Opioid receptor blockade in rat nucleus tractus solitarius alters amygdala dynorphin gene expression

Preclinical and Clinical Evidence for a Distinct Regulation of Mu Opioid and Type 1 Cannabinoid Receptor Genes Expression in Obesity

Among endogenous signaling networks involved in both rewarding and homeostatic mechanisms of obesity, a relevant role is played by the endocannabinoid (ECS) and the opioid (EOS) systems. We here studied the transcriptional regulation of ECS and EOS genes in the hypothalamus of Diet-induced obesity rats, a preclinical model of obesity, as well as in humans with obesity and healthy controls. A significant and selective increase in type 1 cannabinoid receptor gene (Cnr1) expression was observed at the beginning of obesity development (5 weeks on high fat diet) as well as after 21 weeks of high diet exposure. After 5 weeks on high fat diet, selective up-regulation of mu opioid receptor gene (Oprm1) expression was also observed. Consistently, epigenetic studies showed a selective and significant decrease in DNA methylation at specific CpG sites at both gene promoters in overweight rats, but only after 5 weeks on high fat diet. Moreover, significantly lower levels of DNA methylation were observed at selected CpG sites of both receptor gene promoters, analyzed in peripheral blood mononuclear cells from younger (<30 years old) humans with obesity, as well as in those with shorter time length from disease onset. Taken together, we here provide evidence of selective, synergistic and time-dependent transcriptional regulation of CNR1 and OPRM1 genes in overweight rats, as well as in human subjects. These alterations in genes regulation could contribute to the development of the obese phenotype, and we thus suggest CNR1 and OPRM1 epigenetic modulation as possible biomarkers of obesity development. Due to the reversible nature of the epigenetic hallmark, our data might also open new avenue to early environmental strategies of intervention.

Low-frequency electroacupuncture suppresses focal epilepsy and improves epilepsy-induced sleep disruptions

Background

The positive effects of acupuncture at Feng-Chi acupoints on treating epilepsy and insomnia have been well-documented in ancient Chinese literature. However, there is a lack of scientific evidence to elucidate the underlying mechanisms behind these effects. Our previous study demonstrated that high-frequency (100 Hz) electroacupuncture (EA) at Feng-Chi acupoints deteriorates both pilocarpine-induced focal epilepsy and sleep disruptions. This study investigated the effects of low-frequency (10 Hz) EA on epileptic activities and epilepsy-induced sleep disruptions.

Results

In rats, the Feng-Chi acupoint is located 3 mm away from the center of a line between the two ears. Rats received 30 min of 10 Hz EA stimuli per day before each day’s dark period for three consecutive days. Our results indicated that administration of pilocarpine into the left CeA at the beginning of the dark period induced focal epilepsy and decreased both rapid eye movement (REM) sleep and non-REM (NREM) sleep during the consequent light period. Low-frequency (10 Hz) EA at Feng-Chi acupoints suppressed pilocarpine-induced epileptiform EEGs, and this effect was in turn blocked by naloxone (a broad-spectrum opioid receptor antagonist), but not by naloxonazine (a μ-receptor antagonist), naltrindole (a δ-receptor antagonist) and nor-binaltorphimine (a κ-receptor antagonist). Ten Hz EA enhanced NREM sleep during the dark period, and this enhancement was blocked by all of the opioid receptor antagonists. On the other hand, 10 Hz EA reversed pilocarpine-induced NREM suppression during the light period, and the EA’s effect on the sleep disruption was only blocked by naloxonazine.

Conclusions

These results indicate that low-frequency EA stimulation of Feng-Chi acupoints is beneficial in improving epilepsy and epilepsy-induced sleep disruptions, and that opioid receptors in the CeA mediate EA’s therapeutic effects.

