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: μ, κ₁ and complex (maltose dextrin: μ) carbohydrates as well as deprivation-induced water intake (μ) under real-feeding and real-drinking conditions. An identical pattern of μ and κ₁ 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 μg), μ (β-funaltrexamine: 1–20 μg), μ₁ (naloxonazine: 50 μg), κ₁ (nor-binaltorphamine: 1–20 μg), δ₁ ([d-Ala², Leu⁵, Cys⁶]-enkephalin: 10–40 μg) or δ₂2 (naltrindole isothiocyanate: 20 μg) 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 κ₁ (69%) or δ₁ (59%) opioid antagonism, was significantly increased by μ₁ antagonism (43%), and was not significantly affected by either μ or δ₂ opioid antagonism. Deprivation-induced water intake in sham drinking rats was significantly reduced by either μ (41%), μ₁ (28%), δ₁ (48%) or δ₈ (28%) opioid antagonism, but was not significantly affected by κ₁ opioid antagonism. The difference in opioid receptor subtype mediation of maltose dextrin intake in real feeding and sham feeding conditions suggest that κ₁ and δ₁ receptors are involved in the orosensory mechanisms supporting maltose dextrin intake, while μ 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 μ-mediated prerestorative satiety mechanism, and the unmasking of other opioid-mediated signalling mechanisms.     

Multiple opioid receptors mediate feeding elicited by mu and delta opioid receptor subtype agonists in the nucleus accumbens shell in rats

The nucleus accumbens, and particularly its shell region, is a critical site at which feeding responses can be elicited following direct administration of opiate drugs as well as micro-selective and delta-selective, but not kappa-selective opioid receptor subtype agonists. In contrast to observations of selective and receptor-specific opioid antagonist effects upon corresponding agonist-induced actions in analgesic studies, ventricular administration of opioid receptor subtype antagonists blocks feeding induced by multiple opioid receptor subtype agonists. The present study examined whether feeding responses elicited by either putative mu ([D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO)), delta(1) ([D-Pen(2), D-Pen(5)]-enkephalin (DPDPE)) or delta(2) ([D-Ala(2), Glu(4)]-deltorphin (Deltorphin)) opioid receptor subtype agonists administered into the nucleus accumbens shell were altered by accumbens pretreatment with either selective mu (beta-funaltrexamine), mu(1) (naloxonazine), delta(1) ([D-Ala(2), Leu(5), Cys(6)]-enkephalin (DALCE)), delta(2) (naltrindole isothiocyanate) or kappa(1) (nor-binaltorphamine) opioid receptor subtype antagonists. Similar magnitudes and durations of feeding responses were elicited by bilateral accumbens administration of either DAMGO (2.5 microg), DPDPE (5 microg) or Deltorphin (5 microg). DAMGO-induced feeding in the nucleus accumbens shell was significantly reduced by accumbens pretreatment of mu, delta(1), delta(2) and kappa(1), but not mu(1) opioid receptor subtype antagonists. DPDPE-induced feeding in the accumbens was significantly reduced by accumbens pretreatment of mu, delta(1), delta(2) and kappa(1), but not mu(1) opioid receptor subtype antagonists. Deltorphin-induced feeding in the accumbens was largely unaffected by accumbens delta(2) antagonist pretreatment, and was significantly enhanced by accumbens mu or kappa(1) antagonist pretreatment. These data indicate different opioid pharmacological profiles for feeding induced by putative mu, delta(1) and delta(2) opioid agonists in the nucleus accumbens shell, as well as the participation of multiple opioid receptor subtypes in the elicitation and maintenance of feeding by these agonists in the nucleus accumbens shell.     

Inhibition of spinal opioid analgesia by supraspinal administration of selective opioid antagonists

The effect of intracerebroventricular administration of a selective mu- (CTOP) or delta- (ICI 174,864) opioid receptor antagonist on the antinociceptive effects produced by intrathecal administration of selective mu- (DAMGO), delta- (DPDPE) and kappa- (U50-488H) opioid receptor agonists was evaluated using the Randall-Selitto paw-withdrawal test, in the rat. While the intracerebroventricular administration of CTOP or ICI 174,864, alone, had no effect on nociceptive thresholds, intracerebroventricular administration of CTOP and ICI 174,864 produced marked antagonism of the antinociceptive effects of intrathecal DAMGO. The antinociceptive effects of intrathecal administration of DPDPE or U50,488H were not antagonized by intracerebroventricular administration of CTOP or ICI 174,864. These data suggest that, in the rat, along with the established descending antinociceptive pathways, there is an ascending antinociceptive control mechanism projecting from the spinal cord to the brainstem. The ascending antinociceptive control involves mu- and delta-opioid agonism at supraspinal sites and appears to be mediated selectively by mu-, but not by delta- or kappa-opioid agonism at the spinal level.     

