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Quantitative changes in G proteins do not mediate ethanol-induced downregulation of adenylyl cyclase in mouse
B Tabakoff1, J P Whelan, L Ovchinnikova
1Department of Pharmacology, University of Colorado Health Sciences Center, Denver 80262, USA.
Abstract:
Our prior work, and the work of others, demonstrated that chronic administration of ethanol to cells in culture or to mice resulted in decreased responsiveness of adenylyl cyclase (EC4.6.1.1) to a number of stimulatory agents. In this study, we substantiated the ethanol-induced changes in cerebral cortical adenylyl cyclase activity in alcohol-tolerant and alcohol-dependent mice, and we examined whether chronic ethanol treatment of mice altered the quantity of heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) in cerebral cortex and other mouse brain areas. Amounts of various G protein subunits--including the alpha subunits of GS (GS alpha), Gi alpha 1-3, G(o) alpha, and beta subunits--were examined by Western blot analysis. There was no change in quantity of these G protein subunits in cerebral cortex, hippocampus, or cerebellum of ethanol-fed mice, compared with controls. In striatum of ethanol-fed mice, small increases in Gi alpha 1 and G(o) alpha were observed, but these changes could not explain the ethanol-induced desensitization of adenylyl cyclase in brain areas such as the cerebral cortex. Forskolin activation of cerebral cortical adenylyl cyclase activity showed two components of activation, with high and low "affinity" for forskolin. Ethanol treatment caused a decrease in the efficacy of forskolin for both components, whereas the EC50 of forskolin for each component did not change. Adenylyl cyclase activity measured in the presence of manganese was also diminished in cortical membranes of ethanol-treated mice.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Chronic ethanol exposure reduces brain adenylyl cyclase responsiveness by decreasing its efficacy, not by altering G protein levels. This desensitization impacts alcohol tolerance and dependence.
Area of Science:
- Neuroscience
- Biochemistry
Background:
- Chronic ethanol administration decreases adenylyl cyclase responsiveness.
- This desensitization is observed in both cell cultures and animal models.
Purpose of the Study:
- To investigate ethanol's effect on cerebral cortical adenylyl cyclase activity in alcohol-tolerant and dependent mice.
- To determine if chronic ethanol alters G protein quantity in mouse brain regions.
Main Methods:
- Western blot analysis to quantify G protein subunits (GS alpha, Gi alpha, G(o) alpha, beta subunits).
- Measurement of adenylyl cyclase activity and forskolin activation in mouse brain cortical membranes.
Main Results:
- No significant changes in G protein subunit quantities were found in cerebral cortex, hippocampus, or cerebellum.
- Small increases in Gi alpha 1 and G(o) alpha were noted in the striatum.
- Ethanol treatment decreased the efficacy of forskolin activation of adenylyl cyclase without altering its EC50.
- Adenylyl cyclase activity in the presence of manganese was also reduced in ethanol-treated mice.
Conclusions:
- Ethanol-induced adenylyl cyclase desensitization in the cerebral cortex is not due to changes in G protein quantity.
- The reduced efficacy of adenylyl cyclase activation by forskolin is a key mechanism underlying ethanol's effects on brain signaling.