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

A 40-year analysis of central neuroanatomical and neurochemical circuits mediating homeostatic intake and hedonic intake and preferences in rodents

This perspective review was written in response to the celebration of the 60th anniversary of the journal, Brain Research, and covers the evolving focus of my laboratory's work over 40 years in the neurobiological substrates of ingestive behavior in rodents. Following our initial work examining the effects of systemic and ventricular administration of general and selective opioid receptor agonists and antagonists on food intake under spontaneous, deprivation, glucoprivic and hedonic conditions, my laboratory in close collaboration with Drs. Gavril Pasternak and Ying-Xian Pan utilized an antisense oligodoxynucleotide knock-down technique affecting MOR-1, DOR-1, KOR-1 and ORL-1 genes as well as against G-protein subunits to study receptor mediation of opioid receptor agonist-induced feeding as well as feeding following regulatory challenges. Our laboratory employed intracerebral microinjection techniques to map limbic nucleus accumbens and ventral tegmental area central brain circuits mediating homeostatic and hedonic feeding responses through the use of selective mu, delta1, delta2 and kappa opioid receptor subtype agonists in combination with general and selective opioid, dopamineric, glutamatergic and GABAergic antagonists administered into the same site or the reciprocal site, allowing for the identification of a distributed brain network mediating these ingestive effects. Our laboratory in close collaboration with Dr. Anthony Sclafani then focused on the pharmacological, neuroanatomical and learning mechanisms related to the development of sugar- (sucrose, glucose and fructose) and fat- (corn oil) conditioned flavor preferences (CFP) in rats, and on murine genetic variance in food intake, preferences and the process of appetition.

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.

Alterations of naltrexone-induced conditioned place avoidance by pre-exposure to high fructose corn syrup or heroin in Sprague-Dawley rats

RATIONALE

It has been suggested that withdrawal from sugar produces a set of symptoms that resemble those observed following withdrawal from opiate drugs.

OBJECTIVES

This study explored naltrexone-induced withdrawal in animals pre-exposed to acute, chronic, and intermittent high fructose corn syrup (HFCS) or acute and chronic heroin administration.

METHODS

Experiment 1 examined conditioned place avoidance (CPA) induced by different doses of naltrexone (0.01-1 mg/kg) in naïve male Sprague-Dawley rats. In experiment 2, rats received continuous or intermittent home cage HFCS access (0 or 50 %) prior to conditioning with 1 mg/kg naltrexone. In experiment 3, HFCS ingestion was increased by food restriction and rats were conditioned with 3 mg/kg naltrexone. In experiment 4, the timing and quantity of HFCS ingestion (0, 0.5, 1, 2 g/kg) was controlled by intragastric administration, and rats were conditioned with 1 mg/kg naltrexone. In experiment 5, rats received acute (2 mg/kg) or chronic heroin (3.5 mg/kg/day) prior to conditioning with 1 mg/kg naltrexone.

RESULTS

Administration of naltrexone produced moderate conditioned place avoidance in naïve rats. Importantly, acute, continuous, and intermittent HFCS pre-exposure did not significantly amplify this effect, but acute and chronic heroin pre-exposure did.

CONCLUSIONS

As assessed by CPA, these results in rats fail to support the hypothesis that an opioid antagonist can precipitate similar affective withdrawal states following pre-exposure to sugars and opiates.

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.

Combined dieting and stress evoke exaggerated responses to opioids in binge-eating rats

The authors developed an animal model of binge eating where history of caloric restriction with footshock stress (R + S) causes rats to consume twice the normal amount of palatable food. The authors tested the hypothesis that binge eating is mediated by changes in opioid control of feeding by comparing rats' anorectic and orexigenic responses to naloxone and butorphanol, respectively, and by testing the ability of butorphanol to elicit binge eating of chow when palatable food was absent. Mu/kappa opioid-receptor blockade and activation had exaggerated responses in the R + S rats with naloxone suppressing binge eating to control levels, and although butorphanol did not trigger chow binge eating, it enhanced binge eating of palatable food. These responses in sated normal-weight rats strengthen evidence that reward, over metabolic need, drives binge eating.

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.

Family History of Alcoholism and Response to Sweets

BACKGROUND

The relationship between a hedonic response to sweet tastes and a propensity to excessive alcohol drinking is supported by both animal and human studies. This study was designed to test the hypothesis that the genetic risk for alcoholism as measured by a paternal history of alcoholism in young social drinkers is associated with sweet-liking, defined as rating the strongest offered sucrose solution (i.e., 0.83 M) as the most palatable during the standard sweet test.

METHODS

Participants were 163 subjects (39% male) without a lifetime history of alcohol or drug abuse or dependence. Eighty-one subjects had a paternal history of alcoholism (FH+), and 82 did not (FH-). Each subject rated a series of sucrose solutions for intensity of sweetness and palatability. Subjects were categorized as sweet-likers if they rated the highest sucrose concentration as the most pleasurable.

RESULTS

The estimated odds of being a sweet-liker were 2.5 times higher for FH+ than for FH- subjects. FH+ subjects disliked the tastes of the two weakest offered sucrose concentrations (0.05 and 0.10 M), whereas FH- subjects reported these tastes to be neutral.

CONCLUSIONS

The results of this study support the hypothesis that sweet-liking is associated with a genetic vulnerability to alcoholism.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.