Heterogeneous distribution and upregulation of μ, δ and κ opioid receptors in the amygdala

Levels of μ, δ and κ opioid receptors in 4 subnuclei of the rat amygdala were determined by quantitative autoradiography following chronic treatment with naloxone or saline. A different distribution of each receptor subtype was observed, with μ binding greatest in the lateral nucleus (La), δ greatest in the basolateral (B1), and κ greatest in the medial (Me). Levels of all 3 receptors were very low in the central nucleus. Receptor upregulation following chronic naloxone treatment was also anatomically heterogeneous. Increases in μ receptors were statistically significant in the Me, Bl and La, while increases in δ and κ receptors were significant only in the Bl.     

Potentiation of angiotensin II-induced drinking by glucocorticoids is a specific glucocorticoid Type II receptor (GR)-mediated event

Earlier studies showed that pretreatment (3 and 6 h) of rats with the glucocorticoid hormone, dexamethasone, potentiated the drinking response to either central or peripheral administration of angiotensin II (AII). In the present study the specificity and mechanisms of this potentiation were examined. Intraperitoneal (i.p.) injection of rats with the pure glucocorticoid agonist, RU 28362 (0.4-1.6 mg/kg; 3-24 h), resulted in a time- and dose-dependent potentiation of the drinking responses to either peripherally (100 micrograms/kg, s.c.) or centrally (10 ng) injected AII, similar to the effects of dexamethasone. Drinking induced by central injection of carbachol (200 ng) was unaltered by pretreatment with RU 28362, suggesting that potentiation by this compound was specific for AII. The potentiation of AII-induced drinking by either dexamethasone or RU 28362 was completely abolished by pretreatment with the glucocorticoid Type II receptor (GR) antagonist, RU 38486 (2 mg/kg, i.p.), but not by the mineralocorticoid Type I receptor (MR) blocker, mespirenone (2 mg/kg, i.p.). Taken together, these results indicate that the glucocorticoid-induced potentiation of AII-induced drinking is mediated via GR. Associated with the fact that glucocorticoids potentiate AII-induced drinking is the observation that these steroids also potentiate AII-induced urine output. This enhancement of urine output may explain in part the potentiation in drinking behavior. Possible mechanisms are discussed.     

Antinociception produced by receptor selective opioids: modulation of spinal antinociceptive effects by supraspinal opioids

The effect of intracerebroventricular administration of low-antinociceptive doses of selective μ-(DAMGO) or δ-(DPDPE) opioid agonists on the dose-dependent antinociceptive effects produced by intrathecal administration of sequentially increasing doses of selective μ-, δ-, or κ- (U50, 488H) opioid agonists was evaluated, in the rat, using the Randall-Selitto paw-withdrawal test. When DPDPE or U50,488H was administered intrathecally, the low doses of both intracerebroventricular DAMGO and intracerebroventricular DPDPE markedly enhanced the antinociceptive effects of both intrathecal opiods. In contrast, when DAMGO was administered intrathecally, both intracerebroventricular DAMGO and intracerebroventricular DPDPE, administered in low doses, markedly antagonized the antinociceptive effects of the intrathecal opioid. In addition, the intracerebroventricular administration of low-antinociceptive dose of a second μ-opioid agonist, morphiceptin, antagonized the antinociceptive effects of intrathecal morphiceptin. The antagonism of the antinociceptive effects observed with spinal administration of DAMGO is dose-dependent, with the effect observed only at low doses. Furthermore, the antagonism cannot be explained by a reduction in motor deficits produced by intrathecal administration of DAMGO, because there were no differences in motor deficits, measured with an accelerating Rotarod treadmill, between intrathecal DAMGO administered as a single agent or as part of a combination regimen. The differences in antinociceptive effects obtained with the various supraspinal and spinal combinations are discussed in terms of the interactions that may occur between brainstem and spinal opioid receptor sites.     

