Hormones of choice: the neuroendocrinology of partner preference in animals.

Partner preference behavior can be viewed as the outcome of a set of hierarchical choices made by an individual in anticipation of mating. The first choice involves approaching a conspecific verses an individual of another species. As a rule, a conspecific is picked as a mating partner, but early life experiences can alter that outcome. Within a species, an animal then has the choice between a member of the same sex or the opposite sex. The final choice is for a specific individual. This review will focus on the middle choice, the decision to mate with either a male or a female. Available data from rats, mice, and ferrets point to the importance of perinatal exposure to steroid hormones in the development of partner preferences, as well as the importance of activational effects in adulthood. However, the particular effects of this hormone exposure show species differences in both the specific steroid hormone responsible for the organization of behavior and the developmental period when it has its effect. Where these hormones have an effect in the brain is mostly unknown, but regions involved in olfaction and sexual behavior, as well as sexually dimorphic regions, seem to play a role. One limitation of the literature base is that many mate or 'partner preference studies' rely on preference for a specific stimulus (usually olfaction) but do not include an analysis of the relation, if any, that stimulus has to the choice of a particular sexual partner. A second limitation has been the almost total lack of attention to the type of behavior that is shown by the choosing animal once a 'partner' has been chosen, specifically, if the individual plays a mating role typical of its own sex or the opposite sex. Additional paradigms that address these questions are needed for better understanding of partner preferences in rodents.

Mother counts: how effects of environmental contaminants on maternal care could affect the offspring and future generations.

Various compounds of anthropogenic origin represent environmental contaminants (EC) that penetrate the food chain and are frequently detected in human milk and maternal blood at the time of delivery. These ECs can affect the development of the fetus and can be transferred to the newborn during lactation. Many studies have used animal models to study the impact of ECs on the development of the nervous system and have reported effects of early exposure on neural and neuroendocrine systems and on behavior, when the exposed animals are tested as adults. Some of these effects persist across generations and may involve epigenetic mechanisms. The majority of these studies in developmental toxicology treat the pregnant or lactating animal with ECs in order to deliver the contaminants to the developing offspring. Almost universally, the mother is viewed as a passive conduit for the ECs, and maternal behavior is rarely assessed. Here we review the literature on the effects of ECs on maternal care and find mounting evidence that important components of the care given to the offspring are affected by maternal exposure to different ECs. Some of these changes in maternal behavior appear to be secondary to changes in the behavior and/or stimulus properties of the exposed offspring, but others are likely to be direct effects of the ECs on the maternal nervous and endocrine systems. Considering the extent to which the quality of maternal care affects the development of the offspring, it becomes imperative to determine the contributions that changes in maternal behavior make to the deficits traditionally ascribed solely to direct effects of ECs on the developing organism. Given the complexity and importance of mother-infant interactions, future research on developmental toxicology must consider the effects of ECs not only on the offspring, but also on the mother and on the interactions and social bond between mother and infant.

Exogenous androgen during development alters adult partner preference and mating behavior in gonadally intact male rats.

In the rat, neonatal administration of testosterone propionate to a castrated male causes masculinization of behavior. However, if an intact male is treated neonatally with testosterone (hyper-androgen condition), male sexual behavior in adulthood is disrupted. There is a possibility that the hyper-androgen treatment is suppressing male sexual behavior by altering the male's partner preference and thereby reducing his motivation to approach the female. If so, this would suggest that exposure to supra-physiological levels of androgen during development may result in the development of male-oriented partner preference in the male. To test this idea, male rats were treated either postnatally or prenatally with testosterone, and partner preference and sexual behavior were examined in adulthood. The principal finding of this study was that increased levels of testosterone during early postnatal life, but not prenatal, decreased male sexual behavior and increased the amount of time a male spent with a stimulus male, without affecting the amount of time spent with a stimulus female during partner preference tests. Thus, the reduction in male sexual behavior produced by early exposure to high levels of testosterone is not likely due to a reduction in the male's motivation to approach a receptive female.

