The androgen receptor in the hypothalamus positively regulates hind-limb muscle mass and voluntary physical activity in adult male mice

Abstract

We have previously shown that expression of the androgen receptor (AR) in neurons within the brain positively regulates hind-limb muscle mass and physical activity in male mice. To further investigate the region of the brain responsible for mediating these effects of testosterone and to determine whether they are only important for muscle mass accrual during development or whether they are also important for the maintenance of muscle mass in the adult, we deleted the AR specifically in the hypothalamus of adult male mice (Hyp-ARKOs). Hyp-ARKO mice were generated by bilateral stereotaxic microinjection of an adeno-associated virus (AAV) expressing GFP and iCre recombinase under the control of the e-synapsin promoter into the hypothalamus of 10-week-old exon 3-AR floxed male mice. AR mRNA was deleted by 45% in the hypothalamus of Hyp-ARKOs at 5 weeks post-AAV-eSyn-iCre injection. This led to an increase in the mass of the androgen-dependent organs, seminal vesicles and kidneys, by 30% (P\u202f<\u202f0.01) and 10% (P\u202f<\u202f0.05) respectively, and an increase in serum luteinizing hormone (LH) by 2 fold (P\u202f<\u202f0.05). Whilst the mean value for serum testosterone was higher in the Hyp-ARKOs, this did not reach statistical significance. Despite a phenotype consistent with increased androgen bioactivity in Hyp-ARKOs, which would be expected to increase muscle mass, the mass of the hind-limb muscles, gastrocnemius (Gast) (P\u202f=\u202f0.001), extensor digitorum longus (EDL) (P\u202f<\u202f0.001) and soleus (Sol) (P\u202f<\u202f0.01) were paradoxically decreased by 12-19% compared to controls. Voluntary physical activity was reduced by 65% (P\u202f<\u202f0.05) in Hyp-ARKO male mice and was associated with a reduction in gene expression of Drd1a and Maob (P\u202f≤\u202f0.05) in the hypothalamus, suggesting involvement of the brain dopaminergic system. These data provide compelling evidence that androgen signalling via the AR in the hypothalamus acts to positively regulate the maintenance of hind-limb muscle mass and voluntary activity in adult male mice, independent of AR signalling in peripheral tissues.