiology of psychiatric disorders that originate in development and show sex differences.     

A role for Src kinase in progestin facilitation of estrous behavior in estradiol-primed female rats

This study tested the hypothesis that the Src/Raf/MAPK signaling pathway is involved in the facilitation of the lordosis and proceptive behaviors induced by progesterone (P) and its ring A-reduced metabolites in ovariectomized, estradiol-primed rats. Intraventricular (icv) infusion of PP2 (7.5, 15 and 30 µg), a Src kinase inhibitor, significantly depressed P-dependent estrous behavior (lordosis and proceptivity) in estradiol-primed rats. Icv infusion of 30 µg of PP2 also significantly attenuated estrous behavior induced by the ring A-reduced P metabolites 5α-dihydroprogesterone (5α-DHP) and 5α-pregnan-3α-ol-20-one (allopregnanolone). PP2 did not inhibit estrous behavior induced by administration of high doses of estradiol alone to ovariectomized rats. We also assessed if the ventromedial hypothalamus (VMH) is one of the neural sites at which progestins activate Src signaling to facilitate estrous behavior. Bilateral administration of 15 µg of PP2 into the VMH inhibited the stimulation of both lordosis and proceptive behaviors elicited by subcutaneous P administration to estradiol-primed rats. These results suggest that progestins act through Src/Raf/MAPK signaling to initiate estrous behaviors in estrogen-primed rats. This event is one component of the cellular pathways leading to the display of estrous behaviors induced by P and its ring A-reduced metabolites in female rats.     

Impact of pubertal and adult estradiol treatments on cocaine self-administration

Estradiol is thought to play a critical role in the increased vulnerability to psychostimulant abuse in women. Sex differences in the ability of estradiol to influence cocaine self-administration in adult rats have been hypothesized to depend upon pubertal estradiol exposure. The current study investigated whether the presence of gonadal hormones during puberty affected cocaine self-administration behavior and its sensitivity to adult estradiol treatment in male and female Sprague–Dawley rats. Subjects were gonadectomized or SHAM-operated at postnatal day (PD) 22, and received either OIL or estradiol benzoate (EB) during the approximate time of puberty (PD27 to PD37). Adult rats were subsequently treated with either EB or OIL 30 min before cocaine self-administration (0.3 mg/kg/inf) in order to examine the effects of pubertal manipulations on the estradiol sensitivity of acquisition on a fixed ratio (FR) 1 schedule, total intake on a FR5 schedule and motivation on a progressive ratio schedule. Adult EB treatment only affected cocaine self-administration in females, which is consistent with previous research. Adult EB treatment enhanced acquisition in all females irrespective of puberty manipulations. All females, except those treated with EB during puberty, displayed increased cocaine intake following adult EB treatment. Adult EB treatment only enhanced motivation in females that were intact during puberty, whereas those treated with EB during puberty showed reduced motivation. Therefore, the sensitivities of different self-administration behaviors to adult estradiol treatment are organized independently in females, with pubertal estradiol exerting a greater influence over motivational processes, and negligible effects on learning/acquisition.     

Vaginocervical stimulation induces Fos in glutamate neurons in the ventromedial hypothalamus: Attenuation by estrogen and progesterone

Vaginocervical stimulation (VCS) induces the immediate-early gene product Fos in the ventromedial hypothalamus (VMH) of female rats. However, this induction is lower in ovariectomized rats that receive estradiol benzoate (EB) and progesterone (P) relative to an oil vehicle. We have observed that a substantial proportion of cells activated in the VMH by VCS stain for glutamate, and infusions of glutamate or its selective receptor agonists to the VMH inhibit both appetitive and consummatory sexual behaviors in females. This raises the possibility that VCS activates an inhibitory glutamate system in the VMH, and that ovarian steroids blunt the activation, although it is not known whether EB or P, alone or in combination, lead to this effect. The present experiment examined the ability of VCS to induce Fos in glutamate neurons in the VMH of ovariectomized rats under 4 hormonal regimens: oil, EB alone, P alone, or EB + P, following 1 or 50 distributed VCSs administered with a lubricated glass rod over the course of 1 h. Treatment with EB or P alone significantly reduced the number of glutamate neurons activated by 1 VCS, with P being more effective than EB. Treatment with EB + P also produced a significant reduction, but not to the extent of EB or P alone. Although EB and P work in synergy to activate sexual behavior in female rats, actions of EB or P alone are sufficient to blunt the ability of VCS to activate glutamate neurons in the VMH. It thus appears that ovarian steroids may “disinhibit” sexual responding, in part, by dampening the ability of VCS to activate glutamate neurons in the VMH. In turn, this may allow females to receive a sufficient number of intromissions for the activation of sexual reward and the facilitation of pregnancy.     

