Endorphins and food intake: kappa opioid receptor agonists and hyperphagia

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.

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.

Chronic administration of morphine decreases level of dynorphin A in the rat nucleus accumbens

The effect of chronically administered morphine on the levels of dynorphin A in distinct regions of the brain (including medial frontal cortex, olfactory tubercule, nucleus accumbens, dorsal and medial striatum), was determined in male Sprague-Dawley rats. The drug was delivered through a subcutaneously implanted Azlet miniosmotic pump over a period of 5 days. The concentration of peptide was probed by radioimmunoassay, following pre-separation of tissue extracts by reversed phase separation on a SepPak C-18 cartridge. The result showed that the level of dynorphin A remained unaltered in all regions studied immediately before (tolerance) and 20 hr after (withdrawal) the pump was removed. A significant decrease in the level of dynorphin was found in the n. accumbens 48 hr (abstinence) after removal of the pump. It is suggested that previously observed changes in the reward system during abstinence may be connected with dynorphinergic neurones in the limbic system.

Diet and estrous cycle influence pain sensitivity in rats

Effects of estrous cycle and acute and chronic access to palatable fluids on tail-flick latency and opiate-induced analgesia were assessed in 124 female Long Evans rats. Following three consistent cycles, rats were water deprived for 8 h and then given ad lib access to 20 ml of either water, a 32% sucrose solution, or corn oil for 5 h. Nociceptive testing was conducted immediately preceding and 30, 60, and 90 min following an SC injection of morphine sulfate (7.5 mg/kg). Diestrus rats had prolonged premorphine tail-flick latencies compared to rats in proestrus. Rats that consumed corn oil had longer tail-flick latencies preceding and 30 min following morphine injections than rats that drank water or the sucrose solution. Rats were retested after they had ad lib access to the same fluid for 3 weeks. No estrous cycle differences were noted following chronic consumption. Rats with chronic access to sucrose showed increased baseline pain sensitivity and increased morphine-induced analgesia at 30, 60, and 90 min postinjection. These data support the notion that palatable fluid consumption attenuates estrous cycle-dependent differences in pain sensitivity.

Hormonal milieu affects tailflick latency in female rats and may be attenuated by access to sucrose

The steroid hormones estrogen (E) and progesterone (P) are known to modify pain sensitivity; however, the relative role of each of these hormones in this process is not well understood. To systematically investigate the effects of E and P on nociception, pain sensitivity was assessed under several hormone conditions. Tailflick (TF) latencies were measured every other day in 10 cycling female rats and 10 female rats during luteal functioning (pseudopregnancy). Thirty ovariectomized (OVX) rats were tested for TF latency following administration of 10 micrograms estradiol benzoate (EB) and either 0.0, 0.5, or 1.0 mg of P. Significant differences in TF latency were seen across days of the estrous cycle but not during luteal functioning. Tailflick latencies during luteal functioning were elevated relative to latencies in normally cycling animals. Among OVX rats, those administered EB and P (1.0 mg) displayed significant reductions in TF latency compared to vehicle controls. As a separate line of research indicated that consumption of highly palatable foods modified pain sensitivity, whether chronic sucrose consumption might overide the influence of hormones on nociception was examined. Ovariectomized rats given EB and P (0.0, 0.5, or 1.0 mg) were allowed chronic exposure to a 32% sucrose solution. Our preliminary findings suggest that chronic sucrose consumption attenuates hormonally induced differences in nociception.

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.

The α1-blocker dapiprazole inhibits diuresis but not drinking and feeding induced by U-50,488H

To further explore the interaction between opiates and catecholamines in the control of water balance, we studied the effects of the alpha 1-adrenoceptor antagonist dapiprazole on the modifications in drinking and diuresis produced by U-50,488H (a selective kappa-opiate agonist), morphine, naloxone, and amphetamine in rats. Because animals were maintained in a free-feeding paradigm and water intake is also controlled by feeding (prandial drinking), food intake was also measured. At doses administered (3-6 mg/kg, IP), dapiprazole had no effect on basal food and water intake or on diuresis. Nor did it modify changes in feeding and drinking produced by U-50,488H, morphine, naloxone, and amphetamine. It did, however, antagonize the diuretic effect of both U-50,488H and amphetamine. In addition, suppression of diuresis was obtained by combining doses of dapiprazole and morphine or naloxone that were devoid of antidiuretic effects when administered independently. A further experiment showed that diuresis produced by water load was also prevented by dapiprazole. alpha 1-Adrenoceptors thus appear to play a role in the regulation of water balance in a condition of free access to water, inhibiting diuresis without affecting drinking.

