Dynorphin, stress, and depression

Corticotropin-releasing factor (CRF)-induced disruption of attention in rats is blocked by the κ-opioid receptor antagonist JDTic

Stress often disrupts behavior and can lead to psychiatric illness. Considerable evidence suggests that corticotropin-releasing factor (CRF) plays an important role in regulating the effects of stress. CRF administration produces stress-like effects in humans and laboratory animals, and CRF levels are elevated in individuals with stress-related illness. Recent work indicates that κ-opioid receptor (KOR) antagonists can block CRF effects, raising the possibility that at least some of the effects of stress are mediated via KORs. Here we examined the effects of CRF on performance in the 5-choice serial reaction time task (5CSRTT), a test used to quantify attention in rodents, as well as functional interactions between CRF and KORs. Male Sprague-Dawley rats were trained in the 5CSRTT and then each was implanted with an intracerebroventricular (ICV) cannula. After recovery and restabilization of performance, they received a single intraperitoneal (IP) injection of vehicle or JDTic (10 mg/kg), a KOR antagonist with long-lasting (>14 days) effects. In subsequent sessions, rats received ICV infusions of CRF (0.25-1.0 μg) or vehicle and were tested 60 min later. CRF dose-dependently disrupted performance as reflected by decreases in correct responding, increases in omission errors, increases in latencies to respond correctly, and increases in time to complete the session. JDTic attenuated each of these CRF-induced deficits while having no effects on its own. The persistent ability of JDTic to disrupt KOR function was confirmed using the tail immersion assay. These findings indicate that KOR antagonists can prevent acute stress-related effects that degrade performance in tasks requiring attention.

The permeation of dynorphin A 1-6 across the blood brain barrier and its effect on bovine brain microvessel endothelial cell monolayer permeability

Dynorphin A 1-17 (Dyn A 1-17) is an endogenous neuropeptide known to act at the kappa opioid receptor; it has been implicated in a number of neurological disorders, including neuropathic pain, stress, depression, and Alzheimer's and Parkinson's diseases. The investigation of Dyn A 1-17 metabolism at the blood-brain barrier (BBB) is important since the metabolites exhibit unique biological functions compared to the parent compound. In this work, Dyn A 1-6 is identified as a metabolite of Dyn A 1-17 in the presence of bovine brain microvessel endhothelial cells (BBMECs), using LC-MS/MS. The transport of Dyn A 1-6 at the BBB was examined using this in vitro cell culture model of the BBB. Furthermore, the permeation of the BBB by the low molecular weight permeability marker fluorescein was characterized in the presence and absences of Dyn A 1-6.

Repeated stress dysregulates κ-opioid receptor signaling in the dorsal raphe through a p38α MAPK-dependent mechanism

Repeated stress releases dynorphins and causes subsequent activation of κ-opioid receptors (KORs) in limbic brain regions. The serotonergic dorsal raphe nucleus (DRN) has previously been found to be an important site of action for the dysphoric effects of dynorphin-κ-opioid receptor system activation during stress-evoked behaviors, and KOR-induced activation of p38α mitogen-activated protein kinase (MAPK) in serotonergic neurons was found to be a critical mediator of the aversive properties of stress. Yet, how dynorphins and KORs functionally regulate the excitability of serotonergic DRN neurons both in adaptive and pathological stress states is poorly understood. Here we report that acute KOR activation by the selective agonist U69,593 [(+)-(5α,7α,8β)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]benzeneacetamide] inhibits serotonergic neuronal excitability within the DRN through both presynaptic inhibition of excitatory synaptic transmission and postsynaptic activation of G-protein-gated inwardly rectifying potassium channels (GIRKs) electrophysiologically recorded in brain slices. C57BL/6 mice subjected to repeated swim, stress sessions had significantly reduced KOR-mediated GIRK currents recorded in serotonergic neurons in DRN postsynaptically, without significantly affecting presynaptic KOR-mediated regulation of excitatory transmission. This effect was blocked by genetic excision of p38α MAPK selectively from serotonergic neurons. An increase in phospho-immunoreactivity suggests that this functional dysregulation may be a consequence of tyrosine phosphorylation of GIRK (K(IR)3.1) channels. These data elucidate a mechanism for stress-induced dysregulation of the excitability of neurons in the DRN and identify a functional target of stress-induced p38α MAPK activation that may underlie some of the negative effects of pathological stress exposure.

