Naloxone suppresses insulin-induced food intake in novel and familiar environments, but does not affect hypoglycemia

Endogenous opioid modulation of food intake and body weight: Implications for opioid influences upon motivation and addiction

This review is part of a special issue dedicated to Opioid addiction, and examines the influential role of opioid peptides, opioid receptors and opiate drugs in mediating food intake and body weight control in rodents. This review postulates that opioid mediation of food intake was an example of "positive addictive" properties that provide motivational drives to maintain opioid-seeking behavior and that are not subject to the "negative addictive" properties associated with tolerance, dependence and withdrawal. Data demonstrate that opiate and opioid peptide agonists stimulate food intake through homeostatic activation of sensory, metabolic and energy-related In contrast, general, and particularly mu-selective, opioid receptor antagonists typically block these homeostatically-driven ingestive behaviors. Intake of palatable and hedonic food stimuli is inhibited by general, and particularly mu-selective, opioid receptor antagonists. The selectivity of specific opioid agonists to elicit food intake was confirmed through the use of opioid receptor antagonists and molecular knockdown (antisense) techniques incapacitating specific exons of opioid receptor genes. Further extensive evidence demonstrated that homeostatic and hedonic ingestive situations correspondingly altered the levels and expression of opioid peptides and opioid receptors. Opioid mediation of food intake was controlled by a distributed brain network intimately related to both the appetitive-consummatory sites implicated in food intake as well as sites intimately involved in reward and reinforcement. This emergent system appears to sustain the "positive addictive" properties providing motivational drives to maintain opioid-seeking behavior.

Blockade of central kappa-opioid receptors inhibits the antidipsogenic effect of interleukin-1beta

The objective of the present study was to investigate the role of brain kappa-opioid receptors (KOR) in the antidipsogenic effect promoted by third ventricle injections of interleukin-1beta (IL-1beta). Wistar male rats were submitted to three different, thirst-inducing, physiological conditions: dehydration induced by water deprivation, hyperosmolarity induced by salt-load and hypovolemia induced by polyethylene glycol subcutaneous injection. Third ventricle injections of IL-1beta significantly inhibited the increase in water intake observed in those situations. The pharmacological blockade of central KOR by the selective KOR antagonist nor-binaltorphimine (BNI) at different doses significantly inhibited the antidipsogenic effect induced by the central administration of IL-1beta in all conditions tested: dehydration, hypovolemia and hyperosmolarity. The central administration of IL-1beta failed to induce any locomotor deficit, as verified in an open field test. Stimulation of the central interleukinergic component did not result in any general suppression of ingestive behavior since no change in saccharin intake was recorded during a dessert test in animals receiving central injections of IL-1beta. Furthermore, the inhibitory effects of IL-1beta on water intake cannot be attributed to sickness-like effects induced by these compounds, since an aversion test excluded this possibility. In summary, the data shown in the present study clearly show that the antidipsogenic effect observed in rats following third ventricle injections of IL-1beta depend on the functional integrity of a brain kappa-opioid-dependent component.

Choice of lard, but not total lard calories, damps adrenocorticotropin responses to restraint

Although rats given the choice of eating high-density calories as concentrated sucrose solutions or lard exhibit reduced responsivity in the hypothalamo-pituitary-adrenal axis, rats fed high-fat diets have normal or augmented responses to stressors. To resolve this apparent discrepancy, we compared in adult male rats the effects of 7-d feeding with lard + chow (choice) to feeding a 50% lard-chow mixture (no-choice) and to chow only. Rats with choice composed diets with 50-60% total calories from lard. Rats were exposed to 30 min of restraint on d 7. In the choice group, there was a robust inhibition of ACTH and corticosterone responses to restraint compared with chow or no-choice groups. Total caloric intake was less with choice than no-choice. Fat depot weights and body weight gain were similar in the high-fat groups. Leptin concentrations were equal but insulin was higher in the choice group. We conclude the following: 1) choice of eating high-density calories strongly damps hypothalamo-pituitary-adrenal responses to stress; without choice, high-density diet is ineffective; and 2) insulin may signal metabolic well-being, and may act through hypothalamic sites to reduce caloric intake but through forebrain sites to damp stress responses.

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.

