Evidence that opiate receptors mediate suppression of hypertonic saline-induced drinking in the mouse by narcotic antagonists

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.

Interactions between angiotensin II and delta opioid receptor subtype agonists upon water intake in rats

Delta opioid receptor agonists, like those of mu and kappa receptors, stimulate water intake. To assess the relative contributions of delta1 and delta2 receptors in the modulation of water intake stimulated by Angiotensin II (AII), the present study examined the respective actions of [D-Pen2, D-Pen5]-enkephalin (DPDPE: 5-20 ug, i.c.v.) and [D-Ala2, Glu4]-Deltorphin (Delt II: 5-20 ug, i.c.v.) upon water intake per se, and upon AII (0.02-20 ng, i.c.v.)-induced hyperdipsia in rats. Both DPDPE and Delt II dose-dependently stimulated spontaneous water intake. An ineffective (5 ug) dose of DPDPE differentially altered water intake when paired with AII, significantly increasing intake at the 0.02 ng dose, not changing intake at the 0.2 ng dose and significantly decreasing intake at the 2 ng dose. In contrast, pairing ineffective doses of Delt II (5 ug) and AII (0.2 ng) significantly increased water intake that persisted when either the Delt II dose (1 ug) or the AII dose (0.02 ng) was lowered. However, neither delta1 nor delta2 opioid agonists significantly altered the ED50 for AII-induced drinking. Thus, delta2 opioid agonism appeared more consistent and reliable than delta1 opioid agonism in stimulating water intake when paired with AII, and these data indicate interactions between delta1 and delta2 agonists and AII in mediating water intake.

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

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

Differential modulation of angiotensin II and hypertonic saline-induced drinking by opioid receptor subtype antagonists in rats

Opioid modulation of ingestion includes general opioid antagonism of different forms of water intake, mu 2 receptor modulation of deprivation-induced water intake and delta 2 receptor modulation of saccharin intake. Water intake is stimulated by both central administration of angiotensin II (ANG II) and peripheral administration of a hypertonic saline solution; both responses are reduced by general opioid antagonists. The present study examined whether specific opioid receptor subtype antagonists would selectively alter each form of water intake in rats. Whereas systemic naltrexone (0.1-2.5 mg/kg, s.c.) reduced water intake induced by either peripheral ANGII (500 micrograms/kg, s.c.) or hypertonic saline (3 ml/kg, 10%), intracerebroventricular (i.c.v.) naltrexone (1-50 micrograms) only inhibited central ANGII (20 ng)-induced hyperdipsia. Both forms of drinking were significantly and dose-dependently inhibited by the selective kappa antagonist, nor-binaltorphamine (Nor-BNI, 1-20 micrograms). Whereas both forms of drinking were transiently reduced by the mu-selective antagonist, beta-funaltrexamine (beta-FNA, 1-20 micrograms), the mu 1 antagonist, naloxonazine (40 micrograms) stimulated drinking following hypertonic saline. The delta 1 antagonist, [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, 1-40 micrograms) significantly reduced drinking following ANGII, but not following hypertonic saline; the delta antagonist, naltrindole failed to exert significant effects. These data indicate that whereas kappa opioid binding sites modulate hyperdipsia following hypertonic saline, mu 2, delta 1, and kappa opioid binding sites modulate hyperdipsia following ANGII. The mu 1 opioid binding site may normally act to inhibit drinking following saline.

Role of the opioid system in the hypodipsia associated with aging

OBJECTIVE

To compare the effects of the opioid antagonist, naloxone, on fluid ingestion in young and older males, in order to estimate the role of the opioid system in hypodipsia of older men.

DESIGN

Single-blinded, randomized, cross-over, placebo-controlled study.

SETTING

Outpatient Department of Veterans Affairs.

STUDY PARTICIPANTS

Sixteen young subjects (aged 23 to 39) and eight older subjects (aged 69 to 75). All subjects were healthy, community-dwelling, non-smoking men, on no medications.

INTERVENTION

Subjects were randomized to receive either placebo or naloxone on day one. Whichever substance the subject did not receive was given on day two with days one and two separated by at least a 2-week washout period.

MAIN OUTCOME MEASURES

Fluid intake after overnight food and fluid deprivation, with placebo or naloxone injected in the morning.

RESULTS

After overnight fluid deprivation, older individuals consumed 29% less fluid in 2 hours compared with younger individuals (ns). After overnight fluid deprivation and injection with naloxone 100 micrograms/kg, fluid intake was diminished by 42% (P less than 0.05) in young subjects compared with placebo, but only by 7% (ns) in older subjects. Subjective ratings did not differ significantly between young and old subjects.

CONCLUSIONS

These preliminary data suggest that the opioid system plays a role in the drinking response in young subjects while failing to alter fluid intake in older subjects. Hypodipsia in older individuals may be due to a deficit in the opioid drinking drive.

