Glycine transporters are differentially expressed among CNS cells

Abstract

Glycine is the major inhibitory neurotransmitter in the spinal cord and brainstem and is also required for the activation of NMDA receptors. The extracellular concentration of this neuroactive amino acid is regulated by at least two glycine transporters (GLYT1 and GLYT2). To study 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 expressed at the highest concentrations in the spinal cord, brainstem, diencephalon, and retina, and, in a lesser degree, to the olfactory bulb and brain hemispheres, whereas it is not detected in peripheral tissues. Pre-embedding light and electron microscopic immunocytochemistry show 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 band corresponding to GLYT2 is restricted to the spinal cord, brain-stem, and cerebellum; in addition, very low levels occur in the diencephalon. GLYT2 is found in presynaptic elements of neurons thought to be glycinergic. However, in the cerebellum, GLYT2 is expressed both in terminal boutons and in glial elements. The physiological consequences of the 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.

Glycine Uptake Regulates Hippocampal Network Activity via Glycine Receptor-Mediated Tonic Inhibition

“…A rightward shift of the E-S curve was evident after addition of taurine (14.9774.65%, t ¼ 3.3, df ¼ 6, po0.05; Figure 2c and d). Taken together, these results suggest that in intact slices, the concentration of endogenous agonist is not high enough to activate the GlyR so that plays a major role in neurotransmission or excitability, due probably to the presence of efficient glycine reuptake mechanisms in the hippocampus (Zafra et al, 1995). However, raising the agonist concentration overcame the clearance of glycine by GlyTs, thus unmasking its effect on E-S coupling.…”

“…Similar to sarcosine, NFPS (1 mM) shifted the E-S curve to the right (15.9776.90%, t ¼ 2.3, df ¼ 10, po0.05; Figure 3b and c), and this was reversed by subsequent application of STN (1 mM, À15.6574.13%, t ¼ À3.8, df ¼ 8, po0.01 compared with the NFPS group; Figure 3b and d). In view of the sparse distribution of GlyT2 in the hippocampus (Zafra et al, 1995), the effects of a GlyT2 antagonist, O-[(2-benzyloxyphenyl-3-flurophenyl)methyl]-Lserine (ALX-1393), was also tested. Unlike GlyT1 antagonists, ALX-1393 (1 mM) did not shift the E-S curve (À0.3272.00%, t ¼ À0.16, df ¼ 5, p40.1; Figure 3c), indicating that GlyT2 blockade had no effect on GlyRs or neuronal excitability.…”

“…In view of the lack of GlyT2 expression in hippocampal neurons but abundant distribution of GlyT1 in hippocampal astrocytes (Zafra et al, 1995), we hypothesized that astrocyte-derived glycine is responsible for the sarcosineinduced E-S depression. To test this possibility, we used a glia-specific metabolic inhibitor, fluoroacetate (FAC), to interfere with glial cell functions (Figure 6).…”

“…In the hippocampus, where rare GlyT type 2 (GlyT2) (Jursky and Nelson, 1995) but robust GlyT type 1 (GlyT1) (Zafra et al, 1995) is expressed and colocalized with N-methyl-D-aspartate receptors (NMDAR) (Smith et al, 1992), the blockade of high-affinity GlyT1 modulates NMDAR-mediated responses and long-term potentiation (LTP) by increasing extracellular glycine levels (Martina et al, 2004;Kinney et al, 2003). Several other observations from rat prefrontal cortical neurons (Chen et al, 2003), spinal cord lamina X neurons (Bradaia et al, 2004), and hypoglossal motorneurons (Lim et al, 2004) also support a role of elevated glycine levels in NMDAR activity through GlyT1-dependent processes.…”

Differential Effects of Antipsychotic and Glutamatergic Agents on the phMRI Response to Phencyclidine

“…Alternatively, or in addition, clozapine may potentiate NMDAR activation by elevating synaptic levels of glycine through inhibition of type 1 glycine transporters (GlyT-1) (Javitt et al, 2005). Such a mechanism could also be implicated in the focal effect of clozapine in the thalamus, given the high level of Gly-T1 receptor expression in this region, with respect to the cortex and other limbic areas (Zafra et al, 1995). Thus the efficacy of clozapine in the present study might arise through a direct or indirect enhancement of NMDAR function, counteracting the effect of NMDAR block by PCP.…”