Subjects

The subjects were 27 6–8 months-old male GDNF-deficient (HET, n = 12) mice and their WT (n = 15) littermate controls from a GDNF colony (Granholm et al., 1997) maintained on C57BL/6 background for at least 10 generations. Genotypes were confirmed by PCR amplification from tail biopsy samples. Mice were housed in a temperature-controlled room under a 12-h light-dark cycle. Mice were maintained at 85% of their ad libitum weights by restricting access to food (Teklad Diet 8064, Harlan Laboratories Inc., Indianapolis, IN, USA); weight did not differ between groups either before or after stress manipulations (ts₍₂₅₎ < 0.44, p > 0.05). All manipulations were performed in compliance with ethical standards for the treatment of animals National Research Council [USA] (2011) and were approved by Utah State University IACUC committee.