Cardiovascular and dipsogenic effects of angiotensin II administered i.c.v. in Long-Evans and Brattleboro rats

The concurrent cardiovascular and dipsogenic effects produced by i.c.v. administration of angiotensin II (AII) have been investigated in vasopressin-deficient (Brattleboro) and control (Long-Evans) rats. When animals were allowed to drink during testing, the pressor effect of i.c.v. AII (500 ng) in Long-Evans rats (26 +/- 3/26 +/- 3 mm Hg) was significantly greater than that produced when drinking water was not available (19 +/- 2/18 +/- 2 mm Hg). There was a significant decrease in heart rate only when water was available. There was no pressor response to i.c.v. AII in Brattleboro rats not allowed to drink, whereas blood pressure increased by 17 +/- 3/14 +/- 1 mm Hg in response to i.c.v. AII when drinking water was present. There were no significant changes in heart rate following i.c.v. AII in Brattleboro rats. When baseline drinking was taken into account, Brattleboro rats still drank significantly more water than Long-Evans rats in response to i.c.v. AII. Pretreatment of Long-Evans rats with the V1 vasopressin antagonist, D(CH2)5Tyr(Et)DAVP, decreased the pressor effect of i.c.v. AII to a level not significantly different from that of Brattleboro rats allowed to drink. Under these conditions the amount drunk by Long-Evans rats was not significantly less than that drunk by Brattleboro rats. These results confirm that the central pressor actions of AII are mediated, in part, by release of vasopressin and suggest that the greater dipsogenic effect of i.c.v. AII in Brattleboro compared with Long-Evans rats may be due, partly, to its lesser pressor activity in these animals.     

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

Ventricular microinjection studies found that whereas μ (β-funaltrexamine, B-FNA), μ₁ (naloxonazine) and κ (nor-binaltorphamine, Nor-BNI) opioid receptor antagonists, but not δ antagonists, reduce deprivation-induced intake, κ and μ, but not μ₁ or δ antagonists reduce both 2-deoxy- d-glucose (2DG) hyperphagia and sucrose intake. Since opioid agonists stimulate spontaneous food intake in the accumbens, the present study examined whether administration of either naltrexone, B-FNA or Nor-BNI in the accumbens altered intake under deprivation (24 h), glucoprivic (2DG: 500 mg/kg, i.p.) or palatable sucrose (10%) conditions. Naloxonazine's effects in the accumbens were also evaluated for deprivation-induced intake. Deprivation-induced intake was significantly decreased over 4 h by naltrexone (5–20 μg, 44%), B-FNA (1–4 μg, 55%) and Nor-BNI (4 μg, 31%), but not naloxonazine (10 μg) in the accumbens. 2DG hyperphagia was significantly decreased by naltrexone (10–20 μg, 79%), B-FNA (1–4 μg, 100%) and Nor-BNI (1–4 μg, 75%) in the accumbens. Sucrose intake was significantly decreased by naltrexone (50 μg, 27%) and B-FNA (1–4 μg, 37%), but not Nor-BNI in the accumbens. These data suggest that μ receptors, and particularly the μ₂ binding site in the accumbens are responsile for the opioid modulation of these forms of intake in this nucleus, and that this control may be acting upon the amount of intake per se.     

Quantitative autoradiography of μ-,δ- and κ1 opioid receptors in κ-opioid receptor knockout mice

Mice deficient in the κ-opioid receptor (KOR) gene have recently been developed by the technique of homologous recombination and shown to lack behavioural responses to the selective κ₁-receptor agonist U-50,488H. We have carried out quantitative autoradiography of μ-, δ- and κ₁ receptors in the brains of wild-type (+/+), heterozygous (+/−) and homozygous (−/−) KOR knockout mice to determine if there is any compensatory expression of μ- and δ-receptor subtypes in mutant animals. Adjacent coronal sections were cut from the brains of +/+, +/− and −/− mice for the determination of binding of [³H]CI-977, [³H]DAMGO ( -Ala²-MePhe⁴-Gly-ol⁵ enkephalin) or [³H]DELT-I ( -Ala² deltorphin I) to κ₁-, μ- and δ-receptors, respectively. In +/− mice there was a decrease in [³H]CI-977 binding of approximately 50% whilst no κ₁-receptors could be detected in any brain region of homozygous animals confirming the successful disruption of the KOR gene. There were no major changes in the number or distribution of μ- or δ-receptors in any brain region of mutant mice. There were, however some non-cortical regions where a small up-regulation of δ-receptors was observed in contrast to an opposing down-regulation of δ-receptors evident in μ-knockout brains. This effect was most notable in the nucleus accumbens and the vertical limb of the diagonal band, and suggests there may be functional interactions between μ- and δ-receptors and κ₁- and δ-receptors in mouse brain.     

