Evolutionary insights into sexual behavior from whiptail lizards

Is the brain bipotential or is sex-typical behavior determined during development? Thirty years of research in whiptail lizards transformed the field of behavioral neuroscience to show the brain is indeed bipotential, producing behaviors along a spectrum of male-typical and female-typical behavior via a parliamentary system of neural networks and not a predetermined program of constrained behavioral output. The unusual clade of whiptail lizards gave these insights as there are several parthenogenetic all-female species that display both male-typical and female-typical sexual behavior. These descendant species exist alongside their ancestors, allowing a unique perspective into how brain-behavior relationships evolve. In this review, we celebrate the over 40-year career of David Crews, beginning with the story of how he established whiptails as a model system through serendipitous behavioral observations and ending with advice to young scientists formulating their own questions. In between these personal notes, we discuss the discoveries that integrated hormones, neural activity, and gene expression to provide transformative insights into how brains function and reshaped our understanding of sexuality.

The non-aromatizable androgen dihydrotestosterone (DHT) facilitates sexual behavior in ovariectomized female rats primed with estradiol

It is still unclear whether Testosterone (T) increases sexual desire through a stimulation of the androgen receptor in relevant brain regions or through its conversion to estrogens. The aim of this study was to clarify the mechanisms of T facilitation of female sexual desire by assessing the effect of a non-aromatizable androgen (Dihydrotestosterone, DHT) in a validated animal model. Ovariectomized (OVX) Long-Evans rats were treated with oil (O) + O, 10 mcg Estradiol Benzoate (EB) + O, 10 mcg EB + 500 mcg Progesterone (P), O + 500 mcg DHT or 10 mcg EB + 500 mcg DHT (n = 12 per group). EB was administered 48 h, while P and DHT 4 h, prior to 4 sexual behavioral testing sessions in bisected unilevel pacing chambers. Appetitive behaviors (the frequencies of hops/darts and solicitations) were considered as the main outcome measure. Sexual receptivity indexes [lordosis magnitude, expressed as lordosis rating (LR), and lordosis quotient (LQ)], rejection responses, as well as mounts, intromissions and ejaculations received from the male were also coded. The probability of transition among sexual behaviors was evaluated by Transition Matrices; T-Pattern analysis was performed to detect hidden repeated temporal behavioral sequences. Preliminary analyses found no statistically significant differences between the O + O and EB + O groups, therefore we excluded the EB + O group from further analyses. Rats treated with EB + DHT displayed significantly more appetitive behaviors compared to negative controls (O + O and O + DHT), whereas no difference was observed between EB + DHT rats and positive controls (EB + P); noteworthy, a higher number of appetitive behaviors was observed in the O + DHT group compared to the O + O group. Furthermore, rats treated with EB + DHT showed significantly higher receptivity measures (LR and LQ) and received more mounts, intromissions and ejaculations compared to negative controls (O + O and O + DHT), to levels equivalent to EB + P. No differences were detected in female-male mounts or rejection responses among the 4 groups. Under a qualitative perspective, full solicitation was found exclusively in T-patterns of the EB + DHT group, which was also the only one to display T-patterns of higher order encompassing appetitive behaviors-only events. In conclusion, the administration of DHT in EB-primed OVX Long-Evans rats enhances sexual behavior measures. Specifically, DHT seems to stimulate sequences of appetitive behaviors separated from copulative/reproductive measures. Our data support an independent role of androgens in the facilitation of female sexual desire.

Androgens coordinate neurotransmitter-related gene expression in male whiptail lizards

Sex steroid hormones coordinate neurotransmitter systems in the male brain to facilitate sexual behavior. Although neurotransmitter release in the male brain has been well documented, little is known about how androgens orchestrate changes in gene expression of neurotransmitter receptors. We used male whiptail lizards (Cnemidophorus inornatus) to investigate how androgens alter neurotransmitter-related gene expression in brain regions involved in social decision making. We focused on three neurotransmitter systems involved in male-typical sexual behavior, including the N-methyl-d-aspartate (NMDA) glutamate receptor, nitric oxide and dopamine receptors. Here, we show that in androgen-treated males, there are coordinated changes in neurotransmitter-related gene expression. In androgen-implanted castrates compared with blank-implanted castrates (control group), we found associated increases in neuronal nitric oxide synthase gene expression in the nucleus accumbens (NAcc), preoptic area and ventromedial hypothalamus, a decrease of NR1 gene expression (obligate subunit of NMDA receptors) in the medial amygdaloid area and NAcc and a decrease in D1 and D2 dopamine receptor gene expression in the NAcc. Our results support and expand the current model of androgen-mediated gene expression changes of neurotransmitter-related systems that facilitate sexual behavior in males. This also suggests that the proposed evolutionarily ancient reward system that reinforces sexual behavior in amniote vertebrates extends to reptiles.

