R-isovaline: a subtype-specific agonist at GABA_B-receptors?

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• 作者：J.E. Cooke^a ; D.A. Mathers^a ; ^b ; E. Puil^a ; ^{puil@mail.ubc.ca}
• 关键词：isovaline ; GABAB receptors ; K+ channels ; thalamus ; ventrobasal nuclei ; analgesia
• 刊名：Neuroscience
• 出版年：2012
• 出版时间：10 January 2012
• 年：2012
• 卷：201
• 期：Complete
• 页码：85-95
• 全文大小：3096 K

文摘

The R-enantiomer of isovaline, an analgesic amino acid, has a chemical structure similar to glycine and GABA. Although its actions on thalamic neurons are strychnine-resistant and independent of the Cl^{?/sup> gradient, R-isovaline increases membrane conductance for K+. The purpose of this study was to determine if R-isovaline activated metabotropic GABA_B receptors. We used whole-cell voltage-clamp recordings to characterize the effects of R-isovaline applied by bath perfusion and local ejection from a micropipette to thalamic neurons in 250 ¦Ìm thick slices of rat brain. The immunocytochemical methods that we employed to visualize GABA_B1 and GABA_B2 receptor subunits showed extensive staining for both subunits in ventrobasal nuclei, which were the recording sites. Bath or local application of R-isovaline caused a slowly developing increase in conductance and outward rectification in 70 % (54/77) of neurons, both effects reversing near the K+ Nernst potential. As with the GABA_B agonist baclofen, G proteins likely mediated the R-isovaline effects because they were susceptible to blockade by non-hydrolyzable substrates of guanosine triphosphate. The GABA_B antagonists CGP35348 and CGP52432 prevented the conductance increase induced by R-isovaline, applied by bath or local ejection. The GABA_B allosteric modulator CGP7930 enhanced the R-isovaline induced increase in conductance. At high doses, antagonists of GABA_A, GABA_C, glycine_A, ¦Ì-opioid, and nicotinic receptors did not block R-isovaline responses. The observations establish that R-isovaline increases the conductance of K+ channels coupled to metabotropic GABA_B receptors. Remarkably, not all neurons that were responsive to baclofen responded to R-isovaline. The R-isovaline-induced currents outlasted the fast baclofen responses and persisted for a 1-2-h period. Despite some similar actions, R-isovaline and baclofen do not act at identical GABA_B receptor sites. The binding of R-isovaline and baclofen to the GABA_B receptor may not induce the same conformational changes in receptor proteins or components of the intracellular signaling pathways.}