Opioids and consummatory behavior

Morphine reduces the interest for natural rewards

RATIONALE

Alongside a pathological, excessive, motivation for substances of abuse, substance use disorder (SUD) patients often show a dramatic loss of interest for naturally rewarding activities, such as positive peer social interaction and food intake. Yet, pre-clinical evidence of the latter SUD features remains scarce and inconsistent.

OBJECTIVES

In the current study, we investigated the effect of non-rewarding and rewarding doses of morphine upon social behaviour, motivation for and intake of palatable food, in male and female C57BL/6J mice.

METHODS

First, the rewarding effects of two relatively low morphine doses (1.25 and 2.5 mg/kg) were assessed using a newly established single substance administration/conditioning trial conditioned place preference (CPP) paradigm. Then, morphine (1.25 and 2.5 mg/kg) effects upon social behaviour, motivation for and intake of palatable food were examined by the three-chamber (3-CH), an operant behaviour and a palatable food preference test, respectively.

RESULTS

Morphine (2.5 mg/kg) induced CPP in both male and female mice, whereas morphine (1.25 mg/kg) induced CPP only in female mice. Both morphine doses (1.25 and 2.5 mg/kg) reduced sociability, motivation for and intake of palatable food in male and female mice, independently of cognitive function or locomotor activity.

CONCLUSIONS

Female mice were more sensitive than male mice to the rewarding effects of morphine. Moreover, both a non-rewarding and a rewarding dose of morphine impaired the interest for naturally rewarding activities, indicating that brain reward systems might be more sensitive to the deleterious than to the rewarding effects of substances of abuse.

Roles of β-Endorphin in Stress, Behavior, Neuroinflammation, and Brain Energy Metabolism

β-Endorphins are peptides that exert a wide variety of effects throughout the body. Produced through the cleavage pro-opiomelanocortin (POMC), β-endorphins are the primarily agonist of mu opioid receptors, which can be found throughout the body, brain, and cells of the immune system that regulate a diverse set of systems. As an agonist of the body's opioid receptors, β-endorphins are most noted for their potent analgesic effects, but they also have their involvement in reward-centric and homeostasis-restoring behaviors, among other effects. These effects have implicated the peptide in psychiatric and neurodegenerative disorders, making it a research target of interest. This review briefly summarizes the basics of endorphin function, goes over the behaviors and regulatory pathways it governs, and examines the variability of β-endorphin levels observed between normal and disease/disorder affected individuals.

Operant Social Reward Decreases Incubation of Heroin Craving in Male and Female Rats

BACKGROUND

We recently reported that operant social choice-induced voluntary abstinence prevents incubation of methamphetamine craving. Here, we determined whether social choice-induced voluntary abstinence would prevent incubation of heroin craving. We also introduce a fully automatic social reward self-administration model that eliminates the intense workload and rat-human interaction of the original semiautomatic model.

METHODS

In experiment 1, we trained male and female rats for social self-administration (6 days) and then for heroin self-administration (12 days). Next, we assessed relapse to heroin seeking after 1 and 15 abstinence days. Between tests, the rats underwent either forced or social choice-induced abstinence. In experiment 2, we developed a fully automatic social self-administration procedure by introducing a screen between the self-administration chamber and the social-peer chamber; the screen allows physical contact but prevents rats from crossing chambers. Next, we compared incubation of craving in rats with a history of standard (no-screen) or automatic (screen) social self-administration and social choice-induced abstinence.

RESULTS

The time-dependent increase in heroin seeking after cessation of drug self-administration (incubation of craving) was lower after social choice-induced abstinence than after forced abstinence. There were no differences in social self-administration, social choice-induced abstinence, and incubation of craving in rats trained in the standard semiautomatic procedure versus the novel fully automatic procedure.

CONCLUSIONS

Our study demonstrates the protective effect of rewarding social interaction on heroin self-administration and incubation of heroin craving and introduces a fully automatic social self-administration and choice procedure to investigate the role of volitional social interaction in drug addiction and other psychiatric disorders.

The Behavioral Effects of the Antidepressant Tianeptine Require the Mu-Opioid Receptor

Depression is a debilitating chronic illness that affects around 350 million people worldwide. Current treatments, such as selective serotonin reuptake inhibitors, are not ideal because only a fraction of patients achieve remission. Tianeptine is an effective antidepressant with a previously unknown mechanism of action. We recently reported that tianeptine is a full agonist at the mu opioid receptor (MOR). Here we demonstrate that the acute and chronic antidepressant-like behavioral effects of tianeptine in mice require MOR. Interestingly, while tianeptine also produces many opiate-like behavioral effects such as analgesia and reward, it does not lead to tolerance or withdrawal. Furthermore, the primary metabolite of tianeptine (MC5), which has a longer half-life, mimics the behavioral effects of tianeptine in a MOR-dependent fashion. These results point to the possibility that MOR and its downstream signaling cascades may be novel targets for antidepressant drug development.

Incubation of Methamphetamine but not Heroin Craving After Voluntary Abstinence in Male and Female Rats

We recently introduced an animal model of incubation of methamphetamine craving after choice-based voluntary abstinence in male rats. Here we studied the generality of this phenomenon to (1) female rats, and (2) male and female rats with a history of heroin self-administration. We first trained rats to self-administer palatable food pellets for 6 days (6 h per day) for either methamphetamine (0.1 mg/kg/infusion) or heroin (0.1 mg/kg/infusion) for 12 days (6 h/day). We then assessed relapse to drug seeking under extinction conditions after 1 and 21 abstinence days. Between tests, the rats underwent either voluntary abstinence (achieved via a discrete choice procedure between drug and palatable food; 20 trials/day) or home-cage forced abstinence. We found no sex differences in methamphetamine self-administration or in the strong preference for the palatable food over methamphetamine during the choice-based voluntary abstinence. In both sexes, methamphetamine seeking in the relapse tests was higher after 21 days of either voluntary or forced abstinence than after 1 day (incubation of methamphetamine craving). We also found no sex differences in heroin self-administration or the strong preference for the palatable food over heroin during the choice-based voluntary abstinence. However, male and female rats with a history of heroin self-administration showed incubation of heroin craving after forced but not voluntary abstinence. Our results show that incubation of methamphetamine craving after voluntary abstinence generalizes to female rats. Unexpectedly, prolonged voluntary abstinence prevented the emergence of incubation of heroin craving in both sexes.

