Relationship between abnormal cholesterol synthesis and retarded learning in rats.

We examined the relationship between brain sterol composition and associative learning (classical conditioning of the eyeblink response) in newly weaned rats fed BM 15.766 (BM) for 4 months. This compound inhibits 7-dehydrocholesterol-delta7-reductase, which catalyzes the conversion of 7-dehydrocholesterol to cholesterol, the last step in the synthetic pathway. As countertreatment, half of the BM-treated rats were fed 2% cholesterol during the last 2 months. With BM, cholesterol concentrations declined 91% in plasma, but with cholesterol feeding, the levels increased 50% compared with baseline values. 7-Dehydrocholesterol, which was not detected at baseline, increased to 55% of plasma sterols with BM but decreased to 5% of total plasma sterols when cholesterol was added. With BM, brain cholesterol levels decreased 60% and did not increase after cholesterol was added. However, 7-dehydrocholesterol, which comprised 39% of brain sterols with BM, decreased to 31% (P < .05) when cholesterol was fed. Hydroxymethyl glutaryl coenzyme A (HMG-CoA) reductase activity in the liver increased 2.2-fold with BM and declined 95% after adding cholesterol, but did not change in the brain. BM treatment for 4 months prevented learning of the conditioned eyeblink response as compared with controls. In contrast, BM-treated rats supplemented with cholesterol acquired the conditioned eyeblink response. Chronic inhibition of 7-dehydrocholesterol-delta7-reductase reduced cholesterol and increased 7-dehydrocholesterol levels in plasma and brain, and was associated with impaired learning. Cholesterol feeding corrected plasma and hepatic sterol levels and reduced brain 7-dehydrocholesterol concentrations to reestablish normal learning.