Estrogen treatment during development alters adult partner preference and reproductive behavior in female laboratory rats.

There is broad acceptance for the idea that during development estradiol 'organizes' many aspects of reproductive behavior including partner preferences in the laboratory rat. With respect to partner preference, this idea is drawn from studies where estrogen action was in someway blocked, either through aromatase or estrogen receptor inhibition, during development in male rats. The lack of estrogens neonatally results in a decrease in the male rat's preference for females. In this study, the effect of early postnatal estradiol treatment on the partner preferences of female rats was examined as a further test of the hypothesis that male-typical partner preference is dependent upon early exposure to estrogens. Our principal finding was that increased postnatal estradiol exposure during development affected partner preference in the expected direction, and this effect was seen under several adult hormonal and behavioral testing conditions. Female rats that received exogenous estradiol during development spent more time with an estrous female and less time with a sexually active male than did cholesterol treated females. The estradiol treatment also disrupted normal female sexual behavior, receptivity, and proceptivity.

Exposure to PCB 77 affects partner preference but not sexual behavior in the female rat.

In rats, exposure to the polychlorinated biphenyl congener 3, 4, 3', 4'-tetrachlorobiphenyl (PCB 77) affects the brain and behavior of the offspring as well as the maternal behavior of the dams. In the present study, a cross-fostering design was used to examine the effects of pre- and/or postnatal exposure to PCB 77 on sexual behavior and partner preference in female rats, and to determine the role of altered maternal behavior in the mediation of these effects. Pregnant rats were treated with oil or PCB dissolved in oil (2 mg/kg b.w.) on gestation days 6-18 and then given pups that had been exposed to either the oil vehicle or PCB during gestation. As adults, the female offspring were tested for partner preference (that is, whether they preferred to spend time with a sexually receptive female or a sexually active male) and sexual behavior. None of the treatments affected female sexual behavior. However, both double exposure and postnatal exposure diminished the animals' preference for a male over a female stimulus, but partner preference was not affected by prenatal exposure alone. There were no significant correlations between the changes in partner preferences due to PCB exposure and the amount of maternal grooming and licking received by the treated litters. Thus, female partner preference is affected by early PCB exposure, and the effects depend upon whether the exposure is in utero or via lactation and may be independent of any effects of the PCB on maternal care.

A cross-fostering analysis of the effects of PCB 77 on the maternal behavior of rats.

Polychlorinated biphenyls (PCBs) are environmental contaminants known to cause multiple behavioral and developmental problems in humans and animals. In rats, gestational exposure to the PCB congener 3, 4, 3', 4'-tetrachlorobiphenyl (PCB 77) affects the brain and behavior of the offspring as well as the maternal behavior of the dams. Whether the behavior of dams is affected by direct effects of the contaminant or indirectly by actions of the PCB on the developing offspring is not known. We investigated this question using a cross-fostering paradigm in which pregnant rats were exposed to either oil vehicle or 2 mg/kg of PCB 77 on gestational days 6 through 18, and then raised pups that had been exposed to either oil or PCB 77 during gestation. Maternal behavior was observed on postnatal days 1, 2, 4 and 6. Some of the effects on maternal behavior, including an increase in the frequency of nursing bouts and in the amount of maternal autogrooming, can be ascribed to prenatal exposure of the litters to the PCB. Other behavioral effects, including an increase in time on the nest and in the amount of pup grooming as well as a reduction in high-crouch nursing, appear to be due to both direct effects of the PCB on the dams and effects mediated by changes in the offspring. Our results show that exposure to PCB 77 can have complex effects on the behavioral interactions between the dams and their litters with a potential impact on the development of the offspring.

Exposure to PCB 77 affects the maternal behavior of rats.

