Role of hypothalamic serotonergic receptors in the control of lordosis behavior in the female rat

The role of serotonin in mediating hypothalamic control of sexual behavior in estrone-primed ovariectomized (OVX) rats was studied by comparing the lordotic patterns following medial preoptic (MPOA) and arcuate-ventromedial (ARC-VM) infusions of serotonin (5-HT), methysergide (MS), and vehicle. In the initial experiments, low receptivity (preinfusion receptivity: mean lordosis/mount ratio = 0.164) was maintained by priming each animal with a low dose of estrone 48 hr prior to mating. The infusion of MS in either the MPOA or ARC-VM area resulted in a significant enhancement of lordotic behavior from initial low receptivity, 5-HT infusions were found to have no statistically significant effect upon lordotic behavior. In order to corroborate the findings observed in the low preinfusion receptivity protocol, OVX rats were primed with higher doses of estrone to maintain a high level of receptivity (preinfusion receptivity: mean lordosis/mount ratio = 0.787). Using this protocol, significant depressions in lordotic behavior were observed following MPOA or ARC-VM infusions of 5-HT, It was thus proposed that serotonergic receptors within the MPOA or ARC-VM areas have inhibitory effects upon lordotic behavior. In addition to the effects of 5-HT upon estrogen-induced sexual receptivity, serotonergic influences upon luteinizing hormone-releasing hormone (LRH)-facilitated mating behavior were also evaluated. Comparisons were made between the lordotic responses following MPOA or ARC-VM infusions of vehicle, LRH, or LRH with 5-HT in OVX rats primed with low doses of estrone. The infusion of LRH into the MPOA or ARC-VM significantly enhanced lordotic behavior above vehicle levels. However, the addition of 5-HT to the LRH infusate abolished this behavioral enhancement. These findings indicated that LRH and 5-HT have opposing effects within forebrain areas known to be important for the control of lordotic behavior.     

Comparison of some CNS effects of luteinizing hormone-releasing hormone and progesterone

Luteinizing hormone-releasing hormone (LHRH) has been reported to facilitate lordotic behavior in estrogen-primed ovariectomized (OVX) female rats in a manner similar to progesterone (P). This study compared P and LHRH with respect to their behavioral effects and site of action within the brain. The hormones were compared using two different components of sexual behavior, receptivity and proceptivity. To test for receptivity, OVX females were given behaviorally ineffective estradiol benzoate (EB) injections sc 48 hr before testing. They were then treated with either P, LHRH, or vehicle by various routes. Two and/or four hours later, receptivity (LQ) was measured. Treatments for the proceptivity test were similar except that a larger EP-priming dose, which facilitates preceptive behavior, was used. Four hours later, LQ and hopping, darting, and earwiggling were scored. In the receptivity test, sc administration of 1 mg P or 1 μg LHRH (but not 0.5 or 5.0 μg) significantly elevated LQ with respect to vehicle injection 4 hr after treatment. In the proceptivity test, 0.5, 1.0, and 5.0 μg of LHRH given sc failed to alter significantly either LQ or soliciting behavior. Progesterone facilitated both parameters. Implantation of crystalline P into the midbrain reticular formation (MRF) has been shown to elicit both the receptive and preceptive effects of the steroid. Microinjection of as much as 100 ng of LHRH in 1.0 μl saline into the same region failed to enhance lordotic behavior compared to saline injection alone, while a 200-ng intracerebroventricular dose significantly facilitated lordosis at 4 hr. The data indicate that LHRH does not induce proceptive behavior. The effects of peripherally administered LHRH on receptive behavior are similar but less pronounced than those of P. The two hormones elicit this effect from different sites in the brain.     

