An investigation of the role of kappa opiate receptor agonists in the initiation of feeding

A large body of evidence has suggested a role for the endogenous opiates and their receptors in the regulation of appetite. In this study we have examined the relative effects of ketocyclazocine (KC), cyclazocine and ethylketocyclazocine, all putative kappa opiate receptor agonists, and morphine, a putative mu receptor agonist, on food consumption. All the kappa agonists induced feeding when administered at 8 AM as did morphine. KC failed to induce feeding during the nocturnal feeding period (2000 and 0200 hours) and morphine suppressed feeding at these times. KC and morphine suppressed starvation induced feeding when food was made available immediately after injection and had no effect when food was presented 2 and 4 hours after injection. High doses of naloxone (5 mg/kg) suppressed KC induced feeding while actually enhancing high dose morphine (25 mg/kg) induced feeding. Repeated injections of KC or morphine for 5 days resulted in enhancement of the feeding response with initiation of feeding occuring earlier. Taken together with the studies showing that the endogenous kappa ligand, dynorphin, enhabces feeding the most parsimonious interpretation of these studies is that kappa agonists are endogenous initiators of feeding and that kappa receptors are maximally saturated at times of food deprivation and during spontaneous feeding. The mu (or one of the other) opiate receptors inhibit feeding due to their sedative effect and antagonism of this effect leads to enhancement of the feeding response. It is postulated that kappa opiate receptors represent an important component of the natural feeding drive.     

Slugs and snails and opiate tales: opioids and feeding behavior in invertebrates

There is accumulating evidence that opioid systems are involved in the regulation of fundamental behavioral and physiological processes in invertebrates. Feeding is a basic physiological function that is essential for maintaining homeostasis. Results of studies examining the feeding responses of molluscs and arthropods treated with various opiate agonists and antagonists indicate that delta, kappa, mu, and possibly sigma opioid systems differentially and selectively mediate the components of their natural feeding behavior. Moreover, it appears that at an early evolutionary stage the mu and kappa systems have developed to selectively affect the components of feeding behavior associated with the acquisition and ingestion of food. In addition, evidence suggests that neuropeptides that have been proposed as possible endogenous antagonists of opioid-mediated feeding in mammals may also be involved in the control of feeding in invertebrates. This indicates that there may be an interplay of opioid agonists and antagonists in the regulation of feeding and satiation in invertebrates analogous to that proposed for vertebrates. Moreover, these findings indicate that opioid influences on feeding have been conserved through evolution.     

Opioid involvement in the control of feeding in an insect, the American cockroach

Administration of the kappa opiate agonist, U-50,488H (0.10-10 mg/kg), produced over three hours a significant dose-dependent increase in the ingestive responses of free feeding American cockroaches, Periplaneta americana. These effects could be decreased by the opiate antagonist, naloxone (1.0 mg/kg), with naloxone by itself blocking the augmented feeding responses of food-deprived cockroaches. The mu opiate agonist, morphine (1.0-20 mg/kg) caused a significant dose-dependent and naloxone-reversible increase in the locomotory activity of cockroaches. These results suggest that opioid systems may be involved in the control of the feeding in cockroaches in a manner analogous to that proposed for vertebrates.     

Different types of opiate agonists interact distinguishably with mu, delta and kappa opiate binding sites

The present studies were undertaken to evaluate whether different types of opiate agonists interact in a distinguishable manner with mu, delta and kappa opiate binding sites. Two approaches were employed: (a) the well known effects of metal ions on opiate agonist binding affinities of subsite selective ligands were studied at mu, delta and kappa sites in rat brain homogenates. Binding parameters were obtained by simultaneous computeranalysis of displacement curves using the prototypic ligands dihydromorphine (DHM), (D-Ala2, D-Leu5) enkephalin (DADL) and ethylketocyclazocine (EKC) of the mu, delta and kappa binding sites respectively. The results show that the effects of metal ions depend not only on the binding site, but also on the ligand under investigation. (b) The interaction of the delta agonist DADL with the mu agonist DHM was investigated at mu binding sites by characterizing the type of competition occurring between the two ligands. The interaction was of the noncompetitive type. It therefore appears that the various opiate agonists either interact preferentially with different parts of a larger receptor site area or bind to topographically distinct sites on a single receptor molecule which are coupled allosterically.     

