Effect of naloxone and naltrexone on the development of satiation measured in the runway: comparisons with d-amphetamine and d-fenfluramine

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

Behavioural profile of exendin-4/naltrexone dose combinations in male rats during tests of palatable food consumption

RATIONALE

The glucagon-like peptide 1 receptor (GLP-1R) agonist exendin-4 potently suppresses food intake in animals and humans. However, little is known about the behavioural specificity of this effect either when administered alone or when co-administered with another anorectic agent.

OBJECTIVES

The present study characterises the effects of exendin-4, both alone and in combination with naltrexone, on behaviours displayed by male rats during tests with palatable mash.

METHODS

Experiment 1 examined the dose-response effects of exendin-4 (0.025-2.5 μg/kg, IP), while experiment 2 profiled the effects of low-dose combinations of the peptide (0.025 and 0.25 μg/kg) and naltrexone (0.1 mg/kg).

RESULTS

In experiment 1, exendin-4 dose dependently suppressed food intake as well as the frequency and rate of eating. However, these effects were accompanied by dose-dependent reductions in all active behaviours and, at 2.5 μg/kg, a large increase in resting and disruption of the behavioural satiety sequence (BSS). In experiment 2, while exendin-4 (0.25 μg/kg) and naltrexone each produced a significant reduction in intake and feeding behaviour (plus an acceleration in the BSS), co-treatment failed to produce stronger effects than those seen in response to either compound alone.

CONCLUSION

Similarities between the behavioural signature of exendin-4 and that previously reported for the emetic agent lithium chloride would suggest that exendin-4 anorexia is related to the aversive effects of the peptide. Furthermore, as low-dose combinations of the peptide with naltrexone failed to produce an additive/synergistic anorectic effect, this particular co-treatment strategy would not appear to have therapeutic significance.

On the behavioural specificity of hypophagia induced in male rats by mCPP, naltrexone, and their combination

RATIONALE

Serotonergic (5-hydroxytryptamine, 5-HT) and opioidergic mechanisms are intimately involved in appetite regulation.

OBJECTIVES

In view of recent evidence of positive anorectic interactions between opioid and various non-opioid substrates, our aim was to assess the behavioural specificity of anorectic responses to the opioid receptor antagonist naltrexone, the 5-HT2C/1B receptor agonist mCPP and their combination.

METHODS

Behavioural profiling techniques, including the behavioural satiety sequence (BSS), were used to examine acute drug effects in non-deprived male rats tested with palatable mash. Experiment 1 characterised the dose-response profile of mCPP (0.1-3.0 mg/kg), while experiment 2 assessed the effects of combined treatment with a sub-anorectic dose of mCPP (0.1 mg/kg) and one of two low doses of naltrexone (0.1 and 1.0 mg/kg).

RESULTS

Experiment 1 confirmed the dose-dependent anorectic efficacy of mCPP, with robust effects on intake and feeding-related measures observed at 3.0 mg/kg. However, that dose was also associated with other behavioural alterations including increased grooming, reductions in locomotion and sniffing, and disruption of the BSS. In experiment 2, naltrexone dose-dependently reduced food intake and time spent feeding, effects accompanied by a behaviourally selective acceleration in the BSS. However, the addition of 0.1 mg/kg mCPP did not significantly alter the behavioural changes observed in response to either dose of naltrexone given alone.

CONCLUSIONS

In contrast to recently reported positive anorectic interactions involving low-dose combinations of opioid receptor antagonists or mCPP with cannabinoid CB1 receptor antagonists, present results would not appear to provide any support for potentially clinically relevant anorectic interactions between opioid and 5-HT2C/1B receptor mechanisms.

Acute behavioural effects of bupropion and naltrexone, alone and in combination, in non-deprived male rats presented with palatable mash

RATIONALE

In appetite research, drugs frequently progress to clinical trials on the basis of outcome (reduced food intake/body weight gain) with insufficient attention to process (behavioural analysis). Although bupropion and naltrexone (alone and in combination) reduce food consumption in rodents and humans, their effects on behaviour during feeding tests have not been thoroughly investigated.

OBJECTIVES

This study aimed to assess the behavioural specificity of anorectic responses to bupropion, naltrexone and their combination.

METHODS

Video analysis was employed to characterise the behavioural effects of acute systemic treatment with bupropion (10.0-40.0 mg/kg), naltrexone (0.1-3.0 mg/kg) and combined bupropion (20 mg/kg) plus naltrexone (0.1-1.0 mg/kg) in non-deprived male rats exposed for 1 h to palatable mash. Particular attention was paid to the behavioural satiety sequence (BSS).

RESULTS

In experiment 1, the anorectic response to 40 mg/kg bupropion was associated with significant psychomotor stimulation and a complete disruption of the BSS. In experiment 2, the anorectic response to 3 mg/kg naltrexone was associated with an accelerated but otherwise normal BSS. In experiment 3, the co-administration of 20 mg/kg bupropion and naltrexone (0.1 and 1.0 mg/kg) not only produced an additive anorectic profile (including a reduced rate of eating), but the addition of the opioid receptor antagonist also concurrently attenuated the psychomotor stimulant response to the atypical antidepressant.

CONCLUSIONS

Low-dose co-treatment with naltrexone and bupropion produces a stronger suppression of appetite than that seen with either agent alone and has the additional advantage of reducing some of the unwanted effects of bupropion.

Cognitive and neuronal systems underlying obesity

Since the late 1970s obesity prevalence and per capita food intake in the USA have increased dramatically. Understanding the mechanisms underlying the hyperphagia that drives obesity requires focus on the cognitive processes and neuronal systems controlling feeding that occurs in the absence of metabolic need (i.e., "non-homeostatic" intake). Given that a portion of the increased caloric intake per capita since the late 1970s is attributed to increased meal and snack frequency, and given the increased pervasiveness of environmental cues associated with energy dense, yet nutritionally depleted foods, there's a need to examine the mechanisms through which food-related cues stimulate excessive energy intake. Here, learning and memory principles and their underlying neuronal substrates are discussed with regard to stimulus-driven food intake and excessive energy consumption. Particular focus is given to the hippocampus, a brain structure that utilizes interoceptive cues relevant to energy status (e.g., neurohormonal signals such as leptin) to modulate stimulus-driven food procurement and consumption. This type of hippocampal-dependent modulatory control of feeding behavior is compromised by consumption of foods common to Western diets, including saturated fats and simple carbohydrates. The development of more effective treatments for obesity will benefit from a more complete understanding of the complex interaction between dietary, environmental, cognitive, and neurophysiological mechanisms contributing to excessive food intake.

Consumption of palatable food decreases the anorectic effects of serotonergic, but not dopaminergic drugs in baboons

We examined the effects of periodic access to a palatable, high sugar content food (candy) in 8 male baboons on the anorectic response to d-amphetamine, which increases dopamine, and dexfenfluramine, which increases serotonin. During candy access, up to 200 candies containing 75% of energy as sugar were available during the morning on Mondays, Wednesdays and Fridays; food pellets (19% of energy as sugar) were available in the afternoon and throughout the remaining days of the week. During candy access, baboons consumed a mean of 177 pieces of candy containing 696 kcal (2.91 MJ) in the morning compared to 44 food pellets and 150 kcal (0.63 MJ) in the morning on non-candy days. Food pellet intake was lower during candy access. Complete dose-response functions for the effects of the drugs on food pellet intake on days that candy was not available were determined before, during, and after the period of access to candy. Dexfenfluramine and amphetamine produced dose-dependent decreases in food pellet intake and increases in latency to eat food pellets before, during, and after candy access. During access to candy, the dose-response function for dexfenfluramine was shifted to the right indicating the development of tolerance, while that for amphetamine was shifted to the left indicating sensitization. Only the dose-response function for dexfenfluramine returned to baseline after candy access suggesting that the difference was specific to concurrent palatable food consumption. We hypothesize that tolerance to the effects of dexfenfluramine reflects a decrease in the satiating effect of serotonin release due to repeatedly eating large amounts of palatable food.

