Pharmacological assessment of the role of the glycine transporter GlyT-1 in mediating high-affinity glycine uptake by rat cerebral cortex and cerebellum synaptosomes.

Two distinct types of glycine transporter, GlyT-1 and GlyT-2, have been characterised. GlyT-1 and GlyT-2 are known to be differentially expressed amongst CNS areas, but direct functional evidence for their relative contributions to high-affinity glycine uptake by brain tissues is lacking. In the present study, we have used the selective GlyT-1 inhibitor N[3-(4"-fluorophenyl)-3-(4"-phenylphenoxy)propyl]sarcosine (NFPS) to investigate the role of GlyT-1 in mediating glycine uptake. HEK293 cells expressing human GlyT-1c or GlyT-2 showed high levels of Na(+)-dependent glycine uptake, with K(m) values of 117+/-13 and 200+/-22 microM, respectively. NFPS potently inhibited uptake in GlyT-1c cells (IC(50) value 0.22+/-0.03 microM), being around 500-fold more potent than glycine or sarcosine, but had no effect on uptake in GlyT-2 cells (IC(50) >10 microM). Efflux of pre-loaded [3H]-glycine from GlyT-1c cells was increased by glycine or sarcosine, whereas NFPS had no effect on its own but blocked the effects of glycine or sarcosine. These results confirm that NFPS is a potent, selective and non-transportable GlyT-1 inhibitor. Rat cortex and cerebellum synaptosomes also showed a high-affinity Na(+)-dependent component of glycine uptake, with affinities similar to those observed for uptake in GlyT-1c or GlyT-2 cells. In cortex synaptosomes, NFPS and sarcosine produced the same maximal inhibition of uptake as glycine itself. However, in cerebellum synaptosomes, the maximal inhibition produced by NFPS and sarcosine was only half that produced by glycine. In both tissues NFPS was around 1000-fold more potent than glycine or sarcosine. Overall, our findings indicate that high-affinity glycine uptake in cerebral cortex occurs predominantly via GlyT-1. However, in cerebellum, only a part of the high-affinity uptake is mediated by GlyT-1, with the remaining NFPS-insensitive component most likely mediated by GlyT-2.

Use of the radioligand [(125)I]-SB-217644 in the characterisation and solubilisation of the novel binding site for the anticonvulsant SB-204269 in rat brain membranes and cell lines.

SB-204269 (trans-(+)-6-acetyl-4S-(4-fluorobenzoylamino)-3, 4-dihydro-2,2-dimethyl-2H-benzo[b]pyran-3R-ol) shows anticonvulsant activity in a range of animal seizure models, with a high therapeutic index and a lack of side-effects. We have previously reported the characterisation of a novel binding site for [(3)H]-SB-204269 in rat forebrain, which has a unique profile unrelated to other known anticonvulsant sites of action. We now describe the use of a [(125)I]-labelled form of SB-217644 (trans-6-acetyl-4S-(3-iodobenzoylamino)-3,4-dihydro-2, 2-dimethyl-2H-benzo[b]pyran-3R-ol), an analogue of SB-204269, for studies on this novel binding site. In rat forebrain membranes, [(125)I]-SB-217644 shows a similar binding profile to that of [(3)H]-SB-204269, with a maximum specific binding capacity (B(max)) of 286+/-12 fmol/mg protein, but has twenty-fold higher affinity (K(d) value 1.7+/-0.1 nM). The high affinity and high specific activity of [(125)I]-SB-217644 allowed it to be used for detection and characterisation of the detergent-solubilised form of the binding site. Specific [(125)I]-SB-217644 binding to cholate-solubilised rat cerebellum showed a K(d) value of 2.7+/-0.3 nM and a B(max) value of 55+/-11 fmol/mg protein, with a 7.3+/-0.3% yield of solubilised binding sites. [(125)I]-SB-217644 was also used in whole-cell binding assays for investigation of the properties of the novel binding site in a range of cell lines. Both rat brain neuronal and glial primary cultures and several CNS-related cell lines were found to have levels of specific [(125)I]-SB-217644 binding similar to those present in rat forebrain membranes. The solubilisation of this novel binding site, and the ability to quantify and characterise it in solubilised tissues and whole cells using [(125)I]-SB217644, will allow further studies towards the ultimate identification of the molecular target of SB-204269.