Effects of abuse pattern of gestational toluene exposure on metabolism, feeding and body composition.

AIMS: Inhalant abuse during pregnancy lowers birth weight and impedes early development. These studies explored the effects of brief, repeated, prenatal toluene exposures in pregnant female rats on body weight, metabolic rate, body composition, and food intake in their offspring. METHOD: Rats were exposed to 0, 8000, 12,000, or 16,000 ppm of toluene twice daily for 15 min from gestational days 8 to 20. The effects of such exposures on post-weaning litter weights, oxygen consumption, carbon dioxide output, and body fat content were determined in 2 cohorts (n=23, n=24) of offspring. Food intakes and weight changes in response to 3 different diets (regular chow, purified diet, purified high fat diet) were examined in another cohort (n=24) from postnatal days 72 to 116. RESULTS: Litter weights showed a significant linear decrease as a function of toluene dose. Offspring exposed to the 16,000 ppm toluene dose displayed statistically lower energy expenditures than control rats. Male rats exposed to 8000 or 16,000 ppm toluene had significantly greater percentage of body fat as well as total body fat than the other groups. Toluene also significantly suppressed weight gain over the time chow was consumed compared to the 0 ppm control group. Finally there were trends for a main effect of toluene dose on food intake during chow and during high fat diet consumption, with rats in the 12,000 ppm group consuming more than the 0 ppm group on both diets. DISCUSSION: These data suggest that, in addition to other previously documented abnormalities in neurological development and behavior, the physiological regulation of metabolism and body composition in males as well as food intake and weight gain in both sexes may be altered by prenatal exposure to toluene.

Feeding microstructure in diet-induced obesity susceptible versus resistant rats: central effects of urocortin 2.

With one billion people overweight worldwide, the need to identify risk factors and treatments for obesity is urgent. The present study determined whether rats genetically prone to diet-induced obesity (DIO) show preexisting differences in meal microstructure and are sensitive to central anorectic effects of corticotropin-releasing factor type 2 (CRF(2)) receptor stimulation. Male, selectively bred DIO rats and their diet resistant (DR) counterparts (n = 9/genotype) were weaned onto low-fat chow and compared as young adults for spontaneous or intracerebroventricular urocortin 2 administration-induced (0, 0.3, 1, 3 microg) differences in ingestion. DIO rats were hyperphagic selectively at the dark cycle onset, showing shorter latencies to initiate feeding, faster returns to eating following meal completion, and a lower satiety ratio than DR rats. At other times, DIO rats had briefer postmeal intervals, but ate smaller and briefer meals, resulting in normal intake. DIO rats also ate faster than DR rats. Urocortin 2 was less potent in DIO rats, ineffective at the 0.3 microg dose, but produced CRF(2) antagonist-reversible anorexia at higher doses. Though heavier, chow-maintained DIO rats were proportionately as or more lean than DR rats. Thus, DIO rats showed signs of a preexisting, heritable deficit in the maintenance of postmeal satiety and a reduced sensitivity to anorectic CRF(2) agonist stimulation. The meal patterns of DIO rats temporally resemble human 'snacking' behaviour, which predicts adult obesity. Because central CRF(2) stimulation retains full anorectic efficacy at higher doses in the DIO model, manipulating this neuropeptidergic system might yield new therapeutic approaches for diet-induced obesity.

Conditioned place preferences (CPPs) to high-caloric "snack foods" in rat strains genetically prone vs. resistant to diet-induced obesity: resistance to naltrexone blockade.

A previous study in our laboratory using Sprague-Dawley (SD) male rats showed that conditioned place preferences (CPPs) can be learned to two different high-caloric "snack foods"--one high in sugar (Froot Loops cereal: FL) vs. one high in fat (Cheetos: C), and that both preferences were mediated by endogenous opioids. Using the same CPP apparatus and procedures, two genetic sub-strains of SD rats, one selectively bred for diet-induced obesity (DIO) vs. another bred for diet resistance to obesity (DR), were used in this investigation. The experiment determined if (a) CPPs can be created in both strains using the same high-caloric "snack foods" and, (b) if CPPs existed, were they opioid dependent. Four non-deprived groups of eight male rats, half being of each strain, were given 20 min sessions to eat either FL or C in one side of a three-chamber CPP apparatus vs. chow on the opposite side over alternating days of a 20 day period. Each predetermined side had distinctly different environmental cues. Following conditioning, rats were tested during 10 min sessions to see if CPPs existed to the "snack food" trained sides. During conditioning and testing, bodyweights, intakes of foods, and activity were measured. Both FL and C generated strong CPPs that were equivalent in both strains. In contrast to our previous study in the parent strain, doses of 0, 0.50, 1.0, 2.5, and 5.0 mg/kg of the opioid antagonist, naltrexone, had no effect on blocking these CPPs. These results show that (a) DIO and DR rats can learn CPPs (i.e., "exhibit food cravings") as well as their parent strain after periodic access to high-caloric palatable foods, but imply that (b) some physiological system other than the endogenous opioid system mediates such learning.