Effects of the opiate antagonists diprenorphine and naloxone and of selected opiate agonists on feeding behavior in guinea pigs

Opiate-sensitive feeding behavior has now been demonstrated in a number of species. We sought information on which opioid receptors might be involved in the observed feeding behaviors. Guinea pigs are known to have higher concentrations of the opioid kappa receptor than any other laboratory animal, so we compared the feeding suppressive potency of the general opiate antagonist, diprenorphine to that of the relatively more mu-specific antagonist, naloxone in that species. We found that diprenorphine was over twenty times more effective than naloxone in suppressing feeding in guinea pigs, suggesting the importance of receptors other than mu in feeding initiation in the guinea pig. Confirmatory evidence for the role of kappa receptors was sought, but not found, in comparisons of the effectiveness of different types of opiate agonists in promoting feeding in these animals. These agonists suppressed, rather than stimulated feeding. We conclude that no feeding stimulatory effects of opiates can be demonstrated in guinea pigs. This observation may indicate that opioids play little role in the natural regulation of feeding in this species or that opioids result in prolonged sedation during which the animals fail to eat. The greater feeding suppressive potency of diprenorphine, a general opiate antagonist, versus naloxone, a mu-preferential antagonist, indicates that to whatever extent opiates are involved in guinea pig feeding, the opiate effect is probably not a mu receptor effect.     

Orally administered kappa as well as mu opiate agonists delay gastrointestinal transit time in the guinea pig

When an orally administered opiate agonist is systemically bioavailable, the relative activity of that opioid in delaying gastrointestinal transit (GIT) depends on its relative action at central and peripheral sites. This in turn depends on the density of opioid receptor specific subtypes at those sites of action in the species under study. In rats the kappa selective agonist U-50,488H has no effect on GIT. We have found that this same agonist is equipotent to mu agonists morphine and 1-methadone in delaying the orocecal transit of a charcoal meal when administered orally to guinea pigs. Thus, both kappa as well as mu receptor subtypes are involved in the mechanisms of opiate induced slowing of GIT in the guinea pig in contrast to the rat. Interspecies differences must be considered when determining the contribution of opiate receptor subtypes to the mechanisms of opiate-induced constipation.     

Involvement of opioid receptor subtypes in rat feeding behavior

The short-acting opiate antagonist naloxone decreases food intake in three models of ingestive behavior: free feeding, food-deprivation induced feeding and deoxyglucose-induced feeding. Twenty-four hours after administration, the long-acting, mu1 selective antagonist naloxonazine inhibits food intake to the same extent as naloxone in freely feeding and food-deprived rats, but not in animals treated with 2-deoxyglucose. These results indicate that 1) opiates modulate feeding through multiple opioid receptor mechanisms, one of which is the mu subtype, and 2) the feeding observed in various experimental paradigms are modulated by different receptor subtypes. Furthermore, these results illustrate the usefulness of naloxone in defining a behavior as opioid but point out its limitations in discriminating between opioid receptor subtypes.     

Opioids, feeding, and anorexias

This review summarizes recent work that focuses on the role of endogenous opioids (EOs) and opiate receptors in the control of food intake. Although the anorexic effect of opiate antagonists are now well accepted, the exact EO, site(s), and mechanism(s) of action remain to be established. However, accumulating evidence suggests that dynorphin, an endogenous ligand for kappa-type opiate receptors, is an important regulator (stimulant) of appetite. The roles of other EOs, such as beta-endorphin, are less clear. EOs appear to be involved in maintaining normal feeding behavior and are likely responsible for the overconsumption of fat in genetically obese and stressed subjects. Opiate antagonists block overconsumption of palatable foods, thus offering a promising approach to weight reduction for some overweight individuals. Anorexias may follow from a deficiency of kappa-type opioid activity, and surprisingly, can also result from excess opioid activity. Indeed, opiate antagonists of the mu type (naloxone) can enhance eating and weight gain in certain anorexic conditions. Therefore, it appears that excess opioid agonist activity may result in hyperphagia or anorexia (depending on the opiate receptor type). Finally, opiate antagonists may help normalize both types of pathological feeding states.     