Serum 5 alpha-androstane-3 alpha, 17 beta-diol increases in response to paced coital stimulation in cycling female rats

The cycling female rat spontaneously regulates (paces) the amount and timing of cervical-vaginal stimulation received during mating. Female pacing of coital contacts increases the ability of vaginal intromissions to induce luteal function and to abbreviate the period of behavioral estrus. In the experiments reported here, we examined whether paced mating results in alterations in serum steroid concentrations, which might contribute to these processes. In Experiment 1, estradiol (E2), progesterone (P), testosterone (T), 5 alpha-dihydrotestosterone (DHT), and 5 alpha-androstane-3 alpha, 17 beta-diol (3 alpha-androstanediol, 3 alpha-diol) were measured in sera obtained from independent groups of animals between 0 min and 12 h after mating onset. Levels of 3 alpha-diol were significantly higher at 30 min in females pacing coital stimulation (Paced) than in females receiving mounts-without-intromission (Mounts-Only) or solitary exposure to the test chamber. Serum T, E2, P, and DHT did not differ between these groups at any time. P and 3 alpha-diol levels declined significantly for all groups across the 12-h post-treatment period. In Experiment 2, serum 3 alpha-diol measured over 3h after mating was compared in females receiving Paced stimulation with females receiving temporally unregulated coital stimulation (Non-Paced), or Mounts-Only. At 20 min, serum 3 alpha-diol concentrations were significantly higher in Paced than in Non-Paced and Mounts-Only females. In Experiment 3, Paced and Non-Paced stimulation did not differentially effect the proportion of females becoming pseudopregnant/pregnant. The selective increase in serum 3 alpha-diol in females that pace matings is discussed with regard to the known inhibitory effects of 5 alpha-reduced androgens on sexual receptivity.     

Progesterone-Facilitated Lordosis in Medial Preoptic Area-Lesioned, Juvenile Guinea Pigs

Juvenile female guinea pigs rarely display lordosis in response to estradiol and progesterone treatments that elicit sexual receptivity in adults. To test the hypothesis that the medial preoptic area (MPOA) tonically inhibits the display of steroid-induced lordosis in juveniles, 11-day-old guinea pigs were ovariectomized (OVX) and received bilateral, sham, or electrolytic lesions aimed at the MPOA 3–4 days later. At 20–22 days of age, these females were tested for the expression of sexual receptivity following injections of estradiol benzoate (EB, 10 μg sc) and progesterone (0.5 mg sc, 40 h after EB). The lesions damaged portions of the MPOA, the nucleus of the diagonal band of Broca, the lateral aspect of the medial preoptic nucleus, the medial part of the preventricular portion of the periventricular nucleus, and the anterior commissure. The lesions did not alter the display of estradiol-induced lordosis. However, after treatment with EB plus progesterone, 20% of the sham-lesioned females displayed lordosis, as compared to 80% of the MPOA-lesioned animals. These data are consistent with the hypothesis that neurons originating in and/or traversing the MPOA tonically suppress the display of progesterone-facilitated lordosis in juvenile guinea pigs. Removal of this inhibitory input allows prepubertal females to respond behaviorally to estradiol and progesterone in an adult-typical fashion.     

Only Self-Paced Mating Is Rewarding in Rats of Both Sexes

When rats are mated in a traditional mating chamber (with one male and one female) in which the male dictates the pace of the copulatory sequence, males develop a reward state as evaluated by conditioned place preference (CPP). In this mating situation no reward state is induced in females. However, when female rats are able to control (pace) the rate of sexual stimulation, thereby reducing the aversive consequences associated with mating, a clear CPP is observed. In the present study the CPP paradigm was used to determine whether if the reinforced state induced by coital interactions in male rats can be maintained when females pace the sexual interaction. Adult male and female rats were mated in one of two different conditions: (1) where subjects were able to pace their coital interactions or (2) where subjects were not able to pace their sexual contacts. The results showed that when males had control over the sexual interaction they developed a clear place preference while males that mated with females that paced their coital contacts did not develop CPP. Similarly, only females that were able to pace their sexual contacts developed place preference. These results suggest that coital interactions in males, as well as in females, can induce a reward state only when they are able to control the sexual interaction. Under seminatural conditions sexual behavior in rats is highly promiscuous, they mate in groups and repeatedly change partners in the middle of copulation. This behavioral sequence allows both, male and female to control the rate of sexual interaction, assuring the induction of a reward state outlasting the actual performance of coital responses.     