Opioids as facilitators of feeding: can any food be rewarding?

Palatability is one of the most rewarding aspects of consummatory behavior. Opioids, potent facilitators of intake of sweet and fat tastants, are thought to mediate hedonics of feeding. However, the rewarding context of consumption is not limited to palatability, and gratification can be achieved through other means, e.g., eating to satisfy hunger. The current review discusses the role of opioid peptides in food intake regulation by incorporating this expanded concept of feeding reward. We present evidence that, aside from increasing sugar/fat consumption, opioids propel the intake of diets whose gustatory value is low but are nonetheless consumed under circumstances allowing feeding gratification to occur. Opioids enhance reward-driven consumption by acting within the classical reward circuitry and also by signaling reward at sites that regulate other aspects of food intake, such as satiety and aversion. We conclude that, due to the complexity of neural and functional interactions, opioids are capable of enhancing pleasure of eating any food--palatable, bland or even aversive--making any meal into a more rewarding experience, despite possible consequences.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Reward systems and food intake: role of opioids

Humans eat for many reasons, including the rewarding qualities of foods. A host of neurotransmitters have been shown to influence eating behavior and some of these appear to be involved in reward-induced eating. Endogenous opioid peptides and their receptors were first reported more than 30 years ago, and studies suggesting a role of opioids in the regulation of food intake date back nearly as far. Opioid agonists and antagonists have corresponding stimulatory and inhibitory effects on feeding. In addition to studies aimed at identifying the relevant receptor subtypes and sites of action within the brain, there has been a continuing interest in the role of opioids on diet/taste preferences, food reward, and the overlap of food reward with others types of reward. Data exist that suggest a role for opioids in the control of appetite for specific macronutrients, but there is also evidence for their role in the stimulation of intake based on already-existing diet or taste preferences and in controlling intake motivated by hedonics rather than by energy needs. Finally, various types of studies indicate an overlap between mechanisms mediating drug reward and palatable food reward. Preference or consumption of sweet substances often parallels the self-administration of several drugs of abuse, and under certain conditions, the termination of intermittent access to sweet substances produces symptoms that resemble those observed during opiate withdrawal. The overconsumption of readily available and highly palatable foods likely contributes to the growing rates of obesity worldwide. An understanding of the role of opioids in mediating food reward and promoting the overconsumption of palatable foods may provide insights into new approaches for preventing obesity.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

A novel procedure for assessing the effects of drugs on satiation in baboons: effects of memantine and dexfenfluramine

RATIONALE

Procedures for studying the effects of medications on satiation will assist the development of obesity medications.

OBJECTIVES

Develop a procedure for measuring satiation during consumption of bland and highly palatable food and determine the effect of acute intramuscular administration of dexfenfluramine (DFEN), which increases serotonin levels, and memantine (MEM), which blocks N-methyl-D: -aspartate receptors.

MATERIALS AND METHODS

A modified progressive ratio (PR) procedure was used to track changes in reinforcing strength when a food was consumed. The response requirement increased after each reinforcement, and reinforcing strength was estimated using the breakpoint (BP), which was the last completed response cost. There was one preferred food (sweet candy) and one chow pellet PR session per week. During each session, four male and four female adult baboons experienced three 1-h PR trials, separated by 30 min. Chow pellets were available at all other times. We examined the BP for one to 20 candies or chow pellets. Drug effects were examined when baboons had access to one and ten candies or chow pellets.

RESULTS

BPs for candy were greater than for pellets. Varying the pellet/candy pieces per delivery produced an inverted U-shaped function on the first trial, i.e., maximal BP was observed for three items, and the BP for multiple items, but not a single item, decreased across trials, i.e., BP decreased with food intake and satiation. DFEN and MEM decreased responding with the greatest effects at ten deliveries, suggesting that DFEN and MEM enhanced satiation.

CONCLUSION

Drugs that enhance satiation for several types of food may be particularly effective for decreasing food intake.

Genetic variance contributes to naltrexone-induced inhibition of sucrose intake in inbred and outbred mouse strains

The study of genetic variance in opioid receptor antagonism of sucrose and other forms of sweet intake has been limited to reductions in sweet intake in mice that are opioid receptor-deficient or lacking either pre-pro-enkephalin or beta-endorphin. Marked genetic variance in inbred mouse strains has been observed for sucrose intake across a wide array of concentrations in terms of sensitivity, magnitude, percentages of kilocalories consumed as sucrose and compensatory chow intake. The present study examined potential genetic variance in systemic naltrexone's dose-dependent (0.01-5 mg/kg) and time-dependent (5-120 min) ability to decrease sucrose (10%) intake in eleven inbred (A/J, AKR/J, BALB/cJ, CBA/J, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J, SWR/J, 129P3/J) and one outbred (CD-1) mouse strains. A minimum criterion sucrose intake (1 ml) under vehicle treatment, designed to avoid "floor effects" of antagonist treatment was not achieved in three (A/J, AKR/J, CBA/J) inbred mouse strains. Marked genetic variance in naltrexone's ability to inhibit sucrose intake was observed in the remaining strains with the greatest sensitivity observed in the C57BL/10J and C57BL/6J strains, intermediate sensitivity in BALB/cJ, C3H/HeJ, CD-1 and DBA/2J mice, and the least sensitivity in 129P3/J, SWR/J and SJL/J strains with a 7.5-36.5 fold range of greater effects in the ID(50) of naltrexone-induced inhibition in C57BL/10J relative to the three less-sensitive strains across the time course. Naltrexone primarily affected the maintenance, rather than the initiation of intake in BALB/cJ, CD-1, C3H/HeJ, DBA/2J and SJL/J mice, but significantly reduced sucrose intake at higher doses across the time course in C57BL/6J, C57BL/10J and 129P3/J mice. Whereas SWR/J mice failed to display any significant reduction in sucrose intake at any time point following any of the naltrexone doses, naltrexone's maximal magnitude of inhibitory effects was small (35-40%) in 129P3/J and SJL/J mice, moderate ( approximately 50%) in BALB/cJ, C3H/HeJ, CD-1 and DBA2/J mice, and profound (70-80%) in C57BL/6J and C57BL/10J mice. Indeed, the latter two strains displayed significantly greater percentages of naltrexone-induced inhibition of sucrose intake than virtually all other strains. These data indicate the importance of genetic variability in opioid modulation of sucrose intake.

