Endophinergic modulation of acceptability of putative reinforcers

Optimisation of cognitive performance in rodent operant (touchscreen) testing: Evaluation and effects of reinforcer strength

Operant testing is a widely used and highly effective method of studying cognition in rodents. Performance on such tasks is sensitive to reinforcer strength. It is therefore advantageous to select effective reinforcers to minimize training times and maximize experimental throughput. To quantitatively investigate the control of behavior by different reinforcers, performance of mice was tested with either strawberry milkshake or a known powerful reinforcer, super saccharin (1.5% or 2% (w/v) saccharin/1.5% (w/v) glucose/water mixture). Mice were tested on fixed (FR)- and progressive-ratio (PR) schedules in the touchscreen-operant testing system. Under an FR schedule, both the rate of responding and number of trials completed were higher in animals responding for strawberry milkshake versus super saccharin. Under a PR schedule, mice were willing to emit similar numbers of responses for strawberry milkshake and super saccharin; however, analysis of the rate of responding revealed a significantly higher rate of responding by animals reinforced with milkshake versus super saccharin. To determine the impact of reinforcer strength on cognitive performance, strawberry milkshake and super saccharin-reinforced animals were compared on a touchscreen visual discrimination task. Animals reinforced by strawberry milkshake were significantly faster to acquire the discrimination than animals reinforced by super saccharin. Taken together, these results suggest that strawberry milkshake is superior to super saccharin for operant behavioral testing and further confirms that the application of response rate analysis to multiple ratio tasks is a highly sensitive method for the detection of behavioral differences relevant to learning and motivation.

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.

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.

Endocannabinoids and food consumption: comparisons with benzodiazepine and opioid palatability-dependent appetite

The endocannabinoid system consists of several endogenous lipids, including anandamide and 2-arachidonoyl-glycerol (2-AG), and constitute a retrograde signalling system, which modulates neurotransmitter release and synaptic plasticity. Specific brain-type cannabinoid receptors (CB(1)) are widely distributed in the central nervous system, and are localized presynaptically. Mounting evidence, reviewed here, indicates that cannabinoids can act to increase food consumption, and cannabinoid CB(1) receptor antagonists/inverse agonists reduce food intake and suppress operant responding for food rewards. Hence, endocannabinoids provide the first example of a retrograde signalling system, which is strongly implicated in the control of food intake. Benzodiazepine and opioid palatability-dependent appetite are well-established processes supported by several sources of convergent evidence; they provide pharmacological benchmarks against which to evaluate the endocannabinoids. To date, evidence that endocannabinoids specifically modulate palatability as an affective evaluative process is insufficient and not compelling. Endocannabinoids may have important clinical utility in the treatment of human obesity and forms of eating disorders.

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.

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.

Opioid modulation of taste hedonics within the ventral striatum

There is a long-standing interest in the role of endogenous opioid peptides in feeding behavior and, in particular, in the modulation of food reward and palatability. Since drugs such as heroin, morphine, alcohol, and cannabinoids, interact with this system, there may be important common neural substrates between food and drug reward with regard to the brain's opioid systems. In this paper, we review the proposed functional role of opioid neurotransmission and mu opiate receptors within the nucleus accumbens and surrounding ventral striatum. Opioid compounds, particularly those selective for the mu receptor, induce a potent increase in food intake, sucrose, salt, saccharin, and ethanol intake. We have explored this phenomenon with regard to macronutrient selection, regional specificity, role of output structures, Fos mapping, analysis of motivational state, and enkephalin gene expression. We hypothesize that opioid-mediated mechanisms within ventral striatal medium spiny neurons mediate the affective or hedonic response to food ('liking' or food 'pleasure'). A further refinement of this hypothesis is that activation of ventral striatal opioids specifically encodes positive affect induced by tasty and/or calorically dense foods (such as sugar and fat), and promotes behaviors associated with this enhanced palatability. It is proposed that this brain mechanism was beneficial in evolutionary development for ensuring the consumption of relatively scarce, high-energy food sources. However, in modern times, with unlimited supplies of high-calorie food, it has contributed to the present epidemic of obesity.

Naltrexone fails to block the acquisition or expression of a flavor preference conditioned by intragastric carbohydrate infusions

The effects of naltrexone on the expression and acquisition of flavor preferences conditioned by the postingestive actions of carbohydrates were investigated. Food-restricted rats (Experiment 1) were given one-bottle training with one flavored saccharin solution (CS+) paired with intragastric (IG) infusions of 16% sucrose, and another flavored saccharin solution (CS-) paired with water infusions. In two-bottle tests CS+ was preferred to CS-, and naltrexone (1.0-5.0 mg/kg) reduced total intake but not CS+ preference. In Experiment 2 food-restricted rats that received naltrexone (0.1 or 1.0 mg/kg; NTX group) throughout one-bottle training consumed less CS+ and CS- than did saline-treated control rats. Yet, the NTX and control groups displayed similar CS+ preferences during two-bottle tests when treated with saline or naltrexone (0.1-5.0 mg/kg). In Experiment 3, rats were trained to accept more CS+ than CS- in one-bottle tests. Naltrexone (0.1-2.5 mg/kg) reduced the one-bottle intakes of both solutions, and the rats continued to consume more CS+ than CS-. We conclude that the opioid system modulates the consumption of flavored solutions, but is not critically involved in the acquisition or expression of flavor preferences conditioned by IG carbohydrate.

