A bi-directional mu-opioid-opioid connection between the nucleus of the accumbens shell and the central nucleus of the amygdala in the rat

The central nucleus of the amygdala (CeA) and the nucleus of the accumbens shell (NAc) have been shown to be involved in opioid-mediated feeding behavior. The present study examined whether mu-opioid signalling between the CeA and NAc affected feeding. Male Sprague-Dawley rats were fitted with one cannula placed in the CeA and two cannulae placed in the NAc, which allowed for coadministration of the mu-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO) in one site and the opioid antagonist naltrexone (NTX) in the other site. Single injection of DAMGO (2.4 nmol) into the CeA and bilateral injections of DAMGO (2.4 nmol) into the NAc stimulated feeding (P<0.05). The DAMGO-induced increase of food intake following injection into the CeA was decreased by bilateral injection of NTX (13.2 and 26.5 nmol) into the NAc at 2- and 4-h postinjections (P<0.05). In the reverse situation, the DAMGO-induced increase of food intake following injection into the NAc was decreased by injection of NTX (13.2 and 26.5 nmol) into the CeA at 1-, 2-, and 4-h postinjections (P<0.05). These results suggest that a bi-directional mu-opioid-opioid signalling pathway exists between the CeA and the NAc, which influences feeding.     

Differential effects of mu and kappa opioid antagonists on Fos-like immunoreactivity in extended amygdala

It was previously reported that systemic administration of the nonselective opioid antagonist, naltrexone, induces Fos-like immunoreactivity (FLI) within the central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (lateral-dorsal division; BSTLD), nucleus accumbens shell (NACshell) and ventral tegmental area (VTA) of free-feeding rats. These findings suggest that cellular activity in these brain regions is subject to opioid-mediated inhibitory control under basal conditions. Considering the involvement of mesoaccumbens dopamine neurons and components of the 'extended amygdala' in motivated behavior and reward, it was hypothesized that the induction of c-Fos by naltrexone accounts for the motivational-affective consequences of opioid antagonism. In Experiment 1, naltrexone was administered intracerebroventricularly (i.c.v.; 100 microg) to determine whether results obtained in the prior immunohistochemical studies could be attributed to blockade of opioid receptors in brain as opposed to peripheral tissues that convey visceral sensory inputs to the CeA and BSTLD. Naltrexone produced a marked increase in FLI within the CeA and BSTLD, and a moderate increase in NACshell. In Experiment 2, the kappa opioid antagonist, nor-binaltorphimine (Nor-BNI; 20.0 microg, i.c.v.) reproduced the effect of naltrexone in BSTLD and CeA, suggesting that the induction of c-Fos in these two structures is a consequence of kappa receptor blockade. The selective mu antagonist, CTAP (2.0 microg, i.c.v.), reproduced the effect of naltrexone in NACshell, suggesting that the induction of c-Fos in this structure is a consequence of mu receptor blockade. The functional implications of these results are discussed in terms of the known functions of these brain regions and opioid receptor types, and the prior observation that chronic food restriction eliminates the FLI induced by naltrexone in CeA and BSTLD. It is suggested that tonic mu opioid-mediated inhibition in NACshell has a predisposing effect on goal-approach behavior in general while kappa opioid-mediated inhibition in CeA and BSTLD has a predisposing effect on palatability-driven feeding in particular. Finally, a possible relationship between food restriction-induced suppression of the kappa opioid mechanism in CeA/BSTLD, local CRH function, and sensitization of the neural substrate for incentive-motivating effects of abused drugs is discussed.     

Identification of central sites involved in butorphanol-induced feeding in rats

Butorphanol (BT), a mixed kappa- and mu-opioid receptor agonist, induces vigorous food intake in rats. Peripheral injection of BT seems to increase food intake more effectively than intracerebroventricular administration. To further elucidate the nature of BT's influence on consummatory behavior, we examined which feeding-related brain areas exhibit increased c-Fos immunoreactivity (IR) following subcutaneous injection of 4 mg/kg body weight BT, a dose known to induce a maximal orexigenic response. We also evaluated whether direct administration of BT into the forebrain regions activated by peripheral BT injection affects food intake. Peripheral BT administration induced c-Fos-IR in the hypothalamic paraventricular nucleus (PVN), central nucleus of the amygdala (CeA), and nucleus of the solitary tract (NTS). However, 0.1-30 microg BT infused into the CeA, failed to increase food intake 1, 2, and 4 h after injection. Only the highest dose of BT (30 microg) injected into the PVN increased feeding. These results suggest that the PVN, CeA, and NTS mediate the effects of peripherally-injected BT. The PVN or CeA are probably not the main target sites of immediate BT action.     

gamma-Aminobutyric acid receptor subtype antagonists differentially alter opioid-induced feeding in the shell region of the nucleus accumbens in rats.

