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

Dopamine, activation of ingestion and evaluation of response efficacy: a focus on the within-session time-course of licking burst number

RATIONALE

Evidence on the effect of dopamine D1-like and D2-like receptor antagonists on licking microstructure and the forced swimming response led us to suggest that (i) dopamine on D1-like receptors plays a role in activating reward-directed responses and (ii) the level of response activation is reboosted based on a process of evaluation of response efficacy requiring dopamine on D2-like receptors. A main piece of evidence in support of this hypothesis is the observation that the dopamine D2-like receptor antagonist raclopride induces a within-session decrement of burst number occurring after the contact with the reward. The few published studies with a detailed analysis of the time-course of this measure were conducted in our laboratory.

OBJECTIVES

The aim of this review is to recapitulate and discuss the evidence in support of the analysis of the within-session burst number as a behavioural substrate for the study of the mechanisms governing ingestion, behavioural activation and the related evaluation processes, and its relevance in the analysis of drug effects on ingestion.

CONCLUSIONS

The evidence gathered so far suggests that the analysis of the within-session time-course of burst number provides an important behavioural substrate for the study of the mechanisms governing ingestion, behavioural activation and the related evaluation processes, and might provide decisive evidence in the analysis of the effects of drugs on ingestion. However, further evidence from independent sources is necessary to validate the use and the proposed interpretation of this measure.

Effects of nucleus accumbens insulin inactivation on microstructure of licking for glucose and saccharin in male and female rats

Insulin of pancreatic origin enters the brain where several regions express a high density of insulin receptors. Functional studies of brain insulin signaling have focused predominantly on hypothalamic regulation of appetite and hippocampal regulation of learning. Recent studies point to involvement of nucleus accumbens (NAc) insulin signaling in a diet-sensitive response to glucose intake and reinforcement of flavor-nutrient learning. The present study used NAc shell microinjection of an insulin inactivating antibody (InsAb) to evaluate effects on the microstructure of licking for flavored 6.1% glucose. In both male and female rats, InsAb had no effect on the number of lick bursts emitted (a measure of motivation and/or satiety), but decreased the size of lick bursts (a measure of reward magnitude) in a series of five 30 min test sessions. This effect persisted beyond microinjection test sessions and was shown to depend on previous flavored glucose consumption under InsAb treatment rather than InsAb treatment alone. This suggests learning of diminished reward value and aligns with the previous finding that InsAb blocks flavor-nutrient learning. Specificity of the InsAb effect for nutrient reward was indicated by failure to affect any parameter of licking for flavored 0.25% saccharin solution. Finally, maintenance of rats on a 'Western' diet for twelve weeks produced a decrease in lick burst size for glucose in male rats, but an increase in lick burst size in females. Possible implications of these results for flavor-nutrient learning, maladaptive consequences of NAc insulin receptor subsensitivity, and the plausible involvement of distinct insulin-regulated mechanisms in NAc are discussed.

Associations between opioid dependence and sweet taste preference

RATIONALE

Past research suggests that people with opioid dependence show increased consumption of sweet food, but it is unclear if this is influenced by altered taste preference and/or taste perception.

OBJECTIVES

We tested whether people prescribed opioid substitution therapy (OST) exhibited a shift in preference towards sweeter flavours, and altered perception of sweetness, and explored whether these measures of taste preference/perception were associated with measures of opioid use.

METHODS

Three groups of participants (people prescribed OST, n=36; people with past opioid dependence, but now abstinent from all opioids, n=18; and controls with no history of substance dependence other than nicotine, n=29) provided ratings of "sweetness", "liking", and "desire" of 4 solutions with varying concentrations of sucrose.

RESULTS

We did not find significant differences between groups in the effect of sucrose concentration on "sweetness", "liking", or "desire" ratings. However, among those prescribed OST, frequency of recent illicit opioid use was associated with reduced perception of "sweetness" of low sucrose concentrations. Higher methadone dose was associated with a shift towards liking sweeter concentrations. Among those with past opioid dependence, longer duration of abstinence from opioids was associated with a shift towards liking sweeter concentrations.

CONCLUSIONS

Among people currently dependent on opioids, reduced sensitivity to low levels of sweetness and increased preference for sweeter flavours may be associated with increased dependence on opioids. Among those who have ceased opioid use, the association between preference for sweeter flavours and duration of abstinence is a novel finding that deserves further investigation.