Fetuin-null mice are protected against obesity and insulin resistance associated with aging.

alpha2-HS glycoprotein (AHSG), also known as fetuin-A, inhibits insulin receptor autophosphorylation and tyrosine kinase activity in vitro and in vivo. Earlier we have shown that fetuin-null (KO) mice demonstrate improved insulin sensitivity and resistance to diet-induced obesity. Since aging is associated with insulin resistance and impaired glucose handling, we tested the hypothesis that fetuin-null (KO) mice are resilient to changes in insulin sensitivity associated with aging. Aged (80-week-old) fetuin-null mice were leaner and demonstrated significantly lower body weights compared to age- and sex-matched wild-type (WT) littermates. Leanness in aged fetuin KO mice was accompanied by a significant increase in dark-onset energy expenditure, without marked alteration of respiratory quotient. In comparison to WT mice, fetuin KO mice demonstrated a lower fasting insulin resistance index, and significantly lower blood glucose and insulin levels, following a 4h fast. Interestingly, despite significantly decreased insulin levels during a glucose tolerance test, aged fetuin-null mice demonstrated a similar glucose excursion as WT mice, indicative of improved insulin sensitivity. Analysis of aldehyde-fuchsin stained pancreas from aged fetuin KO mice indicated no difference in islet beta-cell size or number. An insulin tolerance test confirmed the increased insulin sensitivity of aged fetuin KO mice. Further, compared to WT mice, aged fetuin-null mice demonstrated increased skeletal muscle and liver IR autophosphorylation and TK activity. Taken together, this study suggests that the absence of fetuin may contribute to the improvement of insulin sensitivity associated with aging.

Effect of opioid antagonism on conditioned place preferences to snack foods.

Previous research has shown that food-deprived rats acquire conditioned place preferences (CPPs) to sweet liquids that are largely attenuated by the opioid antagonist naltrexone (NAL). This study determined if ad libitum Chow-fed rats can learn CPPs when given relatively brief exposures to different solid snack foods (SFs) -- one high in sugar (Froot Loops cereal: FL) vs. one high in fat (Cheetos: C). Two groups of 16 male rats were trained during 20-min sessions to eat either FL or C in one side of a three-chambered CPP apparatus vs. Chow in the opposite side on alternating days for 20 days. Rats ate considerably more SFs of both types than Chow during the conditioning sessions (SFs: about 23 kcal versus Chow: about 7 kcal). Ten-minute tests for CPPs in the absence of SFs showed that the time spent on SF-conditioned sides increased significantly compared to pre-conditioning tests. Analyses of variance for re-tested CCPs after 0.1, 1.0, 2.5, and 5.0mg/kg NAL showed dose-dependent suppressions of CPPs to both SFs. These data show that consuming sweet or fatty SFs can become reliably associated with environmental cues in the non-deprived state. The endogenous opioid system, which mediates hedonic aspects of palatable food intake, appears to mediate these learned associations.

Resistance to diet-induced obesity in mu-opioid receptor-deficient mice: evidence for a "thrifty gene".

Using pharmacological tools, a role for opioid receptors in the regulation of food intake has been documented. However, the involvement of specific receptor subtypes remains questionable, and little information is available regarding a role for opioid receptors in energy metabolism. Using adult male mice lacking the mu-opioid receptor (MOR) gene (MOR-/-), we show that the MOR is not essential for the maintenance of normal levels of ad libitum food intake but does modulate the efficiency of energy storage during high-fat diets through the regulation of energy partitioning. When fed a regular diet, MOR-/- mice displayed only subtle alterations in energy homeostasis, suggesting a relative overuse of fat as a fuel source in the fed state. When fed a high-fat diet, MOR-/- mice were resistant to obesity and impaired glucose tolerance, despite having similar energy intake to wild-type mice. This resistance to obesity was associated with a strong induction of the expression of key mitochondrial enzymes involved in fatty acid oxidation within skeletal muscle. This metabolic role of the MOR, which is consistent with the properties of a "thrifty gene," suggests that the MOR pathway is a potential target for pharmacological intervention in the treatment of obesity associated with the intake of fatty diets.

Systemic nicotine alters whole-body fat utilization in female rats.

Cigarette smoking produces weight suppression in humans that often rebounds following smoking cessation. Nicotine (NIC) administration produces similar effects in rats. While changes in food intake are thought to account for some of the body weight changes, few reports have investigated how NIC affects whole-body metabolism. In the present study, measures of respiratory quotient (RQ) and energy expenditure (EE) were used to investigate metabolic changes that may contribute to NIC's effects on body weight. Female rats (n=46) were implanted for 14 days with subcutaneous Alzet minipumps containing NIC (6 mg/kg/day) or its vehicle. One-hour metabolic test sessions occurred four times: 2 and 12 days after pump implant and 2 and 8 days after pump removal. NIC initially suppressed body weight gain, followed by steady recovery that was briefly exaggerated after withdrawing NIC. Daily food intake was acutely suppressed by NIC and acutely potentiated after NIC cessation. RQ, but not EE, was suppressed by NIC 2 days after pump implant indicating increased fat utilization. Conversely, RQ was increased 2 days after pump removal signaling increased fat storage. These findings indicate that acute changes in whole-body metabolism may contribute to the weight modulating effects of NIC.

