Testosterone, the primary male sex hormone, plays complex and multifaceted roles in brain function and neurodegeneration. Beyond its well-known effects on reproductive tissues, testosterone exerts profound influences on neuronal survival, synaptic plasticity, cognitive function, and neuroprotection through both genomic and non-genomic signaling pathways. The androgen receptor (AR), a ligand-activated transcription factor, mediates most of testosterone's genomic effects, while rapid signaling through membrane-associated receptors accounts for acute neuroprotective actions. Understanding the role of testosterone in neurodegenerative diseases has become increasingly important as research reveals sex-dependent differences in disease incidence, progression, and response to therapy in conditions like Alzheimer's disease (AD), Parkinson's disease (PD), and Amyotrophic Lateral Sclerosis (ALS) [38977069](https://pubmed.ncbi.nlm.nih.gov/38977069/). [@longitudinal2025]
Testosterone, the primary male sex hormone, plays complex and multifaceted roles in brain function and neurodegeneration. Beyond its well-known effects on reproductive tissues, testosterone exerts profound influences on neuronal survival, synaptic plasticity, cognitive function, and neuroprotection through both genomic and non-genomic signaling pathways. The androgen receptor (AR), a ligand-activated transcription factor, mediates most of testosterone's genomic effects, while rapid signaling through membrane-associated receptors accounts for acute neuroprotective actions. Understanding the role of testosterone in neurodegenerative diseases has become increasingly important as research reveals sex-dependent differences in disease incidence, progression, and response to therapy in conditions like Alzheimer's disease (AD), Parkinson's disease (PD), and Amyotrophic Lateral Sclerosis (ALS) [38977069](https://pubmed.ncbi.nlm.nih.gov/38977069/). [@longitudinal2025]
The relationship between testosterone and neurodegeneration is inherently bidirectional. On one hand, testosterone deficiency has been associated with increased risk of cognitive decline and neurodegenerative diseases in elderly men. On the other hand, supraphysiological levels or inappropriate timing of testosterone exposure may promote neurotoxicity through mechanisms including oxidative stress, pro-inflammatory signaling, and excitotoxicity. This delicate balance underscores the importance of understanding testosterone's pleiotropic effects on the central nervous system and highlights the complexity of developing therapeutic strategies targeting the androgenic system [32223745](https://pubmed.ncbi.nlm.nih.gov/32223745/). [@testosterone2024]
The androgen receptor (AR) is a member of the nuclear receptor superfamily, functioning as a ligand-dependent transcription factor. In the brain, AR is expressed in multiple regions including the hippocampus, cortex, basal ganglia, and hypothalamus. The receptor consists of distinct functional domains: an N-terminal transactivation domain, a DNA-binding domain, a hinge region, and a C-terminal ligand-binding domain. Upon testosterone binding, the receptor undergoes conformational changes, dimerization, and translocation to the nucleus where it binds to androgen response elements (AREs) in the promoter regions of target genes [29137141](https://pubmed.ncbi.nlm.nih.gov/29137141/). [@exercise2025]
Genomic signaling through AR regulates numerous genes critical for neuronal function and survival. These include genes involved in antioxidant defense (e.g., superoxide dismutase, glutathione peroxidase), anti-apoptotic proteins (e.g., Bcl-2, Bcl-xL), neurotrophic factors (e.g., BDNF), and proteins governing synaptic plasticity. The diversity of AR target genes explains testosterone's wide-ranging effects on neuronal homeostasis and its potential protective roles in neurodegeneration. [@novel2025]
Testosterone also exerts rapid, non-genomic effects through interactions with membrane-associated receptors and signaling molecules. These effects occur within minutes to hours of hormone exposure and do not require gene transcription or protein synthesis. Key non-genomic mechanisms include: [@presenilin2026]
cAMP/PKA signaling: Testosterone can activate adenylate cyclase through membrane-associated receptors, increasing intracellular cAMP levels and activating protein kinase A (PKA). This pathway influences neuronal excitability, neurotransmitter release, and synaptic plasticity.
MAPK/ERK signaling: Testosterone rapidly activates the extracellular signal-regulated kinase (ERK) pathway in neurons, promoting cell survival through phosphorylation of pro-apoptotic proteins and activation of transcription factors like CREB.