Naloxone benzoylhydrazone, a kappa 3 opioid agonist, stimulates food intake in rats

Naloxone benzoylhydrazone (NalBzoH) is a selective, short-acting agonist at the kappa 3 opioid receptor and a slowly dissociating potent antagonist at the mu opioid receptor. Given the important role of kappa receptors in the opioid control of food intake, the present study examined the central and peripheral effects of NalBzoH upon food intake. Central administration of NalBzoH (1-20 micrograms, i.c.v.) significantly increased food intake for up to 12 h, but failed to alter intake or body weight after 24 or 48 h. The 12 h duration of NalBzoH-mediated effects may be due to either persistent kappa 3 receptor occupancy, and/or activation of an ingestive system which maintains its activity. Peripheral administration of NalBzoH (20 mg/kg, s.c.) significantly increased food intake for up to 1 h. To distinguish kappa 1 (U50,488H) and kappa 3 (NalBzoH) hyperphagic effects, these agonist effects were compared following pretreatment with either naltrexone or the kappa 1 antagonist, nor-binaltorphamine (Nor-BNI). Whereas naltrexone significantly reduced both U50,488H and NalBzoH hyperphagia, Nor-BNI blocked U50,448H, but not NalBzoH hyperphagia. These data indicate a distinct role for the kappa 3 receptor in ingestive behavior separable from that of kappa 1 effects.

Effects of acute and chronic ketocyclazocine and its modulation by oxytocin or vasopressin on food intake in rats

The effects of acute and chronic ketocyclazocine (KCZ, a kappa receptor agonist) and its interactions with oxytocin (OXY) or vasopressin (AVP) were investigated on food intake in free-fed rats. Acute treatment with KCZ (1 mg/kg) produced a generalized hyperphagia during the light phase (0-6 h) without influencing dark phase (6-24 h) food intake. On chronic administration, tolerance developed to hyperphagic effect during light phase, whereas an enhancement in the food intake was seen during dark phase. OXY or AVP (both at 10 micrograms/kg) per se, did not affect the food intake response during either the light or the dark phase, after acute as well as chronic treatment. In the interaction studies, acute AVP or OXY attenuated the hyperphagia of KCZ during the light phase. On chronic treatment, both AVP and OXY blocked (a) the tolerance, and (b) the "reverse tolerance" to the food intake response to KCZ during light and dark phases, respectively. These results are discussed in light of complex opioid-OXY/AVP interactions during food intake in rats.

The kappa-opioid U-50,488H suppresses the initiation of nocturnal spontaneous drinking in normally hydrated rats

The effect of a systemic (IP) treatment with 1.0, 3.0 and 9.0 mg/kg U-50,488H (U50), a highly selective kappa-agonist, on spontaneous, nocturnal ingestive behavior of the rat was studied using a microcomputer controlled data acquisition system. The latency to initiate drinking was increased and drinking behavior was suppressed in the first hour after injection in a dose-dependent manner. The consummatory indices of drinking were not affected. After this period of adipsia, a phase of polydipsia, that was probably due to the diuretic effect of U50, was evident. This prophagic effect of U50 was evident only at the dose of 3 mg/kg and was accompanied by an increased duration of feeding episodes but not by a reduced latency to feed. These results suggest that kappa-receptors play a pivotal role in modulating spontaneous drinking in the normally hydrated rat and that this control is mainly exerted on the motivational aspect of drinking.

Role of diurnal variation and receptor specificity in the opioidergic regulation of food intake in free-fed and food-deprived rats

The effects of opioid agonists, morphine (MOR) and ketocyclazocine (KCZ), and antagonists, naltrexone (NALTX) and Mr2266, were investigated on food intake under various conditions, i.e., during light and dark phases of diurnal cycle and free-fed and fasting states in rats. NALTX showed a greater anorexic effect during dark phase, whereas Mr2266 produced such effect during light phase. This suggests that mu-receptors play a major role during dark phase while kappa-receptors are more important in light phase. The comparison of effects of different opioidergic drugs in fasted and free-fed rats showed that NALTX and Mr2266 reduced the elevated basal food intake in 18-h fasted rats to free-fed control levels. Therefore, it appears that enhanced endogenous mu- and kappa-directed neural mechanisms are one of the factors responsible for enhancing food intake in fasted rats. Differential role of MOR and KCZ on food intake in free-fed and fasted rats is also indicated in our study. Both agonists produced a biphasic response in fasted rats, i.e., hyperphagia (0-1 h) followed by hypophagia (1-6 h). However, a generalized hyperphagic effect is observed in free-fed rats (except during 3-6 h by MOR). The initial hyperphagic effect is more prominent in fasted rats which may be due to additive effects of endopioid mechanisms. Specificity of the response at various intervals is confirmed by blockade with NALTX and Mr2266. NALTX appears more potent than Mr2266 in antagonising the effects of MOR but markedly less potent than Mr2266 in inhibiting the effects of KCZ. This suggests that both MOR and KCZ have a mu as well as kappa component in food intake response.

Ingestive behavior following central [D-Ala2, Leu5, Cys6]-enkephalin (DALCE), a short-acting agonist and long-acting antagonist at the delta opioid receptor

DALCE (1-40 micrograms, ICV), a short-acting agonist and long-acting antagonist at the delta opioid receptor, was examined for its effects upon food intake in rats under spontaneous, deprivation, glucoprivic and palatable conditions. DALCE (10 micrograms) significantly stimulated free feeding for up to 10 h but only minimally decreased (40 micrograms) food intake and body weight after 24-72 h. DALCE, administered prior to food deprivation (24 h), failed to affect subsequent 24-h intake and sporadically decreased intake and body weight change after 48-72 h. 2-Deoxy-D-glucose (650 mg/kg, IP) hyperphagia was transiently (2 h) decreased by long-term DALCE (10 micrograms) pretreatment. Hyperphagia following exposure to a high-fat diet was significantly potentiated by long-term DALCE (1 microgram) pretreatment. DALCE (10 micrograms) hyperphagia (2-10 h) was eliminated by central pretreatment with either naltrexone (20 micrograms) or the kappa antagonist, nor-binaltorphamine (20 micrograms) but was minimally affected by central pretreatment with the mu antagonist, beta-funaltrexamine (20 micrograms) or long-term DALCE (40 micrograms). The general inability of the antagonist actions of DALCE to alter these forms of feeding argues against a role for the delta opioid receptor in these responses.