Food intake is significantly increased by administration of mu-selective opioid agonists into the nucleus accumbens, particularly its shell region. Pretreatment with either opioid (mu, delta(1), delta(2) or kappa(1)) or dopaminergic (D(1)) receptor antagonists in the nucleus accumbens shell reduce mu opioid agonist-induced feeding. Selective GABA(A) (muscimol) and GABA(B) (baclofen) agonists administered into the nucleus accumbens shell each stimulate feeding which is respectively and selectively blocked by GABA(A) (bicuculline) and GABA(B) (saclofen) antagonists. The present study investigated whether feeding elicited by the mu-selective opioid agonist, [D-Ala(2),NMe(4),Gly-ol(5)]-enkephalin in the nucleus accumbens shell was decreased by intra-accumbens pretreatment with an equimolar dose range of either GABA(A) or GABA(B) antagonists, and further, whether general opioid or selective GABA antagonists decreased feeding elicited by GABA(A) or GABA(B) agonists in the nucleus accumbens shell. Feeding elicited by the mu-selective opioid agonist was dose-dependently increased following intra-accumbens pretreatment with GABA(A) (bicuculline) antagonism; this enhancement was significantly blocked by pretreatment with general or mu-selective opioid antagonists. In contrast, mu opioid agonist-induced feeding elicited from the nucleus accumbens shell was dose-dependently decreased by GABA(B) (saclofen) antagonism. Neither bicuculline nor saclofen in the nucleus accumbens shell altered baseline food intake. Whereas muscimol-induced feeding elicited from the nucleus accumbens shell was reduced by bicuculline and naltrexone, but not saclofen pretreatment, baclofen-induced feeding elicited from the nucleus accumbens shell was reduced by saclofen, but not by bicuculline or naltrexone. These data indicate that GABA(A) and GABA(B) receptor subtype antagonists differentially affect feeding elicited by mu opioid receptor agonists within the nucleus accumbens shell in rats.     

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.     

Differential effects of μ and κ opioid antagonists on Fos-like immunoreactivity in extended amygdala

It was previously reported that systemic administration of the nonselective opioid antagonist, naltrexone, induces Fos-like immunoreactivity (FLI) within the central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (lateral–dorsal division; BSTLD), nucleus accumbens shell (NACshell) and ventral tegmental area (VTA) of free-feeding rats. These findings suggest that cellular activity in these brain regions is subject to opioid-mediated inhibitory control under basal conditions. Considering the involvement of mesoaccumbens dopamine neurons and components of the ‘extended amygdala' in motivated behavior and reward, it was hypothesized that the induction of c-Fos by naltrexone accounts for the motivational-affective consequences of opioid antagonism. In Experiment 1, naltrexone was administered intracerebroventricularly (i.c.v.; 100 μg) to determine whether results obtained in the prior immunohistochemical studies could be attributed to blockade of opioid receptors in brain as opposed to peripheral tissues that convey visceral sensory inputs to the CeA and BSTLD. Naltrexone produced a marked increase in FLI within the CeA and BSTLD, and a moderate increase in NACshell. In Experiment 2, the κ opioid antagonist, nor-binaltorphimine (Nor-BNI; 20.0 μg, i.c.v.) reproduced the effect of naltrexone in BSTLD and CeA, suggesting that the induction of c-Fos in these two structures is a consequence of κ receptor blockade. The selective μ antagonist, CTAP (2.0 μg, i.c.v.), reproduced the effect of naltrexone in NACshell, suggesting that the induction of c-Fos in this structure is a consequence of μ receptor blockade. The functional implications of these results are discussed in terms of the known functions of these brain regions and opioid receptor types, and the prior observation that chronic food restriction eliminates the FLI induced by naltrexone in CeA and BSTLD. It is suggested that tonic μ opioid-mediated inhibition in NACshell has a predisposing effect on goal–approach behavior in general while κ opioid-mediated inhibition in CeA and BSTLD has a predisposing effect on palatability-driven feeding in particular. Finally, a possible relationship between food restriction-induced suppression of the κ opioid mechanism in CeA/BSTLD, local CRH function, and sensitization of the neural substrate for incentive-motivating effects of abused drugs is discussed.     

