Testosterone aromatization in rat brain is modulated by social environment

Pubertal stress decreases sexual motivation and supresses the relation between cerebral theta rhythms and testosterone levels in adult male rats

This study evaluated the effect of stress during puberty on sexual motivation and the correlation between serum testosterone levels (T) and the absolute power of the theta electroencephalographic rhythms, recorded in the medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) of adult male rats. Thirty males of the stressed group (SG, housed 1 per cage from days 25-50) and 30 controls (CG, housed 5 per cage), were tested in copulatory interactions at 90 days of age. The above mentioned physiological parameters were obtained during the awake-quiet state in a sub-group without sexual motivation (WSM, n = 15, stimulated with a nonreceptive female) and a sub-group with sexual motivation (SM, n = 15, stimulated with a receptive-female). Pearson correlations (r) between these parameters were calculated for each sub-group and brain structure and then compared between sub-groups. SG presented higher mount and intromission latencies than CG. While CG-WSM showed a positive r between T levels and theta band (0.23-0.59), those CG-SM presented a negative r (-0.23 to -0.67). An r that tended towards zero (-0.31 to 0.29) was obtained in both stressed sub-groups. This study shows that pubertal stress suppresses the relation between serum T levels and theta rhythms in the mPFC and BLA in adult male rats. This is one of the first studies evaluating the association between these two physiological parameters specifically in the context of sexual motivation; thus increasing our understanding of the effect of pubertal stress on prefrontal-amygdaline functioning during the sexually-motivated state in male rats.

Double helix: reciprocity between juvenile play and brain development

This review summarizes what is presently known about the function, sexual differentiation, and neural circuitry of juvenile rough-and-tumble play. Juvenile rough-and-tumble play is a unique motivated behavior that is widespread throughout the mammalian order and usually occurs more often in males. Immediate early gene studies indicate that cortical and subcortical circuits, many of which are sensitive to sex steroid hormones, mediate juvenile play. Sex differences in rough-and-tumble play are controlled in part by neonatal exposure to androgens or their estrogenic metabolites. Studies indicate that testicular androgens during play are also necessary to stimulate male-like levels of play initiation. The resemblance of rough-and-tumble play to aggression and sexual behavior has led some to question whether male-typical adult behavior is contingent upon the experience of play. Attempts to control the amount of play through social isolation show that social experience during adolescence is critical for male-typical adult behaviors to be expressed. This well-established finding, together with evidence that play induces neural plasticity, supports the hypothesis that juvenile play contributes to male-typical brain development that ultimately enables the expression of adult social and reproductive behavior.

New perspectives on rodent models of advanced paternal age: relevance to autism

Offspring of older fathers have an increased risk of various adverse health outcomes, including autism and schizophrenia. With respect to biological mechanisms for this association, there are many more germline cell divisions in the life history of a sperm relative to that of an oocyte. This leads to more opportunities for copy error mutations in germ cells from older fathers. Evidence also suggests that epigenetic patterning in the sperm from older men is altered. Rodent models provide an experimental platform to examine the association between paternal age and brain development. Several rodent models of advanced paternal age (APA) have been published with relevance to intermediate phenotypes related to autism. All four published APA models vary in key features creating a lack of consistency with respect to behavioral phenotypes. A consideration of common phenotypes that emerge from these APA-related mouse models may be informative in the exploration of the molecular and neurobiological correlates of APA.

Social influences on neurobiology and behavior: epigenetic effects during development

The quality of the social environment can have profound influences on the development and activity of neural systems with implications for numerous behavioral and physiological responses, including the expression of emotionality. Though social experiences occurring early in development may be particularly influential on the developing brain, there is continued plasticity within these neural circuits amongst juveniles and into early adulthood. In this review, we explore the evidence derived from studies in rodents which illustrates the social modulation during development of neural systems, with a particular emphasis on those systems in which a long-term effect is observed. One possible explanation for the persistence of dynamic changes in these systems in response to the environment is the involvement of epigenetic mechanisms, and here we discuss recent studies which support the role of these mechanisms in mediating the link between social experiences, gene expression, neurobiological changes, and behavioral variation. This literature raises critical questions about the interaction between neural systems, the concordance between neural and behavioral changes, sexual dimorphism in effects, the importance of considering individual differences in response to the social environment, and the potential of an epigenetic perspective in advancing our understanding of the pathways leading to variations in mental health.