Neonatal agonism of ERβ impairs male reproductive behavior and attractiveness

The organization of the developing male rodent brain is profoundly influenced by endogenous steroids, most notably estrogen. This process may be disrupted by estrogenic endocrine disrupting compounds (EDCs) resulting in altered sex behavior and the capacity to attract a mate in adulthood. To better understand the relative role each estrogen receptor (ER) subtype (ERα and ERβ) plays in mediating these effects, we exposed male Long Evans rats to estradiol benzoate (EB, 10 μg), vehicle, or agonists specific for ERβ (DPN, 1 mg/kg) or ERα (PPT, 1 mg/kg) daily for the first four days of life, and then assessed adult male reproductive behavior and attractiveness via a partner preference paradigm. DPN had a greater adverse impact than PPT on reproductive behavior, suggesting a functional role for ERβ in the organization of these male-specific behaviors. Therefore the impact of neonatal ERβ agonism was further investigated by repeating the experiment using vehicle, EB and additional DPN doses (0.5 mg/kg, 1 mg/kg, and 2 mg/kg bw). Exposure to DPN suppressed male reproductive behavior and attractiveness in a dose dependent manner. Finally, males were exposed to EB or an environmentally relevant dose of genistein (GEN, 10 mg/kg), a naturally occurring xenoestrogen, which has a higher relative binding affinity for ERβ than ERα. Sexual performance was impaired by GEN but not attractiveness. In addition to suppressing reproductive behavior and attractiveness, EB exposure significantly lowered the testis to body weight ratio, and circulating testosterone levels. DPN and GEN exposure only impaired behavior, suggesting that disrupted androgen secretion does not underlie the impairment.     

Organizational effects of testosterone,estradiol, and dihydrotestosterone on vasopressin mRNA expression in the bed nucleus of the stria terminalis

In adulthood, male rats express higher levels of arginine vasopressin (AVP) mRNA in the bed nucleus of the stria terminalis (BST) than do female rats. We tested whether this sex difference is primarily due to differences in neonatal levels of testosterone. Male and female rats were gonadectomized on the day of birth and treated with testosterone propionate (TP) or vehicle on postnatal days 1, 3, and 5 (P1, P3, and P5). Three months later, all rats were implanted with testosterone-filled capsules. Two weeks later, brains were processed for in situ hybridization to detect AVP mRNA. We found that neonatal TP treatment significantly increased the number of vasopressinergic cells in the BST over control injections. We then sought to determine the effects of testosterone metabolites, estradiol and dihydrotestosterone, given alone or in combination, on AVP expression in the BST. Rat pups were treated as described above, except that instead of testosterone, estradiol benzoate (EB), dihydrotestosterone propionate (DHTP), a combination of EB and DHTP (EB+DHTP), or vehicle was injected neonatally. Neonatal treatment with either EB or EB+DHTP increased the number of vasopressinergic cells in the BST over that of DHTP or oil treatment. However, treatment with DHTP also significantly increased the number of vasopressinergic cells over that of oil treatment. Hence, in addition to bolstering evidence that estradiol is the more potent metabolite of testosterone in causing sexual differentiation of the brain, these data provide the first example of a masculinizing effect of a nonaromatizable androgen on a sexually dimorphic neuropeptide system.     

