Chris Barton

Microinjection of opioid antagonists into the substantia nigra reduces stress-induced eating in rats

Stress produced by pinching the tail has been shown to cause satiated animals to eat and to display oral stereotypies. Endogenous opioids and central dopamine systems have been implicated in the mediation of these effects. In order to test the possibility that the substantia nigra (SN) might be involved, the amount of food intake and gnawing produced by mild tail pinch were assessed following bilateral microinjections of opioid antagonists into the SN. Evaluations of nociceptive thresholds were also conducted using tail flick and hot plate tests. Eating induced by tail pinch was reduced by microinjections of the non-selective opioid antagonist naloxone (3, 10, 20 and 30 nmol) and by the mu-selective antagonist Cys2, Tyr3, Orn5, Pen7 Amide (CTOP) (1, 3 and 10 nmol). These effects on eating occurred in the absence of effects on gnawing. kappa- and delta-antagonists (10 nmol) had no effect on eating or gnawing. Naloxone did not alter either tail flick or hot-plate response latencies. The highest dose of CTOP increased response latency on the hot-plate test only. The results are interpreted as suggesting that the SN may be an important central site of action for opioid antagonists in reducing stress-induced eating. The possibility that the SN may be a central site mediating the effects of dopamine on this phenomenon is also discussed.

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.

Differential Response to Kappa-Opioidergic Agents in Dietary Fat Selection Between Osborne-Mendel and S5B/Pl Rats

We have investigated the central effect of a kappa-opioid agonist and an antagonist on the macronutrient preference in two strains of rat, the Osborne Mendel (OM) and S5B/P1 rats, that have different susceptibility to obesity and differential preference for dietary fat intake. OM rats prefer diets high in fat and are sensitive to diet-induced obesity, whereas S5B/P1 prefer a low fat diet and are resistant to high-fat diet-induced obesity. Rats adapted to a two-choice high fat (HF)/low fat (LF) diet were food deprived (20 h) and then infused into the third cerebroventricle with 10 micrograms nor-binaltorphimine (nor-BNI), a selective kappa-antagonist. Nor-BNI preferentially suppressed HF intake, but not LF intake in OM rats, whereas it affected neither diet in S5B rats. Infusion of U50488, a selective kappa-agonist (33 nmol), into the third cerebroventricle in sated rats, potently stimulated the intake of HF only in the OM rats, whereas it induced a significant but moderate stimulation of intake of both HF and LF diets in the S5B/P1 rats. Total energy intake following U50488 was not significantly different between the two strains. These findings suggest that the enhanced sensitivity of the OM rats to kappa-opioid stimulation for dietary fat may contribute to their preference for dietary fat and possibly their increased susceptibility for obesity.

Bombesin-induced hypothermia in rats tested at normal ambient temperatures: Contribution of the sympathetic nervous system

Rats infused centrally with bombesin become hypothermic at normal ambient temperatures when acutely deprived of food, but not while allowed unrestrained access to food. Ad lib-fed rats, tested at normal ambient temperatures, become hypothermic after receiving intracerebroventricular (ICV) bombesin when they have ventromedial hypothalamic lesions or when administered insulin or 2-deoxy-D-glucose peripherally. All of these conditions have been linked to reductions of sympathetic nervous system activity to brown adipose tissue (BAT), a major thermogenic mechanism of many homeotherms. A between group design was used to examine the effects of ICV bombesin infusions on the response to peripheral injections of a) the sympathetic ganglionic blocker chlorisondamine (2.5 mg/kg, IP) in ad lib-fed rats, b) the nonspecific beta-agonist isoproterenol (30 mg/kg, IP) in food-deprived rats, and c) the combination of isoproterenol and chlorisondamine in ad lib-fed rats. Ad lib-fed rats receiving ICV bombesin (100 ng/5 microliters), in combination with peripheral chlorisondamine injection, became hypothermic 60 min postbombesin administration (-2.84 +/- 0.33 degrees C), while ad lib-fed rats receiving ICV bombesin infusion and peripheral injections of saline did not (-0.08 +/- 0.37 degrees C). Isoproterenol blocked hyperthermia in ad lib-fed rats injected with chlorisondamine and ICV bombesin. Food-derived rats receiving ICV bombesin infusion and peripheral saline injection exhibited hypothermia 60 min postbombesin administration (-2.51 +/- 0.29 degrees C). Peripheral injections of isoproterenol prevented bombesin-induced hypothermia in food-deprived rats. These data suggest that bombesin induces hypothermia at normal ambient temperatures when the sympathetic nervous system drive to BAT cannot be (or is not) activated.

Microinfusion of bombesin into the hypothalamic paraventricular nucleus produces hypothermia in the insulin-pretreated rat

Bombesin-like peptides are widely distributed in the mammalian central nervous system and participate in the regulation of a variety of autonomic functions. Central injection of bombesin produces hypothermia at normal ambient temperatures, but only if the rat has been food-deprived or made hypoglycemic with insulin. Two experiments were conducted to reevaluate the impact of bombesin microinfusion into the hypothalamic paraventricular nucleus (PVN) on core body temperature and feeding behavior. In Experiment 1, bombesin (0.05 and 0.1 microgram/1.0 microliter) produced hypothermia, but not hypophagia, in rats (n = 5) pretreated with insulin (10 U/kg; IM). Since a similar response was observed in rats with injection sites adjacent to the PVN, a smaller injection volume was evaluated in Experiment 2. Hypothermia, but not hypophagia, was observed in rats (n = 5) pretreated with insulin following bombesin (0.025 and 0.05 micrograms/0.5 microliter). Bombesin did not produce hypothermia in rats with injection sites outside of the PVN. These findings suggest that the PVN is a sensitive site for bombesin-induced hypothermia.