Excitatory amino acid receptor subtype agonists induce feeding in the nucleus accumbens shell in rats: opioid antagonist actions and interactions with mu-opioid agonists.

Administration of mu-opioid receptor subtype agonists into the nucleus accumbens shell elicits feeding which is dependent upon the normal function of mu-, delta- and kappa-opioid receptors, D(1) dopamine receptors and GABA(B) receptors in the nucleus accumbens shell for its full expression. Whereas the AMPA antagonist, DNQX administered into the nucleus accumbens shell elicits a transient, though intense feeding response, feeding is elicited by excitatory amino acid agonists administered into the lateral hypothalamus. The present study examined whether excitatory amino acid agonists elicited feeding following administration into the nucleus accumbens shell of rats, whether such feeding responses were altered by opioid antagonist pretreatment, and whether such feeding responses interacted with feeding elicited by mu-opioid agonists. Both AMPA (0.25-0.5 microg) and NMDA (1 microg) in the nucleus accumbens shell significantly and dose-dependently increased food intake over 4 h. Both feeding responses were blocked by naltrexone pretreatment in the nucleus accumbens shell. The mu-opioid agonist, [D-Ala(2),NMe-Phe(4),Gly-ol(5)]-enkephalin in the nucleus accumbens shell significantly increased food intake which was significantly enhanced by AMPA cotreatment. This enhanced feeding response was in turn blocked by pretreatment with either general or mu-selective opioid antagonists. In contrast, cotreatment of NMDA and the mu-opioid agonist in the nucleus accumbens shell elicited feeding which was significantly less than that elicited by either treatment alone. These data indicate the presence of important interactions between excitatory amino acid receptors and mu-opioid receptors in the nucleus accumbens shell in mediating feeding responses in nondeprived, ad libitum-fed rats.     

Effect of opioid receptor ligands injected into the rostral lateral hypothalamus on c-fos and feeding behavior

The lateral hypothalamic area (LHa) is an important brain site for the regulation of food intake. Central injection of opioids increases food intake, and the LHa contains mu and kappa opioid receptors, both of which are involved in feeding behavior. It is unclear whether opioids impact feeding when injected directly into the rostral portion of the LHa (rLHa) in rats. We performed a series of studies in which free-feeding rLHa-cannulated rats were injected with opioid agonists (DAMGO, morphine, dynorphin, U-50488H) followed by the measurement of food intake at 1, 2, and 4 h postinjection. To determine whether opioid receptor ligands administered into the rLHa affect neuronal activation in this brain site, we studied cFos immunoreactivity (cFos IR) in response to rLHa stimulation with naltrexone. We found that the only compound that stimulated feeding behavior was morphine. The other agonists had no effect on food consumption. Naltrexone injection into the rLHa increased neural activation in the LHa, indicating the presence of functional opioid receptors in this region. These data suggest that although neuronal activity is affected by opioid agents acting in the rLHa, administration of selective mu and kappa opioid ligands in this subdivision of the LHa does not have a reliable effect on feeding behavior.     

Functional and anatomical localization of mu opioid receptors in the striatum, amygdala, and extended amygdala of the nonhuman primate

The subregional distribution of mu opioid receptors and corresponding G-protein activation were examined in the striatum, amygdala, and extended amygdala of cynomolgus monkeys. The topography of mu binding sites was defined using autoradiography with [(3)H]DAMGO, a selective mu ligand. In adjacent sections, the distribution of receptor-activated G proteins was identified with DAMGO-stimulated guanylyl 5'(gamma-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding. Within the striatum, the distribution of [(3)H]DAMGO binding sites was characterized by a distinct dorsal-ventral gradient with a higher concentration of binding sites at more rostral levels of the striatum. [(3)H]DAMGO binding was further distinguished by the presence of patch-like aggregations within the caudate, as well as smaller areas of very dense receptor binding sites, previously identified in human striatum as neurochemically unique domains of the accumbens and putamen (NUDAPs). The amygdala contained the highest concentration of [(3)H]DAMGO binding sites measured in this study, with the densest levels of binding noted within the basal, accessory basal, paralaminar, and medial nuclei. In the striatum and amygdala, the distribution of DAMGO-stimulated G-protein activation largely corresponded with the distribution of mu binding sites. The central and medial nuclei of the amygdala, however, were notable exceptions. Whereas the concentration of [(3)H]DAMGO binding sites in the central nucleus of the amygdala was very low, the concentration of DAMGO-stimulated G-protein activation in this nucleus, as measured with [(35)S]GTPgammaS binding, was relatively high compared to other portions of the amygdala containing much higher concentrations of [(3)H]DAMGO binding sites. The converse was true in the medial nucleus, where high concentrations of binding sites were associated with lower levels of DAMGO-stimulated G-protein activation. Finally, [(3)H]DAMGO and [(35)S]GTPgammaS binding within the amygdala, particularly the medial nucleus, formed a continuum with the substantia innominata and bed nucleus of the stria terminalis, supporting the concept of the extended amygdala in primates.     

