Membrane fluidization increases low-affinity muscarinic receptor binding in brain: changes with aging.

Specific cholinergic muscarinic receptor binding was determined with L-[3H]quinuclidinyl benzilate ([3H]QNB) in homogenates from crude synaptosomal pellets prepared from mouse whole-brain homogenates. Specific total (high- and low-affinity) binding was determined in the absence of the agonist carbachol and low-affinity binding in its presence. These membrane preparations were fluidized by adding in vitro aliphatic alcohols ranging from ethanol to hexanol and by increasing the incubation temperatures. At 23 degrees C hexanol (14.7 mM) nearly doubled the low-affinity binding in the presence of carbachol (0.32 mM) and decreased high-affinity binding by the same amount. This suggested a change of muscarinic receptors from high- to low-affinity conformation. Increase of incubation temperature from 24 degrees C to 37 degrees C nearly tripled low-affinity binding. Brain homogenates from female C57BL/6J mice, ages 6, 12, 18, and 30 months, showed a progressively lower stimulation by hexanol of low-affinity [3H]QNB binding in the presence of carbachol. We postulate that this diminished change with age of [3H]QNB-receptor binding in response to alcohols may be a result of increasing membrane rigidity with advancing age. Rigidity of membranes may link aging at the membrane level, synaptic receptors, and impaired learning behavior.