Effects of acute and chronic morphine on food intake in rats: modulation by oxytocin and vasopressin

Pain-suppressed consumption of highly palatable liquid in rats

This study determined whether consumption of a highly palatable liquid is a reliable measure of inflammatory pain and antinociception in male and female rats. After a 10-day acquisition period, the impact of intraplantar oil vs. complete Freund adjuvant (CFA) on consumption of vanilla-flavored Ensure was assessed, with a sipper tube height 12 or 19 cm above the floor. CFA significantly decreased Ensure consumption, which completely recovered within 4-7 days to levels in oil-treated controls; neither sex nor sipper tube height significantly influenced Ensure consumption. CFA also significantly suppressed Ensure consumption in rats not exposed to the 10-day acquisition period, but only in males. To test the predictive validity of Ensure consumption as a measure of pain, separate rats were pretreated with a vehicle, an opioid, a nonsteroidal anti-inflammatory drug, or a cannabinoid the day after CFA treatment. Morphine and ibuprofen significantly attenuated CFA-suppressed drinking in at least one sex, and tetrahydrocannabinol did not. Neither ibuprofen nor tetrahydrocannabinol significantly altered drinking in oil-injected, 'pain-free' controls, but morphine increased drinking. These results demonstrate that CFA decreases consumption of a highly palatable liquid regardless of previous exposure (training) to the consumption procedure, but only in males. Although standard analgesics attenuate CFA-suppressed drinking, nonspecific hyperphagic effects can confound the interpretation of results. Thus, consumption of a highly palatable liquid is not an optimal measure for candidate analgesic screening.

Sucrose intake by rats affected by both intraperitoneal oxytocin administration and time of day

RATIONALE

Daily limited access to palatable food or drink at a fixed time is commonly used in rodent models of bingeing. Under these conditions, entrainment may modulate intake patterns. Oxytocin is involved in circadian patterns of intake and, when administered peripherally, reduces sucrose intake. However, oxytocin's effects on intake under limited-access conditions and its potential interaction with entrainment have not been explored.

OBJECTIVES

This study examined the role of entrainment on intake patterns, oxytocin's effects on sucrose intakes and locomotor activity and whether oxytocin's effects were mediated by its actions at the oxytocin receptor.

METHODS

Sated rats received daily 1-h access to 10% sucrose solution either at a fixed or varied time of day. Rats received intraperitoneal oxytocin (0 mg/kg, 0.3 mg/kg, 1 mg/kg, 3 mg/kg) prior to sucrose access, and spontaneous locomotor activity was assessed in an open-field test. Rats were then pre-treated with an oxytocin receptor antagonist, L368,899, prior to oxytocin before sucrose access.

RESULTS

Intake patterns did not differ between fixed- or varied-time presentations; rats consumed more sucrose solution in the middle as opposed to the early-dark phase. Oxytocin dose-dependently reduced sucrose intakes, but also reduced locomotor activity. There was some evidence of partial blockade of oxytocin-induced sucrose intake reductions by L368,899, but the results were unclear.

CONCLUSIONS

Time of day and oxytocin impact sucrose solution intake under daily limited access in rats and the sedative-like effects of oxytocin should be considered in future studies on oxytocin and food intake.

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.

Sex differences in hedonic and homeostatic aspects of palatable food motivation

Feeding behaviors can be modified via homeostatic and hedonic mechanisms. Homeostasis, while primarily concerned with maintaining energy balance via food consumption and energy expenditure, can alter food reward and motivation in response to food deprivation. Alternatively, reward and motivation of food is also driven by its palatability or hedonic nature, and this process can be augmented by opioid receptor activation. The present study examined sex differences in the motivational properties of sucrose pellets through manipulation of homeostatic and hedonic processes via acute food deprivation and acute systemic administration of morphine, respectively. The results showed that regardless of sex, systemic injections of morphine did not alter the motivation to obtain a sucrose pellet on a progressive ratio schedule of reinforcement but does significantly increase consumption of sucrose pellets when freely available. Male and female rats demonstrated similar increased consumption of sucrose pellets under free feeding conditions following acute (24-hours) food deprivation, compared to the non-deprived conditions. Overall, the findings from these experiments indicate that female rats work harder in order to obtain a sucrose pellet (under a Progressive Ratio (PR) schedule of reinforcement) and consume more sucrose pellets than males. However, while acute morphine administration causes similar increases on feeding in males and females, it does not alter motivation as measured by breakpoint on a PR schedule of reinforcement.

Chronic sugar intake dampens feeding-related activity of neurons synthesizing a satiety mediator, oxytocin

Increased tone of orexigens mediating reward occurs upon repeated consumption of sweet foods. Interestingly, some of these reward orexigens, such as opioids, diminish activity of neurons synthesizing oxytocin, a nonapeptide that promotes satiety and feeding termination. It is not known, however, whether consumption-related activity of the central oxytocin system is modified under chronic sugar feeding reward itself. Therefore, we examined how chronic consumption of a rewarding high-sucrose (HS) vs. bland cornstarch (CS) diet affected the activity of oxytocin cells in the hypothalamus at the time of meal termination. Schedule-fed (2h/day) rats received either a HS or CS powdered diet for 20 days. On the 21st day, they were given the same or the opposite diet, and food was removed after the main consummatory activity was completed. Animals were perfused 60 min after feeding termination and brains were immunostained for oxytocin and the marker of neuronal activity, c-Fos. The percentage of c-Fos-positive oxytocin cells in the hypothalamic paraventricular nucleus was significantly lower in rats chronically exposed to the HS than to the CS diet, regardless of which diet they received on the final day. A similar pattern was observed in the supraoptic nucleus. We conclude that the chronic rather than acute sucrose intake reduces activity of the anorexigenic oxytocin system. These findings indicate that chronic consumption of sugar blunts activity of pathways that mediate satiety. We speculate that a reduction in central satiety signaling precipitated by regular intake of foods high in sugar may lead to generalized overeating.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

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.