Naloxone Decreases Responding for Conditioned Reinforcement in Rats

That opioids can mediate unconditioned reinforcement is well established, but there is little evidence indicating whether they modify conditioned reinforcement. Here, a tone which initially served as a discriminative stimulus for the availability of water reinforcement was established as a conditioned stimulus. When later given a choice between pressing a lever producing the tone (but not water) or one which produced no effect, rats chose the tone-producing lever 66% of the time. Naloxone (3.0 mg/kg) reduced overall responding and completely eliminated the preference for the tone-producing lever. Results are briefly discussed in terms of the importance of understanding mechanisms serving conditioned reinforcement.

Inflated reward value in early opiate withdrawal

Through incentive learning, the emotional experience of a reward in a relevant need state (e.g. hunger for food) sets the incentive value that guides the performance of actions that earn that reward when the need state is encountered again. Opiate withdrawal has been proposed as a need state in which, through experience, opiate value can be increased, resulting in escalated opiate self-administration. Endogenous opioid transmission plays anatomically dissociable roles in the positive emotional experience of reward consumption and incentive learning. We, therefore, sought to determine if chronic opiate exposure and withdrawal produces a disruption in the fundamental incentive learning process such that reward seeking, even for non-opiate rewards, can become maladaptive, inconsistent with the emotional experience of reward consumption and irrespective of need. Rats trained to earn sucrose or water on a reward-seeking chain were treated with morphine (10-30 mg/kg, s.c.) daily for 11 days prior to testing in withdrawal. Opiate-withdrawn rats showed elevated reward-seeking actions, but only after they experienced the reward in withdrawal, an effect that was strongest in early (1-3 days), as opposed to late (14-16 days), withdrawal. This was sufficient to overcome a negative reward value change induced by sucrose experience in satiety and, in certain circumstances, was inconsistent with the emotional experience of reward consumption. Lastly, we found that early opiate withdrawal-induced inflation of reward value was blocked by inactivation of basolateral amygdala mu opioid receptors. These data suggest that in early opiate withdrawal, the incentive learning process is disrupted, resulting in maladaptive reward seeking.

Toward a radically embodied neuroscience of attachment and relationships

Attachment theory (Bowlby, 1969/1982) posits the existence of internal working models as a foundational feature of human bonds. Radical embodied approaches instead suggest that cognition requires no computation or representation, favoring a cognition situated in a body in an environmental context with affordances for action (Chemero, 2009; Barrett, 2011; Wilson and Golonka, 2013; Casasanto and Lupyan, 2015). We explore whether embodied approaches to social soothing, interpersonal warmth, separation distress, and support seeking could replace representational constructs such as internal working models with a view of relationship cognition anchored in the resources afforded to the individual by their brain, body, and environment in interaction. We review the neurobiological bases for social attachments and relationships and attempt to delineate how these systems overlap or don’t with more basic physiological systems in ways that support or contradict a radical embodied explanation. We suggest that many effects might be the result of the fact that relationship cognition depends on and emerges out of the action of neural systems that regulate several clearly physically grounded systems. For example, the neuropeptide oxytocin appears to be central to attachment and pair-bond behavior (Carter and Keverne, 2002) and is implicated in social thermoregulation more broadly, being necessary for maintaining a warm body temperature (for a review, see IJzerman et al., 2015b). Finally, we discuss the most challenging issues around taking a radically embodied perspective on social relationships. We find the most crucial challenge in individual differences in support seeking and responses to social contact, which have long been thought to be a function of representational structures in the mind (e.g., Baldwin, 1995). Together we entertain the thought to explain such individual differences without mediating representations or computations, but in the end propose a hybrid model of radical embodiment and internal representations.

Alterations in phosphorylated CREB expression in different brain regions following short- and long-term morphine exposure: relationship to food intake

BACKGROUND

Activation of the cyclic adenosine monophosphate (cAMP)/phosphorylated CREB (P-CREB) system in different brain regions has been implicated in mediating opioid tolerance and dependence, while alteration of this system in the lateral hypothalamus (LH) has been suggested to have a role in food intake and body weight.

METHODS

Given that opioids regulate food intake, we measured P-CREB in different brain regions in mice exposed to morphine treatments designed to induce different degrees of tolerance and dependence.

RESULTS

We found that a single morphine injection or daily morphine injections for 8 days did not influence P-CREB levels, while the escalating dose of morphine regimen raised P-CREB levels only in the ventral tegmental area (VTA). Chronic morphine pellet implantation for 7 days raised P-CREB levels in the LH, VTA, and dorsomedial nucleus of the hypothalamus (DM) but not in the nucleus accumbens and amygdala. Increased P-CREB levels in LH, VTA, and DM following 7-day treatment with morphine pellets and increased P-CREB levels in the VTA following escalating doses of morphine were associated with decreased food intake and body weight.

CONCLUSION

The morphine regulation of P-CREB may explain some of the physiological sequelae of opioid exposure including altered food intake and body weight.

Polydrug abuse: a review of opioid and benzodiazepine combination use

This paper reviews studies examining the pharmacological interactions and epidemiology of the combined use of opioids and benzodiazepines (BZDs). A search of English language publications from 1970 to 2012 was conducted using PubMed and PsycINFO(®). Our search found approximately 200 articles appropriate for inclusion in this paper. While numerous reports indicate that the co-abuse of opioids and BZDs is ubiquitous around the world, the reasons for the co-abuse of these medications are not entirely clear. Though the possibility remains that opioid abusers are using BZDs therapeutically to self-medicate anxiety, mania or insomnia, the data reviewed in this paper suggest that BZD use is primarily recreational. For example, co-users report seeking BZD prescriptions for the purpose of enhancing opioid intoxication or "high," and use doses that exceed the therapeutic range. Since there are few clinical studies investigating the pharmacological interaction and abuse liability of their combined use, this hypothesis has not been extensively evaluated in clinical settings. As such, our analysis encourages further systematic investigation of BZD abuse among opioid abusers. The co-abuse of BZDs and opioids is substantial and has negative consequences for general health, overdose lethality, and treatment outcome. Physicians should address this important and underappreciated problem with more cautious prescribing practices, and increased vigilance for abusive patterns of use.

Endogenously released opioids mediate meal-induced gastric relaxation via peripheral mu-opioid receptors

BACKGROUND

The centrally acting mu-opioid receptor antagonist naloxone inhibits meal-induced gastric accommodation.