Pharmacology of flavor preference conditioning in sham-feeding rats: effects of naltrexone

Relatively little is known about the neurochemical and pharmacological mechanisms involved in flavor preference learning. The present study examined the ability of the opioid antagonist, naltrexone to alter the acquisition and expression of flavor preferences conditioned by the sweet taste of sucrose. This was accomplished by adding a novel flavor (the CS+) to a sucrose solution, and a different flavor (the CS-) to a less-preferred saccharin solution. Rats were trained to drink these solutions with an open gastric fistula (sham-feeding), which minimized postingestive actions. Food-restricted (Experiments 1 and 2A) and ad lib-fed (Experiment 2B) rats were given either limited (Experiment 1) or unlimited (Experiment 2) access to the CS+ and CS- solutions during one-bottle training. Preferences were assessed in two-bottle tests (with the CS+ and CS- flavors presented in mixed sucrose-saccharin solutions) following vehicle or naltrexone (0.1-10 mg/kg, SC) treatment. The rats displayed significant CS+ preferences following vehicle, particularly after unlimited access training. In four of five experiments, naltrexone significantly reduced total intakes during the two-bottle, sham-feeding tests. Except for one instance, however, the drug failed to block the preference for the CS+ flavor over the CS- flavor. The effects of naltrexone (0.1 mg/kg) on the acquisition of flavor preferences were studied in sham-feeding rats under limited (Experiment 3A) and unlimited (Experiment 3B) training access conditions. Rats treated with naltrexone during training displayed similar CS+ preferences as did saline-treated rats, even though they consumed less CS+ during training. The naltrexone-trained rats also displayed smaller reductions in total or CS+ intakes than did saline-trained rats when all rats were treated with a 2.5 mg/kg dose of naltrexone during testing. As in previous studies, these results show that naltrexone significantly reduces the intake of sweet solutions, yet it has little or no effect on the acquisition or expression of flavor preferences conditioned by sucrose in sham-feeding rats.

Opioids and food intake: distributed functional neural pathways?

Agonists of the mu, delta, kappa and ORL(1)opioid receptors increase food intake while opioid receptor blockade decreases food intake. The majority of the collected data related to opioids and feeding has led to the speculation that opioids are involved in meal maintenance and orosensory reward; however, some data suggest that opioids may impact feeding associated with energy needs. Based on the wide distribution of CNS opioid receptors and the presence of other neuropeptides in the vicinity of opioidergic pathways, it seems likely that opioids affect multiple feeding systems. For example, opioids in the hindbrain might be involved in both sensory and metabolic aspects of food intake, those in the amygdala in processing of 'emotional' properties of foods, and those in the hypothalamus in energy needs. In this review we present data which support functional diversity of opioids in feeding behavior.

Evaluation of opioid receptor subtype antagonist effects in the ventral tegmental area upon food intake under deprivation, glucoprivic and palatable conditions

Opioid receptor subtype antagonists differentially alter food intake under deprivation (24 h), glucoprivic (2-deoxy-D-glucose, 500 mg/kg, i.p.) or palatable (10% sucrose) conditions with mu (beta-funaltrexamine) and kappa (nor-binaltorphamine), but not delta1 ([D-Ala2,Leu5,Cys6]enkephalin) opioid antagonists reducing each form of intake following ventricular microinjection. Both mu and kappa opioid antagonists microinjected into either the hypothalamic paraventricular nucleus or the nucleus accumbens reduce intake under deprivation and glucoprivic conditions. Palatable intake is reduced by both antagonists in the paraventricular nucleus, but only mu antagonists are active in the accumbens. Food intake is stimulated by mu and delta, but not kappa, opioid agonists microinjected into the ventral tegmental area. The present study examined whether food intake under either deprivation, glucoprivic or palatable conditions was altered by bilateral administration of general (naltrexone), mu, kappa, delta1 or delta2 (naltrindole isothiocyanate) opioid antagonists into the ventral tegmental area. Deprivation (24 h)-induced feeding was significantly reduced by high (50 microg), but not lower (10-20 microg) doses of naltrexone (21%), and by delta2 (4 microg, 19%) antagonism in the ventral tegmental area. 2-Deoxy-D-glucose (500 mg/kg, i.p.)-induced hyperphagia was significantly reduced by high (50 microg), but not lower (20 microg) doses of naltrexone (64%), and by delta2 (4 microg, 27%) antagonism in the ventral tegmental area. Sucrose (10%) intake was significantly reduced by naltrexone (20-50 microg, 25-39%) and delta2 (4 microg, 25%) antagonism in the ventral tegmental area. Neither mu, kappa nor delta1 antagonists were effective in reducing any form of intake following microinjection into the ventral tegmental area. These data indicate that the ventral tegmental area plays a relatively minor role in the elicitation of these forms of food intake, and that delta2, rather than mu, kappa or delta1 opioid receptors appear responsible for mediation of these forms of intake by this nucleus.

Opioid-receptor subtype agonist-induced enhancements of sucrose intake are dependent upon sucrose concentration

Selective mu ([D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO)), delta1 ([D-Pen2, D-Pen5]-enkephalin (DPDPE)), delta2 ([D-Ala2, Glu4]-Deltorphin (Delt II)), kappa1 (U50488H) and kappa3 (naloxone benzoylhydrazone (NalBzOH)) opioid agonists each stimulate food intake in rats. Whereas studies with selective opioid antagonists implicate mu and kappa1 receptors in the mediation of sucrose intake, studies with selective opioid agonists implicate mu and delta receptors in the mediation of saccharin intake. The present study determined if specific delta1, delta2, kappa1, kappa3 and mu opioid-receptor subtype agonists produced similar alterations in sucrose intake as a function of sucrose concentration (0.5%, 2.5%, 10%) across a 1-h time-course. Each of these agonists significantly increased sucrose intake with variations in pattern, magnitude, and consistency as a function of sucrose concentration. Whereas the mu opioid agonist, DAMGO, and the delta1 opioid agonist, DPDPE, each enhanced sucrose intake at higher (2.5%, 10%), but not lower (0.5%), concentrations, the delta2 opioid agonist, Delt II, increased sucrose intake at lower (0.5%, 2.5%), but not higher (10%), concentrations. Kappa opioid agonists produced less consistent effects. The kappa1 opioid agonist, U50488H, increased sucrose intake at high (10%) concentrations and decreased sucrose intake at low (0.5%) concentrations, and the kappa3 opioid agonist, NalBzOH, inconsistently increased sucrose intake at the 0.5% (20 microg) and 10% (1 microg) concentrations. Thus, these data further implicate mu, delta1, and delta2 opioid mediation of palatable intake, particularly of its orosensory characteristics.

