Endogenous opiates and behavior: 2011

Obesity-mediated Lipoinflammation Modulates Food Reward Responses

Accumulation of white adipose tissue (WAT) during obesity is associated with the development of chronic low-grade inflammation, a biological process known as lipoinflammation. Systemic and central lipoinflammation accumulates pro-inflammatory cytokines including IL-6, IL-1β and TNF-α in plasma and also in brain, disrupting neurometabolism and cognitive behavior. Obesity-mediated lipoinflammation has been reported in brain regions of the mesocorticolimbic reward circuit leading to alterations in the perception and consumption of ultra-processed foods. While still under investigation, lipoinflammation targets two major outcomes of the mesocorticolimbic circuit during food reward: perception and motivation ("Wanting") and the pleasurable feeling of feeding ("Liking"). This review will provide experimental and clinical evidence supporting the contribution of obesity- or overnutrition-related lipoinflammation affecting the mesocorticolimbic reward circuit and enhancing food reward responses. We will also address neuroanatomical targets of inflammatory profiles that modulate food reward responses during obesity and describe potential cellular and molecular mechanisms of overnutrition linked to addiction-like behavior favored by brain lipoinflammation.

Periaqueductal gray and emotions: the complexity of the problem and the light at the end of the tunnel, the magnetic resonance imaging

The periaqueductal gray (PAG) is less referred in relationship with emotions than other parts of the brain (e.g. cortex, thalamus, amygdala), most probably because of the difficulty to reach and manipulate this small and deeply lying structure. After defining how to evaluate emotions, we have reviewed the literature and summarized data of the PAG contribution to the feeling of emotions focusing on the behavioral and neurochemical considerations. In humans, emotions can be characterized by three main domains: the physiological changes, the communicative expressions, and the subjective experiences. In animals, the physiological changes can mainly be studied. Indeed, early studies have considered the PAG as an important center of the emotions-related autonomic and motoric processes. However, in vivo imaging have changed our view by highlighting the PAG as a significant player in emotions-related cognitive processes. The PAG lies on the crossroad of networks important in the regulation of emotions and therefore it should not be neglected. In vivo imaging represents a good tool for studying this structure in living organism and may reveal new information about its role beyond its importance in the neurovegetative regulation.

Systemic morphine treatment derails normal uterine receptivity, leading to embryo implantation failure in mice

Morphine is the oldest worldwide well-known opioid agonist used for pain treatment in clinic, and its illicit use is often associated with adverse pregnancy outcomes in humans. Because of recent dramatic increases in nonmedicinal morphine abuse, one emerging issue is the further revelation of the dark side of illicit opioid uses, particularly in early pregnancy events. In this respect, we have demonstrated that opioid signaling is functionally operative during preimplantation embryo development in mice. However, the pathophysiological significance of the opioid system on uterine functions at peri-implantation remained elusive. In the present study, we demonstrated that opioid receptors were spatiotemporally expressed in the uterus during the peri-implantation period. Employing a pharmacological approach combined with embryo transfer experiments, we further observed that although systemic morphine treatment exerts no apparent adverse influence on preimplantation ovarian secretion of progesterone and estrogen, this aberrant activation of opioid signaling by morphine induces impaired luminal epithelial differentiation, decreased stromal cell proliferation, and poor angiogenesis, and thus hampers uterine receptivity and embryo implantation. These novel findings add a new line of evidence to better understand the causes for obvious adverse effects of opioid abuse on pregnancy success in women.

Stress-induced activation of ventral tegmental mu-opioid receptors reduces accumbens dopamine tone by enhancing dopamine transmission in the medial pre-frontal cortex

RATIONALE

Endogenous opioids could play a major role in the mesocorticolimbic dopamine (DA) responses to stress challenge. However, there is still no direct evidence of an influence of endogenous opioids on any of these responses.

OBJECTIVE

We assessed whether and how endogenous opioids modulate fluctuations of mesocortical and mesoaccumbens DA tone in rats during a first experience with restraint stress.

METHOD

We first evaluated the effects of systemic naltrexone (NTRX) on DA outflow in the medial prefrontal cortex (mpFC) and in the nucleus accumbens (NAc) through dual-probe microdialysis. Second, we assessed the effect of perfusion, through reverse microdialysis, of direct DA receptor agonists in mpFC on NAc DA outflow in NTRX-pretreated stressed rats. Finally, we tested the effects of ventral tegmental area (VTA) perfusion of NTRX, the selective mu1 antagonist naloxonazine and the selective delta antagonist naltrindole on mpFC and NAc DA outflow in stressed rats, with multiple probe experiments.