Amygdala opioid receptors mediate the electroacupuncture-induced deterioration of sleep disruptions in epilepsy rats

BACKGROUND

Clinical and experimental evidence demonstrates that sleep and epilepsy reciprocally affect each other. Previous studies indicated that epilepsy alters sleep homeostasis; in contrast, sleep disturbance deteriorates epilepsy. If a therapy possesses both epilepsy suppression and sleep improvement, it would be the priority choice for seizure control. Effects of acupuncture of Feng-Chi (GB20) acupoints on epilepsy suppression and insomnia treatment have been documented in the ancient Chinese literature, Lingshu Jing (Classic of the Miraculous Pivot). Therefore, this study was designed to investigate the effect of electroacupuncture (EA) stimulation of bilateral Feng-Chi acupoints on sleep disruptions in rats with focal epilepsy.

RESULTS

Our result indicates that administration of pilocarpine into the left central nucleus of amygdala (CeA) induced focal epilepsy and decreased both rapid eye movement (REM) sleep and non-REM (NREM) sleep. High-frequency (100 Hz) EA stimulation of bilateral Feng-Chi acupoints, in which a 30-min EA stimulation was performed before the dark period of the light:dark cycle in three consecutive days, further deteriorated pilocarpine-induced sleep disruptions. The EA-induced exacerbation of sleep disruption was blocked by microinjection of naloxone, μ- (naloxonazine), κ- (nor-binaltorphimine) or δ-receptor antagonists (natrindole) into the CeA, suggesting the involvement of amygdaloid opioid receptors.

CONCLUSION

The present study suggests that high-frequency (100 Hz) EA stimulation of bilateral Feng-Chi acupoints exhibits no benefit in improving pilocarpine-induced sleep disruptions; in contrast, EA further deteriorated sleep disturbances. Opioid receptors in the CeA mediated EA-induced exacerbation of sleep disruptions in epileptic rats.

Activation of amygdala opioid receptors by electroacupuncture of Feng-Chi (GB20) acupoints exacerbates focal epilepsy

BACKGROUND

The effect of seizure suppression by acupuncture of Feng-Chi (GB20) acupoints has been documented in the ancient Chinese literature, Lingshu Jing (Classic of the Miraculous Pivot), however, there is a lack of scientific evidence to prove it. This current study was designed to elucidate the effect of electroacupuncture (EA) stimulation of bilateral Feng-Chi (GB20) acupoints on the epileptic activity by employing an animal model of focal epilepsy.

METHODS

Administration of pilocarpine into the left central nucleus of amygdala (CeA) induced the focal epilepsy in rats. Rats received a 30-min 100 Hz EA stimulation of bilateral Feng-Chi acupoints per day, beginning at 30 minutes before the dark period and performing in three consecutive days. The broad-spectrum opioid receptor antagonist (naloxone), μ-receptor antagonist (naloxonazine), δ-receptor antagonist (naltrindole) and κ-receptor antagonist (nor-binaltorphimine) were administered directly into the CeA to elucidate the involvement of CeA opioid receptors in the EA effect.

RESULTS

High-frequency (100 Hz) EA stimulation of bilateral Feng-Chi acupoints did not suppress the pilocarpine-induced epileptiform electroencephalograms (EEGs), whereas it further increased the duration of epileptiform EEGs. We also observed that epilepsy occurred while 100 Hz EA stimulation of Feng-Chi acupoints was delivered into naïve rats. EA-induced augmentation of epileptic activity was blocked by microinjection of naloxone, μ- (naloxonazine), κ- (nor-binaltorphimine) or δ-receptor antagonists (natrindole) into the CeA, suggesting that activation of opioid receptors in the CeA mediates EA-exacerbated epilepsy.

CONCLUSIONS

The present study suggests that high-frequency (100 Hz) EA stimulation of bilateral Feng-Chi acupoints has no effect to protect against pilocarpine-induced focal epilepsy; in contrast, EA further exacerbated focal epilepsy induced by pilocarpine. Opioid receptors in the CeA mediated EA-induced exacerbation of focal epilepsy.

