Abdelsalam, H. M. “The Impact of Sildenafil Citrate on Neurotransmitter Amino Acid Levels in Brain Tissue of Albino Rats”. International Journal of Clinical and Experimental Physiology, Vol. 5, no. 2, Sept. 2018, pp. 87-93, https://ijcep.org/index.php/ijcep/article/view/305.