Prior exposure to palatable solutions enhances the effects of naltrexone on food intake in rats

Previous research has suggested that chronic intake of palatable foods and fluids enhances the activity of the endogenous opioid system. To examine this suggestion, the effect of naltrexone on food intake was examined in male Long-Evans rats with or without prior exposure to palatable solutions. In Experiment 1, rats were fed laboratory chow alone or laboratory chow and a 32% sucrose solution, and in Experiment 2, were fed chow alone, chow and a 32% Polycose solution, or chow and a 0.15% saccharin solution for three weeks. The sucrose, Polycose, and saccharin solutions were removed 18 h prior to drug administration. Rats then received injections of naltrexone hydrochloride (0.0, 0.3 or 3.0 mg/kg. sc) and chow intakes were measured during the subsequent 1, 2, 4, 6 and 24 h. Naltrexone injections had minimal effects on intakes of animals which previously had consumed only chow. In contrast, naltrexone led to significant dose-related decreases in chow intakes in rats which had previously consumed the sucrose, Polycose, or saccharin solutions. These results provide confirmation for the suggestion that chronic intake of palatable solutions alters the activity of the endogenous opioid system.

Different central opioid receptor subtype antagonists modify maltose dextrin and deprivation-induced water intake in sham feeding and sham drinking rats

Different central opioid receptor subtypes participate in the mediation of intakes of simple (sucrose: mu, kappa 1) and complex (maltose dextrin: mu) carbohydrates as well as deprivation-induced water intake (mu) under real-feeding and real-drinking conditions. An identical pattern of mu and kappa 1 mediation of sucrose intake was observed in sham-feeding rats as well, suggesting their actions on orosensory mechanisms supporting sucose intake. The present study examined whether centrally administered general (naltrexone: 1-50 micrograms), mu (beta-funaltrexamine: 1-20 micrograms), mu 1 (naloxonazine: 50 micrograms), kappa 1 (nor-binaltorphamine: 1-20 micrograms), delta 1 ([D-Ala2, Leu5, Cys6]-enkephalin: 10-40 micrograms) or delta 2 (naltrindole isothiocyanate: 20 micrograms) opioid subtype antagonists altered either maltose dextrin (10%) intake during sham feeding or deprivation (24 h)-induced water intake during sham drinking in rats with gastric fistulas. Sham feeding significantly increased maltose dextrin intake (180%) and sham drinking significantly increased deprivation-induced water intake (256%) over a 60 min time course. Naltrexone significantly and dose-dependently reduced maltose dextrin intake (78%) in sham feeding rats, and deprivation-induced water intake (51%) in sham drinking rats. Maltose dextrin intake in sham feeding rats was significantly reduced by either kappa 1 (69%) or delta 1 (59%) opioid antagonism, was significantly increased by mu 1 antagonism (43%), and was not significantly affected by either mu or delta 2 opioid antagonism. Deprivation-induced water intake in sham drinking rats was significantly reduced by either mu (41%), mu 1 (28%), delta 1 (48%) or delta 2 (28%) opioid antagonism, but was not significantly affected by kappa 1 opioid antagonism. The difference in opioid receptor subtype mediation of maltose dextrin intake in real feeding and sham feeding conditions suggest that kappa 1 and delta 1 receptors are involved in the orosensory mechanisms supporting maltose dextrin intake, while mu receptors are involved in the ingestive and post-ingestive mechanisms supporting maltose dextrin intake. The different patterns of opioid involvement in sucrose and maltose dextrin intake in sham feeding and real feeding conditions provide further support for the hypothesis that at least two different carbohydrate taste systems exist. The difference in opioid receptor subtype mediation of deprivation-induced water intake in real drinking and sham drinking conditions may reflect the removal in the sham drinking condition of a mu-mediated prerestorative satiety mechanism, and the unmasking of other opioid-mediated signalling mechanisms.

General, mu and kappa opioid antagonists in the nucleus accumbens alter food intake under deprivation, glucoprivic and palatable conditions

Ventricular microinjection studies found that whereas mu (beta-funaltrexamine, B-FNA), mu1 (naloxonazine) and kappa (nor-binaltorphamine, Nor-BNI) opioid receptor antagonists, but not delta antagonists, reduce deprivation-induced intake, kappa and mu, but not mu1 or delta antagonists reduce both 2-deoxy-D-glucose (2DG) hyperphagia and sucrose intake. Since opioid agonists stimulate spontaneous food intake in the accumbens, the present study examined whether administration of either naltrexone, B-FNA or Nor-BNI in the accumbens altered intake under deprivation (24 h), glucoprivic (2DG: 500 mg/kg, i.p.) or palatable sucrose (10%) conditions. Naloxonazine's effects in the accumbens were also evaluated for deprivation-induced intake. Deprivation-induced intake was significantly decreased over 4 h by naltrexone (5-20 micrograms, 44%), B-FNA (1-4 micrograms, 55%) and Nor-BNI (4 micrograms, 31%) but not naloxonazine (10 micrograms) in the accumbens. 2DG hyperphagia was significantly decreased by naltrexone (10-20 microgram, 79%), B-FNA (1-4 micrograms, 100%) and NOR-BNI (104 micrograms, 75%) in the accumbens. Sucrose intake was significantly decreased by naltrexone (50 micrograms, 27%) and B-FNA (1-4 micrograms, 37%), but not NOR-BNI in the accumbens. These data suggest that mu receptors, and particularly the mu2 binding site in the accumbens are responsible for the opioid modulation of these forms of intake in this nucleus, and that this control may be acting upon the amount of intake per se.

