ALZHEIMER'S DISEASE - ESTROGENS AND SELECTIVE ESTROGEN RECEPTOR MODULATORS, FRIENDS OR FOES?

New Insights Into Pharmacology of GABAA Receptor Alpha Subunits-Selective Modulators

BACKGROUND

Benzodiazepines have long held a leading position in medical therapeutics, known for their multiple common therapeutic properties and primarily being prescribed for anxiety and insomnia. However, their lack of specificity and various side effects have led to a reevaluation of their long-term use, resulting in a rapid growth in the literature focusing on targeted therapies.

AREAS OF UNCERTAINTY

Despite many efforts, uncertainties persist and there are heterogeneous findings across studies regarding the pharmacological effects attributed to gamma-aminobutyric acid type A (GABAA) receptor subunits. Selective compounds targeting GABAA receptor alpha subunits are currently under active research and definitive conclusions have not been reached yet. Some compounds have not progressed to clinical trials, while others, if advanced, have been halted. These challenges emphasize the difficulty in translating preclinical findings into clinical use.

DATA SOURCES

A literature review was conducted using the PubMed database, searching for articles discussing GABAA receptor subunits. The search was refined by including only selective compounds with potential anxiolytic and cognitive enhancement properties.

RESULTS

Findings reveal compounds with promising anxiolytic and antidepressant effects with minimal sedation and absence of tolerance development. Moreover, some compounds show potential in alleviating cognitive dysfunction. There is a broad spectrum of potential therapeutic applications for selective compounds, ranging from neurological disorders such as epilepsy and neuropathic pain to cognitive dysfunction-related conditions. Currently, the leading selective compounds with the most promising results in ongoing clinical trials are basmisanil and darigabat. Basmisanil holds further exploration potential in the treatment of cognitive impairment and related conditions, while darigabat shows progress in the advancement of adjunctive therapy of focal onset seizures and for the treatment of panic disorder, respectively.

CONCLUSIONS

Future drug discovery efforts are encouraged to focus on positive allosteric modulators that selectively target the α2, α3 subunits and negative/positive allosteric modulators that target the α5 subunit of the GABAA receptor. The pursuit of ligands possessing only anxiolytic effects or those enhancing cognition continues to be an important focus for future research, with promising advancements depicted in recent studies.

The Role of Silymarin in Mitigating Inflammation and Cognitive Impairment Induced by Ovariectomy in Wistar Rats

Background. Silymarin, a polyphenolic flavonoid found in milk thistle, has been used to treat liver and brain injuries in humans and animals. The study aims to investigate the protective effects of silymarin on spatial and passive avoidance memory, oxidative stress, and inflammatory factors in the brain and liver tissues of ovariectomized (OVX) Wistar rats. Methods. The study involved 30 female Wistar rats divided into control, sham, and three silymarin-treated groups. After ovariectomy, rats underwent CT scan, and some of them were administered silymarin via gavage for 2 months. Memory and learning were assessed using Morris water maze and shuttle box tests. Brain and liver tissues were analyzed for inflammatory factors (IL-1β, TNFα, and IL-6) and oxidative stress markers (CAT, SOD, and MDA) after sacrifice. Results. Silymarin improved spatial memory and fear learning compared to the sham group ($P\le 0.05$ to $P\le 0.001$). It also significantly reduced IL-1β, TNF-α, and IL-6 levels in the cortex, hippocampus, and liver ($P\le 0.05$ to $P\le 0.0001$) and increased CAT and SOD while decreasing MDA levels ($P\le 0.05$ to $P\le 0.001$) compared to control and sham groups. Conclusion. Long-term administration of silymarin extract can improve learning and memory, reverse cognitive impairment caused by ovariectomy, and reduce oxidative stress and inflammatory factors induced by ovariectomy in the liver and brain of Wistar rats. This is due to the reduction in MDA levels and an increase in CAT activity, although silymarin has some effect on SOD at high doses.

The novel estrogen receptor modulator STX attenuates Amyloid-β neurotoxicity in the 5XFAD mouse model of Alzheimer's disease

Based on previous evidence that the non-steroidal estrogen receptor modulator STX mitigates the effects of neurotoxic Amyloid-β (Aβ) in vitro, we have evaluated its neuroprotective benefits in a mouse model of Alzheimer's disease. Cohorts of 5XFAD mice, which begin to accumulate cerebral Aβ at two months of age, were treated with orally-administered STX starting at 6 months of age for two months. After behavioral testing to evaluate cognitive function, biochemical and immunohistochemical assays were used to analyze key markers of mitochondrial function and synaptic integrity. Oral STX treatment attenuated Aβ-associated mitochondrial toxicity and synaptic toxicity in the brain, as previously documented in cultured neurons. STX also moderately improved spatial memory in 5XFAD mice. In addition, STX reduced markers for reactive astrocytosis and microgliosis surrounding amyloid plaques, and also unexpectedly reduced overall levels of cerebral Aβ in the brain. The neuroprotective effects of STX were more robust in females than in males. These results suggest that STX may have therapeutic potential in Alzheimer's Disease.

Estrogen Deficiency Induces Mitochondrial Damage Prior to Emergence of Cognitive Deficits in a Postmenopausal Mouse Model

Background: Estrogen deficiency contributes to the development of Alzheimer's disease (AD) in menopausal women. In the current study, we examined the impact of estrogen deficiency on mitochondrial function and cognition using a postmenopausal mouse model. Methods: Bilateral ovariectomy was conducted in adult females C57BL/6J. Cognitive function was examined using the Morris water maze (MWM) test at 2 weeks, 1, 2, and 3 months after ovariectomy. Neurodegeneration was assessed using an immunofluorescence assay of microtubule-associated protein 2 (MAP2) in the hippocampus and immunoblotting against postsynaptic density-95 (PSD95). Mitochondrial function in the hippocampus was assessed using immunoblotting for NDUFB8, SDHB, UQCRC2, MTCO1, and ATP5A1. Mitochondrial biogenesis was examined using immunoblotting for PGC-1α, NRF1, and mtTFA. Mitochondrion fission was assessed with immunoblotting for Drp1, whereas mitochondrion fusion was analyzed with immunoblotting for OPA1 and Mfn2. Mitophagy was examined with immunoblotting for PINK1 and LC3B. Mice receiving sham surgery were used as controls. Results: Ovariectomy resulted in significant learning and memory deficits in the MWM test at 3 months, but not at any earlier time points. At 2 weeks after ovariectomy, levels of Drp1 phosphorylated at Ser637 decreased in the hippocampus. At 1 month after ovariectomy, hippocampal levels of NDUFB8, SDHB, PGC-1α, mtTFA, OPA1, and Mfn2 were significantly reduced. At 2 months after ovariectomy, hippocampal levels of MAP2, PSD95, MTCO1, NRF1, and Pink1 were also reduced. At 3 months, levels of LC3B-II were reduced. Conclusions: The cognitive decline associated with estrogen deficiency is preceded by mitochondrial dysfunction, abnormal mitochondrial biogenesis, irregular mitochondrial dynamics, and decreased mitophagy. Thus, mitochondrial damage may contribute to cognitive impairment associated with estrogen deficiency.