Pre- and postnatal bisphenol A treatment results in persistent deficits in the sexual behavior of male rats, but not female rats, in adulthood

Perinatal administration of the endocrine disruptor bisphenol A (BPA) reportedly inhibits the sexual behavior of sexually naïve adult male rats. In order to evaluate the effects of BPA administration during early development on later reproductive behavior, we administered one of five doses of bisphenol A daily to pregnant female rats throughout gestation and lactation, and quantified the appetitive and consummatory sexual behaviors of the resultant male and female offspring over multiple sexual encounters in adulthood. Males receiving low dose perinatal BPA (50 μg/kg bw/day) showed persistent deficits in sexual behavior in adulthood. Males receiving the highest dose (5 mg/kg bw/day), however, were indistinguishable from controls with respect to consummatory sexual behaviors but showed decreased latencies to engage in those behaviors when sexually naïve, with significant non-linear, or U-shaped, dose-response relationships observed on the first and last day of testing. Adult female sexual behavior was not affected by early BPA administration at any dose tested. These results are consistent with previous reports that BPA exerts behavioral effects especially at low doses, and further indicates that BPA can cause lasting impairment of sexual behavior in males, but does not alter the normal development of female appetitive or consummatory sexual behaviors. To our knowledge, this is the first report indicating that adult sexual performance is impaired in sexually experienced animals following perinatal exposure to bisphenol A.     

Organizational effects of testosterone,estradiol, and dihydrotestosterone on vasopressin mRNA expression in the bed nucleus of the stria terminalis

In adulthood, male rats express higher levels of arginine vasopressin (AVP) mRNA in the bed nucleus of the stria terminalis (BST) than do female rats. We tested whether this sex difference is primarily due to differences in neonatal levels of testosterone. Male and female rats were gonadectomized on the day of birth and treated with testosterone propionate (TP) or vehicle on postnatal days 1, 3, and 5 (P1, P3, and P5). Three months later, all rats were implanted with testosterone‐filled capsules. Two weeks later, brains were processed for in situ hybridization to detect AVP mRNA. We found that neonatal TP treatment significantly increased the number of vasopressinergic cells in the BST over control injections. We then sought to determine the effects of testosterone metabolites, estradiol and dihydrotestosterone, given alone or in combination, on AVP expression in the BST. Rat pups were treated as described above, except that instead of testosterone, estradiol benzoate (EB), dihydrotestosterone propionate (DHTP), a combination of EB and DHTP (EB+DHTP), or vehicle was injected neonatally. Neonatal treatment with either EB or EB+DHTP increased the number of vasopressinergic cells in the BST over that of DHTP or oil treatment. However, treatment with DHTP also significantly increased the number of vasopressinergic cells over that of oil treatment. Hence, in addition to bolstering evidence that estradiol is the more potent metabolite of testosterone in causing sexual differentiation of the brain, these data provide the first example of a masculinizing effect of a nonaromatizable androgen on a sexually dimorphic neuropeptide system. © 2003 Wiley Periodicals, Inc. J Neurobiol 54: 502–510, 2003     

Role of progesterone receptors during postpartum estrus in rats

We studied the role of progesterone receptor (PR) in the display of female sexual behavior during postpartum estrus in rats. Adult female rats were treated with the PR antagonist, RU486 (1.25 and 5 mg), 3 h after parturition and sexual behavior was evaluated throughout the first postpartum day. Estradiol and progesterone serum levels changed during the first 24 h postpartum. The highest estradiol and progesterone levels were found at 9 and 12 h postpartum, respectively. The predominant PR isoform in the hypothalamus and the preoptic area was PR-A during postpartum day. The content of PR-A increased at 6 h postpartum in the hypothalamus and the preoptic area, and decreased in both regions at 9 h. PR-B content only increased in the preoptic area at 12 h postpartum. The highest display of lordotic and proceptive behaviors were found at 12 h postpartum. The treatment with 1.25 and 5 mg of RU486 respectively reduced lordosis by 61% and 92% at 12 h postpartum. These results suggest that PR is essential in the display of postpartum estrus in rats.     