AIM

To study the role of peripheral mu-opioid receptors in the regulation of gastric tone and food intake by comparing the effects of naloxone with the peripherally restricted mu-opioid receptor antagonist methylnaltrexone.

METHODS

Methylnaltrexone (12 mg s.c.), naloxone (20 μg/kg/h intravenous infusion after 0.4 mg bolus) and placebo were studied in 23 healthy volunteers. Gastric volume was recorded using an intragastric bag held at constant pressure connected to a barostat, with administration of a nutrient drink after 30 min. Pressure in the stomach was measured during intragastric nutrient drink infusion until the volunteers scored maximal satiation.

RESULTS

Methylnaltrexone inhibited significantly the volume increase after food intake as assessed with the barostat (P < 0.01). During nutrient drink infusion the intragastric pressure significantly decreased as compared with the preprandial pressure after placebo treatment. Both methylnaltrexone and naloxone significantly inhibited this intragastric pressure decrease (P < 0.001 and P < 0.05, respectively). Volunteers scored maximal satiation after 979 ± 96, 958 ± 84 and 1124 ± 107 mL nutrient drink infused (for naloxone, methylnaltrexone and placebo treatment, respectively; P < 0.05).

CONCLUSIONS

These results indicate that endogenous opioids mediate gastric accommodation and satiation via peripheral mu-opioid receptors. Effects were less pronounced after naloxone treatment, which indicates that centrally involved mu-opioid receptors mediate an opposing effect.

Differential expression and sensitivity of presynaptic and postsynaptic opioid receptors regulating hypothalamic proopiomelanocortin neurons

Hypothalamic proopiomelanocortin (POMC) neurons release the endogenous opioid beta-endorphin and POMC neuron activity is inhibited by opioids, leading to the proposal that beta-endorphin acts to provide feedback inhibition. However, both intrinsic properties and synaptic inputs contribute to the regulation of POMC neurons such that attributing an autoregulatory role to opioids must include consideration of opioid receptor localization and sensitivity at both presynaptic and postsynaptic sites. In the present study, whole-cell recordings were made in POMC cells in mouse brain slices and the presynaptic and postsynaptic regulation of POMC neurons was examined using selective agonists for mu, kappa, and delta opioid receptors. Activation of mu, but not kappa or delta, receptors induced a direct postsynaptic outward current. Agonists for each of the receptors inhibited the frequency of spontaneous IPSCs. Mu and kappa, but not delta, agonists reduced the amplitude of evoked IPSCs and appeared to colocalize in a significant portion of GABAergic terminals onto POMC neurons. The presynaptic inhibition caused by the mu agonist DAMGO had an EC(50) of 80 nM, whereas the EC(50) was 350 nM when measuring the postsynaptic outward current. This differential sensitivity adds an unexpected component of opioid-dependent feedback regulation, where low levels of opioid receptor activation would likely disinhibit POMC neuron activity and higher concentrations would result in an overall inhibition. The results may help explain why it has been difficult to clearly discern the role that opioids play in the regulation of food intake and other processes involving POMC neurons.

Gene expression profiling following short-term and long-term morphine exposure in mice uncovers genes involved in food intake

Addictive drugs including opioids activate signal transduction pathways that regulate gene expression in the brain. However, changes in CNS gene expression following morphine exposure are poorly understood. We determined changes in gene expression following short- and long-term morphine treatment in the hypothalamus and pituitary using genome-wide DNA microarray analysis and confirmed those alterations in gene expression by real-time reverse transcriptase polymerase chain reaction (RT-PCR) analysis. In the hypothalamus, short-term morphine administration up-regulated (at least twofold) 39 genes and down-regulated six genes. Long-term morphine treatment up-regulated 35 genes and down-regulated 51 genes. In the pituitary, short-term morphine administration up-regulated 110 genes and down-regulated 29 genes. Long-term morphine treatment up-regulated 85 genes and down-regulated 37 pituitary genes. Microarray analysis uncovered several genes involved in food intake (neuropeptide Y, agouti-related protein, and cocaine and amphetamine-regulated transcript) whose expression was strongly altered by morphine exposure in either the hypothalamus or pituitary. Subsequent RT-PCR analysis confirmed similar regulation in expression of these genes in the hypothalamus and pituitary. Finally, we found functional correlation between morphine-induced alterations in food intake and regulation of genes involved in this process. Changes in genes related to food intake may uncover new pathways related to some of the physiological effects of opioids.

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.

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.

Changes in mouse mu opioid receptor Exon 7/8-like immunoreactivity following food restriction and food deprivation in rats

Opioid agonists and antagonists respectively increase and decrease food intake. That selective mu opioid antagonists are more effective than antisense probes directed against the mu opioid receptor (MOR-1) gene in reducing deprivation-induced feeding suggests a role for isoforms. Both food restriction and deprivation alter protein and mRNA levels of opioid peptides and receptors. Antisera directed against Exon 4 of the MOR-1-like immunoreactivity (LI) (Exon 4) clone or directed against mouse Exons 7/8 (mE7/8-LI) revealed high levels of immunoreactivity in brain nuclei related to feeding behavior. Therefore, the present study assessed MOR-1LI and mE7/8-LI in hypothalamic and extrahypothalamic sites in rats exposed to ad libitum feeding, food restriction (2, 7, 14 days), or food deprivation (24, 48 h). MOR-1-LI displayed robust reactivity, but was insensitive to food restriction or deprivation. mE7/8-LI, both in terms of cell counts and relative optical density, was significantly and selectively increased in the dorsal and ventral parvocellular subdivisions of the hypothalamic paraventricular nucleus in food-restricted (14 days) rats, but all other restriction or deprivation regimens were ineffective in other hypothalamic nuclei. In contrast, significant and site-specific decreases in relative optical density in the rostral part of the nucleus tractus solitarius (NTS) were observed in food-restricted (2, 7 days) or food-deprived (24, 48 h) animals, but these regimens were ineffective in the other extrahypothalamic sites. This study indicates the sensitivity of this mE7/8-LI probe in the hypothalamic parvocellular paraventricular nucleus and rostral NTS to food restriction and deprivation in rats.