RESULTS

Systemic NTRX, at behaviorally effective doses, selectively prevented the increase of mpFC DA levels and the reduction of NAc DA levels observable during prolonged restraint. Local co-perfusion of D1 and D2 agonists in mpFC recovered inhibition of NAc DA in NTRX-pretreated restrained rats. Finally, intra-VTA perfusion of either NTRX or the mu1 antagonist, but not the delta antagonist, mimicked the effects of systemic NTRX.

CONCLUSION

During prolonged experience with a novel unavoidable/uncontrollable stressor, endogenous opioids, through stimulation of mu1 receptors in the VTA, elevate mesocortical DA tone thus reducing DA tone in the NAc DA.

Assessment of mechanisms involved in antinociception produced by the alkaloid caulerpine

In previous works we showed that oral administration of caulerpine, a bisindole alkaloid isolated from algae of the genus Caulerpa, produced antinociception when assessed in chemical and thermal models of nociception. In this study, we evaluated the possible mechanism of action of this alkaloid in mice, using the writhing test. The antinociceptive effect of caulerpine was not affected by intraperitoneal (i.p.) pretreatment of mice with naloxone, flumazenil, l-arginine or atropine, thus discounting the involvement of the opioid, GABAergic, l-arginine-nitric oxide and (muscarinic) cholinergic pathways, respectively. In contrast, i.p. pretreatment with yohimbine, an α2-adrenoceptor antagonist, or tropisetron, a 5-HT3 antagonist, significantly blocked caulerpine-induced antinociception. These results suggest that caulerpine exerts its antinociceptive effect in the writhing test via pathways involving α2-adrenoceptors and 5-HT3 receptors. In summary, this alkaloid could be of interest in the development of new dual-action analgesic drugs.

Drug abuse and the neurovascular unit

Drug abuse continues to create a major international epidemic affecting society. A great majority of past drug abuse research has focused mostly on the mechanisms of addiction and the specific effects of substance use disorders on brain circuits and pathways that modulate reward, motivation, craving, and decision making. Few studies have focused on the neurobiology of acute and chronic substance abuse as it relates to the neurovascular unit (brain endothelial cell, neuron, astrocyte, microglia, and pericyte). Increasing research indicates that all cellular components of the neurovascular unit play a pivotal role in both the process of addiction and how drug abuse affects the brain response to diseases. This review will focus on the specific effects of opioids, amphetamines, alcohol, and nicotine on the neurovascular unit and its role in addiction and adaption to brain diseases. Elucidation of the role of the neurovascular unit on the neurobiology associated with drug addiction will help to facilitate the development of better therapeutic approaches for drug-dependent individuals.

K-Lysine acetyltransferase 2a regulates a hippocampal gene expression network linked to memory formation

Neuronal histone acetylation has been linked to memory consolidation, and targeting histone acetylation has emerged as a promising therapeutic strategy for neuropsychiatric diseases. However, the role of histone-modifying enzymes in the adult brain is still far from being understood. Here we use RNA sequencing to screen the levels of all known histone acetyltransferases (HATs) in the hippocampal CA1 region and find that K-acetyltransferase 2a (Kat2a)--a HAT that has not been studied for its role in memory function so far--shows highest expression. Mice that lack Kat2a show impaired hippocampal synaptic plasticity and long-term memory consolidation. We furthermore show that Kat2a regulates a highly interconnected hippocampal gene expression network linked to neuroactive receptor signaling via a mechanism that involves nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). In conclusion, our data establish Kat2a as a novel and essential regulator of hippocampal memory consolidation.