Ovarian morphology and insulin sensitivity in women with bulimia nervosa

OBJECTIVE

Hyperinsulinaemia has a role in the development of hyperandrogenism and polycystic ovary syndrome in women of normal weight. Polycystic ovaries are common in women with bulimia nervosa and this study aimed to determine whether women with bulimia nervosa are insulin resistant and to examine the relation between insulin sensitivity and ovarian morphology.

DESIGN

A short intravenous insulin tolerance test was used as a direct measure of insulin sensitivity in a group of women with bulimia nervosa and a control group.

PATIENTS

A series of 12 women with bulimia nervosa and normal weight was compared with a control group of 9 healthy women who had no clinical signs of eating disorder or hyperandrogenism and did not have polycystic ovaries.

MEASUREMENTS

Bulimic behaviour was assessed using the BITE (Bulimia Investigation Test, Edinburgh) questionnaire and clinical interviews. Ovarian morphology was assessed using transabdominal ultrasonography. Insulin sensitivity and serum insulin, fasting glucose, LH, FSH, prolactin, testosterone, androstenedione and sex hormone binding globulin (SHBG) were measured and compared between the two groups.

RESULTS

Ten of the 12 women with bulimia nervosa underwent ovarian ultrasound examination and they all had polycystic ovaries. There was no difference in serum LH, FSH, testosterone, androstenedione of SHBG concentrations between the women with bulimia nervosa and the non-bulimic control group. Fasting blood glucose concentrations were normal in all the women studied and did not differ between the women with bulimia nervosa and the control women. There was also no difference in fasting serum insulin or insulin sensitivity between the women with bulimia nervosa and the nonbulimic women.

CONCLUSIONS

Bulimia nervosa is not associated with insulin resistance and chronic hyperinsulinaemia.

Norbinaltorphimine blocks the feeding but not the reinforcing effect of lateral hypothalamic electrical stimulation

The role of central kappa opioid receptors in the regulation of feeding and reward was evaluated using electrical brain stimulation paradigms in combination with the selective kappa antagonist, norbinaltorphimine (nor-BNI). Lateral ventricular injection of 10.0 and 50.0 nmol doses of nor-BNI increased the lateral hypothalamic stimulation frequency threshold for eliciting feeding behavior but had no effect on threshold for self-stimulation in the absence of food. This result is identical to those previously reported for naloxone and antibodies to dynorphin A and suggests that opioid activity is associated with feeding behavior rather than the eliciting brain stimulation. A further similarity between naloxone, dynorphin antiserum, and nor-BNI is their preferential effect on feeding threshold values obtained later, rather than initially, in a post-injection test session. This pattern of threshold elevation is shown to differ from that of the appetite suppressants, amphetamine and phenylpropanolamine, which elevate threshold uniformly throughout a post-injection test. The signature pattern of threshold elevation produced by opioid antagonism is consistent with the hypothesis that opioid activity is involved in the maintenance rather than the initiation of feeding. Specifically, it is hypothesized that a dynorphin A/kappa receptor mechanism is triggered by food taste and sustains feeding behavior by facilitating incentive reward.

Naltrexone, serotonin receptor subtype antagonists, and glucoprivic intake: 2. Insulin

Opiate antagonist inhibition of deprivation-induced intake and 2-deoxy-D-glucose (2DG) hyperphagia is significantly enhanced by the 5-hydroxytryptamine3 (5-HT3) antagonist, ICS-205,930. Interactions between opiate antagonists and either 5-HT or 5-HT2 antagonists produced smaller effects. The present study evaluated whether insulin (5 U/kg) hyperphagia was affected by methysergide (0.5-5 mg/kg), ritanserin (0.25-2.5 mg/kg), and ICS-205,930 (0.5-5 mg/kg) alone or in combination with naltrexone (2.5-10 mg/kg). Whereas ICS-205,930 stimulated insulin hyperphagia across the 6-h time course, ritanserin and, to a lesser degree, methysergide reduced insulin hyperphagia. Naltrexone marginally (19-33%) reduced insulin hyperphagia. Pairing naltrexone with either ICS-205,930 or ritanserin significantly suppressed insulin hyperphagia after 2 h. Pairing naltrexone with each of the serotonin antagonists significantly enhanced insulin hyperphagia after 4 and 6 h. These data suggest that 5-HT2 and 5-HT3 receptor subtypes interact with opioid systems to modulate insulin hyperphagia. Given that central insulin reduces food intake and body weight, the interaction between serotonergic and opioid systems may occur peripherally.

