Why does a steep caudal-rostral gradient exist in glycine content in the brain?

Glycine is an important amino acid in the central nervous system. Interestingly, the content of glycine is about 9 times higher in the spinal cord grey matter than in the telencephalon. And this kind of caudal to rostral gradient is never seen in any other neurotransmitters. However, the cause of this phenomenon remains unknown. In the present report, I, thus, postulate the following theory. Glycine has dual roles as a neurotransmitter, one is the agonist for inhibitory glycine receptors (GlyRs), and the other is a co-agonist for excitatory NMDA receptors (NMDARs). Inhibitory GlyRs are concentrated in the lower brain and the affinity of glycine to GlyRs is low, leading to the high content of glycine in the lower brain. In contrast, in the upper brain, there are little glycinergic neurons and the affinity of glycine to NMDARs is very high, leading to the low content of glycine in the forebrain. These different roles of glycine as a neurotransmitter between in the upper brain and in the lower brain make this steep caudal-rostral gradient in glycine content.