Activation of delta-opioid receptors reduces excitatory input to putative gustatory cells within the nucleus of the solitary tract

The rostral nucleus of the solitary tract (NST) is the first central relay in the gustatory pathway and plays a key role in processing and modulation of gustatory information. Here, we investigated the effects of opioid receptor agonists and antagonists on synaptic responses of the gustatory parabrachial nuclei (PbN)-projecting neurons in the rostral NST to electrical stimulation of the solitary tract (ST) using whole cell recordings in the hamster brain stem slices. ST-evoked excitatory postsynaptic currents (EPSCs) were significantly reduced by met-enkephalin (MetE) in a concentration-dependent fashion and this effect was eliminated by naltrexone hydrochloride, a nonselective opioid receptor antagonist. Bath application of naltrindole hydrochloride, a selective delta-opioid receptor antagonist, eliminated MetE-induced reduction of EPSCs, whereas CTOP, a selective mu-opioid receptor antagonist had no effect, indicating that delta-opioid receptors are involved in the reduction of ST-evoked EPSCs induced by MetE. SNC80, a selective delta-opioid receptor agonist, mimicked the effect of MetE. The SNC80-induced reduction of ST-evoked EPSCs was eliminated by 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist but not by naltriben mesylate, a selective delta2-opioid receptor antagonist, indicating that delta1-opioid receptors mediate the reduction of ST-evoked EPSCs induced by SNC80. Single-cell reverse transcriptase-polymerase chain reaction analysis revealed the presence of delta1-opioid receptor mRNA in cells that responded to SNC80 with a reduction in ST-evoked EPSCs. Moreover, Western blot analysis demonstrated the presence of 40-kDa delta-opioid receptor proteins in the rostral NST tissue. These results suggest that postsynaptic delta1-opioid receptors are involved in opioid-induced reduction of ST-evoked EPSCs of PbN-projecting rostral NST cells.

Involvement of specific orexigenic neuropeptides in sweetener-induced overconsumption in rats

Palatability is one of the factors that regulates food and fluid intake and contributes to overconsumption in turn contributing to obesity. To elucidate the brain mechanisms of the palatability-induced ingestion, we explored the roles of six hypothalamic orexigenic neuropeptides, orexin, melanin-concentrating hormone (MCH), neuropeptide Y (NPY), agouti-related protein (AgRP), ghrelin and dynorphin, in the intake of a palatable solution, saccharin. Of the six peptides, intracerebroventricular (i.c.v.) administrations of orexin, MCH and NPY increased the intake of saccharin. Drinking of saccharin in turn elevated the mRNA levels of orexin and NPY, but not MCH. Pre-treatments of naloxone, an opioid antagonist, blocked the orexigenic effects of orexin and NPY. Specific gastric motor responses induced by central orexin-A and NPY are well known, however, MCH did not induce such responses. The i.c.v. administration of orexin-A facilitated gastric emptying. These results suggest that the overconsumption promoted by sweet and palatable tastes is attributed to the activation of orexigenic neuropeptides, such as orexin and NPY, and a downstream opioid system together with enhanced digestive functions.

Self-injection of flunitrazepam alone and in the context of methadone maintenance in baboons

Patients in methadone maintenance programs use benzodiazepines to "boost" methadone's subjective effects, and flunitrazepam has been prominent in this context. Self-administration of flunitrazepam (0.001-0.32 mg/kg i.v.) alone and during daily oral methadone administration was evaluated in three baboons. Flunitrazepam maintained self-injection as an inverted U-shaped function of dose at rates higher than those maintained by most other benzodiazepines under the same procedure. In the context of demonstrated physical dependence on 3.2 mg/kg/day p.o. methadone, flunitrazepam doses on the ascending limb of the dose-effect curve maintained greater rates of self-injection than before methadone in two baboons. When the methadone dose decreased to 1.8 mg/kg/day, self-injection remained higher for those baboons and became higher than before methadone for the third baboon. Self-injection remained higher when methadone decreased to 1.0 mg/kg/day, except self-injection of the lowest flunitrazepam dose returned to or below the pre-methadone rate for two baboons. After methadone was discontinued, the dose-effect curve shifted to the right in one baboon but remained to the left in two. Flunitrazepam thus served as a reinforcer alone and in the context of methadone maintenance. Lower doses maintained higher self-injection during and shortly after methadone maintenance. Further research should explore the duration of higher self-injection rates following methadone maintenance.

Age-related changes in brain proDynorphin gene expression in the rat

Dynorphin has a well-established role in feeding and gustation. Alterations in taste perception and feeding behavior are common with age. We hypothesized that proDynorphin gene expression in brain areas involved in taste and feeding declines with age. Male Sprague-Dawley rats were housed individually with ad libitum access to food and water. Brain punches of the selected regions were dissected out in groups of rats aged 4-6, 12-14 and 18-21 months. ProDynorphin mRNA (measured using a cDNA probe) decreased significantly with age in arcuate nucleus and amygdala; increased significantly with age in hippocampus; and was not significantly affected in nucleus of the solitary tract, cortex, caudate putamen or hypothalamic paraventricular nucleus. These data suggest an age-related decrease in the synthesis of dynorphin in two brain regions strongly associated with feeding behavior, and an increase in dynorphin synthesis in a brain region associated with learning and memory.

Sugar-dependent rats show enhanced intake of unsweetened ethanol

Rats show signs of dependence on sugar when it is available intermittently, including bingeing, withdrawal, and cross-sensitization with amphetamine. In the current study, we sought to determine whether sugar-dependent rats would show increased intake of unsweetened ethanol and, conversely, whether intermittent access to ethanol would augment sugar consumption. In Experiment 1, with intermittent versus ad libitum access to ethanol, Sprague-Dawley rats were given escalating concentrations of ethanol (1%, 2%, 4%, 7%, and 9%) over the course of 20 days. Rats in the intermittent ethanol access group, with 12-h daily access, consumed more 4%, 7%, and 9% ethanol during the first hour of access, and more 9% ethanol daily, than did rats in the ad libitum ethanol access group. In Experiment 2, with ethanol as a gateway to sugar intake, the rats from Experiment 1 were switched to 10% sucrose with 12-h daily access for 1 week. Rats in the intermittent ethanol access group consumed significantly more sugar than was consumed by rats in a control group with no prior ethanol experience. In Experiment 3, with sugar as a gateway to ethanol to determine whether sugar dependence leads to increased ethanol intake, four groups were maintained for 21 days according to the following designations: intermittent access to sugar and chow, ad libitum access to sugar and chow, intermittent access to chow, or ad libitum access to chow. Four days later, all groups were switched to intermittent ethanol access, as described in Experiment 1. The group with intermittent access to sugar and chow consumed the most 9% ethanol, supporting the suggestion that sugar dependence alters a rat's proclivity to drink ethanol. These results may relate to the co-morbidity between binge-eating disorders and alcohol intake and the tendency of people abstaining from alcohol to consume excessive amounts of sugar. In conclusion, bingeing on either ethanol or sugar fosters intake of the other.

