The role of 5alpha-reductase activity in sexual behaviors of the green anole lizard

Seasonal regulation of steroidogenic enzyme expression within the green anole lizard (Anolis carolinensis) brain and gonad

Steroid hormones, such as testosterone and estradiol, are necessary for reproductive behavior. Seasonally breeding animals have increased sex steroid hormone levels during the breeding compared to non-breeding season, with increased reproductive behaviors and altered brain morphology in breeding individuals. Similar to other seasonally breeding animals, green anole lizards (Anolis carolinensis) have high sex steroid hormone levels and increased reproductive behaviors in the breeding season. Relatively less is known regarding the regulation of steroidogenesis in reptiles and this experiment examined whether enzymes involved in sex steroid hormone synthesis vary seasonally within the brain and gonads in wild-caught anole lizards. Specifically, we examined mRNA expression of steroidogenic acute regulatory protein (StAR), P450 17α-hydroxylase/C17-20lyase (Cyp17α1), 17 beta-hydroxysteroid dehydrogenase type 3 (17βHSD 3), and aromatase (Cyp19α1). We found that the mRNA for each of these genes was expressed in the lizard brain. Interestingly, Cyp19α1 mRNA expression in the brain was increased during the non-breeding season, potentially revealing a role for aromatase expression in the non-breeding brain. In the anole gonads, StAR mRNA expression levels were increased in both males and females during the breeding season, while the mRNA expression levels of CYP17α1 and 17βHSD 3 are increased when StAR mRNA expression was decreased, suggesting that the enzymes in the steroidogenic pathway are potentially regulated independently of StAR. This work reveals the seasonal regulation of steroidogenesis in the reptilian brain and gonad, although more work is necessary to determine the regulatory mechanisms that control these expression patterns.

To fight or mate? Hormonal control of sex recognition, male sexual behavior and aggression in the gecko lizard

Squamate reptiles are a highly diversified vertebrate group with extensive variability in social behavior and sexual dimorphism. However, hormonal control of these traits has not previously been investigated in sufficient depth in many squamate lineages. Here, we studied the hormonal control of male sexual behavior, aggressiveness, copulatory organ (hemipenis) size and sex recognition in the gecko Paroedura picta, comparing ovariectomized females, ovariectomized females treated with exogenous dihydrotestosterone (DHT), ovariectomized females treated with exogenous testosterone (T), control females and males. The administration of both T and DHT led to the expression of male-typical sexual behavior in females. However, in contrast to T, increased circulating levels of DHT alone were not enough to initiate the full expression of male-typical offensive aggressive behavior and development of hemipenes in females. Ovariectomized females were as sexually attractive as control females, which does not support the need for the demasculinization of the cues used for sex recognition by ovarian hormones as suggested in other sauropsids. On the other hand, our results point to the masculinization of the sex recognition cues by male gonadal androgens. Previously, we also demonstrated that sexually dimorphic growth is controlled by ovarian hormones in P. picta. Overall, it appears that individual behavioral and morphological sexually-dimorphic traits are controlled by multiple endogenous pathways in this species. Variability in the endogenous control of particular traits could have permitted their disentangling during evolution and the occurrence of (semi)independent changes across squamate phylogeny.

Testicular steroidogenic enzymes in the lizard Podarcis sicula during the spermatogenic cycle

Steroidogenic Acute Regulatory protein (StAR), 3β-hydroxysteroid dehydrogenase (3β-HSD), 17β-hydroxysteroid dehydrogenase (17β-HSD), 5α-Reductase (5α-Red), P450 aromatase are key enzymes involved in steroidogenesis. Recently, we showed the expression and the localization of P450 aromatase in Podarcis sicula testis during the different phases of the reproductive cycle, showing its involvement in the control of steroidogenesis, particularly in 17β-estradiol synthesis. Now, we have investigated the presence and distribution of the other enzymes involved in steroidogenesis, i.e. StAR, 3β-HSD, 17β-HSD and 5α-Red, during three significant periods of the reproductive cycle: summer stasis (July-August), autumnal resumption (November) and reproductive period (May-June). We demonstrated for the first time that all these enzymes are always present in somatic cells (Leydig and Sertoli) and germ cells (spermatogonia, spermatocytes I and II, spermatids and spermatozoa) of Podarcis testis, mainly in spermatids and spermatozoa. The present results strongly suggest that in Podarcis testis both somatic and germ cells could be involved in local sex hormone synthesis and that 5α-Red and P450 could carry out a pivot role.