Mu opioid receptor signaling in the nucleus accumbens shell increases responsiveness of satiety‐modulated lateral hypothalamus neurons

Opioid signaling in the nucleus accumbens shell (sNAcc) has been implicated in hedonic feeding and binge eating behavior. The sNAcc projects to the lateral hypothalamus (LH), and this pathway has been suggested to modulate palatability‐driven feeding behavior. In this study, we investigated the effects of sNAcc mu opioid receptor (MOR) stimulation on firing rates of LH neurons in previously sated rats. Neural firing in the LH was recorded while food‐deprived rats performed an operant task to obtain sweetened Intralipid (a 4% fat emulsion containing 5% sucrose) before and after bilateral sNAcc infusion of either a MOR agonist [D‐Ala2, N‐MePhe4, Gly‐ol]‐enkephalin (DAMGO) or a saline control solution. During sessions in which saline was infused into the sNAcc, the number of trials completed after infusion were significantly lower than the number completed before infusion, likely reflecting animals’ increased satiety state. During sessions in which DAMGO was infused into the sNAcc, the decrease in the number of trials completed (comparing post‐ vs. pre‐infusion trials) was significantly attenuated. Electrophysiological recording showed that the percentage of LH neurons showing an excitatory response due to behavioral events (cue presentation, lever press, lever retraction, and consumption) was reduced in post vs. pre‐saline infusion period. However, the percentage of LH neurons showing excitatory responses to the same behavioral events was similar in pre‐ and post‐DAMGO infusion periods. These findings suggest that MOR stimulation in sNAcc leads to an increase in stimulus‐evoked excitatory signaling in LH neurons which could contribute to preventing satiety‐induced decline in palatable food intake.     

Opioid receptor subtype antagonists differentially alter GABA agonist-induced feeding elicited from either the nucleus accumbens shell or ventral tegmental area regions in rats

Food intake is significantly increased by administration of either GABAA (e.g., muscimol) or GABAB (e.g., baclofen) agonists into either the shell region of the nucleus accumbens (NAC) or the ventral tegmental area (VTA); these responses are selectively blocked by pretreatment with corresponding GABAA and GABAB antagonists. Previous studies found that a single dose (5 microg) of the general opioid antagonist, naltrexone reduced feeding elicited by muscimol, but not baclofen in the NAC shell, and reduced feeding elicited by baclofen, but not muscimol in the VTA. The present study compared feeding responses elicited by either muscimol or baclofen in either the VTA and NAC shell following pretreatment with equimolar doses of selective mu (0.4, 4 microg), delta (0.4, 4 microg), or kappa (0.6, 6 microg) opioid receptor subtype antagonists. Muscimol (25 ng) and baclofen (200 microg) each significantly and equi-effectively increased food intake over 4 h following VTA or NAC shell microinjections. Muscimol-induced feeding elicited from the VTA was significantly enhanced by mu or delta antagonists, and was significantly reduced by kappa antagonists. Baclofen-induced feeding elicited from the VTA was significantly reduced by mu or kappa, but not delta antagonists. Muscimol-induced feeding elicited from the NAC was significantly reduced by either mu, kappa or delta antagonists. Baclofen-induced feeding elicited from the NAC was significantly reduced by kappa or delta, but not mu antagonists. These data indicate differential opioid receptor subtype antagonist-induced mediation of GABA receptor subtype agonist-induced feeding elicited from the VTA and NAC shell. This is consistent with previously demonstrated differential GABA receptor subtype antagonist-induced mediation of opioid-induced feeding elicited from these same sites. Thus, functional relationships exist for the elaborate anatomical and physiological interactions between these two neurochemical systems in the VTA and NAC shell.     

