Stress-induced changes in brain Met-enkephalin, Leu-enkephalin and dynorphin concentrations

Protective Role and Functional Engineering of Neuropeptides in Depression and Anxiety: An Overview

Behavioral disorders, such as anxiety and depression, are prevalent globally and touch children and adults on a regular basis. Therefore, it is critical to comprehend how these disorders are affected. It has been demonstrated that neuropeptides can influence behavior, emotional reactions, and behavioral disorders. This review highlights the majority of the findings demonstrating neuropeptides' behavioral role and functional engineering in depression and anxiety. Gut-brain peptides, hypothalamic releasing hormone peptides, opioid peptides, and pituitary hormone peptides are the four major groups of neuropeptides discussed. Some neuropeptides appear to promote depression and anxiety-like symptoms, whereas others seem to reduce it, all depending on the receptors they are acting on and on the brain region they are localized in. The data supplied here are an excellent starting point for future therapy interventions aimed at treating anxiety and depression.

Regulation of Cortico-Thalamic JNK1/2 and ERK1/2 MAPKs and Apoptosis-Related Signaling Pathways in PDYN Gene-Deficient Mice Following Acute and Chronic Mild Stress

The crosstalk between the opioidergic system and mitogen-activated protein kinases (MAPKs) has a critical role in mediating stress-induced behaviors related to the pathophysiology of anxiety. The present study evaluated the basal status and stress-induced alterations of cortico-thalamic MAPKs and other cell fate-related signaling pathways potentially underlying the anxiogenic endophenotype of PDYN gene-deficient mice. Compared to littermates, PDYN knockout (KO) mice had lower cortical and or thalamic amounts of the phospho-activated MAPKs c-Jun N-terminal kinase (JNK1/2) and extracellular signal-regulated kinase (ERK1/2). Similarly, PDYN-KO animals displayed reduced cortico-thalamic densities of total and phosphorylated (at Ser191) species of the cell fate regulator Fas-associated protein with death domain (FADD) without alterations in the Fas receptor. Exposure to acute restraint and chronic mild stress stimuli induced the robust stimulation of JNK1/2 and ERK1/2 MAPKs, FADD, and Akt-mTOR pathways, without apparent increases in apoptotic rates. Interestingly, PDYN deficiency prevented stress-induced JNK1/2 and FADD but not ERK1/2 or Akt-mTOR hyperactivations. These findings suggest that cortico-thalamic MAPK- and FADD-dependent neuroplasticity might be altered in PDYN-KO mice. In addition, the results also indicate that the PDYN gene (and hence dynorphin release) may be required to stimulate JNK1/2 and FADD (but not ERK1/2 or Akt/mTOR) pathways under environmental stress conditions.

Plasma metabolites were associated with spatial working memory in major depressive disorder

Major depressive disorder (MDD) is a common disease with both affective and cognitive disorders. Alterations in metabolic systems of MDD patients have been reported, but the underlying mechanisms still remains unclear. We sought to identify abnormal metabolites in MDD by metabolomics and to explore the association between differential metabolites and neurocognitive dysfunction.Plasma samples from 53 MDD patients and 83 sex-, gender-, BMI-matched healthy controls (HCs) were collected. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) system was then used to detect metabolites in those samples. Two different algorithms were applied to identify differential metabolites in 2 groups. Of the 136 participants, 35 MDD patients and 48 HCs had completed spatial working memory test. Spearman rank correlation coefficient was applied to explore the relationship between differential metabolites and working memory in these 2 groups.The top 5 metabolites which were found in sparse partial least squares-discriminant analysis (sPLS-DA) model and random forest (RF) model were the same, and significant difference was found in 3 metabolites between MDD and HCs, namely, gamma-glutamyl leucine, leucine-enkephalin, and valeric acid. In addition, MDD patients had higher scores in spatial working memory (SWM) between errors and total errors than HCs. Valeric acid was positively correlated with working memory in MDD group.Gamma-glutamyl leucine, leucine-enkephalin, and valeric acid were preliminarily proven to be decreased in MDD patients. In addition, MDD patients performed worse in working memory than HCs. Dysfunction in working memory of MDD individuals was associated with valeric acid.

Potential for Kappa-Opioid Receptor Agonists to Engineer Nonaddictive Analgesics: A Narrative Review

A serious adverse effect of prescription opioid analgesics is addiction, both to these analgesics and to illicit drugs like heroin that also activate the µ-opioid receptor (MOR). Opioid use disorder (OUD) and opioid overdose deaths represent a current American health crisis, and the prescription of opioid analgesics has contributed significantly to this crisis. While prescription opioids are highly effective analgesics, there currently exists no facile way to use them for extended periods without the risk of addiction. If addiction caused by MOR-targeting analgesics could be blocked by blending in a new "antiaddiction" ingredient that does not diminish analgesia and does not introduce its own therapeutically limiting side effects, then continued clinical use of prescription opioids for treating pain could be maintained (or even enhanced) instead of curtailed. In this narrative review, we contextualize this hypothesis, first with a brief overview of the current American opioid addiction crisis. The neurobiology of 2 key receptors in OUD development, MOR and the κ-opioid receptor (KOR), is then discussed to highlight the neuroanatomical features and circuitry in which signal transduction from these receptors lie in opposition-creating opportunities for pharmacological intervention in curtailing the addictive potential of MOR agonism. Prior findings with mixed MOR/KOR agonists are considered before exploring new potential avenues such as biased KOR agonists. New preclinical data are highlighted, demonstrating that the G protein-biased KOR agonist nalfurafine reduces the rewarding properties of MOR-targeting analgesics and enhances MOR-targeting analgesic-induced antinociception. Finally, we discuss the recent discovery that a regulator of G protein signaling (namely, RGS12) is a key component of signaling bias at KOR, presenting another drug discovery target toward identifying a single agent or adjuvant to be added to traditional opioid analgesics that could reduce or eliminate the addictive potential of the latter drug.

κ Opioid Receptor-Dynorphin Signaling in the Central Amygdala Regulates Conditioned Threat Discrimination and Anxiety

Neuropeptides within the central nucleus of the amygdala (CeA) potently modulate neuronal excitability and have been shown to regulate conditioned threat discrimination and anxiety. Here, we investigated the role of κ opioid receptor (KOR) and its endogenous ligand dynorphin in the CeA for regulation of conditioned threat discrimination and anxiety-like behavior in mice. We demonstrate that reduced KOR expression through genetic inactivation of the KOR encoding gene, Oprk1, in the CeA results in increased anxiety-like behavior and impaired conditioned threat discrimination. In contrast, reduction of dynorphin through genetic inactivation of the dynorphin encoding gene, Pdyn, in the CeA has no effect on anxiety or conditioned threat discrimination. However, inactivation of Pdyn from multiple sources, intrinsic and extrinsic to the CeA phenocopies Oprk1 inactivation. These findings suggest that dynorphin inputs to the CeA signal through KOR to promote threat discrimination and dampen anxiety.

On the Nature of the Mother-Infant Tie and Its Interaction With Freudian Drives

The affective bond between an infant and its caregiver, the so-called mother-infant tie, was analyzed by various reputable psychologists (e.g., Ainsworth, Clark, Erikson, Anna Freud, Harlow, Klein, Spitz, and Winnicott) but both the basic tenets of the bond and the importance of the trauma of maternal deprivation for personality disorders in adults were introduced by Bowlby. Although Bowlby was a trained psychoanalyst, he rejected central cornerstones of Freudian theory (esp. drive theory) and used concepts promulgated by renowned ethologists (Tinbergen and Lorenz) to establish his framework of "instinctive behavior" that has been developed further into the concept of "attachment theory" under the influence of Mary Ainsworth. However, since any precise experimental facts were lacking when Bowlby formulated his ideas on the concept of instinctive behavior, the whole framework is a descriptive, category-driven approach (like the ones of Freudian drives). In order to connect the mother-infant tie - as propounded by Bowlby - with experimental data, this manuscript undertakes a biochemical analysis of it because this strategy proved somewhat successful in relation to Freudian drives. The analysis unfolded that the neurochemical oxytocin, released by the action of sensory nerves, is of utmost importance for the operation of the mother-infant tie. Furthermore, multiple evidences have been presented to the fact that there is strong interaction between unconsciously operating Freudian drives and the consciously acting mother-infant tie (that is now classified as a drive). The outlined interaction in conjunction with the classification of attachment urges as drives gave a very detailed insight into how a SEEKING-derived reward can be evoked during operation of the mother-infant tie. In summary, there is no need to marginalize either the mother-infant tie or Freudian drives but rather there is need to respect both (principally different) impulses in moving toward a more extensive description.