Inhibition of drinking by naltrexone in the rat: interaction with the dopamine D-1 antagonist SCH 23390 and the D-2 antagonist sulpiride

The involvement of dopamine receptors in water intake was investigated in the rat deprived of water for 24 hr. A 0.03 mg/kg dose of SCH 23390 markedly enhanced naltrexone (0.1 and 10.0 mg/kg)-induced hypodipsia, whilst the drug alone significantly decreased water intake at doses of 0.01 to 3.0 mg/kg, accompanied by marked motor dysfunction. Sulpiride (20.0 and 40.0 mg/kg) did not markedly affect water intake and naltrexone-induced hypodipsia. Consistent with previous results, apomorphine (0.3 mg/kg) alone was without marked effects, while it produced a marked potentiation of naltrexone (1.0 and 10.0 mg/kg)-induced hypodipsia. SCH 23390 (0.003 mg/kg) and sulpiride (40.0 mg/kg) completely antagonized the enhancing effects of apomorphine on naltrexone-induced hypodipsia. Similar effects were also seen in the latency to begin drinking. In contrast to the effects on naltrexone-induced hypodipsia, it appears that both dopamine D-1 and D-2 receptors play a key role in the effects of apomorphine on naltrexone-induced hypodipsia in the rat.

The opioid antagonist, MR2266, specifically decreases saline intake in the mouse

The effects of the opioid antagonist, Mr2266 [(-)-(1R,5R,9R)-5,9-diethyl-2-(3-furyl-methyl)-2'-hydroxy-6,7-benzomo rph an] on the intake of water and saline (0.9%) were investigated in the mouse, deprived of water for 24 hr. In an attempt to evaluate motor functions, the behavior after treatment with Mr2266 was also examined by using multi-dimensional behavioral analyses. Although smaller doses (1.0, 3.0 and 10.0 mg/kg) of Mr2266 failed to affect significantly the intake of water, a larger dose (30.0 mg/kg) elicited a significant attenuation in the intake of water. During a 30 min observation, Mr2266 (30.0 mg/kg) depressed markedly linear locomotion, while other behavioral responses, such as rearing and grooming, remained unchanged. In contrast, 1.0-30.0 mg/kg doses of the drug produced a significant reduction in the intake of saline. The drug Mr2266 had no significant effects on the latency to start drinking at any doses tested. These results suggest that Mr2266 specifically blocks the intake of saline of the mouse through the mediation of opioid systems.

Apomorphine markedly potentiates naltrexone-induced hypodipsia in the rat

This article describes the effects of apomorphine on naltrexone-induced decreases in water intake of the rat deprived of water for 24 h. Apomorphine alone at reasonable doses (0.03, 0.1, 0.3 and 1.0 mg/kg) failed to affect water intake of the rat, but a higher dose (3.0 mg/kg) abolished water intake completely, accompanied by marked stereotypy. Naltrexone (0.1, 1.0 and 10.0 mg/kg) produced a dose-dependent reduction in water intake. A 0.3-mg/kg dose of apomorphine which is considered to activate preferentially presynaptic dopamine autoreceptors enhanced markedly naltrexone (1.0 and 10.0 mg/kg)-induced decreases in water intake. Only apomorphine at 1.0 mg/kg caused a significant prolongation of the latency to start drinking. Apomorphine (0.3 mg/kg), naltrexone (0.1, 1.0 and 10.0 mg/kg) or their combinations did not produce a marked effect on locomotor activity in the rat. These results suggest that apomorphine is capable of potentiating naltrexone-induced decreases in water intake through the mediation of presynaptic dopamine autoreceptors without causing motor dysfunction.

Opiate antagonists and rewarding brain stimulation

This review examines the literature on the effects of opiate antagonists on brain stimulation (ICSS) reward. Antagonists should have predictable effects if endogenous opioids modulate ICSS. Naloxone is the antagonist most often used, and it has produced inconsistent results in some ICSS paradigms. When schedules of intermittent reinforcement are used, however, naloxone reliably reduces the rate of responding. It reverses the effects of opiate agonists on ICSS behavior, and it also attenuates the effects of psychomotor stimulants, such as amphetamine. The results produced by naloxone are consistent with a modulatory effect of endogenous opioid systems on reward, and suggest that the opiate and dopamine systems together exert significant control over ICSS. Further research is needed to characterize better the actions of the antagonists on ICSS behavior, and productive research directions are proposed. Data obtained in future studies might suggest how the endogenous opioid systems modulate both natural and brain stimulation reward.

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

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

Naloxone blockade of growth hormone-releasing factor-induced feeding

The opiate antagonist naloxone was used to examine the possibility that endogenous opioid function is involved in the expression of the increased feeding observed following intracerebroventricular (i.c.v.) administration of rat hypothalamic growth hormone-releasing factor (GRF). It was found that systemically administered naloxone (0.125, 0.25 and 0.50 mg/kg) significantly suppressed the increased food intake observed following i.c.v. GRF (4.0 pmol) treatment. Though potent enough to eliminate ingestive effects of GRF, baseline food intake was unaffected by 0.125 mg/kg naloxone. Examination of 0.4, 4.0 and 40.0 pmol i.c.v. administered GRF-(3-40), a structurally related but physiologically inactive peptide, revealed no effect on food intake. The present results suggest involvement of endogenous opioid function in GRF-stimulated feeding.