Mediation of insulin hyperphagia by specific central opiate receptor antagonists

The hyperphagic properties of insulin (10 U/kg, s.c.) were transiently (2h) and dose-dependently inhibited (30%) by central pretreatment with naltrexone (20-50 micrograms, i.c.v.). The irreversible mu opioid antagonist, beta-funaltrexamine (B-FNA, 20 micrograms, i.c.v.) significantly inhibited insulin hyperphagia by 28-54% over the 6-h time course. In contrast, insulin hyperphagia was only transiently (2 h) inhibited (27-30%) by either the irreversible mu 1 antagonist, naloxonazine (50 micrograms, i.c.v.) or the selective kappa antagonist, nor-binaltorphamine (NorBNI, 20 micrograms, i.c.v.). The delta-antagonistic actions of [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, 40 micrograms, i.c.v.) failed to affect insulin hyperphagia. These data suggest that the mu 2 opioid receptor subtype modulates insulin hyperphagia.

Enantiomers of phenylpropanolamine suppress food intake in hyperphagic rats

Phenylpropanolamine (PPA, d,l-norephedrine), available in many over-the-counter nasal decongestants and appetite suppressants, is a racemic mixture of the enantiomers d- and l-norephedrine. The present study evaluates the effects of the individual PPA enantiomers on a variety of nondrug (food deprivation) and drug-induced hyperphagias (2-deoxyglucose and insulin). Racemic PPA has been shown to significantly suppress food intake in these hyperphagic models. Both l-norephedrine (5-50 mg/kg) and d-norephedrine (5-150 mg/kg), administered intraperitoneally, significantly suppressed feeding after a 4-hr fast during the dark cycle. During the light period, l-norephedrine (7.5, 10, 15 mg/kg) and d-norephedrine (75, 100, 150 mg/kg) significantly reduced food intake at the 1-hr and 3-hr time intervals in the 24-hr food deprivation-, insulin- and 2-deoxyglucose-induced hyperphagic models. Only 7.5 mg/kg l-norephedrine in the insulin-induced hyperphagia at 3 hr failed to significantly suppress feeding. These results indicate that each individual PPA enantiomer possesses the ability to suppress food intake in rats made hyperphagic by various stimuli.

Effect of morphine and nalmefene on energy balance in diabetic and non-diabetic rats

Male rats made diabetic by intravenous injection of streptozotocin were used to evaluate the effect of the diabetic state on morphine- and nalmefene-induced changes in food intake and body weight. Morphine increased 4 hour food intake in non-diabetic rats after an initial injection, but increased intake in diabetic rats only after repeated injections. Unlike short term measurements, morphine decreased food intake when measured over 24 or more hours in both groups. Chronic injection of morphine decreased body weight only in non-diabetic rats. Feed efficiency data suggest that morphine had a more potent effect on energy balance in the non-diabetic rats. The opioid antagonist, nalmefene, did not alter body weight in either group and only altered food intake in the diabetic animals. These data are in concert with other reports indicating that the diabetic state renders animals less responsive to the effects of morphine on nociception and smooth muscle contraction.

Antibodies to dynorphin A(1-13) but not beta-endorphin inhibit electrically elicited feeding in the rat

Highly specific antibodies to dynorphin A(1-13), infused into the lateral ventricle, elevated brain stimulation threshold for eliciting feeding behavior. Antibodies to beta-endorphin had little or no effect. Temporal analysis of the anorectic action indicated a striking similarity to the effect of systemically administered naloxone. These findings suggest that central dynorphin is involved in the control of ingestive behavior and that the anorectic action of naloxone may result from antagonism of dynorphinergic transmission.

