Adaptation to excess acetylcholine by downregulation of adrenoceptors and muscarinic receptors in lungs of acetylcholinesterase knockout mice

The acetylcholinesterase knockout mouse has elevated acetylcholine levels due to the complete absence of acetylcholinesterase. Our goal was to determine the adaptive changes in lung receptors that allow these animals to tolerate excess neurotransmitter. The hypothesis was tested that not only muscarinic receptors but also α₁-adrenoceptors and β-adrenoceptors are downregulated, thus maintaining a proper balance of receptors and accounting for lung function in these animals. The quantity of α_1A, α_1B, α_1D, β₁, and β₂-adrenoceptors and muscarinic receptors was determined by binding of radioligands. G-protein coupling was assessed using pseudo-competition with agonists. Phospholipase C activity was measured by an enzymatic assay. Cyclic AMP (cAMP) content was measured by immunoassay. Muscarinic receptors were decreased to 50%, α₁-adrenoceptors to 23%, and β-adrenoceptors to about 50% of control. Changes were subtype specific, as α_1A, α_1B, and β₂-adrenoceptors, but not α_1D-adrenoceptor, were decreased. In contrast, receptor signaling into the cell as measured by coupling to G proteins, cAMP content, and PI-phospholipase C activity was the same as in control. This shows that the nearly normal lung function of these animals was explained by maintenance of a correct balance of adrenoceptors and muscarinic receptors. In conclusion, knockout mice have adapted to high concentrations of acetylcholine by downregulating receptors that bind acetylcholine, as well as by downregulating receptors that oppose the action of muscarinic receptors. Tolerance to excess acetylcholine is achieved by reducing the levels of muscarinic receptors and adrenoceptors.