Measurement of CSF dynorphin A 1-8 immunoreactivity in anorexia nervosa and normal-weight bulimia

Twenty-one patients with anorexia nervosa and 35 normal-weight patients with bulimia underwent a series of CSF studies involving measurement of CSF dynorphin A 1-8 immunoreactivity during hospitalization in an eating-disorder treatment and research program. The control group consisted of 17 healthy volunteers. There were no statistically significant differences in CSF dynorphin A 1-8 measurements among groups or within a group at various stages of treatment. These results regarding dynorphin A 1-8 immunoreactivity are discussed in light of other evidence for altered opiate function in some eating-disorder patients.

Reduction by central beta-funaltrexamine of food intake in rats under freely-feeding, deprivation and glucoprivic conditions

The present study evaluated the central effects of beta-funaltrexamine (B-FNA), a non-equilibrium antagonist of mu-opioid receptors and a reversible agonist of kappa-opioid receptors upon food intake in rats under freely-feeding, deprivation and glucoprivic conditions. B-FNA elicited distinct short-term and long-term actions, consistent with binding studies demonstrating its reversible kappa agonist actions and its irreversible mu receptor blockade. Whereas B-FNA (1-20 micrograms, i.c.v.) significantly stimulated free feeding for up to 6 h, B-FNA (10-20 micrograms) significantly inhibited (35-41%) free feeding at 24, 48 and 72 h after injection, a pattern temporally similar to its biochemical opioid effects. Pretreatment (24 h) with B-FNA (10-20 micrograms) significantly inhibited (33-49%) the increased intake following 24 h of food deprivation. Pretreatment (24 h) with B-FNA (10-20 micrograms) also significantly inhibited (75-100%) the increased glucoprivic intake induced by 2-deoxy-D-glucose. The short-term stimulation of food intake by central B-FNA was antagonized by the selective kappa antagonist, nor-binaltorphamine, but was unaffected by pretreatment 24 h earlier with the mu antagonist, B-FNA. Significant reductions in striatal (89%) and hypothalamic (46%) mu-opioid binding occurred in rats pretreated (24 h) with B-FNA; the low levels of delta binding in these structures precluded interpretation of B-FNA effects. These data indicate the importance of the mu-opioid receptor in the modulation of different forms of feeding behavior, and underscores the ability of selective opioid antagonists to delineate precise functional roles for different opioid receptor subtypes.

Multiple cholecystokinin (CCK) receptors and CCK-monoamine interactions are instrumental in the control of feeding

Almost two decades ago, exogenous cholecystokinin (CCK) was shown to suppress food consumption in rats. Since then, CCK has been detected not only in peripheral tissue but extensively throughout the central nervous system. Furthermore, specific CCK receptors have been described, and a distinction drawn between CCK-A and CCK-B receptors. Recently, potent, orally active CCK antagonists, which show a high degree of selectivity for either CCK-A or CCK-B receptors, have been introduced. The present report reviews recent evidence obtained in studies using devazepide (a selective CCK-A receptor antagonist) and L-365,260 (a selective CCK-B/gastrin receptor antagonist). Both compounds increased food consumption and postponed the onset of satiety in well-satiated rats. L-365,260 was more potent, suggesting that central CCK-B type receptors may mediate the satiety effects of endogenously released CCK. Only devazepide was effective in blocking the feeding-suppressant effect of exogenous CCK, indicating that CCK-A type receptors mediate this effect. In a second series of studies, devazepide but not L-365,260 antagonized the anorectic effect of either d-fenfluramine or systemically administered 5-HT. Hence, CCK-A type receptors appear to be involved in the anorectic effects of these serotonergic drugs. We propose that CCK and 5-HT mechanisms involved in mediating satiety are mutually interdependent. Possible interactions between CCK and catecholaminergic mechanisms are also briefly considered.

Effects of intracerebroventricular injection of dynorphin, leumorphin and alpha neo-endorphin on operant feeding in pigs

Young pigs, which are useful experimental animals for biomedical research, were prepared with lateral intracerebroventricular (ICV) cannulae and housed individually in cages fitted with operant panels, with food and water ad lib. ICV injection of 200 micrograms of dynorphin A 1-17 or 1-13 resulted in a significant meal commencing within 2-5 min. Shorter fragments of dynorphin (1-10, 1-9, 1-8) were ineffective at inducing feeding as was dynorphin B (rimorphin). In the same situation, leumorphin and alpha neo-endorphin (200 micrograms) elicited significant feeding but beta neo-endorphin did not. Dynorphin 1-17 or 1-13, administered 5 min before feeding started, increased meal size when pigs were fed after 4-h deprivation. Naloxone ICV (0.4 mg) significantly reduced food intake in pigs feeding after 4-h deprivation and its main effect was in the second half of the meal. Naloxone also abolished the effect of ICV dynorphin. It is concluded that dynorphin and related endogenous opioids are involved in the regulation of food intake in pigs.

