cAMP and Forskolin Decrease γ -aminobutyric Acid-Gated Chloride Flux in Rat Brain Synaptoneurosomes

The effects of the cyclic nucleotide cAMP on γ -aminobutyric acid-gated chloride channel function were investigated. The membrane-permeant cAMP analog N6, O2′ -dibutyryladenosine 3′, 5′-cyclic monophosphate inhibited muscimol-induced36Cl-uptake into rat cerebral cortical synaptoneurosomes in a concentration-dependent manner (IC50= 1.3 mM). The inhibition was due to a decrease in the maximal effect of muscimol, with no change in potency. Similar effects were observed with 8-(4-chlorophenylthio)adenosine 3′, 5′-cyclic monophosphate, 8-bromoadenosine 3′, 5′-cyclic monophosphate, and the phosphodiesterase inhibitor isobutylmethylxanthine. The effect of endogenous cAMP accumulation on the γ -aminobutyric acid-gated Cl-channel was studied with forskolin, an activator of adenylate cyclase. Under identical conditions, in the intact synaptoneurosomes, forskolin inhibited muscimol-induced36Cl-uptake and generated cAMP with similar potencies (IC50= 14.3 μ M; EC50= 6.2 μ M, respectively). Surprisingly, 1,9-dideoxyforskolin, which does not activate adenylate cyclase, also inhibited the muscimol response, suggesting that forskolin and its lipophilic derivatives may interact with the Cl-channel directly. Indeed, forskolin inhibition of muscimol-induced36Cl-uptake was extremely rapid (within 5 sec), preceding the accumulation of sufficient levels of cAMP. After 5 min, a slower phase of inhibition was seen, similar to the time course for cAMP accumulation. The data suggest that γ -aminobutyric acid (GABAA) receptor function in brain can be regulated by cAMP-dependent phosphorylation.