Glycine is typically described as an inhibitory neurotransmitter, although it may also have excitatory effects; when it binds to the NMDA receptors, it makes them more sensitive. Glycine receptors, however, are inhibitory.
The interneurons that release glycine are mostly restricted to the spinal cord and brain stem, unlike those that release GABA which are more widely spread in the CNS.
Glycine receptors are ionotropic, and are pentameric structures that are selectively permeable to Cl–, and are thus inhibitory.
Strychnine, which is a spinal cord convulsant, selectively blocks glycine receptors and has been used in rat poison.
1. Katzung BG. Basic & clinical pharmacology. 10th ed. New York: McGraw-Hill Medical; 2007.