PI3K/Akt signaling: The phosphatidylinositol 3-kinase (PI3K)/Akt pathway mediates testosterone's anti-apoptotic effects in neurons. Akt phosphorylation inhibits pro-apoptotic proteins including Bad, caspase-9, and forkhead transcription factors.
Calcium signaling: Testosterone modulates voltage-gated calcium channels and intracellular calcium homeostasis, affecting neuronal excitability and synaptic transmission [33094209](https://pubmed.ncbi.nlm.nih.gov/33094209/).
The relationship between testosterone and Alzheimer's disease pathology involves complex interactions with amyloid-beta (Aβ) metabolism and toxicity. Research demonstrates that testosterone can modulate amyloid precursor protein (APP) processing toward non-amyloidogenic pathways through activation of α-secretases and inhibition of β-secretase (BACE1) activity. This shift reduces Aβ production and may decrease amyloid plaque formation in the brain.
Furthermore, testosterone protects neurons against Aβ-induced toxicity through multiple mechanisms. Testosterone pre-treatment reduces Aβ-induced reactive oxygen species (ROS) generation, mitochondrial dysfunction, and caspase activation. The hormone also promotes Aβ clearance through upregulation of amyloid-degrading enzymes including neprilysin and insulin-degrading enzyme (IDE) [41642483](https://pubmed.ncbi.nlm.nih.gov/41642483/).
Hyperphosphorylation and aggregation of tau protein into neurofibrillary tangles represents another key pathological feature of AD. Testosterone exerts protective effects against tau pathology through several pathways. The hormone activates protein phosphatases (particularly PP2A) that dephosphorylate tau, while simultaneously inhibiting tau-phosphorylating kinases including GSK-3β and CDK5.
Animal studies demonstrate that androgen depletion accelerates tau hyperphosphorylation and aggregation, while testosterone supplementation reduces tau pathology in mouse models of AD. These findings suggest that age-related testosterone decline may contribute to tau pathogenesis in elderly men.
Testosterone's effects on cognition in AD involve modulation of synaptic plasticity, neurotransmitter systems, and hippocampal function. The hormone enhances long-term potentiation (LTP) in the hippocampus, a cellular correlate of learning and memory. Testosterone also modulates cholinergic neurotransmission, which is prominently affected in AD and critical for cognitive function.
Clinical studies reveal that low testosterone levels correlate with poorer cognitive performance in men, and testosterone replacement therapy trials have shown promise for improving cognitive function in hypogonadal men with or without AD. However, the timing and dosage of testosterone administration appear critical, as inappropriate treatment may worsen outcomes [41490157](https://pubmed.ncbi.nlm.nih.gov/41490157/).
Testosterone exerts notable protective effects on dopaminergic neurons, the cell type primarily lost in Parkinson's disease. In experimental models, testosterone protects against 1-methyl-4-phenylpyridinium (MPP+)-induced dopaminergic toxicity, a neurotoxin used to model PD. The protective mechanisms include antioxidant effects, mitochondrial preservation, and inhibition of apoptosis.
The hormone modulates mitochondrial function in dopaminergic neurons, preserving complex I activity and reducing ROS generation. This is particularly relevant given the well-established mitochondrial dysfunction in PD pathogenesis. Testosterone also upregulates expression of neurotrophic factors including brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF), which support dopaminergic neuron survival [36971787](https://pubmed.ncbi.nlm.nih.gov/36971787/).
Testosterone exhibits anti-inflammatory properties in the brain that may be beneficial in PD. The hormone inhibits microglial activation and reduces production of pro-inflammatory cytokines including TNF-α, IL-1β, and IL-6. This immunomodulatory effect may reduce neuroinflammation-driven dopaminergic degeneration.
Studies demonstrate that male sex is associated with increased PD risk, suggesting that testosterone may have complex, possibly dual roles in PD pathogenesis. While testosterone appears neuroprotective in some contexts, it may also promote neuroinflammation under certain conditions, highlighting the need for careful consideration of therapeutic approaches [36572757](https://pubmed.ncbi.nlm.nih.gov/36572757/).
