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Although women spend their lives in various phases of the reproductive cycle, including menstrual, pregnancy, postpartum, lactation and menopause, few studies have examined immune responses to stress in women as a function of events associated with reproduction. The objective of this study was to evaluate differential effects of breastfeeding (n = 16), bottlefeeding (n = 10) and non-postpartum (n = 10) status on lymphocyte responses to stressful tasks (public speaking and mental arithmetic). To measure cellular immune responses, lymphocyte proliferation to plant lectins, poke weed mitogen (PWM) and phytohemagglutinin (PHA) were used. The autonomic measures, heart rate, vagal tone, blood pressure and the hormones of the HPA axis, ACTH and cortisol, were measured and their possible roles in mediating lymphocyte proliferation responses were examined. Recently parturient women who were breastfeeding or bottlefeeding had attenuated stress-induced change in lymphocyte responses to PWM compared with non-postpartum women, tested in the follicular phase of their cycle (P0.05). Also, lymphocyte responses to PHA were higher in the breastfeeding group compared with non-postpartum controls (P0.05). Regression analyses revealed that an index of cardiac vagal tone, but not other autonomic or endocrine measures, was positively predictive of lymphocyte proliferation to PWM. To summarize, these findings suggest that lactation and parturition can influence lymphocyte proliferation and that activity in the vagal system may influence lymphocyte responses to stress.

This study tested the hypothesis that intraspecific variations in mating systems are correlated with differences in the capacity of peripheral arginine vasopressin (AVP) to facilitate partner preferences. It has been hypothesized that differences in environmental conditions, Kansas being more xeric than Illinois, are responsible for some of the intraspecific differences in the mating systems between Kansas (KN) and Illinois (IL) prairie voles. We predicted that prairie voles from KN would be more behaviorally sensitive to peripheral AVP than prairie voles from IL. To test this hypothesis 60- to 120-day-old male and female, lab-reared, prairie voles originating from KN and IL received three subcutaneous injections of AVP or isotonic saline. Animals were then placed with an adult member of the opposite sex, designated a "partner," for a 1-hour period of cohabitation and subsequently tested for preference for the familiar partner versus a comparable stranger. Only KN males treated with AVP displayed a significant preference for the partner. Using the same experimental paradigm we also examined the ability of peripheral oxytocin (OT) to facilitate partner preference in KN prairie voles. OT facilitated partner preference in females, but not males. This finding was consistent with previous results describing the effects of peripheral OT in IL prairie voles. We also examined the hypothesis that the differential response of KN and IL males would be associated with differences in the distribution of AVP (V1a) receptors. However, there was no apparent difference in the distribution of V(1a) receptors between KN and IL males. The results of this study indicate that there is both intraspecific and intersexual variation in the regulation of social behavior in prairie voles. In addition, these findings suggest that the proximate causes of intraspecific variation may be predicted by knowledge of the habitat of origin.

Although the biological roots of aggression have been the source of intense debate, the precise physiological mechanisms responsible for aggression remain poorly understood. In most species, aggression is more common in males than females; thus, gonadal hormones have been a focal point for research in this field. Although gonadal hormones have been shown to influence the expression of aggression, in many cases aggression can continue after castration, indicating that testicular steroids are not completely essential for the expression of aggression. Recently, the mammalian neuropeptide arginine vasopressin (AVP) has been implicated in aggression. AVP plays a particularly important role in social behavior in monogamous mammals, such as prairie voles ( Microtus ochrogaster ). In turn, the effects of social experiences may be mediated by neuropeptides, including AVP. For example, sexually naïve prairie voles are rarely aggressive. However, 24 h after the onset of mating, males of this species become significantly aggressive toward strangers. Likewise, in adult male prairie voles, central (intracerebroventricular) injections of AVP can significantly increase intermale aggression, suggesting a role for AVP in the expression of postcopulatory aggression in adult male prairie voles. In this paper, we demonstrate that early postnatal exposure to AVP can have long-lasting effects on the tendency to show aggression, producing levels of aggression in sexually naïve, adult male prairie voles that are comparable to those levels observed after mating. Females showed less aggression and were less responsive to exogenous AVP, but the capacity of an AVP V 1a receptor antagonist to block female aggression also implicates AVP in the development of female aggression.

