GABA, the major inhibitory neurotransmitter in the CNS, acts via GABAa, GABAb, and putative GABAc receptors. GABAa and GABAc receptors are ligand-gated ion channels, while GABAB receptors are 7TM-GPCRs. The GABAb receptor is of most interest as a target for AD. Both autoreceptors and heteroreceptors exist for the GABAb receptor family that modulate the presynaptic release of GABA as well as glutamate, various monoamines, and neuropeptides. GABA receptor activation inhibits learning; thus receptor antagonists may be useful in reversing this deficit. SGS-742 (CGP-36742: 23), is a low-potency GABAg/GABAc antagonist (IC50 values = 36 and 63 mM, respectively) currently in the clinic.39 Chronic administration of SGS-742 upregulated GABAb receptor expression, increased protein levels of nerve growth factor (NGF) and brain-derived neuronal factor (BDNF) in rat cortex and hippocampus, and facilitated the release of glutamate, aspartate, glycine, and somatostatin. In mice and rats, SGS-742 improved learning and memory tasks at doses of 3-30 mgkg_ 1 (i.p. or p.o.).
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