Muscarinic agonist receptor subtypes in aging rat brain.

Whole brain homogenates from rats aged 6 months (young) and 24 months (old) showed a decline with age of the pre-synaptic cholinergic marker, choline acetyltransferase, and also of total specific binding sites for the muscarinic antagonist L(-)quinuclidinyl benzilate (L-QNB). However, neither the proportion nor the inhibition constants of high and low affinity muscarinic agonist binding sites (defined by displacement of L-QNB binding with carbachol) changed with age. These findings may be relevant to the central cholinergic deficit reported to be associated with cognitive impairment in aging man.