EFFECTS OF DIHYDROERGOTOXINE ON CENTRAL CHOLINERGIC NEURONAL SYSTEMS AND DISCRIMINATION-LEARNING TEST IN AGED RATS

We evaluated changes in the cholinergic neuronal system and learning ability with aging. Choline acetyltransferase (ChAT) activity, a presynaptic index of the cholinergic system, was decreased in the cerebral cortex. hippocampus. striatum, and hypothalamus in the brain of aged rats compared with young adults. Muscarinic cholinergic binding sites (receptors. MCR), a postsynaptic index of the cholinergic system, were markedly decreased in all areas of the brain. However, intraperitoneal injection of 1 mg/kg of dihydroergotoxine (DHET) for 14 days normalized both ChAT and MCR in the cerebral cortex and hippocampus. In the striatum, ChAT was normalized, but MCR did not recover. Aged rats showed marked learning impairment in a 30-day operant type brightness discrimination learning test. Daily DHET administration restored the discrimination ability in the aged rats to nearly the young adult level. DHET had no effects on central cholinergic indices or learning test results in young adult rats. These findings suggest that learning is impaired in aged rats due to impairment in the central cholinergic neuronal system, and that DHET normalizes the decreased function in this system, restoring the learning ability.