Nucleus accumbens opioid signaling conditions short-term flavor preferences

Opioid signaling in the nucleus accumbens (NAcc) strongly modulates flavor-based food choice. To further investigate the role of opioid signaling in taste reward, we used a sensory specific satiety (SSS) paradigm to devalue specific flavors of nutritionally identical food pellets in rats. In the NAcc, infusion of a mu opioid (MOP) receptor selective agonist selectively increased consumption of a pre-fed flavor, thus reversing the SSS effect. Conversely, blockade of endogenous opioid signaling with the opioid antagonist naltrexone selectively decreased consumption of a recently consumed flavor, potentiating the SSS effect. No enhancement of consumption was observed if a delay of 3 h was imposed following the intra-NAcc MOP agonist indicating that there were no long-term changes in flavor preference. If a delay was introduced between the initial flavor exposure and the intra-NAcc MOP agonist infusion, pellet consumption was increased non-selectively (irrespective of flavor) suggesting that close temporal contiguity between flavor experience and NAcc opioid action is critical for the opioid effect on flavor preference. In contrast to opioid effects, inactivating NAcc neurons by local microinjection of muscimol (a GABAA agonist) increased consumption of both the pre-fed and non-pre-fed flavors equally. These results demonstrate that opioids released in the NAcc during consumption of palatable foods produce a selective and transient increase in preference for a recently sampled flavor.

Striatal opioid peptide gene expression differentially tracks short-term satiety but does not vary with negative energy balance in a manner opposite to hypothalamic NPY

It has long been known that central opioid systems play an important role in certain aspects of appetite and food intake, particularly with regard to the hedonic or rewarding impact of calorically dense food, such as fat and sugar. Ventral striatal enkephalin may be a key component of this system, as infusions of mu-opiate agonists into this region strongly increase feeding, whereas infusions of opiate antagonists decrease food intake. While pharmacological analysis has consistently supported such a role, direct measurement of enkephalin gene expression in relation to differing food motivational conditions has not been examined. In this study, the effects of a restricted laboratory chow diet (resulting in negative energy balance) as well has recent consumption of chow (short-term satiety) on striatal preproenkephalin (PPE) and prodynorphin (PD) mRNA expression were measured in rats, using both Northern blot analysis and in situ hybridization methods. As a comparison, hypothalamic (arcuate nucleus) neuropeptide Y (NPY) was also measured in these conditions. PPE expression was broadly downregulated throughout the striatum in animals that had recently consumed a meal, whereas it was unaffected by negative energy balance. Expression of an additional striatal peptide gene, PD, did not follow this pattern, although diet restriction caused a decrease in accumbens core dynorphin mRNA. Conversely, as expected, arcuate nucleus NPY mRNA expression was markedly upregulated by negative energy balance, but was unchanged by recent food consumption. This double dissociation between striatal and hypothalamic peptide systems suggests a specific role for striatal PPE in relatively short-term food motivational states, but not in long-term metabolic responses to diet restriction.

Differential regulation of the consummatory, motivational and anticipatory aspects of feeding behavior by dopaminergic and opioidergic drugs

Various aspects of feeding behavior (eg consumption, motivation and anticipation) are regulated by homeostatic and hedonic systems, and are modulated by dopaminergic and opioid brain systems. Here, we have studied the modulation of these aspects of feeding behavior by opioid and dopaminergic neurotransmission while taking into account food palatability and homeostatic state. Foods that varied in palatability were presented to either food sated or food restricted rats following injections of different doses of naloxone, an opioid receptor antagonist, or flupenthixol, a dopaminergic receptor antagonist, in behavioral paradigms that measured different aspects of feeding. Naloxone decreased food intake in a dose-dependent manner in sated rats given access to palatable food, without modifying food intake in food restricted rats. Flupenthixol did not have any effect on food intake. With regard to motivation, which was tested in a straight alley, naloxone increased the latency to reach the food only in sated rats presented with palatable food. Flupenthixol did not modify the latency of any group. Conditioned locomotor activity to repeated food presentation, a measure of anticipation, is expressed only in food restricted rats. Naloxone did not modify anticipatory activity, whereas flupenthixol decreased it only in food restricted rats presented with palatable food. These results reinforce the idea that the opioid system regulates feeding through the modulation of the perceived palatability of food. The dopaminergic system seems to be more important for the regulation of anticipatory activity related to motivationally relevant stimuli.

Opposite effects of MK-801 on the expression of food and morphine-induced conditioned place preference in rats

Behavioural studies have provided strong evidence for common substrates in the rewards of natural and addictive substances, but it is still unclear whether there is a common glutamatergic NMDA receptor mechanism involved in the processing of reward for both. The present study was designed to investigate the effects of MK-801 (0.1mg/kg) on the expression of place preference conditioned with food and morphine (5.0mg/kg) in rats. The data indicates that MK-801 potentiates the expression of food-induced conditioned place preference (CPP) but retards that of morphine CPP. It also demonstrates that the opposite effects of MK-801 on food and morphine CPP expression were caused neither by hyperactivity nor by the impairment of memory retrieval. These results suggest that MK-801 enhances food craving and inhibits morphine craving in rats, and that the roles of glutamatergic NMDA receptor mechanisms in the reward processing of natural reinforcers and addictive drugs may be dissociable.

Opioids as agents of reward-related feeding: a consideration of the evidence

Gerard Smith was one of the pioneers in the field of neuropeptidergic control of food intake. He established methodology and criteria used to determine whether a neuropeptide acts as an endogenous satiety factor. More recently, he theorized that there are direct and indirect controls of meal size. Direct controls include those that depend upon contact of food with preabsorptive receptors from the tip of the tongue to the end of the small intestine, and indirect controls include those that do not depend upon direct contact of mucosal receptors, such as learning and metabolism. In this review, we consider the evidence that opioids are mediators of reward-related feeding. We address these issues adopting Smith's approach to problem solving, including an evaluation of the opioids as controllers of the meal. We also present a novel concept of "hedonic restriction," resulting in a change in opioid gene expression. Overall, we believe the evidence supporting opioid participation in reward-driven and other types of ingestion is very strong, but much work remains before we understand how opioids contribute to the widely distributed neural network that controls ingestive behavior.

Minireview: Characterization of influence of central nociceptin/orphanin FQ on consummatory behavior

Nociceptin/orphanin FQ (N/OFQ), a peptide closely related to dynorphin A, is the endogenous agonist of the NOP receptor that moderately increases food intake under various conditions. Its orexigenic properties are mediated by the brain circuitry. In the present review, we focus on discussing the nature of hyperphagic effects of N/OFQ with special emphasis on its function within feeding-related neural networks. Although some of N/OFQ's orexigenic effects resemble those induced by opioids, reward-dependent feeding appears to be affected in a different manner by agonists of the NOP and classical opioid receptors. Also, data suggest that N/OFQ may not only promote feeding initiation, but rather its role may be to inhibit signaling responsible for inhibition of consummatory behavior. Central systems involved in termination of feeding that seem to be influenced by N/OFQ encompass oxytocin, alpha-MSH, and CRH.

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

Effects of baclofen on feeding behaviour examined in the runway

The motivational mechanisms underlying the effects of systemic administration of the GABA-B agonist baclofen on feeding were examined using a runway. Food-deprived male hooded Lister rats were trained to traverse a runway for food reinforcement. Baclofen (1 mg/kg i.p.) significantly increased food intake and this was most evident on the final two blocks of testing. The 2 mg/kg dose of baclofen increased running speed without significantly altering intake. At the highest dose tested (4 mg/kg), no significant effects on either consummatory or appetitive measures were observed. These data suggest that low doses of baclofen enhance the consummatory phase of ingestion by attenuating the natural signals associated with onset of satiation. The data also suggest that baclofen has complex effects on appetitive behaviour that may interfere with its effects on consumption.

