Estrogen in the medial preoptic area of male rats facilitates copulatory behavior

Mating was studied in sexually experienced, gonadally intact male rats assigned to two surgical groups matched on the basis of mean mounting frequency during behavioral screening trials conducted prior to the study. Estradiol (E(2)) was delivered bilaterally into the medial preoptic area (MPO) of experimental males by means of hormone-coated implants, and fadrozole was given sc (0.25 mg/kg/day) via osmotic minipumps to block E(2) formation from testicular testosterone throughout the brain. Control males received blank bilateral implants in the MPO and sc fadrozole. After the completion of behavioral testing, immunocytochemical comparisons of the brains from experimental and control rats were made using the H222 antiestrogen receptor (ER) antibody, whose labeling is inhibited by the presence of E(2). The histology demonstrated that E(2) was confined exclusively to the MPO of experimental males but was absent throughout the brains of controls. In controls, mounting decreased significantly by the 7th day after surgery compared with presurgical levels and did not recover. In contrast, on all postsurgical days, the mounting frequency of the experimental group was significantly higher than that of controls. Although experimental males also showed an initial, significant postsurgical decline in mounting frequency, it recovered completely by the 28th postoperative day. Ejaculations declined significantly after surgery in both groups but, unlike in controls whose performance remained low, ejaculations in experimental males partially recovered and were significantly higher than in controls during the postoperative period. Results showed that ER-containing neurons in the MPO influence male rat copulatory behavior.     

Testosterone restoration of copulatory behavior correlates with medial preoptic dopamine release in castrated male rats

The medial preoptic area (MPOA) is an important integrative site for male sexual behavior. We have reported an increase in dopamine (DA) release in the MPOA of male rats shortly before and during copulation. Postcastration loss of copulatory ability mirrored the loss of the precopulatory DA response to an estrous female. The present study investigated the time courses of restoration, rather than loss, of the MPOA DA response to a receptive female and of copulation in long-term castrates. Male rats were castrated and tested for loss of copulatory ability 21 days later. They then received 2, 5, or 10 daily subcutaneous injections of testosterone propionate (TP, 500 microg) or oil. Microdialysate samples were collected from the MPOA during baseline, exposure to a female behind a barrier, and copulation. Extracellular DA was measured using HPLC-EC. None of the six 2-day-TP-treated animals copulated, nor did they show elevated DA release in the MPOA in the presence of a receptive female. Five of the nine 5-day-TP-treated animals ejaculated; three intromitted without ejaculating; and one failed to copulate, with all but the noncopulating animal showing elevated DA release. All of the six 10-day-TP-treated animals copulated and also demonstrated an increase in MPOA DA. None of the oil controls copulated or showed an increase in DA release. Therefore, a consistent relationship between MPOA DA release during exposure to a receptive female and the subsequent ability of the male to copulate was observed.     

Sterility and delayed vaginal opening following electrolytic lesions placed neonatally in medial preoptic area in female rats.

Electrolytic lesions were placed in the preoptic area or in the middle and posterior hypothalamus of female rats within 48 hrs after birth. Vaginal opening was delayed significantly in rats having the medial preoptic area (mPOA) destructed as compared to sham-operated animals and those given lesions in the other parts of the hypothalamus. Only rats with the mPOA completely destructed developed the anovulatory syndrome. In these animals, unilateral ovariectomy was not followed by enlargement of its fellow. The present results are not in agreement with those of previous workers that electrolytic destructions of the diencephalon induced precocious puberty. The difference may be ascribable to difference in size and locus of lesions.     

The effect of medial preoptic area lesions on sexually stimulated hormone release in the male rat

Male rats were subjected to bilateral electrolytic lesions in the medial preoptic area (mPOA). These lesions disrupted sexual behavior without affecting basal levels of luteinizing hormone (LH), prolactin (PRL), or testosterone (T). During exposure to an estrous female, intact and sham-operated rats mated; these rats showed elevations in LH, PRL, and T levels. Lesioned rats, which did not mate, showed elevations in LH but not PRL or T levels. These results demonstrate that the mPOA is not required for sexually stimulated LH release. The failure of lesioned rats to release PRL and T may be secondary to their failure to mate. Alternatively, the mPOA may participate in sexually stimulated PRL release, while T release may depend on prior elevations in both LH and PRL levels. LH release may be related to arousal, and PRL release to consummation, providing a hormonal analogy for the dual mechanism theory of sexual behavior.     

