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

Neural mechanisms underlying the role of fructose in overfeeding

Fructose consumption has been linked with metabolic syndrome and obesity. Fructose-based sweeteners like high fructose corn syrup taste sweeter, improve food palatability, and are increasingly prevalent in our diet. The increase in fructose consumption precedes the rise in obesity and is a contributing driver to the obesity epidemic worldwide. The role of dietary fructose in obesity can be multifactorial by promoting visceral adiposity, hypertension, and insulin resistance. Interestingly, one emergent finding from human and animal studies is that dietary fructose promotes overfeeding. As the brain is a critical regulator of food intake, we reviewed the evidence that fructose can act in the brain and elucidated the major brain systems underlying fructose-induced overfeeding. We found that fructose acts on multiple interdependent brain systems to increase orexigenic drive and the incentive salience of food while decreasing the latency between food bouts and reducing cognitive control to disinhibit feeding. We concluded that the collective actions of fructose may promote feeding behavior by producing a hunger-like state in the brain.

The effects of morphine withdrawal and conditioned withdrawal on memory consolidation and c-Fos expression in the central amygdala

The current study tested the hypothesis that drug withdrawal contributes to the addiction cycle in part because of an action on memory consolidation. Hence, four experiments in male Sprague-Dawley rats compared the effects of precipitated morphine withdrawal and conditioned morphine withdrawal on the consolidation of object memory and on activation of c-Fos in the central nucleus of the amygdala (CeA). It was found that immediate, but not 6 h delayed, post sample administration of 3 mg/kg of naltrexone significantly enhanced object memory in rats maintained, or previously maintained, on 10 mg/kg/day of morphine via osmotic minipumps. To establish whether conditioned withdrawal could also alter object memory, a contextual conditioning procedure was employed whereby morphine-maintained (10 mg/kg/day) animals received naltrexone (3 mg/kg) in a distinctive context (CS+) and vehicle in a separate context (CS-) for 10 days. During conditioning in the CS+, naltrexone suppressed locomotor activity, caused a rapid body weight loss and increased frequency of wet dog shakes. Interestingly, confinement to this CS+ immediately, but not 6 h, after the sample phase, also enhanced object memory. Finally, posttraining naltrexone and exposure to the CS+ both induced significant expression of c-Fos in the CeA. Therefore, this study reports for the first time that both acute precipitated withdrawal and conditioned withdrawal can facilitate memory consolidation, possibly through a common neural pathway that involves the central amygdala.

Effects of high fructose corn syrup on ethanol self-administration in rats

OBJECTIVE

The addition of sweeteners to alcoholic beverages is thought to facilitate heavy alcohol consumption, and this may be of particular concern when the additive is high fructose corn syrup (HFCS).

METHODS

Four experiments in male Sprague-Dawley rats were performed to investigate whether the addition of 25% HFCS to ethanol (5%, 10%, and 20% v/v ethanol) would alter its intraoral operant self-administration, palatability, and sensitivity to food deprivation stress.

RESULTS

As anticipated, HFCS drastically increased ethanol intake, and this effect appeared driven by its caloric value. Importantly, HFCS increased the persistence of operant responding following extinction in animals trained to self-administer the combination, and the addition of HFCS to ethanol changed subsequent responses to ethanol, including increased palatability and intake.

CONCLUSIONS

These results in rats suggest that the addition of HFCS to the list of ingredients in sweetened alcoholic beverages could play a significant role in the harmful consumption of ethanol-containing beverages.

Opioid withdrawal and memory consolidation

It is well established that learning and memory are central to substance dependence. This paper specifically reviews the effect of opioid withdrawal on memory consolidation. Although there is evidence that opioid withdrawal can interfere with initial acquisition and retrieval of older memories, there are several reasons to postulate a facilitatory action on the consolidation of newly acquired memories. In fact, there is substantial evidence that memory consolidation is facilitated by the release of stress hormones, that it requires the activation of the amygdala, of central noradrenergic and cholinergic pathways, and that it involves long-term potentiation. This review highlights evidence that very similar neurobiological processes are involved in opioid withdrawal, and summarizes recent results indicating that naltrexone-precipitated withdrawal enhanced consolidation in rats. From this neurocognitive perspective, therefore, opioid use may escalate during the addiction cycle in part because memories of stimuli and actions experienced during withdrawal are strengthened.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