Suppression of nocturnal, palatable and glucoprivic intake in rats by the kappa opioid antagonist, nor-binaltorphamine

The increased food intake in the rat during the first two hours of the dark cycle was significantly inhibited by central pretreatment with either the selective kappa opioid antagonist, nor-binaltorphamine (NorBNI, 20 micrograms, i.c.v., 53-54%) or naltrexone (NTX, 20 micrograms, i.c.v., 47-60%). Short-term (2 h) intake of a high-fat diet was significantly inhibited by central NorBNI (1-20 micrograms, 33-79%) and NTX (20 micrograms, 47-51%). Hyperphagia induced by the anti-metabolic glucose analogue, 2-deoxy-D-glucose was significantly inhibited by central NorBNI (20 micrograms, 40-68%) and NTX (20 micrograms, 28-69%). These data suggest that the kappa receptor subtype, in addition to other opioid receptor subtypes, influence these forms of feeding behavior.

Opiatergic modulation of preparatory and consummatory components of feeding and drinking

We present data here indicating that stimulation of kappa but not mu opiate receptors influences motivational and consummatory aspects of feeding and drinking. To differentiate mu and kappa mechanisms controlling preparatory (appetitive) and consummatory components of ingestive behavior, the effects of morphine (MORPH), compound U50488H (U50) and naloxone (NAL) were studied in rats trained to negotiate a straight runway using food or water as a reinforcer. At doses that increase feeding and drinking in conditions of free access to food and water (i.e., 1-2 mg/kg IP), MORPH affected neither food- nor water-maintained runway performance. Since 1 mg/kg of NAL is also devoid of effects, mu-opiate mechanisms are probably not involved in food- or water-maintained behavior. Pharmacological manipulation of kappa-opiate mechanisms had complex effects. At 5 mg/kg, NAL accelerated satiation, depressing food intake, without affecting running. U50 did not increase food intake, but accelerated running for food, an effect that was antagonized by a high dose of NAL (5 mg/kg). These findings suggest that motivational and consummatory components of food-maintained runway performance are both activated by kappa-opiate mechanisms. NAL also reduced water intake but had minimal influences on running. In contrast, U50 depressed both water intake and runway performance; rather than being antagonized, these effects were slightly enhanced by NAL. The combined antidipsic and diuretic effects of U50 suggest that kappa-opiate mechanisms play a dissipatory role in water balance. However, the similar antidipsic effects of U50 and NAL, and the fact that NAL did not antagonize the antidipsic effects of U50, suggest that U50 may reduce drinking by mechanisms other than kappa-opiate agonism.

Enhanced anorectic potency of naloxone in rats sham feeding 30% sucrose: reversal by repeated naloxone administration

The time-course of naloxone-anorexia was monitored in gastric-fistulated rats sham feeding sucrose (10%, 20%, 30%) solutions. Naloxone reduced sham intake dose dependently without affecting feeding initiation and in a manner which resembled the effects of progressive sucrose dilution. However, when rats sham fed 30% sucrose there was a 2-fold increase in the anorectic potency of naloxone. This exaggerated response was prevented by prior repeated naloxone treatment (5 mg/kg IP, bi-daily), concurrent with the stabilization of sham intake levels (4 days). A further experiment ruled out the possibility that tolerance develops to naloxone effects on this treatment schedule, since intact rats showed a suppression of wet mash consumption following repeated naloxone treatment which was equivalent to an acute naloxone challenge. It is proposed that 1) repeated sucrose sham feeding enhances opioid release and leads to opioid receptor adaptation (down-regulation); 2) repeated (chronic) naloxone treatments have an opposite effect on opioid receptors (up-regulation); 3) the two manipulations, in combination, counteract each other's effects. These behavioural data demonstrate dynamic changes in sham-feeding performance as a function of sucrose concentration and naloxone treatments, reinforce the importance of palatability in naloxone-anorexia, and support opioid involvement in orosensory reward.

Selective opioid receptor antagonist effects upon intake of a high-fat diet in rats

Short-term (2 h) intake of a high-fat diet in rats was significantly inhibited by intravenous (0.1-10 mg/kg: 39-67%) and central (1-5 micrograms, i.c.v.: 51%) naloxone. The irreversible mu opioid antagonist, beta-funaltrexamine (10 micrograms, i.c.v.: 37%), but not the irreversible mu 1 antagonist, naloxonazine (10 mg/kg, i.v.) inhibited intake, suggesting mu 2 receptor mediation. The delta antagonist, ICI 174864 (1-10 micrograms, i.c.v.: 41%) inhibited high-fat diet intake only at doses that also produced motor dysfunction.

