Soleus Muscle Hypertrophy And Neurobehavioral Responses To Environmental Enrichment

PURPOSE: Environmental enrichment (EE) has been shown to positively impact brain function by enhancing neuroprotection and neuroplasticity. Recently, we demonstrated that EE can lead to hypertrophy of the soleus muscle (Sudo et al., 2023). However, the relationship between soleus muscle hypertrophy, brain function, physical activity (PA), and stress response under EE conditions remains unclear. The purpose of this study was to clarify the relationship between the soleus muscle hypertrophy and the effects of EE exposure under multiple conditions. METHODS: Rats were divided into four groups: EE group (wheel and objects), EE-R group (wheel only), EE-O group (objects only), and CONT group (standard environment) (N = 6 each). PA was measured using a triaxial accelerometer. After 4 weeks, spatial learning and memory was evaluated using the Morris Water Maze (MWM) test over four consecutive days. The concentration of serum corticosterone (CS) was analyzed using an electrochemiluminescence immunoassay method. The cross-sectional area (CSA) of soleus muscle was quantified from histological analysis. RESULTS: PA was significantly higher in the EE (1082603 ± 149225 a.u.) and EE-R (972414 ± 102069 a.u.) groups compared with the CONT group (618068 ± 70028 a.u., P < 0.0001), but not in the EE-O (565921 ± 110803 a.u.) group. Soleus CSA showed a significant increase in all EE groups (EE 2733 ± 305; EE-R 2560 ± 144; EE-O 2732 ± 200 μm2, interaction P = 0.006, Ps <0.01 vs. CONT 2205 ± 261 μm2). In the MWM test, escape latency times was significantly shorter in all EE groups (EE 15.6 ± 3.8; EE-R 15.9 ± 11.2; EE-O 15.2 ± 5.9 s) compared with the CONT group (30.0 ± 3.2 s) in day3 (interaction P = 0.006, Ps <0.01). CS levels were also lower in all EE groups (EE 399 ± 72; EE-R 409 ± 114; EE-O 420 ± 87 ng/mL) than the CONT group (574 ± 81 ng/mL, Ps <0.01). The CSA of the soleus muscle correlated negatively with CS levels (r2 = 0.31, P = 0.005) and escape latency times (r2 = 0.30, P = 0.006). CONCLUSIONS: These findings suggest that soleus muscle hypertrophy may contribute to environmental enrichment related improvements in brain function and stress response. Supported by: This work was supported by JSPS KAKENHI Grant Numbers 24 K22262