David A. York

Differential Effects of Baseline Macronutrient Preferences on Macronutrient Selection After Galanin, NPY, and an Overnight Fast

Rats display individual patterns of fat and carbohydrate intakes when allowed to self-select among individual macronutrient diets. We investigated whether these individual preferences in macronutrient selection could be modified by an overnight fast or by two orexigenic peptides, galanin and neuropeptide Y (NPY), which may selectively stimulate fat and carbohydrate intake. Rats were grouped by preference based on the ratio of average baseline fat:carbohydrate intake. In counterbalanced tests conducted on separate days, saline, galanin, or NPY was infused into the paraventricular nucleus of the hypothalamus and 60-min food intake was measured. When the macronutrient intakes were expressed as percent of total caloric intake, galanin administered into the PVN did not increase fat consumption compared to saline injection in either preference group. NPY slightly enhanced the proportion of carbohydrate intake, but only in carbohydrate-preferring rats. When all three feeding stimuli were compared to baseline preferences, the only condition that significantly altered macronutrient selection was an overnight fast, which augmented fat intake. These data demonstrate that baseline preferences for fat or carbohydrate are not significantly modified by galanin or NPY but that an overnight fast increases fat preference.

Activation of hypothalamic serotonin receptors reduced intake of dietary fat and protein but not carbohydrate

Systemic treatment with dexfenfluramine (dF), fluoxetine, or serotonin (5-hydroxytryptamine, 5-HT) recently was shown to suppress fat and occasionally protein but not carbohydrate intake in rats when a macronutrient selection paradigm was employed. These reports contrast with the prevailing literature, which for the past decade has described a role for serotonin neurotransmission in the modification of dietary carbohydrate consumption. To test the hypothesis that the suppression of fat selection and/or consumption by systemic serotonin agonists involves stimulation of central 5-HT receptors, a series of experiments was performed in nondeprived rats. In experiment 1, third cerebroventricular (3V) infusion of the nonselective 5-HT antagonist metergoline prevented the reduction in fat but not carbohydrate feeding caused by systemic dF. Furthermore, 3V metergoline alone increased fat intake. In experiments 2 and 3, 3V infusion of 5-HT(1B/2C) receptor agonists D-norfenfluramine (DNF) or quipazine inhibited fat intake exclusively. Next, the infusion of DNF or 5-HT into the region of the paraventricular nucleus (PVN) reduced both fat and protein intake (experiments 4 and 5). Finally, in experiment 6, when rats were grouped by baseline diet preference, 5-HT infused into the PVN led to a dose-related decrease in fat intake in both carbohydrate- and fat-preferring rats. In contrast, there were no dose effects of 5-HT on carbohydrate or protein intake in either preference group. However, in fat-preferring rats, the highest dose of 5-HT reduced intake of all three macronutrient diets. These results demonstrate a selective effect of exogenous serotonergic drugs in the hypothalamus to reduce fat rather than carbohydrate intake and suggest that higher baseline fat intake enhances responsivity to serotonergic drugs.

Differential expression of insulin receptor tyrosine kinase inhibitor (fetuin) gene in a model of diet-induced obesity

The Differential Display technique has been used to identify differences in mRNA expression in adipose tissue after the introduction of a high fat diet to two strains of rat (OM and S5B/PI) that differ in their susceptibility to develop obesity on this diet. The insulin receptor tyrosine kinase inhibitor protein (fetuin) was shown to be differentially expressed in OM but not S5B/PI rats. This circulating protein may play a role in the development of peripheral insulin resistance associated with high fat diets.

Chronic ICV enterostatin preferentially reduced fat intake and lowered body weight.