Affective biases and their interaction with other reward-related deficits in rodent models of psychiatric disorders

Major depressive disorder (MDD) is one of the leading global causes of disability. Symptoms of MDD can vary person to person, and current treatments often fail to alleviate the poor quality of life that patients experience. One of the two core diagnostic criteria for MDD is the loss of interest in previously pleasurable activities, which suggests a link between the disease aetiology and reward processing. Cognitive impairments are also common in patients with MDD, and more recently, emotional processing deficits known as affective biases have been recognised as a key feature of the disorder. Studies in animals have found similar affective biases related to reward. In this review we consider these affective biases in the context of other reward-related deficits and examine how affective biases associated with learning and memory may interact with the wider behavioural symptoms seen in MDD. We discuss recent developments in how analogues of affective biases and other aspects of reward processing can be assessed in rodents, as well as how these behaviours are influenced in models of MDD. We subsequently discuss evidence for the neurobiological mechanisms contributing to one or more reward-related deficits in preclinical models of MDD, identified using these behavioural assays. We consider how the relationships between these selective behavioural assays and the neurobiological mechanisms for affective bias and reward processing could be used to identify potential treatment strategies.

Characterizing ingestive behavior through licking microstructure: Underlying neurobiology and its use in the study of obesity in animal models

Ingestive behavior is controlled by multiple distinct peripheral and central physiological mechanisms that ultimately determine whether a particular food should be accepted or avoided. As rodents consume a fluid they display stereotyped rhythmic tongue movements, and by analyzing the temporal distribution of pauses of licking, it is possible through analyses of licking microstructure to uncover dissociable evaluative and motivational variables that contribute to ingestive behavior. The mean number of licks occurring within each burst of licking (burst and cluster size) reflects the palatability of the consumed solution, whereas the frequency of initiating novel bouts of licking behavior (burst and cluster number) is dependent upon the degree of gastrointestinal inhibition that accrues through continued fluid ingestion. This review describes the analysis of these measures within a context of the behavioral variables that come to influence the acceptance or avoidance of a fluid, and the neurobiological mechanisms that underlie alterations in the temporal distribution of pauses of licks. The application of these studies to models of obesity in animals is also described.

GluA1 AMPAR subunit deletion reduces the hedonic response to sucrose but leaves satiety and conditioned responses intact

The GluA1 subunit of the AMPA receptor has been implicated in schizophrenia. While GluA1 is important for cognition, it is not clear what the role of GluA1 is in hedonic responses that are relevant to the negative symptoms of disorders such as schizophrenia. Here, we tested mice that lack GluA1 (Gria1^−/− mice) on consumption of sucrose solutions using a licking microstructure analysis. GluA1 deletion drastically reduced palatability (as measured by the mean lick cluster size) across a range of sucrose concentrations. Although initial lick rates were reduced, measures of consumption across long periods of access to sucrose solutions were not affected by GluA1 deletion and Gria1^−/− mice showed normal satiety responses to high sucrose concentrations. GluA1 deletion also failed to impair flavour conditioning, in which increased intake of a flavour occurred as a consequence of prior pairing with a high sucrose concentration. These results demonstrate that GluA1 plays a role in responding on the basis of palatability rather than other properties, such as the automatic and learnt post-ingestive, nutritional consequences of sucrose. Therefore, Gria1^−/− mice provide a potential model of anhedonia, adding converging evidence to the role of glutamatergic dysfunction in various symptoms of schizophrenia and related disorders.

Emotional Eating, Binge Eating and Animal Models of Binge-Type Eating Disorders

PURPOSE OF REVIEW

The objective of this paper is to review the role that hedonic factors, emotions and self-regulation systems have over eating behaviours from animal models to humans.

RECENT FINDINGS

Evidence has been found to suggest that for some high-risk individuals, obesity/binge eating may develop as an impulsive reaction to negative emotions that over time becomes a compulsive habit. Animal models highlight the neural mechanisms that might underlie this process and suggest similarities with substance use disorders. Emotional difficulties and neurobiological factors have a role in the aetiology of eating and weight disorders. Precise treatments targeted at these mechanisms may be of help for people who have difficulties with compulsive overeating.