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.     

Beta-endorphin suppression of lordosis behavior in female rats; Lack of effect of peripherally-administered naloxone

Endogenous opioid peptides have been implicated in the control of copulatory behavior of the male rat. In order to assess the possible role of opioids in modulation of sexual receptivity in the female rat, lordosis behavior of ovariectomized (OVX) steroid-primed rats was tested after administration of beta-endorphin (B-END) or naloxone (NAL). Lordosis-to-mount ratio (L/M) of estrogen (E)- and progesterone (P)-primed rats was suppressed 15 and 45 minutes after intraventricular infusion of 100 ng B-END. This suppressive effect was blocked by subcutaneous injections of NAL (2 mg/kg). NAL alone, however, failed to enhance L/M in E-primed rats when administered in subcutaneous doses of 2 or 40 mg/kg. Thus, B-END is capable of suppressing lordotic responsiveness, but endogenous B-END does not appear to tonically suppress responsiveness in the E-primed rat.     

Effect of forebrain implants of testosterone or estradiol on scent-marking and sexual behavior in male and female rabbits

Chinning consists of rubbing the chin against an object, thereby depositing secretions from the submandibular glands. As mating, chinning is stimulated in male and female rabbits by testosterone and estradiol, respectively. To investigate the brain sites where steroids act to stimulate chinning and mating we implanted into the ventromedial hypothalamus (VMH) or the medial preoptic area (MPOA) of gonadectomized male and female rabbits testosterone propionate (TP; males) or estradiol benzoate (EB; females) and quantified chinning and sexual behavior. EB implants into the VMH or MPOA reliably stimulated chinning in females. Most of those implanted into the VMH and around half of the ones receiving EB into MPOA or diagonal band of Broca (DBB) showed lordosis. Chinning, but not sexual behavior, was stimulated in males by TP implants into the MPOA or DBB. Neither chinning nor mounting were reliably displayed by males following TP implants into the VMH. Results indicate that, in females, the VMH is an estrogen-sensitive brain area that stimulates both chinning and lordosis while the MPOA seems to contain subpopulations of neurons involved in either behavior. In males, androgen-sensitive neurons of the MPOA, but not the VMH, are involved in chinning stimulation but it is unclear if these areas also participate in the regulation of copulatory behavior.     

Effect of forebrain implants of testosterone or estradiol on scent-marking and sexual behavior in male and female rabbits

Chinning consists of rubbing the chin against an object, thereby depositing secretions from the submandibular glands. As mating, chinning is stimulated in male and female rabbits by testosterone and estradiol, respectively. To investigate the brain sites where steroids act to stimulate chinning and mating we implanted into the ventromedial hypothalamus (VMH) or the medial preoptic area (MPOA) of gonadectomized male and female rabbits testosterone propionate (TP; males) or estradiol benzoate (EB; females) and quantified chinning and sexual behavior. EB implants into the VMH or MPOA reliably stimulated chinning in females. Most of those implanted into the VMH and around half of the ones receiving EB into MPOA or diagonal band of Broca (DBB) showed lordosis. Chinning, but not sexual behavior, was stimulated in males by TP implants into the MPOA or DBB. Neither chinning nor mounting were reliably displayed by males following TP implants into the VMH. Results indicate that, in females, the VMH is an estrogen-sensitive brain area that stimulates both chinning and lordosis while the MPOA seems to contain subpopulations of neurons involved in either behavior. In males, androgen-sensitive neurons of the MPOA, but not the VMH, are involved in chinning stimulation but it is unclear if these areas also participate in the regulation of copulatory behavior.     