Expression of 5α- and 5β-reductase in spinal cord and muscle of birds with different courtship repertoires

Background

Through the actions of one or more isoforms of the enzyme 5α-reductase in many male reproductive tissues, circulating testosterone (T) undergoes metabolic conversion into 5α-dihydrotestosterone (DHT), which binds to and activates androgen receptors (AR) with greater potency than T. In birds, T is also subject to local inactivation into 5β-DHT by the enzyme 5β-reductase. Male golden-collared manakins perform an androgen-dependent and physically elaborate courtship display, and these birds express androgen receptors in skeletal muscles and spinal cord at levels far greater than those expressed in species with more limited courtship routines, including male zebra finches. To determine if local T metabolism facilitates or impedes activation of male manakin courtship, we examined expression of two isoforms of 5α-reductase, as well as 5β-reductase, in forelimb muscles and spinal cords of males and females of the two aforementioned species.

Results

We found that all enzymes were expressed in all tissues, with patterns that partially predict a functional role for 5α-reductase in these birds, especially in both muscle and spinal cord of male manakins. Moreover, we found that 5β-reductase was markedly different between species, with far lower levels in golden-collared manakins, compared to zebra finches. Thus, modification to neuromuscular deactivation of T may also play a functional role in adaptive behavioral modulation.

Conclusions

Given that such a role for 5α-reductase in androgen-sensitive mammalian skeletal muscle is in dispute, our data suggest that, in birds, local metabolism may play a key role in providing active androgenic substrates to peripheral neuromuscular systems. Similarly, we provide the first evidence that 5β-reductase is expressed broadly through an organism and may be an important factor that regulates androgenic modulation of neuromuscular functioning.

Hormonal regulation of steroid receptor coactivator-1 mRNA in the male and female green anole brain

Green anole lizards are seasonal breeders, with male sexual behaviour primarily regulated by an annual increase in testosterone. Morphological, biochemical and behavioural changes associated with reproduction are activated by testosterone, generally with a greater effect in the breeding season (BS) than in the nonbreeding season (NBS). The present study investigates the possibility that differences in a steroid receptor coactivator may regulate this seasonal difference in responsiveness to testosterone. In situ hybridisation was used to examine the expression of steroid receptor coactivator-1 (SRC-1) in the brains of gonadally intact male and female green anoles across breeding states. A second experiment examined gonadectomised animals with and without testosterone treatment. Gonadally intact males had more SRC-1 expressing cells in the preoptic area and larger volumes of this region as defined by these cells than females. Main effects of both sex and season (males > females and BS > NBS) were present in cell number and volume of the ventromedial hypothalamus. An interaction between sex and season suggested that high expression in BS males was driving these effects. In hormone-manipulated animals, testosterone treatment increased both the number of SRC-1 expressing cells in and volumes of the preoptic area and amygdala. These results suggest that testosterone selectively regulates SRC-1, and that this coactivator may play a role in facilitating reproductive behaviours across both sexes. However, changes in SRC-1 expression are not likely responsible for the seasonal change in responsiveness to testosterone.

Relationships among sex, season and testosterone in the expression of androgen receptor mRNA and protein in the green anole forebrain

Sexual behavior in male green anole lizards is regulated by a seasonal increase in testosterone (T). However, T is much more effective at activating behavioral, morphological and biochemical changes related to reproduction in the breeding season (BS; spring) compared to nonbreeding season (NBS; fall). An increase in androgen receptor (AR) during the BS is one potential mechanism for this differential responsiveness. AR expression has not been investigated in specific brain regions across seasons in anoles. The present studies were designed to determine relative AR expression in areas important for male (preoptic area, ventromedial amygdala) and female (ventromedial hypothalamus) sexual behavior, as well as whether T upregulates AR in the anole brain. In situ hybridization and Western blot analyses were performed in unmanipulated animals across sex and season, as well as in gonadectomized animals with and without T treatment. Among hormone-manipulated animals, more cells expressing AR mRNA were detected in females than males in the amygdala. T treatment increased the volume of the ventromedial hypothalamus of gonadectomized animals in the BS, but not the NBS. AR protein in dissections of the hypothalamus and preoptic area was increased in males compared to females specifically in the BS. Additionally, among females, it was increased in the NBS compared to the BS. Collectively, these results indicate that differences in central AR expression probably do not facilitate a seasonal responsiveness to T. However, they are consistent with a role for AR in regulating some differences between sexes in the display of reproductive behaviors.