Comparison of G-protein activation in the brain by μ-, δ-, and κ-opioid receptor agonists in μ-opioid receptor knockout mice

Mice lacking the μ-opioid receptor gene have been developed by a gene knockout procedure. In this study, the activity of opioid receptor coupled G-proteins was examined to investigate whether there is a change in the extent of coupling for μ-, δ-, and κ-opioid receptors in μ-opioid receptor knockout mice. Selective agonists of μ- (DAMGO), δ- (DPDPE), and κ- (U-69,593) opioid receptors stimulated [³⁵S]GTPγS binding in the caudate putamen and cortex of wild-type mice. In contrast, only U-69,593 stimulated [³⁵S]GTPγS binding in these regions of μ-opioid receptor knockout mice. These results confirmed the absence of G-protein activation by a μ-opioid receptor agonist in μ-opioid receptor knockout mice, and demonstrated that coupling of the κ-opioid receptor to G-proteins is preserved in these mice. However, G-protein activation by the δ-opioid receptor agonist, DPDPE, was reduced in the μ-opioid receptor knockout mice, at least in the brain regions studied using autoradiography.     

Modulation of humoral immune response by central administration of leucine-enkephalin: Effects of μ, δ and κ opioid receptor antagonists

The effect of leucine-enkephalin (Leu-Enk) on primary humoral immune response was investigated following intracerebroventricular (i.c.v.) administration of the peptide in the rat. Leu-Enk stimulated plaque-forming cell (PFC) response in rats i.c.v. injected with 0.1 and 1 μg/kg, whereas doses of 20 and 50 μg/kg exerted immunosuppressive effects. I.c.v. treatment of rats with δ opioid receptor antagonist ICI 174864 and κ opioid receptor antagonist nor-binaltorphimine (nor-BNI) blocked stimulation and suppression of PFC response induced by Leu-Enk, respectively. The μ opioid receptor antagonist β-funaltrexamine (β-FNA) reversed both immunomodulatory effects produced by Leu-Enk. Since β-FNA alone had no effect on PFC response (unlike ICI 174 864 and nor-BNI), these data showed that central effects of Leu-Enk on PFC response were mediated by brain μ opioid receptors, and suggested a possible involvement of δ and κ opioid receptors.     

Opioid receptor affinities of kappa agonists, agonist/antagonists and antagonists in vitro and in vivo

1. 1. The CMS opioid receptors consist of a major triad: μ, δ and κ.

2. 2. The κ agonists presently available act as κ agonists, δ antagonists and μ₂ isoreceptor antagonists.

3. 3. Pure opiate antagonists possess a broad spectrum of activity at all 3 opioid receptor populations.

4. 4. The Ag/Ant analgesics also possess a broad spectrum of receptor affinities which awaits the definition of complex species differences in their actions.

5. 5. The design of more specific opioid agonists and antagonists will lead to a greater understanding of the many possible roles opioids serve within the CNS.

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

Intake of either hypotonic or hypertonic saline solutions is modulated in part by the endogenous opioid system. Morphine and selective mu and delta opioid agonists increase saline intake, while general opioid antagonists reduce saline intake in rats. The present study evaluated whether intracerebroventricular administration of general (naltrexone) and selective mu (beta-funaltrexamins, 5–20 μg), mu₁ (naloxonazine, 50 μg), kappa (nor-binaltorphamine, 5–20 μg), delta (naltrindole, 20 μg), or delta, (DALCE, 40 μg) opioid receptor subtype antagonists altered water intake and either hypotonic (0.6%) or hypertonic (1.7%) saline intake in water-deprived (24 h) rats over a 3-h time course in a two-bottle choice test. Whereas peripheral naltrexone (0.5–2.5 mg/kg) significantly reduced water intake and hypertonic saline intake, central naltrexone (1–50 μg) significantly reduced water intake and hypotonic saline intake. Water intake was significantly reduced following mu and kappa receptor antagonism, but not following mu₁, delta, or delta₁ receptor antagonism. In contrast, neither hypotonic nor hypertonic saline intake was significantly altered by any selective antagonist. These data are discussed in terms of opioid receptor subtype control over saline intake relative to the animal's hydrational state and the roles of palatability and/or salt appetite.     