Anti-estrogenic suppression of the lordosis response in female rats

The anti-estrogen, CI 628, was used to suppress the lordosis response induced by sequential injections of estrogen and progesterone in ovariectomized (OVX) female rats. Appropriate doses of CI 628 completely abolished sexual receptivity in females administered estradiol benzoate (EB) in sesame oil. This behavioral effect could be attenuated by providing increased quantities of EB or decreased quantities of CI 628. Anti-estrogenic effects on lordosis induced by free estradiol in saline (E) were assessed after first establishing behaviorally equivalent doses of EB and E. This was accomplished by determining thresholds for E-induced lordosis. OVX females were approximately seven times less sensitive to E than to EB. CI 628 had no significant effects on E-induced lordosis, in contrast to the complete abolition of lordosis in females treated with behaviorally equivalent EB doses. A possible mechanism to explain this differential responsiveness of EB- and E-treated females is discussed.     

Inhibition of Estrogen-Induced Sexual Receptivity by Androgens: Role of the Androgen Receptor

Both naturally occurring and synthetic androgens have been shown to inhibit estrogen-induced sexual receptivity when administered to ovariectomized (OVX) rats. The mechanisms by which androgens exert these effects, however, remain unclear. Experiments were conducted to determine the role of the androgen receptor in the inhibition of estrogen-induced sexual receptivity in OVX rats by using flutamide, an androgen receptor antagonist. In each experiment, OVX Long–Evans rats received 6 consecutive days of estradiol benzoate (EB; 2.0 μg/day) followed by 15 days of EB concurrent with flutamide (10.0 mg/kg; twice daily) or the vehicle and one of the following androgens or the vehicle: dihydrotestosterone propionate (7.5 mg/kg), 3α-androstanediol (3.75 mg/kg), 17α-methyltestosterone (7.5 mg/kg), stanozolol (7.5 mg/kg), or nandrolone decanoate (7.5 mg/kg). On Day 15, all female rats received progesterone (P; 1.0 mg/rat) 4 h before testing. Tests for sexual receptivity were conducted on Days 3, 6, 14, and 15 of androgen/flutamide treatment. Each androgen inhibited sexual receptivity as expected, and concurrent treatment with flutamide reversed the inhibitory effects of all androgens on sexual receptivity on all test days. High levels of sexual receptivity were displayed in response to P on Day 15, regardless of experimental treatment. These results suggest that naturally occurring and synthetic androgens act at the androgen receptor to inhibit estrogen-induced sexual receptivity in OVX rats.     

Progesterone receptor in the forebrain of female gray short-tailed opossums: Effects of exposure to male stimuli

Progesterone receptor immunoreactivity (PRir) in brain areas involved in reproductive behavior in eutherian species was examined for the first time in a female marsupial, the gray short-tailed opossum (Monodelphis domestica, hereinafter, opossum). PRir in nuclei of neurons, measured as area covered by stained nuclei, was seen in the arcuate nucleus (Arc); anteroventral periventricular nucleus (AVPv); bed nucleus of the stria terminalis (BST); medial preoptic area (MPOA), and ventromedial hypothalamus (VMH), but not in control areas adjacent to the hypothalamus or cortex. Female opossums are induced into cytological, urogenital sinus (UGS), estrus by male pheromones and into behavioral estrus, i.e., receptivity, by pairing with a male, and both estradiol (E) and progesterone (P) are involved in induction of receptivity in intact and ovariectomized females. PRir in the AVPv, MPOA, and VMH was very low in females that had never been exposed to males or their scent marks, i.e., naïve anestrous (NVA) females, and either previous or current exposure to males or their scent marks was associated with elevated PRir. PRir was significantly higher in the AVPv and MPOA of anestrous females with previous but no current exposure to males and their scent marks, i.e., experienced anestrous (EXPA) females, than in NVA females, but PRir was significantly lower in the MPOA and VMH of EXPA females than in females that were behaviorally receptive and had recently copulated, i.e., behavioral receptive estrous (BRE) females. PRir was higher in the VMH of both UGS estrous (UGSE) and BRE females compared to that in EXPA animals, but PRir did not differ between UGSE and BRE females in any of the 3 brain areas examined, including the MPOA These results provide evidence that pheromonal induction of estrus and sexual receptivity in opossums is associated with elevation of PRir in the VMH and MPOA and that prior exposure to males or their pheromones, even in the absence of current male stimuli, is associated with persistent elevation of PRir in the AVPv and MPOA.     