Nucleus accumbens opioid, GABaergic, and dopaminergic modulation of palatable food motivation: contrasting effects revealed by a progressive ratio study in the rat

The current studies were designed to evaluate whether incentive motivation for palatable food is altered after manipulations of opioid, GABAergic, and dopaminergic transmission within the nucleus accumbens. A progressive ratio schedule was used to measure lever-pressing for sugar pellets after microinfusion of drugs into the nucleus accumbens in non-food-deprived rats. The mu opioid agonist D-Ala2, NMe-Phe4, Glyo15-enkephalin and the indirect dopamine agonist amphetamine induced a marked increase in break point and correct lever-presses; the GABA(A) agonist muscimol did not affect breakpoint or lever-presses. The data suggest that opioid, dopaminergic, and GABAergic systems within the accumbens differentially modulate food-seeking behavior through mechanisms related to hedonic evaluation of food, incentive salience, and control of motor feeding circuits, respectively.

Naltrexone suppresses the late but not early licking response to a palatable sweet solution: opioid hedonic hypothesis reconsidered

Opioid antagonists suppress the intake of sweet solutions, but typically have little effect on the initial rate of drinking. The lack of an early drug response was investigated in the present study because it questions the general idea that opioid antagonists reduce the hedonic response to sweets. The first experiment, which measured the rat's licking response to a sucrose+saccharin (S+s) solution, revealed that naltrexone suppressed S+s intake but not initial lick rates. Experiment 2A indicated that the drug's delayed behavioral effect was not due to the 10-min injection-test interval used. Increasing the interval to 20 min did not reduce the latency of drug action. Experiment 2B tested the idea that rats require several minutes to detect that naltrexone has reduced the hedonic value of the S+s solution. The S+s solution was presented either for 30 min without interruption or for 3 min followed, after a 6-min delay, by another 27-min access. In both test conditions, naltrexone did not suppress S+s licking until 7-9 min of drinking had occurred. However, the drug blocked an "appetizer effect"; a post-delay increase in licking rate produced by the split-session test procedure. Microstructure analysis indicated that in all cases, naltrexone reduced S+s licking by reducing the number of lick clusters rather than lick cluster size. In contrast to these drug effects, Experiment 2C showed that reducing the concentration of the S+s solution decreased initial lick rates. Together, these findings suggest that opioid antagonists do not affect all aspects of flavor hedonics, but may primarily alter the intake-maintaining action of palatable flavors.

Naloxone's effect on meal microstructure of sucrose and cornstarch diets

The opioid receptor antagonist naloxone decreases consumption of high-sucrose diets but does not reduce cornstarch diet intake in energy-restricted rats. Sucrose-fed rats eat at a much higher rate, consuming more food than cornstarch-fed rats. We examined meal microstructure using an automated weighing system in food-restricted rats eating either a high-sucrose or high-cornstarch diet. Sucrose-fed rats exhibited a higher rate of eating during their first meal compared with cornstarch-fed rats (0.34 vs. 0.20 g/min, respectively). However, naloxone did not reduce eating rate in either group. Naloxone decreased the size of the first meal in both diet groups by shortening the length of the meal. Naloxone's anorectic effect was more potent in the sucrose-fed rats. These results indicate that naloxone's heightened anorectic effect on sucrose diet consumption is not "rate dependent." Naloxone's anorectic actions may be modulated by two conditions, the sensory properties of food and the energy state of the animal. Thus the elevated anorectic potency of naloxone in energy-restricted sucrose-fed rats may reflect actions on neural systems that mediate orosensory and/or postingestive signals.

Pharmacology of flavor preference conditioning in sham-feeding rats: effects of naltrexone

Relatively little is known about the neurochemical and pharmacological mechanisms involved in flavor preference learning. The present study examined the ability of the opioid antagonist, naltrexone to alter the acquisition and expression of flavor preferences conditioned by the sweet taste of sucrose. This was accomplished by adding a novel flavor (the CS+) to a sucrose solution, and a different flavor (the CS-) to a less-preferred saccharin solution. Rats were trained to drink these solutions with an open gastric fistula (sham-feeding), which minimized postingestive actions. Food-restricted (Experiments 1 and 2A) and ad lib-fed (Experiment 2B) rats were given either limited (Experiment 1) or unlimited (Experiment 2) access to the CS+ and CS- solutions during one-bottle training. Preferences were assessed in two-bottle tests (with the CS+ and CS- flavors presented in mixed sucrose-saccharin solutions) following vehicle or naltrexone (0.1-10 mg/kg, SC) treatment. The rats displayed significant CS+ preferences following vehicle, particularly after unlimited access training. In four of five experiments, naltrexone significantly reduced total intakes during the two-bottle, sham-feeding tests. Except for one instance, however, the drug failed to block the preference for the CS+ flavor over the CS- flavor. The effects of naltrexone (0.1 mg/kg) on the acquisition of flavor preferences were studied in sham-feeding rats under limited (Experiment 3A) and unlimited (Experiment 3B) training access conditions. Rats treated with naltrexone during training displayed similar CS+ preferences as did saline-treated rats, even though they consumed less CS+ during training. The naltrexone-trained rats also displayed smaller reductions in total or CS+ intakes than did saline-trained rats when all rats were treated with a 2.5 mg/kg dose of naltrexone during testing. As in previous studies, these results show that naltrexone significantly reduces the intake of sweet solutions, yet it has little or no effect on the acquisition or expression of flavor preferences conditioned by sucrose in sham-feeding rats.

Opioids and food intake: distributed functional neural pathways?

Agonists of the mu, delta, kappa and ORL(1)opioid receptors increase food intake while opioid receptor blockade decreases food intake. The majority of the collected data related to opioids and feeding has led to the speculation that opioids are involved in meal maintenance and orosensory reward; however, some data suggest that opioids may impact feeding associated with energy needs. Based on the wide distribution of CNS opioid receptors and the presence of other neuropeptides in the vicinity of opioidergic pathways, it seems likely that opioids affect multiple feeding systems. For example, opioids in the hindbrain might be involved in both sensory and metabolic aspects of food intake, those in the amygdala in processing of 'emotional' properties of foods, and those in the hypothalamus in energy needs. In this review we present data which support functional diversity of opioids in feeding behavior.

Diet-induced enhancement of naloxone sensitivity is independent of changes in body weight

Intake of palatable solutions can enhance the anorectic potency of opioid antagonists. This experiment examined the relative contributions of orosensory experience and body weight gain to the enhanced anorectic potency of naloxone (0.125, 0.25, 0.5, and 1.0 mg/kg i.p.). Four groups of male hooded Lister rats (Charles River) were maintained on separate feeding regimes for 3 months. S-ADLIB rats were nondeprived with free access to lab chow and 20% (w/v) sucrose solution. S-RESTRICT rats received limited sucrose (50 ml/day) and chow (15 g/day) access, yoking their body weights to ADLIB rats receiving free access to lab chow only. RESTRICT rats received approx. 15 g of chow/day to maintain their body weights at 90% of the ADLIB rats. Fifteen-minute sucrose intake tests revealed marked differences between naloxone sensitivity of chronic sucrose drinkers and sucrose-naive groups. Intakes of S-ADLIB and S-RESTRICT were suppressed at all doses (max suppression >60%). In comparison to animals given sucrose, ADLIB and RESTRICT animals were significantly less sensitive (maximum suppression = 35%). Naloxone potency was independent of body weight differences. The data demonstrate that overconsumption of palatable ingesta, and not diet-induced weight gain, is sufficient to enhance antagonist potency. The study confirms that orosensory stimulation can induce plasticity in opioid systems, supporting an important role for opioids in intake regulation and general reward processes.