Neuropeptides in eating disorders

The past decade has witnessed a dramatic acceleration in research on the role of the neuropeptides in the regulation of eating behavior and body weight homeostasis. This expanding research focus has been driven in part by increasing public health concerns related to obesity and the eating disorders anorexia nervosa (AN) and bulimia nervosa (BN). Preclinical advances have been facilitated by the development of new molecular and behavioral research methodologies. With a focus on clinical investigations in AN and BN, this article reviews research on selected hypothalamic and gut-related peptide systems with prominent effects on eating behavior. Studies of the orexigenic peptides neuropeptide Y and the opioid peptides have shown state-related abnormalities in patients with eating disorders. With respect to gut-related peptides, there appears to be substantial evidence for blunting in the meal-related release of the satiety promoting peptide cholecystokinin in BN. Fasting plasma levels of the orexigenic peptide ghrelin have been found to be elevated in patients with AN. As discussed in this review, additional studies will be needed to assess the role of nutritional and body weight changes in neuropeptide alterations observed in symptomatic eating disorder patients, and to identify stable trait-related abnormalities in neuropeptide regulation that persist in individuals who have recovered from an eating disorder.

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.

Evidence for a mu-opioid-opioid connection between the paraventricular nucleus and ventral tegmental area in the rat

The paraventricular nucleus (PVN) and the ventral tegmental area (VTA) have been shown to be involved in opioid mediated feeding behavior. The present study examined whether mu-opioid signalling between the PVN and VTA affected feeding behavior. Male Sprague-Dawley rats were cannulated with one cannula placed in the PVN and two cannulae placed in the VTA, which allowed for co-administration of the mu-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO) in one site and the opioid antagonist naltrexone (NTX) in the other site. Bilateral administration of DAMGO (1.2, 2.4 and 4.9 nmol) into the VTA stimulated feeding dose dependently at 2.4 and 4.9 nmol (P<0.05). The DAMGO (2.4 nmol)-induced increase of food intake following injection into the PVN was blocked by bilateral injection of NTX (6.6, 13.2 and 26.5 nmol) into the VTA at 2 and 4 h (P<0.05). In the reverse situation, the DAMGO (2.4 nmol)-induced increase of food intake following injection into the VTA was blocked by injection of NTX (13.2 and 26.5 nmol) into the PVN at 2 and 4 h (P<0.05). The present study suggests that a bidirectional mu-opioid-opioid signalling pathway exists between the PVN and the VTA which influences feeding.

A diet promoting sugar dependency causes behavioral cross-sensitization to a low dose of amphetamine

Previous research in this laboratory has shown that a diet of intermittent excessive sugar consumption produces a state with neurochemical and behavioral similarities to drug dependency. The present study examined whether female rats on various regimens of sugar access would show behavioral cross-sensitization to a low dose of amphetamine. After a 30-min baseline measure of locomotor activity (day 0), animals were maintained on a cyclic diet of 12-h deprivation followed by 12-h access to 10% sucrose solution and chow pellets (12 h access starting 4 h after onset of the dark period) for 21 days. Locomotor activity was measured again for 30 min at the beginning of days 1 and 21 of sugar access. Beginning on day 22, all rats were maintained on ad libitum chow. Nine days later locomotor activity was measured in response to a single low dose of amphetamine (0.5 mg/kg). The animals that had experienced cyclic sucrose and chow were hyperactive in response to amphetamine compared with four control groups (ad libitum 10% sucrose and chow followed by amphetamine injection, cyclic chow followed by amphetamine injection, ad libitum chow with amphetamine, or cyclic 10% sucrose and chow with a saline injection). These results suggest that a diet comprised of alternating deprivation and access to a sugar solution and chow produces bingeing on sugar that leads to a long lasting state of increased sensitivity to amphetamine, possibly due to a lasting alteration in the dopamine system.

Effect of morphine, naloxone and histamine system on water intake in adult male rats

The present study investigated the interaction between histamine and opioid systems on water intake in adult male rats. Intracerebroventricular (i.c.v.) injections were carried out in all experiments. Water intake was measured 1 h after drug injections. Administration of histamine (40-80 microg/rat) and naloxone (0.5-1 microg/rat) increased, while morphine (2.5 microg/rat), pyrilamine (25-50 microg/rat), the histamine H1 receptor antagonist, and ranitidine (10-20 microg/rat), the histamine H2 receptor antagonist, decreased water intake in isolated rats. Blockade of histamine H1 and H2 receptors attenuated the histamine-induced response. Pyrilamine, but not ranitidine, increased the inhibitory effect induced by morphine. Also, pharmacological blockade of histamine H1 and H2 receptors decreased the naloxone-induced effect on water intake. It is concluded that the histaminergic system may have a close interaction with morphine and naloxone on drinking behavior.

Beta-endorphin modulates the acute response to a social conflict in male mice but does not play a role in stress-induced changes in sleep

Beta-endorphin is an endogenous opioid peptide that is released during stress and has been associated with many physiological functions. In this experiment beta-endorphin deficient mice were used to study the role of endorphins in the acute physiological and behavioral responses to a social conflict, as well as their role in social stress-induced changes in sleep. Adult male beta-endorphin deficient and wild type mice were subjected to the stress of a 1 h social conflict with an aggressive dominant conspecific. After the conflict, the beta-endorphin deficient mice had higher corticosterone levels but the peak increase in body temperature was not different from that in wild type animals. In fact, body temperature returned to baseline levels faster in the beta-endorphin deficient mice. During their interaction with the aggressive conspecific several of the beta-endorphin deficient mice showed clear signs of counter aggression whereas this was not seen in any of the wild type mice. Overall, the beta-endorphin deficient mice and wild type mice had fairly similar sleep patterns under baseline conditions and also showed similar amounts of NREM sleep, REM sleep and EEG slow-wave energy after the social conflict. In addition, no differences were found in the sleep patterns of mice that showed counter aggression and mice that did not. In conclusion, the results suggest that beta-endorphin modulates the acute endocrine, thermoregulatory and behavioral response to a social conflict but the data do not support a major role for beta-endorphin in the regulation of sleep or social stress-induced alterations in sleep.