Within-session decrement of the emission of licking bursts following reward devaluation in rats licking for sucrose

We previously observed that dopamine D2-like receptor blockade in rats licking for sucrose produced a within-session decrement of the emission of licking bursts similar to the effect of either reward devaluation, or neuroleptics, on operant responding for different rewards, which, accordingly, we interpreted as an extinction-like effect. This implies that exposing animals to reward devaluation would result in a drop of burst number taking place only after the contact with the devalued reward. To test this prediction, we compared the difference in the within-session time course of burst number in response to high (10%) versus low (2%) concentration sucrose solutions, either in a condition of reward devaluation (exposure to 2% after daily 10%), or in a condition which does not involve changes in the reward value (two groups of subjects each repeatedly exposed to only one of the two concentrations). Reward devaluation resulted in a within-session decrement of the burst number, with the response rate dropping only after the contact with the devalued reward, as predicted. This response pattern was reliably observed only in subjects at their first devaluation experience. In contrast, exposure of separate groups of animals to the two different concentrations yielded lower levels of burst number in the low concentration group apparent since the beginning of the session, as previously observed with dopamine D1-like receptor blockade. These results show that the analysis of burst number, but not of burst size, reveals a specific activation pattern in response to reward devaluation, which differs from the pattern observed comparing the response to two different sucrose concentrations in separate groups of subjects, i.e. in a condition not involving reward devaluation. Finally, the characterisation of the experimental measures of the analysis of licking microstructure in behaviourally (and psychologically) meaningful functional terms, might be relevant for the investigation of the mechanisms underlying behavioural activation and the related evaluation processes.

Activation of mu-opioid receptors in the ventral pallidum decreases the negative hedonic evaluation of a conditioned aversive taste in rats

Conditioned taste aversion (CTA) causes a shift in the hedonic evaluation of a conditioned stimulus (CS) from positive to negative, and reduces the CS intake. Mu-opioid receptors (MORs) in the ventral pallidum (VP) are known to be involved in the hedonic evaluation of positive rewarding stimuli; however, their involvement in evaluation of a negative aversive stimulus is still unclear. To explore the neural mechanisms of the negative hedonic evaluation of the CS in CTA, we examined the effects of the activation of VP MORs on the behavioral responses of rats to a CS. Rats implanted with guide cannulae into the bilateral VP received a pairing of 5mM saccharin solution as a CS with an intraperitoneal injection of 0.15M lithium chloride as an unconditioned stimulus. On the test day, after microinjections of MOR agonist [D-Ala², N-MePhe⁴, Gly-ol]-enkephalin (DAMGO) into the VP, we observed the behavioral responses to the intraorally infused CS solution. The DAMGO injections caused a larger number of ingestive taste reactivity responses to the CS solution. We also measured the consumption of the CS solution in a separate group of rats, using a single-bottle test. The DAMGO injected rats drank a higher volume of the CS solution than the saline injected rats. These results indicate that the activation of MORs in the VP results in the attenuation of aversion to the CS solution, thereby inducing the larger CS intake. Therefore, it is likely that VP MORs are involved in not only positive but also negative hedonic evaluation.

Midazolam impairs the retrieval of conditioned taste aversion via opioidergic transmission in mice

Midazolam is a benzodiazepine agonist that affects the acquisition, retention, and retrieval of malaise-induced conditioned taste aversion (CTA) in rats. Our previous study suggested that the palatability-enhancing rather than amnesic effects of midazolam were responsible for impaired retrieval of conditioned aversion to palatable conditioned stimuli (CSs). However, it remains unclear whether this effect is opioid-dependent. In the present study, we examined the involvement of opioid signaling with the ability of peripheral midazolam administration to transiently impair CTA retrieval in mice. CTA was established by pairing 5mM saccharin ingestion (conditioned stimulus, CS) with an intraperitoneal (i.p.) injection of 0.15M lithium chloride (LiCl, 2% body weight) (unconditioned stimulus) for two consecutive days. Conditioned mice that received midazolam (1.5mg/kg, i.p.) before the first retention test consumed significantly more saccharin (CS) than conditioned mice that received vehicle (phosphate-buffered physiological saline, PBS; i.p.). On the next day, both conditioned groups showed strong aversions to the CS. Next, naloxone, an opioid receptor antagonist, was peripherally administered prior to the midazolam injection before the retention test. Pre-administration of naloxone but not PBS attenuated midazolam-induced increases in CS intake. Finally, we examined aversive orofacial taste reactions (TRs) to an oral infusion of the CS with pre-administration of naloxone or PBS prior to midazolam using a taste reactivity test. Conditioned mice that received midazolam showed significantly longer latencies to express aversive orofacial TRs than those that received PBS. Pre-administration of naloxone eliminated the effect of midazolam on latency to express aversive TRs. Taken together, these data suggest that midazolam activates opioidergic transmission and opioid-dependent palatability enhancement of the CS to eliminate conditioned aversion to a sweet taste.