SUMMARY The purpose of this paper is to review existing behavioral and neuroendocrine perspectives on social attachment and love. Both love and social attachments function to facilitate reproduction, provide a sense of safety, and reduce anxiety or stress. Because social attachment is an essential component of love, understanding attachment formation is an important step toward identifying the neurobiological substrates of love. Studies of pair bonding in monogamous rodents, such as prairie voles, and maternal attachment in precocial ungulates offer the most accessible animal models for the study of mechanisms underlying selective social attachments and the propensity to develop social bonds. Parental behavior and sexual behavior, even in the absence of selective social behaviors, are associated with the concept of love; the analysis of reproductive behaviors, which is far more extensive than our understanding of social attachment, also suggests neuroendocrine substrates for love. A review of these literatures reveals a recurrent association between high levels of activity in the hypothalamic‐pituitary‐adrenal (HPA) axis and the subsequent expression of social behaviors and attachments. Positive social behaviors, including social bonds, may reduce HPA axis activity, while in some cases negative social interactions can have the opposite effect. Central neuropeptides, and especially oxytocin and vasopressin have been implicated both in social bonding and in the central control of the HPA axis. In prairie voles, which show clear evidence of pair bonds, oxytocin is capable of increasing positive social behaviors and both oxytocin and social interactions reduce activity in the HPA axis. Social interactions and attachment involve endocrine systems capable of decreasing HPA reactivity and modulating the autonomic nervous system, perhaps accounting for health benefits that are attributed to loving relationships. © 1998 Elsevier Science Ltd. All rights reserved.

Mammalian monogamy is characterized by pair bonding and a relative absence of sexual dimorphism in body size. Alloparental behaviour is a characteristic of mammalian cooperative breeding systems. Studies of prairie voles, Microtus ochrogaster, from stock captured in a resource-abundant habitat in Illinois have supported the assumption that this species is a monogamous, cooperative breeder, while other studies of prairie voles from a more arid habitat in Kansas have called this assumption into question. We hypothesized that reported differences between these populations represented true intraspecific variation. Patterns of sexual dimorphism in body size, partner preferences and parental contact behaviour were compared in prairie voles from stocks originating in Illinois or Kansas. Both Illinois and Kansas voles showed a strong preference for a familiar partner, which is suggestive of monogamy. Sexual dimorphism in body size was observed in Kansas, but not Illinois voles. Illinois voles displayed significantly higher levels of parental contact behaviour than did voles from Kansas. When animals from Illinois and Kansas were crossed, the expression of parental contact behaviour of the 'hybrid' offspring followed the pattern seen in the population of origin of the sire. Removal of the sire prior to the birth of the litter increased alloparenting in Kansas voles, but removal of the sire was associated with lower levels of alloparenting in Illinois voles. Thus, some traits associated with the social system may show intraspecific variation and can be influenced by the presence or absence of the sire during rearing. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.

ROBERTS, R. L., A. S. ZULLO AND C. S. CARTER. Sexual differentiation in prairie voles: The effects of corticosterone and testosterone. PHYSIOL BEHAV 62(6) 1379–1383, 1997.—Prairie voles (Microtus ochrogaster) exhibit low levels of physical sexual dimorphism and have endogenous basal corticosterone levels that are 5–10 times higher than those measured in rats; prairie voles also do not show a postnatal period of adrenal hyporesponsivity. On the basis of studies in rats suggesting that adrenal hyperactivity during the perinatal period could reduce masculinization or feminize sexual behavior, we hypothesized that adrenal hormones might influence sexual differentiation in prairie voles. We also examined the hypothesis that the effects of testosterone in prairie voles might differ from those reported in other rodents. Treatments with either corticosterone or testosterone propionate (TP) were given prenatally (gestational Days 12-20), via maternal injection, or postnatally (Days 1-6), by directly injecting the pups. Additional groups of males were castrated or sham-operated on postnatal Day 1, and a subgroup of castrated males received postnatal TP. Male and female sexual behavior was observed in adulthood following gonadectomy and hormone treatments. Corticosterone treatment was associated with high levels of mounting in both sexes and did not inhibit lordosis behavior in females. Postnatal TP treatment inhibited lordosis in females but did not facilitate mounting in either sex. Males that were castrated at birth showed unexpectedly high levels of mounting in response to adult androgens. The results of this study suggest that in prairie voles corticosterone is capable of masculinizing without defeminizing sexual behavior, whereas postnatal testicular secretions are not essential for, and may actually inhibit, masculinization in this species.

