This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Which of the following neurotransmitter is having only inhibitory action? 
Select and explain. 
A. GABA, B. Glutamine, C. Aspaate, D. All of the above
A: GABA
Answer: A: GABA 
Explanation: GABA is the major inhibitory mediator in the brain and mediates both presynaptic and postsynaptic inhibition. GABA, which exists as -aminobutyrate in the body fluids, is formed by decarboxylation of glutamate by the enzymeglutamate decarboxylase (GAD), which is present in nerve endings in many pas of the brain. Glutamate is the main excitatory transmitter in the brain and spinal cord and has been calculated to be responsible for 75% of the excitatory transmission in the CNS. Glycine has both excitatory and inhibitory effects in the CNS. Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 7. Neurotransmitters & Neuromodulators. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong's Review of Medical Physiology, 24e.