Opioid-receptor subtype agonist-induced enhancements of sucrose intake are dependent upon sucrose concentration

Selective mu ([D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO)), delta1 ([D-Pen2, D-Pen5]-enkephalin (DPDPE)), delta2 ([D-Ala2, Glu4]-Deltorphin (Delt II)), kappa1 (U50488H) and kappa3 (naloxone benzoylhydrazone (NalBzOH)) opioid agonists each stimulate food intake in rats. Whereas studies with selective opioid antagonists implicate mu and kappa1 receptors in the mediation of sucrose intake, studies with selective opioid agonists implicate mu and delta receptors in the mediation of saccharin intake. The present study determined if specific delta1, delta2, kappa1, kappa3 and mu opioid-receptor subtype agonists produced similar alterations in sucrose intake as a function of sucrose concentration (0.5%, 2.5%, 10%) across a 1-h time-course. Each of these agonists significantly increased sucrose intake with variations in pattern, magnitude, and consistency as a function of sucrose concentration. Whereas the mu opioid agonist, DAMGO, and the delta1 opioid agonist, DPDPE, each enhanced sucrose intake at higher (2.5%, 10%), but not lower (0.5%), concentrations, the delta2 opioid agonist, Delt II, increased sucrose intake at lower (0.5%, 2.5%), but not higher (10%), concentrations. Kappa opioid agonists produced less consistent effects. The kappa1 opioid agonist, U50488H, increased sucrose intake at high (10%) concentrations and decreased sucrose intake at low (0.5%) concentrations, and the kappa3 opioid agonist, NalBzOH, inconsistently increased sucrose intake at the 0.5% (20 microg) and 10% (1 microg) concentrations. Thus, these data further implicate mu, delta1, and delta2 opioid mediation of palatable intake, particularly of its orosensory characteristics.

Why do we eat? A neural systems approach

Neuroregulators found at various brain sites are involved in controlling food intake, a behavior that occurs for many reasons. Different neuroregulators may affect different stimuli that impact eating behavior. For example, neuropeptide Y may initiate feeding for energy needs, opioid peptides may provide the rewarding aspects of eating, and corticotropin releasing factor may affect stress-induced eating. We know that the neural networks regulating feeding also impact other components of energy balance. Neuropeptide Y not only increases eating, it also decreases energy expenditure in brown fat and increases enzymatic activity associated with fat storage in white fat, resulting in a more obese animal. What the sites of action are of these neuroregulators and how they interact with regulators at other sites are of utmost importance. Different regions of the brain, together with the periphery, communicate via signals acting in coordinated fashion, which leads to the final outcome: eating less or more and expending less or more energy.

Behavioral effects of naloxone on neuropeptide Y-induced feeding

We evaluated the effect of naloxone on neuropeptide Y (NPY)-induced feeding behavior using two methods; operant chambers and observational analysis. In the first study rats were trained on a FR 80 (first pellet) FR 3 (subsequent pellets) reinforcement schedule. Following training, rats were injected with NPY (intraventricular, 5 micrograms) and various doses of naloxone (subcutaneous, 0, 0.1, 0.3, 1, 3, and 10 mg/kg). NPY significantly increased the number of pellets consumed during the one hour session and naloxone (1, 3, and 10 mg/kg) blocked this effect. NPY failed to alter the time to the first response, but did decrease the time needed to complete the first ratio (FR 80). Naloxone (3 and 10 mg/kg) increased the latency to the first response and blocked NPY's effect on completion of the first ratio. In the second study, we observed rats continually following injection of saline. NPY (5 micrograms i.c.v.) and/or naloxone (1 mg/kg s.c.). NPY increased food intake during the 1-h session and naloxone blocked this effect. NPY decreased the latency to eat, but naloxone failed to significantly antagonize this effect. The amount of time spent eating was greater in the NPY group compared to the saline group and naloxone antagonized this effect. Lag sequential analysis indicated that NPY induced a move-eat-move behavioral sequence that disappeared following naloxone administration. These data lend support to the notion that opioids are involved in maintenance of NPY-induced feeding but affect meal initiation in a minor way. Only relatively high doses of naloxone (3 and 10 mg/kg) altered NPY-induced changes in meal initiation.

Naltrexone increases the latency to drink alcohol in social drinkers

We investigated the effects of naltrexone (NTX) on alcohol drinking, urge to drink alcohol, and alcohol-induced sensations and mood states in social drinkers consuming alcohol ad libitum in a cocktail bar. Sixteen college-age men and women participated in a double-blind, placebo-controlled, within-subjects, cross-over study. Subjects were tested during each of three drug conditions: NTX, 50 mg/ day, po; inactive placebo; and no drug. Each treatment condition lasted 8 to 11 days. Small groups of subjects consumed alcohol ad libitum during three 2-hr evening drinking sessions, separated by approximately-2 weeks. NTX treatment significantly increased the latency (time in seconds) to first sip the first (p < 0.05) and second alcoholic beverages consumed (p < 0.01). Moreover, the mean blood alcohol concentration at the end of the session was significantly lower when subjects were treated with NTX (p < 0.05). No differences were found on self-report urge to drink alcohol. Subjects reported more fatigue and tension on the Profile of Mood States (p < 0.05), before drinking, and increases in nausea on the Alcohol Sensation Scale (p < 0.05) when treated with NTX. The increase in the latency to sip the first and second alcoholic beverages may reflect the capacity of NTX to block urge for alcohol elicited from external cues (before consuming alcohol), as well as urge for alcohol after priming from ingested alcohol. Thus, the effectiveness of NTX for reducing drinking behaviors of alcoholics may be partially caused by anticraving properties of NTX.

Steroid saponins from fenugreek seeds: extraction, purification, and pharmacological investigation on feeding behavior and plasma cholesterol

The seeds of fenugreek (Trigonella foenum graecum L.) are traditionally assumed to have restorative properties. We have recently shown that a fenugreek seed extract containing steroid saponins increased food consumption and induced hypocholesterolemia in rats. This study aims to investigate the specific role of purified steroid saponins in these properties. For this purpose, an original technique for extraction and purification of steroid saponins was carried out. Thereafter, the effects of these steroid saponins were investigated on feeding behavior and metabolic endocrine changes in normal and diabetic rats. All the steroid saponins (furostanol type) were extracted from the seeds and separated from all other constituents of the entire extract by using several purification procedures to give an extract containing at least 90% of steroid saponins. Pharmcological experiments were performed in vivo in normal and streptozotocin diabetic rats: steroid saponins were administered chronically mixed with food (12.5 mg/day per 300 g body weight). Our data show that the treatment with steroid saponins significantly increased food intake and the motivation to eat in normal rats, while modifying the circadian rhythm of feeding behavior; it also stabilized the food consumption in diabetic rats, which resulted in a progressive weight gain in these animals, in contrast to untreated diabetic controls. Both in normal and diabetic rats, steroid saponins decreased total plasma cholesterol without any change in triglycerides. In conclusion, the present work reports a clear methodology to obtain all the steroid saponins and demonstrates that these saponins enhance food consumption and motivation to eat, and reduce plasma cholesterol levels in rats.