Opioids inhibit visceral afferent activation of catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 catecholamine (CA) neurons within the solitary tract nucleus (NTS) influence many homeostatic functions, including food intake, stress, respiratory and cardiovascular reflexes. They also play a role in both opioid reward and withdrawal. Injections of opioids into the NTS modulate many autonomic functions influenced by catecholamine neurons including food intake and cardiac function. We recently showed that NTS-CA neurons are directly activated by incoming visceral afferent inputs. Here we determined whether opioid agonists modulate afferent activation of NTS-CA neurons using transgenic mice with EGFP expressed under the control of the tyrosine hydroxylase promoter (TH-EGFP) to identify catecholamine neurons. The opioid agonist Met-enkephalin (Met-Enk) significantly attenuated solitary tract-evoked excitatory postsynaptic currents (ST-EPSCs) in NTS TH-EGFP neurons by 80%, an effect reversed by wash or the mu opioid receptor-specific antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP). Met-Enk had a significantly greater effect to inhibit afferent inputs onto TH-EGFP-positive neurons than EGFP-negative neurons, which were only inhibited by 50%. The mu agonist, DAMGO, also inhibited the ST-EPSC in TH-EGFP neurons in a dose-dependent manner. In contrast, neither the delta agonist DPDPE, nor the kappa agonist, U69,593, consistently inhibited the ST-EPSC amplitude. Met-Enk and DAMGO increased the paired pulse ratio, decreased the frequency, but not amplitude, of mini-EPSCs and had no effect on holding current, input resistance or current-voltage relationships in TH-EGFP neurons, suggesting a presynaptic mechanism of action on afferent terminals. Met-Enk significantly reduced both the basal firing rate of NTS TH-EGFP neurons and the ability of afferent stimulation to evoke an action potential. These results suggest that opioids inhibit NTS-CA neurons by reducing an excitatory afferent drive onto these neurons through presynaptic inhibition of glutamate release and elucidate one potential mechanism by which opioids could control autonomic functions and modulate reward and opioid withdrawal symptoms at the level of the NTS.

Learning and Memory Impairment Induced by Salvinorin A, the Principal Ingredient of Salvia divinorum, in Wistar Rats

The effects of salvinorin A ( Salvia divinorum principal ingredient), a potent κ-opioid natural hallucinogen, on learning and memory were investigated. Wistar rats were tested in the 8-arm radial maze, for object recognition and passive avoidance tasks for spatial, episodic, and aversive memory. Attention was assessed using a latent inhibition task. Salvinorin A (80-640 μg/kg subcutaneous [sc]) did not affect short-term memory, but it impaired spatial long-term memory. Episodic and aversive memories were impaired by salvinorin A (160-640 μg/kg). Memory impairment was blocked by the selective κ-opioid receptor antagonist, nor-binaltorphimine ([nor-B]; 0.5-1 mg/kg, intraperitoneal [ip]). Salvinorin A (160 μg/kg) disrupted latent inhibition, after LiCl treatment, such as reduced sucrose intake, suggesting an attention would result in an impairment of cognitive behavior. These findings demonstrate for the first time that salvinorin A has deleterious effects on learning and memory, through a κ-opioid receptor mechanism.

Opioid receptor modulation of hedonic taste preference and food intake: a single-dose safety, pharmacokinetic, and pharmacodynamic investigation with GSK1521498, a novel μ-opioid receptor inverse agonist

Endogenous opioids and µ-opioid receptors have been linked to hedonic and rewarding aspects of palatable food intake. The authors examined the safety, pharmacokinetic, and pharmacodynamic profile of GSK1521498, a µ-opioid receptor inverse agonist that is being investigated primarily for the treatment of overeating behavior in obesity. In healthy participants, GSK1521498 oral solution and capsule formulations were well tolerated up to a dose of 100 mg. After single doses (10-150 mg), the maximum concentration (C(max)) and area under the curve (AUC) in plasma increased in a dose-proportional manner. GSK1521498 selectively reduced sensory hedonic ratings of high-sugar and high-fat dairy products and caloric intake of high-fat/high-sucrose snack foods. These findings provide encouraging data in support of the development of GSK1521498 for the treatment of disorders of maladaptive ingestive behavior or compulsive consumption.