The effects of opioid antagonists on ingestive behavior in the domestic fowl

The effects of opioid antagonists on food and water intake in commercial stocks of chickens were investigated. Four experiments were conducted to examine the effects of naloxone (N-allylnoroxymorphone) and naltrexone (N-cyclopropylnoroxymorphone) in broiler and Single-Comb White Leghorn cockerels. Birds were injected intramuscularly with either naloxone HCl or naltrexone HCl at doses from 2.5 to 10 mg/kg. Food and water were offered ad lib 15 min post-injection. In broilers, naloxone dose-dependently attenuated food and water consumption for 300 min, while in Leghorns naloxone attenuated food and water intake for 240 and 300 min, respectively. Naltrexone dose-dependently reduced food and water consumption for 300 min in both broilers and Leghorns. Neither naloxone nor naltrexone significantly altered food or water intake at 24 hr. A fifth experiment was conducted to verify the specificity of opioid antagonism for water intake. Broiler cockerels received an intraperitoneal injection of either isotonic saline (0.15 M NaCl) or hypertonic saline (2.5 M NaCl) followed by an intramuscular injection of either isotonic saline or naloxone HCl (5 mg/kg). Food was withheld for the entire experiment while water was offered ad lib 15 min following the second injection. Naloxone significantly attenuated drinking in normally hydrated and osmotically challenged birds for 150 min. The results suggest a role for endogenous opioid peptides in the regulation of food and water intake in meat and egg-laying stocks of chickens.

Suppression of deprivation-induced water intake in the rat by opioid antagonists: central sites of action

The effects of naltrexone methobromide, a quaternary derivative of the opioid antagonist naltrexone, were investigated on deprivation (24 h)-induced water intake in the unilaterally cannulated rats. Naltrexone methobromide reduced post-deprivational water intake with an ED50 of 7.3 micrograms when tested at 30 min (peak effect) after intracerebroventricular administration. It also dose-dependently (0.3-10 micrograms) depressed water intake, with peak effects at 15 min, after microinjection into the paraventricular hypothalamic nucleus and into the supraoptic hypothalamic nucleus. The drug did not produce any other effects on behaviors. The ED50S were 1.4 micrograms when given into the paraventricular nucleus, and 3.3 micrograms when given into the supraoptic nucleus, respectively. Although injections of higher doses (1.0, 3.0 and/or 10 micrograms) of the drug into the preoptic area, zona incerta, and corpus callosum significantly suppressed water intake, other behavioral manifestations, such as rotational behaviors, convulsions, body shakes, head swaying, and/or backward locomotion were manifested simultaneously with the reduction in drinking. When injected into the lateral hypothalamic area, water intake was not significantly affected by the drug. These findings suggest that the paraventricular and supraoptic hypothalamic nuclei are important sites of action in the naltrexone-induced suppression of water intake.

Naloxone's effects on intake of sequentially presented fluids

Fluid-deprived rats were presented with one of three types of palatable solutions for 15 min, followed by access to either the same solution or one of the other types for 15 min. The solutions were 5.3% sucrose, 0.9% salt solution, and tap water. Naloxone reduced intake of all solutions, as compared to placebo, regardless of type of fluid or order of presentation. Rats receiving water followed by sucrose solution reduced their intake of water under naloxone, but did subsequently take a considerable amount of sucrose solution showing that they were capable of drinking more. These findings indicate that naloxone's effects closely track ordinary satiation-effects, merely enhancing satiation-like functions at each instance of opportunity to take fluids.

The role of the mediobasal arcuate hypothalamus in relation to opioid systems in the control of ingestive behaviour in the rat

Bilateral, radiofrequency lesions of the mediobasal arcuate hypothalamus (MBH) strongly depleted levels of immunoreactive (ir)-beta-endorphin (beta-EP) in the hypothalamus and other brain tissues: these changes reflect destruction of those beta-EP-containing perikarya which are located in the MBH. No change in plasma ir-beta-EP was seen. The ir-dynorphin (DYN) content of the hypothalamus was also depressed while that of ir-Met-enkephalin was unaffected. The fall in hypothalamic ir-beta-EP was correlated with the fall in that of ir-DYN. Lesioned rats displayed only a minor, transient reduction in rate of weight gain between days 3 and 9 postsurgery: this disappeared thereafter. Further, the lesion did not affect the pattern of weight loss and regain associated with 24 h food and water deprivation. Indeed, the total 24 h (daily) food intake (FI) and water intake (WI) of lesioned rats did not differ from that of sham animals while deprivation-induced hyperphagia and hyperdipsia was not attenuated by the lesions. Moreover, the ability of naltrexone to decrease FI and WI (during both dark and light phases of the daily cycle) was not altered by the lesions. These observations indicate that central beta-EP may not be essential for the maintenance of a normal 24 h FI and WI and that opioid antagonists do not act upon the MBH or upon central beta-EP neurones in their suppression of FI and WI. Further, they suggest that central beta-EP may not fulfil an essential role in the control of body weight in the rat. Lesioned rats did, however, reveal a shift in the diurnal rhythmicity of FI and WI reflected in a reduction in the dark:light ratios of these. An alteration in the diurnal rhythmicity of sleeping and core temperature, but not locomotor activity, was also seen. The shifts in hypothalamic ir-beta-EP and ir-DYN (but no other tissue levels of any peptide) were correlated with the magnitude of the shifts in diurnal rhythmicity of ingestive behaviour. Moreover, lesions caudal to the MBH (not affecting hypothalamic ir-beta-EP or ir-DYN) or dexamethasone treatment (which affects pituitary pools of ir-beta-EP and ir-DYN) did not modify these rhythms. Thus, in these respects, the effects are 'particular' to MBH lesions modifying hypothalamic ir-beta-EP and ir-DYN. The data suggest that the MBH may play a role in the modulation of the diurnal scheduling of ingestive behaviour in the rat.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased delta, but not mu, opiate receptor binding in the medulla oblongata of Long-Evans rats following 5-day water deprivation

