All of the following neurotransmitters are excitatory and inhibitory in function, EXCEPT:

Glycine has both excitatory and inhibitory effects in the CNS. When it binds to NMDA receptors, it makes them more sensitive to the actions of glutamate. It is also responsible in pa for direct inhibition, primarily in the brainstem and spinal cord. Like GABA, it acts by increasing Cl- conductance. Its action is antagonized by strychnine. Glutamate and aspaate are found in very high concentrations in brain, and both amino acids have powerful excitatory effects on neurons in viually every region of the CNS. NO acts as a signal by which postsynaptic neurons communicate with presynaptic endings to enhance release of glutamate. 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.