The effects of breeding protocol in C57BL/6J mice on adult offspring behaviour

Animal experiments have demonstrated that a wide range of prenatal exposures can impact on the behaviour of the offspring. However, there is a lack of evidence as to whether the duration of sire exposure could affect such outcomes. We compared two widely used methods for breeding offspring for behavioural studies. The first involved housing male and female C57Bl/6J mice together for a period of time (usually 10–12 days) and checking for pregnancy by the presence of a distended abdomen (Pair-housed; PH). The second involved daily introduction of female breeders to the male homecage followed by daily checks for pregnancy by the presence of vaginal plugs (Time-mated; TM). Male and female offspring were tested at 10 weeks of age on a behavioural test battery including the elevated plus-maze, hole board, light/dark emergence, forced swim test, novelty-suppressed feeding, active avoidance and extinction, tests for nociception and for prepulse inhibition (PPI) of the acoustic startle response. We found that length of sire exposure (LSE) had no significant effects on offspring behaviour, suggesting that the two breeding protocols do not differentially affect the behavioural outcomes of interest. The absence of LSE effects on the selected variables examined does not detract from the relevance of this study. Information regarding the potential influences of breeding protocol is not only absent from the literature, but also likely to be of particular interest to researchers studying the influence of prenatal manipulations on adult behaviour.

Epigenetics and the origins of paternal effects

Though there are multiple routes through which parents can influence their offspring, recent studies of environmentally induced epigenetic variation have highlighted the role of non-genomic pathways. In addition to the experience-dependent modification of DNA methylation that can be achieved via mother-infant interactions, there has been increasing interest in the epigenetic mechanisms through which paternal influences on offspring development can be achieved. Epidemiological and laboratory studies suggest that paternal nutritional and toxicological exposures as well as paternal age and phenotypic variation can lead to variations in offspring and, in some cases, grand-offspring development. These findings suggest a potential epigenetic germline inheritance of paternal effects. However, it may be important to consider the interplay between maternal and paternal influences as well as the experimental dissociation between experience-dependent and germline transmission when exploring the role of epigenetic variation within the germline as a mediator of these effects. In this review, we will explore these issues, with a particular focus on the potential role of paternally induced maternal investment, highlight the literature illustrating the transgenerational impact of paternal experiences, and discuss the evidence supporting the role of epigenetic mechanisms in maintaining paternal effects both within and across generations.

Post-weaning social isolation of male rats reduces the volume of the medial amygdala and leads to deficits in adult sexual behavior

At 21 days of age, gonadally intact male Long Evans rats were weaned and placed into standard laboratory conditions (three per cage) or housed singly. They were tested for noncontact erections and sexual performance at 90 and 220 days of age. Rats raised in isolation displayed significantly fewer noncontact erections in response to sensory cues from an estrous female and fewer intromissions when allowed to mate with a female than did males raised in groups. The volume of the posterodorsal component of the medial amygdala (MePD) and the size of neurons within the MePD were significantly smaller in the isolated males than in socially housed males. Similarly, neurons in the sexually dimorphic nucleus of the preoptic area (SDN-POA) were smaller in isolate animals than in controls. As both MePD volume and SDN-POA soma size are responsive to sex steroids, these differences could result if the isolates experienced lower testosterone levels. Finally, the volume of the overall medial amygdala (MeA) correlated significantly with the number of noncontact erections, a relationship that was not explained by housing condition. These findings highlight the role of social experience as a factor in the sexual differentiation of the brain and suggest a positive relationship between the volume of a brain structure and the display of sexual behaviors.