Nicotine and its withdrawal modify dorsal raphe 8-hydroxy-2-(di-n-propylamino) tetralin feeding.

Nicotine (NIC) and its withdrawal modify dorsal raphe (DR) serotonin (5-HT) neurotransmission in ways that may contribute to the body weight loss vs. gain associated with cigarette smoking vs. cessation, respectively. Modifications in feeding to DR infusions of the 5-HT-1A receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), were used to characterize these potential relationships in the DR-5-HT system during NIC administration vs. withdrawal. Two groups of female rats (total N=45) were implanted for 14 days with subcutaneous Alzet minipumps containing NIC (6 mg/kg/day) or saline. Mid-light cycle (1300-1500 h) 8-OH-DPAT feeding tests occurred three times: (1) 2 days after pump implant, (2) 12 days after pump implant, and (3) 2 days after pump removal. Each feeding test consisted of a 1-h measure of pre-feeding, then a 1-h measure of feeding after DR injection of 8-OH-DPAT (0.6 nmol) or 0.4 microl saline. NIC administration produced acute hypophagia, weight loss, and attenuated 8-OH-DPAT-induced feeding. NIC withdrawal produced acute hyperphagia, weight gain, and a transient increase in 8-OH-DPAT feeding. These findings provide behavioral evidence that systemic NIC modifies the DR 5-HT system in ways that may contribute to NIC's ability to alter feeding and body weight.

Nicotine and its withdrawal alter feeding induced by paraventricular hypothalamic injections of neuropeptide Y in Sprague-Dawley rats.

RATIONALE: Cigarette smoking produces feeding and weight suppression in humans that often rebound following cessation. Nicotine (NIC) administration produces similar effects in rats, but the neural mechanisms responsible are not fully known. Recent evidence shows that hypothalamic levels of neuropeptide Y (NPY) change with NIC administration. Infusions of NPY into the paraventricular nucleus of the hypothalamus (PVN), which normally produce robust feeding, were used to investigate changes in the PVN-NPY system that may contribute to NIC's effects on energy balance. OBJECTIVE: To characterize potential differences in PVN-NPY-induced feeding during NIC treatment versus withdrawal. METHODS: Three groups of female rats ( n=66) bearing unilateral PVN cannulae were implanted for 14 days with subcutaneous Alzet mini-pumps containing NIC (0, 6, or 12 mg/kg per day). Dark-onset (1800-2000 hours) NPY feeding tests occurred five times: pre-implant, 2 days and 12 days post-implant and 2 days and 8 days after implant removal. Feeding tests consisted of 1 h of pre-feeding prior to lights off, then two 1-h measures of feeding after PVN injections of 0.4 microl saline or NPY (78 pmol, 235 pmol). RESULTS: NIC initially suppressed body weight gain, followed by steady recovery that was briefly exaggerated after withdrawing NIC. Daily feeding was acutely suppressed by NIC but acutely potentiated after NIC cessation. PVN-NPY-induced feeding was suppressed by both doses of NIC 2 days after pump implant, elevated 2 days after pump removal, but returned to pre-NIC levels 8 days after pump removal. CONCLUSIONS: These findings provide behavioral support that changes in PVN-NPY neurotransmission may play a functional role in the food intake and weight-modulating effects of NIC.

Estrous cycle and food availability affect feeding induced by amygdala 5-HT receptor blockade.

We have recently reported that bilateral infusions of the 5-HT receptor antagonist metergoline (MET) into the posterior basolateral amygdala (pBLA) elicit feeding in female rats tested at mid-light cycle. The present study was performed to determine whether (1) testing at two different phases of the estrous cycle, and/or (2) the palatability of the food might modify this effect. Subjects were 18 adult females with bilateral pBLA cannulae. Following familiarization with Froot Loops cereal, a within-subjects design tested all animals for 1- and 2-h food intake under 2 Drug (0.3 nmol MET vs. Vehicle), 2 Estrous Cycle (diestrus vs. estrus) and 2 Food (lab chow vs. Froot Loops) conditions. Rats weighed more at diestrus than at proestrus (P<.05) or estrus (P<.005). Multivariate analyses of variance (MANOVAs) revealed a preference for Froot Loops over lab chow (P<.0001). MET increased feeding regardless of food type (P<.0001). Rats ate more Froot Loops (P<.01), but not lab chow, at diestrus vs. estrus. A three-way interaction (P<.05) showed rats ate more during the first hour in estrus than in diestrus to lab chow but not Froot Loops. These data suggest pBLA MET differentially affects feeding over the estrous cycle depending on the palatability of food available.