Parsing the hedonic and motivational influences of nociceptin on feeding using licking microstructure analysis in mice

Opioid peptides are implicated in processes related to reward and aversion; however, how specific opioid peptides are involved remains unclear. We investigated the role of nociceptin (NOC) in voluntary licking for palatable and aversive tastants by studying the effect of intracerebroventricularly administered NOC on licking microstructure in wild-type and NOC receptor knockout (NOP KO) mice. Compared with the wild-type mice, NOP KO mice emitted fewer bouts of licking when training to lick for a 20% sucrose solution. Correspondingly, intracerebroventricular administration of NOC increased the number of licking bouts for sucrose and sucralose in wild-type, but not in NOP KO mice. The ability of NOC to initiate new bouts of licking for sweet solutions suggests that NOC may drive motivational aspects of feeding behavior. Conversely, adulterating a sucrose solution with the aversive tastant quinine reduced licking bout lengths in wild-type and NOP KOs, suggesting that NOC signaling is not involved in driving voluntary consumption of semiaversive tastants. Interestingly, when consuming sucrose following 20 h of food deprivation, NOP KO mice emitted longer bouts of licking than wild types, suggesting that under hungry conditions, NOC may also contribute toward hedonic aspects of feeding. Together, these results suggest differential roles for NOC in the motivational and hedonic aspects of feeding.

Involvement of Endogenous Enkephalins and β-Endorphin in Feeding and Diet-Induced Obesity

Studies implicate opioid transmission in hedonic and metabolic control of feeding, although roles for specific endogenous opioid peptides have barely been addressed. Here, we studied palatable liquid consumption in proenkephalin knockout (PENK KO) and β-endorphin-deficient (BEND KO) mice, and how the body weight of these mice changed during consumption of an energy-dense highly palatable 'cafeteria diet'. When given access to sucrose solution, PENK KOs exhibited fewer bouts of licking than wild types, even though the length of bouts was similar to that of wild types, a pattern that suggests diminished food motivation. Conversely, BEND KOs did not differ from wild types in the number of licking bouts, even though these bouts were shorter in length, suggesting that they experienced the sucrose as being less palatable. In addition, licking responses in BEND, but not PENK, KO mice were insensitive to shifts in sucrose concentration or hunger. PENK, but not BEND, KOs exhibited lower baseline body weights compared with wild types on chow diet and attenuated weight gain when fed cafeteria diet. Based on this and related findings, we suggest endogenous enkephalins primarily set a background motivational tone regulating feeding behavior, whereas β-endorphin underlies orosensory reward in high need states or when the stimulus is especially valuable. Overall, these studies emphasize complex interplays between endogenous opioid peptides targeting μ-receptors, such as enkephalins and endorphins, underlying the regulation of feeding and body weight that might explain the poor efficacy of drugs that generally target μ-opioid receptors in the long-term control of appetite and body weight.

Intact Hedonic Responses to Sweet Tastes in Autism Spectrum Disorder

The Sweet Taste Test (STT) is a standardized measure designed to index the ability to detect differences in sweet tastes (sweet taste sensitivity) and hedonic responses to sweet tastes (sweet taste liking). Profiles of response on the STT suggest enhanced hedonic responses to sweet tastes in psychiatric disorders characterized by dysfunctional reward processing systems, including binge-eating disorders and substance use disorders, and a putative mechanism governing STT responses is the brain opioid system. The present study examined STT responses in 20 adults with autism spectrum disorder (ASD) and 38 healthy control adults. There were no differences in sweet taste sensitivity or hedonic response to sweet tastes between the ASD and control groups. Within the ASD sample, ASD symptom severity was associated with sweet taste sensitivity, but not hedonic response to sweet taste. Results may ultimately shed light on brain opioid system functioning in ASD.

