Chronic Standard Scheduled-Diet Improves Memory Performance and Is Associated with Positive Correlation between Plasma Ghrelin and Hippocampal Dopamine Level in Rats

Abstract

Dietary content/habit is known to play a prominent role in memory function, in part via a direct effect of ghrelin on the hippocampus. Given the role of ghrelin in regulation of central dopamine release, ghrelin impact on learning and memory may also be indirect via dopamine. Here, we examined effect of scheduled-diets and different amounts of calorie intakes on hippocampal dopamine content and memory performance as well as correlation between plasma ghrelin and hippocampal dopamine. Forty male Wistar rats (180-200 g) were divided into four groups (n = 10): freely fed (control) and three scheduled-fed groups with different caloric intakes; high fat, standard and restricted diets, and were maintained on each feeding program for 16 days. At day 16, memory performance (using passive avoidance test) and circulating ghrelin were evaluated. The hippocampal homogenates were analyzed using HPLC with electrochemical detection to quantitate dopamine levels. The hippocampal dopamine was increased in scheduled-feeding rats compared to controls, with the highest level in the scheduled-restricted group (p < 0.01). There was a significant positive relationship between circulating ghrelin and hippocampal dopamine in anticipation of food in all scheduled-feeding rats compared to controls (p < 0.05 and p < 0.01). Memory improvement in standard scheduled-diet was significantly greater than non-scheduled-diet (control)> scheduled-restricted diet> scheduled-high fat diet (p < 0.001). These data indicate a differential effect of increased plasma ghrelin and hippocampal dopamine on memory performance in scheduled-diet groups, with enhanced memory in standard scheduled-diet and impaired memory in scheduled-high fat and restricted diets, suggesting nutritional balance and incentive signals of diet related to dopamine may be critical determinants of memory performance in scheduled-diets.