Phenylpropanolamine decreases food intake in rats made hyperphagic by various stimuli

Phenylpropanolamine (PPA, d,l-norephedrine), found in many over-the-counter appetite suppressants and nasal decongestants, induces anorexia by a yet unidentified mechanism. The present study evaluates the effects of PPA on different types of non-drug- and drug-induced hyperphagias (i.e., food deprivation, 2-deoxy glucose, ketocyclazocine and insulin). Phenylpropanolamine (15, 25 and 35 mg/kg IP) significantly reduced food intake in a dose-related fashion at the 1 hr and 3 hr time intervals in the food deprivation-, insulin- and 2-deoxy glucose-induced hyperphagic models. Phenylpropanolamine produced a non-dose-related 99% reduction of food intake in the ketocyclazocine-induced model at the 1 and 3 hr measurement, which was most likely due to a combination of the appetite suppressant activity of PPA and the sedation produced by ketocyclazocine in combination with PPA. We conclude that PPA is capable of suppressing appetite in rats made hyperphagic by various stimuli.

Opioid and non-opioid components of insulin-induced feeding

The present study was initiated to clarify the involvement of endogenous opioids in insulin-induced feeding. Naloxone (3 mg/kg) was injected in male Sprague Dawley rats every hour for 2 hours after insulin injection (10 U/kg). Only the first hour food intake was depressed (68% reduction). When naloxone was given only 1 hour after insulin administration, depression of food intake was not noted. When food was withheld for 2 hours after insulin injection, both naloxone and its long acting congener, naltrexone (3 mg/kg) were able to depress only the first hour feeding subsequent to food presentation. These data suggest that insulin-induced feeding can be divided into two pharmacologically distinct phases: the early phase being naloxone-sensitive while the late phase is naloxone-insensitive. Furthermore, the early phase begins with the presentation of food and not with the administration of insulin.

Opioids and consummatory behavior

Since the second decade of this century it has been known that opiates can influence ingestive behaviors. Generally, opioid agents enhance feeding and opioid antagonists decrease feeding. The present paper reviews the responsiveness of different animal species to opiates in relation to ingestive behaviors, the opioid receptors involved in such consummatory behaviors, the site of action of opioid modulation of feeding, the role of glucose in opioid induced feeding, and endocrine effects on opioid feeding systems. We emphasize the finding that more than one opioid receptor is involved in the modulation of feeding. A large body of evidence indicates a major role for the dynorphin/alpha-neo-endorphin kappa opioid receptor as one of the receptors involved in feeding modulation. Opioids appear to exert their effect predominantly within the central nervous system, though peripheral effects on taste and gastrointestinal function may play a role in opioid-induced feeding. Although opioid blockade acutely blocks food intake, chronic administration of opiate antagonists to humans and laboratory animals has not proven to be an effective means of decreasing body weight. Chronic opiate administration decreases body weight and autosensitization of beta-endorphin increases body weight. Thus, although it is clear that opioids can effect food intake, it is not clear what effect chronic administration of opioids has no food intake or body weight.

Sugar, opioids and binge eating

There is evidence that endogenous opiates are involved in the control of feeding in experimental animals. Several types of experimental obesity are associated with increased opiate production and/or increased numbers and sensitivity of opiate receptors. Research with experimental animals suggests that nutrients, particularly sugar, have an effect on feeding behavior that is mediated by opiates. For instance, the obesity-producing effect of a palatable diet in rodents is blocked by opiate antagonists. Stress induced feeding in rodents leads to preferential sucrose ingestion and is blocked by opiate antagonists and beta-endorphin. The effect of nutrients on the endogenous opiate system of humans is less clear. Clinical experience suggest that carbohydrates (sugar in particular) play a role in binge eating and obesity. Many binge eaters preferentially eat sweets during a binge. Many obese individuals consume more than half of their total daily calories as carbohydrates. Sweet snacking is a frequent behavior at times of stress. Recent evidence suggests that sugar can lead to increased beta-endorphin production in obese subjects.