Blockade of central delta-opioid receptors inhibits salt appetite in sodium-depleted rats

Various studies have investigated the role of central opioid peptides in feeding behavior; however, only a few have addressed the participation of opioids in the control of salt appetite. The present study investigated the effect of intracerebroventricular injections of the δ-opioid antagonist, naltrindole (5, 10 and 20 nmol/rat) and the agonist, deltorphin II (2.5, 5, 10 and 20 nmol/rat) on salt intake. Two protocols for inducing salt intake were used: sodium-depletion and the central injection of angiotensin II. In addition, the effect of a central δ-opioid receptor blockade on locomotor activity, on palatable solution intake (0.1% saccharin) and on blood pressure was also studied. The blockade of central δ-opioid receptors inhibits salt intake in sodium-depleted rats, while the pharmacological stimulation of these receptors increases salt intake in sodium-replete animals. Furthermore, the blockade of central δ-opioid receptors inhibits salt intake induced by central angiotensinergic stimulation. These data suggest that during sodium-depletion activation of the δ-opioid receptors regulates salt appetite to correct the sodium imbalance and it is possible that an interaction between opioidergic and angiotensinergic brain system participates in this control. Under normonatremic conditions, δ-opioid receptors may be necessary to modulate sodium intake, a response that could be mediated by angiotensin II. The decrease in salt intake following central δ-opioid receptors blockade does not appear to be due to a general inhibition of locomotor activity, changes in palatability or in blood pressure.

Neurochemical mechanisms of alcohol withdrawal

Alcohol dependence encompasses a serious medical and societal problem that constitutes a major public health concern. A serious consequence of dependence is the emergence of symptoms associated with the alcohol withdrawal syndrome when drinking is abruptly terminated or substantially reduced. Clinical features of alcohol withdrawal include signs of central nervous system hyperexcitability, heightened autonomic nervous system activation, and a constellation of symptoms contributing to psychologic discomfort and negative affect. The development of alcohol dependence is a complex and dynamic process that ultimately reflects a maladaptive neurophysiologic state. Perturbations in a wide range of neurochemical systems, including glutamate, γ-aminobutyric acid, monoamines, a host of neuropeptide systems, and various ion channels produced by the chronic presence of alcohol ultimately compromise the functional integrity of the brain. These neuroadaptations not only underlie the emergence and expression of many alcohol withdrawal symptoms, but also contribute to enhanced relapse vulnerability as well as perpetuation of uncontrolled excessive drinking. This chapter highlights the hallmark features of the alcohol withdrawal syndrome, and describes neuroadaptations in a wide array of neurotransmitter and neuromodulator systems (amino acid and monoamine neurotransmitter, neuropeptide systems, and various ion channels) as they relate to the expression of various signs and symptoms of alcohol withdrawal, as well as their relationship to the significant clinical problem of relapse and uncontrolled dangerous drinking.

Stress modulates intestinal secretory immunoglobulin A

Stress is a response of the central nervous system to environmental stimuli perceived as a threat to homeostasis. The stress response triggers the generation of neurotransmitters and hormones from the hypothalamus pituitary adrenal axis, sympathetic axis and brain gut axis, and in this way modulates the intestinal immune system. The effects of psychological stress on intestinal immunity have been investigated mostly with the restraint/immobilization rodent model, resulting in an up or down modulation of SIgA levels depending on the intensity and time of exposure to stress. SIgA is a protein complex formed by dimeric (dIgA) or polymeric IgA (pIgA) and the secretory component (SC), a peptide derived from the polymeric immunoglobulin receptor (pIgR). The latter receptor is a transmembrane protein expressed on the basolateral side of gut epithelial cells, where it uptakes dIgA or pIgA released by plasma cells in the lamina propria. As a result, the IgA-pIgR complex is formed and transported by vesicles to the apical side of epithelial cells. pIgR is then cleaved to release SIgA into the luminal secretions of gut. Down modulation of SIgA associated with stress can have negative repercussions on intestinal function and integrity. This can take the form of increased adhesion of pathogenic agents to the intestinal epithelium and/or an altered balance of inflammation leading to greater intestinal permeability. Most studies on the molecular and biochemical mechanisms involved in the stress response have focused on systemic immunity. The present review analyzes the impact of stress (mostly by restraint/immobilization, but also with mention of other models) on the generation of SIgA, pIgR and other humoral and cellular components involved in the intestinal immune response. Insights into these mechanisms could lead to better therapies for protecting against pathogenic agents and avoiding epithelial tissue damage by modulating intestinal inflammation.