Butorphanol tartrate induces feeding in rats

Peripheral administration of butorphanol tartrate markedly enhanced feeding from 0800 to 1400 hours when compared with vehicle controls. Butorphanol tartrate feeding was not antagonized by doses of naloxone as high as 10 mg/kg. These data support the concept that the kappa or sigma opiate receptors are involved in feeding behavior.It is well recognized that the endogenous opiates play a role in the central regulation of appetite (1, 2, 3, 4). Numerous studies have shown that The endogenous opioid peptides and morphine can initiate feeding under various conditions (5–12) whereas the opiate antagonist, naloxine can reduce food consumption (13–20). Recently, the endogenous opiod peptide, dynorphin, has been reported to enhance food intake (12–25).Much evidence has been accumulated indicating that a number of opiate receptors are present in the brain, each one having a high affinity for a specific endogenous opioid peptide (26, 27). Both the cyclazocine related compounds (28) and the feeding enhancer, dynorphin (29–32), have been reported to be specific kappa receptor agonists. In the present study, we report on the effect of the morphinan congener, butorphanol tartrate (33), on ingestive behaviour.     

Involvement of dynorphin and the kappa opioid receptor in feeding

Dynorphin-(1–17) produces a highly specific increase in food ingestion. Similar enhancement of food ingestion is found with dynorphin fragments (1–10), (1–11), (1–13) and (3–13) but not with (1–8) and (1–9). Dynorphin B (rimorphin) also enhances food intake. The highly specific kappa agonist U-50,488 also enhances food intake as do a number of other kappa-opiate receptor agonists. These studies provided further support for the role of a highly specific dynorphin-kappa opioid receptor in the modulation of feeding.     

FMRFamide suppresses kappa opiate induced feeding in the mouse

Intracerebroventricular administration of 0.01-1.0 micrograms of the peptide FMRFamide (Phe-Met-Arg-Phe-NH2) to mice suppressed feeding induced by the specific kappa opiate agonist, U-50, 488H. This suggests that FMRFamide, or FMRFamide-like neuropeptides, may have a role in the control of kappa opioid mediated feeding in the mouse.     

Magnetic fields differentially inhibit mu, delta, kappa and sigma opiate-induced analgesia in mice

An exposure for 60 min to a 0.5 Hz rotating magnetic field (1.5-90 G) significantly attenuated the daytime analgesic effects of the mu and kappa opiate agonists, morphine and U50,488H, respectively, and significantly inhibited the analgesic actions of the delta agonist, D-Ala2-D-Leu5-enkephalin, in mice. The magnetic stimuli had no significant effects on the analgesic effects of the prototypic sigma opiate agonist (+/-) SKF-10,047. These results show that exposure to relatively weak magnetic stimuli has significant and differential inhibitory influences on various opioid systems.     

Reciprocal opioid-opioid interactions between the ventral tegmental area and nucleus accumbens regions in mediating mu agonist-induced feeding in rats

Feeding elicited by the mu-selective agonist, [D-Ala2, M-Phe4, Gly-ol5]-encephalin administered into the nucleus accumbens is blocked by accumbal pre-treatment with mu, delta1, delta2 and kappa, but not mu1 opioid antagonists. Correspondingly, mu-agonist-induced feeding elicited from the ventral tegmental area is blocked by ventral tegmental area pre-treatment with mu and kappa, but not delta opioid antagonists. A bi-directional opioid-opioid feeding interaction has been firmly established such that mu-agonist-induced feeding elicited from the ventral tegmental area is blocked by accumbal naltrexone, and that accumbal mu-agonist-induced feeding is blocked by naltrexone pre-treatment in the ventral tegmental area. To determine which opioid receptor subtypes mediate the regional bi-directional opioid-opioid feeding interactions between these two sites, the present study examined the dose-dependent ability of either general (naltrexone), mu (beta-funaltrexamine), kappa (nor-binaltorphamine) or delta (naltrindole) opioid antagonists administered into one site to block mu-agonist-induced feeding elicited from the other site. General, mu and kappa, but not delta opioid receptor antagonist pre-treatment in the ventral tegmental area dose-dependently reduced mu-agonist-induced feeding elicited from the nucleus accumbens. General, mu and delta, and to a lesser degree kappa, opioid receptor antagonist pre-treatment in the nucleus accumbens dose-dependently reduced mu-agonist-induced feeding elicited from the ventral tegmental area. Thus, multiple, but different opioid receptor subtypes are involved in mediating opioid-opioid feeding interactions between the nucleus accumbens and ventral tegmental area regions.     

