Age-related changes in the density of muscarinic cholinergic M1 and M2 receptor subtypes in pyramidal neurons of the rat hippocampus.

Age-dependent changes in the expression of muscarinic M1 and M2 cholinergic receptors were assessed in the CA1 and CA3 fields of hippocampus using radioligand binding and autoradiographic techniques with [3H]-pirenzepine and [3H]-AF-DX 116 as ligands. Male Wistar rats of 2 months (young), 12 months (adult) and 27 months (old) of age were examined. Radioligand binding analysis revealed a significant decrease of the density of muscarinic M1 cholinergic receptors with increasing age and no change in muscarinic M2 cholinergic receptors. Autoradiographic evaluation of the number of silver grains developed within the cell body of pyramidal neurons of the CA1 and CA3 fields revealed a decrease of muscarinic M1 cholinergic receptors in the 27-month-old rats in comparison with younger cohorts and no changes in muscarinic M2 cholinergic receptors. These findings suggest that the reduction of muscarinic M1 sites noticed between rats of 2- and 12-months of age using radioligand binding techniques is probably dependent on the loss of hippocampal neurons rather than on the reduction of receptor density per neuron. Our data also indicate that appropriate morphological techniques associated with quantitative analysis may be useful in assessing age-dependent changes in the expression of neurotransmitter receptors by specific neuronal populations. The possible pharmaco-therapeutic relevance of the decreased expression of muscarinic M1 cholinergic receptors by pyramidal neurons of the hippocampus of old rats is discussed.