Reduced nucleus accumbens enkephalins underlie vulnerability to social defeat stress

Enkephalins, endogenous ligands for delta opioid receptors (DORs), are highly enriched in the nucleus accumbens (NAc). They are implicated in depression but their role in the NAc, a critical brain region for motivated behavior, is not fully investigated. To provide insight into enkephalin function we used a chronic social defeat stress paradigm, where animals are either categorized as susceptible or resilient to stress based on their performance in a social interaction test. Compared to controls, susceptible animals showed reduced enkephalin levels in the NAc. Such decrease in enkephalin levels is not due to a change in mRNA of its precursor protein, proenkephalin, in susceptible mice but is consistent with increased mRNA levels of enkephalinases in the NAc of susceptible animals. Systemic administration of enkephalinase inhibitors or NAc infusion of the DOR agonist, SNC80, caused a resilient outcome to CSDS. Both treatments increased phosphorylation of ERK, which was downregulated by social defeat stress. To further validate these results, we also used Q175 knock-in mice, an animal model of Huntington's disease in which enkephalin levels are reduced in striatum and comorbidity with mood disorders is common. Consistent with data in wild-type mice, Q175 animals showed reduced enkephalin levels in the NAc and enhanced susceptibility to a social defeat stress. Overall, our data implicate that depression-like behavior induced by social defeat stress arises from disrupted DOR signaling resulting from lowered levels of enkephalins, which is partly mediated through elevated expression of enkephalinases.

Effects of the Alpha-1 Antagonist Prazosin on KOR Agonist-Induced Reinstatement of Alcohol Seeking

BACKGROUND

Stress is associated with relapse to alcohol seeking during abstinence, but the processes underlying this relationship are poorly understood. Noradrenaline is a key transmitter in stress responses and in stress-induced drug seeking. The alpha-1 adrenoceptor antagonist prazosin has been investigated as a treatment for alcoholism and for chronic stress disorders that are frequently comorbid with alcoholism. In rats, we previously showed that prazosin blocks reinstatement of alcohol seeking induced by footshock and yohimbine stressors and reduces yohimbine-induced brain activation. The role of alpha-1 adrenoceptors in reinstatement induced by other stressors is not known. Our most recent work is on the role of kappa opioid receptors in stress-induced reinstatement of alcohol seeking and have reported that the selective kappa opioid receptor agonist U50,488 induces reinstatement and neuronal activation in stress- and relapse-related brain regions. Here we determine the involvement of alpha-1 receptors in reinstatement and brain activation induced by U50,488.

METHODS

We trained male Long-Evans rats to self-administer alcohol (12% w/v), extinguished alcohol-reinforced responding, and then determined the effects of prazosin (1 mg/kg) on U50,488 (2.5 mg/kg)-induced reinstatement and regional Fos expression.

RESULTS

Prazosin blocked U50,488-induced reinstatement and decreased U50,488-induced Fos expression in the orbitofrontal cortex, nucleus accumbens core, ventral bed nucleus of the stria terminalis, central and basolateral amygdalar nuclei and ventral tegmental area.

CONCLUSIONS

These findings suggest that prazosin may reduce U50,488-induced relapse by inhibiting activity in 1 or more of these brain areas.

Isolation stress impacts Met-enkephalin in the hypothalamo-pituitary-adrenocortical axis in growing Polish Mountain sheep: a possible role of the opioids in modulation of HPA axis

It was hypothesized that there is cross-talk between the classical constituents of the hypothalamo-pituitary-adrenocortical axis (HPA) and Met-enkephalin in the HPA axis. The study examined effects of isolation stress, sex, and age on concentrations of native Met-enkephalin and pro-enkephalin (PENK) gene expression in tissues of the HPA (hypothalamus, pituitary gland and adrenal cortex) in 3-, 6- and 9-month old female and male lambs. In addition, the effects of isolation stress on in vitro release Met-enkephalin from fragments of the hypothalamus or adrenal cortex were examined. Isolation stress was followed by decreases in the concentration of Met-enkephalin in both the pituitary gland and adrenal cortex. There were also increases in the hypothalamic concentration of Met-enkephalin together with increases in PENK gene expression in the HPA in 6- and 9-months old females and males. There were reductions in release of Met-enkephalin from hypothalamic and adrenocortical tissue in vitro after isolation stress. In the presence of naltrexone, there were increases in basal release in vitro of Met-enkephalin from hypothalamic tissue from control and stressed female lambs but a decrease in tissue from stressed male lambs. In a somewhat similar manner, the presence of naltrexone was associated with increases in the basal release of Met-enkephalin from adrenocortical tissue from control female lambs but a decrease with tissue from stressed female and both stressed and control male lambs. Lay summary   The present studies examine the impact of isolation stress on Met-enkephalin in growing female and male lambs. The results clearly showed the involvement of Met-enkephalin modulation of the psychological stress response in growing female and male lambs.

Effect of chronic alcohol vapor exposure on reinstatement of alcohol seeking induced by U50,488

Alcohol dependence and stress are associated with relapse to alcohol during abstinence, but the underlying mechanisms are poorly understood. Kappa opioid receptors (KOR) are involved in alcohol reward and in the effects of stress. Previously, in non-dependent rats, we showed that KOR in the bed nucleus of the stria terminalis (BNST) mediate reinstatement of alcohol seeking induced by the selective KOR agonist U50,488. Here, we determine the effects of chronic, intermittent exposure to alcohol vapor on U50,488-induced reinstatement of alcohol seeking. We also study brain mechanisms involved using the neuronal activity marker Fos and phosphorylated p38 MAPK (p-p38), an intracellular messenger implicated in the effects of KOR stimulation. We trained male Long-Evans rats to self-administer alcohol (12% w/v) and exposed them to alcohol vapor (14 h vapor/10 h air) daily for 24 d or to the control condition, extinguished alcohol-reinforced responding and determined the dose response for U50,488-induced reinstatement. We then determined the effects of vapor exposure on U50,488-induced Fos and p-p38 expression. Vapor-exposed rats were more sensitive to U50,488-induced reinstatement. U50,488 increased Fos expression in brain areas involved in stress-induced relapse, and Fos activation in the ventral BNST was greater in vapor exposed rats. Vapor exposed rats had increased basal p-p38 expression in the dorsal BNST, LC and NTS. Our findings suggest that changes in the neuronal responses to KOR stimulation in the ventral BNST may be involved in the increased sensitivity to U50,488 accompanying dependence.

Glucocorticoids stimulate hypothalamic dynorphin expression accounting for stress-induced impairment of GnRH secretion during preovulatory period

Stress-induced reproductive dysfunction is frequently associated with increased glucocorticoid (GC) levels responsible for suppressed GnRH/LH secretion and impaired ovulation. Besides the major role of the hypothalamic kisspeptin system, other key regulators may be involved in such regulatory mechanisms. Herein, we identify dynorphin as a novel transcriptional target of GC. We demonstrate that only priming with high estrogen (E2) concentrations prevailing during the late prooestrus phase enables stress-like GC concentrations to specifically stimulate Pdyn (prodynorphin) expression both in vitro (GT1-7 mouse hypothalamic cell line) and ex vivo (ovariectomized E2-supplemented mouse brains). Our results indicate that stress-induced GC levels up-regulate dynorphin expression within a specific kisspeptin neuron-containing hypothalamic region (antero-ventral periventricular nucleus), thus lowering kisspeptin secretion and preventing preovulatory GnRH/LH surge at the end of the prooestrus phase. To further characterize the molecular mechanisms of E2 and GC crosstalk, chromatin immunoprecipitation experiments and luciferase reporter gene assays driven by the proximal promoter of Pdyn show that glucocorticoid receptors bind specific response elements located within the Pdyn promoter, exclusively in presence of E2. Altogether, our work provides novel understanding on how stress affects hypothalamic-pituitary-gonadal axis and underscores the role of dynorphin in mediating GC inhibitory actions on the preovulatory GnRH/LH surge to block ovulation.