Injections of phentolamine into the anterior hypothalamus-preoptic area of rats blocks both pressor and drinking responses produced by central administration of angiotensin II

There are conflicting reports of a possible contribution of noradrenergic projections to the rostral hypothalamus to drinking and blood pressure regulation. The present study investigated the effects of injecting phentolamine into the anterior hypothalamus-preoptic region on drinking and blood pressure responses elicited by injecting angiotensin II into a lateral cerebral ventricle of the rat. Angiotensin II (250 ng or 25 ng) elicited water intakes averaging 9.25 +/- 0.52 ml and 4.35 +/- 0.44 ml respectively in 15 min with latencies of less than 3 min. Phentolamine, an alpha-adrenergic antagonist, injected into the rostral hypothalamus produced a dose-dependent reduction in water intake and number of laps taken accompanied by an increased latency to drink. In the urethane anaesthetized rat, angiotensin II produced significant increases in blood pressure. Injections of phentolamine into rostral hypothalamic sites in which drinking responses to angiotensin II were attenuated, also attenuated the pressor response to angiotensin II. These results indicate that alpha-adrenergic input to the rostral hypothalamus is involved in both the pressor and drinking responses elicited by central angiotensin II.     

Reductions in body weight following chronic central opioid receptor subtype antagonists during development of dietary obesity in rats

Acute administration of long-acting general opioid antagonists reduces body weight and food intake in rats. In contrast, chronic administration of short-acting general opioid antagonists produces transient effects. The present study evaluated whether chronic central administration of selective long-acting antagonists of mu (beta-funaltrexamine, BFNA, 20 μg), mu₁ (naloxonazine, 50 μg), delta₁([d-Ala²,Leu⁵,Cys⁶]-enkephalin, DALCE, 50 μg), delta₂ (naltrindole isothiocyanate, NTII, 20 μg) or kappa (nor-binaltorphamine, NBNI, 20 μg) opioid receptor subtypes altered weight and intake of rats exposed to a palatable diet of pellets, fat, milk and water, relative to pellet-fed and diet-fed controls. Diet-fed rats receiving chronic vehicle injections significantly increased weight (7–10%) and intake over the 11-day time course. Weight was significantly reduced over the time course in rats administered either BFNA (9%), naloxonazine (12%), DALCE (7%) or NTII (6%). Initial weight reductions failed to persist following chronic NBNI. All antagonists chronically reduced fat intake, but did not systematically alter total intake, pellet intake or milk intake relative to the pattern of weight loss. These data indicate that central mu, mu₁, delta₁, delta₂, and, to a lesser degree, kappa receptors mediate long-term opioid modulation of weight even in animals maintained on diets that ultimately result in dietary obesity.     

Role of theα1-andα2-adrenoceptors of the lateral hypothalamus in the dipsogenic response to central angiotensin II in rats

The present experiments were conducted to investigate the role of theα₁-andα₂-adrenergic receptors of the lateral hypothalamus (LH) on the drinking response elicited by intracerebroventricular (i.c.v) injections of carbachol and angiotensin II (AII) in rats. Clonidine (anα₂-adrenergic agonist) injected into the LH produced a dose-dependent reduction of the drinking responses elicited by i.c.v. administration of carbachol and AII. Theα₁-adrenergic agonist phenylephrine injected into the LH reduced the dipsogenic response to i.c.v. AII, but not to carbachol. Injection of yohimbine (anα₂-adrenergic antagonist) and prazosin (anα₁-adrenergic antagonist) into the LH also reduced the water intake produced by i.c.v. injection of AII. Previous injection ofα₁-orα₂-adrenergic antagonists into the LH increased the antidipsogenic effect of clonidine or phenylephrine injected into the same area on the water intake induced by i.c.v. AII. These results show that theα₁-andα₂-adrenergic receptors of the LH are involved in the control of drinking responses elicited by i.c.v. injection of AII in rats. They also show that clonidine, but not phenylephrine, suppresses the drinking induced by i.c.v. carbachol. The data suggest that the discharge of central α-adrenergic receptors has a dual (inhibitory and excitatory) effect on water intake induced by central AII.     

