Brain fos-like immunoreactivity in chronic decerebrate and neurologically intact rats given 2,5-anhydro-D-mannitol

Reduced liver mitochondrial energy metabolism impairs food intake regulation following gastric preloads and fasting

OBJECTIVE

The capacity of the liver to serve as a peripheral sensor in the regulation of food intake has been debated for over half a century. The anatomical position and physiological roles of the liver suggest it is a prime candidate to serve as an interoceptive sensor of peripheral tissue and systemic energy state. Importantly, maintenance of liver ATP levels and within-meal food intake inhibition is impaired in human subjects with obesity and obese pre-clinical models. Previously, we have shown decreased hepatic mitochondrial energy metabolism (i.e., oxidative metabolism & ADP-dependent respiration) in male liver-specific, heterozygous PGC1a mice results in increased short-term diet-induced weight gain with increased within meal food intake. Herein, we tested the hypothesis that decreased liver mitochondrial energy metabolism impairs meal termination following nutrient oral pre-loads.

METHODS

Liver mitochondrial respiratory response to changes in ΔGATP and adenine nucleotide concentration following fasting were examined in male liver-specific, heterozygous PGC1a mice. Further, food intake and feeding behavior during basal conditions, following nutrient oral pre-loads, and following fasting were investigated.

RESULTS

We observed male liver-specific, heterozygous PGC1a mice have reduced mitochondrial response to changes in ΔGATP and tissue ATP following fasting. These impairments in liver energy state are associated with larger and longer meals during chow feeding, impaired dose-dependent food intake inhibition in response to mixed and individual nutrient oral pre-loads, and greater acute fasting-induced food intake.

CONCLUSIONS

These data support previous work proposing liver-mediated food intake regulation through modulation of peripheral satiation signals.

Reduced Liver Mitochondrial Energy Metabolism Impairs Food Intake Regulation Following Gastric Preloads and Fasting

Objective

The capacity of the liver to serve as a peripheral sensor in the regulation of food intake has been debated for over half a century. The anatomical position and physiological roles of the liver suggest it is a prime candidate to serve as an interoceptive sensor of peripheral tissue and systemic energy state. Importantly, maintenance of liver ATP levels and within-meal food intake inhibition is impaired in human subjects with obesity and obese pre-clinical models. Previously, we have shown decreased hepatic mitochondrial energy metabolism (i.e., oxidative metabolism & ADP-dependent respiration) in male liver-specific, heterozygous PGC1a mice results in increased short-term diet-induced weight gain with increased within meal food intake. Herein, we tested the hypothesis that decreased liver mitochondrial energy metabolism impairs meal termination following nutrient oral pre-loads.

Methods

Liver mitochondrial respiratory response to changes in ΔGATP and adenine nucleotide concentration following fasting were examined in male liver-specific, heterozygous PGC1a mice. Further, food intake and feeding behavior during basal conditions, following nutrient oral pre-loads, and following fasting were investigated.

Results

We observed male liver-specific, heterozygous PGC1a mice have reduced mitochondrial response to changes in ΔGATP and tissue ATP following fasting. These impairments in liver energy state are associated with larger and longer meals during chow feeding, impaired dose-dependent food intake inhibition in response to mixed and individual nutrient oral pre-loads, and greater acute fasting-induced food intake.

Conclusion

These data support previous work proposing liver-mediated food intake regulation through modulation of peripheral satiation signals.

Neural mechanisms underlying the role of fructose in overfeeding

Fructose consumption has been linked with metabolic syndrome and obesity. Fructose-based sweeteners like high fructose corn syrup taste sweeter, improve food palatability, and are increasingly prevalent in our diet. The increase in fructose consumption precedes the rise in obesity and is a contributing driver to the obesity epidemic worldwide. The role of dietary fructose in obesity can be multifactorial by promoting visceral adiposity, hypertension, and insulin resistance. Interestingly, one emergent finding from human and animal studies is that dietary fructose promotes overfeeding. As the brain is a critical regulator of food intake, we reviewed the evidence that fructose can act in the brain and elucidated the major brain systems underlying fructose-induced overfeeding. We found that fructose acts on multiple interdependent brain systems to increase orexigenic drive and the incentive salience of food while decreasing the latency between food bouts and reducing cognitive control to disinhibit feeding. We concluded that the collective actions of fructose may promote feeding behavior by producing a hunger-like state in the brain.