Kappa opiate agonists inhibit Ca2+ influx in rat spinal cord-dorsal root ganglion cocultures. Involvement of a GTP-binding protein

The aim of the present study has been to characterize the regulation by opiates of 45Ca2+ influx in rat spinal cord-dorsal root ganglion cocultures. We have demonstrated that K+-induced depolarization, in the presence of the Ca2+ channel agonist Bay K8644, stimulated Ca2+ influx (3-4-fold) via the dihydropyridine class of voltage-dependent Ca2+ channels. While mu and delta opiates had no effect, kappa opiate agonists (e.g. U50488, dynorphin) profoundly depressed the stimulated Ca2+ influx (86% inhibition at 100 microM U50488). The kappa agonist action was stereospecific and could be reversed by the opiate antagonist naloxone. The inhibition produced by kappa agonists was greatly diminished following pertussis toxin treatment, and this effect was accompanied by toxin-induced ADP-ribosylation of a 40-41-kDa protein. This suggests that kappa opiate receptors are negatively coupled to voltage-dependent Ca2+ channels, via a pertussis toxin-sensitive GTP-binding protein. Basal 45Ca2+ uptake, stimulated by adenylate cyclase activators (forskolin and cholera toxin), was potently inhibited by kappa opiates suggesting that, under conditions of neurohormonal stimulation of adenylate cyclase, kappa receptors are coupled to Ca2+ channels indirectly via the adenylate cyclase complex. In addition, cAMP-independent coupling pathways may also be involved.     

ACTH-(1-24) and alpha-MSH antagonize feeding behavior stimulated by kappa opiate agonists

ACTH-(1-24) and alpha-MSH, intracerebroventricularly (ICV) injected at the doses of 4 and 10 micrograms/animal, respectively, markedly inhibited spontaneous feeding in adult Sprague-Dawley rats, the effect remaining significant for 6-9 hours. At these same doses, ACTH-(1-24) and alpha-MSH abolished the feeding-stimulatory effect of the kappa opiate receptor agonist pentazocine, intraperitoneally (IP) injected at the dose of 10 mg/kg. The same antagonism was obtained by ICV injection of ACTH-(1-24) into rats IP treated with other kappa opiate agonists, bremazocine and tifluadom, at the doses of 1 and 5 mg/kg, respectively. These data suggest that melanocortin peptides play an inhibitory role in the complex regulation of food intake, and further support and extend the hypothesis of a melanocortin-opioid homeostatic system, its two neuropeptide components usually having opposite, mutually-balancing effects.     

Spinal analgesia: Comparison of the mu agonist morphine and the kappa agonist ethylketazocine

The sites of analgesic action of the mu agonist morphine and the purported kappa agonist ethylketazocine (EKC) were compared. Using local drug injections and parenteral administration of drugs to spinalized rats, our data support a predominantly spinal site of action for EKC and a major supraspinal action for morphine in antinociceptive tests. This spinal analgesic action of EKC was dose dependent and naloxone reversible indicating opiate receptor involvement. The possibility that EKC activates a spinal kappa receptor population is under further study.     

Role of NPY and its receptor subtypes in foraging, food hoarding, and food intake by Siberian hamsters

Fasting has widespread physiological and behavioral effects such as increases in arcuate nucleus neuropeptide Y (NPY) gene expression in rodents, including Siberian hamsters. Fasting also stimulates foraging and food hoarding (appetitive ingestive behaviors) by Siberian hamsters but does relatively little to change food intake (consummatory ingestive behavior). Therefore, we tested the effects of third ventricular NPY Y1 ([Pro(34)]NPY) or Y5 ([D-Trp(34)]NPY) receptor agonists on these ingestive behaviors using a wheel running-based food delivery system coupled with simulated burrow housing. Siberian hamsters had 1) no running wheel access and free food, 2) running wheel access and free food, or 3) foraging requirements (10 or 50 revolutions/pellet). NPY (1.76 nmol) stimulated food intake only during the first 4 h postinjection ( approximately 200-1,000%) and mostly in hamsters with a foraging requirement. The Y1 receptor agonist markedly increased food hoarding (250-1,000%), increased foraging as well as wheel running per se, and had relatively little effect on food intake (<250%). Unlike NPY, the Y5 agonist significantly increased food intake, especially in foraging animals ( approximately 225-800%), marginally increased food hoarding (250-500%), and stimulated foraging and wheel running 4-24 h postinjection, with the distribution of earned pellets favoring eating versus hoarding across time. Across treatments, food hoarding predominated early postinjection, whereas food intake tended to do so later. Collectively, NPY stimulated both appetitive and consummatory ingestive behaviors in Siberian hamsters involving Y1/Y5 receptors, with food hoarding and foraging/wheel running (appetitive) more involved with Y1 receptors and food intake (consummatory) with Y5 receptors.     