Characterization of neurons in the rat central nucleus of the amygdala: cellular physiology, morphology, and opioid sensitivity

The central nucleus of the amygdala (CeA) orchestrates autonomic and other behavioral and physiological responses to conditioned stimuli that are aversive or elicit fear. As a related CeA function is the expression of hypoalgesia induced by conditioned stimuli or systemic morphine administration, we examined postsynaptic opioid modulation of neurons in each major CeA subdivision. Following electrophysiological recording, biocytin-filled neurons were precisely located in CeA regions identified by chemoarchitecture (enkephalin-immunoreactivity) and cytoarchitecture (DAPI nuclear staining) in fixed adult rat brain slices. This revealed a striking distribution of physiological types, as 92% of neurons in capsular CeA were classified as late-firing, whereas no neurons in the medial CeA were of this class. In contrast, 60% or more of neurons in the lateral and medial CeA were low-threshold bursting neurons. Mu-opioid receptor (MOPR) agonists induced postsynaptic inhibitory potassium currents in 61% of CeA cells, and this ratio was maintained in each subdivision and for each physiological class of neuron. However, MOPR agonists more frequently inhibited bipolar/fusiform cells than triangular or multipolar neurons. A subpopulation of MOPR-expressing neurons were also inhibited by delta opioid receptor agonists, whereas a separate population were inhibited kappa opioid receptors (KOPR). The MOPR agonist DAMGO inhibited 9/9 CeM neurons with projections to the parabrachial nucleus identified by retrograde tracer injection. These data support models of striatopallidal organization that have identified striatal-like and pallidal-like CeA regions. Opioids can directly inhibit output from each subdivision by activating postsynaptic MOPRs or KOPRs on distinct subpopulations of opioid-sensitive neurons.     

γ-Aminobutyric acid receptor subtype antagonists differentially alter opioid-induced feeding in the shell region of the nucleus accumbens in rats

Food intake is significantly increased by administration of μ-selective opioid agonists into the nucleus accumbens, particularly its shell region. Pretreatment with either opioid (μ, δ₁, δ₂ or κ₁) or dopaminergic (D₁) receptor antagonists in the nucleus accumbens shell reduce μ opioid agonist-induced feeding. Selective GABA_A (muscimol) and GABA_B (baclofen) agonists administered into the nucleus accumbens shell each stimulate feeding which is respectively and selectively blocked by GABA_A (bicuculline) and GABA_B (saclofen) antagonists. The present study investigated whether feeding elicited by the μ-selective opioid agonist, [ -Ala²,NMe⁴,Gly-ol⁵]-enkephalin in the nucleus accumbens shell was decreased by intra-accumbens pretreatment with an equimolar dose range of either GABA_A or GABA_B antagonists, and further, whether general opioid or selective GABA antagonists decreased feeding elicited by GABA_A or GABA_B agonists in the nucleus accumbens shell. Feeding elicited by the μ-selective opioid agonist was dose-dependently increased following intra-accumbens pretreatment with GABA_A (bicuculline) antagonism; this enhancement was significantly blocked by pretreatment with general or μ-selective opioid antagonists. In contrast, μ opioid agonist-induced feeding elicited from the nucleus accumbens shell was dose-dependently decreased by GABA_B (saclofen) antagonism. Neither bicuculline nor saclofen in the nucleus accumbens shell altered baseline food intake. Whereas muscimol-induced feeding elicited from the nucleus accumbens shell was reduced by bicuculline and naltrexone, but not saclofen pretreatment, baclofen-induced feeding elicited from the nucleus accumbens shell was reduced by saclofen, but not by bicuculline or naltrexone. These data indicate that GABA_A and GABA_B receptor subtype antagonists differentially affect feeding elicited by μ opioid receptor agonists within the nucleus accumbens shell in rats.     