An exploration of the aversive properties of 2-deoxy-D-glucose in rats

Hypoglycemia can alter arousal and negatively impact mood. This study tests the hypothesis that acute drops in glucose metabolism cause an aversive state mediated by monoamine activity. In experiment 1, male Sprague-Dawley rats were either food deprived (FD) or pre-fed (PF) and tested on conditioned place avoidance (CPA; biased place conditioning design; 3 pairings drug/vehicle, each 30 min-long) induced by the glucose antimetabolite 2-deoxy-D-glucose (2-DG; 0, 300 or 500 mg/kg, SC). Locomotion and blood glucose were also assessed. Experiment 2 examined whether clonidine (noradrenergic α2 agonist, 0, 10 or 40 μg/kg, SC) or bupropion (monoamine reuptake blocker, 0, 10 or 30 mg/kg, SC) could alter CPA induced by 500 mg/kg 2-DG. In experiment 3, blood corticosterone (CORT) was measured in response to 500 mg/kg 2-DG, alone or in combination with 40 μg/kg clonidine or 30 mg/kg bupropion. Finally, experiment 4 controlled for possible place conditioning induced by 10 or 40 μg/kg clonidine, or 10 or 30 mg/kg bupropion injected without 2-DG. It was found that 2-DG increased blood glucose and produced a robust CPA. The feeding status of the animals modulated these effects, including CORT levels. Both clonidine and bupropion attenuated the effects of 2-DG on CPA and CORT, but only bupropion reversed suppression of locomotion. Taken together, these results in rats suggest that impaired glucose metabolism can negatively impact arousal and mood via effects on HPA and monoamine systems.

Effect of steady-state methadone on high fructose corn syrup consumption in rats

Patients undergoing methadone maintenance treatment self-report enhanced preferences for, and excessive consumption of, foods rich in sugar. However, it is unclear whether these are direct pharmacological effects of methadone or the consequences of metabolic dysfunctions induced by addiction to illicit opiates. Hence, the current study in drug-naïve male Sprague-Dawley rats explored the effects of steady-state methadone delivered by osmotic mini-pumps (13 days; 0, 10, 30 mg/kg/day) on consumption of rat chow and a palatable, sweet, liquid high fructose corn syrup solution. Six days after the removal of the pumps, mRNA expression of genes involved in responses to stress and rewards were quantified: pro-opiomelanocortin in the hypothalamus, mu-opioid receptor in the nucleus accumbens, and dopamine D2 receptor in the dorsal striatum. Taste reactivity and locomotion tests were also performed throughout the study. It was found that methadone increased caloric intake from high fructose corn syrup and reduced caloric intake from chow, effects that could not be directly ascribed to changes in high fructose corn syrup taste reactivity or motor functions. However, the changes in caloric intake displayed significant tolerance, and mRNA expression analysis suggested that methadone attenuated the effect of high fructose corn syrup on pro-opiomelanocortin mRNA, and possibly on dopamine D2 receptor mRNA. These findings in rats suggest that the pharmacological effect of methadone, administered to achieve steady-state maintenance, may not be the primary cause of dietary alterations reported by patients maintained on methadone.

The combination of escitalopram and aripiprazole: Investigation of psychomotor effects in rats

Pre-clinical and clinical evidence suggests that the antidepressant efficacy of the selective serotonin reuptake inhibitor escitalopram can be enhanced by the dopamine and serotonin partial agonist aripiprazole. Given the range of possible neurochemical interactions between these drugs, the current study investigated whether aripiprazole alters the hedonic and psychomotor effects of escitalopram. Male Sprague Dawley rats ( n=116) received 10 mg/kg/day escitalopram (subcutaneous), 2 mg/kg/day aripiprazole (subcutaneous), or combined aripiprazole + escitalopram, and were tested for consumption of incentive nutritional stimuli (high-fructose corn syrup and chow), stereotypy and locomotor activity. At the conclusion of behavioral testing, mRNAs of two genes involved in reward processes were quantified: hypothalamic pro-opiomelanocortin and hippocampal brain-derived neurotrophic factor. Escitalopram produced a selective, but temporary, decrease in high fructose corn syrup consumption that was not altered by aripiprazole co-administration. Escitalopram had no significant effect on locomotion, but aripiprazole co-administration produced a persistent increase in stereotypy. Both brain-derived neurotrophic factor and pro-opiomelanocortin mRNA levels were lower in the aripiprazole + escitalopram group relative to the escitalopram group. Taken together, these results suggest that aripiprazole may enhance the antidepressant efficacy of escitalopram through improvement of psychomotor functions.