Opioidergic consequences of dietary-induced binge eating

Endogenous opioids are involved in the hedonic aspects of eating. Opioid impairments and alterations have been implicated in the pathophysiology of bulimia nervosa and binge eating disorder. Specific contributions by Bartley G. Hoebel have furthered the understanding how cyclical caloric restriction and intermittent optional access to sugar solutions result in opioid-like forebrain neural alterations and dependency in rodents. The present study sought to investigate caudal brainstem and nodose ganglion mu-opioid receptor mRNA alterations in a rodent model of dietary-induced binge eating of sweetened fat (vegetable shortening blended with 10% sucrose). Five groups (n=7 or 8) of adult female Sprague Dawley rats were exposed to various dietary conditions for 6 weeks. As measured by in situ hybridization, there was reduced (approximately 25% from naive) mu-opioid receptor mRNA in the nucleus of the solitary tract (NTS) in the binge access group, which had intermittent calorie restriction and optional limited access to the sweetened fat. A similar reduction in expression was demonstrated in the continuous access group, which has unlimited optional sweetened fat and an obese phenotype. In the nodose ganglion, mu-opioid receptor mRNA was increased (approximately 30% from groups with sweetened fat access) in rats with intermittent caloric restriction alone. Our findings and the body of work from the Hoebel laboratory suggest that dietary-induced binge eating can consequentially alter opioidergic forebrain and hindbrain feeding-related neural pathways. Future work is needed to determine whether similar alterations are involved in the maintenance and progression of binge eating and other related eating pathologies.

Hedonic and motivational roles of opioids in food reward: implications for overeating disorders

Food reward can be driven by separable mechanisms of hedonic impact (food 'liking') and incentive motivation (food 'wanting'). Brain mu-opioid systems contribute crucially to both forms of food reward. Yet, opioid signals for food 'liking' and 'wanting' diverge in anatomical substrates, in pathways connecting these sites, and in the firing profiles of single neurons. Divergent neural control of hedonic and motivational processes raises the possibility for joint or separable modulation of food intake in human disorders associated with excessive eating and obesity. Early findings confirm an important role for 'liking' and 'wanting' in human appetitive behaviors, and suggest the intriguing possibility that exaggerated signals for 'wanting,' and perhaps 'liking,' may contribute to forms of overeating.

Reward processing by the opioid system in the brain

The opioid system consists of three receptors, mu, delta, and kappa, which are activated by endogenous opioid peptides processed from three protein precursors, proopiomelanocortin, proenkephalin, and prodynorphin. Opioid receptors are recruited in response to natural rewarding stimuli and drugs of abuse, and both endogenous opioids and their receptors are modified as addiction develops. Mechanisms whereby aberrant activation and modifications of the opioid system contribute to drug craving and relapse remain to be clarified. This review summarizes our present knowledge on brain sites where the endogenous opioid system controls hedonic responses and is modified in response to drugs of abuse in the rodent brain. We review 1) the latest data on the anatomy of the opioid system, 2) the consequences of local intracerebral pharmacological manipulation of the opioid system on reinforced behaviors, 3) the consequences of gene knockout on reinforced behaviors and drug dependence, and 4) the consequences of chronic exposure to drugs of abuse on expression levels of opioid system genes. Future studies will establish key molecular actors of the system and neural sites where opioid peptides and receptors contribute to the onset of addictive disorders. Combined with data from human and nonhuman primate (not reviewed here), research in this extremely active field has implications both for our understanding of the biology of addiction and for therapeutic interventions to treat the disorder.

From taste hedonics to motivational drive: central μ-opioid receptors and binge-eating behaviour

Endogenous opioids and μ-opioid receptors (MORs) have long been implicated in the mechanism of appetite control and, in particular, hedonic processes associated with food evaluation, consumption and orosensory reward processes. In animal models of binge eating, selective MOR antagonists suppress food consumption. In humans, non-selective opioid receptor antagonists reduce hedonic taste preferences and food intake, particularly for palatable foods, and cause short-term weight loss. These effects have been linked to direct stimulation of MORs and modulation of dopamine release within the reward circuitry including the nucleus accumbens. These findings suggest that reduction of MOR-mediated hedonic and motivation processes driving consumption of highly palatable foods may be a promising therapeutic approach and provide a strong rationale for developing safer and more selective MOR antagonists or inverse agonists for disorders of 'appetitive motivation' including obesity and binge-eating disorder.