ALS exhibits a notable sex bias, with approximately 60% of cases occurring in males. This male predominance suggests a potential role for testosterone or other sex hormones in disease pathogenesis. However, the relationship between testosterone and ALS appears complex, with both protective and potentially harmful effects reported.
Some studies suggest that testosterone may accelerate disease progression in ALS through mechanisms including enhanced excitotoxicity and promotion of neuroinflammation. The hormone can potentiate glutamate-induced excitotoxicity, a recognized contributor to motor neuron degeneration in ALS.
The complex role of testosterone in ALS presents therapeutic challenges. While testosterone deficiency in aging men may contribute to neurodegeneration, supraphysiological levels may promote toxicity. Careful monitoring of hormone levels and individualized treatment approaches appear necessary for any testosterone-based therapeutic intervention in ALS patients.
Testosterone replacement therapy (TRT) represents a potential approach for neuroprotection in aging-related testosterone deficiency. Clinical trials have demonstrated that TRT can improve cognitive function, mood, and quality of life in hypogonadal men. However, TRT carries risks including prostate cancer, cardiovascular events, and polycythemia that require careful patient selection and monitoring.
For neurodegenerative disease applications, TRT would need to be carefully timed and dosed. Early intervention before significant neurodegeneration may be more beneficial than treatment in advanced disease states. The concept of "hormonal prevention" suggests that maintaining optimal testosterone levels throughout aging may reduce neurodegenerative disease risk [33094209](https://pubmed.ncbi.nlm.nih.gov/33094209/).
Selective androgen receptor modulators (SARMs) offer a potentially safer alternative to traditional testosterone therapy. These compounds produce tissue-selective anabolic effects with reduced androgenic side effects. Some SARMs may provide neuroprotective benefits without the risks associated with supraphysiological testosterone levels.
Research directions for testosterone-based neuroprotective therapies include:
Testosterone signaling in the brain represents a complex, multifaceted system with important implications for neurodegenerative disease. The hormone exerts both protective and potentially harmful effects depending on context, dosage, timing, and individual factors. Understanding the molecular mechanisms underlying testosterone's actions in the brain provides opportunities for developing novel therapeutic strategies while highlighting the importance of personalized approaches to hormone-based interventions. As the population ages and neurodegenerative diseases become increasingly prevalent, clarifying testosterone's role in brain health and disease becomes ever more critical.
Testosterone's effects on synaptic plasticity represent a critical mechanism underlying its cognitive effects. In the hippocampus, testosterone modulates dendritic spine density and morphology, with important implications for learning and memory. Research demonstrates that testosterone increases spine density on CA1 pyramidal neurons through both androgen receptor-dependent and independent mechanisms [29452160](https://pubmed.ncbi.nlm.nih.gov/29452160/).
The hormone influences long-term potentiation (LTP), the cellular basis for memory formation, through multiple signaling pathways. Testosterone enhances NMDA receptor function and promotes AMPA receptor trafficking to synaptic membranes, facilitating synaptic strengthening. Additionally, the hormone modulates GABAergic neurotransmission, reducing inhibition and enhancing excitability conducive to LTP induction [29194818](https://pubmed.ncbi.nlm.nih.gov/29194818/).
Beyond genomic signaling, testosterone exerts rapid effects on synaptic structure and function. Within minutes of exposure, testosterone can modulate spine dynamics through activation of signaling cascades including PI3K/Akt and MAPK/ERK. These rapid effects involve modulation of actin cytoskeleton dynamics and local protein synthesis at dendritic spines.
Studies in animal models demonstrate that testosterone's effects on spines are region-specific and depend on the functional state of the hippocampus. Testosterone's enhancement of synaptic plasticity may underlie its beneficial effects on spatial memory and cognitive function in both young and aged animals [30632505](https://pubmed.ncbi.nlm.nih.gov/30632505/).
Testosterone may influence motor symptoms in Huntington's disease (HD), a neurodegenerative disorder characterized by progressive motor dysfunction, cognitive decline, and psychiatric disturbances. Some evidence suggests that testosterone levels correlate with motor phenotype severity in HD patients, with higher levels potentially associated with more severe chorea.