Opiate receptors of the mu type were labeled with [125I]D-Ala2,N-Me-Phe4,Met-(O)5-ol-enkephalin (FK-33824). delta receptors were labeled with [125I]D-Ala2-D-Leu5-enkephalin (DADLE) in the presence of excess (N-Me-Phe3,D-Pro4)-morphiceptin (PL017). Since DADLE binds mu and delta receptor sites, and PL017 blocks mu receptors, this protocol improves specific labeling of delta receptors. Quantitative autoradiography showed that chronic dehydration causes no changes in mu receptor binding in the medulla oblongata of Long-Evans rats. However, there is increased delta receptor binding in the solitary, hypoglossal and gracilis nuclei, and the spinal nucleus of trigeminal system of dehydrated animals, suggesting that delta opiate receptors participate in the physiological response to dehydration.

Naloxone and fluid consumption in rats: dose-response relationships for 15 days

Rats were given daily intraperitoneal injections of 10.0, 1.0, 0.1, 0.01, 0.001 or 0.0 mg/kg naloxone for 15 days. Each day after the injections, animals were allowed access to a 20% sucrose solution for two hours and to tap water for the subsequent 10 hours. Consumption of the sucrose solution by the group that received 1.0 mg/kg was reliably decreased on Day 1 and 2, reflecting the suppressive effect of naloxone at that dose. By Day 3 until the end of the experiment, however, the suppression was no longer significant, suggesting that tolerance had developed. A similar effect was seen with the group given the highest dose, 10.0 mg/kg; although drinking was significantly less than the control in each of the 15 sessions, this group showed a trend to increase intake over the days of the experiment, thus also indicating possible tolerance to the effect of naloxone. Drinking patterns of the other groups did not differ statistically from the control. Thus, the low doses had no ability to suppress consumption, and the lowest dose that did lower it soon lost that ability; the highest dose continued to suppress drinking throughout the study but with decreasing efficacy. High performance liquid chromatography (HPLC) demonstrated that the naloxone remained intact over the 15 days of the experiment, supporting the suggestion that tolerance to naloxone might have developed.

Effects of systemic or intracerebroventricular naloxone injection on basal and 2-deoxy-D-glucose-induced ingestive behavior

In order to examine the role and site of action of opiates on both hunger and thirst and food and water intake in rats after short term (3 hr.) food deprivation alone or in combination with 2DG-induced glucoprivic stress, naloxone was given to rats in either the jugular vein or the lateral ventricle. Both basal and 2DG-induced food and water intake were reduced by naloxone injected either peripherally or centrally. Latencies to eat and drink were used as measures of hunger and thirst respectively. Only central injection of naloxone significantly reduced 2DG-induced but not basal hunger. These results suggest a central site of action of naloxone on both food and water intake even if some peripheral effects cannot be totally ruled out. Our findings indicate central nervous system opiate receptor involvement in the hunger response to 2DG-induced glucoprivation. In all other treatment conditions, decreases in food intake cannot be related to reduction of hunger but may be due to potentiation of satiation during opiate receptor blockade.

Suppression of food and water intake after intracerebroventricular infusion of morphine and naloxone in rabbits

The effect of intracerebroventricular infusion of morphine and naloxone on food and water intake was investigated in rabbits. Morphine hydrochloride at a dose of 120, 10 and 5 micrograms produced statistically significant suppression of 24-h food and water intake. The same effect ensued after infusion of naloxone at dose of 120 and 10 micrograms. Postmorphine aphagia was accompanied by a rise in blood free fatty acids and normal glucose levels. No changes were seen after 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.