Nicotinic Cholinergic System in the Hypothalamus Modulates the Activity of the Hypothalamic Neuropeptides During the Stress Response

BACKGROUND

The hypothalamus harbors high levels of cholinergic neurons and axon terminals. Nicotinic acetylcholine receptors, which play an important role in cholinergic neurotransmission, are expressed abundantly in the hypothalamus. Accumulating evidence reveals a regulatory role for nicotine in the regulation of the stress responses. The present review will discuss the hypothalamic neuropeptides and their interaction with the nicotinic cholinergic system. The anatomical distribution of the cholinergic neurons, axon terminals and nicotinic receptors in discrete hypothalamic nuclei will be described. The effect of nicotinic cholinergic neurotransmission and nicotine exposure on hypothalamic-pituitaryadrenal (HPA) axis regulation at the hypothalamic level will be analyzed in view of the different neuropeptides involved.

METHODS

Published research related to nicotinic cholinergic regulation of the HPA axis activity at the hypothalamic level is reviewed.

RESULTS

The nicotinic cholinergic system is one of the major modulators of the HPA axis activity. There is substantial evidence supporting the regulation of hypothalamic neuropeptides by nicotinic acetylcholine receptors. However, most of the studies showing the nicotinic regulation of hypothalamic neuropeptides have employed systemic administration of nicotine. Additionally, we know little about the nicotinic receptor distribution on neuropeptide-synthesizing neurons in the hypothalamus and the physiological responses they trigger in these neurons.

CONCLUSION

Disturbed functioning of the HPA axis and hypothalamic neuropeptides results in pathologies such as depression, anxiety disorders and obesity, which are common and significant health problems. A better understanding of the nicotinic regulation of hypothalamic neuropeptides will aid in drug development and provide means to cope with these diseases. Considering that nicotine is also an abused substance, a better understanding of the role of the nicotinic cholinergic system on the HPA axis will aid in developing improved therapeutic strategies for smoking cessation.

Tyr1-ψ[( Z)CF═CH]-Gly2 Fluorinated Peptidomimetic Improves Distribution and Metabolism Properties of Leu-Enkephalin

Opioid peptides are key regulators in cellular and intercellular physiological responses, and could be therapeutically useful for modulating several pathological conditions. Unfortunately, the use of peptide-based agonists to target centrally located opioid receptors is limited by poor physicochemical (PC), distribution, metabolic, and pharmacokinetic (DMPK) properties that restrict penetration across the blood-brain barrier via passive diffusion. To address these problems, the present paper exploits fluorinated peptidomimetics to simultaneously modify PC and DMPK properties, thus facilitating entry into the central nervous system. As an initial example, the present paper exploited the Tyr¹-ψ[( Z)CF═CH]-Gly² peptidomimetic to improve PC druglike characteristics (computational), plasma and microsomal degradation, and systemic and CNS distribution of Leu-enkephalin (Tyr-Gly-Gly-Phe-Leu). Thus, the fluoroalkene replacement transformed an instable in vitro tool compound into a stable and centrally distributed in vivo probe. In contrast, the Tyr¹-ψ[CF₃CH₂-NH]-Gly² peptidomimetic decreased stability by accelerating proteolysis at the Gly³-Phe⁴ position.

Role of κ-Opioid Receptors in the Bed Nucleus of Stria Terminalis in Reinstatement of Alcohol Seeking

κ-Opioid receptors (KORs) and their endogenous ligand dynorphin are involved in stress-induced alcohol seeking but the mechanisms involved are largely unknown. We previously showed that systemic injections of the KOR agonist U50,488, which induce stress-like aversive states, reinstate alcohol seeking after extinction of the alcohol-reinforced responding. Here, we used the neuronal activity marker Fos and site-specific injections of the KOR antagonist nor-BNI and U50,488 to study brain mechanisms of U50,488-induced reinstatement of alcohol seeking. We trained male Long-Evans rats to self-administer alcohol (12% w/v) for 23-30 days. After extinction of the alcohol-reinforced responding, we tested the effect of U50,488 (0, 1.25, 2.5, and 5 mg/kg) on reinstatement of alcohol seeking. Next, we correlated regional Fos expression with reinstatement induced by the most effective U50,488 dose (5 mg/kg). Based on the correlational Fos results, we determined the effect of bed nucleus of the stria terminalis (BNST) injections of nor-BNI (4 μg/side) on U50,488-induced reinstatement of alcohol seeking, and reinstatement induced by injections of U50,488 (0, 0.3, 1, and 3 μg/side) into the BNST. U50,488-induced reinstatement of alcohol seeking was associated with increased Fos expression in multiple brain areas, including the BNST, where it was significantly correlated with lever pressing. U50,488-induced reinstatement was blocked by BNST nor-BNI injections, and BNST U50,488 injections partially mimicked the drug's systemic effect on reinstatement. Our data indicate that the BNST is a critical site for U50,488-induced reinstatement of alcohol seeking and suggest that KOR/dynorphin mechanisms in this brain area play a key role in stress-induced alcohol seeking.

Cross-Species Alterations in Synaptic Dopamine Regulation After Chronic Alcohol Exposure

Alcohol use disorders are a leading public health concern, engendering enormous costs in terms of both economic loss and human suffering. These disorders are characterized by compulsive and excessive alcohol use, as well as negative affect and alcohol craving during abstinence. Extensive research has implicated the dopamine system in both the acute pharmacological effects of alcohol and the symptomology of alcohol use disorders that develop after extended alcohol use. Preclinical research has shed light on many mechanisms by which chronic alcohol exposure dysregulates the dopamine system. However, many of the findings are inconsistent across experimental parameters such as alcohol exposure length, route of administration, and model organism. We propose that the dopaminergic alterations driving the core symptomology of alcohol use disorders are likely to be relatively stable across experimental settings. Recent work has been aimed at using multiple model organisms (mouse, rat, monkey) across various alcohol exposure procedures to search for commonalities. Here, we review recent advances in our understanding of the effects of chronic alcohol use on the dopamine system by highlighting findings that are consistent across experimental setting and species.

Addressing Structural Flexibility at the A-Ring on Salvinorin A: Discovery of a Potent Kappa-Opioid Agonist with Enhanced Metabolic Stability

Previous structure-activity studies on the neoclerodane diterpenoid salvinorin A have demonstrated the importance of the acetoxy functionality on the A-ring in its activity as a κ-opioid receptor agonist. Few studies have focused on understanding the role of conformation in these interactions. Herein we describe the synthesis and evaluation of both flexible and conformationally restricted compounds derived from salvinorin A. One such compound, spirobutyrolactone 14, was synthesized in a single step from salvinorin B and had similar potency and selectivity to salvinorin A (EC₅₀ = 0.6 ± 0.2 nM at κ; >10000 nM at μ and δ). Microsomal stability studies demonstrated that 14 was more metabolically resistant than salvinorin A. Evaluation of analgesic and anti-inflammatory properties revealed similar in vivo effects between 14 and salvinorin A. To our knowledge, this study represents the first example of bioisosteric replacement of an acetate group by a spirobutyrolactone to produce a metabolically resistant derivative.

Early-Life Social Isolation Stress Increases Kappa Opioid Receptor Responsiveness and Downregulates the Dopamine System

Chronic early-life stress increases vulnerability to alcoholism and anxiety disorders during adulthood. Similarly, rats reared in social isolation (SI) during adolescence exhibit augmented ethanol intake and anxiety-like behaviors compared with group housed (GH) rats. Prior studies suggest that disruption of dopamine (DA) signaling contributes to SI-associated behaviors, although the mechanisms underlying these alterations are not fully understood. Kappa opioid receptors (KORs) have an important role in regulating mesolimbic DA signaling, and other kinds of stressors have been shown to augment KOR function. Therefore, we tested the hypothesis that SI-induced increases in KOR function contribute to the dysregulation of NAc DA and the escalation in ethanol intake associated with SI. Our ex vivo voltammetry experiments showed that the inhibitory effects of the kappa agonist U50,488 on DA release were significantly enhanced in the NAc core and shell of SI rats. Dynorphin levels in NAc tissue were observed to be lower in SI rats. Microdialysis in freely moving rats revealed that SI was also associated with reduced baseline DA levels, and pretreatment with the KOR antagonist nor-binaltorphimine (nor-BNI) increased DA levels selectively in SI subjects. Acute ethanol elevated DA in SI and GH rats and nor-BNI pretreatment augmented this effect in SI subjects, while having no effect on ethanol-stimulated DA release in GH rats. Together, these data suggest that KORs may have increased responsiveness following SI, which could lead to hypodopaminergia and contribute to an increased drive to consume ethanol. Indeed, SI rats exhibited greater ethanol intake and preference and KOR blockade selectively attenuated ethanol intake in SI rats. Collectively, the findings that nor-BNI reversed SI-mediated hypodopaminergic state and escalated ethanol intake suggest that KOR antagonists may represent a promising therapeutic strategy for the treatment of alcohol use disorders, particularly in cases linked to chronic early-life stress.