Leptin-induced increase in body fat content of rats

We previously reported that peripheral leptin infusions in chronically decrebrate rats, in which the forebrain is neurally isolated from the hindbrain, increased body fat and decreased energy expenditure. Any central leptin response in decerebrate rats would depend upon the hindbrain. Here, we tested whether selective activation of hindbrain leptin receptors increased body fat. Fourth ventricle infusion of 0.6 μg leptin/day for 12 days increased body fat by 13% with no increase in food intake. Third ventricle leptin infusions decreased food intake, body fat, and lean tissue with a maximal response at 0.3 μg leptin/day. To test whether hindbrain receptors opposed activity of hypothalamic receptors, rats received peripheral infusions of 40 μg leptin/day and increasing 4th ventricle doses of the leptin receptor antagonist mutein protein. Mutein (3.0 μg/day) reduced body fat in PBS-infused rats to the same level as leptin-infused rats and reduced lean tissue in all rats. Leptin, but not mutein, inhibited food intake. By contrast, 3.0 μg/day mutein in the 3rd ventricle increased food intake and body fat in both PBS- and leptin-infused rats. In basal conditions, hindbrain leptin receptors may antagonize activity of forebrain receptors to protect lean and fat tissue, but there is no evidence for an anabolic role for hindbrain receptors when leptin is elevated. In a dietary study, rats increased energy intake when offered lard and 30% sucrose solution in addition to chow. Peripheral leptin infusion exaggerated the gain in body fat without altering energy intake confirming the potential for leptin to increase adiposity.

Central structures necessary and sufficient for ingestive and glycemic responses to Urocortin I administration

CNS delivery of Urocortin I (UcnI), a member of the corticotropin-releasing factor family, suppresses feeding behavior and increases plasma glucose. The sites of action necessary and sufficient for these responses remain unclear. The contribution of the caudal brainstem was explored using chronically maintained decerebrate (CD) and neurologically intact control rats given fourth-ventricle injections of UcnI. Ingestive and glycemic responses were evaluated, and Fos immunoreactivity was measured in the paraventricular nucleus of the hypothalamus (PVN), the parabrachical nucleus (PBN), the rostral ventrolateral medulla (RVLM), and the nucleus of the solitary tract (NTS). CD rats, like the neurologically intact controls, decreased intraoral food intake and had elevated plasma glucose in response to Unc1 injections, indicating that forebrain structures are not required for these behavioral and physiological actions of UcnI. Fos immunohistochemistry, however, revealed notable differences in the pattern of UcnI-induced activation between intact and CD rats. UcnI-related activation was observed in each of the four aforementioned brain areas of neurologically intact rats but only in the NTS of CD rats. The intact behavioral and physiological responses to UcnI in the absence of neural activation in the PVN, PBN, and RVLM help limit the list of structures necessary for the stimulation and mediation of these responses to UcnI and suggest that the NTS may serve as a primary site of UcnI action.

The neuroanatomical axis for control of energy balance

The hypothalamic feeding-center model, articulated in the 1950s, held that the hypothalamus contains the interoceptors sensitive to blood-borne correlates of available or stored fuels as well as the integrative substrates that process metabolic and visceral afferent signals and issue commands to brainstem mechanisms for the production of ingestive behavior. A number of findings reviewed here, however, indicate that sensory and integrative functions are distributed across a central control axis that includes critical substrates in the basal forebrain as well as in the caudal brainstem. First, the interoceptors relevant to energy balance are distributed more widely than had been previously thought, with a prominent brainstem complement of leptin and insulin receptors, glucose-sensing mechanisms, and neuropeptide mediators. The physiological relevance of this multiple representation is suggested by the demonstration that similar behavioral effects can be obtained independently by stimulation of respective forebrain and brainstem subpopulations of the same receptor types (e.g., leptin, CRH, and melanocortin). The classical hypothalamic model is also challenged by the integrative achievements of the chronically maintained, supracollicular decerebrate rat. Decerebrate and neurologically intact rats show similar discriminative responses to taste stimuli and are similarly sensitive to intake-inhibitory feedback from the gut. Thus, the caudal brainstem, in neural isolation from forebrain influence, is sufficient to mediate ingestive responses to a range of visceral afferent signals. The decerebrate rat, however, does not show a hyperphagic response to food deprivation, suggesting that interactions between forebrain and brainstem are necessary for the behavioral response to systemic/ metabolic correlates of deprivation in the neurologically intact rat. At the same time, however, there is evidence suggesting that hypothalamic-neuroendocrine responses to fasting depend on pathways ascending from brainstem. Results reviewed are consistent with a distributionist (as opposed to hierarchical) model for the control of energy balance that emphasizes: (i) control mechanisms endemic to hypothalamus and brainstem that drive their unique effector systems on the basis of local interoceptive, and in the brainstem case, visceral, afferent inputs and (ii) a set of uni- and bidirectional interactions that coordinate adaptive neuroendocrine, autonomic, and behavioral responses to changes in metabolic status.

