Neurometabolic substrate utilization governs oxidative phosphorylation conductance in cortex and hippocampus

Neurometabolism is increasingly recognized as a pathogenic contributor to neurodegenerative disease. However, commonly reported mitochondrial functional outcomes (e.g., respiration) often lack specificity with respect to energetic demand, carbon substrate utilization, and key bioenergetic parameters such as mitochondrial membrane potential. To address this limitation, the present study sought to determine whether oxidative phosphorylation conductance differs across brain regions and as a function of carbon substrate. Oxidative phosphorylation conductance was investigated in permeabilized frontal cortex and hippocampus of female and male C57BL/6J mice using pyruvate/malate substrate (PM, supporting complex I) or succinate with rotenone complex-I inhibition (SR, supporting complex II). Both mitochondrial volume (multiphoton microscopy) and abundance (flow cytometry) assessments showed no regional differences (P > 0.05 in both sexes). Mitochondria's ability to titer respiration to clamped energetic demands was lower with SR compared to PM in both sexes, regardless of brain region (P < 0.001). The production of ATP-to-respiration ratio (P/O ratio) was less at low energetic demands with SR compared to PM in males (P < 0.001) and less regardless of energetic demand with SR compared to PM in females (P < 0.05). This study, utilizing otherwise healthy, young brain tissue, demonstrates the necessity for greater precision in mitochondrial bioenergetic approaches to rigorously advance understanding of neurometabolism.NEW & NOTEWORTHY By integrating multi-modal imaging and high-resolution respirometry, this study reveals a critical divergence in regional brain mitochondrial bioenergetics. Although succinate-supported (Complex II) respiration yields higher absolute flux, it exhibits significantly lower oxidative phosphorylation conductance and enzymatic activity than pyruvate/malate-supported (Complex I) states. This substrate-specific inefficiency provides a novel mechanistic basis for mitochondrial failure in neurodegenerative diseases where Complex I is dysfunctional, highlighting the necessity for precise bioenergetic profiling in brain health.