Effects of acute and chronic social defeat stress are differentially mediated by the dynorphin/kappa-opioid receptor system

Accumulating evidence indicates that kappa-opioid receptors (KORs) and their endogenous ligand, dynorphin (DYN), can play important roles in regulating the effects of stress. Here, we examined the role of KOR systems in the molecular and behavioral effects of acute (1-day) and chronic (10-day) social defeat stress (SDS) in mice. We found that acute SDS increased DYN mRNA levels within the nucleus accumbens, a key element of brain dopamine (DA) systems. In contrast, chronic SDS produced long-lasting decreases in DYN mRNA levels. We then examined whether disruption of KOR function would affect development of SDS-induced depressive-like behaviors, as measured in the intracranial self-stimulation and social interaction tests. Ablation of KORs from DA transporter-expressing neurons delayed the development of SDS-induced anhedonia in the intracranial self-stimulation test, suggesting increased stress resilience. However, administration of the long-lasting KOR antagonist JDTic (30 mg/kg, intraperitoneally) before the SDS regimen did not affect anhedonia, suggesting that disruption of KOR function outside DA systems can oppose stress resilience. Social avoidance behavior measured after the 10-day SDS regimen was not altered by ablation of KORs in DA transporter-expressing neurons or by JDTic administration before testing. Our findings indicate that KORs expressed in DA systems regulate the effects of acute, but not chronic, social stress.

The role of the dynorphin/κ opioid receptor system in anxiety

Anxiety disorders are the most common and prevalent forms of psychiatric disease, although the biological basis of anxiety is not well understood. The dynorphin/κ opioid receptor system is widely distributed in the central nervous system and has been shown to play a critical role in modulating mood and emotional behaviors. In the present review, we summarize current literature relating to the role played by the dynorphin/κ opioid receptor system in anxiety and κ opioid receptor antagonists as potential therapeutic agents for the treatment of anxiety disorders.

Glucocorticoid Regulation of Reproduction

It is well accepted that stress, measured by increased glucocorticoid secretion, leads to profound reproductive dysfunction. In times of stress, glucocorticoids activate many parts of the fight or flight response, mobilizing energy and enhancing survival, while inhibiting metabolic processes that are not necessary for survival in the moment. This includes reproduction, an energetically costly procedure that is very finely regulated. In the short term, this is meant to be beneficial, so that the organism does not waste precious energy needed for survival. However, long-term inhibition can lead to persistent reproductive dysfunction, even if no longer stressed. This response is mediated by the increased levels of circulating glucocorticoids, which orchestrate complex inhibition of the entire reproductive axis. Stress and glucocorticoids exhibits both central and peripheral inhibition of the reproductive hormonal axis. While this has long been recognized as an issue, understanding the complex signaling mechanism behind this inhibition remains somewhat of a mystery. What makes this especially difficult is attempting to differentiate the many parts of both of these hormonal axes, and new neuropeptide discoveries in the last decade in the reproductive field have added even more complexity to an already complicated system. Glucocorticoids (GCs) and other hormones within the hypothalamic-pituitary-adrenal (HPA) axis (as well as contributors in the sympathetic system) can modulate the hypothalamic-pituitary-gonadal (HPG) axis at all levels-GCs can inhibit release of GnRH from the hypothalamus, inhibit gonadotropin synthesis and release in the pituitary, and inhibit testosterone synthesis and release from the gonads, while also influencing gametogenesis and sexual behavior. This chapter is not an exhaustive review of all the known literature, however is aimed at giving a brief look at both the central and peripheral effects of glucocorticoids on the reproductive function.

Stress and VTA synapses: implications for addiction and depression

While stressful experiences are a part of everyone's life, they can also exact a major toll on health. Stressful life experiences are associated with increased substance abuse, and there exists significant co-morbidity between mental illness and substance use disorders [N.D. Volkow & T.K. Li (2004) Nat. Rev. Neurosci., 5, 963-970; G. Koob & M.J. Kreek (2007) Am. J. Psych., 164, 1149-1159; R. Sinha (2008) Annals N.Y. Acad. Sci., 1141, 105-130]. The risk for development of mood or anxiety disorders after stress is positively associated with the risk for substance use disorders [R. Sinha (2008) Annals N.Y. Acad. Sci., 1141, 105-130], suggesting that there are common substrates for vulnerability to addictive and affective disorders. Understanding the molecular and physiological substrates of stress may lead to improved therapeutic interventions for the treatment of substance use disorders and mental illnesses.

Does the kappa opioid receptor system contribute to pain aversion?

The kappa opioid receptor (KOR) and the endogenous peptide-ligand dynorphin have received significant attention due the involvement in mediating a variety of behavioral and neurophysiological responses, including opposing the rewarding properties of drugs of abuse including opioids. Accumulating evidence indicates this system is involved in regulating states of motivation and emotion. Acute activation of the KOR produces an increase in motivational behavior to escape a threat, however, KOR activation associated with chronic stress leads to the expression of symptoms indicative of mood disorders. It is well accepted that KOR can produce analgesia and is engaged in chronic pain states including neuropathic pain. Spinal studies have revealed KOR-induced analgesia in reversing pain hypersensitivities associated with peripheral nerve injury. While systemic administration of KOR agonists attenuates nociceptive sensory transmission, this effect appears to be a stress-induced effect as anxiolytic agents, including delta opioid receptor agonists, mitigate KOR agonist-induced analgesia. Additionally, while the role of KOR and dynorphin in driving the dysphoric and aversive components of stress and drug withdrawal has been well characterized, how this system mediates the negative emotional states associated with chronic pain is relatively unexplored. This review provides evidence that dynorphin and the KOR system contribute to the negative affective component of pain and that this receptor system likely contributes to the high comorbidity of mood disorders associated with chronic neuropathic pain.

Role of the kappa-opioid receptor system in stress-induced reinstatement of nicotine seeking in rats

RATIONALE

The correlation between stress and smoking is well established. The mechanisms that underlie this relationship are, however, unclear. Recent data suggest that the kappa-opioid system is involved in the mediation of negative affective states associated with stress thereby promoting drug addiction and relapse. Pharmacological treatments targeting the kappa-opioid system and this mechanism may prove to be useful therapeutics for nicotine addiction in the future.

OBJECTIVES

We sought to determine whether there was a stress-specific role of the kappa-opioid system in nicotine seeking behavior.

METHOD

Groups of male Long Evans rats were trained to self-administer nicotine intravenously; their operant responding for nicotine was extinguished prior to tests of reinstatement. Pretreatment with systemic injections of the kappa-opioid receptor (KOR) antagonist nor-binaltorphimine (nor-BNI) was given prior to tests of stress (systemic injections of yohimbine (YOH)) or cue-induced reinstatement of nicotine seeking. Systemic injections of the KOR agonist U50,488 were also given in a test for reinstatement of nicotine seeking.

RESULTS

Nor-BNI pretreatment at 1h and 24h prior to testing was able to block YOH-induced, but not cue-induced reinstatement of nicotine seeking. U50,488 reinstated nicotine seeking behavior in a dose-dependent manner.

CONCLUSIONS

These findings support the hypothesis that the kappa-opioid system is involved in relapse to nicotine seeking induced by stress, but not by conditioned cues. KOR antagonists such as nor-BNI may therefore be useful novel therapeutic agents for decreasing the risk of stress-induced drug relapse.

Kappa opioid receptors regulate stress-induced cocaine seeking and synaptic plasticity

Stress facilitates reinstatement of addictive drug seeking in animals and promotes relapse in humans. Acute stress has marked and long-lasting effects on plasticity at both inhibitory and excitatory synapses on dopamine neurons in the ventral tegmental area (VTA), a key region necessary for drug reinforcement. Stress blocks long-term potentiation at GABAergic synapses on dopamine neurons in the VTA (LTPGABA), potentially removing a normal brake on activity. Here we show that blocking kappa opioid receptors (KORs) prior to forced-swim stress rescues LTPGABA. In contrast, blocking KORs does not prevent stress-induced potentiation of excitatory synapses nor morphine-induced block of LTPGABA. Using a kappa receptor antagonist as a selective tool to test the role of LTPGABA in vivo, we find that blocking KORs within the VTA prior to forced-swim stress prevents reinstatement of cocaine seeking. These results suggest that KORs may represent a useful therapeutic target for treatment of stress-triggered relapse in substance abuse.