Drinking behavior is modulated by CNS administration of opioids in the rat

While opiate antagonists have been shown to reliably attenuate drinking following both central and peripheral administration, relatively few data exist on the effects of agonist agents on this behavior. To address this issue, two opiate agonists, morphine sulfate, a mu agonist, and [D-ala2, D-leu5]-enkephalin (DADLE), a semi-synthetic delta analog of a delta agonist, were administered into several CNS sites in rats. There was a dose-related, naloxone-reversible reduction of water intake following morphine injections into the lateral hypothalamus (LH) and preoptic area (POA). In addition, injections of DADLE also attenuated drinking when injected into LH and POA, but not following the ventral tegmental area or zona incerta administration. These data are discussed in view of a role for the endogenous opioid peptides in the regulation of drinking behavior.

Morphine and naloxone modulate intake of ethanol

Rats, deprived of food and water for 18 hr, were given an opportunity to drink water and sweetened ethanol solution for 1 hr prior to being fed and watered for 5 hr, daily. One group received water and sucrose solution without ethanol and other groups received water and sucrose solution with 3, 6, 12 or 24% ethanol. Prior to some days' opportunities to drink, rats were injected with morphine (2.5 mg/kg), naloxone (10 mg/kg), or saline. Morphine increased intake of solutions containing ethanol as compared to intake under placebo. Naloxone reduced intakes of both fluids. Since morphine only increased sucrose solution intake when it contained ethanol, it was concluded that increments in opioid activity increase rats' avidity for ethanol.

Feeding, drinking, urine osmolality in DI Brattleboro rats: changes by morphine, naloxone, D-amino acids, prolyl-leucyl-glycinamide (PLG)

Brattleboro rats placed in metabolism cages were injected with morphine (Mor), naloxone (Nal), D- and L-aspartic acid (D- and L-Asp), D-phenylalanine (D-Phe), D-leucine (D-Leu) and prolyl-leucyl-glycinamide (PLG), alone and in suitable combinations. Food and fluid intake, urine outflow, faeces weight, rectal temperature and urinary osmolality were determined at the end of seven hours period of time. Mor, Nal, D-Asp and PLG alone caused a significant decrease in food and fluid intake, urine volume and faeces weight and a significant increase in urinary osmolality being the osmolality of the Mor, D-Asp and PLG injected groups higher than 300 mOsmol/kg. The combination of Nal with Mor, D-Asp and PLG appeared to intensify the changes induced by Mor, D-Asp and PLG whereas L-Asp antagonized the majority of changes caused by Mor or PLG. The results were discussed in the light of the previous experimental findings.

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.

Patterns of drinking in the rat following the administration of opiate antagonists

Durations of drinking were recorded for water-deprived rats as they drank to satiety, following SC injections of naloxone (0.1-10.0 mg/kg), naltrexone (0.1-10.0 mg/kg) or saline vehicle. The results provided evidence for the effects of opiate antagonists on the temporal pattern of drinking exhibited by water-deprived animals. A separate, time-sampling procedure was used to supplement the drinking duration data, and showed that the opiate antagonists may suppress water consumption during a period 2.5-7.5 min after the start of the initial drinking bout. A second experiment confirmed that the pattern of drinking displayed during schedule-induced polydipsia in the rat is resistant to any suppressant effect of a moderate dose of an opiate antagonist. The similarity between opiate receptor blockade and water preloading in their effect on drinking in response to water deprivation, and lack of effect on schedule-induced polydipsia is discussed. Opiate antagonists may affect drinking principally by imposing a thirst satiety signal.

Naloxone-induced hypodipsia: a CNS mapping study

Opiate antagonists have been shown to reliably attenuate drinking behavior. Recent research points to a central site of action for this antidipsogenic effect. To pursue this issue of site specificity, naloxone, a specific opiate antagonist, was delivered into a number of discrete subcortical areas in 23 hour water-deprived rats. Water intake was measured at 5, 15, 30 and 60 minutes post drug injection. Compared to saline control injections, naloxone reliably depressed water intake, in a dose-related manner, in lateral hypothalamus, preoptic area and zona incerta. Previous research has repeatedly implicated these areas in drinking behavior. Placements which were not generally effective included lateral ventricle, nucleus accumbens, substantia nigra and cortex/corpus callosum. Latency to drink was never affected by any dose of naloxone injected into any site, suggesting an opioid influence on mechanisms involved in termination and/or maintenance rather than on initiation of drinking.

Opioid modulation of appetite

The discovery of opiate receptors and endogenous opioid peptides within the central nervous system has resulted in a number of speculations concerning the physiological significance of these peptides. In the present article, we review the evidence suggesting a primary role for some of the opioid peptides as regulators of ingestive behavior. In particular, we elaborate a hypothesis in which we suggest that in some species opioid peptides may play a role as a tonic inducer of ingestive behaviors, held in check by a variety of neuropeptides and monoamines. This review explores in detail the role of the opioid peptides as major mediators of the reward system and as a link between reward and feeding behaviors. Finally, a teleological role for opioid peptides in species preservation, which may explain the discrepancies in the role of the opioid peptides in feeding behavior in different species is proposed. It is suggested that the feeding profile of the animal provides important clues as to whether or not the animal has an opiate-sensitive feeding system. We stress that interactions with ingested nutrients and the milieu interieur provide an important means by which animals modulate the opiate-entrained feeding drives.