Seasonal and sexual dimorphisms in expression of androgen receptor and its coactivators in brain and peripheral copulatory tissues of the green anole

Green anoles are seasonally breeding lizards, with an annual rise in testosterone (T) being the primary activator of male sexual behaviors. Responsiveness to T is decreased in the non-breeding season (NBS) compared to breeding season (BS) on a variety of levels, including displays of reproductive behavior and the morphology and biochemistry of associated tissues. To evaluate the possibility that seasonal changes in responsiveness to T are regulated by androgen receptors (AR) and/or two of its coactivators, CREB binding protein (CBP) and steroid receptor coactivator-1 (SRC-1), we tested whether they differ in expression across season in brains of both sexes and in peripheral copulatory tissues of males (hemipenis and retractor penis magnus muscle). AR mRNA was increased in the brains of males compared to females and in copulatory muscle in the BS compared to NBS. In the hemipenis, transcriptional activity appeared generally diminished in the NBS. T-treatment increased AR mRNA in the copulatory muscle and AR protein in the hemipenis, the latter to a greater extent in the BS than the NBS. T also decreased SRC-1 protein in hemipenis. Interpretations are complicated, in part because levels of mRNA and protein expression were not correlated and multiple sizes of the AR and CBP proteins were detected, with some tissue specificity. However, the results are consistent with the idea that differences in receptor and coactivator expression at central and peripheral levels may play roles in regulating sex and seasonal differences in the motivation or physical ability to engage in sexual behavior.

Expression of aromatase and two isozymes of 5α-reductase in the developing green anole forebrain

Neural steroids, as well as the enzymes that produce these hormones, are important for sexual differentiation of the brain during development. Aromatase converts testosterone into oestradiol. 5α-reductase converts testosterone to 5α-dihydrotestosterone and occurs in two isozymes: type 1 (5αR1) and type 2 (5αR2). Each of these enzymes is present in the developing brain in many species, although no work has been carried out examining the expression of all three enzymes in non-avian reptiles with genetic sex determination. In the present study, we evaluated mRNA expression of neural aromatase, 5αR1 and 5αR2, on the day of hatching and at day 50 in one such lizard, the green anole. We describe the distribution of these enzymes throughout the brain and the quantification of mRNA expression in three regions that control adult sexual behaviours: the preoptic area (POA) and ventromedial amygdala (AMY), which are involved in male displays, as well as the ventromedial hypothalamus, which regulates female receptivity. Younger animals had a greater number (POA) and density (AMY) of 5αR1 mRNA expressing cells. We detected no effects of sex or age on aromatase or 5αR2. In comparison with data from adults, the present results support the idea that the green anole forebrain has not completely differentiated by 50 days after hatching and that 5αR1 may play a role in the early development of regions important for masculine function.

Aromatase and 5α-reductase type 2 mRNA in the green anole forebrain: an investigation of the effects of sex, season and testosterone manipulation

Aromatase and 5α-reductase (5αR) catalyze the synthesis of testosterone (T) metabolites: estradiol and 5α-dihydrotestosterone, respectively. These enzymes are important in controlling sexual behaviors in male and female vertebrates. To investigate factors contributing to their regulation in reptiles, male and female green anole lizards were gonadectomized during the breeding and non-breeding seasons and treated with a T-filled or blank capsule. In situ hybridization was used to examine main effects of and interactions among sex, season, and T on expression of aromatase and one isozyme of 5αR (5αR2) in three brain regions that control reproductive behaviors: the preoptic area, ventromedial nucleus of the amygdala and ventromedial hypothalamus (VMH). Patterns of mRNA generally paralleled previous evaluations of intact animals. Although no main effects of T were detected, interactions were present in the VMH. Specifically, the density of 5αR2 expressing cells was greater in T-treated than control females in this region, regardless of season. Among breeding males, blank-treated males had a denser population of 5αR2 positive cells than T-treated males. Overall, T appears to have less of a role in the regulation of these enzymes than in other vertebrate groups, which is consistent with the primary role of T (rather than its metabolites) in regulation of reproductive behaviors in lizards. However, further investigation of protein and enzyme activity levels are needed before specific conclusions can be drawn.

Relationships among hormones, brain and motivated behaviors in lizards

Lizards provide a rich opportunity for investigating the mechanisms associated with arousal and the display of motivated behaviors. They exhibit diverse mating strategies and modes of conspecific communication. This review focuses on anole lizards, of which green anoles (Anolis carolinensis) have been most extensively studied. Research from other species is discussed in that context. By considering mechanisms collectively, we can begin to piece together neural and endocrine factors mediating the stimulation of sexual and aggressive behaviors in this group of vertebrates.

Testosterone selectively affects aromatase and 5alpha-reductase activities in the green anole lizard brain

Testosterone (T) and its metabolites are important in the regulation of reproductive behavior in males of a variety of vertebrate species. Aromatase converts T to estradiol and 5alpha-reductase converts T to 5alpha-dihydrotestosterone (DHT). Male green anole reproduction depends on androgens, yet 5alpha-reductase in the brain is not sexually dimorphic and does not vary with season. In contrast, aromatase activity in the male brain is increased during the breeding compared to non-breeding season, and males have higher levels than females during the breeding season. Aromatase is important for female, but not male, sexual behaviors. The present experiment was conducted to determine whether 5alpha-reductase and aromatase are regulated by T. Enzyme activity was quantified in whole brain homogenates in both the breeding and non-breeding seasons in males and females that had been treated with either a T or blank implant. In males only, T increased 5alpha-reductase activity regardless of season and up-regulated aromatase during the breeding season specifically. Thus, regulation of both enzymes occurs in males, whereas females do not show parallel sensitivity to T. When considered with previous results, the data suggest that aromatase might influence a male function associated with the breeding season other than sexual behavior. 5alpha-Reductase can be mediated by T availability, but this regulation may not serve a sex- or season-specific purpose.

