SLC6A5 and major depressive disorder: Returning to glycinergic neurotransmission, the pharmacological inhibition of GlyT1 and/or GlyT2 and the modulation of Gly receptors are currently being studied as potential therapeutic strategies against serious CNS pathologies that include pain, schizophrenia, epilepsy, depression, alcohol abuse, and neurodegenerative disorders, and some ongoing studies also include novel clinical trials [19,20,21,46,63,89,92,101,103,107,110,112,127,133,134,135,136,137,138].