Effects of opiate agonists and an antagonist on food intake and brain neurotransmitters in normophagic and obese "cafeteria" rats

The relationship between the effects of opiates on food intake and on central monoamines in various brain areas was investigated in normophagic and obese "cafeteria" rats. Three agonists, beta-endorphin, dynorphin, and D-Ser2-Leu-Enk-Thr6 (DSLET) and an antagonist, naltrexone, were used. The three agonists enhanced feeling in normophagic rats but had different dopaminergic effects. Serotonergic metabolism increased concomitantly with the enhancement of feeding by the agonists, whereas it decreased following treatment with the antagonist naltrexone. In the cafeteria rats, although the feeding effects of dynorphin and DSLET occurred earlier, there was a complete lack of monoaminergic effects. beta-Endorphin was completely devoid of effects in this model. There would, thus, appear to be a positive correlation between the behavioural effects of these opiates and serotonergic metabolism in normophagic rats, while stimulated feeding situations ("cafeteria" rats) the disruption of a monoaminergic modulation does not prohibit a direct effect on feeding.

Effects of kappa-opioid receptor agonists and morphine on food intake and urinary output in food-deprived and nondeprived rats

The effects of kappa-opioid receptor agonists, bremazocine, U-50, 488H and tifluadom and of a mu-opioid receptor agonist, morphine, on food intake and urinary output in food-deprived and nondeprived Sprague-Dawley rats was determined. In food-deprived animals, intraperitoneal administration of bremazocine at 0.1 mg/kg increased food intake but at 1.0 and 10.0 mg/kg doses decreased it. Tifluadom (0.1-10.0 mg/kg) had no effect on food intake. U-50,488H at 1.0 mg/kg increased food intake, whereas 10.0 mg/kg dose decreased the food consumption. In nondeprived rats, the kappa-opioid receptor agonists failed to produce any effect on food consumption. In food-deprived rats, all the three kappa-opioid receptor agonists increased the urinary output at the highest dose (10 mg/kg). In nondeprived rats similar effects as in food-deprived rats were observed except bremazocine increased urinary output at all the doses used. These results with kappa-opioid agonists may be related to either the existence of more than one population of kappa-opioid receptors or their differential actions at the opioid receptor types.

Central mu, delta, and kappa opioid binding sites, and brain and pituitary beta-endorphin and met-enkephalin in genetically obese (ob/ob) and lean mice

The equilibrium dissociation constants and maximal binding capacities of 3H-dihydromorphine (DHM), 3H-D-Ala2-D-leu3-enkephalin (DADL), and 3H-dynorphin A(1-8) for their respective mu, delta, and kappa opiate binding sites were studied in brain membrane preparations from lean and genetically obese-hyperglycaemic (Aston ob/ob) mice. The concentration of kappa binding sites was 2.7 fold higher in obese compared with lean mouse brain (231 +/- 44.6 versus 83.8 +/- 10.3 fmoles 3H-dynorphin/mg protein respectively, mean +/- SEM). The concentration of delta binding sites in obese was 1.6 fold that in lean mouse brain (94.5 +/- 8.6 versus 59.5 +/- 6.5 fmoles 3H-DADL/mg protein). In contrast, the concentration of brain mu receptors was 40% lower in obese compared with lean mice (20.8 +/- 2.19 and 34.8 +/- 3.1 fmoles 3H-DHM/mg protein respectively). Binding affinities of delta and kappa sites for their respective ligands were not significantly different in lean v. obese mice. However, for mu sites, lean mouse binding data showed two affinities, one was not significantly different from obese (0.35 nM) the second was lower (1.18 nM) for DHM. Increases of approximately 5 fold and 3 fold in the brain content of beta-endorphin and met-enkephalin respectively, and no differences in brain dynorphin levels, were demonstrated in obese mice compared with lean controls. In obese mice, pituitary beta-endorphin content was 9 fold higher, met-enkephalin 4 fold higher and dynorphin 12 fold higher than in lean mice. The striking differences in opioid binding-site characteristics and in endogenous opioid peptide levels in obese compared with lean mice may contribute to the hyperphagia and, directly or indirectly, to the development of hyperglycaemia and hyperinsulinaemia in obese mice.

Effects of the selective kappa opioid antagonist, nor-binaltorphimine, on electrically-elicited feeding in the rat

Lateral ventricular injections of the 'nonspecific' opioid antagonist naloxone (100 micrograms) and the kappa-selective opioid antagonist nor-binaltorphimine (50 micrograms) elevated the electrical brain stimulation frequency threshold for eliciting feeding behavior. Mesopontine aqueductal injections of nor-binaltorphimine, on the other hand, lowered the feeding threshold while naloxone still elevated threshold. These findings suggest the existence of forebrain kappa receptors at which endogenous opioid activity results in a facilitation of feeding while kappa receptors in the brainstem seem to mediate an inhibitory effect.

Effects of naloxone and picrotoxin on diazepam- or pentobarbital-induced hyperphagia in nondeprived rats

Diazepam and pentobarbital administered intravenously increased food intake in a dose-dependent manner in nondeprived rats. Low doses of naloxone inhibited diazepam-induced feeding, but did not inhibit pentobarbital-induced feeding. On the other hand, picrotoxin inhibited feeding induced by both drugs. These findings suggest that diazepam-induced hyperphagia is related to endogenous opioid mechanisms, but pentobarbital-induced hyperphagia is not. Hyperphagia induced by both drugs may be related to GABAergic neurons.