Behavioral diversity and neurochemical plasticity: selection of stress coping strategies that define social status

Social interactions include a variety of stimulating but challenging factors that are the basis for strategies that allow individuals to cope with novel or familiar stressful situations. Evolutionarily conserved strategies have been identified that reflect specific behavioral and physiological identities. In this review we discuss a unique model for social stress in the lizard Anolis carolinensis, which has characteristics amenable to an investigation of individual differences in behavioral responses via central and sympathetic neurochemical adaptation. Profiles of proactive and reactive phenotypes of male A. carolinensis are relatively stable, yet retain limited flexibility that allows for the development of the social system over time. For male A. carolinensis, the celerity of social signal expression and response translate into future social standing. In addition, proactive aggressive, courtship, and feeding behaviors also predict social rank, but are not as important as prior interactions and memories of previous opponents to modify behavioral output and affect social status. The central neurotransmitters dopamine and serotonin, and the endocrine stress axis (HPA) appear to be the fundamental link to adaptive stress coping strategies during social interactions. Only small adaptations to these neural and endocrine systems are necessary to produce the variability measured in behavioral responses to stressful social interactions. These neuroendocrine factors are also manifest in responses to other stimuli and form the basis of heritable strategies for coping with stress.

Androgen dependent seasonal changes in muscle fiber type in the dewlap neuromuscular system of green anoles

Green anoles (Anolis carolinensis) possess two sexually dimorphic neuromuscular systems involved in reproductive behaviors. One controls extension of a red throat fan (dewlap), which males employ during courtship, and the other controls intromission of copulatory organs (hemipenes). Although seasonal changes in circulating androgens mediate both courtship and copulatory behaviors, testosterone has differential effects on the underlying neuromuscular morphology. The present experiments were designed to test whether changes in muscle fiber type correspond to seasonal and androgenic regulation of reproductive behaviors in gonadally intact males (Experiment 1) or castrated males treated with either testosterone propionate or vehicle (Experiment 2). Gonadally intact males housed in breeding environmental conditions had a higher percentage of fast oxidative glycolytic fibers in the dewlap muscle than non-breeding males, but no effect of season on copulatory fibers was detected. Interestingly, testosterone treatment increased the percentage of fast oxidative glycolytic dewlap fibers independent of season, suggesting that routine changes in this hormone may mediate fiber type in gonadally intact males. In contrast, testosterone manipulation had little to no effect on copulatory muscle fiber type, demonstrating that a change in this feature is not the primary mediator for seasonal changes in male copulatory behaviors.

Androgen receptor expression and morphology of forebrain and neuromuscular systems in male green anoles displaying individual differences in sexual behavior

Investigating individual differences in sexual performance in unmanipulated males is important for understanding natural relationships between behavior and morphology, and the mechanisms regulating them. Among male green anole lizards, some court and copulate frequently (studs) and others do not (duds). To evaluate potential factors underlying differences in the level of these behaviors, morphology and androgen receptor expression in neuromuscular courtship and copulatory structures, as well as in the preoptic area and amygdala, were compared in males displaying varying degrees of sexual function. This study revealed that individual differences in behavior among unmanipulated males, in particular the extension of a throat fan (dewlap) used during courtship, were positively correlated with the size of fibers in the associated muscle and with soma size in the amygdala. The physiological response to testosterone, as indicated by the height of cells in an androgen-sensitive portion of the kidney, was also correlated with male sexual behavior, and predicted it better than plasma androgen levels. Androgen receptor expression was not related to the display of courtship or copulation in any of the tissues examined. The present data indicate that higher levels of male courtship behavior result in (or are the result of) enhanced courtship muscle and amygdala morphology, and that androgen-sensitive tissue in studs may be more responsive to testosterone than duds. However, some mechanism(s) other than androgen receptor expression likely confer this difference in responsiveness.