Glucose and calcium ions may modulate the efficiency of bovine β-casomorphin-7 permeability through a monolayer of Caco-2 cells

Milk and dairy products provide a lot of valuable nutritive elements. They are also sources of biologically active peptides, including β-casomorphins that manifest the properties of morphine. An activity of DPPIV seems to be most crucial factor decreasing the efficiency of the β-casomorphin-7 (BCM7) transport. The increase of BCM7 concentration in blood may intensify symptoms of apparent life threatening events (ALTE), autism, schizophrenia, and allergy. This study aimed at identifying the influence of several selected substances on a transport efficiency of bovine BCM7 through an intestinal monolayer in a Caco-2 cell model system. Applying the ELISA method, the permeability coefficient of BCM7 through the Caco-2 monolayer was calculated. TEER values were used to evaluate the integrity of Caco-2 cell monolayers. An increase of glucose and Ca(2+) concentrations in the culture medium was accompanied by an increase of the BCM7 transport efficiency. The lowest permeability coefficients of BCM7 were observed for the membranes with high electrical resistances. The transport was enhanced in the presence of milk infant formulas, whereas no changes were observed when using μ-opioid receptor antagonist (casoxin-6). The results may be useful in understanding the pathogenesis of inflammation and food allergy in infants.

Opioid therapy for chronic pain in the United States: promises and perils

Opioid therapy offers the promise of reducing the burden of chronic pain in not just individual patients, but among the broad population of patients with chronic pain. Randomized trials have demonstrated that opioid therapy for up to 12-16weeks is superior to placebo, but have not addressed longer-term use. In the United States, opioid sales have quadrupled during 2000-2010, with parallel increases in opioid accidental overdose deaths and substance abuse admissions. Clinical use of long-term opioid therapy is characterized by a pattern of adverse selection, where high-risk patients are prescribed high-risk opioid regimens. This adverse selection may link these trends in use, abuse, and overdose. Long-term opioid therapy appears to be associated with iatrogenic harm to the patients who receive the prescriptions and to the general population. The United States has, in effect, conducted an experiment of population-wide treatment of chronic pain with long-term opioid therapy. The population-wide benefits have been hard to demonstrate, but the harms are now well demonstrated.

15 years of genetic approaches in vivo for addiction research: Opioid receptor and peptide gene knockout in mouse models of drug abuse

The endogenous opioid system is expressed throughout the brain reinforcement circuitry, and plays a major role in reward processing, mood control and the development of addiction. This neuromodulator system is composed of three receptors, mu, delta and kappa, interacting with a family of opioid peptides derived from POMC (β-endorphin), preproenkephalin (pEnk) and preprodynorphin (pDyn) precursors. Knockout mice targeting each gene of the opioid system have been created almost two decades ago. Extending classical pharmacology, these mutant mice represent unique tools to tease apart the specific role of each opioid receptor and peptide in vivo, and a powerful approach to understand how the opioid system modulates behavioral effects of drugs of abuse. The present review summarizes these studies, with a focus on major drugs of abuse including morphine/heroin, cannabinoids, psychostimulants, nicotine or alcohol. Genetic data, altogether, set the mu receptor as the primary target for morphine and heroin. In addition, this receptor is essential to mediate rewarding properties of non-opioid drugs of abuse, with a demonstrated implication of β-endorphin for cocaine and nicotine. Delta receptor activity reduces levels of anxiety and depressive-like behaviors, and facilitates morphine-context association. pEnk is involved in these processes and delta/pEnk signaling likely regulates alcohol intake. The kappa receptor mainly interacts with pDyn peptides to limit drug reward, and mediate dysphoric effects of cannabinoids and nicotine. Kappa/dynorphin activity also increases sensitivity to cocaine reward under stressful conditions. The opioid system remains a prime candidate to develop successful therapies in addicted individuals, and understanding opioid-mediated processes at systems level, through emerging genetic and imaging technologies, represents the next challenging goal and a promising avenue in addiction research. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Expression of substance P, calcitonin gene-related peptide, β-endorphin and methionine-enkephalin in human dental pulp tissue after orthodontic intrusion: A pilot study

Objective:

To determine the levels of two sensory neuropeptides (substance P [SP] and calcitonin gene-related peptide [CGRP]) and two endogenous opioids (methionine-enkephalin [Met-Enk] and β-endorphin [β-End]) in dental pulp tissue samples subjected to controlled orthodontic intrusive forces.

Materials and Methods:

Sixteen healthy premolars were selected from eight patients who were undergoing extraction for orthodontic purposes. Eight were randomly used as controls, and the other eight were assigned to an experimental group (controlled orthodontic intrusive forces applied for 24 hours). After this period, teeth were extracted, and pulp samples were obtained. All samples were processed to quantify the expression levels of SP, CGRP, Met-Enk, and β-End using commercial radioimmunoassay kits.