Selective actions of central mu and kappa opioid antagonists upon sucrose intake in sham-fed rats

Intake of a palatable sucrose solution in real-fed rats is mediated in part by central mu and kappa opioid receptors. Since general opioid antagonists still inhibit sucrose intake in sham-fed rats, the present study examined whether centrally administered mu (beta-funaltrexamine: 5, 20 micrograms), mu1 (naloxonazine: 50 micrograms), kappa (nor-binaltorphamine: 1, 5, 20 micrograms), delta (naltrindole: 20 micrograms) or delta 1 (DALCE: 40 micrograms) opioid subtype antagonists altered sucrose intake in sham-fed rats in a similar manner to systemic naltrexone (0.01-1 mg/kg) and whether such effects were equivalent to altering the sucrose concentration. Sucrose (20%) intake in sham-fed rats was significantly and dose-dependently reduced by naltrexone (59%), beta-funaltrexamine (44%) and nor-binaltorphamine (62%), but not by naloxonazine, naltrindole or DALCE. The reductions in sham sucrose (20%) intake by general, mu and kappa antagonism were similar in pattern and magnitude to diluting sucrose concentration from 20% to 10% in untreated sham-fed rats. Since both real-fed and sham-fed rats share similar patterns of specificity of opioid effects, magnitudes and potencies of inhibition, it suggests that central mu and kappa antagonism acts on orosensory mechanisms supporting sucrose intake.

The delta opioid receptor antagonist naltrindole attenuates both alcohol and saccharin intake in rats selectively bred for alcohol preference

This study demonstrates that the selective delta receptor antagonists ICI 174864 and naltrindole (NTI) attenuate alcohol intake in a dose-dependent manner, without altering water intake, in rats selectively bred for alcohol preference. ICI 174864 had a very limited duration of action, as evidenced by the fact that suppression of alcohol intake lasted for only an hour following ICI 174864 administration. NTI, when administered in a dose of 10 mg/kg, suppressed alcohol intake by 28%. Increasing the dose of NTI to 15 mg/kg produced a 44% suppression of alcohol intake, but a further increase to 20 mg/kg did not produce greater suppression than was seen with a dose of 15 mg/kg (46% versus 44%, respectively). This suggests that NTI is maximally effective in suppressing alcohol intake at a dose of 15.0 mg/kg. NTI displayed a long duration of action, as evidenced by attenuation of alcohol drinking that lasted for at least 8 h following drug treatment. Administering the maximally effective dose of NTI (15 mg/kg) in two parts, separated by 4 h, served to prolong the duration of action of NTI and produced an attenuation of alcohol intake, but not water intake, that lasted for at least 28 h. The effect of NTI on alcohol intake was not specific for alcohol, as evidenced by the fact that NTI reduced the intake of saccharin solutions with and without alcohol.

Analysis of central opioid receptor subtype antagonism of hypotonic and hypertonic saline intake in water-deprived rats

Intake of either hypotonic or hypertonic saline solutions is modulated in part by the endogenous opioid system. Morphine and selective mu and delta opioid agonists increase saline intake, while general opioid antagonists reduce saline intake in rats. The present study evaluated whether intracerebroventricular administration of general (naltrexone) and selective mu (beta-funaltrexamine, 5-20 micrograms), mu, (naloxonazine, 50 micrograms), kappa (nor-binaltorphamine, 5-20 micrograms), delta (naltrindole, 20 micrograms), or delta 1 (DALCE, 40 micrograms) opioid receptor subtype antagonists altered water intake and either hypotonic (0.6%) or hypertonic (1.7%) saline intake in water-deprived (24 h) rats over a 3-h time course in a two-bottle choice test. Whereas peripheral naltrexone (0.5-2.5 mg/kg) significantly reduced water intake and hypertonic saline intake, central naltrexone (1-50 micrograms) significantly reduced water intake and hypotonic saline intake. Water intake was significantly reduced following mu and kappa receptor antagonism, but not following mu 1, delta, or delta 1 receptor antagonism. In contrast, neither hypotonic nor hypertonic saline intake was significantly altered by any selective antagonist. These data are discussed in terms of opioid receptor subtype control over saline intake relative to the animal's hydrational state and the roles of palatability and/or salt appetite.

Naltrexone, serotonin receptor subtype antagonists, and carbohydrate intake in rats

Functional interactions between serotonergic (5-HT) and opioid drugs have been observed with 5-HT3 receptor antagonism enhancing the inhibitory actions of naloxone and naltrexone in both food-deprived and glucoprivic rats; 5-HT2A/C receptor antagonism enhanced naltrexone's inhibition of insulin hyperphagia. The present study examined whether pretreatment with either general 5-HT (methysergide: 0.5-5 mg/kg), 5-HT2A/C (ritanserin: 0.25-2.5 mg/kg), or 5-HT3 (ICS 205930: 0.5-5 mg/kg) antagonists altered the pattern and magnitude of ad lib intake of simple (sucrose: 10%) or more complex (maltose dextrin: MD, 10%) carbohydrate solutions, or naltrexone's (0.25-2.5 mg/kg) inhibition of these forms of intake. Methysergide significantly increased the pattern and magnitude of sucrose intake at low (0.5-2.5 mg/kg) doses, and transiently delayed the pattern of MD intake at high (5 mg/kg) doses. Ritanserin significantly accelerated the pattern, but not the magnitude of sucrose intake at low (0.25-1.25 mg/kg) doses without affecting MD intake. ICS 205930 reduced the magnitude of sucrose intake at the highest (5 mg/kg) dose, and transiently reduced MD intake. Naltrexone dose dependently altered the pattern and magnitude of both sucrose and MD intake. Coadministration of ritanserin and naltrexone either eliminated or delayed the pattern of opioid antagonist inhibition of both sucrose and MD intake. Methysergide and ICS 205930 pretreatment produced minor changes in the pattern of naltrexone-induced inhibition. These data indicate that 5-HT receptor differentially modulate the pattern of carbohydrate intake, and indicate differential ingestive interactions between 5-HT and opioid antagonists under challenge and palatable conditions.