Antinociception following application of DAMGO to the basolateral amygdala results from a direct interaction of DAMGO with Mu opioid receptors in the amygdala

Previous studies from our laboratory have shown that application of the mu opioid agonist DAMGO into the basolateral region of the amygdala (BLA) suppresses the radiant heat tail flick (TF) reflex in anesthetized rats. This antinociceptive effect can be blocked by lesions of brainstem regions such as the periaqueductal gray (PAG) or the rostral ventromedial medulla (RVM) or by functional inactivation of neurons in these regions, suggesting the activation of brainstem-descending antinociceptive systems from the amygdala. However, little is known about the direct interaction of DAMGO with mu receptors in the amygdala. In the present series of experiments, the BLA was pretreated with opioid receptor antagonists and a G protein inhibitor prior to TF testing with application of DAMGO into the same site. Rats pretreated with the non-selective opioid antagonist naltrexone (1.25-3.75 microg/0.25 microl per side) or the G protein inhibitor pertussis toxin (0.25 microg) failed to show inhibition of TF reflexes following infusion of DAMGO (0.168-0.50 microg), indicating that DAMGO works through G-protein-coupled opioid receptors in the BLA. Furthermore, pretreatment with the mu antagonist beta-FNA (1.00-2.00 microg) attenuated antinociception induced by DAMGO injection, suggesting DAMGO's action on mu receptors in the BLA. Accordingly, we confirm a direct interaction of DAMGO with G-protein-coupled mu receptors in the BLA contributing to induction of opioid antinociception in the amygdala.     

Excitatory amino acid receptor subtype agonists induce feeding in the nucleus accumbens shell in rats: opioid antagonist actions and interactions with μ-opioid agonists

Administration of μ-opioid receptor subtype agonists into the nucleus accumbens shell elicits feeding which is dependent upon the normal function of μ-, δ- and κ-opioid receptors, D₁ dopamine receptors and GABA_B receptors in the nucleus accumbens shell for its full expression. Whereas the AMPA antagonist, DNQX administered into the nucleus accumbens shell elicits a transient, though intense feeding response, feeding is elicited by excitatory amino acid agonists administered into the lateral hypothalamus. The present study examined whether excitatory amino acid agonists elicited feeding following administration into the nucleus accumbens shell of rats, whether such feeding responses were altered by opioid antagonist pretreatment, and whether such feeding responses interacted with feeding elicited by μ-opioid agonists. Both AMPA (0.25–0.5 μg) and NMDA (1 μg) in the nucleus accumbens shell significantly and dose-dependently increased food intake over 4 h. Both feeding responses were blocked by naltrexone pretreatment in the nucleus accumbens shell. The μ-opioid agonist, [D-Ala²,NMe-Phe⁴,Gly-ol⁵]-enkephalin in the nucleus accumbens shell significantly increased food intake which was significantly enhanced by AMPA cotreatment. This enhanced feeding response was in turn blocked by pretreatment with either general or μ-selective opioid antagonists. In contrast, cotreatment of NMDA and the μ-opioid agonist in the nucleus accumbens shell elicited feeding which was significantly less than that elicited by either treatment alone. These data indicate the presence of important interactions between excitatory amino acid receptors and μ-opioid receptors in the nucleus accumbens shell in mediating feeding responses in nondeprived, ad libitum-fed rats.     

The amygdala is critical for opioid-mediated binge eating of fat

Endogenous opioid peptides within the nucleus accumbens are thought to mediate the hedonic aspects of food intake, particularly foods such as fat and sugar. In view of evidence that the amygdala also regulates positive affect, we hypothesized this brain region participates in the control of opioid-mediated food intake. The robust increase in the intake of fat following intra-accumbens administration of the mu-opioid agonist D-Ala2,Nme-Phe4,Glyol5-enkephalin (DAMGO) was completely blocked by concurrent temporary inactivation (muscimol, GABA(A) agonist) of either the basolateral or central nucleus of the amygdala. In summary, we demonstrate that amygdala inactivation prevents the intra-accumbens opioid induced "binge" eating of fat, possibly through reducing the hedonic properties of high-fat palatable food.     

Muscarinic receptor antagonism causes a functional alteration in nucleus accumbens mu-opiate-mediated feeding behavior

Intra-nucleus accumbens (Acb) infusion of cholinergic muscarinic antagonist, scopolamine (10 microg/0.5 microl), markedly reduced fat intake elicited by intra-Acb treatment of the mu-opioid receptor agonist, DAMGO, with 30 min and 4h pretreatment intervals. Intra-Acb scopolamine infusions also reduced food intake in food-deprived rats, but not water intake in water-deprived rats. Hence, Acb muscarinic manipulations exhibit some specificity for feeding, perhaps via interactions with the striatal opioid system.     