Polychlorinated biphenyls are environmental contaminants known to affect neurobehavioral development in many laboratory studies using different animal models. Because of their bioaccumulation and long half-life they are a serious concern for our own species. The dioxin-like PCB congener 3,4,3',4'-tetrachlorobiphenyl (PCB 77) has estrogenic and anti-estrogenic properties, and has been shown to affect brain chemistry and behavior of developing rats when administered during gestation. Since many developmental outcomes in mammals depend upon the type of maternal care provided by the dams, we investigated the effects of two doses of PCB 77 (2 and 4 mg/kg administered during gestational days 6-18) on the maternal behavior of the treated dams. Both doses of PCB 77 reduced the amount of nursing time in which the dams displayed the high-crouch posture over postnatal days 1-6. In addition, the high dose increased the amount of maternal licking and grooming of the litters and the amount of time the dams spent on the nest. The high dose also increased pup mortality, and both doses reduced the weight gain of the litters during the first 6 days of life. These results document effects of PCB 77 on maternal behavior and serve to raise questions about the importance of maternal contributions to the developmental effects of this and similar contaminants.

Developmental exposure to polychlorinated biphenyls affects sexual behavior of rats.

Polychlorinated biphenyls (PCBs) are persistent environmental contaminants that have the potential to disrupt reproduction through a variety of different pathways. In the present study, we investigated the effects of fetal and lactational PCB exposure on reproductive behavior in male and female laboratory rats. These pregnant rats were injected daily with either 2,4,2',4'-tetrachlorobiphenyl (PCB 47) at the dosage of 1 or 20 mg/kg body weight or 3,4,3',4'-tetrachlorobiphenyl (PCB 77) at the dosage of 0.25 or 1 mg/kg body weight or sesame oil (control group) from gestational days 7 to 18. Offspring were then tested for sexual behavior as adults. Exposure to both PCB 77 and PCB 47 reduced the level of sexual receptivity in the female offspring, but had no detectable effects on the sexual behavior of the male offspring. In addition to changes in adult sexual behavior in the females, both PCBs produced a significant increase in the females' anogenital distance, suggesting a modification of androgen responsiveness in females resulting from PCB exposure during development. Similar effects were not seen with the males.

Effects of neonatal polychlorinated biphenyl exposure on female sexual behavior.

The effects of polychlorinated biphenyls (PCBs) on development and reproduction are well documented. However, very little is known about the effects of PCBs on sexual behavior. In this study, we examined the effects of two commercial PCB mixtures, Aroclor 1221 (A1221) and Aroclor 1254 (A1254), on the development of female sexual behavior and of the incertohypothalamic dopaminergic cells (A11 and A13) in Long-Evans rats. Neonatal exposure to A1254 significantly reduced sexual receptivity and reduced the female's latency to approach a male after an intromission. Neonatal treatment with A1221 did not affect female sexual behavior nor did treatment of adult females with A1221 or A1254. Since sexual behavior is affected by dopamine and since PCBs have been reported to alter dopamine content in the brain, we examined the effects of A1221 or A1254 on dopaminergic cells in the incertohypothalamic region of neonatally exposed rats. None of the treatments significantly affected the number of A11 or A13 neurons that were immunoreactive for tyrosine hydroxylase (TH) or the expression of Fos (i.e., the product of the immediate early gene c-fos) in these dopaminergic neurons. Therefore, the disruption of behavior induced by neonatal treatment with A1254 does not appear to be mediated by toxic effects of the mixture on incertohypothalamic dopaminergic systems.

Effects of bisphenol A on energy balance and accumulation in brown adipose tissue in rats.