Naloxone blocks exercise-stimulated water intake in the rat

To examine whether opiate receptors modulate exercise-induced water intake, we measured water intake during four consecutive hours after a one-hour swim stress in male, Sprague-Dawley rats. Increased cumulative water intake was found four hours following exercise and this response was naloxone-reversible (P = 0.06). Suppression of water intake in the naloxone-treated, exercised group was most marked in the first two hours after exercise (P less than 0.05). Non-exercised rats consumed water at a constant, linear rate (P less than 0.05) whether treated with naloxone or saline. These results indicate an endogenous opioid role in regulating exercise-induced water intake in the rat, but do not delineate whether this role reflects a non-specific stress behavior or specific physiological processes related to thirst.

Morphine and acceptability of putative reinforcers

Rats were given the opportunity to take one of five concentrations of saccharin solutions. Intake across concentrations generated a preference-aversion curve. Morphine, 2 mg/kg, increased intake of saccharin solutions when rats were 12-hr water deprived or were not deprived. These effects with morphine are opposite to those of naloxone and strengthen the idea that there is opioid involvement in incentive motivation.

Effects of opiate agonists and antagonists on fluid intake and saccharin choice in the rat

Both naloxone (3 and 10 mg X kg-1) and naltrexone (1-10 mg X kg-1) abolished the preference for a highly palatable 0.05% sodium saccharin solution in rats that had been adapted to a 22 hr water-deprivation schedule. The effect occurred as a result of a selective decrease in the consumption of the saccharin solution, since the intake of water, which was concurrently available in the two-fluid choice test, remained unaffected. When a less preferred saccharin solution was used (0.01%), naltrexone exerted a similar suppressant effect on the sodium preference, whilst naloxone failed to produce significant effects on the intake of saccharin solution or water. The data for the opiate agonists were interpreted in terms of a drug-induced blockade of the natural reward of highly palatable fluids in thirsty rats. In the same choice test, morphine and a stabilised enkephalin analogue, with a selective agonist action at mu-opiate receptors (RX783030), failed to influence the preference for the palatable saccharin solutions. In water-deprived animals, at least, exogenous opiate agonists, active at mu-receptors, did not appear to influence the reward of the palatable solutions.

Antidipsogenic action of naloxone under different water deprivation conditions

Naloxone (1 mg/Kg s.c.), in the light phase of the daily light-dark cycles during the 30 and 60 min of the test, shows an antidipsogenic effect of the same intensity in chronically water-deprived rats which drink more (+ 176% and + 162%) than the acutely water-deprived animals. In chronically water-deprived rats, the time interval from the drug introduction (5, 30, 60 and 90 min) is not critical for the intensity of the antidipsogenic action of naloxone. When water intake is controlled by a 20% variable ratio schedule that does not allow the animals to satisfy, during the session, more than 50% of their thirst, the time interval from the introduction of naloxone is critical. When the rats can drink freely for 30 min in their home-cage after they satisfied their thirst partially by means of bar-pressing, naloxone, paradoxically shows a weaker and not significant antidipsogenic action. The results and the usefulness of the behavioural test are briefly discussed.

Effect of D- and/or L-aspartic acids on feeding, drinking, urine outflow and core temperature

Rats were given D- and/or L-aspartic acids (Asp) in saccharin solution for one week. Body weight gain, daily food and fluid intake, weight of faeces, urine outflow and osmolality, and rectal temperature were compared with those of the pretreatment period. After the rats had been sacrificed the weights of liver, spleen and kidney were estimated and compared with those of the control. The long-term oral administration of D-Asp caused a significant decrease in the weights of total body, liver and kidney, in the daily food and fluid intake, in the weight of faeces and in the volume of urine. The osmolality of urine of the rats administered D-Asp was significantly higher than that of the control. The concomitant oral administration of L-Asp seemed to antagonize the effects of D-Asp.

Effects of opiate antagonists and putative mu- and kappa-agonists on milk intake in rat and squirrel monkey

The effects of a number of relatively pure opiate antagonists (naloxone, naltrexone, diprenorphine), and putative mu- (morphine, etorphine) and kappa- (ketocyclazocine, ethylketocyclazocine) receptor agonists on sweetened condensed milk intake were examined over a broad range of doses in non-deprived rats and squirrel monkeys. The antagonists consistently decreased milk intake in both the rat and squirrel monkey. There were, however, species differences: diprenorphine was 30 times more potent than either naloxone or naltrexone in the squirrel monkey, but was of similar potency in the rat. The effects of the opiate agonists were more variable than those of the antagonists. In both species, all agonists decreased milk intake at high doses that also produced behavioral depression. Significant increases in drinking were produced only by low doses of ketocyclazocine and ethylketocyclazocine in the rat. The suppression of milk intake by the antagonists supports a modulatory role of opiate receptors in the control of drinking behavior, however, the effects of the agonists on drinking are less easily interpreted within this conceptual framework.