The developmental psychobiology of behavioural plasticity in mice: the role of social experiences in the family unit

Small perturbations of young animals' sensory experience or hormonal milieu have been shown to alter ontogenetic pathways and to potentially produce huge effects on CNS functioning and behaviour later in life. From a social point of view, variables such as the expression of affiliative bonding and of playful interactions among littermates, the quantity/quality of maternal care, or episodes of maternal or sibling deprivation during critical phases in development, seem to interfere as epigenetic factors with the rigidly ordered temporal sequences of events that occur during the ontogenesis of CNS. This leads to the onset of adaptive neurodevelopmental changes, which are observable within a continuum that encompasses both "normal" individual variability and potential behavioural disorganisation, which in turn will probably be related to profound alteration in the establishment of adult social competence. The present review summarises the more recent work in mice dealing with short-term, as well as long-term modifications, in naturally occurring species-typical social and non-social responses as a function of the early manipulation of social characteristics of the family unit (such as litter gender composition and time of weaning). These analyses were carried out on infant animals, i.e. during the ontogenetic stage of the establishment of social bonding, as well as on pre-pubertal and adult mice and on lactating adult females. Critical issues, such as the respective roles of sibling-sibling and dam-offspring interactions in the shaping of "sibling effects", are also addressed. Overall, these studies indicate that, within their natural range of variation, early patterns of social stimulation are powerful determinants of subsequent behaviour of developing altricial rodents, and confirm that early social life events warrant attention because they can strongly affect neurobehavioural development. Evidence of a relationship between social events occurring during early rearing (i.e. when dramatic transitions in neuroendocrine and neurochemical CNS systems occur) and individual behavioural variability in the infant and adult response to the effects of psychostimulants abused by humans is presented. A better understanding of the mechanisms that mediate such remarkable plasticity might have great psychobiological as well as clinical importance, especially when considering the issue of vulnerability to drug abuse.

On mouse pups and their lactating dams: behavioral consequences of early exposure to oxazepam and interacting factors

Behavioral analysis in animal models appears to be a valuable and sensitive tool for detecting subtle alterations in CNS function, which can be produced by early exposure to small perturbations of sensory experience, hormonal milieu, or exposure to psychotropic agents devoid of major teratogenic potential. Concerning anxiolytics, the more recent work in mice, which is here summarized, was carried out by putting the emphasis on changes in naturally occurring species-typical social responses as a function of early exposure to benzodiazepines. For adult females, on the behavior expressed during the early postpartum period, whereas for infant subjects, on the ontogenetic stage of the establishment of social bonding. Critical issues such as the choice of fostering procedure and rearing conditions are also addressed. Evidences of relationships between changes in social milieu taking place during early rearing-i.e., when dramatic transitions in the neurochemical target system occur- and the adult behavioral response to challenges with BDZ agents are presented. These data strengthen the notion that the modes of reaction of adult animals to the joint influence of physiological and environmental (stimulus) variables are under the influence of events in early ontogenesis. Therefore, a better understanding of the mechanisms-as unveiled by an appropriate use of drug tools-that mediate such a plasticity might have considerable psychobiological and clinical-therapeutical relevance.

Sexual segregation in infancy and bi-directional benzodiazepine effects on hot-plate response and neophobia in adult mice

In the present experiment, the hypothesis that rearing animals in conditions of sexual segregation in infancy (ISS) would affect their adult behavioral reactivity to drug or environmental challenges was tested. Outbred Swiss CD-1 mouse litters were reduced at birth to six pups according to three conditions: MM (all males), MF (sex-balanced composition), and FF (all females). At weaning (day 21), all mice were rehoused in unisexual groups. At adulthood (day 70), animals were challenged either with BDZ agonist chlordiazepoxide (CDP at 2.5- or 5.0-mg/kg dose) or BDZ receptor partial inverse agonist Ro 15-3505 (RO at 3-, 10-, or 30-mg/kg dose) and assessed in sequence for pain reactivity in a hot-plate apparatus (set at 55 +/- 1 degrees C), for locomotor activity in a Varimex apparatus, and finally for neophobia level by measuring the latency to first approach a novel object. As concerns the hot-plate test, lick latency was significantly shortened in MF females receiving CDP (5.0 mg/kg), while RO was either ineffective in MF females or induced a prominent dose-dependent analgesia in FF females. Activity was decreased by CDP (2.5 mg/kg) and enhanced by RO (3.0 mg/kg). For latency to approach a novel object, males as a whole exhibited shorter times than females. Mixed-sex animals of both sexes were less fearful, being also more explorative than their corresponding unisexually reared groups. In particular, MF males receiving either a 5.0-mg/kg CDP dose or a 3.0-mg/kg RO dose explored the object more often than MM males.(ABSTRACT TRUNCATED AT 250 WORDS)