For mammalian reproduction to succeed, self-defense and asociality must be subjugated to positive social behaviors, at least during birth, lactation, and sexual behavior. Perhaps the important task of regulating the interaction between social and agonistic behaviors is managed, in part, by interactions between two related neurochemical systems that incorporate oxytocin and vasopressin in their functions. The neuropeptides oxytocin and vasopressin participate in important reproductive functions, such as parturition and lactation, and homeostatic responses, including modulation of the adrenal axis. Recent evidence also implicates these hormones in social aspects of reproductive behaviors. For example, oxytocin is important for a variety of positive social behaviors, including the regulation of maternal-infant interactions. In adult animals, oxytocin may facilitate both social contact and selective social interactions associated with social attachment and pair bonding, and it participates in the regulation of parasympathetic functions. Vasopressin, in contrast, is associated with behaviors that might be broadly classified as "defensive" including enhanced arousal, attention, or vigilance, increased aggressive behavior, and a general increase in sympathetic functions. On the basis of the literature on the functions of these hormones and our own recent findings, we propose that dynamic interactions between oxytocin and vasopressin are components of a larger system which integrates the neuroendocrine and autonomic changes associated with mammalian social behaviors and the concurrent regulation of the stress axis. In addition, studies of lactating females provide a valuable model for understanding the more general neuroendocrinology of the stress axis. Peptide hormones, including oxytocin and vasopressin, do not readily cross the blood-brain barrier and must be administered centrally (i.c.v.) to reach the brain. Nasal sprays have been used to promote milk let down and have been used in some behavioral studies, but the extent to which such compounds reach the brain is not known. Therefore, virtually nothing is known regarding the effects in humans of centrally administered oxytocin. The study of human lactation, in conjunction with animal research, provides an opportunity to begin to develop viable hypotheses regarding the behavioral effects of oxytocin.

Research described in a recent issue of PNAS by Wismer Fries, Ziegler, Kurian, Jacoris, and Pollak (1) reveals significant group differences in two brain-derived neuropeptides, oxytocin and vasopressin, between family-versus orphanage-reared children. These intriguing results may stimulate a new chapter in the ongoing scientific dialogue concerning the neuro-biological effects of early experience. Fascination with the developmental effects of social deprivation is not new. Consider Romulus and Remus, the legendary founders of Rome who were purportedly nursed by a shewolf; or Kasper Hauser, a mysterious 19th century German boy, believed to have been isolated from human contact from early childhood. Modern interest in the impact of maternal separation or neglect on human behavior can be traced to two proteges of Sigmund Freud, Rene Spitz and John Bowlby (2, 3), who described children reared in orphanages or by inconsistent caregivers as at risk for emotional and social disorders. Behavioral studies, including research in nonhuman primates, have documented vulnerabilities in socially deprived infants (4–6). From birth onward, social interactions are essential for normal development. In all human cultures, including nonhuman animals, we find mechanisms for transmitting social experiences from one generation to the next. These experiences may in turn alter future behavior. Attempts to study the neuroendocrine consequences of early experience have tended to focus on changes in the hypothalamic–pituitary–adrenal (HPA) axis (7 …

Based on research with prairie voles, we hypothesize that the unusual patterns of reproduction and social behavior associated with mammalian monogamy may arise as a consequence of normal developmental exposure to high levels of glucocorticoids and/or other hormones of the HPA axis. Increased HPA activity could functionally inhibit some of the masculinizing processes expected during the perinatal period. We hypothesize that the unique behavioral, physiological, and anatomical changes associated with monogamy may reflect the adaptive consequences of reduced exposure to the masculinizing actions of HPG hormones, such as testosterone. Reproductively naive, unpaired adult prairie voles also show unusual patterns of adrenal activity, including a marked decline in corticosterone levels within minutes following exposure to novel animals of the opposite sex. In females, this decline in corticosterone may contribute to pair bonding, since corticosterone injections inhibit, and adrenalectomy is associated with a facilitation of pair bond formation. In males, corticosterone injections facilitate pair bonding and adrenalectomy has the opposite effect. In animals from established social pairs corticosterone levels also fluctuate according to the social environment of the animal; the absence of a familiar partner is associated with increased corticosterone secretion, and in the presence of the familiar partner corticosterone levels tend to decline. Prairie voles may offer a valuable source of information regarding the behavioral, anatomical, and physiological consequences of long-term and short-term exposure to high levels of adrenal activity in the absence of pathology.