Effects of naloxone and its quaternary form on fluid consumption in rats

Three studies were performed on albino rats to determine the effects of naloxone and its quaternary derivative, naloxone methylbromide, on fluid consumption. The doses of the quaternary naloxone were equated with naloxone by molarity and effectiveness in order to facilitate direct comparisons. All rats were deprived of food and water for 12 hr and exposed to a 20% sucrose solution for a 2 hr period. In Experiment 1, a low (0.01 mg/kg) dose of naloxone or an equated dose of quaternary naloxone was given ICV and immediate access allowed to the fluid on four consecutive days. Animals receiving naloxone were not significantly different from controls, and rats receiving quaternary naloxone exhibited seizures, resulting in decreased consumption. In Experiment 2, the low dose of naloxone or the equated dose of quaternary naloxone was given IP for four consecutive days and neither was significantly different from controls. In Experiment 3, animals were given an IP dose of either 1 mg/kg naloxone, a 1 mg/kg or 50 mg/kg dose of quaternary naloxone, or saline and tested for a single 2 hr period. The doses of 1 mg/kg naloxone and 50 mg/kg quaternary naloxone produced significantly less drinking than controls. In all studies, the initial 30 min period produced the most drinking. Suppression of drinking by a dose of 50 mg/kg quaternary naloxone suggested, in contrast to other studies, that it may cross the blood-brain barrier at high doses.

Enhancement of osmotic- and hypovolemic-induced drinking by chlordiazepoxide in rats is blocked by naltrexone

Recent reports indicate that benzodiazepine-induced hyperphagia can be antagonised by naloxone, an opiate antagonist. Benzodiazepines are also known to facilitate water ingestion in water-deprived rats, and the present study showed that in addition, benzodiazepine treatment can enhance drinking which is elicited by an osmotic thirst stimulus (2 M hypertonic saline) or by a hypovolemic thirst stimulus (20% polyethylene glycol). In both cases, low dose levels of naltrexone (also an opiate antagonist) dose-dependently suppressed the facilitation of thirst-aroused drinking by chlordiazepoxide. Taken with recent biochemical data these behavioral results indicate that the enhancement of ingestive responses by benzodiazepines may depend upon a naloxone-reversible release of endogenous opioid peptides.

Naloxone decreases consumption of liquid and solid sucrose in vagotomized rats

Intraperitoneal injections of the opiate antagonist naloxone decreased food intake in both vagotomized and sham-vagotomized rats. Consumption of liquid and solid sucrose, which were used in order to equate baseline intake, was equally suppressed in both groups under food-deprivation and appetitively-motivated conditions at all doses of naloxone (1, 2, 4, and 8 mg/kg). It is concluded that, in contrast to previous findings, the vagus nerve does not mediate the suppressive effects of naloxone on feeding behavior.

Effects of opiate antagonists and of morphine on chlordiazepoxide-induced hyperdipsia in the water-deprived rat

The effects of naloxone hydrochloride (0.01-10 mg X kg-1), naltrexone hydrochloride (0.01-10 mg X kg-1), and morphine sulphate (0.01-10 mg X kg-1) on the increased water consumption provoked by administration of chlordiazepoxide (10 mg X kg-1) were investigated in male rats which had been adapted to a 22 hr water-deprivation schedule. As previously reported, both naloxone and naltrexone dose-relatedly reduced water ingestion. Naloxone at 1 mg X kg-1 and naltrexone at 0.1 mg X kg-1 completely blocked the chlordiazepoxide-induced hyperdipsia. Since both opiate antagonists removed the chlordiazepoxide-induced effects in small doses, their effect can plausibly be attributed to opiate receptor blockade. Hence, chlordiazepoxide-induced hyperdipsia may depend upon the activation of endogenous opioid mechanisms. Morphine had little effect on drinking, unless a large dose (10 mg X kg-1) was used, when the thirst-induced and chlordiazepoxide-induced drinking were attenuated. The data provided no evidence that morphine, a mu opiate agonist, enhanced chlordiazepoxide-induced water consumption. The results are considered in relation to other relevant behavioural and biochemical findings.

Opiate receptor blockade in man reduces 2-deoxy-D-glucose-induced food intake but not hunger, thirst, and hypothermia

Opioid peptides may act as neuromodulators in the central nervous system to conserve energy stores and water in mammals. To examine this hypothesis in man, the effect of opiate receptor blockade with naloxone on the hunger, thirst, and hypothermic response to 2-deoxy-D-glucose-induced glucoprivic stress was assessed. Opiate receptor blockade decreased stress-induced food intake but did not reduce marked increases in hunger produced by glucoprivation. Naloxone infusions did not change the hypercortisolemic, polydipsic, hypothermic, and thermogenic response to 2-deoxy-D-glucose. While these results do not suggest a major role for a beta-endorphin modulation of stress-induced hunger, hypothermia and water conservation, the reduction of food intake could be due to augmented satiety, perhaps associated with retardation of gastric emptying during opiate receptor blockade.