The pancreatic peptide enterostatin will acutely reduce fat intake in rats provided a choice of diets. Chronic ICV infusions of enterostatin suppress the intake of high fat diet. However, the effects of chronic ICV enterostatin on diet choice has not previously been studied. To investigate this, enterostatin (0.5 microgram/h) or artificial cerebrospinal fluid (CSF) was infused for 9 days into the lateral ventricle of rats adapted to a two-choice high-fat (HF) and low-fat (LF) diet regime. Enterostatin reduced intake of HF diet with the maximum depression at day 4, but there was no compensatory increase in LF intake. The body weight of enterostatin-infused rats declined. This was associated with a reduction in fat pad and liver weights compared to the CSF-infused control rats. Serum triglycerides and insulin were decreased and corticosterone was elevated in enterostatin-infused rats. The data show that enterostatin will chronically reduce fat intake and body weight and suggest that enterostatin may attenuate the appetite for fat.

Regulation of feeding behavior, gastric emptying, and sympathetic nerve activity to interscapular brown adipose tissue by galanin and enterostatin: the involvement of vagal-central nervous system interactions

Galanin and enterostatin, which are distributed in both the central nervous system and the gastrointestinal tract, regulate the feeding behavior. In the first set of experiments, we investigated the effects of galanin and enterostatin, injected into the third ventricle, on food intake, gastric emptying, and the sympathetic activity of nerves innervating interscapular brown adipose tissue in rats. Galanin dose-dependently increased the intake of a high-fat diet after overnight starvation, but it did not affect low-fat diet intake. In contrast, enterostatin suppressed the intake of the high-fat diet, while intake of the low-fat diet was not affected. Galanin significantly and dose-dependently suppressed gastric emptying rate. However, gastric emptying showed no response to enterostatin. Galanin produced a dose-dependent suppression of sympathetic firing rate. In rats fed a high-fat diet, the injection of enterostatin showed a dose-dependent increase in firing rate. In contrast, animals fed a chow diet showed almost no response. In the second set of experiments, we investigated the role of the hepatic vagus nerve in modulating the peripheral response to enterostatin in rats. Intraperitoneal (i.p.) enterostatin reduced the intake of a high-fat diet. Immunohistochemical identification indicated that the Fos protein was present in the nucleus tractus solitarius, and parabrachial, paraventricular, and supraoptic nuclei after IP enterostatin. These responses to i.p. enterostatin were blocked by hepatic vagotomy.These results suggest that galanin and enterostatin coordinate to regulate feeding behavior, gastric emptying, and sympathetic activity to interscapular brown adipose tissue via central and peripheral sites of action, one of which was the interaction which was found to exist through the vagal system.

Macronutrient diet intake of the lethal yellow agouti (Ay/a) mouse.

To examine the effect of chronic endogenous melanocortin receptor (MC-R) antagonism on macronutrient diet selection, Ay/a mice that ectopically overexpress the MC-R antagonist, agouti, were fed a three-choice macronutrient diet of pure fat, carbohydrate, and protein. Ay/a mice gained more weight and consumed a greater proportion of their daily intake from fat and less from carbohydrate than wild-type littermates did. The increased fat preference was present immediately, and persisted throughout the 7-week long experiment. Protein intake was greater for Ay/a mice; however, the proportion of protein intake to total intake was similar between mouse types. Ovarian fat pads of Ay/a mice comprised a greater percentage of total body weight that that from wild-type littermates. These results suggest that endogenous inhibition of MC-Rs mediate the increased fat intake in growing mice.

The peptide enterostatin may produce early satiety

The time course of feeding, grooming, exploration, and sleeping behaviors has been measured following treatment with enterostatin, the signal pentapeptide from procolipase. The peptide was injected intraperitoneally prior to presenting food, and the frequency of feeding and grooming activity, drinking, and rest or sleeping were observed at 10-s intervals for 60 min. Enterostatin did not delay the onset of feeding but shortened the time spent eating compared to saline injected controls. Conversely, grooming activity appeared earlier following enterostatin, activity was reduced, and resting behavior occurred earlier with this peptide. There were no changes in the drinking behavior. For the first hour following enterostatin, eating represented 20.8% of the time, grooming 9.2%, activity 18.3%, and rest or sleep 47.2%, with drinking making up the other 4.4%. In contrast, saline-injected animals ate for 27.1% of the time, groomed for 12.4%, were active 28.5% of the time, had sleep or rest time equal to 27.9%, and drank for 4.1% of the time. In fasted animals, the onset of grooming, the decrease in activity, and the increase in time sleeping occurred earlier than with saline. These studies support the concept that enterostatin decreases food intake by producing early satiety.