Effects of season, testosterone and female exposure on c-fos expression in the preoptic area and amygdala of male green anoles

Expression of the immediate early gene, c-fos, was used to investigate changes in neuronal activity in forebrain regions involved in male sexual behavior following social, hormonal and/or seasonal manipulations in the male green anole. These factors all influence behavior, yet it is unclear how they interact to modify neuronal activity in forebrain regions, including the preoptic area (POA) and ventromedial nucleus of the amygdala (AMY). These regions are involved in the display of sexual behaviors in male green anoles as in many other vertebrates. To determine the effects of seasonal, hormonal and social cues on these brain areas, we investigated c-fos under environmental conditions typical of the breeding or non-breeding season in adult male green anoles that were castrated and implanted with either testosterone (T) or blank (Bl) capsules. We also manipulated social cues by exposing only half of the animals in each group to females. T enhanced courtship and copulatory behaviors, but decreased c-fos expression in the AMY. A similar, although not statistically significant, pattern was observed in the POA, and the density of c-fos+ cells was negatively correlated in that region with the number of extensions of a throat fan (dewlap) used during courtship. Therefore, it appears that in the male green anole, T may diminish c-fos expression (likely in inhibitory neurons) in the POA and AMY to create a permissive environment in which the appropriate behavioral response can be displayed.

Courtship and copulation in the adult male green anole: effects of season, hormone and female contact on reproductive behavior and morphology

Interactions among reproductive season, testosterone (T) and female presence were investigated on the structure and function of forebrain and neuromuscular systems controlling courtship and copulation in the green anole lizard. Under breeding (BS) or non-breeding (NBS) environmental conditions, male green anoles were implanted with either T or blank capsules and exposed to one of three female stimulus conditions: physical, visual or no female contact. T and at least visual exposure to females increased courtship displays (extension of a throat fan, or dewlap), and these effects were greater during the BS than NBS. T also facilitated copulation, and did so to a greater extent in the BS. The hormone increased soma size in the preoptic area (POA) and amygdala (AMY), and in the AMY the effects were greater in the BS than NBS. Cross-sectional areas of copulatory organs and associated muscle fibers were enhanced by T, and more so in the BS than NBS. However, no effects on morphology of dewlap motoneurons or muscles or copulatory motoneurons were detected. Thus, (1) changes in behavior and neural and/or muscular morphology are not always parallel and (2) differences in responsiveness to T exist across seasons and among tissues.

Self-administration of estrogen and dihydrotestosterone in male hamsters

Anabolic-androgenic steroids (AAS) are drugs of abuse. Previous studies have shown that male and female hamsters self-administer testosterone (T) and other AAS, suggesting that androgens are reinforcing in a context where athletic performance is irrelevant. AAS are synthetic derivatives of T, which may be aromatizable to estrogen and/or reducible to dihydrotestosterone (DHT). However, we do not know which metabolites of T are reinforcing. To determine if DHT, estradiol (E(2)), or DHT + E(2) are reinforcing, we tested intracerebroventricular (icv) self-administration in male hamsters. The hypothesis was that androgen reinforcement is sensitive to both androgenic and estrogenic T metabolites. If so, hamsters would self-administer DHT, E(2), and DHT + E(2). Twenty four castrated male hamsters (n = 8/group) received icv cannulas and sc T implants for physiologic androgen replacement. One week later, hamsters self-administered DHT (0.1, 1.0, 2.0 microg/microl), E(2) (0.001, 0.01, 0.02 microg/microl), or DHT + E(2), each for 8 days in increasing concentration (4 h/day). Operant chambers were equipped with an active and inactive nose-poke. At the medium concentration, hamsters self-administered DHT (active nose-poke: 47.9 +/- 13.9 responses/4 h vs. inactive: 18.7 +/- 4.8), E(2) (active: 44.8 +/- 14.9 vs. inactive: 16.6 +/- 2.6), and DHT + E(2) (active: 19.1 +/- 2.4 vs. inactive: 10.4 +/- 2.4, P < 0.05). At the highest concentration, males self-administered DHT (active: 28.3 +/- 7.7 vs. inactive: 15.0 +/- 3.5, P < 0.05) and DHT + E(2) (active: 22.6 +/- 3.8 vs. inactive: 11.6 +/- 2.5, P < 0.05), but not E(2). Hamsters did not self-administer the lowest concentrations of DHT, E(2), or DHT + E(2). These results support our hypothesis that both androgenic and estrogenic T metabolites are reinforcing. Together, they do not exert synergistic effects.

Current research on the behavioral neuroendocrinology of reptiles

Selected reptilian species have been the targets of investigations in behavioral neuroendocrinology for many years. Reptiles offer a particularly powerful set of traits that facilitate comparisons at multiple levels, including those within and between individuals of a particular species, between different environmental and social contexts, as well as across species. These types of studies, particularly as they are considered within the framework of results from other vertebrates, will enhance our understanding of the genetic and hormonal influences regulating changes in the structure and function of the nervous system. Work on the hormonal and environmental factors influencing courtship and copulatory behaviors in green anoles, including the development and maintenance of the neuromuscular structures critical for their display, is highlighted. Some very recent work on other model systems is also discussed to provide a context for suggested future research directions.