Stress, κ manipulations, and aversive effects of ethanol in adolescent and adult male rats

Elevated ethanol use during adolescence, a potentially stressful developmental period, is accompanied by insensitivity to many aversive effects of ethanol relative to adults. Given evidence that supports a role for stress and the kappa opioid receptor (KOR) system in mediating aversive properties of ethanol and other drugs, the present study assessed the role of KOR antagonism by nor-binaltorphimine (nor-BNI) on ethanol-induced conditioned taste aversion (CTA) in stressed (exposed to repeated restraint) and non-stressed male rats (Experiment 1), with half of the rats pretreated with nor-BNI before stressor exposure. In Experiment 2, CTA induced by the kappa agonist U62,066 was also compared in stressed and non-stressed adolescents and adults. A highly palatable solution (chocolate Boost) was used as the conditioned stimulus (CS), thereby avoiding the need for water deprivation to motivate consumption of the CS during conditioning. No effects of stress on ethanol-induced CTA were found, with all doses eliciting aversions in adolescents and adults in both stress conditions. However, among stressed subjects, adults given nor-BNI before the repeated stressor displayed blunted ethanol aversion relative to adults given saline at that time. This effect of nor-BNI was not seen in adolescents, findings that support a differential role for the KOR involvement in ethanol CTA in stressed adolescents and adults. Results from Experiment 2 revealed that all doses of U62,066 elicited aversions in non-stressed animals of both ages that were attenuated in stressed animals, findings that support a modulatory role for stress in aversive effects of KOR activation. Collectively, these results suggest that although KOR sensitivity appears to be reduced in stressed subjects, this receptor system does not appear to contribute to age differences in ethanol-induced CTA under the present test circumstances.

Novel opioid cyclic tetrapeptides: Trp isomers of CJ-15,208 exhibit distinct opioid receptor agonism and short-acting κ opioid receptor antagonism

BACKGROUND AND PURPOSE

The κ opioid receptor antagonists demonstrate potential for maintaining abstinence from psychostimulant abuse, but existing non-peptide κ-receptor selective antagonists show exceptionally long activity. We hypothesized that the L- and D-Trp isomers of CJ-15,208, a natural cyclic tetrapeptide reported to be a κ-receptor antagonist in vitro, would demonstrate short-acting, dose-dependent antagonism in vivo, preventing reinstatement of cocaine-seeking behaviour.

EXPERIMENTAL APPROACH

Affinity, selectivity and efficacy of the L-Trp and D-Trp isomers for opioid receptors were assessed in vitro in radioligand and GTPγS binding assays. Opioid receptor agonist and antagonist activities were characterized in vivo following i.c.v. administration with the 55°C warm water tail-withdrawal assay. The D-Trp isomer, which demonstrated primarily κ-receptor selective antagonist activity, was further evaluated for its prevention of stress- and drug-induced reinstatement of extinguished cocaine conditioned place preference (CPP).

KEY RESULTS

The two isomers showed similar affinity and selectivity for κ receptors (K(i)  30-35 nM) as well as κ receptor antagonism in vitro. As expected, the D-Trp cyclic tetrapeptide exhibited minimal agonist activity and induced dose-dependent κ-receptor selective antagonism lasting less than 18 h in vivo. Pretreatment with this peptide prevented stress-, but not cocaine-induced, reinstatement of extinguished cocaine CPP. In contrast, the L-Trp cyclic tetrapeptide unexpectedly demonstrated mixed opioid agonist/antagonist activity.

CONCLUSIONS AND IMPLICATIONS

The L-Trp and the D-Trp isomers of CJ-15,208 demonstrate stereospecific opioid activity in vivo. The relatively brief κ opioid receptor antagonism, coupled with the prevention of stress-induced reinstatement of extinguished cocaine-seeking behaviour, suggests the D-Trp isomer could be used therapeutically to maintain abstinence from psychostimulant abuse.

The dynorphin/κ-opioid receptor system and its role in psychiatric disorders

The dynorphin/κ-opioid receptor system has been implicated in the pathogenesis and pathophysiology of several psychiatric disorders. In the present review, we present evidence indicating a key role for this system in modulating neurotransmission in brain circuits that subserve mood, motivation, and cognitive function. We overview the pharmacology, signaling, post-translational, post-transcriptional, transcriptional, epigenetic and cis regulation of the dynorphin/κ-opioid receptor system, and critically review functional neuroanatomical, neurochemical, and pharmacological evidence, suggesting that alterations in this system may contribute to affective disorders, drug addiction, and schizophrenia. We also overview the dynorphin/κ-opioid receptor system in the genetics of psychiatric disorders and discuss implications of the reviewed material for therapeutics development.

Endogenous opioid peptides in regulation of innate immunity cell functions

Endogenous opioid peptides comprise a group of bioregulatory factors involved in regulation of functional activity of various physiological systems of an organism. One of most important functions of endogenous opioids is their involvement in the interaction between cells of the nervous and immune systems. Summary data on the effects of opioid peptides on regulation of functions of innate immunity cells are presented.

The influences of reproductive status and acute stress on the levels of phosphorylated mu opioid receptor immunoreactivity in rat hippocampus

Opioids play a critical role in hippocampally dependent behavior and plasticity. In the hippocampal formation, mu opioid receptors (MOR) are prominent in parvalbumin (PARV) containing interneurons. Previously we found that gonadal hormones modulate the trafficking of MORs in PARV interneurons. Although sex differences in response to stress are well documented, the point at which opioids, sex and stress interact to influence hippocampal function remains elusive. Thus, we used quantitative immunocytochemistry in combination with light and electron microscopy for the phosphorylated MOR at the SER375 carboxy-terminal residue (pMOR) in male and female rats to assess these interactions. In both sexes, pMOR-immunoreactivity (ir) was prominent in axons and terminals and in a few neuronal somata and dendrites, some of which contained PARV in the mossy fiber pathway region of the dentate gyrus (DG) hilus and CA3 stratum lucidum. In unstressed rats, the levels of pMOR-ir in the DG or CA3 were not affected by sex or estrous cycle stage. However, immediately following 30 minutes of acute immobilization stress (AIS), males had higher levels of pMOR-ir whereas females at proestrus and estrus (high estrogen stages) had lower levels of pMOR-ir within the DG. In contrast, the number and types of neuronal profiles with pMOR-ir were not altered by AIS in either males or proestrus females. These data demonstrate that although gonadal steroids do not affect pMOR levels at resting conditions, they are differentially activated both pre- and post-synaptic MORs following stress. These interactions may contribute to the reported sex differences in hippocampally dependent behaviors in stressed animals.

Prodynorphin gene deletion increased anxiety-like behaviours, impaired the anxiolytic effect of bromazepam and altered GABAA receptor subunits gene expression in the amygdala

This study evaluated the role of prodynorphin gene in the regulation of anxiety and associated molecular mechanisms. Emotional responses were assessed using the light-dark test, elevated plus maze and social interaction tests in prodynorphin knockout and wild-type mice. Corticotrophin releasing factor and proopiomelanocortin gene expressions in the hypothalamus were evaluated after restraint stress using in situ hybridization. The anxiolytic efficacy of bromazepam and GABA(A) receptor subunits gene expression in the amygdala were also assessed in both genotypes. The deletion of prodynorphin increased anxiety-like behaviours and proopiomelanocortin gene expression in the arcuate nucleus (two-fold). Moreover, the anxiolytic action of bromazepam was significantly attenuated in the mutant mice. Decreased GABA(A)γ(2) and increased GABA(A)β(2) gene expression receptor subunits were found in the amygdala of prodynorphin knockout mice. These results indicate that deletion of prodynorphin gene is associated with increased anxiety-like behaviours, enhanced sensibility response to stress stimuli, reduced anxiolytic efficacy of bromazepam and altered expression of the GABA(A) receptor subunits.

