Modulation of gamma-aminobutyric acid release in cerebral cortex by fluoride, phorbol ester, and phosphodiesterase inhibitors: differential sensitivity of acetylcholine release to fluoride and K+ channel blockers.
In this study we have used fluoride as a tool to investigate the involvement of G protein-coupled effector systems in the regulation of the depolarization-induced release of gamma-aminobutyric acid (GABA) from rat cerebral cortex. To distinguish among the activating effects of NaF on G proteins linked to different effectors, such as adenylate cyclase, polyphosphoinositide phospholipase C, and K+ channels, agents specific to these effectors have been used in parallel. NaF induced a marked dose-dependent facilitation of the K(+)-evoked release of [14C]GABA, with an EC50 of 1.26 mM, increasing release by 103% at 5 mM NaF. No effect on basal release was seen up to 3 mM NaF, and no modulation of [3H]acetylcholine (ACh) release was seen up to 5 mM NaF. Phorbol 12,13-diacetate (PDA) produced a similar dose-dependent facilitation of the K(+)-evoked release of [14C]GABA, potentiating the release of [14C]GABA by 50% at 10 microM PDA. The phosphodiesterase inhibitors, 3-isobutyl-1-methylxanthine (IBMX) and theophylline, inhibited the K(+)-evoked release of [14C]GABA, and IBMX reversed the NaF facilitation of GABA release in a dose-dependent manner (pA2 2.57). The K+ channel blocker (IA current) tetrahydroaminoacridine (THA), which markedly inhibits the K(+)-evoked release of [14C]GABA, also reversed the NaF facilitatory effect, but the release of [3H]ACh was less sensitive to the inhibitory effect of THA. On the other hand, the K+ channel blocker, tetraethylammonium, which has no effect on the release of [14C]GABA, caused a significant facilitation of K(+)-evoked release of [3H]ACh. From these studies, it is concluded that GABA release in cerebral cortex is subject to regulation by G protein-linked effector systems that are distinct from those affecting the release of [3H]ACh in cerebral cortex.
Department of Biochemistry, Charing Cross and Westminster Medical School, London, England.
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Gardiner
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de Belleroche
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D002540:Cerebral Cortex / Q000378:metabolism*; D010703:Phorbol Esters / Q000494:pharmacology; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology*; D011188:Potassium / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D012969:Sodium Fluoride / Q000494:pharmacology*; D013619:Tacrine / Q000494:pharmacology; D013806:Theophylline / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D010703:Phorbol Esters; D010726:Phosphodiesterase Inhibitors; D015221:Potassium Channels; C008874:phorbol-12,13-diacetate; D013619:Tacrine; D005680:gamma-Aminobutyric Acid; D012969:Sodium Fluoride; D013806:Theophylline; D000109:Acetylcholine; D011188:Potassium; D015056:1-Methyl-3-isobutylxanthine