Testosterone's neuroprotective properties may be relevant in HD pathogenesis. The hormone's antioxidant effects and anti-apoptotic signaling could potentially protect striatal neurons, the cell type primarily lost in HD. Additionally, testosterone's modulation of neurotrophic factor expression may support neuronal survival in HD-affected brains [36497038](https://pubmed.ncbi.nlm.nih.gov/36497038/).
Age-related testosterone decline, often termed "andropause" or late-onset hypogonadism, affects a significant proportion of elderly men. This decline has been implicated in increased risk of cognitive impairment, neurodegenerative diseases, and mood disorders. The gradual reduction in testosterone levels with aging coincides with increased incidence of neurodegenerative diseases, suggesting a potential causal relationship [39060901](https://pubmed.ncbi.nlm.nih.gov/39060901/).
Testosterone also influences myelination and myelin maintenance in the central nervous system. In the wobbler mouse model of motor neuron disease, testosterone reduces myelin abnormalities and improves neurological function [38672445](https://pubmed.ncbi.nlm.nih.gov/38672445/). This finding has implications for demyelinating conditions and white matter pathology in neurodegenerative diseases.
Sex differences in neurodegenerative disease incidence and progression are well-established. Alzheimer's disease shows a female predominance, despite men's higher testosterone levels, suggesting complex interactions between sex hormones and disease pathogenesis. Parkinson's disease exhibits a male bias that may relate to testosterone's effects on dopaminergic neurons. ALS shows the strongest male predominance among major neurodegenerative diseases [40281071](https://pubmed.ncbi.nlm.nih.gov/40281071/).
Genetic background influences responses to sex hormones in the brain. Studies reveal sex-specific effects of genetic polymorphisms on brain structure and function, highlighting the importance of considering individual genetic backgrounds in hormone-based therapies [41415377](https://pubmed.ncbi.nlm.nih.gov/41415377/).
Clinical assessment of testosterone's brain effects requires consideration of multiple biomarkers. Serum testosterone levels provide an initial assessment, but brain-specific effects depend on local conversion to dihydrotestosterone (DHT) and estradiol through steroidogenic enzymes. Additionally, androgen receptor polymorphisms influence individual responses to testosterone [40626351](https://pubmed.ncbi.nlm.nih.gov/40626351/).
Testosterone therapy for neurodegenerative applications requires careful monitoring. Serum testosterone levels, PSA, hematocrit, and lipid profiles should be regularly assessed. Neuroimaging and cognitive testing may provide additional monitoring for therapeutic efficacy. The goal is to maintain physiological testosterone levels while avoiding supraphysiological concentrations that may promote toxicity.
Multiple sclerosis (MS) exhibits a clear female predominance, with women comprising approximately 70% of cases. This sex difference points to potential protective or pathogenic roles for sex hormones. Testosterone appears to have immunomodulatory effects that may be relevant in MS pathogenesis, with some studies suggesting protective effects against demyelination and axonal loss [40706904](https://pubmed.ncbi.nlm.nih.gov/40706904/).
In the experimental autoimmune encephalomyelitis (EAE) model of MS, testosterone treatment reduces disease severity and promotes neuroprotective immune responses. The hormone shifts immune responses from pro-inflammatory Th1/Th17 patterns toward protective Th2/Treg phenotypes. These findings have prompted interest in testosterone as a potential therapeutic agent in MS.
Clinical trials of testosterone therapy in MS have demonstrated safety and potential efficacy for reducing brain atrophy and improving cognitive function. Larger trials are needed to confirm these findings and establish optimal dosing strategies.
Testosterone deficiency is associated with depressive symptoms in men, and depression represents a significant risk factor for neurodegenerative diseases. Testosterone's antidepressant effects may involve modulation of monoaminergic neurotransmission, neurotrophic factor expression, and neuroimmune function.
Depression and neurodegeneration share common neuroinflammatory mechanisms. Testosterone's anti-inflammatory properties may benefit both mood and neurodegenerative processes. This intersection suggests potential for testosterone-based interventions in depression associated with neurodegenerative risk [40437391](https://pubmed.ncbi.nlm.nih.gov/40437391/).
Androgen receptors modulate stress responses through effects on corticotropin-releasing hormone (CRH) neurons in the hypothalamus. This interaction has implications for understanding how stress and sex hormones interact in neurodegenerative processes [40106355](https://pubmed.ncbi.nlm.nih.gov/40106355/).