QRFP in female rats: effects on high fat food intake and hypothalamic gene expression across the estrous cycle

Pyroglutamylated arginine-phenylalanineamide peptide (QRFP) is a neuropeptide involved in feeding behavior. Central administration of QRFP selectively increases the intake of a high fat diet in male rats. QRFP administration also stimulates the hypothalamic-pituitary-gonadal axis via gonadotrophin-releasing hormone in male and female rats. Prepro-QRFP mRNA is expressed in localized regions of the mediobasal hypothalamus which are abundant in neurotransmitters, neuropeptides and receptor systems important for food intake regulation and reproductive behaviors. The current experiments were conducted to investigate the effects of centrally administered QRFP-26 on the intake of a high fat diet (HFD, 60% kcal from fat) in female rats and to investigate alterations in hypothalamic prepro-QRFP and its receptors, GPR130a and GPR103b, mRNA levels over the estrous cycle. In Experiment 1, female rats were administered QRFP-26 (intracerebroventricular; 0.3 nmol, 0.5 nmol, 1.0 nmol) in rats consuming either a HFD or a low fat diet. All doses of QRFP-26 selectively increased the intake of the HFD in female rats. These data suggest that QRFP-26 regulates the intake of energy dense foods in female rats, which is similar to previous findings in male rats. In Experiment 2, hypothalamic levels of prepro-QRFP mRNA and its receptors were assessed during diestrus, proestrus, or estrus. The level of prepro-QRFP mRNA in the ventromedial/arcuate nucleus (VMH/ARC) of the hypothalamus was increased during proestrus, which suggests that endogenous estrogen levels regulate QRFP expression in the VMH/ARC. These data suggest that QRFP may play a role in coordinating feeding behaviors with reproductive function when energy demand is increased.     

Modulation of the arcuate nucleus–medial preoptic nucleus lordosis regulating circuit: A role for GABAB receptors

Estradiol rapidly activates a microcircuit in the arcuate nucleus of the hypothalamus (ARH) that is needed for maximal female sexual receptivity. Membrane estrogen receptor-α complexes with and signals through the metabotropic glutamate receptor-1a stimulating NPY release within the ARH activating proopiomelanocortin (POMC) neurons. These POMC neurons project to the medial preoptic nucleus (MPN) and release β-endorphin. Estradiol treatment induces activation/internalization of MPN μ-opioid receptors (MOR) to inhibit lordosis. Estradiol membrane action modulates ARH gamma-aminobutyric acid receptor-B (GABA_B) activity. We tested the hypothesis that ARH GABA_B receptors mediate estradiol-induced MOR activation and facilitation of sexual receptivity. Double-label immunohistochemistry revealed expression of GABA_B receptors in NPY, ERα and POMC expressing ARH neurons. Approximately 70% of POMC neurons expressed GABA_B receptors. Because estradiol initially activates an inhibitory circuit and maintains activation of this circuit, the effects of blocking GABA_B receptors were evaluated before estradiol benzoate (EB) treatment and after at the time of lordosis testing. Bilateral infusions of the GABA_B receptor antagonist, CGP52432, into the ARH prior to EB treatment of ovariectomized rats prevented estradiol-induced activation/internalization of MPN MOR, and the rats remained unreceptive. However, in EB-treated rats, bilateral CGP52432 infusions 30 min before behavior testing attenuated MOR internalization and facilitated lordosis. These results indicated that GABA_B receptors were located within the lordosis-regulating ARH microcircuit and are necessary for activation and maintenance of the estradiol inhibition of lordosis behavior. Although GABA_B receptors positively influence estradiol signaling, they negatively regulate lordosis behavior since GABA_B activity maintains the estradiol-induced inhibition.     