Changes in the subcellular distribution of NADPH oxidase subunit p47phox in dendrites of rat dorsomedial nucleus tractus solitarius neurons in response to chronic administration of hypertensive agents

NADPH oxidase-generated superoxide can modulate crucial intracellular signaling cascades in neurons of the nucleus tractus solitarius (NTS), a brain region that plays an important role in cardiovascular processes. Modulation of NTS signaling by superoxide may be linked to the subcellular location of the mobile NADPH oxidase p47(phox) subunit, which is known to be present in dendrites of NTS neurons. It is not known, however, if hypertension can produce changes in the trafficking of p47(phox) in defined NTS subregions, particularly the preferentially barosensitive dorsomedial NTS (dmNTS), or preferentially gastrointestinal medial NTS (mNTS). We used immunogold electron microscopy to determine if p47(phox) localization was differentially affected in dendritic profiles of neurons from these NTS subregions of the rat in response to distinct models of hypertension, namely chronic 7-day subcutaneous administration of angiotensin II (AngII), or phenylephrine. In small (<1 microm) dendritic processes, both AngII and phenylephrine produced a decrease in intracellular p47(phox) labeling selectively in dmNTS neurons. In intermediate-size (1-2 microm) dendritic profiles in the dmNTS region only, there was an increase in p47(phox) labeling in response to each hypertensive agent, although these changes occurred in different subcellular compartments. There was an increase in non-vesicular labeling in response to AngII, but an increase in surface labeling with phenylephrine. Moreover, each of the changes in p47(phox) targeting mentioned above occurred in dendritic profiles with, or without immunoperoxidase labeling for the AngII AT-1A receptor subtype (AT-1A). These results indicate that chronic administration of agents that induce hypertension can also produce changes in the subcellular localization in p47(phox) in dmNTS neurons. Thus, systemic hypertension may produce alterations in the trafficking of proteins associated with superoxide production in central autonomic neurons, thus revealing a potentially important neurogenic component of free radical production and systemic blood pressure elevation.

Subcellular localization of nicotinamide adenine dinucleotide phosphate oxidase subunits in neurons and astroglia of the rat medial nucleus tractus solitarius: relationship with tyrosine hydroxylase immunoreactive neurons

Superoxide produced by the enzyme nicotinamide adenine dinucleotide phosphate (NADPH) oxidase mediates crucial intracellular signaling cascades in the medial nucleus of the solitary tract (mNTS), a brain region populated by catecholaminergic neurons, as well as astroglia that play an important role in autonomic function. The mechanisms mediating NADPH oxidase (phagocyte oxidase) activity in the neural regulation of cardiovascular processes are incompletely understood, however the subcellular localization of superoxide produced by the enzyme is likely to be an important regulatory factor. We used immunogold electron microscopy to determine the phenotypic and subcellular localization of the NADPH oxidase subunits p47(phox), gp91(phox,) and p22(phox) in the mNTS in rats. The mNTS contains a large population of neurons that synthesize catecholamines. Significantly, catecholaminergic signaling can be modulated by redox reactions. Therefore, the relationship of NADPH oxidase subunit labeled neurons or glia with respect to catecholaminergic neurons was also determined by dual labeling for the superoxide producing enzyme and tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. In the mNTS, NADPH oxidase subunits were present primarily in somatodendritic processes and astrocytes, some of which also contained TH, or were contacted by TH-labeled axons, respectively. Immunogold quantification of NADPH oxidase subunit localization showed that p47(phox) and gp91(phox) were present on the surface membrane, as well as vesicular organelles characteristic of calcium storing smooth endoplasmic reticula in dendritic and astroglial processes. These results indicate that NADPH oxidase assembly and consequent superoxide formation are likely to occur near the plasmalemma, as well as on vesicular organelles associated with intracellular calcium storage within mNTS neurons and glia. Thus, NADPH oxidase-derived superoxide may participate in intracellular signaling pathways linked to calcium regulation in diverse mNTS cell types. Moreover, NADPH oxidase-derived superoxide in neurons and glia may directly or indirectly modulate catecholaminergic neuron activity in the mNTS.