The dynorphin/kappa opioid system as a modulator of stress-induced and pro-addictive behaviors

Stress is a complex experience that carries both aversive and motivating properties. Chronic stress causes an increase in the risk of depression, is well known to increase relapse of drug seeking behavior, and can adversely impact health. Several brain systems have been demonstrated to be critical in mediating the negative affect associated with stress, and recent evidence directly links the actions of the endogenous opioid neuropeptide dynorphin in modulating mood and increasing the rewarding effects of abused drugs. These results suggest that activation of the dynorphin/kappa opioid receptor (KOR) system is likely to play a major role in the pro-addictive effects of stress. This review explores the relationship between dynorphin and corticotropin-releasing factor (CRF) in the induction of dysphoria, the potentiation of drug seeking, and stress-induced reinstatement. We also provide an overview of the signal transduction events responsible for CRF and dynorphin/KOR-dependent behaviors. Understanding the recent work linking activation of CRF and dynorphin/KOR systems and their specific roles in brain stress systems and behavioral models of addiction provides novel insight to neuropeptide systems that regulate affective state.

Peptide kappa opioid receptor ligands: potential for drug development

While narcotic analgesics such as morphine, which act preferentially through mu opioid receptors, remain the gold standard in the treatment of severe pain, their use is limited by detrimental liabilities such as respiratory depression and drug dependence. Thus, there has been considerable interest in developing ligands for kappa opioid receptors (KOR) as potential analgesics and for the treatment of a variety of other disorders. These include effects mediated both by central receptors, such as antidepressant activity and a reduction in cocaine-seeking behavior, and activity resulting from the activation of peripheral receptors, such as analgesic and anti-inflammatory effects. While the vast majority of opioid receptor ligands that have progressed in preclinical development have been small molecules, significant advances have been made in recent years in identifying opioid peptide analogs that exhibit promising in vivo activity. This review will focus on possible therapeutic applications of ligands for KOR and specifically on the potential development of peptide ligands for these receptors.

Endogenous kappa opioid activation mediates stress-induced deficits in learning and memory

We hypothesized that mice subjected to prolonged stress would demonstrate decreased performance in a learning and memory task attributable to the endogenous activation of the kappa opioid receptor (KOR). C57BL/6J mice were tested using the novel object recognition (NOR) assay at various time points after exposure to repeated forced swim stress (FSS). Unstressed mice demonstrated recognition of the novel object at the end of a procedure using three 10-min object interaction phases, with a recognition index (RI) for the novel object of 71.7+/-3.4%. However, 1 h after exposure to FSS, vehicle-pretreated mice displayed a significant deficit in performance (RI=58.2+/-4.1%) compared with unstressed animals. NOR was still significantly reduced 4 but not 24 h after FSS. Treatment with the KOR-selective antagonist norbinaltorphimine (10 mg/kg, i.p.) prevented the decline in learning and memory performance. Moreover, direct activation of the KOR induced performance deficits in NOR, as exogenous administration of the KOR agonist U50,488 [(+/-)-trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide] (0.3 mg/kg, i.p.) suppressed NOR (RI=56.0+/-3.9%). The effect of FSS on NOR performance was further examined in mice lacking the gene for the endogenous KOR agonist dynorphin (Dyn). Dyn gene-disrupted mice exposed to FSS did not show the subsequent learning and memory deficits (RI=66.8+/-3.8%) demonstrated by their wild-type littermates (RI=49.7+/-2.9%). Overall, these results suggest that stress-induced activation of the KOR may be both necessary and sufficient to produce subsequent deficits in novel object recognition.

Effects of neonatal stress and morphine on kappa opioid receptor signaling

BACKGROUND

Critically ill neonates experience multiple stressors during hospitalization. Opioids are commonly prescribed to ameliorate their pain and stress. However, the enduring effects of stress and opioids are not understood. The kappa opioid system is important in the mediation of stress in adults, but little is known about its function in neonates.

OBJECTIVES

To characterize kappa opioid receptor (KOR) distribution in the neonatal mouse brain and test whether neonatal exposure to morphine, stress, or both, change KOR signaling.

METHODS

Five groups of wild-type C57BL/6 or prodynorphin (Pdyn) knockout mice were tested: (1) untreated control (dam-reared, no handling), (2) saline-injected control, (3) morphine-injected control, (4) stressed with saline injections and (5) stressed with morphine injections. Mice were treated from postnatal day 5 to postnatal day 9, after which their brains were immunolabeled with a phospho-specific KOR antibody (KOR-P), glial fibrillary acidic protein or glutamic acid decarboxylase.

RESULTS

There were no effects of saline or morphine injection on KOR-P immunoreactivity. Neonatal stress increased KOR-P labeling in wild-type brains (p < 0.05), but not in Pdyn(-/-) animals. Mice exposed to stress and morphine showed region-specific increases in KOR-P immunoreactivity from 38 to 500% (p < 0.05 to p < 0.001), with marked gliosis. In stressed morphine-treated Pdyn(-/-) animals, KOR-P immunoreactivity was absent, but gliosis increased compared to wild-type animals.

CONCLUSIONS

Neonatal stress increases KOR activation via the dynorphin system. Neonatal stress plus morphine treatment further increased this response and also resulted in hippocampal gliosis. Enhanced gliosis noted in Pdyn(-/-) animals suggests that the endogenous dynorphin may play a role in downregulating this inflammatory response.

Depletion of serotonin decreases the effects of the kappa-opioid receptor agonist U-69593 on cocaine-stimulated activity

Treatment with a kappa-opioid receptor agonist for 5 days decreases locomotor activity and reduces activity in response to a cocaine challenge 3 days later. In addition, chronic cocaine increases kappa-opioid receptor density, striatal dynorphin, and dynorphin gene expression in the striatum. The upregulation of kappa-opioid receptors after cocaine treatment occurs predominantly in brain regions that are highly innervated by serotonin. To determine if serotonin plays a role in the effects of kappa-opioid receptor agonists on cocaine-stimulated activity, parachloroamphetamine (PCA), which depleted serotonin by 53%-66%, or saline, was given prior to a five-day treatment with U-69593 or vehicle. Three days later each rat received a single injection of cocaine and locomotor activity was measured. Treatment with PCA had no effect on the ability of U-69593 alone to decrease locomotor activity. Thus, the behavioral effects of U-69593 alone were not dependent upon serotonin. In rats pretreated with saline, U-69593 treatment significantly blocked the locomotor-activating effects of cocaine. Following PCA pretreatment, however, there were no significant differences in locomotor activity in rats challenged with an injection of cocaine after treatment with U-69593 or vehicle. Thus, serotonin depletion prevented the long-lasting blockade of the locomotor-activating effects of cocaine subsequent to repeated administration of U-69593 but did not alter the effects of cocaine in rats that were treated with vehicle. Thus, the effects of PCA on U-69593 are not due to non-specific alterations in cocaine-induced locomotor activity. These findings suggest that serotonin plays an important role in mediating the effects of kappa-opioid receptor agonists on the behavioral response to cocaine.

Preprodynorphin mediates locomotion and D2 dopamine and mu-opioid receptor changes induced by chronic 'binge' cocaine administration

Evidence suggests that the kappa-opioid receptor (KOP-r) system plays an important role in cocaine addiction. Indeed, cocaine induces endogenous KOP activity, which is a mechanism that opposes alterations in behaviour and brain function resulting from repeated cocaine use. In this study, we have examined the influence of deletion of preprodynorphin (ppDYN) on cocaine-induced behavioural effects and on hypothalamic-pituitary-adrenal axis activity. Furthermore, we have measured mu-opioid receptor (MOP-r) agonist-stimulated [(35)S]GTPgammaS, dopamine D(1), D(2) receptor and dopamine transporter (DAT) binding. Male wild-type (WT) and ppDYN knockout (KO) mice were injected with saline or cocaine (45 mg/kg/day) in a 'binge' administration paradigm for 14 days. Chronic cocaine produced an enhancement of locomotor sensitisation in KO. No genotype effect was found on stereotypy behaviour. Cocaine-enhanced MOP-r activation in WT but not in KO. There was an overall decrease in D(2) receptor binding in cocaine-treated KO but not in WT mice. No changes were observed in D(1) and DAT binding. Cocaine increased plasma corticosterone levels in WT but not in KO. The data confirms that the endogenous KOP system inhibits dopamine neurotransmission and that ppDYN may mediate the enhancement of MOP-r activity and the activation of the hypothalamic-pituitary-adrenal axis after chronic cocaine treatment.