A Single Neonatal Injection of Ethinyl Estradiol Impairs Passive Avoidance Learning and Reduces Expression of Estrogen Receptor α in the Hippocampus and Cortex of Adult Female Rats

Although perinatal exposure of female rats to estrogenic compounds produces irreversible changes in brain function, it is still unclear how the amount and timing of exposure to those substances affect learning function, or if exposure alters estrogen receptor α (ERα) expression in the hippocampus and cortex. In adult female rats, we investigated the effects of neonatal exposure to a model estrogenic compound, ethinyl estradiol (EE), on passive avoidance learning and ERα expression. Female Wistar-Imamichi rats were subcutaneously injected with oil, 0.02 mg/kg EE, 2 mg/kg EE, or 20 mg/kg 17β-estradiol within 24 h after birth. All females were tested for passive avoidance learning at the age of 6 weeks. Neonatal 0.02 mg/kg EE administration significantly disrupted passive avoidance compared with oil treatment in gonadally intact females. In a second experiment, another set of experimental females, treated as described above, was ovariectomized under pentobarbital anesthesia at 10 weeks of age. At 15–17 weeks of age, half of each group received a subcutaneous injection of 5 μg estradiol benzoate a day before the passive avoidance learning test. Passive avoidance learning behavior was impaired by the 0.02 mg/kg EE dose, but notably only in the estradiol benzoate-injected group. At 17–19 weeks of age, hippocampal and cortical samples were collected from rats with or without the 5 μg estradiol benzoate injection, and western blots used to determine ERα expression. A significant decrease in ERα expression was observed in the hippocampus of the estradiol-injected, neonatal EE-treated females. The results demonstrated that exposure to EE immediately after birth decreased learning ability in adult female rats, and that this may be at least partly mediated by the decreased expression of ERα in the hippocampus.     

Paced mating behavior in female rats in response to different hormone priming regimens

A female rat will display a repertoire of behaviors during a sexual encounter with a male rat including sexually receptive (the lordosis response) and proceptive (hopping, darting) behaviors. In addition, when given the opportunity, a sexually receptive female rat will approach and withdraw from the male rat, controlling the timing of the receipt of mounts, intromissions, and ejaculations, a behavior known as paced mating behavior. The present experiments tested the hypotheses (1) that progesterone regulates paced mating behavior, and (2) that multiple hormone regimens used previously to induce sexual receptivity have the same effect on paced mating behavior. Paced mating behavior was assessed in sexually receptive ovariectomized female rats after treatment with: (1) estradiol benzoate (EB; 30.0 mg/kg) followed by a range of doses of progesterone (P; 1.0-8.0 mg/kg), (2) two pulses of unesterified estradiol (E2; 2.0 microg/rat) followed by 1.0 mg/rat of P, and (3) EB alone (5.0 microg/rat) for 6 days. No differences in sexual receptivity or in paced mating behavior were observed across doses of P (1.0-8.0 mg/kg). In contrast, the number of hops and darts per min increased with the dose of P administered. E2 + P administration resulted in slightly, but significantly, lower levels of sexual receptivity along with significantly longer contact-return latencies following an intromission in relation to the other treatment conditions. In addition, female rats exhibited fewer hops and darts per min in response to E2 + P than in response to EB + 8.0 mg/kg of P. The administration of EB alone for 6 days induced levels of receptivity and paced mating behavior indistinguishable from EB + P, while eliciting significantly fewer hops and darts per min than the EB + 8.0 mg/kg P treatment condition. Hormone priming regimen had no effect on the percentage of exits displayed during the paced mating tests in any experimental phase. Dose of P had no effect on paced mating behavior in sexually receptive rats. In addition, P does not appear to be necessary for the display of paced mating behavior following long-term treatment with EB. In contrast, the pulsatile administration of E2 + P induced a different pattern of paced mating behavior in sexually receptive rats.     