Dopaminergic and serotoninergic anorectics differentially antagonize insulin- and 2-DG-induced hyperphagia

The efficacy of anorectic drugs has been studied in rats made hyperphagic by injection of insulin or of 2-deoxy-d-glucose (2-DG). It was found that anorectics that act through a serotoninergic mechanism, i.e., d- and d-l-fenfluramine, p-chloroamphetamine, quipazine and fluoxetine antagonize both insulin- and 2-DG-induced overeating, while anorectics acting through the dopaminergic system, i.e., d-amphetamine, diethylpropion, lisuride, bromocriptine and mazindol, antagonize the hyperphagia induced by 2-DG but not that induced by insulin. Neither serotoninergic nor dopaminergic anorectics modified insulin-induced hypoglycaemia. The serotonin (5-HT) receptor blocker metergoline did not modify the hyperphagic response to insulin or 2-DG. The present results indicate that there are different neuronal or humoral circuits underlying the hyperphagic responses to insulin and 2-DG. In addition, these results, which show different effectiveness of anorectic drugs depending on what has provoked the hyperphagia, suggest that differences in the etiology of the hyperphagia of obese subjects must be taken into consideration when choosing therapy.

Environment modulates naloxone's suppressive effect on feeding in diabetic and non-diabetic rats

We evaluated the effect of environment on naloxone-induced suppression of feeding in streptozotocin rats and sham injected controls. Naloxone was administered to animals fasted for 24 hours and food intake was measured at 30, 60 and 120 minutes. Diabetic rats, in their home cages, were insensitive to naloxone's suppressive effect for the first 30 minutes and the 5 mg/kg dose suppressed feeding only at 120 minutes. In control rats, feeding was suppressed at 1 and 5 mg/kg naloxone during the first 30 minutes. In contrast, when animals were placed in novel plastic cages, control animals were insensitive to naloxone at all time points at doses as high as 5 mg/kg. In novel cages, diabetic rats responded to doses of 1 and 5 mg/kg during the first 30 minute period by lowering food intake. It should also be noted that basal food intake was suppressed (40-53%) when animals were placed in novel cages. These data suggest that stress of a novel environment alters the neuroregulatory system involved in inducing feeding. Lack of response of normal rats to naloxone's suppressive effect in a novel environment suggests that (1) a non-opioid feeding system operates under these conditions, or (2) opioid receptors are occupied as a result of the release of endogenous opioids due to stress. The opposite result observed in the diabetics indicates that glucose has a modulating effect on opioid effects.

Effects of intracerebroventricular injection of naloxone on operant feeding and drinking in pigs

Operant feeding and drinking to satiation were studied in prepubertal pigs deprived of food or water for 18 hours and then given intracerebroventricular (ICV) injections of a solution of naloxone hydrochloride. In feeding tests there was no difference in the amount of food consumed, or in the rate at which reinforcements were obtained, between pigs given ICV injections of 0.4 or 0.8 mg naloxone and those receiving a control injection of saline. However, in drinking tests, injection of both 0.2 and 0.4 mg naloxone significantly (p less than 0.01) reduced the quantity of water drunk and slowed the rate at which reinforcements were obtained. No significant effects on operant water intake were seen after intravenous injection of 0.4 mg naloxone.

Opiate antagonists stereoselectively attenuate the consumption of food but not of water by pigeons

Three experiments were performed to evaluate the influence of the two opiate antagonists, naloxone HCl (NAL) and Mr 2266, on the ingestive behavior of domestic pigeons. In the first and second experiments, these drugs were administered at 3 doses (0.25, 1 and 4 mg) to non-deprived and to 24 hr-fasted pigeons, respectively. Measure of the food and water consumption of the birds for up to 6 hrs post-injection revealed that as compared to control values, administration of both antagonists attenuated feeding without reducing drinking. Administration of both drugs produced a rather similar anorexic effect, with the difference that Mr 2266 tended to decrease the food intake for a longer period of time than did NAL. In the third experiment, the food consumption of fasted pigeons was reduced by the injection of Mr 2266, but not of its (+) stereoisomer Mr 2267, showing that the behavioral influence of Mr 2266 is stereoselective. Confronted with other studies, these results suggest that in pigeons, opiate receptors participate in the regulation of the food consumption without playing a major role in the control of the water intake.