Disruption of the CRF/CRF1 Receptor Stress System Exacerbates the Somatic Signs of Opiate Withdrawal

Escape from the extremely stressful opiate withdrawal syndrome may motivate opiate seeking and taking. The corticotropin-releasing factor receptor-1 (CRF1) pathway mediates behavioral and endocrine responses to stress. Here, we report that genetic inactivation (CRF1-/-) as well as pharmacological antagonism of the CRF/CRF1 receptor pathway increased and prolonged the somatic expression of opiate withdrawal. Opiate-withdrawn CRF1-/- mice also showed aberrant CRF and dynorphin expression in the paraventricular nucleus of the hypothalamus (PVN) and the striatum, indicating profound impairments in stress-responsive brain circuitry. Intake of nonstressful amounts of corticosterone effectively reduced the exaggerated somatic reactions of CRF1-/- mice to opiate withdrawal. Exogenous corticosterone also restored "wild-type-like" patterns of CRF and dynorphin gene expression in the PVN and the striatum of opiate-withdrawn CRF1-/- mice, respectively. The present findings unravel a key role for the hypothalamus-pituitary-adrenal (HPA) system and brain extra-hypothalamic CRF/CRF1 receptor circuitry in somatic, molecular, and endocrine alterations induced by opiate withdrawal.

A Microdialysis Profile of Dynorphin A1-8 Release in the Rat Nucleus Accumbens Following Alcohol Administration

BACKGROUND

Pharmacological studies have implicated the endogenous opioid system in mediating alcohol intake. Other evidence has shown that alcohol administration can influence endorphinergic and enkephalinergic activity, while very few studies have examined its effect on dynorphinergic systems. The aim of the present study was to investigate the effect of alcohol administration or a mechanical stressor on extracellular levels of dynorphin A(1-8) in the rat nucleus accumbens-a brain region that plays a significant role in the processes underlying reinforcement and stress.

METHODS

Male Sprague-Dawley rats were implanted with a microdialysis probe aimed at the shell region of the nucleus accumbens. Artificial cerebrospinal fluid was pumped at a rate of 1.5 microL/min in awake and freely moving animals and the dialysate was collected at 30-minute intervals. In one experiment, following a baseline period, rats were injected intraperitoneally with either physiological saline or 1 of 3 doses of alcohol, 0.8, 1.6, or 3.2 g ethanol/kg body weight. In a second experiment, following a baseline period, rats were applied a clothespin to the base of their tail for 20 minutes. The levels of dynorphin A(1-8) in the dialysate were analyzed with solid-phase radioimmunoassay.

RESULTS

Relative to saline-treated controls, an alcohol dose of 1.6 and 3.2 g/kg caused a transient increase in the extracellular levels of dynorphin A(1-8) in the first 30 minutes of alcohol administration. However, the effect resulting from the high 3.2 g/kg dose was far more pronounced and more significant than with the moderate dose. There was no effect of tail pinch on dynorphin A(1-8) levels in the nucleus accumbens.

CONCLUSIONS

In this experiment, a very high dose of alcohol was especially capable of stimulating dynorphin A(1-8) release in the nucleus accumbens. Dynorphin release in the accumbens has been previously associated with aversive stimuli and may thus reflect a system underlying the aversive properties of high-dose alcohol administration. However, the lack of effect of tail-pinch stress in the present study suggests that dynorphin A(1-8) is not released in response to all forms of stressful/aversive stimuli.

Prior activation of kappa opioid receptors by U50,488 mimics repeated forced swim stress to potentiate cocaine place preference conditioning

Repeated forced-swim stress (FSS) produced analgesia, immobility and potentiation of cocaine-conditioned place preference (CPP) in wild-type C57Bl/6 mice, but not in littermates lacking the kappa opioid receptor (KOR) gene. These results were surprising because kappa agonists are known to produce conditioned place aversion and to suppress cocaine-CPP when coadministered with cocaine. The possibility that disruption of the kappa system blocked the stress response by adversely affecting the hypothalamic-pituitary axis was examined by measuring plasma corticosterone levels. However, disruption of the dynorphin/kappa system by gene deletion or receptor antagonism did not reduce the FSS-induced elevation of plasma corticosterone levels. A second explanation for the difference is that kappa receptor activation caused by FSS occurred prior to cocaine conditioning rather than contemporaneously. To test this hypothesis, we measured the effects of the kappa agonist (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50,488) administered to mice at various intervals preceding cocaine conditioning. The results showed that the interaction between the kappa system and cocaine reinforcement depended on the timing of the drug pairing. Mice given U50,488 60 min prior to cocaine showed a robust, nor-BNI-sensitive potentiation of cocaine-CPP, whereas administration 15 min before cocaine significantly suppressed cocaine-CPP. In the absence of cocaine, U50,488 given 60 min prior to saline conditioning produced no place preference, whereas administration 15 min before saline conditioning produced significant place aversion. The results of this study suggest that kappa receptor activation induced by FSS prior to the cocaine-conditioning session may be both necessary and sufficient for potentiation of the reinforcing actions of cocaine.

Differential effects of the novel kappa opioid receptor antagonist, JDTic, on reinstatement of cocaine-seeking induced by footshock stressors vs cocaine primes and its antidepressant-like effects in rats

RATIONALE

Stress and depression have been linked to relapse of cocaine abuse. Antagonism of the kappa opioid receptor (KOR) has been reported to attenuate some effects of stressors, and antagonism of the KOR has been reported to have antidepressant-like properties.

OBJECTIVES

Our objective was to determine whether the potent and selective KOR antagonist, (3R)-7-hydroxy-N-{(1S)-1-{[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl}-2-methylpropyl}-1,2,3,4-tetrahydro-3-isoquinoline-carboxamide (JDTic), can reduce the ability of a stressor (intermittent footshock) to reinstate cocaine-seeking behavior and to have antidepressant-like effects in the forced swim test (FST).

METHODS

Male Long-Evans hooded rats were trained to lever-press, reinforced with 0.5 mg/kg i.v. infusion of cocaine, according to fixed ratio 1 reinforcement schedules during daily 2-h experimental sessions. After performance had stabilized, lever pressing was extinguished for 12 consecutive sessions, and doses of 0 (vehicle), 3, 10, and 30 mg/kg JDTic were then administered i.g. to separate groups of 12 rats. Twenty four hours later, the rats were given 15 min of intermittent footshock (0.87 mA, 0.5 s activation time, average inter-activation interval of 40 s) or a 17-mg/kg i.p. administration of cocaine prime followed by a 2-h reinstatement test session. JDTic was also evaluated for its ability to block diuresis induced by the KOR agonist, U50,488H (10 mg/kg, s.c.), during 5-h test sessions beginning 1 h after footshock reinstatement tests to verify its KOR antagonist activity. In the FST, male Sprague-Dawley rats were treated with either nor-binaltorphimine (nor-BNI) or JDTic (both at 0.3, 1, 3, or 10 mg/kg, injected s.c. 23 h before), or desipramine (5.6, 10, or 17 mg/kg, injected i.p. 23, 5, and 1 h before) and placed in a cylinder of water, during which the predominance of immobility, swimming, and climbing were scored during 5-s intervals for 5 min.

RESULTS

The 10- and 30-mg/kg doses of JDTic significantly reduced footshock-induced reinstatement of responding previously reinforced by cocaine and significantly attenuated U50,488H-induced diuresis. In contrast, JDTic did not affect cocaine-prime-induced reinstatement. Both nor-BNI and JDTic decreased immobility and increased swimming time in the FST, similar to the antidepressant desipramine.

CONCLUSIONS

Depression and stress are two states during cocaine abstinence which users identify as precipitating relapse, and JDTic may have properties which attenuate both.

Endogenous central κ-opioid systems augment renal sympathetic nerve activity to maximally retain urinary sodium during hypotonic saline volume expansion

Intracerebroventricular injection of κ-opioid agonists produces diuresis, antinatriuresis, and a concurrent increase in renal sympathetic nerve activity (RSNA). The present study examined whether endogenous central κ-opioid systems contribute to the renal excretory responses produced by the stress of an acute hypotonic saline volume expansion (HSVE). Cardiovascular, renal excretory, and RSNA responses were measured during control, acute HSVE (5% body weight, 0.45 M saline over 30 min), and recovery (70 min) in conscious rats pretreated intracerebroventricularly with vehicle or the κ-opioid receptor antagonist nor-binaltorphimine (nor-BNI). In vehicle-pretreated rats, HSVE produced a marked increase in urine flow rate but only a low-magnitude and delayed natriuresis. RSNA was not significantly suppressed during the HSVE or recovery periods. In nor-BNI-treated rats, HSVE produced a pattern of diuresis similar to that observed in vehicle-treated rats. However, during the HSVE and recovery periods, RSNA was significantly decreased, and urinary sodium excretion increased in nor-BNI-treated animals. In other studies performed in chronic bilateral renal denervated rats, HSVE produced similar diuretic and blunted natriuretic responses in animals pretreated intracerebroventricularly with vehicle or nor-BNI. Thus removal of the renal nerves prevented nor-BNI from enhancing urinary sodium excretion during HSVE. These findings indicate that in conscious rats, endogenous central κ-opioid systems are activated during hypotonic saline volume expansion to maximize urinary sodium retention by a renal sympathoexcitatory pathway that requires intact renal nerves.