Social modulation of behavioral reproductive senescence in female rats

During aging, the lordosis reflex of female rats changes from a state of cyclic lordosis, when the lordosis reflex can be elicited by manual palpation only on proestrus, to a state of constant lordosis (CL), when it can be elicited daily. Social signals from other females altered this pattern of behavioral senescence. Group living decreased the lordosis reflex intensity in both old and young rats and delayed the onset of CL. Long-term group living delayed CL until late in the life span, while temporary group living delayed it only until rats returned to isolation. Long-term group living also changed the temporal relationship between CL and the acyclic, anovulatory state of estrus. Finally, the lordosis reflex intensity, measured by manual palpation, predicted the old rats' receptivity to a male, indicating that constant lordosis is a biomarker for other aspects of behavioral reproductive senescence.

Isolation accelerates reproductive senescence and alters its predictors in female rats

The process of reproductive senescence in female rats (Rattus norvegicus, Sprague-Dawley strain) was altered by their social environment during adulthood. The incidence of constant estrus (CE), which marks the end of estrous cyclicity, was nearly twice as high in females living in isolation as it was in females living in groups. Isolated females also entered CE at three times the rate of group-housed females. In addition, the characteristics of a rat's estrous cycle when she was young predicted whether or not she would enter CE during reproductive senescence. However, the characteristics of the cycle that predicted if a rat would enter CE were different for isolated females than for females living in groups. In isolated rats, entry into CE was predicted by a pattern of regular cycles followed by irregular cycles, a lordosis reflex of consistently high intensity during irregular cycles, and an absence of spontaneous pseudopregnancies. Furthermore, a long duration of CE was predicted by early cessation of estrous cycles. In rats living in groups, increased estrogenization of the vaginal smears during irregular cycles was the only predictor of entry into CE. In both environments, the onset of an acyclic lordosis reflex predicted the timing of CE. The potential role of ovarian steroids as mediators of these effects is discussed.

Differential modulation of monoamine levels and turnover rates by estrogen and/or androgen in hypothalamic and vocal control nuclei of male zebra finches

Sexual behaviors, including singing, in male zebra finches are under the combined control of androgens and estrogens. Treating castrates with androstenedione (AE), a hormone which provides both androgenic and estrogenic metabolites, restores catecholamine (CA; i.e., norepinephrine (NE) and dopamine (DA] function in brain areas known to be involved in the control of these behaviors. Since these behaviors can only be activated by the combined action of androgens and estrogens, the present study determined whether estrogen alone, androgen alone, or only their combination would restore CA function in these brain areas. Males were castrated for at least three weeks and then received one of three hormone implants: (1) estradiol (E) alone, (2) dihydrotestosterone (DHT) alone, or (3) E + DHT. Each male was then housed with a female for at least one week. Levels of NE, DA, epinephrine, serotonin and the latter's primary metabolite, 5-hydroxyindoleacetic acid, were measured in 3 hypothalamic, one auditory and 6 vocal control areas in their brains. NE and DA turnover in these areas were also estimated using alpha-methylparatyrosine. CA function proved to be hormone sensitive in 64% of the comparisons in the three hypothalamic and six vocal control nuclei examined. As expected, the treatment which provided both estrogenic + androgenic stimulation (E + DHT) restored CA function in all of these cases. Although E + DHT restored CA function in these areas, only 22% required both estrogenic + androgenic stimulation to restore normal CA function. For 57% of the comparisons, treatment with E alone also restored normal function, while in the remaining 22%, DHT alone was sufficient. All hormone-induced changes in noradrenergic neurotransmission were estrogen dependent; while modulation of DA function was much more variable, and frequently androgen dependent. To the best of our knowledge, this is the first time that these types of functional differences in the role of estrogen versus androgen in the modulation of neurotransmitter systems has been demonstrated.