Objective Autism spectrum disorder (ASD) is a heritable but highly heterogeneous neuropsychiatric syndrome, which poses challenges for research relying solely on behavioral symptoms or diagnosis. Examining biomarkers may give us ways to identify individuals who demonstrate specific developmental trajectories and etiological factors related to ASD. Plasma oxytocin (OT) and whole-blood serotonin (5-HT) levels are consistently altered in some individuals with ASD. Reciprocal relationships have been described between brain oxytocin and serotonin systems during development. We therefore investigated the relationship between these peripheral biomarkers as well as their relationships with age. Method In our first study, we analyzed correlations between these two biomarkers in 31 children and adolescents who were diagnosed with autism and were not on medications. In our second study, we explored whether whole-blood 5-HT levels are altered in mice lacking the oxytocin receptor gene Oxtr . Results In humans, OT and 5-HT were negatively correlated with each other ( p Oxtr showed increased whole-blood 5-HT levels ( p = .05), with this effect driven exclusively by mice less than 4 months old ( p Conclusions Identifying relationships between identified ASD biomarkers may be a useful approach to connect otherwise disparate findings that span multiple systems in this heterogeneous disorder. Using neurochemical biomarkers to perform parallel studies in animal and human populations within a developmental context is a plausible approach to probe the root causes of ASD and to identify potential interventions.

Results of these experiments indicate that females given at least 24 hours of cohabitation with a male develop a social preference for the familiar partner versus a stranger. Mating is not essential for the development of partner preferences, but clearly facilitates the onset of preferences. Females given six hours of cohabitation showed partner preferences only if they mated with the partner during cohabitation (Experiment 2) or if they received oxytocin (Experiment 3). Females that continued to mate during preference tests (Experiment 2, n = 4) mated exclusively with the partner. Oxytocin infusions, even in the absence of mating or estrogen priming, facilitated the development of partner preferences.

Responses to vibrotactile stimuli were examined in men as a function of chronic exposure to either exogenous or endogenous androgens. Psychophysical techniques were used to evaluate thresholds to stimulus detection and perceived stimulus intensities in response to mild vibration applied to either the finger or the penis. Normal men were compared to the following groups: (a) untreated hypogonadal men, (b) androgen-replaced hypogonadal men, or (c) infertile men with androgen levels in the low normal range. Among the four groups, untreated hypogonadal men perceived vibrotactile stimuli as most intense and were slightly more sensitive to touch than were men with higher levels of androgen. Chronic treatment with testosterone enanthate was associated with a decline in the perceived intensity of vibrotactile stimuli in hypogonadal men. The lowest levels of sensitivity to tactile stimuli were observed in the infertile men.

In ovariectomized female Mongolian gerbils (Meriones unguiculatus) estradiol benzoate (6.6 μg/day) injections elicited high levels of lordosis. Lordosis was first observed after 7 days of estrogen treatment and lordosis quotients of greater than 100 occurred after 10 continuous days of estrogen treatment. In females primed with estrogen for 13 days a further facilitation of lordosis frequency was observed 4–6 hr after progesterone injection. In estrogen-primed females, lordosis quotients of over 100 were still possible even after 9 consecutive days of progesterone treatment, although prolonged progesterone exposure (0.4 mg/day for 24 hr or more) may have depressed lordosis. Mating stimuli also interact with hormonal treatments to depress receptivity in this species. Previously tested females showed lower levels of responding than females first tested after 21 consecutive days of estrogen and 9 days of progesterone. Males often footstomped in tests in which females showed lordosis, while pairs fought and males scent marked most often in tests with nonreceptive females.