Chronic d-fenfluramine treatment reduces fat intake independent of macronutrient preference

We investigated the effect of chronic dexfenfluramine (DFEN) treatment on macronutrient selection in a three-choice diet paradigm using Sprague-Dawley rats. Baseline macronutrient intakes were measured for several days before the initiation of treatment. In Experiment 1, daily intraperitoneal injections of DFEN (1.5 mg/kg) or saline were administered 60 min before dark onset for 12 consecutive days and 24 h macronutrient intakes were measured. DFEN significantly reduced absolute fat intake (kcal) by 30% and relative fat intake (% of total energy) by 14% in animals that received dexfenfluramine treatment compared to controls over the 12-day period. Absolute carbohydrate intake was increased 24% compared to controls, but this difference was not significant. These changes in food intake resulted in a 10% lower total energy intake. Upon discontinuation of the drug, fat intake of the DFEN-treated rats rebounded to control levels within 24 h. In Experiment 2, rats were assigned to carbohydrate- or fat-preferring groups based on the ratio of their average daily carbohydrate to fat intake (kcal). All animals then received DFEN. During DFEN treatment, fat-preferring rats reduced their daily fat intake from 62 to 53% of total energy. The low baseline fat intake of carbohydrate-preferring rats was reduced even further by DFEN (from 24 to 15% of total energy). These corresponding effects of DFEN on macronutrient selection in both fat- and carbohydrate-preferring rats indicate that chronic DFEN treatment selectively suppressed fat intake independent of the preferred macronutrient diet.

Opioid receptor subtype control of galanin-induced feeding

This study examined the effects of specific antagonists to kappa- and mu-opioid receptors on the feeding induced by injecting galanin into the lateral cerebral ventricle (LCV). Galanin injected into the lateral cerebral ventricle of sated rats stimulated the consumption of high-fat diet when compared to controls injected with saline vehicle. The mu-opioid receptor antagonist, CTOP, completely abolished galanin-induced feeding in sated rats whereas the kappa-opioid receptor antagonist, nor-BNI, had no effect on galanin-induced feeding. Neither CTOP nor nor-BNI alone produced any change in food consumption in sated rats. In fasted rats, on the other hand, nor-BNI significantly decreased consumption of a high-fat diet (> 83%) when compared to animals treated with the saline vehicle, whereas CTOP had no significant effect. These findings suggest that galanin-induced feeding of a high-fat diet is selectively modulated by a pathway involving mu-opioid receptors whereas feeding induced by fasting is dependent on a pathway mediated by kappa-opioid receptors. These data also suggest that galanin does not mediate the feeding response after fasting.

Changes in the Microstructure of Feeding After Administration of Enterostatin into the Paraventricular Nucleus and the Amygdala

The effects of enterostatin (ENT) injected into the paraventricular nucleus (PVN) and the amygdala (AMYG) on the microstructure of feeding was studied by using an automated feeding apparatus. In rats adapted to a 6-h meal feeding regime, ENT reduced the size and duration of the first meal after injection in both the PVN and the AMYG. Similar effects were observed when ENT was given at the beginning of the dark cycle in rats fed ad libitum although the onset of feeding was also delayed in this situation. The number of meals and the size of subsequent meals was unaffected by ENT but the eating rate within the first meal was reduced after ENT injection into the AMYG of meal-fed rats. Enterostatin injected into the AMYG at a dose that suppressed feeding did not produce a conditioned taste aversion. ENT given centrally therefore appears to reduce food intake by delaying the initiation of feeding and/or advancing meal termination suggesting that it affects both appetite and satiation mechanisms.