Environmental Enrichment Components Required to Reduce Methamphetamine-Induced Behavioral Sensitization in Mice: Examination of Behaviors and Neural Substrates

Environmental enrichment (EE) involves the presentation of various sensory, physical, social, and cognitive stimuli in order to alter neural activity in specific brain areas, which can ameliorate methamphetamine (MAMPH)-induced behavioral sensitization and comorbid anxiety symptoms. No previous studies have comprehensively examined which EE components are critical for effectively reducing MAMPH-induced behavioral sensitization and anxiety. This study examined different housing conditions, including standard housing (SH, No EE), standard EE (STEE), physical EE (PEE), cognitive EE (CEE), and social EE (SEE). In the beginning, mice were randomly assigned to the different combinations of housing conditions and injections, consisting of No EE/Saline, No EE/MAMPH, STEE/MAMPH, PEE/MAMPH, CEE/MAMPH, and SEE/MAMPH groups. Then, the mice received intraperitoneal injections of 1 mg/kg MAMPH or normal saline daily for 7 days, followed by a final injection of 0.5 mg/kg MAMPH or normal saline. After behavioral tests, all mice were examined for c-Fos immunohistochemical staining. The results showed that MAMPH induced behavioral sensitization as measured by distance traveled. MAMPH appeared to induce lowered anxiety responses and severe hyperactivity. All EE conditions did not affect MAMPH-induced lowered anxiety behaviors. STEE was likely more effective for reducing MAMPH-induced behavioral sensitization than PEE, CEE, and SEE. The c-Fos expression analysis showed that the medial prefrontal cortex (i.e., cingulate cortex 1 (Cg1), prelimbic cortex (PrL), and infralimbic cortex (IL)), nucleus accumbens (NAc), basolateral amygdala (BLA), ventral tegmental area (VTA), caudate-putamen (CPu), and hippocampus (i.e., CA1, CA3, and dentate gyrus (DG)) contributed to MAMPH-induced behavioral sensitization. The Cg1, IL, NAc, BLA, VTA, CPu, CA3, and DG also mediated STEE reductions in MAMPH-induced behavioral sensitization. This study indicates that all components of EE are crucial for ameliorating MAMPH-induced behavioral sensitization, as no individual EE component was able to effectively reduce MAMPH-induced behavioral sensitization. The present findings provide insight into the development of non-pharmacological interventions for reducing MAMPH-induced behavioral sensitization.