Hippocampal norepinephrine system regulates exercise habituation in SPORTS rats, a newly established animal model for high voluntary wheel running

Abstract

Male SPORTS (Spontaneously Running Tokushima Shikoku) run voluntarily in a running wheel 6 times longer than male control Wistar rats. To test whether running habit is related to hippocampal catecholamine dynamics, we investigated the level of monoamines in the brain of SPORTS rats. Release of hippocampal norepinephrine (NE) was assessed with in vivo microdialysis, and concentration of NE and monoamine oxidase A (MAOA) expression were measured using tissue homogenate. The extracellular NE levels were higher whereas intracellular NE levels lower in the hippocampus of SPORTS rats compared to those of controls. The mRNA level of MAOA, a critical enzyme for the degradation of monoamines, in hippocampus was not different between SPORTS and control rats. However, the protein levels and activity were lower in the SPORTS strain without any mutation in the nucleotide sequences. Intraperitoneal administration of clorgyline (1 mg/kg BW/day), a specific MAOA inhibitor, to the control rats decreased MAOA activity in hippocampus and increased wheel running activity. Our results suggest that the decreased activity of MAOA and subsequent increase in the extracellular NE levels in the hippocampus are closely linked to the hyper-running phenotype of SPORTS rats. [Jpn J Physiol 55 Suppl:S184 (2005)]