Selective actions of central mu and kappa opioid antagonists upon sucrose intake in sham-fed rats

Intake of a palatable sucrose solution in real-fed rats is mediated in part by central mu and kappa opioid receptors. Since general opioid antagonists still inhibit sucrose intake in sham-fed rats, the present study examined whether centrally administered mu (beta-funaltrexamine: 5, 20 micrograms), mu1 (naloxonazine: 50 micrograms), kappa (nor-binaltorphamine: 1, 5, 20 micrograms), delta (naltrindole: 20 micrograms) or delta 1 (DALCE: 40 micrograms) opioid subtype antagonists altered sucrose intake in sham-fed rats in a similar manner to systemic naltrexone (0.01-1 mg/kg) and whether such effects were equivalent to altering the sucrose concentration. Sucrose (20%) intake in sham-fed rats was significantly and dose-dependently reduced by naltrexone (59%), beta-funaltrexamine (44%) and nor-binaltorphamine (62%), but not by naloxonazine, naltrindole or DALCE. The reductions in sham sucrose (20%) intake by general, mu and kappa antagonism were similar in pattern and magnitude to diluting sucrose concentration from 20% to 10% in untreated sham-fed rats. Since both real-fed and sham-fed rats share similar patterns of specificity of opioid effects, magnitudes and potencies of inhibition, it suggests that central mu and kappa antagonism acts on orosensory mechanisms supporting sucrose intake.

Butorphanol increases food-reinforced operant responding in satiated rats

In the present series of studies we examined the effect of butorphanol tartrate on food-reinforced operant responding in satiated rats. In the first experiment, 8.0 mg/kg butorphanol was administered subcutaneously, once per day for 4 days, to satiated rats responding under an fixed ratio 10 (FR 10) reinforcement schedule. In the second experiment, butorphanol (0, 0.3, 1.0, 3.0, 10.0 mg/kg) was administered to satiated rats responding under an FR 80 (first pellet) FR 3 (subsequent pellets) reinforcement schedule for 4 consecutive days. Repeated butorphanol administration increased total amount of food consumed over sessions in both experiments. Under the FR 80 schedule component, butorphanol initially increased latency to acquire the first pellet, an effect attenuated by repeated administration. Whereas vehicle administration was associated with consumption of relatively large quantities of food within the first 10 min of receiving the first pellet, butorphanol was associated with continued feeding as the session progressed. These data suggest that butorphanol-induced food intake is associated with maintenance rather than initiation of feeding.

Naloxone's effects on operant responding depend upon level of deprivation

Naloxone's effects on initiation, maintenance, and maximal response effort to acquire food were examined in rats maintained under different levels of food deprivation. In Experiment 1, naloxone was administered SC to rats responding under an FR 80 (first pellet) FR 3 (subsequent pellets) reinforcement schedule. Naloxone did not increase time to acquire the first pellet. Naloxone's suppression of subsequent intake and lowest effective dose were inversely related to level of deprivation. In Experiment 2, rats responded for food under a Progressive Ratio 2 reinforcement schedule. Breakpoint was lowered only when rats were maintained with free access to food. Decreases in response and running rate were inversely related to deprivation level. Results are consistent with the hypothesis that opioids are involved in the maintenance but not the initiation of feeding.

Naltrexone, serotonin receptor subtype antagonists, and carbohydrate intake in rats

Functional interactions between serotonergic (5-HT) and opioid drugs have been observed with 5-HT3 receptor antagonism enhancing the inhibitory actions of naloxone and naltrexone in both food-deprived and glucoprivic rats; 5-HT2A/C receptor antagonism enhanced naltrexone's inhibition of insulin hyperphagia. The present study examined whether pretreatment with either general 5-HT (methysergide: 0.5-5 mg/kg), 5-HT2A/C (ritanserin: 0.25-2.5 mg/kg), or 5-HT3 (ICS 205930: 0.5-5 mg/kg) antagonists altered the pattern and magnitude of ad lib intake of simple (sucrose: 10%) or more complex (maltose dextrin: MD, 10%) carbohydrate solutions, or naltrexone's (0.25-2.5 mg/kg) inhibition of these forms of intake. Methysergide significantly increased the pattern and magnitude of sucrose intake at low (0.5-2.5 mg/kg) doses, and transiently delayed the pattern of MD intake at high (5 mg/kg) doses. Ritanserin significantly accelerated the pattern, but not the magnitude of sucrose intake at low (0.25-1.25 mg/kg) doses without affecting MD intake. ICS 205930 reduced the magnitude of sucrose intake at the highest (5 mg/kg) dose, and transiently reduced MD intake. Naltrexone dose dependently altered the pattern and magnitude of both sucrose and MD intake. Coadministration of ritanserin and naltrexone either eliminated or delayed the pattern of opioid antagonist inhibition of both sucrose and MD intake. Methysergide and ICS 205930 pretreatment produced minor changes in the pattern of naltrexone-induced inhibition. These data indicate that 5-HT receptor differentially modulate the pattern of carbohydrate intake, and indicate differential ingestive interactions between 5-HT and opioid antagonists under challenge and palatable conditions.

The role of multiple opioid receptors in the maintenance of stimulation-induced feeding

Feeding induced by lateral hypothalamic electrical stimulation is sensitive to opioid antagonism and has previously been blocked by naloxone and antibodies to dynorphin A fragments. In the present study, high affinity receptor-selective antagonists were used to determine the particular opioid receptor type(s) that mediates stimulation-induced feeding (SIF). Separate groups of rats were used to conduct i.c.v. dose-response studies with TCTAP (mu), naltrindole (delta) and norbinaltorphimine (kappa). TCTAP, at the highest dose tested (i.e. 5.0 nmol) and norbinaltorphimine, at doses of 10.0 and 50.0 nmol, increased the brain stimulation frequency threshold for eliciting SIF. Naltrindole, at doses up to 50.0 nmol, had no effect. Results of another study, recently conducted in this laboratory, indicate that the present doses of TCTAP and norbinaltorphimine have no effect on thresholds for lateral hypothalamic self-stimulation. This suggests that mu and kappa opioid activity are associated with feeding, rather than the eliciting brain stimulation, and excludes non-specific performance deficits as an explanation of elevated SIF thresholds. In the SIF test, where 5 determinations of threshold are obtained in serial order, naloxone characteristically increases thresholds toward the end of a test while conventional appetite suppressants increase thresholds uniformly throughout a test. TCTAP and norbinaltorphimine produced a 'naloxone-like' pattern of threshold elevation, suggesting that mu and kappa receptors are involved in the process whereby endogenous opioid activity sustains feeding once initiated.

Morphine enhances hedonic taste palatability in rats

The question of whether opiates stimulate feeding by enhancing taste pleasure was investigated by examining the effect of morphine upon hedonic and aversive reactions to taste (tongue protrusions, gapes, etc.). Rats (n = 12) were given SC injections of morphine (4 mg/kg) or equal volumes of isotonic saline 2 h after the start of their daily light cycle. Food intake was measured in a 2-h test. On days when they were given morphine, rats ate significantly more food than when given saline. Hedonic and aversive taste reactions were elicited by an infusion of sucrose-quinine solution into the mouth and were measured subsequently in a slow-motion video analysis. The same rats that showed an increase in food intake after treatment with morphine showed a significant increase in their positive hedonic responses. Aversive reactions were unchanged by morphine. The results support the hypothesis that morphine enhances feeding by increasing the hedonic palatability of food.