Androstenedione modulation of monoamine levels and turnover in hypothalamic and vocal control nuclei in the male zebra finch: steroid effects on brain monoamines

Levels of norepinephrine (NE), dopamine (DA), serotonin and the latter's primary metabolite, 5-hydroxyindoleacetic acid, were measured in two hypothalamic and 6 vocal control areas in the brains of male zebra finches. NE and DA turnover in these areas were also estimated using alpha-methylparathyrosine. Males were castrated for at least 3 weeks and then received an implant of androstenedione (AE) or a control implant of cholesterol. Each male was then housed with a female for at least one week. significant quantities of the 4 monoamines were found in all brain areas examined; steady-state levels varied significantly across brain areas. Hormone treatment affected steady-state monoamine levels in the preoptic area (POA), the magnocellular paraventricular nucleus and the vocal control area, area X. Hormone treatment altered both NE and DA turnover in the POA, area X and a second vocal control area, nucleus robustus archistriatalis. NE turnover as altered by hormone treatment ina third vocal area, the dorsomedial portion of the intercollicular nucleus. These data suggest that monoaminergic neurotransmitters may be involved in the mediation of steroid-dependent changes in singing behavior in passerine birds.

The distribution and regulation of aromatase activity in the central nervous system

Our data demonstrate that androgen-dependent AA is found in areas of the brain that are essential for the neuroendocrine control of gonadotropin secretion and sexual behavior. However, until we know more about the neurons that contain AA, e.g., whether they are peptidergic or catecholaminergic, we can not speculate about the neuronal functions that depend on local estrogen formation. In fact, the association of AA with neurons and not glia has only recently been demonstrated. That estrogens and androgens synergize in the regulation of various neuroendocrine functions has been known for many years, but an explanation of the synergism at the cellular level was not available. One explanation for this synergism may lie in our recent observation that the administration of exogenous estradiol to castrated rats increases androgen-receptor concentrations in specific brain nuclei. Perhaps locally formed estrogens work in a similar fashion to regulate androgen receptors in the brain of the intact male.

Testosterone induces hypothalamic aromatase during early development in quail

The effects of testosterone on androgen metabolizing enzymes were examined in the developing hypothalamus of male and female quail using an in vitro radiometric assay which measures metabolite formation in individual brain samples. Testosterone (T) administered by subcutaneous silastic implants to gonadectomized 4-day-old chicks increased formation of estradiol-17 beta (E2) in both preoptic area + anterior hypothalamus (PA) and posterior hypothalamus + tuberal area (HT) to adult levels. The T-induced increase in E2 formation occurred to the same degree in both sexes. The increase was very small in control non-target areas, neostriatum intermediale + hyperstriatum ventrale (VN), of either sex. Testosterone had no effect on formation of 5 alpha-dihydrotestosterone (5 alpha-DHT), 5 beta-dihydrotestosterone (5 beta-DHT) and 5 beta-androstane 3 alpha, 17 beta-diol (5 beta, 3 alpha-diol) in PA. Kinetic analysis of the rate of E2 production by hypothalamic tissue from castrated chicks (CX-chicks) and castrates treated with T (CX + T-chicks) indicates that the increase in hypothalamic aromatase activity by T corresponds to induction of the enzyme: the Vmax (maximum velocity) was increased by T (CX-chicks, 21; CX + T-chicks, 91 fmol/mg FW/h), whereas the Km was unaffected (CX-chicks, 5.5; CX + T-chicks, 4.7 X 10(-8) M). Testosterone treatment, effective for inducing PA and HT aromatase activity, also activated crowing and caused cloacal gland development; neither of these effects were sexually dimorphic. Our results indicate that: (1) T induces aromatase specifically in the hypothalamus during early post-hatching development, other pathways of T metabolism are not affected; and (2) the inducible aromatase is not sexually dimorphic in the developing brain. Since there are sex differences in adult brain aromatase, we conclude that capacity for induction of the hypothalamic aromatase becomes sexually differentiated after the post-hatching period.