Effects of intrahypothalamic administration of opioid peptides selective for mu-, kappa, and delta-receptors on different schedules of water intake in the rat

The effects of opioid peptides selective for mu, kappa, and delta-opioid receptors were investigated on 3 different schedules of water intake in the rat: spontaneous, deprivational (12 h), and hypertonic saline-induced drinking. Peptides were injected into the paraventricular and supraoptic hypothalamic nuclei, D-Ala2-NMePhe4-Gly(ol)-enkephalin, a mu-selective opioid agonist, tended to increase water intake in non-deprived rats, but 0.01 and 0.1 microgram significantly decreased water intake for 45 min in deprived rats, and for up to 60 min in hypertonic saline-injected rats when injected into the paraventricular hypothalamic nucleus. The kappa-selective agonist, dynorphin A1-13 (0.1, 0.3, 1.0 and 3.0 micrograms)and the delta-selective agonist, [D-Pen2,L-Pen5]enkephalin (0.3 and 3.0 micrograms) did not affect spontaneous, deprivational or hypertonic saline-induced water intakes when injected into either the paraventricular or supraoptic hypothalamic nuclei. Thus, a mu-selective opioid peptide produced dose- and time-dependent effects on drinking that were pharmacologically and anatomically specific, and dependent upon the schedule of water intake.

Effects of beta-funaltrexamine on ingestive behaviors in the rat

The effects of beta-funaltrexamine (beta-FNA), an irreversible mu-opioid receptor antagonist, were determined on water and food intake of non-deprived rats. Intracerebroventricular administration of 1.25 or 2.5 micrograms of beta-FNA did not affect drinking or eating. However, 5.0 micrograms first transiently increased food intake and then reduced both water and food intake for at least 72 h. Locomotor activity was unaffected by 5.0 micrograms of beta-FNA; thus, changes in ingestive behavior were not a secondary consequence of drug-induced behavioral stimulation or depression. The early increase in food intake may be due to the short lived and reversible kappa-agonist activity of beta-FNA. On the other hand, selective blockade of mu-opioid receptors appears sufficient to reduce the intake of water and food.

The role of opiate mechanisms in the development of tolerance to the anorectic effects of amphetamines

To study the role played by opiate mechanisms in the tolerance to the anorectic effects of amphetamines, the influence of chronic treatment with d,1-amphetamine (AMPH) on the effects of the selective kappa opiate agonist U50488H (U50), of morphine (MORPH) and of diazepam (DZP) on food and water intake was evaluated in rats. Since diuresis is selectively enhanced by kappa agonists, its sensitivity to chronic AMPH was also evaluated. On the first day of AMPH treatment the feeding response to U50 was depressed. On day 9, when tolerance to the anorectic effects of AMPH had developed, this response was enhanced and prolonged. U50-mediated diuresis was not increased in the AMPH group. AMPH however produced diuresis by itself and this effect may be responsible for the increased water intake that developed during chronic treatment. The administration of MORPH (on day 17), but not of DZP (on day 13), produced a similar pattern of response. Interruption of AMPH treatment brought about a slow normalization of response to U50, that appeared to be completed after 17 days. An increase in feeding response to U50 was also obtained after 14 days of cathinone administration, confirming the amphetamine-like properties of this drug. In order to evaluate the possibility that preventing sensitization of opiate mechanisms could also prevent tolerance to anorectic effects of AMPH, we chronically administered MORPH in combination with AMPH, obtaining a further reduction of feeding and an apparent slowing in tolerance development. However, such a reduction was also obtained acutely, although MORPH alone produced feeding stimulation. We suggest that opiates may both activate and inhibit feeding and that AMPH inhibits the activatory branch and works synergically with the inhibitory branch. The prolonged inhibition of the activatory branch causes its compensatory hypertrophy resulting in hypersensitivity to exogenous opiates.

Long-term blockade of mu-opioid receptors suggests a role in control of ingestive behaviour, body weight and core temperature in the rat

Chronic subcutaneous infusion with a low dose (0.5 mg/kg/h) of naloxone via minipumps blocked the antinociceptive action of the mu-agonist, morphine, without affecting that of the kappa-agonist, U50488H. This dose resulted in a transient suppression in the rate of body weight gain and a sustained reduction in daily food intake (FI) and water intake (WI): this decrease was seen in both the light and dark phases. Naloxone also resulted in a reduction in resting core temperature (TC) in the light but not the dark phase. It did not affect the weight loss or hypothermia which accompanied 24 h food and water deprivation. Naloxone did, however, suppress FI and WI following deprivation and inhibited the recovery of body weight thereafter. The influence of naloxone upon FI, WI, TC and body weight was dose-dependent over 0.05-0.50 mg/kg/h. Increasing the dose to 3.0 mg/kg/h eliminated the antinociceptive action of U50,488H revealing a blockade of kappa- (in addition to mu-) receptors. This higher dose was not more effective in reducing FI, WI, body weight and TC than 0.5 mg/kg/h. Further, treatment with MR 2266, an antagonist (or weak partial agonist) with a higher activity at kappa-receptors than naloxone, was not more effective than naloxone in reducing FI, WI and body weight: further, it did not affect TC. Moreover, chronic infusion of bremazocine, (a kappa-agonist and mu-antagonist) reduced WI, FI, body weight and TC by a magnitude comparable to that of naloxone. Finally, chronic infusion of the mu-agonist, sufentanyl, led to a sustained rise in TC. It is concluded, that: (1) mu-opioid receptors may play a major role in the modulation of daily FI and WI and of body weight in freely behaving rats: this action is expressed in both the light and dark phases of the cycle and maintained following deprivation. The data provide no evidence for (but do not exclude) a particular role of kappa-receptors. (2) mu-Receptors play a physiological role in the modulation of TC in the light but not the dark phase of the daily cycle.