Nitric oxidergic cells related to ejaculation in gerbil forebrain contain androgen receptor and respond to testosterone

Two clusters of forebrain neurons-one in the posterodorsal preoptic nucleus (PdPN) and one in the lateral part of the posterodorsal medial amygdala (MeApd)-are activated at ejaculation in male rats and gerbils as seen with Fos immunocytochemistry. To understand the functions of these cells and how they respond synchronously, it may be useful to identify their neurotransmitters. Nitric oxide (NO) was of interest because its levels in the preoptic area affect ejaculation, and it could synchronize clustered neurons through paracrine/volume transmission. Thus, we determined whether the ejaculation-related cells produce NO by assessing Fos co-localization with NO synthase (NOS) in recently mated male gerbils. We also studied NOS-Fos co-localization in the medial part of the medial preoptic nucleus (MPNm), where half of the neurons that express Fos after mating reflect ejaculation. We also quantified NOS co-localization with androgen receptor (AR) and NOS sensitivity to androgens at these sites. Without quantification, we extended these analyses throughout the hypothalamus and amygdala. Many mating-activated PdPN, lateral MeApd, and MPNm cells contained NOS (32-54%), and many NOS neurons at these sites expressed Fos (34-51%) or AR (25-69%). PdPN and MPNm NOS cells were sensitive to testosterone but not its androgenic metabolite dihydrotestosterone. The overall distribution of NOS and NOS-AR cells was similar to that in rats. These data suggest that NO may help to synchronize the activation of PdPN and lateral MeApd neurons at ejaculation and that NOS in PdPN and MPNm cells is regulated by testosterone acting via estradiol or without undergoing metabolism.

Neuronal nitric oxide synthase as a substrate for the evolution of pseudosexual behaviour in a parthenogenetic whiptail lizard

The evolution of neuroendocrine mechanisms governing sex-typical behaviour is poorly understood. An outstanding animal model is the whiptail lizard (Cnemidophorus) because both the ancestral and descendent species still exist. The ancestral little striped whiptail, Cnemidophorus inornatus, consists of males and females, which exhibit sex-specific mating behaviours. The descendent desert grassland whiptail, Cnemidophorus uniparens, consists only of females that alternately exhibit both female-like and male-like pseudosexual behaviour. Castrated male C. inornatus will mount a conspecific in response to exogenous androgen, although some are also sensitive to progesterone. This polymorphism in progesterone sensitivity in the ancestral species may have been involved in evolution of progesterone-mediated male-typical behaviour in the descendant unisexual lizards. We tested whether progesterone activates a typically androgenic signalling pathway by investigating hormonal regulation of neuronal nitric oxide synthase (nNOS) using in situ hybridisation and NADPH diaphorase histochemistry, a stain for nNOS protein. NADPH diaphorase is widely distributed throughout the brain of both species, although only in the periventricular nucleus of the preoptic area (pvPOA) are there differences between mounting and non-mounting individuals. The number of cells expressing nNOS mRNA and NADPH diaphorase is higher in the pvPOA of individuals that mount in response to progesterone or androgen. Furthermore, the nNOS promoter has both androgen and progesterone response elements, and NADPH diaphorase colocalises with the progesterone receptor in the pvPOA. These data suggest that a polymorphism in progesterone sensitivity in the sexual ancestor reflects a differential regulation of nNOS and may account for the male-typical behaviour in unisexual whiptail lizards.

Molecular characterization and brain distribution of the progesterone receptor in whiptail lizards

Progesterone and its nuclear receptor are critical in modulating reproductive physiology and behavior in female and male vertebrates. Whiptail lizards (genus Cnemidophorus) are an excellent model system in which to study the evolution of sexual behavior, as both the ancestral and descendent species exist. Male-typical sexual behavior is mediated by progesterone in both the ancestral species and the descendant all-female species, although the molecular characterization and distribution of the progesterone receptor protein throughout the reptilian brain is not well understood. To better understand the gene targets and ligand binding properties of the progesterone receptor in whiptails, we cloned the promoter and coding sequence of the progesterone receptor and analyzed the predicted protein structure. We next determined the distribution of the progesterone receptor protein and mRNA throughout the brain of Cnemidophorus inornatus and Cnemidophorus uniparens by immunohistochemistry and in situ hybridization. We found the progesterone receptor to be present in many brain regions known to regulate social behavior and processing of stimulus salience across many vertebrates, including the ventral tegmental area, amygdala, nucleus accumbens and several hypothalamic nuclei. Additionally, we quantified immunoreactive cells in the preoptic area and ventromedial hypothalamus in females of both species and males of the ancestral species. We found differences between both species and across ovarian states. Our results significantly extend our understanding of progesterone modulation in the reptilian brain and support the important role of the nuclear progesterone receptor in modulating sexual behavior in reptiles and across vertebrates.

Future sexual medicine physiological treatment targets

INTRODUCTION

Sexual function in men and women incorporates physiologic processes and regulation of the central and peripheral nervous systems, the vascular system, and the endocrine system. There is need for state-of-the-art information as there is an evolving research understanding of the underlying molecular biological factors and mechanisms governing sexual physiologic functions.

AIM

To develop an evidence-based, state-of-the-art consensus report on the current knowledge of the major cellular and molecular targets of biologic systems responsible for sexual physiologic function.

METHODS

State-of-the-art knowledge representing the opinions of seven experts from four countries was developed in a consensus process over a 2-year period.

MAIN OUTCOME MEASURES

Expert opinion was based on the grading of evidence-based medical literature, widespread internal committee discussion, public presentation, and debate.

RESULTS

Scientific investigation in this field is needed to increase knowledge and foster development of the future line of treatments for all forms of biological-based sexual dysfunction. This article addresses the current knowledge of the major cellular and molecular targets of biological systems responsible for sexual physiologic function. Future treatment targets include growth factor therapy, gene therapy, stem and cell-based therapies, and regenerative medicine.

CONCLUSIONS

Scientific discovery is critically important for developing new and increasingly effective treatments in sexual medicine. Broad physiologic directions should be vigorously explored and considered for future management of sexual disorders.