Loss of calcineurin from the medial preoptic area of primiparous rats

Western blot analyses reveal that calcineurin A (CNA), which is present in the hippocampus, basolateral amygdala, parietal cortex, and MPOA of virgin males and females, is undetectable only in the MPOA of primiparous females regardless of whether they had postpartum pup contact or not. In contrast, CNB was expressed at unchanging levels in the PC and MPOA. Similarly, G(alphao) and PKA(RI) were expressed at high levels in all of the brain regions of virgin males, virgin females, and primiparous females, supporting the concept that this loss of CNA is a specific event. Understanding how and why the expression of CNA, the sole neuronal Ca2+/CaM-dependent protein phosphatase, is down-regulated specifically in the MPOA of primiparous females may yield some insight into the signal transduction events that mediate the onset of mammalian maternal behavior.     

The LHRH neuronal system in female rats: relation to the medial preoptic nucleus

Previously we have found that small lesions confined to the medial preoptic nucleus (MPN) or the suprachiasmatic nucleus (SCN) blocked the cyclic release of gonadotropins in the female rat, inducing a persistent estrous state. Since the MPN is located just caudal to the organum vasculosum of the lamina terminalis (OVLT) where LHRH cell bodies are most concentrated, we applied an immunocytochemical technique to examine the possibility that the lesions had simply disrupted LHRH neurons or fibers. Using a new anti-LHRH provided by Dr. V. D. Ramirez, we found that the distribution pattern of immunoreactive LHRH cell bodies and fibers was similar to that previously reported, although the staining was more intense and extensive with low background. There was no concentration of LHRH cell bodies and fibers in the MPN or SCN and, in fact, these nuclei generally showed a lower density of stained elements than did surrounding tissue. In persistent estrous animals with lesions confined to the MPN there was no detectable reduction of stained fibers in the median eminence. These results, along with the results of other workers, suggest that persistent estrus following lesions of the MPN or SCN is not due to reduction of LHRH neurons or fibers. Rather, they support the hypothesis that these nuclei are critical for triggering the ovulatory release of LHRH.     

Decline of sexual behavior in castrated male rats: Effects of female precopulatory behavior

From the population of 89 adult sexually inexperienced Wistar male rats 20 animals that initiated copulatory behavior with females exhibiting low intensity of precopulatory behavior (presenting females) were preselected. Prior to castration all 20 males had the same sexual experience: three ejaculatory series in four weekly sessions with females exhibiting high intensity of precopulatory behavior (darting females). Following castration, the decline of copulatory behavior was much slower for the nine males tested with darting females as compared to the 11 males tested with presenting females. Male precopulatory behaviors (anogenital sniffing, touching flanks, etc) outlasted the loss of copulatory behavior and seem to be less dependent on both external and internal determinants. It is concluded that intensive external sexual stimuli can function to compensate, and therefore mask, the subnormal operation of androgen-dependent mechanisms in initiating the copulatory behavior.     

Age-related changes in sexual function and steroid-hormone receptors in the medial preoptic area of male rats

Testosterone is the main circulating steroid hormone in males, and acts to facilitate sexual behavior via both reduction to dihydrotestosterone (DHT) and aromatization to estradiol. The mPOA is a key site involved in mediating actions of androgens and estrogens in the control of masculine sexual behavior, but the respective roles of these hormones is not fully understood. As males age they show impairments in sexual function, and a decreased facilitation of behavior by steroid hormones compared to younger animals. We hypothesized that an anatomical substrate for these behavioral changes is a decline in expression and/or activation of hormone receptor-sensitive cells in the mPOA. We tested this by quantifying and comparing numbers of AR- and ERα-containing cells, and Fos as a marker of activated neurons, in the mPOA of mature (4–5 months) and aged (12–13 months) male rats, assessed one hour after copulation to one ejaculation. Numbers of AR- and ERα cells did not change with age or after sex, but the percentage of AR- and ERα-cells that co-expressed Fos were significantly up-regulated by sex, independent of age. Age effects were found for the percentage of Fos cells that co-expressed ERα (up-regulated in the central mPOA) and the percentage of Fos cells co-expressing AR in the posterior mPOA. Interestingly, serum estradiol concentrations positively correlated with intromission latency in aged but not mature animals. These data show that the aging male brain continues to have high expression and activation of both AR and ERα in the mPOA with copulation, raising the possibility that differences in relationships between hormones, behavior, and neural activation may underlie some age-related impairments.     

Mu opioid receptors in the medial preoptic area govern social play behavior in adolescent male rats