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.

Feeding elicited by dynorphin (1-13) microinjections into the ventral tegmental area in rats

Both the endogenous opioid peptide, dynorphin (1-13) (DYN), and morphine elicited dose-dependent feeding when microinjected into the ventral tegmental area of food-satiated rats. DYN was 50,000 times more potent than morphine in producing feeding. Whereas the ED50 for morphine was in the nanomole range, the ED50 for DYN was in the femtomole range. Administration of a narcotic antagonist attenuated DYN-elicited feeding. These data suggest a possible role for DYN in the VTA in opioid modulation of feeding behavior.

Differential sensitivity of opioid-induced feeding to naloxone and naloxonazine

The high-affinity mu-1 opioid binding site has been implicated in some opioid responses (e.g., supraspinal analgesia) but not others (e.g., respiratory depression) by comparing the actions of naloxone, a short-acting, non-selective antagonist, and naloxonazine, an irreversible and selective mu-1 antagonist. The mu-1 site has been implicated in the opioid component modulating free feeding and deprivation-induced feeding, but not glucoprivic feeding. The present study compared naloxone and naloxonazine antagonism of hyperphagia induced by morphine, ethylketocyclazocine (EKC), dynorphin and d-ala2,d-leu5-enkephalin (DADL) in rats. Morphine produced a dose-dependent (0.01-5 mg/kg) hyperphagia in mildly food-deprived rats that was blocked by naloxone (0.01-10 mg/kg). Naloxonazine (10 mg/kg) shifted the morphine hyperphagia dose-response curve to the right. These effects could not be fully accounted for by the intrinsic hypophagic properties of these antagonists. EKC produced a dose-dependent (0.5-5 mg/kg) hyperphagia which was blocked by naloxone (10 mg/kg) only at low effective EKC doses. Naloxonazine (10 mg/kg) failed to affect EKC hyperphagia. Naloxone, but not naloxonazine also blocked dynorphin and DADL hyperphagia. These results indicate that feeding induced by opiate and opioid agonists are differentially mediated by the mu-1 and other opioid binding sites; these data contrast with the modulation by the mu-1 site of the supraspinal analgesia induced by each of these agonists.

Involvement of kappa type opioids on ethanol drinking

The effects of the administration of the kappa agonist dynorphin1-17 and/or the kappa antagonist MR-2266-BS on ethanol preference was investigated using a paradigm by which rats develop alcohol preference. Administration of dynorphin shortly before or after the conditioning session (forced ethanol exposure) failed to affect later ethanol preference. However, dynorphin treatment prior to the first choice session reduced ethanol preference during the three consecutive testing days. This effect was reversed by the simultaneous administration of the kappa antagonist MR-2266-BS. The results of the present study provide further support for evidence of the involvement of dynorphinergic systems on drinking behavior and suggest that kappa-type opioid mechanisms may be involved in the consumption and development of preference to ethanol in rats.

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.

Sex and day-night differences in opiate-induced responses of insular wild deer mice, Peromyscus maniculatus triangularis

We examined the effects of mu and kappa opiate agonists on the day- and night-time nociceptive, locomotory and ingestive behaviors of an island population of wild male and female deer mice, Peromyscus maniculatus triangularis. The prototypical mu opiate agonist, morphine, had significant analgesic and locomotory effects, which were blocked by naloxone, and the specific delta opiate antagonist, ICI 154,129, respectively. The specific kappa opiate agonist, U-50,488, had significant analgesic actions and inhibitory effects on locomotor activity, as well as stimulating feeding. Significant day-night variations occurred in the analgesic and activity responses, with the mu and kappa opiate agonists having significantly greater effects at night. There were also prominent sex differences in responses; male deer mice displaying significantly greater levels of mu and kappa opiate-induced analgesia and alterations in activity than female animals. These sex differences in opiate-induced effects were most pronounced at night, female deer mice displaying reduced day-night rhythms of responsiveness. These results demonstrate the existence of significant day-night rhythms and sex differences in the mu and kappa opiate behavioral responses of a wild population of rodents.

Effects of naloxone and naltrexone on meal patterns of freely-feeding rats

The effects of naloxone and naltrexone on the night-time meal patterning of freely-feeding male rats were investigated using a Kissileff-type eatometer. Naloxone (5.0 mg/kg) and naltrexone (2.5 mg/kg) reduced intake for two hours after IP injection. This effect resulted from a shortening of duration of meals and an extension of postmeal intervals. Unlike other anorexic agents neither drug affected meal frequency or the eating rate within meals. These particular opioid antagonists therefore appear to produce anorexia by advancing meal termination and extending the inhibition of feeding which follows a meal. These specific changes in the structure of the meal pattern consolidate previous findings and support the hypothesis that naloxone and naltrexone reduce food intake in rats by promoting satiation and prolonging satiety.