Dynorphin-(1–13) induces spontaneous feeding in rats

Dynorphin-(1–13), a recently isolated opioid peptide stimulates food ingestion in rats after intracerebroventricular administration at doses of 1 and 10 μg. The latency until food ingestion is 22.4 ± 1.9 min. The ability of dynorphin-(1–13) to initiate food ingestion is antagonized by the opiate antagonist, naloxone. The food ingestion is accompanied by excessive grooming behavior.     

MTII attenuates ghrelin- and food deprivation-induced increases in food hoarding and food intake

Food deprivation triggers a constellation of physiological and behavioral changes including increases in peripherally-produced ghrelin and centrally-produced agouti-related protein (AgRP). Upon refeeding, food intake is increased in most species, however hamsters primarily increase food hoarding. Food deprivation-induced increases in food hoarding by Siberian hamsters are mimicked by peripheral ghrelin and central AgRP injections. Because food deprivation stimulates ghrelin as well as AgRP synthesis/release, food deprivation-induced increases in hoarding may be mediated by melanocortin 3 or 4 receptor (MC3/4-R) antagonism via AgRP, the MC3/4-R inverse agonist. Therefore, we asked: Can a MC3/4-R agonist block food deprivation- or ghrelin-induced increases in foraging, food hoarding and food intake? This was accomplished by injecting melanotan II (MTII), a synthetic MC3/4-R agonist, into the 3rd ventricle in food deprived, fed or peripheral ghrelin injected hamsters and housed in a running wheel-based food delivery foraging system. Three foraging conditions were used: a) no running wheel access, non-contingent food, b) running wheel access, non-contingent food or c) a foraging requirement for food (10 revolutions/pellet). Food deprivation was a more potent stimulator of foraging and hoarding than ghrelin. Concurrent injections of MTII completely blocked food deprivation- and ghrelin-induced increases in food intake and attenuated, but did not always completely block, food deprivation- and ghrelin-induced increases in food hoarding. Collectively, these data suggest that the MC3/4-R are involved in ghrelin- and food deprivation-induced increases in food intake, but other neurochemical systems, such as previously demonstrated with neuropeptide Y, also are involved in increases in food hoarding as well as foraging.     

MTII attenuates ghrelin- and food deprivation-induced increases in food hoarding and food intake

Food deprivation triggers a constellation of physiological and behavioral changes including increases in peripherally-produced ghrelin and centrally-produced agouti-related protein (AgRP). Upon refeeding, food intake is increased in most species, however hamsters primarily increase food hoarding. Food deprivation-induced increases in food hoarding by Siberian hamsters are mimicked by peripheral ghrelin and central AgRP injections. Because food deprivation stimulates ghrelin as well as AgRP synthesis/release, food deprivation-induced increases in hoarding may be mediated by melanocortin 3 or 4 receptor (MC3/4-R) antagonism via AgRP, the MC3/4-R inverse agonist. Therefore, we asked: Can a MC3/4-R agonist block food deprivation- or ghrelin-induced increases in foraging, food hoarding and food intake? This was accomplished by injecting melanotan II (MTII), a synthetic MC3/4-R agonist, into the 3rd ventricle in food deprived, fed or peripheral ghrelin injected hamsters and housed in a running wheel-based food delivery foraging system. Three foraging conditions were used: a) no running wheel access, non-contingent food, b) running wheel access, non-contingent food or c) a foraging requirement for food (10 revolutions/pellet). Food deprivation was a more potent stimulator of foraging and hoarding than ghrelin. Concurrent injections of MTII completely blocked food deprivation- and ghrelin-induced increases in food intake and attenuated, but did not always completely block, food deprivation- and ghrelin-induced increases in food hoarding. Collectively, these data suggest that the MC3/4-R are involved in ghrelin- and food deprivation-induced increases in food intake, but other neurochemical systems, such as previously demonstrated with neuropeptide Y, also are involved in increases in food hoarding as well as foraging.