1. Field and experimental evidence is provided for the existence of pair-bonding or monogamy in prairie voles (Microtus ochrogaster). 2. Multiple-capture live-trap data indicated that male and female pairs of M. ochrogaster were repeatedly captured together. (A comparable analysis for data from M. pennsylvanicus revealed no indications of long-term male-female associations.) 3. Male-female pair captures of M. ochrogaster were equally likely during either the breeding or nonbreeding seasons, further suggesting a relative stability of pairs. 4. In laboratory dyadic encounters, both males and females from breeding pairs tended to show relatively high levels of aggression toward unfamiliar animals of the opposite sex. In contrast, aggression was rarely observed between members of established breeding pairs. Nonpaired animals of either sex infrequently initiated aggressive encounters. 5. When pairs were separated for 8 days (during which time the females lived with a new male) the apparent pair-bond with the original male was broken. These females became aggressive toward the male with which they had previously bred, and rarely fought with their new mate. This suggests that the pair-bonding process is reversible. 6. Females in postpartum estrus preferentially showed high levels of sexual receptivity and low levels of aggression toward familiar males and were less likely to mate with unfamiliar sexually experienced males. The presence of pups at the time of testing did not appear to influence female-initiated aggression.

Behaviour was quantified in pairs of prairie voles preceding and during sexual interactions through the use of time-lapse videotaping. At the onset of testing, females were either anoestrous or were in confirmed behavioural oestrus. These females were paired with either sexually naive or experienced males and behavioural interactions were monitored for at least 72 h. For anoestrus females, brought into oestrus during taping, the median latency to the onset of sexual activity was 52 h with naive males and 41 h with sexually experienced males. Typical mating bouts lasted for more than 24 h with intermittent periods of rest. The pacing of mating behaviour differed only slightly as a function of male sexual experience and did not differ as a function of the oestrous condition of the female at the onset of videotaping. Autogrooming, which may facilitate the transfer of chemosignals, was monitored throughout the experiment. Within 13–24 h following introduction of the pair, autogrooming increased in pairs in which the male was sexually experienced; in contrast, in pairs in which the male was sexually naive, increases in autogrooming were not observed until 25–36 h following the onset of videotaping. The oestrous condition of the female did not significantly affect patterns of autogrooming; however, autogrooming patterns in the female paralleled those of the male with which she was paired.

Dyadic encounters between virgin female prairie voles ( Microtus ochrogaster ) and males with various reproductive histories were analysed using observational techniques and sequential analysis. Unfamiliar, sexually inexperienced males interacted in a manner that would permit physical contact, pheromone transfer and the subsequent induction of female sexual behaviour. Low levels of olfactory investigation were seen among sibling males and females, which may account for the failure of such pairs to reproduce. Agonistic encounters were common between females and unfamilar sexually-experienced males, suggesting that fighting could also provide a behavioural barrier to reproduction.

In the female golden hamster, Mesocricetus auratus, group housing permits the expression of male-like behavioral patterns including mounting and pelvic thrusting toward an estrous stimulus female. Sixty percent of females housed in groups of five showed these behaviors, while singly housed females were rarely observed to show mounting or thrusting. Females weaned at 19 or 35 days of age did not differ in their tendencies to show male-like behavioral patterns. Virtually all observations of mounting and pelvic thrusting behaviors were made on days when the females being tested were in behavioral estrus. These findings differ from previous studies which have reported that masculine behavioral patterns are rare in normal female hamsters. In previous studies females that were evaluated were typically not in behavioral estrus and were not housed for long periods of time with other intact females. Social factors interact with hormonal state to contribute to the expression of male-like behaviors in female hamsters.

In reproductively naive female prairie voles ( Microtus ochrogaster ) direct contact with male urine or housing in a male-soiled cage, in the absence of physical contact, resulted in increased uterine weights, but did not reliably elicit behavioral estrus (defined by lordosis). Physical contact with an unfamiliar male, for 1 hr or more, followed by 30 or 48 hr of continuous access to a male-soiled cage, induced lordosis in approximately two-thirds of the females tested. When females were physically exposed to a male for 18 hr and tested 6 hr later, 70% showed lordosis. However, when females receiving either 1 or 18 hr of male contact were removed from the presence of the male and placed in a clean cage for 24 hr, only 29–37% of the females subsequently showed lordosis. These results suggest that direct physical contact with the male or chemical stimuli from the male may be necessary to induce and maintain behavioral estrus in female prairie voles.