Differential inhibition by propranolol of feeding induced in rats by various stimuli

Opiate receptor blockade, or forced imbibition of 2% NaCl to deplete pituitary dynorphin decreases 2-deoxy-D-glucose (2-DG), but not insulin-induced hyperphagia, indicating a possible role for dynorphin in the eating associated with endogenous opiates. Beta-adrenergic receptor blockade decreases vasopressin release induced by 2-DG but not by insulin. Because vasopressin and dynorphin are sometimes co-localized, it was hypothesized that naloxone-sensitive feeding might be selectively inhibited by beta-adrenergic blockade with propranolol. Propranolol in doses as low as 2.5 mg/kg inhibited 4 hr feeding induced by 2-DG (400 mg/kg). Propranolol did not significantly affect feeding induced by ketocyclazocine administration (3.0 mg/kg) or by 24 hr food deprivation. Feeding stimulated by insulin (10 U/kg) was significantly inhibited by propranolol (2.5 mg/kg) only when the propranolol was reinjected during the period 2 hr after insulin injection, when the induced feeding was greatest. In summary, propranolol inhibited opiate-related (2-DG) as well as opiate-independent (insulin) hyperphagias. It also failed to inhibit food intake resulting from the opiate related stimulus of 24 hr food deprivation. Therefore, naloxone sensitive hyperphagias were not specifically inhibited by beta-adrenergic blockade, indicating that vasopressin-associated dynorphin is not involved in opiate related feeding.

Naloxone treatment attenuates food but not water intake in domestic pigeons

Satiated as well as 24 h food-deprived pigeons showed an attenuated consumption of food in response to peripheral injection of the opiate antagonist naloxone. The minimal effective amount of the drug was found to be 1.5 mg in satiated birds and 0.5 mg in fasted birds. The anorexic effect of naloxone was observed for no longer than 3 h in both situations, and it did not differ at doses of either 1.5 mg or 5 mg of the antagonist. By contrast, doses of up to 5 mg of naloxone failed to depress the water intake of pigeons which were either 24 h water-deprived or were loaded with hypertonic saline. Together with previous studies, these results suggest that, in pigeons, endorphinergic naloxone-sensitive mechanisms are involved in the regulation of feeding but play no major role in the control of drinking. Possible modes of action of opiate antagonists on ingestive behaviour are discussed.

Ingestive behaviour of the pigeon: stereoselective influence of the opiate agonist levorphanol and its antagonism by naloxone

Four experiments evaluated the effect of levorphanol on ingestive behaviour of different groups of non-deprived pigeons. In experiments 1 and 2, levorphanol and its (+)stereoisomer dextrorphan were administered at three doses (0.25, 1 and 2 mg). As compared with control values, levorphanol dose-dependently reduced food intake. This anorexia persisted for at least 5 h post-injection. A late hyperdipsia was also observed. These changes were stereoselective, suggesting that they followed the binding of levorphanol to opiate receptors. In experiments 3 and 4, the anorexic effect of 1 mg levorphanol, but not its hyperdipsic effect, was partly antagonized by the concomitant administration of either 0.25 mg or 1 mg naloxone. Given alone at the dose of 1 mg, naloxone slightly and transiently reduced food, but not water, intake. These results are discussed in terms of the endorphinergic regulation of ingestive behaviour in birds.

Naloxone suppresses intake of highly preferred saccharin solutions in food deprived and sated rats

In repeated tests naloxone (1 mg/kg, sc) suppressed intake of a narrow range of highly preferred saccharin concentrations (0.1 and 1.0%) in nondeprived male rats but a wider range of concentrations (.001-1.0%) following 10 hr. food deprivation. In sated rats a low dose of morphine (0.1 mg/kg, sc) had no effect on intake of low concentrations of saccharin but significantly facilitated intake of the highest (10%) and least preferred concentration. These data support the hypothesis that endogenous opioids can modulate the affective quality of gustatory stimuli.

Endogenous opiates: 1982

This article is the fifth installment in an annual series of reviews of successive year's research dealing with the endogenous opiate peptides. Due to the continuing massive increase in the number of studies in this field, it has become impossible to continue comprehensive reviews of all aspects of this work. As a result we have decided that beginning this year the coverage will be abbreviated to emphasize non-analgesic and behavioral work. The specific areas discussed include stress, tolerance and dependence, consummatory responses, alcohol consumption, schizophrenia and emotional disorders, learning and memory, cardiovascular responses, respiratory effects, thermoregulatory effects, neurological deficits and other disorders, activity, and other, miscellaneous behaviors. As in previous years, we have attempted a relatively comprehensive review of the subjects covered only for the previous year and have not made an attempt to evaluate their contributions relative to those of past years.