Some environmental contaminants have the potential to affect humans or animals by mimicking the effects of hormones. Bisphenol A (BPA) is a weak estrogen agonist when tested using in vitro or in vivo bioassays. In addition to the well documented effects of estrogens on reproductive functions, ovarian hormones also have salient effects on mammalian energy balance and feeding behavior. In this study, we investigated the effects of BPA on body weight and food intake of ovariectomized adult female rats. Treatment with doses of 4 or 5 mg/day for 15 days resulted in a significant reduction of body weight gain with no reduction in food intake. A dose of 1 mg/day did not affect feeding or weight gain. BPA was detected in the blood, brain and adipose tissues of the BPA-treated animals but not in the vehicle control group. There was a preferential concentration of BPA in brown adipose tissue. These results indicate that BPA can affect energy balance and that brown adipose tissue may be a primary tissue into which BPA accumulates in mammals.

Relation of Fos-IR expression in the pelvic ganglion to sexual behavior in laboratory rats.

The pelvic ganglion (PG) provides both sympathetic and parasympathetic innervation to the genitalia and other pelvic structures. To determine whether neuronal activity; of the PG, as detected by Fos-like immunoreactivity (Fos-IR), is related to sexual stimulation, male and female rats were tested under a variety of conditions. In males, Fos-IR expression in the PG was positively correlated with the amount of both genital and noncontact stimulation. In females, only ejaculation preceded by multiple intromissions induced a significant increase in Fos-IR; multiple intromissions or ejaculation preceded by only 0-1 intromission did not affect Fos-IR. Additional experiments comparing Fos-IR expression, in which some females were allowed to pace their sexual contact and others were not, revealed that ejaculation duration was the key factor in the induction of Fos-IR in female rats. Because the conditions under which Fos-IR expression occurred in females are identical to those required for sperm transport, we suggest that, in the female, sperm transport is regulated in part by autonomic outflow from the PG after copulation. These relations between sexual behavior and measures of PG activity are consistent with the idea that the sexually dimorphic organization of the peripheral nervous system plays a major role in mediating the gender-specific outcome of copulation: ejaculation in the male and sperm transport in the female.

Vaginocervical stimulation inhibits female-female mounting in laboratory rats.

The objective of this study was to examine how vaginocervical stimulation (VCS) affects female-female mounting in laboratory rats. After receiving sexual stimulation from the male, the latency for the female to mount another female was significantly longer than that of control females. In the absence of any copulatory stimulation, the latency to initiate mounting of another female was about 3 min. However, following three mounts or three intromissions by a male, the latency for the experimental female to initiate mounting increased to about 10 min, and ejaculation abolished mounting for almost 2 h. Once females began mounting, regardless of the copulatory stimulation they received prior to testing, their mounting rate (mounting frequency/2 h) did not differ from stimulus control females. Artificial VCS also inhibited female mounting and anesthetization of the vaginocervical area diminished the inhibiting effect of ejaculation. Taken together, the present results provide evidence that VCS can temporarily inhibit female mounting, and that the duration of the inhibition is related to the amount of VCS received. These data are interpreted within the perspective that female mounting behavior is not a sexual behavior and is consequently suppressed within the context of normal copulation.

Influence of male-related stimuli on female postejaculatory refractory period in rats.

Female rats "pace" their sexual contacts with the male when tested in situations where they can escape from the male during copulation. The type and quality of vaginocervical stimulation that the females receive during copulation influences their pacing behavior. This study investigated the effect of several male-related stimuli on the female's postejaculatory refractory period (PER). Females were tested in a two-compartment test chamber from which they could escape the male through one of four openings along the bottom of the barrier separating the two compartments. Experiment 1 examined the influence of the seminal plug, the penile cup, and prostate secretions on the female's PER. Results showed that neither the seminal plug, the penile cup, nor prostate secretions contributed to the female's PER. Experiment 2 investigated the relation of pre-ejaculatory intromission frequency, ejaculation duration, and the number of pelvic thrusts during ejaculation to the female's PER. Results indicated that pre-ejaculatory intromission frequency and ejaculation duration but not the number of pelvic thrusts during ejaculation were significantly correlated with the female's PER. In addition, pre-ejaculatory intromission frequency was significantly correlated with ejaculation duration. Partial correlation analysis suggested that pre-ejaculatory intromission frequency affected ejaculation duration which, in turn, influenced the female's PER. This finding was further supported by the evidence that ejaculation duration and the female's PER were significantly shorter in tests in which the male ejaculated on the first or second intromission.