Central opioid receptor subtype antagonists differentially reduce intake of saccharin and maltose dextrin solutions in rats

Opioid modulation of ingestion includes general opioid antagonism of deprivation-induced water intake and intake of sucrose and saccharin solutions. Previous studies using selective subtype antagonists indicated that opioid effects upon deprivation-induced water intake occurred through the mu2 receptor and that opioid effects upon sucrose intake occurred through kappa and mu2 receptors. The present study compared the effects of intracerebroventricular administration of opioid receptor subtype antagonists upon intakes of a saccharin solution and a maltose dextrin (MD) solution to determine which receptor subtypes were involved in modulation of ingestion of different preferred tastants. Significant reductions in saccharin intake (1 h) occurred following naltrexone (20-50 micrograms: 66%) and naltrindole (delta, 20 micrograms: 75%), whereas [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, delta 1, 40 micrograms: 45%) had transient (5 min) effects. Neither beta-funaltrexamine (B-FNA, mu), naloxonazine (mu1), nor nor-binaltorphamine (Nor-BNI, kappa) significantly altered saccharin intake. Significant reductions in MD intake (1 h) occurred following naltrexone (5-50 micrograms: 69%) and B-FNA (1-20 micrograms: 38%). MD intake was not reduced by naltrindole, DALCE, naloxonazine and Nor-BNI. Peak antagonist effects were delayed (20-25 min) to reflect interference with the maintenance, rather than the initiation of saccharin or MD intake. Comparisons of opioid antagonist effects across intake situations revealed that naltrexone had consistently low ID40 values for saccharin (29 nmol), MD (25 nmol), sucrose (6 nmol) and deprivation (38 nmol) intake. Despite its significant effects relative to naloxonazine, B-FNA had significantly higher ID40 values for saccharin (800 nmol), MD (763 nmol) and sucrose (508 nmol) relative to deprivation (99 nmol) intake, suggesting that mu2 receptors may be mediating maintenance of intake rather than taste effects. Nor-BNI had low ID40 values for intake of sucrose (4 nmol), but not for saccharin (168 nmol), MD (153 nmol) and deprivation (176 nmol), suggesting that kappa receptors may mediate ingestion of sweet-tasting stimuli. That delta (naltrindole: ID40 = 60 nmol), but not delta 1 (DALCE: ID40 = 288 nmol) antagonists consistently reduce saccharin intake suggests a role for the delta 2 receptor subtype in the modulation of hedonic orosensory signals.

Responding for oral ethanol after naloxone treatment by alcohol-preferring AA rats

The present study examined the effect of a relatively nonselective opioid antagonist, naloxone, on lever pressing for oral ethanol by the alcohol-preferring AA rats. The AAs, housed continually in operant chambers with free access to food and water, learned to respond for 10% oral ethanol during daily 60-min alcohol access periods indicated by a stimulus light. The rats developed stable ethanol responding, resulting in mean ethanol intakes of 1.2 g/kg/60 min and measurable blood alcohol levels. In the first experiment, single systemic injections of naloxone (0.05-2.5 mg/kg) had no effect on the initial rate of responding; dose-dependent decreases were observed later during the alcohol access. The second experiment examined the effects of repeated injections of 0.5 and 2.5 mg/kg naloxone on 5 consecutive days. Naloxone suppressed responding dose-relatedly over the treatment days. In contrast to the effects of single injections, repeated injections with 2.5 mg/kg naloxone produced progressive decreases within the first minutes of access. The results suggest that naloxone may attenuate the reinforcing actions of ethanol.

Effects of a fenugreek seed extract on feeding behaviour in the rat: metabolic-endocrine correlates

Fenugreek seeds (Trigonella foenum graecum L.) are assumed to have restorative and nutritive properties. The present work was designed to investigate the effects of a fenugreek seed extract on feeding behaviour. Experiments were performed to determine food consumption and motivation to eat as well as metabolic-endocrine changes in chronically treated animals. Male Wistar rats were given the seed extract orally (10 and 100 mg/day per 300 g body weight), mixed together with food, and control animals were monitored in parallel. The results show that chronic oral administration of the fenugreek extract significantly increases food intake and the motivation to eat. The treatment, however, does not prevent the anorexia nor the decreased motivation to eat induced by d-fenfluramine (2 mg/kg, IP). An increase in plasma insulin and a decrease in total cholesterol and very low-density lipoprotein (VLDL)-low-density lipoprotein (LDL) total cholesterol were also observed. In conclusion, chronic administration of a fenugreek seed extract enhances food consumption and motivation to eat in rats and also induces hyperinsulinemia as well as hypocholesterolemia.

Evidence for a direct central anorectic effect of tumor-necrosis-factor-alpha in the rat

This study was designed to investigate the effect of tumor necrosis factor-alpha (TNF) on the control of food intake in rat. The specific aims were: a) to evaluate the effects of central (ICV) or peripheral (IP) microdoses of TNF on food intake; b) to show that the TNF-induced anorexia results from a direct action of the mediator on the central nervous system; c) to demonstrate that the anorexic activity of TNF is not due to nonspecific malaise. In the first experiment, ICV administration (0.5-4.0 micrograms) of recombinant-murine tumor necrosis factor-alpha (rmTNF) significantly reduced food intake in a dose-dependent manner. The maximal effect (-66%) was observed 5 h after a 4.0 micrograms dose. In contrast, rm TNF did not affect feeding behavior when injected IP, indicating that the anorexic activity was centrally mediated. The estimated DE50 was 2.0 micrograms/rat by the ICV route. In the second experiment, the place conditioning paradigm was used to show that TNF administered ICV at 2.0 micrograms/rat did not induce aversive or deleterious effects as compared to naltrexone given IP at the equi-anorectic dose 5.0 mg/kg. It was concluded that TNF reduces food intake in rats by a direct action on the central nervous system.

Norbinaltorphimine blocks the feeding but not the reinforcing effect of lateral hypothalamic electrical stimulation

The role of central kappa opioid receptors in the regulation of feeding and reward was evaluated using electrical brain stimulation paradigms in combination with the selective kappa antagonist, norbinaltorphimine (nor-BNI). Lateral ventricular injection of 10.0 and 50.0 nmol doses of nor-BNI increased the lateral hypothalamic stimulation frequency threshold for eliciting feeding behavior but had no effect on threshold for self-stimulation in the absence of food. This result is identical to those previously reported for naloxone and antibodies to dynorphin A and suggests that opioid activity is associated with feeding behavior rather than the eliciting brain stimulation. A further similarity between naloxone, dynorphin antiserum, and nor-BNI is their preferential effect on feeding threshold values obtained later, rather than initially, in a post-injection test session. This pattern of threshold elevation is shown to differ from that of the appetite suppressants, amphetamine and phenylpropanolamine, which elevate threshold uniformly throughout a post-injection test. The signature pattern of threshold elevation produced by opioid antagonism is consistent with the hypothesis that opioid activity is involved in the maintenance rather than the initiation of feeding. Specifically, it is hypothesized that a dynorphin A/kappa receptor mechanism is triggered by food taste and sustains feeding behavior by facilitating incentive reward.

