GLYCINE TRANSPORTERS ARE DIFFERENTIALLY EXPRESSED AMONG CNS CELLS

Glycine is the major inhibitory neurotransmitter in the spinal cord an d brainstem and is also required for the activation of NMDA receptors. The extracellular concentration of this neuroactive amino acid is reg ulated by at least two glycine transporters (GLYT1 and GLYT2). To stud y the localization and properties of these proteins, sequence-specific antibodies against the cloned glycine transporters have been raised. Immunoblots show that the 50-70 kDa band corresponding to GLYT1 is exp ressed at the highest concentrations in the spinal cord, brainstem, di encephalon, and retina, and, in a lesser degree, to the olfactory bulb and brain hemispheres, whereas it is not detected in peripheral tissu es, Pre-embedding light and electron microscopic immunocytochemistry s how that GLYT1 is expressed in glial cells around both glycinergic and nonglycinergic neurons except in the retina, where it is expressed by amacrine neurons, but not by glia. The expression of a 90-110 kDa ban d corresponding to GLYT2 is restricted to the spinal cord, brainstem, and cerebellum; in addition, very low levels occur in the diencephalon . GLYT2 is found in presynaptic elements of neurons thought to be glyc inergic, However, in the cerebellum, GLYT2 is expressed both in termin al boutons and in glial elements, The physiological consequences of th e regional and cellular distributions of these two proteins as well as the possibility of the existence of an unidentified neuronal form of GLYT1 are discussed.