Opioid antagonists in the periaqueductal gray inhibit morphine and β-endorphin analgesia elicited from the amygdala of rats

In addition to brainstem sites of action, analgesia can be elicited following amygdala microinjections of morphine and μ-selective opioid agonists. The present study examined whether opioid analgesia elicited by either morphine or β-endorphin in the amygdala could be altered by either the general opioid antagonist, naltrexone, the μ-selective antagonist, β-funaltrexamine (BFNA) or theδ₂ antagonist, naltrindole isothiocyanate (Ntii) in the periaqueductal gray (PAG). Both morphine (2.5–5 μg) and β-endorphin (2.5–5 jig) microinjected into either the baso-lateral or central nuclei of the amygdala significantly increased tail-flick latencies and jump thresholds in rats. The increases were far more pronounced on the jump test than on the tail-flick test. Placements dorsal and medial to the amygdala were ineffective. Naltrexone (1–5 μg) in the PAG significantly reduced both morphine (tail-flick: 70–75%; jump: 60–81%) and β-endorphin (tail-flick: 100%; jump: 93%) analgesia elicited from the amygdala, indicating that an opioid synapse in the PAG was integral for the full expression of analgesia elicited from the amygdala by both agonists. Both BFNA (68%) and Ntii (100%) in the PAG significantly reduced morphine, but not β-endorphin analgesia in the amygdala on the tail-flick test. Ntii in the PAG was more effective in reducing morphine (60%) and β-endorphin (79%) analgesia in the amygdala on the jump test than BFNA (15–24%). Opioid agonist-induced analgesia in the amygdala was unaffected by opioid antagonists administered into control misplacements in the lateral mesencephalon, and the small hyperalgesia elicited by opioid antagonists in the PAG could not account for the reductions in opioid agonist effects in the amygdala. These data indicate that PAGδ₂ and to a lesser degree, μ opioid receptors are necessary for the full expression of morphine and β-endorphin analgesia elicited from the amygdala.     

Mercaptoacetate induces feeding through central opioid-mediated mechanisms in rats

The endogenous opioid system has been implicated in the mediation of food intake elicited by such regulatory challenges as glucoprivation induced by 2-deoxy-D-glucose (2DG) or food deprivation in rodents. Administration of the free fatty acid oxidation inhibitor, mercaptoacetate (MA), produces a potent short-term increase in feeding in rats, the mechanisms of which have been dissociated from that elicited by 2DG. The present study evaluated whether MA-induced feeding in rats was mediated by the endogenous opioid system through systemic administration of the general opioid antagonist, naltrexone, through central administration of either general, mu, mu(1), kappa(1) or delta opioid antagonists, and through central administration of antisense oligodeoxynucleotide (AS ODN) probes directed against specific exons of either the mu (MOR-1), kappa (KOR-1), kappa(3) (KOR-3/ORL-1) or delta (DOR-1) opioid receptor clones. MA-induced feeding was significantly and dose-dependently reduced by systemic naltrexone (0.005-5 mg/kg); these ingestive effects were quite selective since neither total, ambulatory nor stereotypic activity was affected by either MA itself or MA paired with naltrexone. MA-induced feeding was significantly reduced by central pretreatment with either naltrexone (0.1-20 microgram) or mu-selective (beta-funaltrexamine, 0.1-20 microgram), mu(1)-selective (naloxonazine, 1-20 microgram), kappa(1)-selective (nor-binaltorphamine, 0.1-20 microgram), or delta-selective (naltrindole, 1-20 microgram) opioid receptor antagonists. MA-induced feeding was significantly reduced by AS ODN probes directed against either exons 1, 2 or 3, but not exon 4 of the MOR-1 clone, exon 3, but not exons 1 or 2 of the KOR-1 clone, exons 1 or 2, but not exon 3 of the KOR-3/ORL-1 clone, and exon 1, but not exons 2 or 3 of the DOR-1 clone. These data are discussed in terms of opioid mediation of ingestive responses related to fat, and in terms of potential central sites of action at which lipoprivic ingestive responses might act.