Acute and chronic stress-like levels of cortisol inhibit the oestradiol stimulus to induce sexual receptivity but have no effect on sexual attractivity or proceptivity in female sheep

Stress-like levels of cortisol inhibit sexual receptivity in ewes but the mechanism of this action is not understood. One possibility is that cortisol interferes with the actions of oestradiol to induce sexual receptivity. We tested this hypothesis in 2 experiments with ovariectomised ewes that were artificially induced into oestrus by 12 days of i.m. injections of progesterone followed by an i.m. injection of oestradiol benzoate (ODB) 48 h later. In Experiment 1, ewes were randomly allocated to the following groups: saline infusion + 25 μg ODB, saline infusion + 50 μg ODB, cortisol infusion + 25 μg ODB or cortisol infusion + 50 μg ODB (n = 5 per group). Saline or cortisol was infused i.v. for 40 h beginning at the ODB injection. In Experiment 2, ewes were infused with saline or cortisol (n = 5 per group) for 5 h beginning 1 h before ODB injection. In both experiments, ewe sexual behaviour (attractivity, proceptivity and receptivity) was quantified every 6 h. Blood samples were also collected. The cortisol infusion yielded plasma concentrations of cortisol similar to those seen during psychosocial stress. In both experiments, cortisol suppressed receptivity index (number of immobilisations by ewe/courtship displays by ram) and the number of times ewes were mounted but had no effect on attractivity or proceptivity, irrespective of the dose of ODB (Experiment 1). Cortisol also suppressed LH pulse amplitude. These results suggest that both an acute (5 h) and chronic (40 h) infusion of cortisol inhibit oestradiol-induced sexual receptivity in ewes and that increasing the dose of ODB does not overcome the inhibitory effects of cortisol.     

Influence of time of mating and paced copulation on induction of pseudopregnancy in cyclic female rats

The present experiment was designed to examine whether changes occur during the course of behavioural oestrus in the sensitivity of the female to copulatory stimulation and in patterns of sexual behaviour which might influence the likelihood of luteal maintenance. Cyclic female rats were mated on the evening of pro-oestrus (21:00 h) or early on the morning of oestrus (05:00 h) and received either 5 or 10 intromissions from males under conditions which allowed or did not allow pacing of contacts with males to occur. During mating, the levels of sexual receptivity, the timing of sexual mounts from males, and pacing behaviours were measured. The occurrence of pseudopregnancy or pregnancy in each animal was determined by examining vaginal smears for 14 days after mating and by examining the uterus for the presence of fetuses at the end of the experiment. At both mating times, pacing of copulation with males increased the likelihood of prolonged luteal activity. However, females were more likely to become pseudopregnant following non-paced mating at 05:00 h than at 21:00 h the previous evening. Of those females receiving an ejaculation during mating, no difference were seen between groups in the incidence of pregnancy. This change in sensitivity to cervical-vaginal stimulation during oestrus was not associated with changes in sexual receptivity or pacing behaviour. The ability of cervical-vaginal stimulation to induce pseudopregnancy therefore increases toward the end of the period of oestrus, but the behavioural mechanisms which regulate receipt of such stimulation remain constant during that time.     