Neural systems underlying important behaviors are usually highly conserved across species. The medial preoptic area (MPOA) has been demonstrated to play a crucial role in reward associated with affiliative, nonsexual, social communication in songbirds. However, the role of MPOA in affiliative, rewarding social behaviors (eg, social play behavior) in mammals remains largely unknown. Here we applied our insights from songbirds to rats to determine whether opioids in the MPOA govern social play behavior in rats. Using an immediate early gene (ie, Egr1, early growth response 1) expression approach, we identified increased numbers of Egr1‐labeled cells in the MPOA after social play in adolescent male rats. We also demonstrated that cells expressing mu opioid receptors (MORs, gene name Oprm1) in the MPOA displayed increased Egr1 expression when adolescent rats were engaged in social play using double immunofluorescence labeling of MOR and Egr1. Furthermore, using short hairpin RNA‐mediated gene silencing we revealed that knockdown of Oprm1 in the MPOA reduced the number of total play bouts and the frequency of pouncing. Last, RNA sequencing differential gene expression analysis identified genes involved in neuronal signaling with altered expression after Oprm1 knockdown, and identified Egr1 as potentially a key modulator for Oprm1 in the regulation of social play behavior. Altogether, these results show that the MPOA is involved in social play behavior in adolescent male rats and support the hypothesis that the MPOA is part of a conserved neural circuit across vertebrates in which opioids act to govern affiliative, intrinsically rewarded social behaviors.     

Role of the medial preoptic area in sexual behaviour of the male rat: a study using repeated cycloheximide infusions

The effect of cycloheximide (CHX), an inhibitor of proteosynthesis, on sexual behaviour was studied in adult male rats in which it was infused into the medial preoptic area of the hypothalamus (MPOA). Sexual interaction took place under control and modification of the precopulatory behaviour of the female. Among the various bilaterally infused amounts of CHX--20 micrograms in 1 microliter, 40 micrograms in 1 microliter, 80 micrograms in 2 microliters--the effect of the largest dose corresponded to a hypothetical state of copulatory readiness of the males; 2 h after administration the males were not capable of initiating copulatory behaviour with a passively receptive female, but did so successfully with a highly soliciting female. The dependence of copulatory readiness of the males on the precopulatory behaviour of the females was confirmed repeatedly after this dose of CHX. At the same time, the precopulatory activity of the males towards a passively receptive female was unimpaired. The effect of CHX was reversible; 48 h after infusion the males displayed high copulatory readiness. Only half the males (n = 7) given bilateral infusions of 80 micrograms CHX fulfilled criterion of copulatory performance. Histological control demonstrated that the MPOA was affected bilaterally by CHX infusion in 10 males out of 14. The results are discussed from the aspect of participation of the MPOA in the regulation of male sexual behaviour. The method allows changes in sexual behaviour to be studied in the same individual in a chronic experiment.     

Effects of lesions in the medial preoptic region on precocial copulation in the male chick

Chicks showing androgen-induced precocial copulation to a prone hand were given bilateral electrolytic lesions in the medial preoptic region. Symmetrical lesions in the medial preoptic area disrupted copulatory behavior over the 10-day postoperative test period. Asymmetrical or sham lesions did not differentially affect copulatory activity. Postoperative records indicated that all animals showed similar increases in weight gain and comb size.     

The effects of nitric oxide-cGMP pathway stimulation on dopamine in the medial preoptic area and copulation in DHT-treated castrated male rats

Dopamine (DA) in the medial preoptic area (MPOA) provides important facilitative influence on male rat copulation. We have shown that the nitric oxide-cGMP (NO-cGMP) pathway modulates MPOA DA levels and copulation. We have also shown that systemic estradiol (E(2)) maintains neuronal NO synthase (nNOS) immunoreactivity in the MPOA of castrates, as well as relatively normal DA levels. This effect of E(2) on nNOS probably accounts for at least some of the previously demonstrated behavioral facilitation by intra-MPOA E(2) administration in castrates. Therefore, we hypothesized that stimulation of the MPOA NO-cGMP pathway in dihydrotestosterone (DHT)-treated castrates should restore DA levels and copulatory behaviors. Reverse-dialysis of a NO donor, sodium nitroprusside (SNP), increased extracellular DA in the MPOA of DHT-treated castrates and restored the ability to copulate to ejaculation in half of the animals. A cGMP analog, 8-Br-cGMP, also increased extracellular DA, though not as robustly, but did not restore copulatory ability. The effectiveness of the NO donor in restoring copulation and MPOA DA levels is consistent with our hypothesis. However, the lack of behavioral effects of 8-Br-cGMP, despite its increase in MPOA DA, suggests that NO may have additional mediators in the MPOA in the regulation of copulation. Furthermore, the suboptimal copulation seen in the NO donor-treated animals suggests the importance of extra-MPOA systems in the regulation of copulation.     

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.     