MDMA and performance under a progressive-ratio schedule of water delivery

This study investigated the possible motivational effects of (+-)3,4-methylenedioxymethamphetamine (MDMA, ecstasy) in water-deprived rats responding under a progressive-ratio 2 schedule of water delivery. Lower doses (1.0 and 1.8 mg/kg i.p.) had inconsistent effects on breakpoints and response rates, whereas higher doses (3.2 and 5.6 mg/kg ip) significantly decreased both response measures relative to vehicle control levels. Increasing the level of water restriction significantly increased both response measures, and decreasing restriction significantly decreased both response measures. This study found no evidence that MDMA increased the reinforcing efficacy of water, although prior findings have suggested that the drug might have such an effect. MDMA-induced changes in motor activity may account for the present results.

Acute and chronic effects of morphine under a progressive-ratio 25 schedule of food delivery

The present study examined the effects of morphine in pigeons responding under a progressive-ratio 25 schedule of food delivery. Morphine initially reduced response rates and breaking points. With chronic exposure, tolerance developed to these effects. The magnitude of the observed tolerance was not obviously different from that previously reported under a PR 5 schedule of food delivery. In addition, when drug effects were compared under the fixed-ratio 25 and fixed-ratio 100 components comprised by the progressive-ratio schedule, comparable tolerance was observed. Although prior studies using other procedures have shown that ratio size modulates the development of tolerance to morphine and other drugs, the present data suggest that this relation is constrained, and is not easily observed under progressive-ratio schedules.

Dopaminergic and opiate agonists and antagonists differentially decrease multiple schedule responding maintained by sucrose/ethanol and sucrose

Similar neurobiological mechanisms are hypothesized to influence ethanol- and food-related reinforcement processes. This study examined the ability of compounds with dopaminergic or opiate activity to selectively alter responding maintained by a sucrose/ethanol solution in comparison to a sucrose solution. Long-Evans rats were trained to press a lever using 5% sucrose/10% ethanol and 5% sucrose as the reinforcers on a multiple Fixed Ratio 4 Fixed Ratio 4 schedule of reinforcement. When stable responding was established, the effects of intraperitoneally administered amphetamine (0.0-3.0 mg/kg), haloperidol (0.0-1.0 mg/kg), morphine (0.0-10.0 mg/kg), and naloxone (0.0-10.0 mg/kg) were examined on total session reinforcer presentation and presentation of each reinforcer within individual multiple schedule components. Prior to drug treatment, the total number of reinforcer presentations of the sucrose/ethanol solution was significantly greater than sucrose reinforcer presentations, suggesting the sucrose/ethanol solution was a more efficacious reinforcer. All agents administered decreased responding maintained by sucrose/ethanol and sucrose. The dose-effect curves for sucrose/ethanol were shifted to the left compared to sucrose, suggesting that although the compounds did not selectively impact sucrose/ethanol-maintained responding, sucrose/ethanol-maintained responding was more sensitive to the effects of these compounds.

Interactions between vasopressin and mu-opioid systems in the rat fetus

Administration of arginine-vasopressin (AVP) into the cisterna magna (IC injection) of the E20 rat fetus increases motor activity and promotes expression of rare patterns of behavior including mouthing, licking, and facial wiping. These effects are mediated by V1 receptors in the brain stem and spinal cord. In this study, AVP-induced changes in motor behavior were measured to characterize interactions within the AVP system and between the AVP and mu-opioid systems in the fetal rat. Injection of AVP into the brain hemispheres (IH injection) diminished the effects of an IC injection of AVP. AVP effects were potentiated by blockade of hemispheric V1 receptors, suggesting that hemispheric V1 receptors inhibit V1 receptor-containing neurons in the brain stem and spinal cord. Intracisternal injection of the mu agonist DAGO suppressed the effects of AVP whereas blockade of mu-opioid receptors in the brain stem and spinal cord with CTOP and activation of mu receptors in the hemispheres with DAGO potentiated the behavioral effects of AVP. The behavioral effects of AVP are mediated by V1 receptors in the brain stem and spinal cord and may be under the inhibitory control of a mu-opioid system localized at the same level of the brain. Facilitation of AVP effects following IH injection of DAGO may involve an inhibition of the inhibitory effects of V1 receptor-containing neurons located in the hemispheres. Interactions between mu-opioid and AVP systems in the caudal and rostral portions of the fetal brain may be based on a common principle.

Acute and chronic effects of morphine in pigeons responding under a progressive-ratio schedule of food delivery

Although progressive-ratio schedules have often been used by behavioral pharmacologists to index the relative reinforcing effects of drugs of abuse, they have been ignored in the study of tolerance to opioids. The present study examined tolerance to morphine in pigeons responding under a progressive-ratio 5 schedule of food delivery. Acute administrations of morphine produced general dose-dependent reductions in response rates and breaking points. Dose-response curves for both measures shifted rightward substantially (roughly fivefold) following chronic (daily) exposure to morphine, indicating that tolerance developed to the drug's effects.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.