Effects of paraventricular lesions on sex behavior and seminal emission in male rats.

Oxytocinergic neurons of the paraventricular nucleus (PVN) of the hypothalamus have been implicated in modulating male sexual responses in rats. Previous investigators have shown that cerebrospinal fluid concentrations of oxytocin (OT) increased after ejaculation and that intraventricular administration of OT and electrolytic lesions of the PVN increased temporal measures of male sexual behavior. Recently, we have demonstrated that OT-immunoreactive neurons in the parvocellular subnuclei of the PVN project to lower levels of spinal cord. In the present study, N-methyl-D-aspartic acid lesions, which have been shown to destroy parvocellular PVN neurons while leaving magnocellular neurons intact, were used to evaluate the role of parvocellular neurons on male copulatory behavior and seminal emissions. OT-immunoreactive fibers were reduced in the lower lumbar spinal cord (L5-L6) following N-methyl-D-aspartic acid lesions in the PVN. This reduction was associated with a significant decrease in seminal emission at the time of ejaculation, but mount, intromission and ejaculatory latencies were unaffected.

Function of intromissions on intromission-return latency of female rats during paced sexual behavior.

The objectives of this study were to examine how multiple intromissions affect the temporal pattern of the female rat's copulatory behavior; in particular, her latency to return to the male following intromission (intromission-return latency, IRL) and if different hormone replacement regimens affect the temporal aspects of female copulatory behavior. Repeated intromissions alone, without ejaculation, often resulted in prolonged IRLs equal to the postej aculatory refractory period (PER). The first prolonged IRL occurred most frequently between the 24th and 44th intromission. The similar pattern of IRLs around the PER and the prolonged IRLs may indicate that the mechanisms mediating the occurrence of the prolonged IRL are similar to those for the PER. One possible function of the prolonged IRLs may be to facilitate the male's ejaculation after the female has received enough vaginocervical stimulation for the induction of the progestational state of pregnancy. Finally, females receiving a single dose of 50 microg estradiol benzoate (EB) followed by an injection of 0.5 mg progesterone (P) 48 h later showed a significantly longer PER than those receiving 3 daily injections of 0.5 microg EB followed by an injection of 0.5 mg P 24 h after the last EB injection.

Relation of intromissions to the female's postejaculatory refractory period in rats.

The objectives of this study were to investigate the temporal aspects of female sexual behavior during single and multiple ejaculatory tests. Females were tested in a two-compartment chamber where they could escape from the male following sexual contacts. In Experiment 1, correlation analysis showed that the number of intromissions received by the female over 3 ejaculatory series was positively correlated with the female's postejaculatory refractory period (PER). In Experiment 2, females receiving 2-4 intromissions before ejaculation had a PER that did not differ from those receiving 5-15 (average 10) intromissions preceding ejaculation. However, if the male ejaculated on the first or second intromission, the female's PER was significantly shorter than the other groups and did not differ from her return latency after an intromission without ejaculation. Females receiving 24-31 intromissions preceding ejaculation exhibited the longest PER. Analysis revealed that the number of intromissions received by females before ejaculation was positively correlated with the female's PER. We concluded that the male's ejaculatory reflex, seminal emission, and postejaculatory behavior alone without at least 2 preceding intromissions were not sufficient to induce a female's PER comparable to that seen after an ejaculation during normal copulation. In addition, the number of intromissions received by the female preceding ejaculation was positively correlated with the female's PER if the range of intromission frequency was large enough.

Levels of serum steroids, aromatase activity, and estrogen receptors in preoptic area, hypothalamus, and amygdala of B6D2F1 male house mice that differ in the display of copulatory behavior after castration.