Evidence for a mu-opioid-opioid connection between the paraventricular nucleus and ventral tegmental area in the rat

The paraventricular nucleus (PVN) and the ventral tegmental area (VTA) have been shown to be involved in opioid mediated feeding behavior. The present study examined whether mu-opioid signalling between the PVN and VTA affected feeding behavior. Male Sprague-Dawley rats were cannulated with one cannula placed in the PVN and two cannulae placed in the VTA, which allowed for co-administration of the mu-opioid receptor agonist [D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO) in one site and the opioid antagonist naltrexone (NTX) in the other site. Bilateral administration of DAMGO (1.2, 2.4 and 4.9 nmol) into the VTA stimulated feeding dose dependently at 2.4 and 4.9 nmol (P<0.05). The DAMGO (2.4 nmol)-induced increase of food intake following injection into the PVN was blocked by bilateral injection of NTX (6.6, 13.2 and 26.5 nmol) into the VTA at 2 and 4 h (P<0.05). In the reverse situation, the DAMGO (2.4 nmol)-induced increase of food intake following injection into the VTA was blocked by injection of NTX (13.2 and 26.5 nmol) into the PVN at 2 and 4 h (P<0.05). The present study suggests that a bidirectional mu-opioid-opioid signalling pathway exists between the PVN and the VTA which influences feeding.     

Induction of fos-like immunoreactivity by opioids in guinea-pig brain

In the present study the effects of intracerebroventricular (i.c.v.) administration of 100 nmol of morphine, the selective μ-receptor agonist DAMGO, the δ-receptor agonist DPDPE and the κ-receptor agonist U50,488H, on the induction of Fos-like immunoreactivity (Fos-LI) in the guinea-pig brain were investigated using immunohistochemical techniques. Guinea-pigs given i.c.v. injection of opioids showed marked increases in the number of Fos-LI nuclei within a large number of brain regions, several of which, including hypothalamic nuclei, paraventricular thalamic nucleus, the amygdala, periaqueductal gray, superior and inferior colliculi, the piriform and entorhinal cortices, have been shown to be activated under stressful or aversive conditions. Pretreatment with the opioid antagonist, naltrexone, before administration of morphine or U50,488H, inhibited Fos-LI induction indicating that the effects of the opioids were mediated by opioid receptors. U50,488H administration resulted in higher numbers of Fos-LI stained neurons compared to morphine in most regions other than the nucleus accumbens and interpeduncular nucleus. Morphine and DAMGO produced significantly higher numbers of Fos-LI neurons in the nucleus accumbens shell region than U50,488H, which may reflect the more powerful reinforcing/rewarding effects of μ-receptor agonists. Thus the present study supports a critical role for the nucleus accumbens shell region in the reinforcing/rewarding effects of opioids.     

Pro-nociceptin/orphanin FQ and NOP receptor mRNA levels in the forebrain of food deprived rats

Forebrain injections of nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the NOP opioid receptor, previously referred to as ORL1 or OP4 receptor, stimulate feeding in freely feeding rats, while the NOP receptor antagonist [Nphe(1)]N/OFQ(1-13)NH(2) inhibits food deprivation-induced feeding. To further evaluate whether the N/OFQ-NOP receptor system plays a physiological role in feeding control, the present study evaluated forebrain mRNA levels for the N/OFQ precursor (pro-N/OFQ), as well as for the NOP receptor in food deprived rats. The results obtained show that food deprived rats have lower mRNA levels for the NOP receptor in several forebrain regions; a significant reduction was found in the paraventricular and lateral hypothalamic nuclei and in the central nucleus of the amygdala. Food deprived rats also exhibited lower pro-N/OFQ mRNA levels in the central amygdala. These results suggest that the N/OFQ-NOP receptor system may have a physiological role in feeding control. The observation that food deprivation reduces gene expression of the N/OFQ-NOP receptor system is apparently not consistent with a direct hyperphagic action for N/OFQ. Taking into account that N/OFQ exerts inhibitory actions at cellular level, the present results may be in keeping with the hypothesis that N/OFQ stimulates feeding by inhibiting neurons inhibitory for food intake; under conditions of food deprivation, these neurons may be silent and the N/OFQ-NOP receptor system, which controls them, may also be regulated at a lower level. Consistently, in the present study N/OFQ stimulated food intake in freely feeding rats, but did not further increase feeding in food deprived rats.