Maternal high-fat diet during pregnancy and lactation reduces the appetitive behavioral component in female offspring tested in a brief-access taste procedure

Maternal high-fat diet appears to disrupt several energy balance mechanisms in offspring. Here, female offspring from dams fed a high-fat diet (HF) did not significantly differ in body weight compared with those fed chow (CHOW), when weaned onto chow diet. Yet when presented with both a chow and a high-fat diet, high-fat intake was significantly higher in HF compared with CHOW offspring. To assess taste-based responsiveness, offspring (12 wk old) were tested in 30-min sessions (10-s trials) to a sucrose concentration series in a brief-access taste test. Compared with CHOW, the HF offspring initiated significantly fewer trials but did not significantly differ in the amount of concentration-dependent licking. Thus, rather than affect lick response (consummatory), maternal diet affects spout approach (appetitive), which may be attributed to motivation-related mechanisms. Consistent with this possibility, naltrexone, an opioid receptor antagonist, further reduced trial initiation, but not licking in both groups. With naltrexone administration, the group difference in trial initiation was no longer evident, suggesting differences in endogenous opioid activity between the two groups. Relative expression of μ-opioid receptor in the ventral tegmental area was significantly lower in HF rats. When trial initiation was not required in one-bottle intake tests, no main effect of maternal diet on the intake of sucrose and corn oil emulsions was observed. Thus, the maternal high-fat diet-induced difference in diet preference is not likely due to changes in the sensory orosensory component of the taste stimulus but may depend on alterations in satiety signals or absorptive mechanisms.

Central effects of ethanol interact with endogenous mu-opioid activity to control isolation-induced analgesia in maternally separated infant rats

Endogenous opioid activity plays an important role in ethanol consumption and reinforcement in infant rats. Opioid systems are also involved in mediation and regulation of stress responses. Social isolation is a stressful experience for preweanling rats and changes the effects of ethanol through opioid-dependent mechanisms. The present study assessed effects of intracisternal (i.c.) administration of a selective mu-opioid antagonist (CTOP) and i.p. administration of a nonspecific opioid antagonist (naloxone) on voluntary intake and behavior in socially isolated 12-day-old (P12) pups treated with 0.5 g/kg ethanol. Voluntary intake of 0.1% saccharin or water, locomotion, rearing activity, paw licking and grooming were assessed during short-term isolation from littermates (STSI; 8-min duration). Thermal nociceptive reactivity was measured before and after this intake test, with normalized differences between pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). Results indicated several effects of social isolation and ethanol mediated through the mu-opioid system. Effects of low dose ethanol (0.5 g/kg) and voluntary consumption of saccharin interacted with endogenous mu-opioid activity associated with STSI. Blockade of mu-opioid receptors on saccharin consumption and paw licking-grooming affected intoxicated animals. Low dose ethanol and ingestion of saccharin blunted effects of CTOP on rearing behavior and nociceptive reactivity. Central injections of CTOP stimulated paw licking and grooming dependent on ethanol dose and type of fluid ingested. Ethanol selectively increased saccharin intake during STSI in females, naloxone and CTOP blocked ethanol-mediated enhancement of saccharin intake. We suggest that enhancement of saccharin intake by ethanol during STSI is the product of synergism between isolation-induced mu-opioid activity that increases the pup's sensitivity to appetitive taste stimulation and the anxiolytic effects of 0.5 g/kg ethanol that decreases behaviors otherwise competing with independent ingestive activity.

Decreased consumption of sweet fluids in μ opioid receptor knockout mice: a microstructural analysis of licking behavior

RATIONALE

Evidence suggests that the palatability of food (i.e., the hedonic impact produced by its sensory features) can promote feeding and may underlie compulsive eating, leading to obesity. Pharmacological studies implicate opioid transmission in the hedonic control of feeding, though these studies often rely on agents lacking specificity for particular opioid receptors.

OBJECTIVES

Here, we investigated the role of mu opioid receptors (MORs) specifically in determining hedonic responses to palatable sweet stimuli.

METHODS

In Experiment 1, licking microstructure when consuming sucrose solution (2 to 20 %) was compared in MOR knockout and wildtype mice as a function of sucrose concentration and level of food deprivation. In Experiment 2, a similar examination was conducted using the palatable but calorie-free stimulus sucralose (0.001 to 1 %), allowing study of licking behavior independent of homeostatic variables.