Systemic ICI 182,780 alters the display of sexual behaviors in the female rat

The present study investigates the effects of the antiestrogen ICI 182,780 (ICI) on the display of sexual behaviors in female rats. ICI 182,780 is a pure anti-estrogen and when given systemically, ICI is thought to act only in the periphery, and is not believed to cross the blood brain barrier. The present study examines the effects of ICI on sexual receptivity and on paced mating behavior following treatment with estradiol benzoate (EB) and progesterone (P) (Experiment 1) or with EB alone (Experiment 2). In Experiment 1, ICI (250.0 microg) did not affect the display of receptivity or paced mating behavior induced by EB and P. In contrast, in Experiment 2 female rats receiving EB alone displayed a decrease in the level of sexual receptivity following treatment with 500.0 and 750.0 microg ICI (but not 250.0 microg ICI). In addition, in Experiment 2 EB-treated female rats receiving 250.0 microg ICI spent more time away from the male rat following an intromission and were more likely to exit from the male compartment following a mount. Last, ICI had potent antiestrogenic effects on vaginal cytology (Experiment 2) and on the uterus (Experiments 1 and 2). The present study supports a role for peripheral estrogen receptors in sexual receptivity and paced mating behavior and suggests that estrogen receptor activation may decrease the aversive sensation associated with sexual stimulation.     

Seminal fluids mediate sexual inhibition and short copula duration in mated female Queensland fruit flies

Molecules in male seminal fluid transferred to female insects during mating can have potent effects on their subsequent sexual and reproductive behaviour. Like many other tephritids, female Queensland fruit flies (Bactrocera tryoni) typically have diminished sexual receptivity after their first mating. Also, copulations of females that do remate tend to be shorter than those of virgins. We here find that virgin females injected with small doses (0.1, 0.2 or 0.5 male equivalents) of extracts from the male reproductive tract accessory tissues, which consist of male accessory glands, ejaculatory apodeme and ejaculatory duct (AG/EA/ED), have diminished receptivity and short copula duration very similar to naturally mated females. In contrast, virgin females injected with saline or with high doses of AG/EA/ED (1 or 2 male equivalents) that likely exceed the range of natural variation retain the higher levels of sexual receptivity and longer copulations of un-injected virgins. We conclude that reduced sexual receptivity and shorter copulations of mated female Q-flies are mediated by products in the male seminal fluid derived from the male reproductive tract accessory tissues.     

The reversible inhibition of steroid-induced sexual behavior by intracranial cycloheximide

Cycloheximide(Cyclo), an inhibitor of protein synthesis by a direct action on protein synthesis at the ribosomal level, was used to reversibly inhibit estrogen-induced sexual receptivity. Cyclo (100 μg per rat) was infused into the preoptic area(POA) of ovariectomized rats at varying times before, simultaneously with, and after 3 μg of subcutaneous estradiol benzoate (EB). All animals received 0.5 mg progesterone (P) 36 hr after EB, and were tested for sexual receptivity 4–6 hr after P. The females were placed with stud males and a lordosis quotient was computed for each female (lordosis quotient = number of lordosis responses/20 mounts by the male × 100). Females receiving Cyclo 6 hr before, simultaneously with, or 12 hr after EB showed significantly lower levels of sexual receptivity when compared to females receiving Cyclo 36 hr before and 18 and 24 hr after EB. When those animals that showed low levels of sexual behavior after Cyclo infusion were reprimed with EB and P 7 days later and presented with a male they showed high levels of sexual receptivity. Thus, the effect of Cyclo was reversible. Only Cyclo infusions into the POA (bilateral) and third ventricle were effective in suppressing sexual behavior. Caudate nucleus, lateral ventricle, and unilateral POA infusions were without effect.The data presented are in agreement with earlier work that utilized actinomycin D to inhibit steroid-induced sexual behavior. Cyclo was found to be less toxic than actinomycin D. All of the available evidence is consistent with the hypothesis that estrogen stimulates RNA and/or protein synthesis in its facilitation of sexual behavior in the female rat.     