The interactive effects of hormones, sexual history and cohabitation on sexual and social behaviors were examined in pairs of ovariectomized female and sexually experienced male prairie voles ( Microtus ochrogaster ). Monitoring with time lapse video tape revealed that females in estradiol benzoate (EB)-induced estrus, and their male partners engaged in high levels of sexual activity which continued intermittently for at least 3 days (until observations were arbitrarily terminated). In conjunction with other studies, these results indicate that the hormonal condition of the female at the time of testing is a major determinant of sexual activity. Prior hormonal, copulatory, or cohabitation experience did not significantly influence sexual responses between females and unfamiliar male partners. However, affiliative behaviors, such as side by side contact, were higher in pairs that were familiar due to prior sexual and cohabitational experience. These results indicate that social and sexual behaviors are independently regulated. Other behaviors, including nasogenital investigation and autogrooming were influenced by the hormonal and sexual history of the female. The implications of these behavioral patterns for reproductive activation, pair bonding, and incest avoidance are dicussed.

The behaviors of intact or ovariectomized, estradiol benzoate-treated or estradiol benzoate followed by progesterone-treated female brown lemmings were compared. Intact, diestrous females engaged in more social interactions with a male than did ovariectomized females (Experiment 1). In the first 5 min of a 1-hr mating exposure (Experiment 2, Test A) intact females in natural estrus engaged in more social and sexual behaviors than did ovariectomized females in estrogen-induced estrus. However, during the last 5 min of the 1-hr exposure (Test B) ovariectomized females receiving estrogen alone continued to show high levels of sexual activity with a male partner, while intact estrous females or females receiving estrogen followed by progesterone showed an apparent drop in sexual receptivity and an increase in aggressivity. Aggressive behaviors, as indexed by threat-leap behaviors on the part of the female may increase in the presence of progesterone. Declines in sexual activity, occurring within 1 hr of progesterone injection, were apparently dependent on the interaction of progesterone and copulatory events which may affect both the male and female.

The environmental factors that arrest breeding in prairie voles during the middle of the breeding season are unknown. The role of water availability on reproductive function was examined by limiting water intake to 50% of ad lib water consumption for 10 weeks. At autopsy, testicular, epididymal and seminal vesicle masses were reduced in water restricted males as compared to animals with ad lib access to water. Body mass was also reduced in water restricted males. Plasma testosterone levels and the number of testicular and epididymal sperm were significantly reduced in water restricted voles as compared to animals drinking water ad lib, but plasma levels of luteinizing hormone were unaffected. Taken together, these data suggest that reduced water availability can inhibit male prairie vole breeding.

The effects of estrogen and progesterone on the social and sexual behavior of brown lemmings, Lemmus trimucronatus , were investigated. The behavior of hormone-treated and untreated ovariectomized females and sexually vigorous males was observed in six consecutive daily 5-min dyadic encounters. Sexual receptivity, as measured by lordosis, and other social behaviors including nasonasal contact, boxing postures, allogrooming, perineal investigation, and male mounting increased following 48 hr of exposure to daily injections of 0.5 μg estradiol benzoate (EB). Lordosis in EB-primed females was not facilitated or inhibited by short-term (4 hr) exposure to 0.5 mg progesterone (P). Long-term (greater than 24 hr) exposure to P apparently inhibited lordosis and other social behaviors in EB-treated females, although males continued to attempt to mount these females. In EB-treated females a dramatic increase in threat-leaps, directed by the female toward the male, was observed within 4 hr of P injection. Threat-leaps declined when P was withdrawn. Threat-leaps were also observed in ovariectomized females after prolonged exposure to P only (0.5 mg/day). Vaginal perforation and cornification were first apparent 48 hr after EB injection. P-alone treated ovariectomized females also showed vaginal perforation but cornified cells were infrequent and these animals did not show lordosis.