Novel phenylpiperidine opioid antagonists and partial agonists: effects on fluid consumption

The effects of five opioid antagonists, a racemate partial agonist and its agonist and antagonist optical isomers were studied on deprivation-induced drinking. All compounds had a phenylpiperidine nucleus. The antagonists produced dose-related decreases in drinking, and the potencies for decreasing drinking correlated with morphine-antagonist doses. The racemic partial agonist and its agonist isomer decreased drinking at doses higher than those which produced marked analgesia. Within the class of phenylpiperidine drugs studied, some had less specificity than naloxone for the mu-receptors as compared to the delta-receptor, but the suppression of drinking was not related to changes in mu-to-delta ratios.

The effects of chronic administration of naltrexone on appetite and water exchange in rats

The effects of chronic administration of naltrexone (200 microgram/kg/hr) on appetitive behaviors and renal water and electrolyte excretions were studied in rats. Naltrexone reduced food and water intake, the renal excretions of water and electrolyte excretions were studied in rats. Naltrexone reduced food and water intake, the renal excretions of water and electrolytes, and osmolar clearance. No changes in plasma levels of electrolytes, plasma and urine Na+-K+ ratios, hematocrit ratio, plasma osmolality, the clearances of K+ and Na+, and the reabsorption of solute free water were found. The changes in appetite were compensated for by appropriate changes in renal excretions, resulting in no change in electrolyte balance or water exchange. These observations are discussed in relation to current theories of the role of endorphins in appetite control.

A comparison of the effects of opiate antagonists on operant and ingestive behavior

Previous studies have found that naloxone and other opiate antagonists will decrease the food and water intake of experimental animals. The present study investigated the possibility that these effects may be due to a generalized action of opiate antagonists to block an endogenous reward system. A direct comparison was made between the effects of naloxone and naltrexone on FR responding maintained by small quantities of milk and on the consumption of milk when it was freely available. Both drugs reduced milk consumption at all doses (0.3-30 mg/kg) but produced only small decreases in FR response rates at the highest doses. These results do not support the view that the actions of opiate antagonists on eating and drinking represent an inhibitory action on central reward mechanisms.

Relationship of the central ACTH-immunoreactive opiocortin system to the supraoptic and paraventricular nuclei of the hypothalamus of the rat

ACTH-immunoreactive (ir) fibers of the central opiocortin system are present in high density in the mid-portion of the paraventricular nucleus (PVN). ACTH-ir fibers, however, do not appear to directly contact oxytocin-ir vasopressin-ir, containing perikarya or axons of vasopressinergic neurons. Magnocellular neurons located more anteriorly and posteriorly in the PVN as well as those located laterally have few ACTH-ir fibers associated with them. No ACTH-ir fibers are present in the supraoptic nucleus.

Comparison of the suppression by naloxone of water intake induced in rats by hyperosmolarity, hypovolemia, and angiotensin

The effect of naloxone upon water consumption by rats was assessed using two intensities each of IV NaCl (Hyperosmolarity), SC polyethylene glycol (hypovolemia), and IV angiotensin II. In each case naloxone produced a dose-related reduction in the amount drunk. Angiotensin-induced drinking was most easily inhibited, and was abolished by only 1 mg/kg naloxone. In contrast, 1 mg/kg naloxone produced only a 50% reduction NaCl-induced drinking, and hypovolemia-induced drinking was not completely reversed by 5 mg/kg. Naloxone was without effect upon the natriuresis after NaCl, or the hypertension during AII administrations. Parallels are drawn between the effects of naloxone on these types of thirst, and of other perturbations including brain damage and taste adulteration.

The anorectic action of naloxone is attenuated by adaptation to a food-deprivation schedule

Recent studies have shown that naloxone and other opiate antagonists can reduce the amounts of food and water consumed by laboratory animals, a finding consistent with a role for endogenous opioids in the control of appetite. Because there have also been some failures to observe an anorectic action of naloxone, a study was carried out in which the effects of the drug on food intake were investigated using two different experimental procedures. In naive rats deprived of food for 24 h, both naloxone (0.1, 1.0 and 10.0 mg/kg) and fenfluramine (1.0, 3.0 and 10.0 mg/kg) produced dose-related decreases in food and water intake. In rats which had been adapted to receiving food for only 6h each day, fenfluoramine produced a similar effect whereas naloxone had no effect on food intake and reduced water consumption only at the highest dose. A second experiment showed that the different actions of a 1.0 mg/kg dose of naloxone in the two procedures were not due to differences in the duration of the immediately preceding period of food deprivation or in the time during which the rats were handled. These results show that the anorectic action of naloxone can be attenuated by adaptation to a schedule of repeated food deprivation.

A temporal analysis of naloxone's suppressant effect on drinking

Licking activity was monitored in water deprived rats following various doses of naloxone. It was found that naloxone, at doses ranging from 0.01 to 10.0 mg/kg, had very little effect on initial drinking. However, naloxone did slow sustained drinking after 2 to 6 min into the bout, dependent upon dose. We takes these results to mean that naloxone is interfering with those processes which serve to sustain continued drinking throughout a drinking bout.