A possible physiological basis for the dud-stud phenomenon

Intact male rats were tested on two successive weekly tests with females to determine their level of sexual activity. Nuclear estrogen receptor content was measured in specific brain regions of individual sexually responsive and sexually nonresponsive males. Sexually nonresponsive male rats had significantly reduced nuclear estrogen receptor levels in the preoptic area compared to sexually responsive males. Sexually active males did not differ from inactive males in nuclear estrogen receptors in the medialbasal hypothalamus.

The effect of medial preoptic-anterior hypothalamic lesions on testosterone plasma levels and testosterone conversion in the hypothalamus of male rats

Male Wistar rats were submitted to bilateral high frequency lesions in the medial preoptic-anterior hypothalamic area or to sham procedure. The behavioral effect of the lesions was observed and plasma testosterone (T) and estradiol (E2) were measured by radioimmunoassay. In vitro metabolism of T was studied in the hypothalamus. Lesions produced a permanent deficit in male sexual behavior, an increase of plasma T and E2, and of hypothalamic T aromatization, and a decrease of T conversion to 5 alpha-dihydrotestosterone (DHT).

Is the 5 alpha-reductase of the hypothalamus and of the anterior pituitary neurally regulated? Effects of hypothalamic deafferentations and of centrally acting drugs

The following experiments have been performed in order to verify whether the conversion of testosterone into its 5 alpha-reduced metabolites, 5 alpha-androstane-17 beta-ol-3-one (DHT), 5 alpha-androstane-3 alpha,17 beta-diol (3 alpha-diol) and 5 alpha-androstane-3 beta,17 beta-diol (3 beta-diol), in the hypothalamus and in the anterior pituitary is controlled by neural stimuli. Long-term castrated male rats have been submitted to anterior and total deafferentations of the hypothalamus and to the administration of the following centrally acting drugs: reserpine, p-chlorophenylalanine pCPA and atropine sulphate. The possible involvement of the central opioid system has also been investigated utilizing morphine and naloxone. Neither hypothalamic deafferentations, nor the treatment with reserpine, pCPA, atropine, morphine or naloxone produce any significant modification in the metabolism of testosterone in the hypothalamus. Hypothalamic deafferentations and treatments with reserpine, morphine and naloxone are also ineffective in changing the pattern of testosterone metabolism in the anterior pituitary. On the contrary, atropine and pCPA seem to affect the conversion of testosterone in the gland, both drugs producing an increased formation of DHT and 3 alpha-diol but not of 3 beta-diol. It is concluded that the 5 alpha-reductase-3-hydroxysteroid-dehydrogenase system of the hypothalamus does not appear to be controlled either neurally by inputs coming from other brain structures, or by variations of the neurotransmitter content in the hypothalamus itself. Serotonin and acetylcholine seem to participate in the control of testosterone metabolism at pituitary level, even if it is not clear whether their action takes place directly on the gland, or is mediated through some hypothalamic factor(s). Moreover, it does not appear that brain opioids are involved in the control of the enzymatic complex under consideration either in the hypothalamus or in the anterior pituitary.

Odour of male and female rats changes hypothalamic aromatase and 5 alpha-reductase activity and plasma sex steroid levels in unisexually reared male rats

Male rats between 25--65 days of age were reared under four different social conditions: (1) In cohabitation with only males; (2) as in (1), but exposed to bedding from a cage containing other males; (3) as in (1), but exposed to bedding of females; (4) in cohabitation with both males and females. At 65 days of age the animals were killed and analyzed for plasma levels of testosterone and estradiol and in vitro studies were undertaken of hypothalamic testosterone metabolism. Males reared in absence of females showed lowered testosterone and estradiol plasma concentrations and increased hypothalamic aromatase and 5 alpha-reductase activity compared to heterosexually reared males. The effects of cohabitation with males only were counteracted by exposure to bedding of other males or of females suggesting an importance of odoriferous stimuli associated with sexually mature males and females during the sexual maturation of the male rat.