Pathophysiology of anorexia

A physiologic decline in food intake occurs with advancing age. The physiologic anorexia of aging and its associated weight loss predispose older persons to develop protein-energy malnutrition. In older persons a variety of social and psychologic factors, diseases, and medications can aggravate the physiologic anorexia and lead to severe weight loss. Many of these factors are amenable to treatment, resulting in a reversal of the underlying malnutrition. This article first reviews the physiologic factors responsible for anorexia in older persons. It then reviews the major pathologic processes responsible for producing protein-energy malnutrition in older persons.

Diurnal rhythm of agouti-related protein and its relation to corticosterone and food intake

In the present study we examined the diurnal patterns of agouti-related protein (AGRP) and proopiomelanocortin (POMC) mRNA expression in the arcuate nucleus and their relation to circulating glucocorticoids and food intake. Animals were killed at 4-h intervals throughout the 24-h diurnal cycle, and the expression of AGRP and POMC mRNA was evaluated by semiquantitative in situ hybridization analysis. We observed a significant diurnal rhythm in AGRP mRNA expression, with a marked peak at 2200 h (4 h after lights off) and a trough at 1000 h (4 h after lights on), consistent with the overall day-night rhythm of food intake. In contrast, POMC mRNA levels did not show a significant fluctuation across the diurnal cycle, although there was a tendency for levels to decrease after the onset of the dark cycle. Corticosterone secretion temporally coincided with the rising phase of AGRP mRNA expression. Depletion of corticosterone by adrenalectomy abolished the AGRP diurnal rhythm by suppressing the nighttime expression, but did not alter the feeding rhythm. Exposure of adrenalectomized rats to constant corticosterone replacement (10 or 50 mg continuous release corticosterone pellet) resulted in fixed AGRP mRNA expression throughout the 12-h light, 12-h dark cycle. A relatively high level of corticosterone (50 mg) significantly increased AGRP mRNA expression, with a positive correlation between these two measures. These results indicate that 1) the diurnal expression of AGRP mRNA is regulated by corticosterone independently of the light/dark cue; and 2) a normal endogenous corticosterone rhythm is required for generating the diurnal AGRP rhythm.

Carbohydrate ingestion, blood glucose and mood

A series of studies have reported that a high carbohydrate meal, or diets high in carbohydrate, were associated with feeling less energetic. However, after a drink containing pure sugar most studies report no effect. Meals almost exclusively carbohydrate increase the availability of tryptophan and hence serotonin synthesis in the brain, however, a small amount of protein blocks this mechanism making it an uncommon response. In many individuals, poor mood stimulates the eating of palatable high carbohydrate/high fat foods that stimulate the release of endorphins. There is a tendency for those with lower blood glucose, when performing cognitively demanding tasks, to report poorer mood. In a range of situations an association between a tendency for blood glucose levels to fall rapidly, and irritability, has been found. Differences in the ability to control blood glucose levels influence the association between carbohydrate intake and mood. There is a need in future research to contrast the impact of carbohydrate on mood in those distinguished because of their pre-existing psychological and physiological functioning.

The effect of naloxone on operant behavior for food reinforcers in DBA/2 mice

Mice are powerful models to investigate the genetic basis of food reward because many spontaneous obesity mutants exist and the murine genome is accessible to selectively targeted manipulations. Experiments in rats have shown that opioid receptor blockade reduces operant responding to food reinforcers. The present study investigated whether DBA/2J mice would display similar behavior in response to an opioid antagonist. Twelve male DBA/2J mice were trained to lever press for food reinforcers and subsequently randomized in a within subjects design for no injection, saline injection, or 10 mg/kg naloxone injection intraperitoneal (i.p.) 20 min before each daily trial under ad lib or food-deprived conditions. A significant main effect of injection occurred to reduce lever pressing by the mice. However, a greater pharmacological effect of naloxone occurred compared with saline on the operant responding only under the food-deprived conditions. Interestingly, the percentage of dispensed food pellets actually consumed was significantly reduced after naloxone injection compared with saline injection for either chow-based or sucrose pellets under ad lib or deprived feeding conditions. These data suggest that opioids specifically influence consumatory behavior in mice, but our findings on instrumental behavior were confounded by an independent inhibitory effect of an i.p. saline injection.

Effects of aging on the opioid modulation of feeding in humans

OBJECTIVES

To determine whether aging is associated with a reduction in the opioid modulation of feeding, which may be important in the pathogenesis of the "anorexia of aging."

DESIGN

Three studies on separate days, in randomized order and double-blind fashion.

SETTING

Clinical Human Research Laboratory, Department of Medicine, RAH, Adelaide, Australia.

PARTICIPANTS

Twelve older (5 male/7 female) (age 65-84) and 12 young (5 male/7 female) (age 20-26) healthy subjects.

INTERVENTION

Subjects received in double-blinded random order, intravenous bolus (10 minutes) and then continuous (140 minutes) infusions of saline (control), naloxone low dose (LD) (bolus 27 microg/kg; continuous 50 microg/kg/hr), or naloxone high dose (HD) (bolus 54.5 microg/kg; continuous 100 microg/kg/hr).

MEASUREMENTS

After 120 minutes, subjects were offered a buffet meal, and their energy intake was quantified. Hunger, fullness, nausea, and drowsiness were assessed using visual analogue scales.

RESULTS

The naloxone LD and HD infusions had no significant effect on ratings of hunger, fullness, or nausea, but increased drowsiness (P < .01) compared with the control infusion in both age groups. Older subjects ate less (P < .001) at the buffet meal than young subjects during all three infusions. Naloxone infusions reduced energy intake compared with control (P < .001), LD by 13.2 +/- 5.0% and HD by 10.7 +/- 5.0%, with no difference between the doses (P = .71). Overall, naloxone suppressed energy intake in both young and older subjects (P < .01). This suppression was slightly, but not significantly, greater in young than in older subjects (mean of LD and HD 16.4 +/- 4.9% vs 7.5 +/- 4.9%, P = .42), because of a trend to reduced suppression in older women.

CONCLUSIONS

We conclude that healthy older adults retain their sensitivity to the suppressive effects of naloxone on food intake. Possible gender differences in this sensitivity warrant further investigation. A decline in opioid activity is unlikely to contribute substantially to the physiological anorexia of aging observed in older people.

