The hypophagic response to heat stress is not mediated by GPR109A or peripheral β-OH butyrate

Abstract

Rising temperatures resulting from climate change will increase the incidence of heat stress, negatively impacting the labor force and food animal production. Heat stress elevates circulating β-OH butyrate, which induces vasodilation through GPR109a. Interestingly, both heat stress and intraperitoneal β-OH butyrate administration induce hypophagia. Thus, we aimed to investigate the role of β-OH butyrate in heat stress hypophagia in mice. We found that niacin, a β-OH butyrate mimetic that cannot be oxidized to generate ATP, also reduces food intake. Interestingly, the depression in food intake as a result of 8-h intraperitoneal niacin or 48-h heat exposure did not result from changes in hypothalamic expression of orexigenic or anorexigenic signals (AgRP, NPY, or POMC). Genetically eliminating GPR109a expression did not prevent the hypophagic response to heat exposure, intraperitoneal β-OH butyrate (5.7 mmol/kg), or niacin (0.8 mmol/kg). Hepatic vagotomy eliminated the hypophagic response to β-OH butyrate and niacin but did not affect the hypophagic response to heat exposure. We subsequently hypothesized that the hypophagic response to heat stress may depend on direct effects of β-OH butyrate at the central nervous system: β-OH butyrate induced hormonal changes (hyperinsulinemia, hypercorticosteronemia, and hyperleptinemia), or gene expression changes. To test these possibilities, we blocked expression of hepatic hydroxyl methyl glutaryl CoA synthase II (HMGCS2) to prevent hepatic β-OH butyrate synthesis. Mice that lack HMGCS2 maintain a hypophagic response to heat stress. Herein, we establish that the hypophagia of heat stress is independent of GPR109a, the hepatic vagus afferent nerve, and hepatic ketone body synthesis.

Keywords: GPR109a; heat stress; hyperketonemia; niacin; β-OH butyrate.

Fig. 1.

Sustained hypophagia in response to repeated intraperitoneal administration of β-OH butyrate and niacin. Intraperitoneal injection of β-OH butyrate (5.7 mmol/kg) and niacin (0.8 mmol/kg) decreased 2, 4, 6, and 8 h food intake in hyperphagic 16-h fasted mice (A; n = 6 or 7). When expressed as a percentage of saline-injected controls, we can see that the relative hypophagia induced by β-OH butyrate and niacin was similar throughout the refeeding duration (B; n = 6 or 7). ^a,b,cBars that do not share a common letter differ significantly (P < 0.05). *Significant decrease in relative food intake for all time points compared with saline (P < 0.05). NS, no significant difference in relative food intake within an injection treatment at any time point (P > 0.1).

Fig. 2.

Phagic, behavioral, and neuroendocrine response to intraperitoneal β-OH butyrate and niacin injections. Intraperitoneal injection of β-OH butyrate (5.7 mmol/kg) and niacin (0.8 mmol/kg) decreased 1 h food intake in 16-h fasted mice (A; n = 8–13) but did not affect either water consumption (B; n = 4) or ambient movement (C; n = 4). Hypothalamic expression of mRNA encoding orexigenic [Agouti-related peptide (AgRP), D; neuropeptide Y (NPY), E] and anorexigenic [proopiomelanocortin (POMC), F] peptides in fed or fasted (16 h) mice injected with saline or niacin (0.8 mmol/kg) every 2 h for the last 8 h of the fast. NS, no significant difference. ^a,bBars that do not share a common letter differ significantly (P < 0.05; n = 6 or 7).

Fig. 3.

Phagic and neuroendocrine response to heat stress. Heat exposure (35^○C) decreased 24 h (A), light cycle (B), and dark cycle food intake (C; n = 6–9). Hypothalamic expression of mRNA encoding orexigenic (AgRP, D; NPY, E) and anorexigenic (POMC, F) peptides after 48-h temperature exposure (25^○C or 35^○C). NS, no significant difference. ^a,bBars that do not share a common letter differ significantly (P < 0.05; n = 5).

Fig. 4.

The role of GPR109a in the hypophagic response to β-OH butyrate, niacin, and heat exposure. β-OH butyrate (5.7 mmol/kg) and niacin (0.8 mmol/kg) depress food intake in GPR109a wildtype (+/+) and null (−/−) sibling mice (A; n = 6). 24 h (B), light cycle (C), and dark cycle (D) food intake in response to heat exposure (35^○C) in GPR109a +/+ and −/− sibling mice. ^a,bBars that do not share a common letter differ significantly (P < 0.05; n = 6–9).

Fig. 5.

The role of the vagus nerve in the hypophagic response to β-OH butyrate, niacin, and heat exposure. A: hepatic vagotomy increases food intake in the light cycle and decreases food intake in the dark cycle, blunting the diurnal variation in food consumption at standard housing temperature (25^○C; n = 8 or 9). B: hepatic vagotomy blocks the hypophagic response to β-OH butyrate (5.7 mmol/kg) and niacin (0.8 mmol/kg; n = 8 or 9). There is no effect of hepatic vagotomy on the 24 h (C), light cycle (D) or dark cycle (E) hypophagic response to heat exposure (35^○C; n = 8 or 9). ^a,bBars that do not share a common letter differ significantly (P < 0.05; n = 8 or 9).

Fig. 6.

Four weeks of biweekly 3-hydroxy-3-methylglutaryl CoA synthase II (HMGCS2) targeted antisense oligonucleotide injection (25 mg/kg) eliminates the hepatic expression of HMGCS2 mRNA (A) and the fast induced rise in serum β-OH butyrate (B), but does not affect the hypophagic response to heat exposure (C; 35^○C; n = 6). ^a,bBars that do not share a common letter differ significantly (P < 0.05).