“The use of pharmacological agents … of nitric oxide), D-arginine (non-precursor of nitric oxide) N(omega_-nitro-L-arginine and N(omega)-nitro-L-…-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (nitric oxide scavenger), demonstrated that nitric oxide modulated this depression phenomenon …. facilitation or inhibition.”