Endogenous opiates: 1985

This paper is the eighth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1985. The specific topics this year include stress, tolerance and dependence, eating, drinking and alcohol consumption, gastrointestinal and renal activity, mental illness, learning and memory, cardiovascular responses, respiration and thermoregulation, seizures and neurological disorders, activity, and some other selected topics.

Food hoarding and ingestion in the deer mouse, Peromyscus maniculatus: selective responses to mu and kappa opiate agonists

The feeding behavior of the deer mouse, Peromyscus maniculatus, includes food hoarding as well as ingestion. Administration of the prototypical mu opiate agonist, morphine sulfate, 1-20 mg/kg, produced over three hours a significant dose-dependent stimulation of hoarding by free feeding deer mice. The specific kappa opiate agonist, U-50,488H, 0.10-10 mg/kg, markedly increased ingestion without having any augmentatory effects on hoarding. The mixed mu and kappa opiate agonist, ketocyclazocine hydrochloride, 1-10 mg/kg, as well as various combinations of morphine sulfate and U-50,488H, augmented both hoarding and ingestion. Food restriction for 24 hr caused a significant, naloxone (1.0 mg/kg) reversible, increase in food intake. Food deprivation also modified the hoarding and ingestion responses of the deer mice to the mu and kappa opiate agonists, reducing the relative amounts of food that were hoarded. These results indicate that mu and kappa opioid systems are differentially involved in the mediation of various aspects of feeding. This also suggests that environmental factors, such as food restriction, can modify the relative roles of mu and kappa opioid systems in the expression of feeding behavior.

Midazolam-induced hyperphagia and FG 7142-induced anorexia: behavioural characteristics in the rat

Non-deprived male rats, familiarized with a highly palatable diet, were treated with 0.3-10.0 mg/kg of the imidazobenzodiazepine midazolam. The increases in consumption of the food observed at larger doses of midazolam were due to increases in the duration of feeding, but not in the rate of eating. These, in turn, were due to increases in the duration of eating bouts, but not in their frequency. The beta-carboline FG 7142, a partial benzodiazepine receptor inverse agonist, reduced the consumption of the diet when it was injected at 10.0 and 15.0 mg/kg (IP). The overall duration of feeding was not affected at these doses in the 30 min test, but rate of eating was reduced. However, during the first 5 min interval of the test, when feeding behaviour was most motivated, FG 7142 did significantly reduce the duration of feeding. The effect depended upon a reduction in the duration of eating bouts, but not upon any change in their frequency. Hence, midazolam and FG 7142 had opposite effects on the duration of bouts of feeding. Both midazolam and FG 7142 reduced the frequencies of concurrent grooming, locomotor activity, and rearing in the test of palatable food consumption. Possible explanations for these effects are briefly considered.

Effect of naloxone and naltrexone on the development of satiation measured in the runway: comparisons with d-amphetamine and d-fenfluramine

A straight runway was used to monitor changes in measures of food motivation and food consumption in order to track and to characterise the development of satiation following administration of equianorectic doses of naloxone (5.0 mgkg-1), naltrexone (2.5 mgkg-1), dexfenfluramine (1.5 mgkg-1) and d-amphetamine (1.0 mgkg-1). Naloxone and naltrexone did not reduce motivational measures or block food consumption during the early trials. These drugs brought about a prompt cessation of intake only after some food had been consumed. Dexfenfluramine displayed an early effect on motivation and hastened the onset of satiation. The anorexic activity of d-amphetamine was virtually abolished. These results indicate that the runway is a useful device for analysing the effects of drugs on eating motivation. This study has further characterised the anorexic actions of naloxone and naltrexone; the profiles of these agents can be distinguished from both dexfenfluramine and d-amphetamine. The suppressive action on food intake exerted by these particular opioid antagonists appears to arise from an intensification of the feedback from food ingestion. The mechanisms through which this effect is achieved are not known.

Effects of kappa opiate agonists on palatable food consumption in non-deprived rats, with and without food preloads

There is increasing evidence to suggest that kappa opiate receptors may be importantly involved in the mediation of feeding responses in the rat. A series of experiments is reported in which the effects of four kappa receptor agonists (ethylketocyclazocine, U-50,488H, tifluadom, bremazocine) on the consumption of a highly palatable diet were investigated. Under one condition, non-deprived male rats were administered drug treatments before a 30 min feeding test. Bremazocine (0.1 mg/kg) and ethylketocyclazocine (3.0 mg/kg) both significantly decreased the level of food consumption. In contrast, U-50,488H and tifluadom each produced significant increases in food intake. In a second condition, non-deprived male rats were first allowed to consume some of the palatable diet to achieve partial satiation, prior to the administration of the drug treatments. In this case, evidence for hyperphagic effects of all four kappa agonists was obtained, within the first 30 min access to the palatable diet. Thus, hyperphagia occurred with 0.01 mg/kg bremazocine and 0.1 mg/kg ethylketocyclazocine. We conclude that some kappa agonists have mixed stimulant/inhibitory effects on food intake, whereas others are more consistent in producing hyperphagia. In neither condition did morphine (0.3-10.0 mg/kg) show any hyperphagic effect. Our data support an involvement of kappa opiate receptors in mechanisms which control palatable food consumption in non-deprived rats.