Animal studies suggest that reproductive behavior fully recovers after puberty-blocking treatment ends

Recent research provides evidence that temporarily pausing puberty with the drug leuprolide delays reproductive development in adolescent rats but allows for full physical and behavioral maturation after the treatment stops. The findings suggest that the brain and body adjust to the medication through specific genetic changes, enabling normal reproductive function to eventually emerge. These two related studies were published in the journals Biology of Sex Differences and Hormones and Behavior.

Adolescents who experience distress because their gender identity does not match their sex assigned at birth are sometimes prescribed puberty-blocking medications. These drugs, known as gonadotropin-releasing hormone agonists, temporarily pause physical changes like breast development or facial hair growth. Leuprolide is one commonly used medication in this category. Delaying these physical changes gives young people time to explore their gender identity without the stress of developing unwanted physical traits.

A research team led by Fay Guarraci at Southwestern University initiated a series of experiments to understand how protracted puberty suppression affects brain and reproductive development. The researchers wanted to track both the behavioral and biological changes that occur during and after leuprolide administration. Because long-term physiological studies on human adolescents are limited, animal models help scientists observe how the brain and reproductive systems respond to these hormone-altering drugs over time.

In the first study, published in 2023, Guarraci and her colleagues tested how leuprolide affected the physical and behavioral development of 40 adolescent Long-Evans rats. The sample included 24 female rats and 16 male rats. Starting on postnatal day 25, which corresponds to early adolescence in rats, the scientists administered daily injections of either 50 micrograms per kilogram of leuprolide or a neutral saltwater solution for 25 days. The researchers monitored the rats daily for physical signs of puberty, such as vaginal opening in females and penile development in males.

Following the treatment period, the authors placed the rats in specialized chambers to observe their sexual motivation and mating behaviors. They recorded how much time the subjects chose to spend with male or female stimulus rats and tracked specific reproductive actions. The female subjects were tested during their first reproductive cycle after the drug was stopped. The male subjects underwent weekly behavioral tests for a month to track their development over a longer timeframe.

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The researchers observed that leuprolide delayed the physical onset of puberty. Female rats receiving the drug reached puberty at an average of 45.9 days of age, compared to 38.1 days for those receiving the saltwater solution. Male rats receiving the drug reached puberty at an average of 45.0 days, compared to 39.0 days for the control group. During the treatment window, the female rats did not experience normal reproductive cycles.

Once the leuprolide injections ended, the female rats rapidly resumed normal reproductive function. Within a week, they displayed typical mating behaviors and became pregnant at absolute rates comparable to the control group. The male rats took longer to recover from the puberty-blocking effects. During the initial post-treatment tests, the leuprolide-treated males showed reduced sexual motivation toward females and engaged in fewer mating behaviors. By the fourth week after treatment ended, the treated males exhibited sexual behavior that matched the control group.

To understand the biological mechanisms driving these behavioral changes, the researchers conducted a second study, published in 2025. This experiment involved 16 female and 17 male adolescent rats, using the exact same 25-day leuprolide treatment protocol. Instead of observing behavior after the drug was stopped, the scientists analyzed the rats’ brains and blood hormone levels on the final day of treatment. They specifically measured the activity of genes related to reproduction in the pituitary gland and the hypothalamus, two brain regions that control hormone production.

Leuprolide once again delayed physical puberty, pushing the onset back by about five days in females (from 37.25 to 42.25 days) and ten days in males (from 39.50 to 49.67 days). Blood tests indicated that the circulating levels of sex hormones like testosterone and estrogen were not statistically significantly different between the treated and untreated rats at the end of the 25-day window. However, the genetic analysis showed distinct changes in how the brain was preparing for reproduction.

In the pituitary gland, both male and female rats treated with leuprolide showed increased activity in genes responsible for producing estrogen receptors and gonadotropin-releasing hormone receptors. In the hypothalamus, the effects differed by sex. Male rats receiving the drug had lower activity of the Kiss1 gene in the preoptic area, a brain region involved in sexual behavior. This gene produces kisspeptin, a protein that helps trigger puberty.

In a different section of the hypothalamus called the mediobasal region, the Kiss1 gene was highly active in both treated males and females. The scientists note that this heightened gene activity likely represents the brain attempting to compensate for the drug’s suppressive effects. The brain continues to mature and build the necessary reproductive architecture even while the physical manifestation of puberty is stalled.

Applying findings from animal models to human biology requires noting that rats and humans mature on vastly different timelines. The precise brain mechanisms governing puberty also feature species-specific variations, meaning a one-week recovery period in rats does not correspond to a specific timeframe in human adolescents.

Animal studies remain highly useful, however, because they allow scientists to directly examine brain tissue and track genetic changes across a full developmental lifespan. Observing these molecular adaptations in a controlled environment provides evidence about how the mammalian brain responds to protracted puberty suppression that would be impossible to gather from human patients.

Drug doses used in experimental models do not always translate exactly to the proportional doses prescribed in clinical settings. Future research could isolate smaller, more specific clusters of cells within the hypothalamus to see exactly where these genetic changes originate. Tracking hormone and gene expression over a longer timeline after the medication is stopped would also help clarify how the brain readjusts. Testing different doses of the medication could provide additional context regarding how the body manages chemical puberty suppression.

The study, “Chronic periadolescent leuprolide exposure affects the development of reproductive physiology and behavior of female and male rats differently, but both mature after treatment termination,” was authored by Fay A. Guarraci, Layla Avendano, Megan Kelly, Cleriza Estoesta, Bernard Sencherey, Hannah S. Valdivia, Amanda Gale, Lily Yepez, Jasmine B. Belfield, Kristen M. Carter, Natalie Williams, and Andrea C. Gore.

The study, “Chronic periadolescent leuprolide exposure affects the expression of multiple genes in the hypothalamus and pituitary gland with a different pattern of expression in female and male Long-Evans rats,” was authored by Fay A. Guarraci, Ian M. Klepcyk, Lindsay M. Thompson, Madeline Streifer, Emily N. Hilz, Grace Hudson, Sarah H. Meerts, and Andrea C. Gore.