RESULTS

In Experiment 1, MOR knockout mice exhibited several alterations in sucrose licking. Although wildtype mice exhibited a twofold increase in the burst length when food deprived, relative to the nondeprived test, this aspect of sucrose licking was generally insensitive to manipulations of food deprivation for MOR knockout mice. Furthermore, during concentration testing, their rate of sucrose licking was less than half that of wildtype mice. During sucralose testing (Experiment 2), MOR knockout mice licked at approximately half the wildtype rate, providing more direct evidence that MOR knockout mice were impaired in processing stimulus palatability.

CONCLUSIONS

These results suggest that transmission through MORs mediates hedonic responses to palatable stimuli, and therefore likely contributes to normal and pathological eating.

μ-Opioid blockade reduces ethanol effects on intake and behavior of the infant rat during short-term but not long-term social isolation

Numerous findings in adult and infant rats have shown that the endogenous opioid system is involved in control of ethanol consumption and its reinforcing effects. Opioid systems are also involved in reactivity to social isolation with several factors (age, duration, and type of isolation) affecting this modulation. The present study investigated the effects of a selective mu-opioid antagonist CTOP (0, 0.1, 0.5mg/kg), ethanol (0, 0.5 g/kg), and the interaction of the two drugs on the behavioral consequences of two types of social isolation given to preweanling rats: 1) short-term social isolation from littermates (STSI, duration 8 min) and 2) relatively long-term (5h) isolation (LTSI) from the dam and littermates. Voluntary intake of saccharin, locomotion, rearing activity, paw licking, and grooming were assessed during an 8-min. intake test. Thermal nociceptive reactivity was also measured before and after the testing session with normalized differences in pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). The results indicate that pharmacological blockade of mu-opioid receptors by CTOP substantially attenuated ethanol's anxiolytic effects on the developing rat's reactions to social isolation. Some of these stress-attenuating effects of CTOP were observed only in animals exposed to short-term isolation (STSI) but not in pups isolated for 5h (LTSI). Ethanol selectively increased saccharin intake during STSI in females and CTOP blocked this effect. Ethanol decreased the magnitude of analgesia associated with STSI but had no effect on pain reactivity during LTSI. CTOP by itself did not affect IIA or saccharin intake in sober animals. The findings of the present experiments indicate that the anxiolytic effects of 0.5 g/kg ethanol on pups exposed to STSI are modulated by endogenous opioid activity.

Ghrelin increases the motivation to eat, but does not alter food palatability

Homeostatic eating cannot explain overconsumption of food and pathological weight gain. A more likely factor promoting excessive eating is food reward and its representation in the central nervous system (CNS). The anorectic hormones leptin and insulin reduce food reward and inhibit related CNS reward pathways. Conversely, the orexigenic gastrointestinal hormone ghrelin activates both homeostatic and reward-related neurocircuits. The current studies were conducted to identify in rats the effects of intracerebroventricular ghrelin infusions on two distinct aspects of food reward: hedonic valuation (i.e., "liking") and the motivation to self-administer (i.e., "wanting") food. To assess hedonic valuation of liquid food, lick motor patterns were recorded using lickometry. Although ghrelin administration increased energy intake, it did not alter the avidity of licking (initial lick rates or lick-cluster size). Several positive-control conditions ruled out lick-rate ceiling effects. Similarly, when the liquid diet was hedonically devalued with quinine supplementation, ghrelin failed to reverse the quinine-associated reduction of energy intake and avidity of licking. The effects of ghrelin on rats' motivation to eat were assessed using lever pressing to self-administer food in a progressive-ratio paradigm. Ghrelin markedly increased motivation to eat, to levels comparable to or greater than those seen following 24 h of food deprivation. Pretreatment with the dopamine D1 receptor antagonist SCH-23390 eliminated ghrelin-induced increases in lever pressing, without compromising generalized licking motor control, indicating a role for D1 signaling in ghrelin's motivational feeding effects. These results indicate that ghrelin increases the motivation to eat via D1 receptor-dependent mechanisms, without affecting perceived food palatability.