Role of septum and preoptic area in regulating masculine and feminine sexual behavior in male rats

The effects of septal or preoptic lesions on both masculine and feminine sexual behaviors were examined in castrated adult male rats. Three weeks after brain surgery, animals were implanted with Silastic tubes containing testosterone (T) and observations of masculine sexual behavior were carried out four times every 5 days. T tubes were removed immediately after the end of the masculine behavioral tests. Two weeks later, animals implanted with Silastic tubes containing estradiol-17 beta(E2) were subjected to three feminine sexual behavioral tests at 5-day intervals. The bilateral lateral septal lesion (LSL) and the medial preoptic lesion (MPOL) effectively suppressed the performance of mounts, intromissions, and ejaculations, whereas the medial septal lesion (MSL), the dorsolateral preoptic lesion (DPOL), and the sham operation did not show any significant suppression of these behaviors. In the feminine sexual behavioral tests, intact and sham-operated control males showed only a low lordotic activity. However, the performance of the lordosis reflex was markedly facilitated by LSL or DPOL, while the lordotic activity of MSL and MPOL males was not significantly different from that of control males. These results suggest that the lateral septum exerts not only a facilitatory influence on masculine sexual behavior but also an inhibitory influence on feminine sexual behavior in male rats. On the other hand, the medial preoptic area may play a critical role in regulating masculine sexual behavior in male rats.     

Remating by female Mediterranean fruit flies (Ceratitis capitata, Diptera: Tephritidae): Temporal patterns and modulation by male condition

We determined the temporal pattern of female remating in the Mediterranean fruit fly, Ceratitis capitata, and how mating with sterile males affects remating. In addition, we examined the hypotheses that sterile male nutrition and age affect the subsequent receptivity of their mates. Temporally, female receptivity varied significantly throughout the experimental period. Relatively high levels of remating (14%) on the days following the first copulation were followed by a decline, with a significantly low point (4.1%) 2 weeks after mating. Subsequently, receptivity is gradually restored (18%) 3 and 4 weeks after the initial copulation. When females were first mated to sterile males, significantly higher remating percentages were recorded. The ability of sterile males to inhibit receptivity of both wild and laboratory reared females on the day of first mating was significantly improved when they were fed a nutrient rich diet. Male age at first mating also affected female receptivity: sterile males of intermediate age (11 days old) inhibited female remating significantly more than younger or older flies. Although further studies are needed to determine the relative roles of natural and sexual selection in modulating patterns of female sexual receptivity, the Sterile Insect Technique may be improved by releasing well nourished, older sterile males.     

Conjugated estradiol increases female sexual receptivity in response to oxytocin infused into the medial preoptic area and medial basal hypothalamus

The ovarian steroid estradiol (E) has been found to increase both receptor affinity and release of the neuropeptide oxytocin (OT) in plasma membrane preparations. Therefore, we hypothesized that E conjugated to bovine serum albumin at position 6 (E-6-BSA) would increase behavioral responsiveness to OT. Preliminary results showed that 200 ng/microl of E-6-BSA increased sexual receptivity slightly, but not significantly. Therefore, this dose was used as a subthreshold dose to test whether it would increase sexual responsiveness when infused in combination with 100 ng/microl OT. After recovery from cannula implantation surgery animals were injected with 0.5 microg E benzoate daily for 3 days before testing. On the fourth day, after a baseline preinfusion test rats were infused bilaterally with E-6-BSA alone or with OT, OT with BSA, or conjugated progesterone, luteinizing hormone-releasing hormone equimolar to OT alone, or with E-6-BSA or conjugated progesterone alone. When infused into either the medial preoptic area-anterior hypothalamus or the medial basal hypothalamus the combination of OT and E-6-BSA significantly increased sexual receptivity over receptivity after artificial cerebrospinal fluid control infusions. Neither bilateral infusions of OT in combination with conjugated progesterone nor E-6-BSA in combination with luteinizing hormone-releasing hormone enhanced sexual receptivity. Results presented here strongly support the conclusion that some of the effects that E has in sensitizing brain systems to the facilitating effects of OT occur at the membrane level in the medial preoptic area-anterior hypothalamus and medial basal hypothalamus.