Exercise contributes to the effects of DHA dietary supplementation by acting on membrane-related synaptic systems

Abstract

Dietary omega-3 fatty acid (i.e. docosohexaenoic acid (DHA)) and exercise are gaining recognition for supporting brain function under normal and challenging conditions. Here we evaluate the possibility that the interaction of DHA and exercise can involve specific elements of the synaptic plasma membrane. We found that voluntary exercise potentiated the effects of a 12-day DHA dietary supplementation regimen on increasing the levels of syntaxin 3 (STX-3) and the growth-associated protein (GAP-43) in the adult rat hippocampus region. STX-3 is a synaptic membrane-bound protein involved in the effects of DHA on membrane expansion. The DHA diet and exercise also elevated levels of the NMDA receptor subunit NR2B, which is important for synaptic function underlying learning and memory. The actions of exercise and DHA dietary supplementation reflected on enhanced learning performance in the Morris water maze as learning ability was associated with higher levels of STX-3 and NR2B. The overall findings reveal a mechanism by which exercise can interact with the function of DHA dietary enrichment to elevate the capacity of the adult brain for axonal growth, synaptic plasticity, and cognitive function.

Fig. 1

Exercise enhanced the effects of DHA dietary supplementation on STX-3, a protein implicated with the action of DHA on synaptic membrane (A). The values were converted to percent of RD-Sed controls (each value represents the mean latency±SEM, two-way ANOVA, **P<0.01). (B) The levels of STX-3 changed proportionally to the amount of exercise in animals receiving DHA diet (r=0.897, p<0.01). (C–F) Immunofluorescence for STX-3 in coronal sections of the hippocampus shows STX-3 (red, Cy3 secondary antibody) in RD-Sed controls (C, E) and animals receiving combined exercise and diet treatment (DHA-Exc; D, F). (D) and (F) are high magnification photomicrographs (of D and F). A marked increase in STX-3 immunofluorescence was shown in the DHA-Exc group (white arrows, F). Myelinated axons were labeled using immunofluorescence for myelin-associated glycoprotein (green, FITC secondary antibody). RD-Sed: regular diet-sedentary; RD-Exc: regular diet-exercise; DHA-Sed: DHA diet-sedentary; DHA-Exc: DHA diet-exercise.

Fig. 2

DHA dietary supplementation and exercise affected synaptic plasticity markers in hippocampus. (A) The DHA diet and exercise elevated levels of NR2B while the combination of both resulted in greater NR2B levels (mean± SEM, ANOVA, *P<0.05, **P<0.01). (B) NR2B levels changed in proportion to STX-3 levels for individual rats receiving the DHA diet and exercise combination (r=0.878, p<0.05). Levels of STX-3 and NR2B values are expressed as a percent of RD-Sed controls.

Fig. 3

Association between plasticity markers and learning performance after DHA dietary supplementation and exercise. The effects of exercise and DHA dietary supplementation on the Morris water maze test over the five consecutive days of training were reflected in changes in the learning speed (slope of the latency, A) to locate the hidden platform. We measured the slope of the escape latency across the five days of learning, and each value represents the mean slope latency±SEM (ANOVA, **P<0.01). A correlation analysis revealed an association between the learning speed and levels of NR2B (B) and syntaxin 3 (C) in the DHA-Exc group but not in the RD-Sed group. RD-Sed: regular diet-sedentary; RD-Exc: regular diet-exercise; DHA-Sed: DHA diet-sedentary; DHA-Exc: DHA diet-exercise.

Fig. 4

(A) The separate applications of exercise or the DHA diet elevated levels of GAP-43 while the concurrent application of both elevated GPA-43 levels further. Each value represents the mean latency±SEM (two-way ANOVA, *p<0.05, **p<0.01). (B) Levels of GAP-43 changed in proportion to the total amount of exercise in animals receiving DHA diet and exercise combined treatment (r=0.908, p<0.01).

Fig. 5

Schematic diagram depicting the effects of the DHA diet and exercise on key elements involved in the maintenance of the synaptic membrane. Dietary DHA and exercise can influence contents of membrane DHA and STX-3 with subsequent effects on membrane stability and fluidity. The function of membrane embedded receptors such as NR2B depends on the fluidity of the membrane affecting synaptic function and learning and memory.