Cholinergic markers in aged cognitively impaired Long-Evans rats.

Aged Long-Evans rats (24-25 months old) were classified into cognitively impaired or unimpaired subgroups based on their performances in the Morris Swim Maze task compared to young controls. Using quantitative in vitro receptor autoradiography, we investigated the status of various cholinergic markers in these two groups and in young adults (six months) animals. The apparent density of [3H]pirenzepine (muscarinic M1) sites was similar in the three groups of rats in various cortical areas, subfields of the hippocampus, medial septum and striatum. Similarly, choline acetyltransferase activity and the density of [3H]hemicholinium-3 (high-affinity choline uptake) and [3H]cytisine (nicotinic) binding sites were also unchanged in the brain regions studied between the aged cognitively impaired, unimpaired and young adult rats. In contrast, significant increases in [3H]AF-DX 384 (muscarinic M2) binding density were observed in various cortical areas and in the molecular layer of the dentate gyrus of aged cognitively impaired versus unimpaired rats and in few cortical regions of old as compared to young animals. Therefore, a selective alteration in the regulation of putative M2 receptor sites is apparent, particularly in the aged cognitively impaired rats. Increases in M2 binding sites could lead to a decrease in the capacity to release acetylcholine, as some of the M2 receptors are believed to act as negative autoreceptors. This could influence cognitive functions as selective M2 blockers have recently been reported to facilitate spatial memory in aged impaired rats [Doods et al. (1993) Life Sci. 52, 497-503: Quirion et al. (1995) J. Neurosci. 15, 1455-1462.