Increased feeding and food hoarding following food deprivation are associated with activation of dopamine and orexin neurons in male Brandt's voles

Abstract

Small mammals usually face energetic challenges, such as food shortage, in the field. They have thus evolved species-specific adaptive strategies for survival and reproductive success. In the present study, we examined male Brandt's voles (Lasiopodomys brandtii) for their physiological, behavioral, and neuronal responses to food deprivation (FD) and subsequent re-feeding. Although 48 hr FD induced a decrease in body weight and the resting metabolic rate (RMR), such decreases did not reach statistical significance when compared to the control males that did not experience FD. During the first 2 hr of re-feeding following 48 hr FD, voles showed higher levels of feeding than controls. However, when permitted to hoard food, FD voles showed an increase in food hoarding, rather than feeding, compared to the controls. Further, both feeding and food hoarding induced an increase in neuronal activation, measured by Fos-ir, in a large number of brain areas examined. Interestingly, feeding and food hoarding also induced an increase in the percentage of tyrosine hydroxylase immunoreactive (TH-ir) cells that co-expressed Fos-ir in the ventral tegmental area (VTA), whereas both FD and feeding induced an increase in the percentage of orexin-ir cells that co-expressed Fos-ir in the lateral hypothalamus (LH). Food hoarding also increased orexin-ir/Fos-ir labeling in the LH. Together, our data indicate that food-deprived male Brandt's voles display enhanced feeding or food hoarding dependent upon an environmental setting. In addition, changes in central dopamine and orexin activities in selected brain areas are associated with feeding and hoarding behaviors following FD and subsequent re-feeding.

Figure 1. Experimental paradigm and food hoarding apparatus.

Experimental paradigm (panel A) illustrating a time line by which food intake, resting metabolic rate (RMR), and food hoarding during re-feeding following 48 hr food deprivation (FD) were measured. The food hoarding apparatus (panel B) consisted of a home cage and a food cage connected by a plastic tube.

Figure 2. Body weight, RMR, food intake and food hoarding.

Panel A showed the changes in body weight over the course of the baseline, 48 hr food deprivation (FD), and subsequent re-feeding (refeeding) period in male Brandt's voles. Panel B: No differences were found in RMR at basal level (Base), during FD, and 72 hr after re-feeding (RF72h) between intact male voles (Control) and voles that went through 48 FD followed by re-feeding (FD-refeeding). Panel C: The FD-refeeding males increased food intake by 33% within 2 hr after they were re-fed ad libitum, compared to the controls. Panel D: The FD-rehoarding males increased the amount of food hoarding by 140% within 2 hr after they were re-fed ad libitum, compared to the controls. Panel E: There was no difference in the amount of food intake during food hoarding between control voles and voles that went through 48 hr FD followed by 2 hr food hoarding (RH2h). Data are presented as mean ± SEM. * P<0.05.

Figure 3. Photomicrographs of Fos-ir cells.

Fos-ir cells were shown in the nucleus accumbens (NAcc; panel A–C), lateral hypothalamus (LH; panel D–F), and ventral tegmental area (VTA; panel G–I) in the control (A, D, G), FD (B, E, H), and re-fed voles (RF2h: C, F, I). f, fornix; OT, optic tract; MM, medial mammillary nucleus. Scale bar = 100 µm in panel A (applies to A–I).

Figure 4. Double-labeled cells for TH-ir and Fos-ir staining.

Photomicrographs of double-labeled cells for TH-ir (brown cytoplasmic staining) and Fos-ir (dark nuclear staining) were shown in the paraventricular nucleus (PVN; panel A), anterior hypothalamus (AH; panel B), arcuate nucleus (ARC; panel C), and ventral tegmental area (VTA; panel D) in the brain of male Brandt's voles. 3 V, third ventricle; OT, optic tract; MM, medial mammillary nucleus. Scale bar = 100 µm in panel A (applies to A–D); 10 µm in inset to panel A (applies to insets to A–D). Panel E: No group differences were found in the number of TH-ir cells in any brain areas examined in male voles that were intact controls, food-deprived for 48 hr (FD), or re-fed for 2 hr following FD (RF2h). Panel F: RF2h males had a higher percentage of TH-ir cells that co-labeled for Fos-ir in the PVN and VTA than did the control and FD males. Panel G: Under the food hoarding condition, control males and males that experienced 2 hr food hoarding following FD (RH2h) showed no differences in the number of TH-ir cells in any of the brain regions examined. Panel H: RH2h males, however, had a higher percentage of TH-ir cells co-labeled for Fos-ir in the PVN, MPOA (medial preoptic area), ARC, and VTA than control males. No difference was found in the AH. Data are presented as mean ± SEM. * P<0.05, ** P<0.01, and *** P<0.001.

Figure 5. Double-labeled cells for orexin-ir and Fos-ir staining.

Schematic drawing (panel A) and photomicrograph (panel B) illustrating the lateral hypothalamus (LH) and cells double-labeled for orexin-ir (brown cytoplasmic staining) and Fos-ir staining in LH in the vole brain. Scale bar = 100 µm in panel B; 10 µm in inset to panel B. Panel C: The males that were re-fed for 2 hr following FD (RF2h) had more orexin-ir cells in the LH than control and food-deprived for 48 hr (FD). Panel D: Both FD and RF2h males had a higher percentage of orexin-ir cells that co-expressed Fos-ir in the LH than the control males. Panel E: Under the food hoarding condition, control males and males that experienced 2 hr food hoarding following FD (RH2h) showed no differences in the number of orexin-ir cells in the LH. Panel F: RH2h males, however, had a higher percentage of orexin-ir cells co-labeled for Fos-ir in the LH than control males. Data are presented as mean ± SEM. * P<0.05, ** P<0.01, and *** P<0.001.