Interaction of morphine and naltrexone on oral ethanol self-administration in rhesus monkeys

Opioid antagonists, such as naltrexone (NTX), reduce ethanol consumption and opioid agonists increase or decrease ethanol consumption in rats depending upon the dose. If the opioid antagonist and agonist effects on ethanol consumption are mediated by mu-opioid receptors, then NTX doses that reduce ethanol consumption should be similar to the doses necessary to antagonize the effects of opioid agonists on ethanol consumption. The purpose of these experiments was: (1) to determine whether morphine increases ethanol consumption in rhesus monkeys as it does in rodents; (2) to determine if the mu-receptor mediates the effects of morphine on ethanol consumption by conducting a pK(B) analysis using NTX; and (3) to determine if the mu-receptor also mediates the NTX-induced decreases in ethanol consumption by making comparisons between the NTX doses that affect ethanol consumption and the NTX doses that block the effects of morphine on ethanol consumption. Three male rhesus monkeys responded for 2% ethanol and water for 2 h/day on a fixed-ratio 4 schedule of reinforcement. Morphine doses as low as 0.0032mg/kg failed to increase ethanol fluid deliveries, whereas higher doses produced a dose-related decrease in ethanol fluid deliveries. Although 0.01 mg/kg NTX alone had no effect on ethanol fluid deliveries, it reduced the suppressant effects of morphine with a mu-receptor pK(B) of 8.21 (8.08-8.34). When given alone, 0.1 mg/kg NTX decreased ethanol fluid deliveries but failed to reverse the suppression caused by 1 mg/kg morphine. Therefore, monkeys may differ from rats in their response to morphine when ethanol consumption is the dependent variable. Furthermore, because the NTX dose that reduced the effects of morphine on responding for ethanol was smaller than the NTX doses that suppressed ethanol-reinforced responding when given alone, NTX may exert these two effects through different mechanisms.

Identification of central sites involved in butorphanol-induced feeding in rats

Butorphanol (BT), a mixed kappa- and mu-opioid receptor agonist, induces vigorous food intake in rats. Peripheral injection of BT seems to increase food intake more effectively than intracerebroventricular administration. To further elucidate the nature of BT's influence on consummatory behavior, we examined which feeding-related brain areas exhibit increased c-Fos immunoreactivity (IR) following subcutaneous injection of 4 mg/kg body weight BT, a dose known to induce a maximal orexigenic response. We also evaluated whether direct administration of BT into the forebrain regions activated by peripheral BT injection affects food intake. Peripheral BT administration induced c-Fos-IR in the hypothalamic paraventricular nucleus (PVN), central nucleus of the amygdala (CeA), and nucleus of the solitary tract (NTS). However, 0.1-30 microg BT infused into the CeA, failed to increase food intake 1, 2, and 4 h after injection. Only the highest dose of BT (30 microg) injected into the PVN increased feeding. These results suggest that the PVN, CeA, and NTS mediate the effects of peripherally-injected BT. The PVN or CeA are probably not the main target sites of immediate BT action.

Anorexia, sarcopenia, and aging

Food intake declines throughout the life span. This physiologic anorexia of aging is caused in part by alterations of stomach-fundus compliance and release and activity of cholecystokinin. In addition, the decline in testosterone in males results in elevated leptin levels that increase the anorexia. There is also evidence that cytokines play a role in the pathogenesis of anorexia and sarcopenia, thus accelerating the development of frailty in older persons. Numerous treatable causes of anorexia and weight loss exist. Depression is the most commonly diagnosed cause of pathologic weight loss in older persons.

Effect of Agouti-related protein delivered to the dorsomedial nucleus of the hypothalamus on intake of a preferred versus a non-preferred diet

Agouti-related protein (Agrp), a high-affinity antagonist of the melanocortin-3/4 receptors, increases feeding when administered centrally. Previous studies have shown that this increase is long-lasting (at least 24 h) and delayed, unless the animal is first stimulated to feed by fasting or onset of the dark phase. The present studies first demonstrate that long-lasting and delayed increases in food intake are also evident when Agrp is microinjected into the dorsomedial nucleus of the hypothalamus (DMH). Next, the effects of DMH-administered Agrp were assessed on intake of two foods, isocaloric but differing in flavor (with or without sucrose). Following exposure to the two diets, rats were injected via cannula aimed at the DMH with 100 pmol Agrp at 10:00 h and allowed ad libitum access to either: (1) a choice of both diets or (2) one of the diets alone. Food intake was determined at 2, 4, and 24 h post-injection. In the first (choice) paradigm, Agrp only increased intake of the sucrose-containing diet. In the second (no-choice) paradigm, animals on either diet showed an Agrp-induced increase in intake 24 h following injection; only animals on the sucrose-containing diet showed an increase in intake 4 h post-injection. The results are discussed in the context of the possible involvement of Agrp/MC4-R in the rewarding characteristics of food intake.

Modulation of food intake by the kappa opioid U-50,488H: evidence for an effect on satiation

The main goal of the present study was to test the hypothesis that the prophagic effect of the kappa opioid agonist U-50,488H (U50) is primarily due to an effect on satiation. In Experiment 1, the feeding effects of U50 (2.0 and 4.0 mg/kg, i.p.) was tested in animals with ad libitum access to ground food and to three sucrose solutions (1, 4, and 20%). In Experiment 2, a classical "one-bottle" test was utilized to test for the effect of U50 (4.0 mg/kg, i.p.) on the intake of five different sucrose solutions (1, 4, 16, 32, and 40%) over a 30-min period. Finally, in Experiment 3 we evaluated the effect of U50 (2.0, 4.0, and 6.0 mg/kg, i.p.) on extracellular dopamine (DA) concentration in the nucleus accumbens. In Experiment 1, U50 enhanced the intake of ground food but not of sucrose. In Experiment 2, U50 increased the intake of high concentration sucrose solutions whereas it decreased that of low concentration solutions. In Experiment 3, U50 produced a dose-dependent decrease in DA concentrations in the absence but not in the presence of food. The most likely explanation for the present results is that U50 enhances feeding by activating mechanisms that block satiety and satiation. In contrast, we found little evidence for an effect of U50 on palatability.