The μ-opioid receptor subtype is required for the anorectic effect of an opioid receptor antagonist

A diaryl ether derivative, (6-(4-{[(3-methylbutyl)amino]methyl}phenoxy)nicotinamide, was prepared and investigated for its biochemical properties at cloned opioid receptors and its pharmacological effects on animal feeding. The compound displaced [(3)H]DAMGO binding of human mu-opioid receptor, [(3)H]U-69593 of human kappa-opioid receptor, and [(3)H]DPDPE of human delta-opioid receptor with IC(50) values of 0.5+/-0.2 nM, 1.4+/-0.2 nM, and 71+/-15 nM, respectively. The compound also potently inhibited [(3)H]DAMGO binding of cloned mouse and rat mu-opioid receptors (IC(50) approximately 1 nM), and acted as a competitive antagonist in a cAMP functional assay using cultured cells expressing human or mouse mu-opioid receptors. Following a single oral administration in diet-induced obese mice (at 10 or 50 mg/kg) or rats (at 1, 3, or 10 mg/kg), the compound caused a dose-dependent suppression of acute food intake and body weight gain in both species. Importantly, the anorectic efficacy of the compound was mostly diminished in mice deficient in the mu-opioid receptor. Our results suggest an important role for the mu-opioid receptor subtype in animal feeding regulation and support the development of mu-selective antagonists as potential agents for treating human obesity.

Caloric restriction and physical activity in zebrafish (Danio rerio)

Understanding the mechanism of energy flux may be critical for explaining how obesity has emerged as a public health epidemic. It is known that changes in caloric intake predictably alter physical activity levels (PA) in mammals. Here, our goal was to test the hypothesis that fasting induces a biphasic pattern of change in PA by measuring PA before and after long-term food deprivation in zebrafish. Compared to control-fed fish, food-deprived fish showed a significant increase in PA levels during the first 2 days of food deprivation. Subsequently, however, fasted fish showed a significant chronic decrease in PA compared to fish fed at weight-maintenance levels. These data are comparable to those seen with mammals, which also show a biphasic response of PA to caloric restriction. In a separate group of fish, long-term food deprivation, associated with decreases in PA, induced a significant increase in brain preproorexin mRNA levels compared to fed controls. No change in orexin mRNA was seen after 2 days of food deprivation. The finding that orexin mRNA expression is altered only after long-term starvation suggests that orexin may be coupled with the changes in PA seen at this time. Thus, the association between negative energy balance and reductions in PA occurs across genera in biology and is associated with predictable neurological changes in brain gene expression.

Chronic administration of nalmefene leads to increased food intake and body weight gain in mice

Nalmefene is an orally available opioid receptor antagonist that has been shown to suppress appetite in humans, but its effects on chronic food intake and body weight remain unclear. Here, we report that chronic (21-day) oral administration of nalmefene at 2 or 10 mg/kg/day in diet-induced obese (DIO) mice led to significant increases (9-11%) in cumulative food intake. Mice in the nalmefene-treated groups also gained body weight at a rate faster than the control. Body composition analysis showed that the extra body weight gains in the treated animals were mostly due to increased fat accumulation. Since acute nalmefene treatment showed a trend toward a decrease rather than an increase in food intake, it is possible that the orexigenic effect of chronic oral administration of nalmefene was caused by pharmacologically active metabolites rather than the drug itself. Our results argue against the potential use of nalmefene for treating human obesity.

Synergistic effects of cannabinoid inverse agonist AM251 and opioid antagonist nalmefene on food intake in mice

Oral administration of the opioid antagonist nalmefene alone (up to 20 mg/kg) failed to show a significant effect on acute food intake in mice. However, combined oral dosing of nalmefene and subthreshold doses of AM251, a cannabinoid CB1 receptor inverse agonist, led to a significant reduction in food intake in both lean and diet-induced obese (DIO) mice. Furthermore, the anorectic effect of a high dose of AM251 was further enhanced when co-administered with nalmefene. The results support a synergistic interaction between opioid and cannabinoid systems in regulating feeding behavior.