Timing of exercise differentially modulates fear memory and hippocampal neurotransmitters in male rats.

Exercise promotes neurogenesis and enhances memory consolidation while reducing the retention of aversive memories and anxiety-like behaviors. While our previous work found that acute exercise alters neurotransmitter concentrations, including dopamine and serotonin, in a time-of-day-dependent manner, the long-term effects of chronically timed exercise on neurotransmitter dynamics and behavioral phenotypes remain unclear. To examine whether the daily timing of a chronic exercise intervention modulates its impact on neurotransmitter profiles and fear responses, male rats were conditioned using a Pavlovian contextual fear approach, then assigned to a 4-week treadmill exercise intervention performed during the early (ZT14) or late (ZT22) active phase or a time-matched sham-exercise control group. One day after completing training, rats underwent a context retrieval test in the middle of active phase (ZT18), and hippocampal neurotransmitters were quantified using UPLC-MRM/MS. Rats subjected to sham-exercise at ZT22 exhibited higher freezing than sham-exercised rats at ZT14, whereas exercise interventions at ZT22 selectively attenuated freezing. Histamine, acetylcholine, and GABA exhibited significant exercise × time interactions. Direct neurotransmitter-freezing correlations were weak after false discovery rate control, consistent with a network-level reorganization rather than a single transmitter driver. These findings suggest that vulnerability to aversive memory expression can be buffered by exercise, if timed appropriately, and that exercise reshapes hippocampal neuromodulatory tone in a circadian-phase-dependent manner, supporting the potential of exercise timing as a chronotherapeutic strategy to enhance stress resilience and mental wellbeing.