New preclinical research from Middlebury College suggests that the timing and duration of testosterone exposure can determine whether newly formed brain cells survive to maturity. In a study published in Neuroscience, male rats showed the most robust survival of young neurons when testosterone was present during the later stages of their development, particularly within a hippocampal region tied to spatial learning and memory.
Adult neurogenesis—the process by which the mature brain generates new neurons—occurs largely in the hippocampus, a seahorse-shaped structure deep in the temporal lobe essential for learning, memory formation and stress regulation. Newly born cells originate in the subgranular zone of the dentate gyrus, migrate into the granule cell layer, and extend connections as they mature. However, about half of these new cells die off in the first month before fully integrating into brain circuits.
Hormones, including testosterone, are major internal regulators of this survival process. Testosterone, the primary male sex hormone, declines with age and has been linked to memory decline in older men. In the brain it can act directly on androgen receptors or be converted into other active hormones such as estradiol or dihydrotestosterone.
Senior author Mark D. Spritzer, a professor of biology at Middlebury College, told PsyPost that testosterone may support memory in aging men by acting as a neuroprotectant in the brain, with the hippocampus being particularly vulnerable to age-related degradation. He added that this region’s role in learning and memory makes it a key target for understanding how testosterone influences cognition.
Earlier work showed that removing the testes did not change the rate at which hippocampal cells were born, but it did reduce how many survived. Restoring testosterone through daily injections for 30 consecutive days reliably rescued these neurons, while shorter windows of 15 to 21 days showed no benefit. A five-day burst of testosterone between days 11 and 15 after cell birth did improve survival in a separate study by the same group.
These mixed results prompted the current investigation into whether testosterone needs a full month to protect developing neurons or whether certain developmental stages render cells more sensitive to the hormone. The researchers focused on a window when neurons mature and form synapses, between 11 and 30 days post-birth, a period that also coincides with a shift in the neurotransmitter GABA from excitatory to inhibitory, aiding maturation.
To test this, the team designed an experiment comparing a full 30-day testosterone treatment with two 15-day regimens, one covering the first half and the other the second half of the 30-day maturation period. They studied 43 adult male Sprague-Dawley rats, about 60 days old at the outset, all surgically castrated to control testosterone levels. BrdU, a marker of newly generated cells, was injected 11 days after castration, tagging cells born during a specific window.
From day after BrdU injection, rats were assigned to six groups receiving either testosterone at a high physiological dose of 0.500 milligrams per rat or an oil control, during one of the three time windows: days 1–15, days 16–30, or days 1–30. Blood samples taken on day 31 confirmed that animals treated during the late or full windows exhibited elevated testosterone levels—roughly 5.7 ng/mL—versus the oil-treated groups and the early-window group, which had fallen to under 0.3 ng/mL.
Analysis of the entire dentate gyrus revealed more surviving BrdU-labeled cells in the neurogenic zone among testosterone-treated animals compared with oil controls. While the full 30-day treatment produced a statistically significant increase across the whole structure, the late-window treatment showed a hopeful trend that did not reach significance, and the early-window treatment had no effect.
When the team examined dorsal and ventral regions separately, a clearer pattern emerged. In the dorsal dentate gyrus, testosterone increased cell survival for the full 30-day course and for the late 16–30-day window, while the early 1–15-day window offered no benefit. In the ventral dentate gyrus, testosterone had no effect in any window, though early injections unexpectedly yielded fewer surviving cells than late or full windows regardless of treatment.
Fluorescent microscopy revealed that roughly 79 to 87 percent of surviving BrdU-tagged cells became NeuN-positive mature neurons across all groups, regardless of hormone exposure. The researchers found no treatment differences in dentate gyrus volume or in counts within the neighbouring hilus, indicating the effects were specific to the neurogenic zone.
The findings imply that maturing neurons in the dorsal hippocampus are particularly responsive to testosterone when they are two to four weeks old. Given the dorsal hippocampus’s role in spatial navigation and memory, the results line up with behavioural data showing testosterone can improve spatial working memory in male rats. The hormone may help preserve new cells during this late window by blocking programmed cell death, boosting growth factors such as brain-derived neurotrophic factor, or aiding the maturation of GABA signals as new neurons integrate into existing networks.
“We demonstrated that testosterone enhances adult neurogenesis, with the effects being strongest during the later stages of neuronal development,” Spritzer said. “These results suggest that testosterone may be a useful therapeutic target for helping to sustain neuroplasticity, among men in particular.”
The researchers caution that the study was conducted in surgically castrated, otherwise isolated rats, a setup that does not mirror the gradual, partial hormone decline seen in ageing men. They also note that blood and brain tissue were collected 24 hours after the final testosterone injection for the full and late groups, but 16 days after the final injection for the early group, a timing difference that could affect results. While this could partly explain the lack of early-window effects, it would require a steady rate of cell death across development—a pattern not supported by earlier work.
Spritzer emphasised that the findings are preclinical and cannot be directly extrapolated to humans. He pointed out that the study used a high physiological dose of testosterone and that prior human trials have not carefully considered dose levels. He added that boosting cell survival indefinitely is not necessarily advantageous, arguing that there is likely an optimal level of adult neurogenesis for brain function.
The authors acknowledge that the timing conclusions rely on separate within-window analyses, each with relatively small group sizes, and that the study did not directly identify the molecular pathways involved. They advocate further research to uncover the receptors and downstream mechanisms at play, and to determine whether similar timing effects occur in older animals or in females.
The study, titled Effects of testosterone upon the survival of new neurons in the dentate gyrus of adult male rats, is credited to Tatum W. Hamilton, Ezra J. LeMole, Grace K. Nadecki, Jordan M. Gardner, Quinn P. Kennedy, Kathryn A. Blek and Mark D. Spritzer.