Central opioid receptor subtype antagonists differentially alter sucrose and deprivation-induced water intake in rats

The present study compared the effectiveness of centrally-administered opioid receptor subtype antagonists to inhibit intake of either a 10% sucrose solution under ad libitum conditions, or water following 24 h of water deprivation. Full dose-response functions were evaluated over a 1 h period for the following antagonists: naltrexone (general: 1-50 micrograms), nor-binaltorphamine (Nor-BNI, kappa: 1-20 micrograms), beta-funaltrexamine (beta-FNA, mu: 1-20 micrograms), naltrindole (delta 2: 1-20 micrograms), [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, delta 1: 10-40 micrograms) and naloxonazine (mu 1: 10-50 micrograms). Naltrexone significantly and dose-dependently inhibited both sucrose intake (64-67%) and deprivation-induced water intake (53-67%). Nor-BNI significantly and dose-dependently inhibited sucrose intake (53-55%), but failed to significantly affect (28%) deprivation-induced water intake. beta-FNA significantly and dose-dependently inhibited both sucrose intake (31-34%) and deprivation-induced water intake (36-50%). Naltrindole failed to significantly alter either sucrose intake (24%) or deprivation-induced water intake (16%). Whereas DALCE significantly, but transiently (15-20 min) inhibited sucrose intake (28%), it failed to significantly alter deprivation-induced water intake (14%). Naloxonazine significantly, but transiently (5-10 min) stimulated sucrose intake at low doses (26%), but non-significantly reduced sucrose intake at higher doses (20%). Naloxonazine failed to significantly alter deprivation-induced water intake (16% reduction). These data indicate that whereas the kappa and mu 2 binding sites participate in the opioid modulation of sucrose intake, the mu 2 binding site participates in the opioid modulation of deprivation-induced water intake.

Dexfenfluramine. A review of its pharmacological properties and therapeutic potential in obesity

Dexfenfluramine stimulates serotoninergic activity by inhibiting serotonin reuptake into presynaptic neurons and by enhancing its release into brain synapses. Based on the serotonin hypothesis of appetite control these effects would be expected to reduce food intake and thus body-weight. Studies in animal models and severely overweight patients have confirmed the effectiveness of dexfenfluramine as a weight-reducing agent which appears to be well tolerated. Permanent weight loss is the goal of weight-reducing strategies and, based on current clinical evidence, dexfenfluramine appears to exert a weight reducing effect over periods of up to 12 months without development of tolerance, a problem that has limited the long term use of other pharmacological agents used in the treatment of this disorder. Dexfenfluramine facilitated weight loss in patients who had not responded satisfactorily to other weight-reducing strategies, prevented relapse in those patients who had achieved weight reduction by other methods, and corrected disturbed eating patterns (and therefore reduced weight gain) in small studies involving patients with premenstrual syndrome, seasonal affective disorder and nicotine withdrawal syndrome. Follow-up of the longest study reported with dexfenfluramine suggests that continued therapy is required in severely overweight patients if weight loss is to be maintained. Dexfenfluramine has not been directly compared with nonpharmacological measures of weight control such as behaviour modification or exercise programmes. The decision that pharmacological means are indicated in overweight patients must be highly individualised, and must consider the many complex factors that often contribute to overweight states, as well as the anticipated magnitude of drug effect. Despite such a cautionary note, and the expected need (at this stage of its development) for an expanded clinical study programme in certain areas, dexfenfluramine is a clear advance in the pharmacological approach to improved management of overweight individuals.

Role of 5-HT receptors in the effect of d-fenfluramine on gastric emptying and feeding behaviour as examined in the runway test

In one experiment, the effect of d-fenfluramine (DF) on gastric emptying was studied in rats treated i.p. with metergoline, a non-selective serotonin (5-HT) receptor antagonist, ritanserin, a selective 5-HT2 and 5-HT1C receptor antagonist, and xylamidine, a 5-HT antagonist which has poor access to the brain. Metergoline (1 mg/kg) but not ritanserin (0.5 mg/kg) or xylamidine (3 mg/kg) blocked the effect of 2.5 mg/kg DF studied 2 and 4 h after injection. In a second experiment, we studied the ability of metergoline to antagonise the effect of DF, administered after a meal, on runway performance, food intake and gastric emptying assessed 4 h later. Metergoline at a dose of 1 mg/kg did not antagonise the effect of DF (2.5 mg/kg) on runway performance but completely blocked the effect on gastric emptying. The data clearly show that DF delays gastric emptying by indirectly activating 5-HT1 receptors; this effect is not important for the ability of DF to reduce runway performance and food intake when the drug is injected after a pre-feeding period. While there is evidence that DF hastens the termination of the meal by a 5-HT mechanism, the data suggest that DF may prolong the satiating effect of food during the post-absorptive phase by mechanisms other than 5-HT.

Effects of dexfenfluramine and opioid peptides, alone or in combination, on food intake and brain serotonin turnover in rats

Dexfenfluramine (d-FF) and opiate agonists both act on food intake but in opposite ways. Serotonin is known to be involved in the pharmacological action of both d-FF and opiates, but not necessarily in the feeding effect of the latter. In order to test this hypothesis, the effects of three opioid agonists, beta-endorphin, dynorphin and D-Ser2-Leu-Enk-Thr6 (DSLET) and of an antagonist, naltrexone, were investigated individually and in combination with d-FF on food intake and brain serotonin turnover. The opioid agonist-d-FF combinations generally produced a similar anorectic effect to that of d-FF alone, with the exception of DSLET which showed a reciprocal antagonism. The serotonergic effects varied according to the opioid tested, alone or in combination with d-FF. This does not allow to highlight a general pattern of serotonin involvement in the feeding effects of these peptides. However, all the treatments which decreased feeding (d-FF, naltrexone and the combinations dynorphin-d-FF and beta-endorphin-d-FF) displayed similar trends in hypothalamic serotonergic variations. This study evidences a role of serotonin in the feeding effect of opiates, although not similar for all of them. The use of d-FF provides a tool for assessing this involvement.

Reversal of a feeding-reward system by dexfenfluramine: neurochemical involvement

In addition to its anorectic properties, dexfenfluramine may inhibit some manifestations of feeding-related reward. We attempted to verify this effect by measuring paw-lick latency on the hot plate test in rats conditioned to expect a palatable food. The involvement of variations in beta-endorphinergic, dopaminergic and serotonergic systems was assessed. Despite an inherent effect of increasing paw-lick latency, dexfenfluramine (1.5 mg/kg IP) partly reversed the expectancy-induced increase in this latency. Saline-treated "expectant" rats displayed elevated plasma beta-endorphin levels and reduced hypothalamic 5-HIAA/5-HT and DOPAC/DA ratios. Only the decrease in the DOPAC/DA ratio was reversed by dexfenfluramine, suggesting an involvement of the dopaminergic system in this dexfenfluramine-sensitive reward system.

Studies on the role of 5-HT receptors in satiation and the effect of d-fenfluramine in the runway test

d-Fenfluranine has previously been shown to reduce food-rewarded runway behaviour in the rat, an effect thought to be mediated through activation of central 5-HT pathways. We now examined in more detail the mechanism by which d-fenfluramine reduces runway performance and food intake in the 15-trial runway test. The non-selective 5-HT receptor antagonist, metergoline (1.0 mg/kg), significantly antagonised the effect of d-fenfluramine (2.5 mg/kg) in the runway test. In contrast, neither ritanserin, the potent 5-HT2 and 5-HT1C receptor antagonist (0.5 mg/kg), nor the peripheral 5-HT receptor antagonist, xylamidine (3.0 mg/kg), attenuated the effect of d-fenfluramine in this test situation. Metergoline, but not ritanserin or xylamidine significantly increased runway performance when administered alone. These data indicate that d-fenfluramine reduces runway performance and food intake through activation of 5-HT1 receptors. In addition, blockade of 5-HT1 receptors can attenuate the development of satiation normally observed under control conditions in the runway test.