Testosterone regulates androgen receptor immunoreactivity in the copulatory, but not courtship, neuromuscular system in adult male green anoles

Androgens regulate the expression of male reproductive behaviour in diverse vertebrate species, often acting on androgen receptors (AR) to induce structural or functional changes in the nervous system and periphery. Male green anoles possess two sexually dimorphic neuromuscular systems, one controlling throat fan (dewlap) extension, which occurs during courtship, and the other mediating copulatory organ function. Although androgens are required for behavioural activation in both systems, testosterone has differential effects on the neuromuscular morphology. It increases the size of copulatory muscle fibres during the breeding season, but significant effects on dewlap muscle fibre size and motoneurone soma size in either system have not been detected. Corresponding to the lack of testosterone-induced morphological effects in the courtship system, relatively low levels of AR are expressed in the associated motoneurones. The present experiment had two goals, aiming to determine whether: (i) the other courtship and copulatory neuromuscular tissues express AR and (ii) testosterone and/or seasonal environmental changes regulate AR expression. The percentage of AR+ nuclei was evaluated in both the breeding and nonbreeding seasons in gonadally intact adult males (Experiment 1) and in castrated males treated with either testosterone or vehicle (Experiment 2). AR was extensively expressed in the dewlap and copulatory muscles, and in a high percentage of the copulatory motoneurones, but immunoreactivity did not vary across season. Testosterone increased the percentage of AR+ nuclei in the copulatory muscles of both breeding and nonbreeding males but not in the dewlap muscle or copulatory motoneurones. Finally, the target structures for both systems (cartilages and hemipenes) expressed AR in all animals. Therefore, the effects of testosterone on AR immunoreactivity suggest that up-regulation of the receptors may be important for morphological change. However, because all structures investigated in the present experiment expressed AR, the data also indicate that the receptors are involved with other functions.

Evolutionary insights into the regulation of courtship behavior in male amphibians and reptiles

Comparative studies of species differences and similarities in the regulation of courtship behavior afford an understanding of evolutionary pressures and constraints shaping reproductive processes and the relative contributions of hormonal, genetic, and ecological factors. Here, we review species differences and similarities in the control of courtship and copulatory behaviors in male amphibians and reptiles, focusing on the role of sex steroid hormones, the neurohormone arginine vasotocin (AVT), and catecholamines. We discuss species differences in the sensory modalities used during courtship and in the neural correlates of these differences, as well as the value of particular model systems for neural evolution studies with regard to reproductive processes. For example, in some genera of amphibians (e.g., Ambystoma) and reptiles (e.g., Cnemidophorus), interspecific hybridizations occur, making it possible to compare the ancestral with the descendant species, and these systems provide a window into the process of behavioral and neural evolution as well as the effect of genome size. Though our understanding of the hormonal and neural correlates of mating behavior in a variety of amphibian and reptilian species has advanced substantially, more studies that manipulate hormone or neurotransmitter systems are required to assess the functions of these systems.

Seasonal plasticity in the copulatory neuromuscular system of green anole lizards: a role for testosterone in muscle but not motoneuron morphology

The copulatory system of green anoles is highly sexually dimorphic. Males possess bilateral copulatory organs called hemipenes, each independently controlled by two muscles: the transversus penis (TPN) and retractor penis magnus (RPM). The TPN everts the hemipene through the cloaca and the RPM retracts it. Adult females do not possess hemipenes or either of these two muscles. The spinal nucleus projecting to the TPN and RPM contains more and larger motoneurons in males than females. Because anoles breed seasonally, two experiments were designed to test whether adult copulatory morphology varies with environmental condition, and if so, whether the effect is mediated by testicular androgens. Three groups of adult males were used in each experiment: males from breeding environmental conditions with reproductive testes (BS); males in breeding conditions with regressed testes (BS-X); and males in nonbreeding conditions with regressed testes (NBS). Experiment 1 compared gonadally intact males and Experiment 2 compared castrated males treated with either testosterone (T) or an empty implant. In both experiments, copulatory and control motoneurons appeared smaller in NBS males, but T did not affect their size. In contrast, while hemipene and RPM muscle fiber size were not plastic across season in gonadally intact males, T in castrated males significantly increased both measures under BS and BS-X, but not NBS, conditions. These results demonstrate that neuron soma size might change on a general level and environmental cues can mediate T-induced changes in peripheral structures, suggesting that plasticity across copulatory system components is regulated by different mechanisms.