Results:

All samples exhibited basal levels of both neuropeptides and endogenous opioids. After 24 hours of the intrusive stimulus, all patients reported a tolerable discomfort localized at the involved premolar. Only SP was significantly increased (P < .05). For the other molecules, no statistically significant differences were observed (P > .05); however, they expressed important increasing trends.

Conclusions:

The expression levels of SP and CGRP in dental pulp samples from the experimental group support the positive correlation between the symptomatic clinical scenario and increased expression levels of neuropeptides, clarifying the role of neurogenic inflammation in early injury response.

Preparation and evaluation at the delta opioid receptor of a series of linear leu-enkephalin analogues obtained by systematic replacement of the amides

Leu-enkephalin analogues, in which the amide bonds were sequentially and systematically replaced either by ester or N-methyl amide bonds, were prepared using classical organic chemistry as well as solid phase peptide synthesis (SPPS). The peptidomimetics were characterized using competition binding, ERK1/2 phosphorylation, receptor internalization, and contractility assays to evaluate their pharmacological profile over the delta opioid receptor (DOPr). The lipophilicity (LogD7.4) and plasma stability of the active analogues were also measured. Our results revealed that the last amide bond can be successfully replaced by either an ester or an N-methyl amide bond without significantly decreasing the biological activity of the corresponding analogues when compared to Leu-enkephalin. The peptidomimetics with an N-methyl amide function between residues Phe and Leu were found to be more lipophilic and more stable than Leu-enkephalin. Findings from the present study further revealed that the hydrogen-bond donor properties of the fourth amide of Leu-enkephalin are not important for its biological activity on DOPr. Our results show that the systematic replacement of amide bonds by isosteric functions represents an efficient way to design and synthesize novel peptide analogues with enhanced stability. Our findings further suggest that such a strategy can also be useful to study the biological roles of amide bonds.

Acetaldehyde involvement in ethanol's postabsortive effects during early ontogeny

Clinical and biomedical studies sustains the notion that early ontogeny is a vulnerable window to the impact of alcohol. Experiences with the drug during these stages increase latter disposition to prefer, use or abuse ethanol. This period of enhanced sensitivity to ethanol is accompanied by a high rate of activity in the central catalase system, which metabolizes ethanol in the brain. Acetaldehyde (ACD), the first oxidation product of ethanol, has been found to share many neurobehavioral effects with the drug. Cumulative evidence supports this notion in models employing adults. Nevertheless very few studies have been conducted to analyze the role of ACD in ethanol postabsorptive effects, in newborns or infant rats. In this work we review recent experimental literature that syndicates ACD as a mediator agent of reinforcing aspects of ethanol, during early ontogenetic stages. We also show a meta-analytical correlational approach that proposes how differences in the activity of brain catalase across ontogeny, could be modulating patterns of ethanol consumption.

Links between breeding readiness, opioid immunolabeling, and the affective state induced by hearing male courtship song in female European starlings (Sturnus vulgaris)

Male courtship vocalizations represent a potent signal designed to attract females; however, not all females find male signals equally attractive. We explored the possibility that the affective state induced by hearing courtship vocalizations depends on the motivational state of a receiver. We used a conditioned place preference test of reward to determine the extent to which the rewarding properties of hearing male courtship song differed in female European starlings categorized as nest box owners (a sign of breeding readiness) or non-owners. Nest box owners developed a preference for a chamber in which they previously heard male courtship song. Non-owners displayed no preference for a chamber in which they previously heard song. Positive correlations were identified between the preference a female developed for the song-paired chamber and female nesting and dominance behaviors observed prior to conditioning (indices of the motivation to breed). Immunolabeling for met-enkephalin (an opioid neuropeptide involved in reward) in the medial preoptic nucleus, ventromedial nucleus of the hypothalamus, nucleus accumbens, and periaqueductal gray was higher in females with compared to those without nest boxes. Both nest box entries and song-induced place preference also correlated positively with met-enkephalin labeling in the ventromedial nucleus of the hypothalamus. These findings indicate that the reward value of vocal signals is linked to individual differences in motivational state; and that differences in enkephalin activity may play a role in modifying an individual's motivational state and/or the reward value of song.