The effects of age and hormonal status on sexual response were quantitatively assessed by the use of a vaginal photoplethysmograph to measure changes in pulse amplitude in response to erotic film and fantasy. In young, regularly cycling women (mean age, 31 years) there were no significant differences in sexual response between the follicular, ovulatory, and luteal phases. The older premenopausal women (mean age, 51 years) showed responses not significantly different from those of the young cycling women. However, both the younger cycling women and older premenopausal women had significantly higher responses to erotic film than the postmenopausal women (mean age, 57 years). It was concluded that the menopause does result in a decrease in sexual response as assessed by vaginal pulse amplitude. However, it was not established that the magnitude of change noted is sufficient to cause any sexual dysfunction.

Neonatal lesions of the cerebral cortex in female rats did not eliminate female sexual behavior as measured by lordosis. However, lordosis in response to prolonged low levels of estradiol benzoate (1.0 μg/day for 6 days) was attenuated in lesioned females. Following estradiol benzoate plus progesterone (0.5 mg) treatment the probability of lordosis increased markedly in the decorticate females, but still remained below control levels. Decorticate females were mounted by the male at least as often as control females. Hopping and darting and rejection behaviors on the part of the female were virtually eliminated in the decorticate group. However, these females continued to direct sniffing behavior toward the male at levels above those of the controls.

Sexual development of female prairie voles does not occur unless females are exposed to stimuli from an unfamiliar male. Three experiments were conducted to determine whether a saturable high-affinity estradiol binding site (ERN) is present in cell nuclei of brain tissue from females exposed to male stimuli and whether serum estradiol and brain ERN are correlated with the duration of male exposure. Brain ERN were detected and found to be correlated with serum estradiol levels. Significant increases in brain ERN were observed within 18 hours after females were exposed to a male for either a continuous (uninterrupted) or transient (1 1/4 hour) period. A significant increase in serum estradiol was observed for females given continuous male exposure. The rate of increase of ERN in brain was significantly higher for females given continuous as opposed to transient male exposure. The results suggest that the continued presence of a male can accelerate the rate of increase of ERN in the brain, presumably through increases in circulating estradiol.

In the induced ovulator, Microtus ochrogaster (the prairie vole), daily injections of estradiol benzoate (EB 0.5 μg or 5.0 μg) facilitated high levels of sexual receptivity in ovariectomized females withing 48 hr of the onset of treatment. A lower level of EB (0.05 μg) with or without progesterone (0.5 mg) for five days did not induce lordosis. Four hr after sexually receptive EB-primed females received progesterone (0.5 mg) they either continued to show lordosis (EB 5.0 μg group) or showed an inhibition of lordosis (EB 0.5 μg group). Within 24 hr lordosis was markedly reduced in both groups. Progesterone-associated inhibitions were no longer apparent 48 hr after progesterone was discontinued. This general pattern of response to ovarian hormones is consistent with data described for other induced ovulators.

Publisher Summary This chapter describes the features of reproduction, which may play important roles as determinants of population fluctuations. It is assumed that the physiological analysis of reproductive mechanisms will be more meaningful, if formulated in the context of natural history and knowledge of mating systems. For these and a number of other reasons given in the chapter, the prairie vole presents an attractive laboratory-compatible animal model for the analysis of the interaction of social and physiological substrates of behavior. Salient features of reproduction in the prairie vole seem to follow patterns observed in other monogamous mammals including taxa as diverse as canids and primates.

In a review of reproduction in the golden hamster published in 1968, Kent devoted less than four pages to a relatively exhaustive survey of the existing literature on female sexual behavior. The current review will summarize findings that have appeared in the intervening 15 years. In that period, research on the female hamster increased exponentially. The hamster has taken over an important position, probably second only to the domestic rat, in its popularity as a laboratory subject for studies of female sexual behavior.

The purpose of this chapter is to review pharmacological effects on male erectile dysfunction. As will be documented in this chapter, a wide variety of pharmacological agents have been reported to have sexual side effects. Drugs may influence male sexual function at various levels including (1) sexual interest or desire (also termed libido), (2) the capacity to achieve and maintain an erection sufficient for coitus, (3) ejaculation, and (4) fertility. Although the issue of fertility is beyond the scope of this review, drugs that inhibit fertility often suppress hormonal secretions and may thus influence sexual behavior. This text is concerned primarly with erectile function. However, libido and ejaculation problems often coexist and interact with erectile problems. The effects of drugs on these sexual activities will also be reviewed.

Additional file 1. Supplementary figures and tables.

Additional file 1. Supplementary figures and tables.