Muscarinic cholinergic receptors in the hippocampus of aged rats: influence of choline alphoscerate treatment

Abstract

The present study was designed to investigate age-dependent changes of muscarinic M 1 and M 2 cholinergic receptors in the rat hippocampus using radioreceptor assay and autoradiographic techniques with [ 3H]pirenzepine and [ 3H]AF-DX 116 as ligands. The analysis was performed on 2-, 12- and 27-month-old male Wistar rats, considered young, adult and old, respectively. Moreover, the influence of a 6-month treatment with choline alphoscerate on the density and pattern of M 1 and M 2 cholinergic receptors was assessed. Choline alphoscerate ( l-α-glyceryl phosphorylcholine) is a precursor in the biosynthesis of several brain phospholipids which increases the availability of acetylcholine in various tissues. Muscarinic M 1 cholinergic receptors were significantly decreased with increasing age whereas M 2 cholinergic receptors did not show changes. Choline alphoscerate treatment countered, in part, the loss of muscarinic M 1 receptor sites in old rats. Light microscope autoradiography revealed a loss of silver grains developed after exposure of sections of hippocampus to [ 3H]pirenzepine in the stratum oriens of CA 1 and CA 3 fields in rats of 12 and 27 months in comparison with young animals. Choline alphoscerate restored, in part, the decrease of silver grains noted in old rats. Quantitative analysis of the density of silver grains developed in the cell body of pyramidal neurons of CA 1 and CA 3 fields processed for the demonstration of muscarinic M 1 receptor sites revealed a decrease of these grains in rats of 27 months in comparison with younger cohorts. These findings suggest that the reduction in muscarinic M 1 sites noticeable between 2- and 12-month rats is probably dependent on the loss of nerve cells and/or terminals in these hippocampal fields rather than to a reduction of their density per neuron. Treatment with choline alphoscerate increased the expression of muscarinic M 1 cholinergic receptors within the cell body of pyramidal neurons of CA 1 and CA 3 fields compared to age-matched control old rats. Consistent with radioreceptor assay data, no changes in the density of muscarinic M 2 cholinergic receptors in the animal groups examined were demonstrated by light microscope autoradiography. The possible pharmacological relevance of the increased expression of muscarinic M 1 cholinergic receptors elicited by choline alphoscerate in the hippocampus of aged rats is discussed.