UC Berkeley

Sex steroids such as testosterone (T) and estradiol (E2) establish structural differences between sexes during sensitive periods of development. These structural changes take place in hormone-sensitive tissues such as the gonads, brain, and larynx, and these structural changes lead to differences in sex and behavior depending on the absence or presence of these sex steroid hormones. For example, the presence of E2 during early development of male rodents masculinizes their brain and can lead to sexually dimorphic brain regions such as the SDN-POA. The difference in brain region size indicates the significance of the SDN in sexually dimorphic traits such as male mating behavior.

Studying the presence or absence of hormones can be done through exposure to hormone-altering chemical reagents. For example, T levels can be increased by exposure to flutamide (an antiandrogen) or decreased by exposure to DHT. Likewise, E2 levels can be increased through tamoxifen (an E2 antagonist) or increased through atrazine, an aromatase inducer. Altering the hormone exposure of individuals will allow us to then observe differences in brain and behavior. Sex steroid effects of hormones on sex differences can be observed through an environmental context. For example, the species Xenopus laevis is a widely studied model system for development. Male X. laevis treated with atrazine were outcompeted by control males for amplexus with females. However, removing competition allowed atrazine-treated males to engage in amplexus behavior.

I observed how altered T and E2 levels affect mating roles in amplexus. In amplexus, the male-typical role is to clasp during mating behavior and the female-typical role is to show receptivity. When treated males were paired with sexually primed females, I tested whether hormone exposure affected the male-typical role to clasp in amplexus. When treated males were sexually primed then paired with males, I tested whether the treated male exhibited the female-typical role (to be receptive in amplexus). Likewise, when treated females were paired with sexually primed females, I tested for alterations to male-typical role in amplexus, and when treated females were sexually primed then paired with males, I tested for alterations to female-typical role in amplexus.

Brain neuropeptide differences were explored to link sex steroid effects of hormones on the brain to behavioral sex differences. Sex steroid effects of hormones may impact sex differences in behavior through structural differences within the brain. The neuropeptide of interest, vasotocin (AVT), is involved in amphibian reproductive behaviors and AVT-IR cell size is sexually dimorphic. AVT-IR cell number and individual cell area were compared across sex and exposure groups and correlated with amplexus behavior to link sex steroid effects of hormones on the brain to behavior.

Lastly, a study site offered the opportunity to study how environmental factors affect reproductive hormones in X. laevis. Two sources of frogs exist in the same river in San Diego, CA where one source lives treated with run-off downstream of a golf course, and the other source lives untreated upstream of the golf course. Hormone levels of frogs were measured over years to establish hormonal differences between sources. These studies provide insight into year-to-year changes between sites and sexes and emphasize the importance for multi-year studies in research related to studying environmental effects on physiology.

The significance of my work on Xenopus laevis will bring attention to the role of sex steroid effects on brain and reproductive behavior in the context of unique study systems through exposure of hormone-altering reagents. Contributions to the fields of endocrinology and reproductive behavior can be made by providing more consideration to sex steroid effects in understudied contexts, such as studying female mating behavior in addition to (and not only) male mating behavior. In addition, careful consideration for multimodal factors of sex that lead to sex differences can encourage studies in research areas that may often overlook or neglect sex differences.