Most male B6D2F1 hybrid house mice continue to copulate after castration (continuers), whereas others do not (noncontinuers). Copulation in continuers appears estrogen dependent. Serum testosterone (T), 17 beta-estradiol (E2), and dihydrotestosterone (DHT), as well as aromatase activity (AA) and estrogen receptor (ER) levels in preoptic area (POA), hypothalamus (HYP), and amygdala (AM) were measured to determine if continuers and noncontinuers differ in estrogen physiology. In general, continuers and noncontinuers did not differ in serum steroid levels, AA, or ER levels. Castration reduced AA in the POA, HYP, and AM. Castration did not affect nuclear ER levels in the POA and HYP but reduced nuclear ER in AM. The data demonstrate that castrated B6D2F1 male mice continue to be under the influence of circulating nongonadal E2 that is important for copulation.

Calcitonin gene-related peptide-like immunoreactivity in spinal motoneurons of the male mouse is affected by castration and genotype.

Calcitonin gene-related peptide (CGRP) is found in motoneurons of the mammalian spinal cord, including motoneurons of the androgen-dependent spinal nucleus of the bulbocavernosus (SNB) of the mouse. Motoneurons of the SNB innervate the bulbocavernosus (BC), a striated muscle involved in penile reflexes. CGRP is though to be a trophic factor produced by motoneurons to regulate the expression of the acetylcholine receptor at the neuromuscular junction. In rats, the number of SNB motoneurons containing CGRP is increased by gonadal steroids. This regulation appears to rely on an activity-dependent factor produced by the BC muscle. The purpose of the present study was to examine, using immunohistochemistry, the steroid dependence of CGRP in the SNB of male house mice. Genotypic differences in the steroid regulation of CGRP immunoreactivity were examined in three strains of mice that differ in their behavioral sensitivity to castration. The results demonstrate that castration reduces the number of CGRP-positive SNB motoneurons in mice. The magnitude of the change in CGRP in response to castration and the length of time required following castration to alter CGRP were dependent on genotype. Interestingly, the effect of castration in mice, to reduce the number of CGRP-immunoreactive SNB motoneurons, is opposite in direction from the increase in CGRP SNB motoneurons observed in rats observed following castration. These experiments suggest that androgens may alter neuromuscular junction function of mouse SNB by regulating the production of CGRP in a species-specific, genotypically dependent fashion.

Neurophysin-containing pathway from the paraventricular nucleus of the hypothalamus to a sexually dimorphic motor nucleus in lumbar spinal cord.

A model that has been widely used in the study of steroid sensitive neurons, the spinal nucleus of the bulbocavernosus (SNB) is a sexually dimorphic motor nucleus in the lower lumbar spinal cord that innervates the striated bulbocavernosus (BC) muscle. The BC is responsible for penile reflexes in the male rat, which are important in ensuring pregnancy in females. The characterization of afferents to the SNB aids in the understanding of the neural circuitry involved in reproductive behavior. We have recently identified the paraventricular nucleus (PVN) as a possible source of afferents to the SNB. Because the PVN is the major source of oxytocin/vasopressin within the central nervous system (CNS), the purpose of the present study was to examine and characterize a neurophysin (NP)-containing pathway from the PVN to the SNB. The results demonstrate that neurons of the lateral parvicellular subnucleus of PVN, which project to levels of spinal cord containing SNB motoneurons, contain NP, the coproduct of oxytocin and vasopressin. NP-containing fibers and putative terminals were found in the region of the SNB and appear to contact the soma and proximal dendrites of SNB motoneurons which were retrogradely identified as BC-innervating. Electrolytic lesions, which destroy the lateral parvicellular subnucleus of PVN, abolish NP-containing fibers in the region of the SNB, suggesting that the PVN is the source of these NP fibers. The results of this study indicate a NP-containing projection from the hypothalamus directly to SNB motoneurons. It is suggested that this pathway may play a role in the integration of penile reflexes with other aspects of male copulatory behavior that are under hypothalamic control.