Evidence for serotonergic involvement in saccharin preference in a two-choice test in rehydrating rats

Adult male rats were adapted to a 20-h water-deprivation schedule and trained to drink a 0.1% sodium saccharin and water in a two-choice test (30 min). Several direct acting serotonergic receptor agonists (putatively agonists at the 5-HT2C receptor), MK212, mCPP, and TFMPP, respectively, blocked the saccharin taste preference normally exhibited in this test. Water intake was unaffected. Taken with earlier evidence that these drugs reduce salt taste preference in rehydrating rats, it appears that they may inhibit taste preferences more generally, and that this effect may be closely related to their well-documented anorectic effect. At 3.0 mg/kg, d-fenfluramine almost completely blocked the saccharin taste preference, although l-fenfluramine (0.3 and 1.0 mg/kg) exhibited only hyperdipsic effects. 5-HT creatinine sulphate (0.3-3.0 mg/kg) also produced hyperdipsic effects, but showed no sign of blocking sweet taste preference. As a positive control, it was also shown that the opioid receptor antagonist, naloxone, reduced saccharin taste preference.

Interactions among aging, gender, and gonadectomy effects upon naloxone hypophagia in rats

The present study examined the dose-dependent (0.25-5 mg/kg) effects of systemic naloxone upon deprivation-induced intake and high-fat intake as functions of age (4, 8, 14, and 20 months), gender, and gonadectomy in rats. Significant increases in body weight were observed as functions of age and gonadectomy. Whereas aging significantly reduced basal deprivation-induced intake, it generally failed to alter basal high-fat intake. Whereas age, gender, and gonadectomy failed to alter the decreases in deprivation-induced intake following low (0.25-2.5 mg/kg) naloxone doses, sham males displayed significantly greater age-related and gender-related inhibition following the 5 mg/kg dose of naloxone. Young gonadectomized rats displayed significant increases in naloxone's inhibition of deprivation-induced intake as well. More dramatic changes occurred in naloxone's inhibition of high-fat intake. Naloxone's potency increased in sham female rats as a function of age, and decreased in sham males and ovariectomized females as a function of age. Whereas sham males and ovariectomized females were most sensitive to naloxone's inhibition of high-fat intake at young ages, sham females were most sensitive at older ages. These data indicate that effects of age, gender, and gonadectomy upon naloxone-induced hypophagia dissociate as a function of the type of intake. Because selective opioid antagonist studies demonstrate that deprivation-induced intake is mediated by the mu1 receptor and high-fat intake is mediated by kappa and mu2 receptors, it is postulated that the differential effects of aging, gender, and gonadectomy variables upon opioid mediation of the two forms of intake may reflect their interaction with different opioid receptor subtypes.

Opioidergic manipulations affect intake of 3% NaCl in sodium-deficient rats

On six weekly occasions, a 3% NaCl solution was presented along with water to rats for 2 h 1 day after being treated with furosemide, a diuretic/natriuretic drug that causes a strong hunger for 3% NaCl. On some of the days, the sodium-hungry rats were injected with morphine in doses ranging from 0.3 to 10.0 mg/kg. Morphine produced biphasic effects on intake of 3% NaCl, with doses of 0.3-3.0 mg/kg increasing intakes dose dependently and 10.0 mg/kg decreasing intakes. The 3.0-mg/kg dose nearly doubled rats' mean intake of 3% NaCl. In contrast, naltrexone, an opioid receptor antagonist, reduced intake of 3% NaCl about 25-40% across doses ranging from 0.1 to 10.0 mg/kg. At some doses of morphine and naltrexone, NaCl ingestion was affected without significant influence of water intake. Therefore, it can be inferred that endogenous opioidergic systems participate in the control of NaCl drinking by sodium-deficient rats. The range of demonstrations of opioid involvement in the control of ingestion can now be extended to the hunger for hypertonic NaCl induced by sodium depletion.

Central opioid receptor subtype antagonists differentially reduce intake of saccharin and maltose dextrin solutions in rats

Opioid modulation of ingestion includes general opioid antagonism of deprivation-induced water intake and intake of sucrose and saccharin solutions. Previous studies using selective subtype antagonists indicated that opioid effects upon deprivation-induced water intake occurred through the mu2 receptor and that opioid effects upon sucrose intake occurred through kappa and mu2 receptors. The present study compared the effects of intracerebroventricular administration of opioid receptor subtype antagonists upon intakes of a saccharin solution and a maltose dextrin (MD) solution to determine which receptor subtypes were involved in modulation of ingestion of different preferred tastants. Significant reductions in saccharin intake (1 h) occurred following naltrexone (20-50 micrograms: 66%) and naltrindole (delta, 20 micrograms: 75%), whereas [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, delta 1, 40 micrograms: 45%) had transient (5 min) effects. Neither beta-funaltrexamine (B-FNA, mu), naloxonazine (mu1), nor nor-binaltorphamine (Nor-BNI, kappa) significantly altered saccharin intake. Significant reductions in MD intake (1 h) occurred following naltrexone (5-50 micrograms: 69%) and B-FNA (1-20 micrograms: 38%). MD intake was not reduced by naltrindole, DALCE, naloxonazine and Nor-BNI. Peak antagonist effects were delayed (20-25 min) to reflect interference with the maintenance, rather than the initiation of saccharin or MD intake. Comparisons of opioid antagonist effects across intake situations revealed that naltrexone had consistently low ID40 values for saccharin (29 nmol), MD (25 nmol), sucrose (6 nmol) and deprivation (38 nmol) intake. Despite its significant effects relative to naloxonazine, B-FNA had significantly higher ID40 values for saccharin (800 nmol), MD (763 nmol) and sucrose (508 nmol) relative to deprivation (99 nmol) intake, suggesting that mu2 receptors may be mediating maintenance of intake rather than taste effects. Nor-BNI had low ID40 values for intake of sucrose (4 nmol), but not for saccharin (168 nmol), MD (153 nmol) and deprivation (176 nmol), suggesting that kappa receptors may mediate ingestion of sweet-tasting stimuli. That delta (naltrindole: ID40 = 60 nmol), but not delta 1 (DALCE: ID40 = 288 nmol) antagonists consistently reduce saccharin intake suggests a role for the delta 2 receptor subtype in the modulation of hedonic orosensory signals.

