Cholesterol-enriched diet affects spatial learning and synaptic function in hippocampal synapses

The aim of the present study was to determine the effect of a cholesterol-rich diet on learning performance and monitor possible related changes in synaptic function. To this purpose, we compared controls with rats fed with a cholesterol-enriched diet (CD). By using a Morris water-maze paradigm, we found that CD rats learned a water-maze task more quickly than rats fed with a regular diet (RD). A longer period of this diet tended to alter the retention of memory without affecting the improvement in the acquisition of the task. Because of the importance of the hippocampus in spatial learning, we hypothesized that these behavioral effects of cholesterol would involve synaptic changes at the hippocampal level. We used whole-cell patch-clamp recording in the CA1 area of a hippocampal rat slice preparation to test the influence of the CD on pre- and postsynaptic function. CD rats displayed an increase in paired-pulse ratio in both glutamatergic synapses (+48 +/- 9%) and GABAergic synapses (+41 +/- 8%), suggesting that the CD induces long-lasting changes in presynaptic function. Furthermore, by recording NMDA-receptor-mediated currents (I(NMDA)) and AMPA-receptor-mediated currents (I(AMPA)) in the same set of cells we found that CD rats display a lower I(NMDA)/I(AMPA) ratio (I(NMDA)/I(AMPA) = 0.75 +/- 0.32 in RD versus 0.10 +/- 0.03 in CD), demonstrating that cholesterol regulates also postsynaptic function. We conclude that a cholesterol-rich diet affects learning speed and performance, and that these behavioral changes occur together with robust, long-lasting, synaptic changes at both the pre- and postsynaptic level.