Neural androgen receptor deletion impairs temporal processing, highlighting the hormone's role in cognitive functions beyond simple memory. This effect may relate to timing deficits observed in various neurodegenerative conditions [26849367](https://pubmed.ncbi.nlm.nih.gov/26849367/).
Exercise modulates testosterone levels and may interact with the hormone's effects on the brain. Physical activity increases testosterone temporarily while also enhancing neuroplasticity and reducing inflammation. The combination of exercise and optimal testosterone levels may provide synergistic neuroprotective benefits [41465180](https://pubmed.ncbi.nlm.nih.gov/41465180/).
Exposure to environmental endocrine disruptors may interfere with testosterone signaling in the brain. Bisphenol-A (BPA), for example, exhibits anti-androgenic effects that could impact spatial memory and synaptic function [28576649](https://pubmed.ncbi.nlm.nih.gov/28576649/).
Testosterone and brain-derived neurotrophic factor (BDNF) interact in complex ways to influence neuronal survival and plasticity. Both molecules promote similar signaling pathways and may have synergistic effects. Understanding this interaction could lead to combined therapeutic approaches for neurodegeneration [40141172](https://pubmed.ncbi.nlm.nih.gov/40141172/) [41677347](https://pubmed.ncbi.nlm.nih.gov/41677347/).
The balance between sex hormones and neurotrophic factors appears critical for brain health. Disruption of this balance may contribute to neurodegenerative processes, while restoration could provide therapeutic benefit.
Translation of findings from animal studies to human applications requires caution. Rodent models may not fully capture the complexity of human testosterone signaling in the brain. Species differences in androgen receptor distribution, steroid metabolism, and brain regional organization must be considered.
Individual variation in testosterone effects is substantial. Genetic polymorphisms in androgen receptor and steroidogenic enzymes, age, baseline hormone levels, and underlying health conditions all influence responses to testosterone. Personalized approaches are essential for effective therapy.
The hippocampus shows particularly high androgen receptor expression and is highly responsive to testosterone. Testosterone's effects on hippocampal function include modulation of neurogenesis, synaptic plasticity, and spatial memory. Age-related hippocampal dysfunction may relate to declining testosterone levels.
Cortical androgen receptor distribution varies by region, with prefrontal cortex showing notable expression. Testosterone influences executive function, decision-making, and working memory through cortical mechanisms. These cognitive domains are prominently affected in neurodegenerative diseases.
Testosterone affects dopaminergic neurons in the substantia nigra and striatum, regions critical for motor control and affected in Parkinson's disease. The hormone's modulation of dopaminergic function has implications for both motor symptoms and reward processing.
Hypothalamic androgen receptors regulate neuroendocrine functions and autonomic responses. This region mediates testosterone's effects on circadian rhythms, sleep, and stress responses, all of which may influence neurodegenerative processes.
Testosterone can be converted to estradiol through aromatase activity in the brain. Many of testosterone's effects on the brain may actually be mediated by locally-produced estrogen. This conversion blurs the boundaries between "male" and "female" hormone effects and suggests potential for combined hormone therapies.
Testosterone and cortisol have antagonistic relationships in stress responses. High cortisol levels may reduce testosterone effects, while adequate testosterone may buffer stress responses. This interaction has implications for stress-related neurodegeneration.
Thyroid hormone and testosterone show complex interactions in brain function. Both hormones affect mitochondrial function and can influence each other's signaling. Hypothyroidism and testosterone deficiency may synergistically promote neurodegeneration.
Testosterone may exert long-term effects through epigenetic modifications. DNA methylation, histone modifications, and non-coding RNA expression could mediate persistent changes in neuronal gene expression. These effects may be particularly relevant during critical developmental periods.
Testosterone signaling may involve extracellular vesicles as communication vectors. These vesicles could transport androgen receptors, testosterone metabolites, or associated proteins between cells, propagating hormonal signals throughout neural networks.
Modern single-cell approaches are revealing cell-type-specific effects of testosterone in the brain. These studies show that different neuronal populations respond differently to testosterone based on their androgen receptor expression patterns and metabolic state.