Central opioid receptor subtype antagonists differentially alter sucrose and deprivation-induced water intake in rats

The present study compared the effectiveness of centrally-administered opioid receptor subtype antagonists to inhibit intake of either a 10% sucrose solution under ad libitum conditions, or water following 24 h of water deprivation. Full dose-response functions were evaluated over a 1 h period for the following antagonists: naltrexone (general: 1-50 micrograms), nor-binaltorphamine (Nor-BNI, kappa: 1-20 micrograms), beta-funaltrexamine (beta-FNA, mu: 1-20 micrograms), naltrindole (delta 2: 1-20 micrograms), [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, delta 1: 10-40 micrograms) and naloxonazine (mu 1: 10-50 micrograms). Naltrexone significantly and dose-dependently inhibited both sucrose intake (64-67%) and deprivation-induced water intake (53-67%). Nor-BNI significantly and dose-dependently inhibited sucrose intake (53-55%), but failed to significantly affect (28%) deprivation-induced water intake. beta-FNA significantly and dose-dependently inhibited both sucrose intake (31-34%) and deprivation-induced water intake (36-50%). Naltrindole failed to significantly alter either sucrose intake (24%) or deprivation-induced water intake (16%). Whereas DALCE significantly, but transiently (15-20 min) inhibited sucrose intake (28%), it failed to significantly alter deprivation-induced water intake (14%). Naloxonazine significantly, but transiently (5-10 min) stimulated sucrose intake at low doses (26%), but non-significantly reduced sucrose intake at higher doses (20%). Naloxonazine failed to significantly alter deprivation-induced water intake (16% reduction). These data indicate that whereas the kappa and mu 2 binding sites participate in the opioid modulation of sucrose intake, the mu 2 binding site participates in the opioid modulation of deprivation-induced water intake.

Development of tolerance to endogenous opiates activated by 24-h food deprivation

Four experiments assessed the effects of exposure to 24-h food deprivation on the tail-flick latency of rats exposed to a temperature stimulus. Confirming previous studies, Experiment 1 showed that food deprivation gave rise to analgesia, as indicated by increased tail-flick latencies, that was antagonized by naloxone. Experiment 2 found that analgesia was greatly reduced after five exposures to periods of 24-h food deprivation (alternating with 24-h free access to food), indicating the development of tolerance. Experiments 3 and 3a examined the development of tolerance to the analgesic effects of morphine following repeated morphine injections, saline injections, and exposure to 24-h food deprivation plus saline injections. The combined results of both experiments provided evidence that repeated exposures to either morphine or food deprivation, produced greater tolerance to morphine than did exposures to saline. That food deprivation was cross-tolerant with morphine indicated that tolerance to food deprivation-induced analgesia involved opioid mechanisms. The relevance of opioid tolerance to psychobiological models of feeding and to the development of an animal model of anorexia nervosa was discussed.

Lower pleasantness of palatable foods in nalmefene-treated human volunteers

The involvement of endogenous opioids in control of human food intake and appetite was investigated in a double-blind placebo-controlled study using a single, 2.5 mg oral dose of the opioid receptor antagonist, nalmefene. Ratings of the pleasantness of the smell and the taste, but not the appearance, of a number of foods was significantly lower in nalmefene-treated subjects. The magnitude of this effect was greater in food items independently rated as highly palatable. Caloric intake of a buffet-style meal was 20% lower in nalmefene-treated subjects, with the proportional reduction in intake of individual food items also depending on their palatability. These results lend further support to recent suggestions that opioids are involved in reward-related aspects of ingestion.

Centrally administered opioid antagonists, nor-binaltorphimine, 16-methyl cyprenorphine and MR2266, suppress intake of a sweet solution

Three opioid antagonists (MR2266, 16-methyl cyprenorphine and nor-binaltorphimine) were tested independently for their ability to suppress the intake of a highly palatable saccharin and glucose (S/G) solution after central administration. MR2266 is an equally potent antagonist at kappa (kappa) and mu (mu) opioid receptors. Nor-binaltorphimine (N-BNI) and 16-methyl cyprenorphine (M80) are two recently developed opioid antagonists that were chosen based upon their ability to act more selectively than naloxone at kappa and delta (delta) opioid receptor types, respectively. Prior research has demonstrated that when dissolved in acid and administered centrally, MR2266 (20 micrograms) fails to suppress S/G intake. Because all three antagonists are rather insoluble in water, they were dissolved in dimethyl sulfoxide (DMSO). Rats with chronic ventricular cannula were allowed to consume S/G for a 0.5 hr bout. They received a single intracerebroventricular (ICV) injection of antagonist (MR2266: 0, 10, 20 and 40 micrograms; M80: 0, 5, 10, 20 and 40 micrograms or N-BNI: 0, 1, 3, and 10 micrograms) 10 min prior to the start of the drinking bout. Administration of DMSO alone failed to alter drinking relative to saline, whereas each antagonist significantly attenuated S/G intake. We conclude that, when dissolved in DMSO, these antagonists suppress drinking by blockade of opioid receptors.