Suppressive effects of milk-derived lactoferrin on psychological stress in adult rats

Lactoferrin (LF) is known as an iron-binding glycoprotein. It has been shown that bovine LF (bLF) is transported into cerebrospinal fluid via blood although its physiological effects in the central nervous system (CNS) are still unclear. In this study, a suppressive effect of bLF on psychological distress was investigated in adult rats. Intraperitoneal injection of bLF (100 mg/kg) reduced stressful behaviors in a conditioned fear-induced freezing test and an elevated plus-maze test. Interestingly, the suppressive effect of bLF was enhanced by pretreatment with electric foot-shock (FS). This suppressive effect of bLF in the elevated plus-maze test was reversed by pretreatment with naloxone, an opioid receptor antagonist, at a dose of 1 mg/kg (ip). N(omega)-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase (NOS) inhibitor, also blocked the suppressive effect of bLF and foot-shock. In addition, combined application of a low dose of bLF (30 mg/kg, ip) and l-arginine (30 and 100 mg/kg, ip) showed significant potentiated effects on psychological stress. These results suggest that bLF has suppressive effects on psychological distress, especially under the condition of moderate stress. Furthermore, it is suggested that bLF possibly activates an endogenous opioidergic system via nitric oxide synthase activation.

Effects of the kappa opioid agonist U50,488 and the kappa opioid antagonist nor-binaltorphimine on choice between cocaine and food in rhesus monkeys

RATIONALE

Selective kappa opioid receptor agonists usually decrease cocaine self-administration in procedures that use rate-based measures of reinforcement; however, the rate-altering effects of kappa agonists complicate interpretation of these findings.

OBJECTIVES

To evaluate the effects of the selective kappa agonist U50,488 and the selective kappa antagonist nor-binaltorphimine (nor-BNI) on concurrent choice between cocaine and food in rhesus monkeys. The concurrent-choice procedure provides a rate-independent measure of the relative reinforcing effects of cocaine in comparison with food.

METHODS

Four rhesus monkeys were trained to respond under a concurrent-choice schedule for food (1-g pellets) or cocaine (0-0.1 mg/kg per injection). Saline and increasing doses of U50,488 (0.0032-0.1 mg/kg per h) were administered by pseudo-continuous i.v. infusion (one infusion every 20 min) during sequential 3-day blocks. In a separate experiment, monkeys were treated with nor-BNI (3.2 mg/kg, i.v.), and cocaine choice was re-determined during pseudo-continuous infusion with saline or U50,488 (0.1 mg/kg per h).

RESULTS

During saline treatment, cocaine maintained a dose-dependent and monotonic increase in cocaine choice. Monkeys responded primarily for food when low cocaine doses were available (0-0.01 mg/kg per injection) and primarily for cocaine when higher cocaine doses were available (0.032-0.1 mg/kg per injection). U50,488 produced a dose-dependent increase in cocaine choice, manifested as leftward shifts in the cocaine-choice, dose-effect curve. U50,488 also dose-dependently decreased overall response rates. Nor-BNI did not alter cocaine choice, but it attenuated the effects of U50,488.

CONCLUSIONS

These results suggest that continuous treatment with U50,488 produces a kappa receptor-mediated increase in the relative reinforcing effects of cocaine in comparison with food.

Kappa opioid receptor antagonism and prodynorphin gene disruption block stress-induced behavioral responses

Previous studies have demonstrated that stress may increase prodynorphin gene expression, and kappa opioid agonists suppress drug reward. Therefore, we tested the hypothesis that stress-induced release of endogenous dynorphin may mediate behavioral responses to stress and oppose the rewarding effects of cocaine. C57Bl/6 mice subjected to repeated forced swim testing (FST) using a modified Porsolt procedure at 30 degrees C showed a characteristic stress-induced immobility response and a stress-induced analgesia observed with a tail withdrawal latency assay. Pretreatment with the kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI; 10 mg/kg, i.p.) blocked the stress-induced analgesia and significantly reduced the stress-induced immobility. The nor-BNI sensitivity of the behavioral responses suggests an activation of the kappa opioid receptor by a stress-induced release of dynorphin peptides. Supporting this hypothesis, transgenic mice possessing a disrupted prodynorphin gene showed no increase in immobility or stress-induced analgesia after exposure to repeated FST. Because both stress and the kappa opioid system can modulate the response to drugs of abuse, we tested the effects of forced swim stress on cocaine-conditioned place preference (CPP). FST-exposed mice conditioned with cocaine (15 mg/kg, s.c.) showed significant potentiation of place preference for the drug-paired chamber over the responses of unstressed mice. Surprisingly, nor-BNI pretreatment blocked stress-induced potentiation of cocaine CPP. Consistent with this result, mice lacking the prodynorphin gene did not show a stress-induced potentiation of cocaine CPP, whereas wild-type littermates did. The findings suggest that chronic swim stress may activate the kappa opioid system to produce analgesia, immobility, and potentiation of the acute rewarding properties of cocaine in C57Bl/6 mice.

Developmental changes in opioid peptides and their receptors in Cpe(fat)/Cpe(fat) mice lacking peptide processing enzyme carboxypeptidase E

Carboxypeptidase E (CPE) is involved in the biosynthesis of a number of neuropeptides including opioid peptides. A point mutation in this gene results in a loss of enzyme activity, decrease in mature neuroendocrine peptides, and development of late onset obesity as seen in Cpe(fat)/Cpe(fat) mice. In this study, we examined the processing of peptides derived from prodynorphin and proenkephalin in various brain regions of these mice during development. At 6 to 8 weeks, an age prior to the onset of obesity, levels of dynorphin peptides are decreased in all brain regions, whereas levels of ir-Met-enkephalin are differentially altered. There is an accumulation of C-terminally extended forms of all three opioid peptides in Cpe(fat)/Cpe(fat) mice, consistent with a lack of CPE activity. Thus, it appears that there is no direct correlation between the level of mature opioid peptides and the development of obesity in these mice. Since altered levels of peptides can influence the opioid receptor system, we examined the functional activity of mu and kappa opioid receptors using [35S]guanosine-5'-O-(gamma-thio)-triphosphate binding assays. We find no differences in kappa receptor activity in Cpe(fat)/Cpe(fat) compared with control littermate mice. In contrast, the mu receptor activity is differentially altered in select regions of Cpe(fat)/Cpe(fat) mice in response to a mu-specific ligand. Taken together, these results suggest that the lack of CPE activity leads to alterations in the level of opioid peptides during development and that changes in peptide levels differentially affect opioid receptor activity in vivo.

Regulation of dynorphin gene expression by kappa-opioid agonist treatment

The effects of K-opioid agonist treatment on prodynorphin mRNA expression in the rat brain were studied. Rats were treated with the selective kappa-opioid agonist U-69593 or vehicle for 5 days and prodynorphin mRNA was measured on day 8 (3 days after the last injection) or 22 (17 days after the last injection). On day 8 prodynorphin mRNA was increased in the hypothalamus and decreased in the striatum, frontal cortex, and hippocampus of rats treated with U-69593. On day 22, prodynorphin mRNA was increased in the hypothalamus, frontal cortex and striatum of U-69593 treated rats. These findings suggests that kappa-opioid receptor agonist treatment has long-term, continually changing effects on prodynorphin mRNA expression.

Alcohol reinforcement and voluntary ethanol consumption

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The organizer/chair was Ting-Kai Li and the co-chair was Rainer Spanagel. The presentations were (1) Genetic differences in alcohol drinking and reinforcement: The sP and sNP Rats, by Giancarlo Colombo; (2) Ventral tegmental area-Neuroanatomical substrate for alcohol reinforcement, by William J. McBride; (3) Metabolic mapping of alcohol reinforcement, by Linda J. Porrino; (4) Role of opioid receptors in the ethanol-induced place preference in rats exposed to conditioned fear stress, by Tsutomu Suzuki; and (5) Repeated deprivations enhance the reinforcing properties of ethanol in alcohol preferring (P) rats, by Zachary A. Rodd-Henricks.