Effects of testosterone metabolites on copulation and medial preoptic dopamine release in castrated male rats

The medial preoptic area (MPOA) is an important integrative site for male sexual behavior. Dopamine (DA) is released in the MPOA of male rats shortly before and during copulation. The recent presence of testosterone (T) may be necessary for this precopulatory increase in release. Previously, the postcastration loss of copulatory ability mirrored the loss of the DA response to an estrous female, and the restoration of copulation with exogenous T was concurrent with the reemergence of this DA response. The present study investigated the effectiveness of the two major metabolites of T in maintaining copulation and basal and female-stimulated DA levels. Adult male rats were castrated and received daily injections of estradiol benzoate (EB), dihydrotestosterone benzoate (DHTB), EB + DHTB, testosterone propionate (TP), or oil vehicle for 3 weeks. Microdialysis samples were collected from the MPOA during baseline conditions, exposure to an estrous female behind a barrier, and copulation testing. EB + DHTB- and TP-treated animals had normal basal DA levels and showed a precopulatory DA response, and most copulated normally. EB-treated castrates had high basal DA levels, but failed to show a female-stimulated increase; most intromitted, but none ejaculated. DHTB- and oil-treated groups had low basal levels of extracellular DA that did not increase during copulation testing; most failed to mount and none ejaculated. These results suggest that E maintains normal basal levels of extracellular DA in the MPOA, which are sufficient for suboptimal copulation, but that androgen is required for the female-stimulated increase in DA release and for facilitation of ejaculation.     

Short-latency effect of testosterone on copulatory behaviour and ejaculation in sexually experienced intact male rats

The effect of a single injection of testosterone on the number of stimuli required to trigger ejaculation and on copulatory behaviour was investigated in sexually experienced intact male rats. An intraperitoneal injection of 100 microgram free testosterone 1 h before testing facilitated the copulatory behaviour and decreased the number of mounts and intromissions needed for ejaculation; the latter effect was obtained in a test in which a constant interval was kept between each intromission.     

Lesions to the medial preoptic area affect singing in the male European starling (Sturnus vulgaris)

The aromatization of testosterone (T) in the medial preoptic nucleus (POM) is known to regulate male courtship and sexual behaviors expressed prior to, and in anticipation of, copulation. Singing in male European starlings is used to attract mates prior to physical sexual contact, suggesting that the POM might be involved. The present study was performed to examine the effects of lesions targeting the POM on singing and courtship behavior in reproductively active male starlings. A significant decrease in song output and the gathering of green nest materials was observed in males with lesions to the POM compared to males with damage to brain areas outside of the POM. Lesions did not affect a male's tendency to remain near a female or to occupy a nestbox, suggesting that the effects of POM lesions were specific to courtship behaviors. Behavioral differences were not related to testis mass or volume, and GnRH immunoreactivity was observed within the hypothalamus and median eminence for each male, suggesting that the effects of POM lesions were related specifically to POM involvement in song expression rather than to a disruption of the GnRH axis. These results suggest a general role for the POM in the expression of behaviors related to sexual arousal or anticipation, including song.     

Effects of discrete medial preoptic hypothalamic lesions on the androgen gonadotropin feedback system in the male rat

The effect of discrete lesions of the Anterior Medial Preoptic Area of the Hypothalamus (MPOA) in the control of pituitary gonadotropins in the adult male Wistar rat has been studied. Electrolytic lesions were made by passing an anodal current through tungsten electrodes. Electrodes were oriented stereotaxically into the MPOA and lesion placement was histologically checked. In sham controls, electrodes were lowered into the MPOA but no current was applied. Serum LH and FSH were measured by RIA. MPOA lesioned animals showed significantly lower plasma LH (p less than 0.01) in comparison to sham lesioned group. Plasma FSH remained unaltered. To test whether these results were related to an alteration in the negative feedback system, the response to administration of Testosterone Propionate (TP) and the secretion patterns of LH and FSH after castration were analyzed. Administration of TP revealed similar LH and FSH 24 and 48 h decrements and the pattern of LH and FSH secretion after gonadectomy was not significantly different in lesioned and sham lesioned animals. Responsiveness to exogenous LHRH was not impaired by MPOA lesions. The results suggest that neural elements within the MPOA are functionally related to pituitary LH secretion in the male rat. The LH control alteration in lesioned animals is not associated with modifications in negative feedback of androgens and pituitary sensitivity to LHRH remains unaltered.     

Sexual responses of the male rat medial preoptic area and medial amygdala to estrogen I: site specific suppression of estrogen receptor alpha

Male rat copulation is mediated by estrogen-sensitive neurons in the medial preoptic area (MPO) and medial amygdala (MEA); however, the mechanisms through which estradiol (E(2)) acts are not fully understood. We hypothesized that E(2) acts through estrogen receptor α (ERα) in the MPO and MEA to promote male mating behavior. Antisense oligodeoxynucleotides (AS-ODN) complementary to ERα mRNA were bilaterally infused via minipumps into either brain area to block the synthesis of ERα, which we predicted would reduce mating. Western blot analysis and immunocytochemistry revealed a knockdown of ERα expression in each brain region; however, compared to saline controls, males receiving AS-ODN to the MPO showed significant reductions in all components of mating, whereas males receiving AS-ODN to the MEA continued to mate normally. These results suggest that E(2) acts differently in these brain regions to promote the expression of male rat sexual behavior and that ERα in the MPO, but not in the MEA, promotes mating.