Nutritional status influences feeding behaviors, food preferences and taste sensations. For example, zinc-deficient rats have been reported to show reduced and cyclic food intake patterns with increased preferences for NaCl. Although some impairments of the central nervous and endocrine systems have been speculated to be involved in these phenomena, the effects of short-term zinc deficiency on the brain have not been well examined to date. In this study, we performed a comprehensive analysis of the gene expression patterns in the rat diencephalon, which is a portion of the brain that includes the hypothalamus and thalamus, after short-term zinc deficiency and also during zinc recovery. The rats showed reduced and cyclic food intake patterns with increased salt preferences after a 10-day dietary zinc deficiency. A comparative analysis of their diencephalons using cDNA microarrays revealed that approximately 1% of the genes expressed in the diencephalons showed significantly altered expression levels. On the other hand, a 6-day zinc supplementation following the deprivation allowed for the recovery to initial food intake behaviors and salt preferences. The expression levels of most of the genes that had been altered by exposure to zinc deficient conditions were also recovered. These results show that feeding behaviors, taste preferences and gene expression patterns in the diencephalon respond quickly to changing zinc levels. This suggests that the gene expression changes observed in the diencephalon and the accompanying functional changes may be related to the development of deviations in feeding behaviors and increased preferences for NaCl in zinc-deficient rats.
Overall design: Four-week-old male Sprague-Dawley rats were purchased from Charles River Japan (Yokohama, Japan). Fifteen rats were individually housed in a stainless steel cage in a room with constant humidity at 22 ± 1 °C under a 12 h light-dark cycle (lights on at 8:00). Zinc-deficient diets (# D19488M, Research Diets, New Brunswick, NJ, USA) were based on the AIN-93G diet and contained 0.6 mg zinc / 1000 g diet (Table 1). For the control zinc-sufficient diets, zinc sulfate supplements were provided with up to 30.0 mg zinc / 1000 g diet. Rats were fed the control diets ad libitum for 1 week for adaptation and then were divided into two groups matched for body weight.
For the zinc-deficient experiment, rats in the ZD group (n =9) were allowed to eat the zinc-deficient diets ad libitum. The rats in the PF group (n = 6) were pair-fed the control diets to match the intake of the ZD rats. Two fluid bottles with deionized water were attached to each cage for the entire experiment with the exception of the 48 h preference test, in which 300 mM NaCl was provided from days 6 to 8. Zinc was limited for 10 days to reduce the effects of differences in salt intake in the preference test between the two groups.
For the zinc recovery experiment, the ZR (n = 9) and ZRPF (n = 6) groups were subjected to the same conditions as above. Briefly, after 1 week of adaptation, ZR rats were fed the zinc-deficient diets ad libitum, and ZRPF rats were pair-fed the control diets for 10 days. After that, the rats in both the ZR and ZRPF groups were fed the fixed amount (13 g) of the control diets for the 6 day zinc recovery period to avoid any differences in food intake between the two groups that would be caused by pair-feeding because the previous report showed rapidly increased food intake following the initiation of the zinc-sufficient diets after zinc deficiency [1]. The diet amounts were restricted to the average consumption of the ZR group on the last day of the zinc-deficient period. The 48 h two-bottle preference tests for 300 mM NaCl were performed from days 6 to 8 during the deficient period and from days 14 to 16 during the recovery period. The analysis removed one rat from the ZRPF group showing more than a 0.5 of preference ratio to the 300 mM NaCl solution and three rats from the ZR group showing less than 0.5 preference ratios as assessed by a preference test that was conducted during days 6 to 8.
At the end of each experiment, the rats were deeply anesthetized with pentobarbital sodium. Blood samples were collected from the carotid arteries and stored at -80 °C. The brains were quickly removed from the bodies after decapitation, and the diencephalons were separated by forceps on ice. The diencephalons were washed in ice-cold phosphate-buffered saline (PBS), treated with RNAlater (Invitrogen, Carlsbad, CA, USA), and stored at -20 °C. The serum zinc concentrations were analyzed with an ICP-AES (SPS 1200 VR, Seiko Instruments, Chiba, Japan). The Animal Care Committee of the University of Tokyo approved all animal experiments.
Four average rats from each group were selected based on body weights, plasma zinc concentrations, and salt preferences.
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