Effects of a selective mu opioid receptor agonist and naloxone on the intake of sodium chloride solutions

Endogenous opioid peptides are thought to play a role in mediating the palatability or rewarding aspects of sweet tastes. There is also evidence, however, which suggests that opioids may influence the preference for the taste of salt as well. In the present studies, we measured the effects of central administration of naloxone and the mu agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) on the ingestion of salt solutions. In non-deprived rats given a choice of water and 0.6% saline, ICV injections of DAGO (1 and 3 nmol) significantly increased the intake of 0.6% saline; baseline water intake was minimal and was unaffected by DAGO. When rats were given a choice between water and 1.7% saline, DAGO stimulated both water and saline intake. Because 1.7% saline is a hypertonic solution, the increase in water intake may have been secondary to saline intake. In rats on a deprivation schedule in which water and 0.6% saline were available for only 2-3 h/day, there was a tendency for DAGO to increase 0.6% saline intake and decrease water intake, though these effects were not significant. In rats given water and 1.7% saline, DAGO increased saline intake and had no effect on water intake. Naloxone was also tested in water-deprived rats. Naloxone (20 and 50 micrograms) significantly decreased 0.6% saline intake; baseline water intake was low (3-5 ml) and was unaffected by naloxone. When rats were given a choice between water and 1.7% saline, naloxone (50 micrograms) significantly reduced water intake, while intake of 1.7% saline was slightly increased. These results suggest a role for central mu opioid receptors in mediating the preference for salt solutions.

Effects of nalmefene on feeding in humans. Dissociation of hunger and palatability

Effects of nalmefene on eating were investigated in two groups of ten male volunteers, in a double-blind placebo-controlled study. The nalmefene treated group ate 22% less, both in terms of absolute weight and caloric intake, of a standardised buffet-meal than did the placebo group. No differences in subjective ratings of hunger or satiety were found between the groups, suggesting that the reduced feeding was not a consequence of any change in motivation to eat. When analysed by nutrient content, nalmefene was found to reduce fat and protein, but not carbohydrate, intakes. Analyses of intakes of individual foods showed a differential effect of nalmefene on foods rated as highly palatable. Thus the apparent nutrient specificity of nalmefene appeared to be an indirect consequence of its effect on palatability. Nalmefene also caused slight increases in self-rated alertness, and decreases in ratings of tiredness and elation, although it was thought unlikely that these accounted for observed changes in eating behaviour. No other side-effects were detected, and performance on a choice reaction time task was unaffected. These results add weight to suggestions that endogenous opioids are involved in reward-related aspects of feeding associated with food palatability.

Centrally administered opioid peptides stimulate saccharin intake in nondeprived rats

Endogenous opioid peptides are thought to play a role in mediating the pleasurable or rewarding aspects of the ingestion of certain foods and liquids. We therefore measured the effects of central administration of selective opioid agonists and naloxone on the intake of two concentrations of saccharin solution. All tests were performed on nondeprived rats, such that the taste of the solutions provided the primary incentive to consume. Intracerebroventricular (ICV) administration of the selective mu agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAGO) and the selective delta agonist Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET) (3 nmol) increased intake of a 0.15% saccharin solution by approximately 10 ml over 3 hr. Water was available simultaneously, but intake was minimal. The selective kappa agonist U-50,488H did not increase intake of the saccharin solution. Naloxone (30 and 100 micrograms, ICV) caused a 44% reduction in saccharin solution intake in the first hour; two- and three-hour cumulative intakes were not different from control. DAGO and DTLET were also tested when rats were given a weaker saccharin solution (0.006%) along with water. Both agonists caused small increases in saccharin and water intake, but the increases above baseline were much smaller than those observed with the more palatable 0.15% saccharin solution. These results are consistent with reports by others which suggest that endogenous opioids influence taste preferences or palatability. Further, they indicate a role for central mu and delta opioid receptors in the mediation of this influence.

Effects of naltrexone on food preference and concurrent behavioral responses in food-deprived rats

Naltrexone (0.05-5.0 mg/kg, SC) was administered to food-deprived rats prior to a 15-min food-preference test. Total food intake and feeding duration was reduced following administration of the opiate antagonist. However, while naltrexone reduced the consumption of the initially-preferred chocolate-coated cookies, the ingestion of the nonpreferred standard laboratory chow pellets was significantly enhanced. These data cannot be explained in terms of a general anorexic effect and nonspecific suppression of feeding responses. Instead, they indicate that naltrexone reduced preference for the highly palatable cookies, so that a feeding response to the chow pellets emerged. Under the conditions of test-familiarity, naltrexone did not reduce grooming, locomotion or rearing duration. An increase in locomotion may have been secondary to the reduction in feeding. The results agree with previous data from animal and human studies in suggesting that endogenous opioid peptide activity is involved in the palatability of preferred foods.

A small dose of morphine increases intake of and preference for isotonic saline among rats

Water-deprived rats were given hourly opportunities to ingest physiological saline and water for a number of days until they were taking substantial amounts of both solutions. Prior to some opportunities to ingest, they were injected with either morphine (2.0 mg/kg) or a placebo. Across a variety of procedures, morphine increased intake of and, in 1-hr tests, increased preference for 0.9% NaCl. Intake of 1.5% NaCl also increased after administration of morphine. These data suggest that endogenous opioids are involved in sodium intake. These data also provide further support for the idea that one or more of the endogenous opioid systems are involved in the regulation of ingestion.

Attenuation of sham feeding by naloxone is stereospecific: evidence for opioid mediation of orosensory reward

The time course of the suppressive action of naloxone on sham feeding was examined in gastric fistulated rats. One hour sham intake of 30% sucrose solution was halved by 1.25 mg/kg IP (-)-naloxone. A maximal 75% reduction was obtained with 2.5 mg/kg. Naloxone's effect appeared to mimic the attenuation of intake rate produced by sucrose dilution. The stereoisomer (+)-naloxone (1.25, 2.5 and 10 mg/kg IP) was ineffective. These data confirm the involvement of opioid receptors in naloxone anorexia and are consistent with opioid involvement in the rewarding aspects of ingestion.

