The Effect of Individual Branched Chain Amino Acids Dietary Restriction on Hippocampal- Related Functions and Neurogenesis in Male Rats

Abstract

Introduction: Neurogenesis is the process by which new neurons are generated from neural stem cells (NSCs) and is essential for brain plasticity. Although it is most prominent during embryonic development, neurogenesis persists in the adult brain within discrete neurogenic niches, particularly the subventricular zone (SVZ) and the subgranular zone (SGZ) of the dentate gyrus in the hippocampus. Adult hippocampal neurogenesis contributes to functions such as learning, memory, and emotional regulation and proceeds through coordinated stages including proliferation, migration, differentiation, and integration, with proliferation being a key step for generating new progenitor cells. This process is regulated by both intrinsic factors (e.g., genetic programming) and extrinsic influences, including environmental and metabolic signals. Among extrinsic modulators, dietary factors have gained attention due to their potential to shape neurogenesis through nutrient availability. Branched-chain amino acids (BCAAs) including leucine (Leu), valine (Val), and isoleucine (Ile) are traditionally known for their role in muscle protein synthesis but are increasingly recognized as neurometabolic signals that may influence brain function. BCAAs regulate pathways relevant to neural proliferation and growth, including mTORC1 signaling, protein synthesis, and amino acid/glutamate metabolism. However, the specific contributions of individual BCAAs to adult neurogenesis remain insufficiently understood, highlighting an important gap in how dietary amino acids may modulate hippocampal plasticity. Objective: This study aims to investigate how selective restriction of individual BCAAs under low-protein conditions differentially affects adult hippocampal neurogenesis, hippocampal gene expression, and behavior (anxiety-like behavior, recognition memory, and spatial working memory). Methods: Forty male Sprague Dawley rats (3–4 weeks old) were randomly assigned to five diet groups: Control (20% total energy), Low Protein (10% total energy), and Low Protein with 50% reduction in Leu, Val, or Ile. The diets were isoenergetic (3.75 kcal/g), with each group receiving the designated BCAA restriction. Behavioral testing was conducted, including elevated plus maze (EPM), novel object recognition (NOR), and Y-maze. For neurogenesis assessment, BrdU+ve cells were quantified in the SGZ of the dentate gyrus using confocal microscopy to measure NSC proliferation. Hippocampal gene expression related to serotonergic, glutamatergic, and neurotrophic pathways (e.g., 5-HT3A, GluR1, BDNF, TrkB) was assessed using RT-qPCR. Results: Selective BCAA restriction produced distinct physiological and neurobehavioral effects, with the greatest growth impairment observed under isoleucine deficiency. In the EPM, the Low-Ile group showed the clearest anxiety-like avoidance profile (with anxiety-like behavior also noted in leucine- and isoleucine-deficient groups). In the NOR, locomotor/immobility parameters were altered by diet, but indices of novel object exploration were not significantly affected, indicating preserved recognition memory across groups. In the Y-maze, spontaneous alternation did not significantly differ between diets, while measures of novelty exploration were modulated (increased novel-arm entries in Low-Protein and Low-Val; increased novel-arm time in Low-Ile). Importantly, BrdU+ cell counts in the SGZ were lowest in the low-protein group, whereas both Low-Val and Low-Ile groups showed significantly higher BrdU+ counts compared with low protein; Low-Val also exceeded Low-Leu. At the molecular level, valine restriction selectively increased 5-HT3A mRNA (Low-Val vs Low-Protein), while GluR1 and neurotrophic factors (BDNF, FGF2, TrkB) did not significantly differ across diets. Conclusion: These findings support the concept that leucine, isoleucine, and valine are not interchangeable in their brain-related effects. Under low protein conditions, selective valine and isoleucine restriction unexpectedly increased SGZ BrdU+ proliferation, while hippocampal serotonergic signalling (5-HT3A) was selectively modulated by valine restriction and basic recognition/spatial working memory remained largely intact. Given the distinct effects of individual BCAAs on neurogenesis and behavior, future research should explore the mechanisms underlying these differential effects, particularly in relation to neurotransmitter systems such as serotonin and glutamate. Additionally, further studies could also investigate the role of dietary BCAA supplementation in promoting neurogenesis in aging populations or individuals with neurological disorders.