·        Testosterone supplementation improves the performance of aged male rats in behavioral tests measuring their exploratory capacity in a novel environment.
·        The sex hormone improves cognitive performance by preserving neurons and improving the function of their mitochondria.
·        These findings could aid the design of testosterone-based therapeutics for sex-specific age-related cognitive decline.

The process of aging differs in men and women. In men, age-related reductions in the male sex hormone testosterone levels as well as dysfunction of mitochondria — the energy generating structures in our cells — in brain cells correlate with the onset of age-related neurodegenerative diseases like Alzheimer’s and Parkinson’s disease, and supplementing with testosterone improves cognitive and motor symptoms. But a clear connection between supplementing males with testosterone and improved mitochondrial function hadn’t been established, until now.

Shi and colleagues from Hebei Medical University in China published a study in Aging showing that testosterone replacement therapy ameliorates age-associated brain mitochondrial dysfunction and promotes enhanced cognition in aged male rats. By injecting male rats daily with 1 mg/kg of testosterone, the Chinese research team observed improved rat cognitive function, attenuated neuronal loss and dysfunction, and restored neuronal mitochondria function.

“These findings suggest that testosterone supplementation may be a viable approach to ameliorating brain mitochondrial dysfunction and thus prevent or treat cognitive-behavioral deficits and neurodegenerative conditions associated with aging,” said Shi and colleagues.

Testosterone Supplementation Attenuates Age-Related Cognitive Decline

To figure out whether testosterone supplementation attenuates cognitive decline, Shi and colleagues saw how age and testosterone affected the performance of male rats in field tests — an experimental test used to assay general locomotor activity levels, anxiety, and willingness to explore in animals (usually rodents) in scientific research. Twenty-four-month-old rats performed substantially worse at exploring their surroundings compared to their younger, six-month-old counterparts. But when aged male rats were supplemented with testosterone for 12 weeks, Shi and colleagues observed that the treatment  restored various exploratory behaviors, such as climbing, rearing, and sniffing, indicative of improved cognition.

(Yan et al., 2021 | Aging) Testosterone supplementation improves exploratory behavior in aged male rats. While testosterone therapy did not affect the amount of walking (A) in field tests, it substantially improved climbing (B), rearing (C), and sniffing (D) behaviors in aged male rats. These findings indicate that overall, testosterone therapy significantly improved exploratory behavior, an indicator of cognitive function, in aged male rats.

Testosterone Replacement Prevents Age-Related Neuron Loss and Dysfunction

Shi and colleagues wondered whether these testosterone-induced cognitive enhancements in aged rats were related to improved structural integrity of brain cells. So, they looked at the condensation of chromosomes in cells’ nuclei (karyopyknosis) that shows neurons are either dead or dying. Although much higher levels of karyopyknosis were seen in aged rat neurons, testosterone therapy reversed the buildup of these karyopyknotic neurons. These findings showed that testosterone therapy mitigates age-related neuronal loss and dysfunction.

(Yan et al., 2021 | Aging) Testosterone supplementation substantially improves neuronal integrity during aging. A region of the brain involved in learning and memory formation — the hippocampus — is shown for young rats (A), old rats (B), and old rats treated with testosterone (C). The top row shows a slice of the hippocampus, and the boxes in these images are magnified 10-fold in the bottom row. The dark blue stains that are more intense and abundant throughout the magnified line of cells called the dentate gyrus in the aged, 24-month-old mice (B) indicate chromosome condensation (karyopyknosis). Testosterone therapy ameliorates karyopyknosis in aged male rat neurons, as quantified in the graph (D).

Supplementing with Testosterone Improves Mitochondrial Function in Aged Rats

Since mitochondrial dysfunction is intimately tied to age-related neuronal malfunction and testosterone therapy improves cognition and neuronal integrity, Shi and colleagues searched for the link between testosterone-enhanced cognition and improved mitochondrial function. By looking at indicators of mitochondrial integrity, such as the ability of the mitochondrial membrane to separate oppositely charged ions, or what’s called the mitochondrial membrane potential, Shi and colleagues found that testosterone supplementation restored faltering membrane potentials in aged male rats. These findings indicate that testosterone therapy improves brain mitochondrial function in aged rats and provides the sought-after link between testosterone therapy-induced cognitive enhancement and improved mitochondrial health.

(Yan et al., 2021 | Aging) Testosterone supplementation improves mitochondrial function in aged male rat neurons. The dye rhodamine123 was used to measure the membrane potential of mitochondrial membranes in the brain regions called the substantia nigra (A) and the hippocampus (B). In both brain regions, ion permeability diminished with age, but testosterone therapy substantially increased mitochondrial membrane potential in aged male rat neurons.

The Findings May Help in Testosterone-Based Therapy Design

“Our study revealed that testosterone supplementation improved exploratory behavior, attenuated neuronal dysfunction and neuronal loss, and ameliorated mitochondrial dysfunction,” stated Shi and colleagues.

Findings from the study provide a clear link between testosterone-induced enhanced cognition and improved mitochondrial function in aged male rats. Figuring out details on how testosterone facilitates enhanced mitochondrial function and improves cognition will require further research.

“Our findings may be relevant for the design of testosterone-based therapies aimed at preventing or treating cognitive-behavioral deficits and diseases associated with aging,” said Shi and colleagues.

Since these findings came from research on male rats, future studies will need to look at how well we can apply testosterone therapy to brain health for aging human males. Moreover, the study didn’t examine hormone replacement therapy in aging females with cognitive decline, so figuring out the intricacies of hormone levels in both sexes will require further work.