Energy regulatory signals and food reward

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

Energy regulatory signals and food reward

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

Genetic variance contributes to dopamine and opioid receptor antagonist-induced inhibition of intralipid (fat) intake in inbred and outbred mouse strains

Preference for and intake of solid and emulsified fat (intralipid) solutions vary across different mouse strains. Fat intake in rodents is inhibited by dopamine and opioid receptor antagonists, but any variation in these responses as a function of genetic background is unknown. Therefore, the present study compared the ability of dopamine D1-like (SCH23390) and general opioid (naltrexone) receptor antagonism to alter intake of fat emulsions (intralipid) in mice. Two-hour intakes of 5% intralipid were measured (5-120 min) in seven inbred (BALB/c, C57BL/6, C57BL/10, DBA/2, SJL, SWR, 129P3) and one outbred (CD-1) mouse strains following treatment with vehicle, SCH23390 (50-1600 nmol/kg, ip) and naltrexone (0.001-5 mg/kg, sc). SCH23390 significantly, dose-dependently and differentially reduced intralipid intake at all five (DBA/2, SWR, CD-1), four (SJL, C57BL/6), three (129P3) and one (C57BL/10) of the doses tested, but failed to affect intralipid intake in BALB/c mice. Naltrexone significantly, dose-dependently and differentially reduced intralipid intake at all four (DBA/2), three (SWR, SJL), two (CD-1, C57BL/10) and one (C57BL/6, 129P3) of the doses tested, and also failed to affect intralipid intake in BALB/cJ mice. SCH23390 and naltrexone were respectively 13.3-fold and 9.3-fold more potent in inhibiting intralipid intake in the most sensitive (DBA/2) relative to the least sensitive (BALB/c) mouse strains. A strong positive relationship (r=0.91) was observed for the abilities of SCH23390 and naltrexone to inhibit intralipid intake across strains. These findings indicate that dopaminergic and opioid signaling mechanisms differentially control intralipid intake across different mouse strains, suggesting important genetic and pharmacological interactions in the short-term control of rewarding and post-ingestive consequences of fat intake.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Dopaminergic control of food choice: Contrasting effects of SKF 38393 and quinpirole on high-palatability food preference in the rat

The aim of the present study was to determine the behavioural effects of the selective dopamine D1 receptor agonist, SKF 38393, and of the selective dopamine D2/D3 receptor agonist, quinpirole, on the feeding performance of food-deprived rats in a model of food-preference behaviour. The animals were familiarised with a choice between a high-palatability, high-fat, high-sugar food (chocolate biscuits/cookies) and their regular maintenance diet. Following administration of either SKF 38393 (1.0-10.0 mg/kg, s.c.) or quinpirole (0.03-0.3 mg/kg, s.c.), the animals were observed throughout a 15-min test period, and their feeding behaviour was carefully monitored. Other behavioural categories were also observed. The resulting data were subject to a microstructural analysis to determine the loci of the behavioural effects. The results indicated that SKF 38393 and quinpirole had contrasting effects on the preference for the high-palatability chocolate food. SKF 38393 enhanced the preference, whereas quinpirole eliminated it. These data reinforce the view that forebrain dopamine mechanisms are closely involved in responses to high-palatability energy-dense food constituents, including chocolate. The data also indicate that pharmacological characterization is important, such that dopamine receptor subtypes appear to mediate contrasting effects on food preference for a high-fat, high-sugar food. Hence, brain dopamine appears to be involved in potentially complex ways in determining food preferences, and this may carry implications in the growing evidence for a link between brain dopamine and human obesity.

Mu-opioid receptor cellular function in the nucleus accumbens is essential for hedonically driven eating

Acute pharmacological studies have implicated mu-opioid receptors (MORs) in the shell of the nucleus accumbens (NAC) in mediating responses for palatable food and other natural and drug-induced rewards. However, the long-term behavioral effects of inactivating signal transduction via accumbal MORs, as quantified by an anatomically defined loss of cellular activity, have never been analysed. We combined microinfusion of the irreversible MOR antagonist, beta-funaltrexamine (beta-FNA; 8.0 nmol/0.8 microL, n=9; controls, n=6) with mapping by [35S]GTPgammaS autoradiography to demonstrate an anatomically specific loss of the coupling of MORs to their G-proteins in the dorsal caudomedial shell of the NAC in rabbits. beta-FNA did not alter the stimulated coupling of kappa-opioid receptors. This selective blockade of the cellular function of MORs persistently decreased consumption of a palatable sucrose solution by 40% during a daily 4-h test conducted 2, 3 and 4 days after infusion. beta-FNA did not alter body weight or 20-h consumption of standard chow or water. In 10 different rabbits, infusion of the selective, competitive MOR antagonist, CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) into the same locus produced a reversible decrease in sucrose consumption, with normal intakes returning on the next day. Together, these data appear to establish that MORs in this accumbal subregion support responding for orosensory reward. Overall, these results visualize a discrete brain locus where cellular actions of endogenous opioids mediate behaviors involved in self-administration of foods and perhaps other hedonically valued substances, such as ethanol and drugs of abuse.