Role of vagal afferent innervation in feeding and brain Fos expression produced by metabolic inhibitors

Hepatic vagal afferent fibers have been implicated in the feeding responses initiated by administration of 2,5-anhydro-D-mannitol (2,5-AM; an inhibitor of hepatic metabolism) and methyl palmoxirate (MP; an inhibitor of fat metabolism). 2,5-AM and MP also increase brain Fos expression, an indicator of neural activity, which suggests that Fos expression can reveal the central neural pathways involved in the stimulation of feeding by these agents. To more closely test the hypothesis that brain Fos expression is related to the effects of 2,5-AM and MP on feeding, the vagus was lesioned by application of capsaicin, which destroys afferent fibers, directly to the cervical vagi. Perivagal capsaicin treatment blocked 2,5-AM-induced eating and attenuated MP-induced eating. Although perivagal capsaicin treatment attenuated MP-induced Fos expression, capsaicin treatment did not affect brain Fos expression produced by 2,5-AM. It is concluded that (1) brain Fos expression is not always related to the effects of 2,5-AM on feeding, (2) capsaicin-sensitive hepatic vagal afferent fibers carry the signal that stimulates feeding following 2,5-AM treatment, and (3) MP-induced feeding and brain Fos expression is mediated in part by capsaicin-sensitive fibers.

Food deprivation does not potentiate glucose taste reactivity responses of chronic decerebrate rats

The chronic supracollicular decerebrate (CD) rat fails to increase meal size in response to systemic/metabolic aspects of food deprivation. Here we asked whether or not deprivation increases immediate oral motor responding to taste stimuli (taste reactivity) in CD rats, as it does in neurologically intact controls. The responses of CD rats were evaluated as functions of glucose concentration and deprivation state, with taste reactivity responses recorded myographically during 15-s intraoral infusions and during 45-s post-infusion periods. Five glucose concentrations (0, 3.2, 6. 25, 12.5, 25%) were each presented three times during each test session. The rats were tested when not-deprived (i.e. receiving their full complement of gavage feedings), deprived (23.5 h) of food and water, and deprived of food but not water. The number of oral motor responses emitted increased monotonically with stimulus concentration; during oral infusions the increase was greatest over the lower half of the concentration range, whereas responding increased linearly with concentration in the post-infusion period. This CD response profile resembled that obtained previously with neurologically intact rats tested according to the same protocols. In contrast to results obtained in intact rats, deprivation did not influence the CD's response to glucose at any concentration or for any observation period. Although the caudal brainstem may receive and process information associated with deprivation state, neural interactions between forebrain and brainstem structures appear necessary for the behavioral expression of deprivation effects on meal size or, as we can now conclude, on immediate oral motor responses to taste stimuli.

Neural substrate for an integrated metabolic control of feeding behavior

Evidence indicates that feeding behavior in rats is controlled by a mechanism that integrates information about different aspects of fuel metabolism. We investigated the neural substrate for this integrated control by measuring the effect of metabolic inhibitors given alone and in combination on food intake and neuronal activity as reflected by the expression of c-Fos protein. Combined administration of methyl palmoxirate (5 mg/kg po), an inhibitor of fatty acid oxidation, and 2,5-anhydro-D-mannitol (150 mg/kg ip), which decreases liver ATP content, increased feeding in rats more than expected on the basis of eating responses after treatment with either inhibitor given alone. Combined treatment also produced a synergistic increase in Fos-like immunoreactivity in several brain areas, including the nucleus of the solitary tract, area postrema, and parvocellular portion of the hypothalamic paraventricular nucleus. These findings provide strong evidence for the involvement of selected brain regions in the metabolic control of food intake and suggest that metabolic information used to control feeding behavior is integrated in the periphery or at the level of the brain stem.