Exploring Developmental, Neurobehavioral, and Transcriptional Effects of Early Embryonic Testosterone Propionate Exposure in Zebrafish Larvae

In the earliest days of life, before a heart beats or a brain forms, a developing organism is exquisitely sensitive to the chemical signals floating around it. Among these signals, steroid hormones act as powerful messengers, instructing cells on how to grow, differentiate, and organize into complex systems. One such hormone, testosterone, is vital for normal development, but its presence in the wrong amounts or at the wrong time can disrupt the delicate construction of the nervous system. This is particularly relevant in the context of maternal health. Conditions like polycystic ovary syndrome, which cause higher-than-average levels of androgens in a mother's blood, have been linked in large population studies to an increased risk of neurodevelopmental challenges in children. While these human studies show a clear connection, they cannot easily prove that the hormone itself is the direct cause, as many other factors vary alongside it. To understand the specific biological mechanisms at play, scientists need a way to isolate the hormone's effect in a controlled setting, watching exactly how an excess of this chemical reshapes a developing brain and body.

Researchers at the University of Petroleum and Energy Studies in India turned to the zebrafish to answer this question. These small, transparent fish are a favorite tool for developmental biologists because their embryos grow outside the mother's body, allowing scientists to watch every stage of development in real time. The team exposed zebrafish embryos to testosterone propionate, a synthetic form of the hormone, starting just a few hours after fertilization. They wanted to see if this early exposure would alter how the fish grew, how they moved, and how the genes inside their cells behaved. By treating the embryos like a controlled experiment, the scientists could observe the direct consequences of an androgen-rich environment without the complications of a maternal body or placenta.

The investigation began by determining how much of the hormone the developing fish could tolerate. The team exposed groups of embryos to a wide range of concentrations, from very low levels up to high doses, and watched them for four days. They found that while low amounts were harmless, higher concentrations began to kill the embryos. The point at which half of the fish died was calculated to be 146.3 micromolar. More importantly, they observed that even doses that did not kill the fish caused physical deformities. At higher levels, the fish developed swollen hearts, curved spines, and fluid-filled sacs where their swim bladders should have been. These physical signs confirmed that the hormone was interfering with normal growth, but the researchers were more interested in what happened at lower, non-lethal levels that might still affect behavior.

To explore these subtler effects, the scientists selected a range of lower concentrations and watched the fish as they grew into larvae. They started by observing the embryos just one day after fertilization. At this very early stage, the fish normally wiggle their tails in a rhythmic pattern. The researchers found that those exposed to the highest safe dose of testosterone wiggled significantly more often than the unexposed fish. This suggested that the hormone was already altering the nervous system's control over movement before the fish had even hatched. As the fish grew older, reaching five and seven days of age, the researchers tested their behavior in new environments. They placed the larvae in open dishes and observed where they chose to swim. Fish that had been exposed to the hormone showed a strong tendency to stick to the walls of the container rather than exploring the open center. In a separate test involving a tank with a light side and a dark side, the exposed fish spent more time in the dark, avoiding the light. They also preferred to stay at the bottom of a new tank rather than swimming near the surface. These behaviors, which indicate anxiety-like responses and a reluctance to explore, were consistent across different concentrations of the hormone, suggesting that even small amounts of early exposure can permanently shift how the fish interact with the world.

To understand why the fish were behaving this way, the team looked inside the cells of the larvae at seven days old. They measured the activity of specific genes that are known to be crucial for brain development and hormone signaling. The results showed a clear pattern of suppression. The genes responsible for receiving and responding to testosterone and estrogen were less active than normal. Similarly, genes that help nerve cells survive and form connections, as well as those involved in the internal signaling pathways that tell cells how to grow and mature, were also downregulated. Essentially, the presence of the extra hormone had turned down the volume on the very genetic machinery needed to build a healthy nervous system. The study did not measure the proteins these genes make, nor did it track the exact chemical pathways inside the cells, but the drop in gene activity provides a strong molecular clue linking the hormone exposure to the behavioral changes.

The findings offer a clearer picture of how an excess of androgens during early development can ripple outward to affect behavior. The research suggests that when a developing organism is flooded with testosterone, it does not just grow differently; its nervous system is rewired at a genetic level, leading to altered movement and a more cautious, less exploratory nature. While the study was conducted in fish, the biological pathways involved are shared with humans, offering a plausible explanation for the neurodevelopmental risks seen in children of mothers with high androgen levels. The work does not prove that this is the sole cause of any specific human condition, nor does it suggest a cure, but it successfully isolates the hormone as a direct agent of change. By showing that a simple chemical imbalance can alter gene expression and behavior in a vertebrate, the study provides a concrete foundation for understanding the complex relationship between maternal health and the developing brain.