Adult hippocampal cell proliferation is suppressed with estrogen withdrawal after a hormone-simulated pregnancy

Estradiol withdrawal after pregnancy is hypothesized to precipitate depressive symptoms in vulnerable women. A hormone-simulated pregnancy was induced in female rats and the effects of a ‘postpartum’ drop in estradiol on hippocampal cell proliferation were examined. All groups were ovariectomized or given sham surgery prior to treatment. Rats were randomly assigned to ‘postpartum’, ‘postpartum’ + EB (estradiol benzoate), ‘postpartum’ + DPN (diarylpropionitrile; an ERβ agonist), ‘postpartum’ + IMI (imipramine; a tricyclic antidepressant), sham, ovariectomized (OVX), sham + IMI or OVX + IMI groups. All ‘postpartum’ groups received hormone injections (estradiol and progesterone) over 23 days to simulate pregnancy, while IMI groups also received daily imipramine injections. After day 23, ‘postpartum’ rats were withdrawn from the hormone-simulated pregnancy (mimicking the postpartum drop in gonadal hormones), while other ‘postpartum’ treatment groups received daily injections of DPN, EB or IMI. On day 3 ‘postpartum’ all rats were injected with bromodeoxyuridine (BrdU; a DNA synthesis marker) and perfused 24 h later to assess cell proliferation and cell death in the dentate gyrus. ‘Postpartum’ hormone withdrawal decreased hippocampal cell proliferation in the ‘postpartum’ and ‘postpartum’ + EB groups only. Chronic imipramine significantly increased hippocampal cell proliferation in sham + IMI, but not OVX + IMI rats suggesting that imipramine's effects to increase hippocampal cell proliferation in female rats is related to reproductive status. Cell death (pyknotic cells) was decreased only in the ‘postpartum’ group. Together, these results suggest an important, though complex, role for gonadal hormones in the cellular changes accompanying this model of postpartum depression.     

Sexual incentive motivation in male rats requires both androgens and estrogens

In Experiment 1 castrated male rats were implanted with a Silastic capsule containing either E or cholesterol (CHOL) 35 days after castration. They were then tested for sexual incentive motivation and copulatory behaviors every 5th day for 3 weeks. None of the treatments affected sexual incentive motivation. After the last test, all subjects were implanted with DHT-containing Silastic capsules, and tests continued for another 3 weeks. While E + DHT enhanced sexual incentive motivation and copulatory behavior, DHT alone failed to do so. In Experiment 2 the aromatase inhibitor fadrozole (F) was combined with testosterone (T). T restored all behaviors to the level seen in intact rats, and F significantly reduced these effects. In fact, T + F was not different from DHT. T and DHT restored the weight of the prostate and seminal vesicles to levels close to those of intact rats. In Experiment 3 a lower dose of E was employed. Also this dose of E failed to affect sexual incentive motivation while E + DHT restored it to the level of intact animals. Castration enhanced the serum concentrations of LH and FSH. E alone caused a marked reduction, and E + DHT brought both gonadotropins back to the level of intact animals. It was concluded that the doses of E and DHT employed in these experiments were within or close to the physiological range, and that such doses of E completely fail to enhance sexual incentive motivation in castrated animals. DHT has small or no effects. It appears that sexual incentive motivation and copulation require simultaneous stimulation of androgen and estrogen receptors.     

Effects of ventromedial nucleus lesions on estradiol induced ovulation in the rat

Small bilateral stereotaxic lesions were made in the hypothalamic ventromedial nucleus (SVMN) to determine: (i) whether estrogen would restore early receptivity in unreceptive SVMN lesioned female rats and (ii) whether SVMN lesions would suppress estrogen induced ovulation in the rat. SVMN lesions were shown to completely suppress spontaneous early receptivity and seriously impair estrous receptivity in 5-day cyclic female Wistar rats. A loss of early receptivity in response to 10 μg estradiol benzoate (EB) was also observed in SVMN lesioned females, in comparison to unoperated, sham VMN and dorsomedial nucleus (DMN) lesioned animals. Isolated SVMN lesioned females exhibited a weak ovulatory response to 10 μg EB, but, where shown to be unreceptive prior to estrogen injection, they never ovulated. On the contrary, ovulation occured in about 50% of cases in isolated unoperated and in sham VMN and DMN lesioned females following estrogen administration. The mechanisms whereby EB brought about precocious ovulation in 5-day cyclic female rats were therefore concluded to be dependent on VMN functional integrity and thereby on the degree of early sexual receptivity in the rat.