Bombesin-induced hypothermia in food-deprived rats

Three separate experiments were conducted to characterize the effects of bombesin on core body temperature in food-deprived rats and further evaluate the role of food deprivation in this response. Previous research has demonstrated that naloxone reverses bombesin-induced hypothermia in cold-exposed rats. The present study was unable to demonstrate a similar reversal under conditions of food deprivation following naloxone. In a second experiment, cold exposure (11 degrees C) was shown to potentiate bombesin-induced hypothermia in food-deprived rats. Taken together, these data support the notion that bombesin-induced hypothermia in food-deprived and cold-exposed rats represent the activation or disruption of different thermoregulatory mechanism(s). The final experiment evaluated the effects of bombesin microinfusion into the preoptic area of the anterior hypothalamus on core body temperature. Animals were tested under conditions of food satiation, food deprivation, and insulin pretreatment. Bombesin produced hypothermia in food-deprived and insulin-treated rats. No hypothermia was observed in food-satiated rats. Our finding supports the notion that factors associated with the fasting state are important for the production of hypothermia by bombesin in food-deprived rats.

Bombesin produces hypothermia in insulin treated rats

Previous studies have shown that central injection of bombesin produces hypothermia in food-deprived, but not food satiated rats at normal ambient temperatures. The present study evaluated the effects of bombesin on core body temperature (Tb) and feeding behavior in rats pretreated with insulin. Administration of bombesin (0.25, 0.5, and 1.0 microgram) into the lateral cerebral ventricle produced hypothermia in rats injected with insulin (10 U/kg; i.m.). No significant change in core temperature was observed in control rats following bombesin. Insulin treatments significantly stimulated feeding behavior and the highest dose of bombesin significantly reduced feeding behavior. The results demonstrate bombesin-induced hypothermia under metabolic conditions similar to acute starvation. These findings are consistent with the hypothesis that bombesin-induced hypothermia in food-deprived rats is directly related to the fasting state.

REFEEDING ATTENUATES BOMBESIN-INDUCED HYPOTHERMIA IN THE RAT

The tetradecapeptide bombesin is a potent agent in producing hypothermia when injected centrally. Bombesin-induced hypothermia at normal ambient temperature occurs under conditions of food deprivation or insulin-induced hypoglycemia. This experiment examined the effect of refeeding on the duration of bombesin-induced hypothermia. Rats (n = 7) received microinfusions of bombesin (0.1 microgram/1.0 microliter) into the preoptic area under separate conditions of food deprivation (18 h) and insulin pretreatment (10 U/kg, IM). Core body temperature was evaluated over a period of 4 h with or without food available during testing. Hypothermia was observed under all conditions during the first 2 h. Food-deprived and insulin-pretreated rats not permitted access to food remained hypothermic until at least 4 h following bombesin. These results are discussed in terms of the possible role of glucose availability in the production and duration of bombesin-induced hypothermia.

Bombesin-induced hypothermia in VMH-lesioned rats.

Microinfusion of bombesin into the lateral ventricles (LV) of rats pretreated with insulin or acutely deprived of food has been demonstrated to reduce core body temperature. Lesions of the ventromedial hypothalamus (VMH) have been shown to produce hyperphagia, hyperinsulinemia, and to alter serum metabolic fuels. The present study examines VMH lesions as a permissive event in bombesin-induced hypothermia in rats tested at normal ambient temperature. A between-group design was used to evaluate the effect of microinjections of bombesin (1, 10, 100 ng) into the LV of rats with bilateral VMH lesions or sham lesions. Core body temperature was recorded over a 240-min period. In animals with lesions of the VMH, hypothermia was demonstrated by 30 min after injection of the 10 ng and 100 ng doses; the hypothermia persisted for 120 min. The 1 ng dose had no effect on body temperature in VMH-lesioned animals. Animals that received sham lesions of the VMH did not demonstrate a reduction in core body temperature at the maximum effective dose (100 ng) of bombesin. These results suggest that some event(s) associated with bilateral VMH lesions acts as a permissive factor in the production of bombesin-induced hypothermia at normal ambient temperature.

Bombesin-induced hypothermia and hypophagia are associated with plasma metabolic fuel alterations in the rat

Microinfusion of bombesin into the preoptic area (POA) has previously been shown to reduce core body temperature and feeding in rats that are food-deprived or made hypoglycemic with insulin. The present study determined the metabolic fuel state of rats under these experimental conditions. In addition, changes in plasma metabolic fuels following the microinfusion of bombesin (50 ng/0.25 microliters) into the POA were evaluated. Rats (n = 8) were tested under conditions of food satiation, food deprivation (20 h), and insulin pretreatment (10 U/kg). Prior to peptide infusion, food-deprived rats exhibited the expected elevation in free fatty acids coupled with a small decline in plasma glucose. Insulin treatment resulted in hypoglycemia which persisted for at least 120 min. Following bombesin infusion, free fatty acids and corticosterone levels were elevated in food-sated rats. Food-deprived rats exhibited elevation in plasma glucose, free fatty acids, and corticosterone following peptide infusion. In insulin-treated rats, bombesin attenuated the hypoglycemia observed in controls and increased corticosterone levels. These findings suggest that bombesin-like peptides localized within the POA may participate in the regulation of metabolic fuels.