Reward deficiency syndrome: a biogenetic model for the diagnosis and treatment of impulsive, addictive, and compulsive behaviors

The dopaminergic system, and in particular the dopamine D2 receptor, has been implicated in reward mechanisms. The net effect of neurotransmitter interaction at the mesolimbic brain region induces "reward" when dopamine (DA) is released from the neuron at the nucleus accumbens and interacts with a dopamine D2 receptor. "The reward cascade" involves the release of serotonin, which in turn at the hypothalmus stimulates enkephalin, which in turn inhibits GABA at the substania nigra, which in turn fine tunes the amount of DA released at the nucleus accumbens or "reward site." It is well known that under normal conditions in the reward site DA works to maintain our normal drives. In fact, DA has become to be known as the "pleasure molecule" and/or the "antistress molecule." When DA is released into the synapse, it stimulates a number a DA receptors (D1-D5) which results in increased feelings of well-being and stress reduction. A consensus of the literature suggests that when there is a dysfunction in the brain reward cascade, which could be caused by certain genetic variants (polygenic), especially in the DA system causing a hypodopaminergic trait, the brain of that person requires a DA fix to feel good. This trait leads to multiple drug-seeking behavior. This is so because alcohol, cocaine, heroin, marijuana, nicotine, and glucose all cause activation and neuronal release of brain DA, which could heal the abnormal cravings. Certainly after ten years of study we could say with confidence that carriers of the DAD2 receptor A1 allele have compromised D2 receptors. Therefore lack of D2 receptors causes individuals to have a high risk for multiple addictive, impulsive and compulsive behavioral propensities, such as severe alcoholism, cocaine, heroin, marijuana and nicotine use, glucose bingeing, pathological gambling, sex addiction, ADHD, Tourette's Syndrome, autism, chronic violence, posttraumatic stress disorder, schizoid/avoidant cluster, conduct disorder and antisocial behavior. In order to explain the breakdown of the reward cascade due to both multiple genes and environmental stimuli (pleiotropism) and resultant aberrant behaviors, Blum united this hypodopaminergic trait under the rubric of a reward deficiency syndrome.

Food intake after diazepam, morphine or muscimol: microinjections In the nucleus accumbens shell

This study examined the effect on food intake of bilateral microinfusions of the benzodiazepine agents, diazepam and midazolam, the opioid agonist, morphine, and the GABA(A) agonist, muscimol into the shell of the nucleus accumbens in rats. Both muscimol (at 0.075 microg, combined bilateral dose) and morphine (1.0 microg) in the nucleus accumbens shell increased feeding as expected. However, it was clear that diazepam (2.5, 5.0, 25, 50 microg) and midazolam (7.5 microg) both failed to enhance feeding even at doses that are effective when microinjected in the brain stem. We conclude that opioid and GABA(A) agents promote feeding behavior by acting on receptors in the nucleus accumbens shell, but that benzodiazepines probably act elsewhere in the brain to increase food intake.

Effects of taste stimulation on beta-endorphin levels in rat cerebrospinal fluid and plasma

Opioids are suggested to be involved in generation of palatability and facilitation of consumption of food and fluid. We measured the level of an endogenous opioid, beta-endorphin, in the cerebrospinal fluid (CSF) and plasma after free drinking of water and taste solutions in Wistar rats. When the water-deprived animals were allowed to drink 10 mL of water, the level of beta-endorphin increased significantly 60 and 90 min after the start of drinking in both samples. beta-Endorphin in the CSF increased most after ingestion of 0.5 M sucrose and 0.005 M saccharin followed by 0.1 M NaCl, 0.1 mM quinine and water. An intragastric infusion of 7 mL of water did not change the beta-endorphin level. Essentially the same results were obtained for plasma samples except that NaCl and quinine solutions did not increase beta-endorphin levels. Sucrose became ineffective in releasing beta-endorphin in both samples after the establishment of conditioned taste aversions to this taste stimulus. These results suggest that the release of beta-endorphin is positively correlated with the palatability of taste stimuli, and that CSF beta-endorphin also reflects the reinforcement of fluid intake in thirsty animals.

Pharmacology of sucrose-reinforced place-preference conditioning: effects of naltrexone

Two experiments investigated the role of the opioid system in sucrose-reinforced conditioned place preferences (CPPs) in rats. Experiment 1 examined the effects of a general opioid antagonist, naltrexone, on the expression of a CPP acquired in the absence of the drug. Subjects were trained to associate one compartment of a two-compartment chamber with sucrose and the other compartment with water. Rats displayed a preference for the sucrose-associated compartment in a choice test without sugar or water available following vehicle saline treatment. Naltrexone doses of 2.5 and 5.0 mg/kg reduced this preference for the sucrose-associated compartment. Experiment 2 examined the effects of naltrexone on the acquisition as well as the expression of CPPS. Different groups of rats received daily injections of either saline, 0.1, 1.0, or 5.0 mg/kg of naltrexone prior to each training session, and then these groups were given a choice test for the CPP after saline or naltrexone injections. Although naltrexone treatment attenuated the expression of CPPs in each group relative to saline treatment, there were no group differences during these tests in the magnitude of the preferences. Moreover, all groups displayed equal acquisition of CPPs despite the fact that naltrexone dose dependently decreased sucrose intake during the training phase. Together, the results indicate that the opioid system modulates the expression but not the acquisition of sucrose-reinforced CPPs.

Opiate modulating properties of nociceptin/orphanin FQ

The recently discovered peptide nociceptin/orphanin FQ (N/OFQ) and its receptor NOR share many structural similarities with the opioid peptides and their receptors. The anatomical distributions of N/OFQ and NOR are similar to those of opioid peptides and receptors. In addition, NOR and opiate receptors couple via the same G-proteins to similar effectors, such as Ca(2+) channels, K(+) channels, adenylyl cyclase, and several protein kinases. Thus, the behavioral effects of N/OFQ have been investigated in the context of known opiate effects, and a possible connection has been sought between the effects of these two homologous signaling systems. Originally characterized as a nociception-producing peptide, N/OFQ has now been shown to have diverse effects on nociception, as well as effects on many other behaviors. With regard to nociception, the peptide has been reported to produce hyperalgesia, reversal of opioid-mediated analgesia, analgesia, and allodynia. N/OFQ also has effects on other behaviors, such as locomotion, feeding, anxiety, spatial attention, reproductive behaviors, and opiate tolerance. The relationship between opiates and N/OFQ is strengthened by the fact that opiates also affect these behaviors. However, the exact nature of the relationship of N/OFQ with opiates-opiate-like versus antiopiate-remains controversial. This review will detail the diverse effects of N/OFQ and suggest that this peptide, like other putative antiopiate peptides, can be described as 'opiate modulating. '