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cAMP and forskolin decrease gamma-aminobutyric acid-gated chloride flux in rat brain synaptoneurosomes
1Department of Pharmacology, Duke University Medical Center, Durham, NC 27710.
Abstract:
The effects of the cyclic nucleotide cAMP on gamma-aminobutyric acid-gated chloride channel function were investigated. The membrane-permeant cAMP analog N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate inhibited muscimol-induced 36Cl- 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 gamma-aminobutyric acid-gated Cl- channel was studied with forskolin, an activator of adenylate cyclase. Under identical conditions, in the intact synaptoneurosomes, forskolin inhibited muscimol-induced 36Cl- uptake and generated cAMP with similar potencies (IC50 = 14.3 microM; EC50 = 6.2 microM, 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-induced 36Cl- 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 gamma-aminobutyric acid (GABAA) receptor function in brain can be regulated by cAMP-dependent phosphorylation.
Insights
Cyclic adenosine monophosphate (cAMP) regulates gamma-aminobutyric acid (GABA) receptor function by inhibiting chloride channel activity. Forskolin demonstrates rapid, direct interaction with the channel, suggesting dual regulatory mechanisms.
Area of Science:
- Neuroscience
- Molecular Biology
- Pharmacology
Background:
- The gamma-aminobutyric acid (GABA) receptor is a key inhibitory neurotransmitter receptor in the central nervous system.
- Cyclic adenosine monophosphate (cAMP) is a crucial second messenger involved in various cellular signaling pathways.
- Understanding the regulation of GABAergic neurotransmission by cAMP is vital for comprehending neuronal function and dysfunction.
Purpose of the Study:
- To investigate the effects of cAMP on the function of GABA-gated chloride channels.
- To elucidate the mechanisms by which cAMP modulates GABAergic signaling.
- To determine if forskolin and its derivatives directly interact with the GABA-gated chloride channel.
Main Methods:
- Measurement of 36Cl- uptake in rat cerebral cortical synaptoneurosomes.
- Application of membrane-permeant cAMP analogs and phosphodiesterase inhibitors.
- Utilized forskolin and 1,9-dideoxyforskolin to study endogenous cAMP effects and direct interactions.
Main Results:
- N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate (cAMP analog) inhibited muscimol-induced 36Cl- uptake in a concentration-dependent manner.
- Forskolin, an adenylate cyclase activator, inhibited 36Cl- uptake and increased cAMP levels with similar potencies.
- 1,9-dideoxyforskolin inhibited the response rapidly, suggesting a direct interaction with the chloride channel independent of cAMP accumulation.
Conclusions:
- GABA receptor function in the brain can be regulated by cAMP-dependent phosphorylation.
- Forskolin exhibits a dual mechanism of action: rapid direct channel inhibition and slower cAMP-mediated effects.
- These findings highlight a complex regulatory network influencing GABAergic neurotransmission.