Naloxone attenuation of sham feeding is modified by manipulation of sucrose concentration

The time course of sucrose (5, 10 and 20%; w/v) sham feeding was monitored in one hour tests. Intake levels increased as a function of concentration. Naloxone (1.25 mg/kg, IP) attenuated the sham feeding of 10% sucrose solution in gastric fistulated rats, without affecting initial intake rates. Furthermore, after naloxone the intake pattern of 10% sucrose was identical to that for 5% sucrose in untreated rats. In a second test, substitution of 10% sucrose by a 20% solution after 15 min of sham feeding reversed the effect of naloxone, restoring intake to 10% baseline levels. Thus naloxone's effect appeared to be behaviourally equivalent to that of sucrose dilution and was counteracted by increasing sucrose concentration. Naloxone was apparently more effective against the lower sucrose concentration, suppressing intake at an earlier stage of testing. The data confirm the importance of oropharyngeal stimulation to the suppressive action of naloxone and support opioid mediation of orosensory reward.

Bidirectional changes in the consumption of food produced by beta-carbolines

beta-Carboline derivatives provide examples of benzodiazepine receptor ligands which span the range: full agonist-partial agonist-antagonist-partial inverse agonist-full inverse agonist. Taken together, the effects of these compounds illustrate two important principles: firstly, the bidirectionality of effects which can be achieved using benzodiazepine receptor ligands; secondly, the selectivity of effects which are produced by partial agonists. Applied to the study of feeding processes, these principles imply that both hyperphagic and anorectic effects can be generated by actions of selected ligands at benzodiazepine receptors. Furthermore, they suggest that a hyperphagic effect may occur in the absence of side-effects (e.g., sedation, muscle-relaxation), which are characteristic of classical benzodiazepines. Experimental data in support of these predictions are presented. A microstructural approach to feeding behavior indicated that a benzodiazepine receptor agonist and an inverse agonist extend and abbreviate, respectively, the duration of individual bouts of eating. Preference for a saccharin solution was attenuated by the beta-carboline inverse agonist, FG 7142, but rejection of a quinine solution was not increased. Adrenalectomy had no effect on the anorectic effect of inverse agonists.

Adrenalectomy and the anorectic effects of benzodiazepine inverse agonists and opiate antagonists in rats fed a palatable diet

The benzodiazepine (BZ) receptor inverse agonists, FG 7142 (1.25-10.0 mg/kg, IP) and CGS 8216 (2.5-20.0 mg/kg, IP), significantly attenuated the consumption of a palatable sweetened diet by non-deprived male rats in a 30 min test. Adrenalectomy failed to affect the reduction in food intake produced by these two drugs. Similarly, the anorectic effects of the opiate antagonists, naltrexone (0.3-3.0 mg/kg, SC) and diprenorphine (0.3-3.0 mg/kg, SC) in the same feeding paradigm were unaffected by adrenalectomy. So far as palatability-induced feeding in concerned, anorectic effects of BZ inverse agonists and opiate-antagonists appear to be adrenal-independent in the rat. The benzodiazepines, clonazepam (0.3 mg/kg, IP) and diazepam (1.0 mg/kg, IP), stimulated food consumption in both adrenalectomized and sham-operated animals.

Demonstrating morphine's potentiating effects on sucrose-intake

Mildly deprived rats (18-hr of deprivation of water) were given the opportunity to take a solution of sucrose daily for periods of either 10, 18, 31, 56, or 100 min. After daily intakes stabilized and prior to a session, rats were given an injection of either morphine sulfate, 1.0 mg/kg, or naloxone hydrochloride, 2.5 mg/kg, an agonist and an antagonist, respectively, at the opioid receptors. Naloxone, as expected, decreased intakes regardless of the test-session's length. Morphine decreased intakes of the shorter sessions, but increased intake of the longest session. Subsequently, injections of morphine were given 56 min into a 100-min session. These injections also increased intake. Morphine's effects in potentiating intake seem to have special relevance with respect to the continuance of ingestion. Variations across experiments, in duration of test-sessions, could account for the variations in conclusions drawn about whether or not morphine and other agonists potentiate intake of ingesta.

Endorphins and food intake: kappa opioid receptor agonists and hyperphagia

Evidence from studies which utilise either opiate receptor agonists and antagonists strongly indicate a role for endorphinergic mechanisms in the control of feeding responses. Two means by which these compounds may exert an effect on feeding can be singled-out. Firstly, emerging evidence suggests that the process of achieving satiety (terminating a meal, or choice of a commodity) may be accelerated following treatments with opiate receptor antagonists. Secondly, the preference for highly palatable solutions (sweet solutions have received most attention) in two-bottle tests is blocked after injection of opiate receptor antagonists. This finding has been interpreted in terms of the abolition of the reward or incentive quality associated with the particularly attractive flavour. These two mechanisms of action may represent two aspects of a single, fundamental process. Following an introduction to rat urination model of in vivo kappa agonist activity, the consistent effect of several kappa agonists (including the highly selective U-50,488H) to stimulate food consumption is described. Recognising that members of the dynorphin group of endogenous opioid peptides are kappa receptor ligands, some with a high degree of selectivity, and the evidence the dynorphins and neo-endorphins produce hyperphagia in rats is particularly interesting. Such lines of evidence lead to the hypothesis that peptides of the dynorphin group may act endogenously to promote the expression of normal feeding behaviour.

Evidence for opiate receptor involvement in the consumption of a high palatability diet in nondeprived rats

Nondeprived adult rats were familiarized with a highly palatable diet (powdered small animal diet mixed with sweetened condensed milk and water). The palatability of food was such that it induced vigorous feeding responses, 15-20 g food consumed within the first 30 min of access. In partially-satiated male rats, the kappa receptor agonists EKC and U-50,488 (subcutaneously administered) produced large increases in food consumption in the first 30 min of access, post-injection. In experiments with naloxone and WIN 44,441-3, we found that the effects of naloxone (0.01-10 mg/kg; S.C.) were crucially dependent on the sex and dietary history of the animals. Male, obese rats were most sensitive to naloxone's anorectic effect. Lean females were completely insensitive. WIN 44,441-3 (0.01-10 mg/kg, S.C.) had no effect on food intake in any group of animals.