Daily increase of fat ingestion mediated via mu-opioid receptor signaling pathway

We investigated the involvement of opioid receptors such as the mu and delta receptors in the predominant elevation of corn oil appetite just after 5-day repeated treatment of corn oil ingestion. Rats were given 5% corn oil emulsified with 0.3% xanthan gum for 20 min at the same hour for 5 consecutive days. A strong appetite for fat was formed after the 5 days presentation, and it was inhibited by naloxonazine, a selective antagonist of the mu-1 receptor, at doses of 3 mg/kg, but not by antagonists of the opioid delta receptor. In days 6, after the formation of a strong appetite for corn oil, an additional injection of naloxonazine suppressed fat intake 0-30, 30-60, 60-90 and 90-150 min after the presentation of the corn oil, but antagonists of the opioid delta receptor did not. These data suggested that the opioid mu receptor is involved in the sharp elevation of corn oil appetite during repeated presentation of corn oil to rats.

Enhanced initial and sustained intake of sucrose solution in mice with an oxytocin gene deletion

Laboratory mice drink little sucrose solution on initial exposure, but later develop a strong preference for sucrose over water that plateaus after a few days. Both the initial neophobia and later plateau of sucrose intake may involve central oxytocin (OT) signaling pathways. If so, then mice that lack the gene for OT [OT knockout (KO)] should exhibit enhanced initial and sustained sucrose intake compared with wild-type (WT) cohorts. To test this hypothesis, female OT KO and WT mice (11–13 mo old) were given a two-bottle choice between 10% sucrose and water available ad libitum for 4 days. On the first day, sucrose intake was 20-fold greater in OT KO mice compared with WT cohorts. The avid sucrose consumption by OT KO mice increased further on day 2 and was sustained at significantly higher levels than intake by WT mice. Enhanced initial and sustained sucrose intake also was observed in 5- to 7-mo-old male OT KO mice. The effect of genotype was observed over a range of sucrose concentrations and was maintained over at least 8 days of continual exposure. However, there was no effect of genotype on daily intake of sucrose-enriched powdered chow. These findings indicate that the genetic absence of OT in mice is associated with enhanced initial and sustained intake of sucrose solutions. Thus central OT pathways may normally participate in limiting initial intake of novel ingesta and may also participate in limiting intake of sweet, highly palatable familiar ingesta.

Naltrexone does not prevent acquisition or expression of flavor preferences conditioned by fructose in rats

The effects of the general opioid antagonist, naltrexone, on the acquisition and expression of flavor preferences conditioned by the sweet taste of fructose were examined. Food-restricted rats were trained over eight daily alternating one-bottle sessions (2 h) to drink an 8% fructose solution containing one novel flavor (CS+/F) and a less preferred 0.2% saccharin solution containing a different flavor (CS-/S). Four groups of rats were treated daily with either saline (control group) or naltrexone doses of 0.1, 1.0, or 5.0 mg/kg during training. Preferences were assessed in two-bottle tests with the CS+/S and CS-/S flavors presented in 0.2% saccharin solutions following saline injections. Naltrexone dose-dependently reduced fructose and saccharin intakes during training, confirming the drug's well-known suppressive effect on the intake of sweet solutions. Despite their reduced training intakes, the naltrexone groups displayed preferences for the CS+/S over the CS-/S (72-86%) that were similar to that of the control group (78%). The effect of naltrexone on the expression of the CS+/S flavor preference was evaluated by treating control rats with naltrexone (0.1-5 mg/kg) prior to CS+/S vs. CS-/S choice tests. The drug doses produced a dose-dependent reduction in CS+/S intake but did not significantly attenuate the CS+/S preference. These data are consistent with the relative inability of naltrexone to reduce flavor-flavor conditioning by sucrose in sham-feeding rats and flavor-nutrient conditioning in rats receiving intragastric sucrose infusions. In contrast, dopamine antagonists reduce both sucrose- and fructose-conditioned flavor preferences, which indicates the sensitivity of these conditioning paradigms to neuropharmacological manipulations. These data indicate that the endogenous opioid system, unlike the dopamine system, does not play a major role in either the acquisition or expression of flavor preference learning as measured in two-bottle choice tests.