Effects of testosterone on the development of neuromuscular systems and their target tissues involved in courtship and copulation in green anoles (Anolis carolinensis)

Male green anole lizards court females using a red throat fan (dewlap) and copulate by intromitting one of two penises (hemipenes). These structures begin sexually monomorphic, but by adulthood males have larger dewlaps, only males have hemipenes, and many of the neuromuscular components of both systems show male-biased dimorphisms. We hypothesized that testosterone (T), which increases in juvenile males but not females about a month after hatching, facilitates masculinization. To test this idea, on post-hatching day 30, gonadally intact females received either a blank or T implant, and males were either castrated or sham-castrated. At day 90, juveniles were euthanized and the length of the cartilage and cross-sectional areas of the muscle fibers and motoneurons required for dewlap extension were examined. We also measured the cross-sectional areas of the hemipenes and associated muscle fibers and motoneurons, and counted the motoneurons. T-treated females had longer cartilages and larger dewlap muscle fibers compared to those with blank implants. No effects on motoneurons were detected, and no females possessed hemipenes or associated musculature. In males, castration produced shorter dewlap cartilages and smaller hemipenes; other measures were not affected by treatment. These data indicate that components of the dewlap system differentiate relatively late in development, that T likely mediates the process, and that although components of the copulatory system are plastic in juvenile males, sexual differentiation of peripheral features is complete before day 30. The data also suggest that target structures (dewlap cartilage and hemipenes), compared to their neuromuscular effectors, are particularly sensitive to developmental T exposure.

The green anole (Anolis carolinensis): a reptilian model for laboratory studies of reproductive morphology and behavior

The green anol (Anolis carolinensis) is an excellent reptilian model for studying reproductive behavior and the neural and muscular morphology that supports it. This lizard has been the subject of behavioral and ecological study for more than 100 yr, and a rich literature exists on its natural history. Both courtship and copulatory behaviors reveal sex and seasonal differences, which allow for the study of mechanisms regulation naturally occurring variation in performance at multiple levels within a single animal model. Green anoles are readily obtained due to their abundance in the wild; once in the laboratory, they are easily maintained, bred, and reared. Background on the natural history and husbandry of this lizard is provided, and the authors' research program on the regulation of reproductive anatomy and behavior is reviewed, Discussion includes the similarities and differences in the mechanisms mediating both structure and function compared with more traditional animal models. This type of comparative research will make it possible to identify the fundamental principles governing reproductive biology, thus advancing both basic and applied knowledge.

Seasonal and sexual dimorphisms in the green anole forebrain

During the breeding season, male green anole lizards extend a throat fan (dewlap) in courtship. This behavior is facilitated by testosterone (T). Females extend a much smaller dewlap less often, even with the same dose of T. During the nonbreeding season when T is low, dewlap extension is reduced. To determine if parallels exist between structure and function, we investigated neuron soma size and density in the preoptic area (POA) and ventromedial nucleus of the amygdala (AMY), which are involved in the display behavior, in breeding and nonbreeding males and females. Cells from breeding animals were larger than cells from nonbreeding animals, but they were not sexually dimorphic. No significant effects existed in neuron density. This experiment indicates that portions of the anole forebrain important for sexual behavior are plastic and might be influenced by seasonal changes in steroid hormones. To investigate whether T can reverse the seasonal difference in soma size in both sexes, gonadectomized nonbreeding anoles were implanted with an empty or T propionate-filled capsule; animals were also tested for male-typical courtship behavior. Males and females treated with T had higher rates of dewlap extension, but across treatment groups these rates were greater in males. Neuron soma size in the POA and AMY was larger in males than females, but no effects of treatment were detected. Taken together, the results indicate that T can stimulate behavior in the nonbreeding season and suggest that a dissociation exists between the regulation of the courtship display and soma size of relevant brain regions.

Effects of testosterone treatment and season on the frequency of dewlap extensions during male-male interactions in the lizard Anolis sagrei

Lizards of the genus Anolis extend and retract a large and often brightly colored throat fan called a dewlap. The dewlap in most anoles is a sexually dimorphic structure. It is larger in males than females and males use the dewlap more frequently and in more contexts than do females. In the present study we investigated whether plasma testosterone (T) levels and season affect the frequency of dewlap use in male--male interactions in the brown anole, Anolis sagrei. We manipulated plasma T levels by implanting adult, castrated males with pellets that delivered no T, a moderate dose of T, or a high dose of T. In tests with stimulus males, castrated males that were treated with a moderate or with a high dose of T had a significantly higher frequency of dewlap extensions than did castrated males that received no T. However, the frequency of dewlap extensions in castrated males that received the high dose of T did not differ significantly from castrated males that received the moderate dose of T or from non-castrated control males. Males captured during the breeding season and tested in the laboratory had a significantly higher frequency of dewlap extensions than did males captured in the postbreeding season and tested in the same manner. These results suggest that plasma T levels affect how frequently males of A. sagrei extend their dewlaps in male--male interactions and that seasonal changes in male dewlap use may be due to seasonal differences in plasma T levels.

Distribution of androgen receptor mRNA expression and immunoreactivity in the brain of the green anole lizard

Male courtship and copulation are androgen dependent in the green anole lizard, and female receptivity can be facilitated by testosterone. However, only a few, and relatively large, regions in the brain have been implicated in the control of these behaviours. In situ hybridization and immunohistochemistry were therefore used to determine in detail where androgens are likely to act in the brains of breeding males and females. A 697-bp fragment of the anole androgen receptor (AR) was cloned from total RNA isolated from the kidney, which contains the highly androgen-sensitive renal sex segment. The cloned fragment spanned part of the C, the entire D, and part of the E domains, and shared a high degree of similarity with the AR of various species. 35S-labelled antisense and sense probes were generated from the 697-bp fragment for use in in situ hybridization, and the AR antibody PG-21 was used for immunohistochemistry. Both sexes consistently had AR mRNA expression and immunoreactivity in areas associated with vertebrate reproductive behaviours and in motor areas of the brainstem. Interestingly, the PG-21 antibody produced labelling in both the nucleus and cytoplasm, including neuronal processes. The distribution of mRNA and immunoreactivity were comparable in males and females, and the amount of labelling was generally similar, although slightly greater in females. The expression pattern of AR in this species supports the idea that distribution is highly conserved among vertebrates, but that it probably does not dictate behavioural differences between the sexes in anoles.

Sexual dimorphism in neuromuscular junction size on a muscle used in courtship by green anole lizards

The green anole lizard exhibits seasonal courtship behavior that is sexually dimorphic. This courtship consists of the extension of a bright red throat fan (dewlap) associated with head-bobbing display behavior. While males extend their dewlaps in aggressive encounters as well as in courtship, females use their considerably smaller dewlaps much less frequently and mainly in agonistic encounters. In parallel, a number of components of the neuromuscular system controlling dewlap extension are greater in males than in females during the breeding season, including dewlap motoneuron soma size and muscle fiber size and number. These features do not seem to change substantially in adulthood, despite a dramatic decline in dewlap use during the nonbreeding season. We explored the morphology of this neuromuscular system in more detail in the present experiment in males and females during both the breeding and nonbreeding seasons. Fiber and whole muscle length (approximately perpendicular to the fibers) were measured. Acetylcholinesterase histochemistry was used to visualize neuromuscular junctions (NMJs), and the surface area and density of NMJs were assessed for each animal. During the breeding season, NMJ size was larger in males than in females, but NMJ density along each fiber was equivalent between the sexes. In addition, whole muscle length and that of individual muscle fibers, was larger in males than in females. However, when corrected for body size, the sex difference in muscle fiber length disappeared. In the nonbreeding season, the sexual dimorphisms were maintained, suggesting that these features do not change substantially due to differences in circulating testosterone or a difference in use across seasons. Overall, these results are consistent with the idea that enhanced NMJ size is a relatively stable feature of the dewlap muscle in adulthood that either facilitates or is a consequence of using a larger muscle to extend a bigger dewlap in males compared to females.

Androgen metabolism in the brain of the green anole lizard (Anolis carolinensis): effects of sex and season

Courtship behavior in male green anoles is partly mediated by the 5alpha-reductase enzyme, which converts testosterone (T) to 5alpha-dihydrotestosterone. This study aimed to determine whether the activity of 5alpha-reductase is enhanced in breeding males compared to females and nonbreeding males who do not normally display masculine behaviors. In some cases, aromatase, which converts T to 17beta-estradiol, also was assessed to determine whether the pattern of its activity in anoles is similar to that in other vertebrates. 5alpha-Reductase is greatest in the brainstem, so its activity was determined separately in homogenates of whole brain and brainstems. The following comparisons were done in different assays: (1) breeding males with breeding females, (2) nonbreeding males with nonbreeding females, and (3) breeding males with nonbreeding males. Aromatase activity was greater in breeding males (mean +/- SE, 0.61 +/- 0.06 fmol/min/mg protein) than in breeding females (0.41 +/- 0.08 fmol/min/mg protein). It was also greater in breeding males (0.84 +/- 0.16 fmol/min/mg protein) than in nonbreeding males (0.33 +/- 0.07 fmol/min/mg protein). In contrast, sex or seasonal differences did not exist in 5alpha-reductase activity. The results are consistent with those of other vertebrate species in which male-biased sex dimorphisms and seasonal differences occur in aromatase, but not in 5alpha-reductase activity. The greater levels of aromatase activity in breeding male anoles suggest that this enzyme might mediate male-specific functions. The equivalently high levels of 5alpha-reductase activity in both sexes suggests that, in addition to facilitating male courtship behavior, the enzyme has a basic function common to both sexes.