Effects of a post-shock injection of the kappa opioid receptor antagonist norbinaltorphimine (norBNI) on fear and anxiety in rats

Exposure of rats to footshocks leads to an enduring behavioral state involving generalized fear responses and avoidance. Recent evidence suggests that the expression of negative emotional behaviors produced by a stressor is in part mediated by dynorphin and its main receptor, the kappa opioid receptor (KOR). The purpose of this study was to determine if a subcutaneous injection of the long-acting KOR antagonist norbinaltorphimine (norBNI; 15.0 and 30.0 mg/kg) given 2 days after an acute exposure of rats to footshooks (5×2 s episodes of 1.5 mA delivered over 5 min) attenuates the expression of lasting fear and anxiety. We report that exposure of rats to acute footshock produced long-lasting (>4 weeks) fear (freezing) and anxiety (avoidance of an open area in the defensive withdrawal test). The 30 mg dose of norBNI attenuated the fear expressed when shock rats were placed in the shock context at Day 9 but not Day 27 post-shock. The same dose of norBNI had no effect on the expression of generalized fear produced when shock rats were placed in a novel chamber at Days 8 and 24. In contrast, the 30 mg dose of norBNI produced consistent anxiolytic effects in shock and nonshock rats. First, the 30 mg dose was found to decrease the latency to enter the open field in the defensive withdrawal test done 30 days after the shock exposure. Second, the same high dose also had anxiolytic effects in both nonshock and shock rats as evidence by a decrease in the mean time spent in the withdrawal box. The present study shows that systemic injection of the KOR antagonist norBNI had mixed effect on fear. In contrast, norBNI had an anxiolytic effect which included the attenuation of the enhanced avoidance of a novel area produced by a prior shock experience.

Dynorphin--still an extraordinarily potent opioid peptide

This issue of Molecular Pharmacology is dedicated to Dr. Avram Goldstein, the journal's founding editor and one of the leaders in the development of modern pharmacology. This article focuses on his contributions to the discovery of the dynorphins and evidence that members of this family of opioid peptides are endogenous agonists for the kappa opioid receptor. In his original publication describing the purification and sequencing of dynorphin A, Avram described this peptide as "extraordinarily potent" ("dyn" from the Greek, dynamis = power and "orphin" for endogenous morphine peptide). The name originally referred to its high affinity and great potency in the bioassay that was used to follow its activity during purification, but the name has come to have a second meaning: studies of its physiologic function in brain continue to provide powerful insights to the molecular mechanisms controlling mood disorders and drug addiction. During the 30 years since its discovery, we have learned that the dynorphin peptides are released in brain during stress exposure. After they are released, they activate kappa opioid receptors distributed throughout the brain and spinal cord, where they trigger cellular responses resulting in different stress responses: analgesia, dysphoria-like behaviors, anxiety-like responses, and increased addiction behaviors in experimental animals. Avram predicted that a detailed molecular analysis of opiate drug actions would someday lead to better treatments for drug addiction, and he would be gratified to know that subsequent studies enabled by his discovery of the dynorphins resulted in insights that hold great promise for new treatments for addiction and depressive disorders.

Discovery and pharmacological evaluation of a diphenethylamine derivative (HS665), a highly potent and selective κ opioid receptor agonist

Here we report on the design, synthesis, and biological characterization of novel κ opioid (KOP) receptor ligands of diphenethylamines. In opioid receptor binding and functional assays, the N-cyclobutylmethyl substituted derivative 4 (HS665) showed the highest affinity and selectivity for the KOP receptor and KOP agonist potency. Compound 4 inhibited acetic acid induced writhing after subcutaneous administration in mice via KOP receptor-mediated mechanisms, being equipotent as an analgesic to the KOP agonist U50,488.

Alcohol-induced plasticity in the dynorphin/kappa-opioid receptor system

Alcoholism is a chronic relapsing disorder characterized by continued alcohol use despite numerous adverse consequences. Alcohol has been shown to interact with numerous neurotransmitter systems to exert its pharmacological effects. The endogenous opioid system (EOS) has been strongly implicated in the positive and negative reinforcing effects of alcohol. Traditionally recognized as dysphoric/anhedonic in nature, the dynorphin/kappa-opioid receptor (DYN/KOR) system has recently received considerable attention due to evidence suggesting that an upregulated DYN/KOR system may be a critical contributor to the complex factors that result in escalated alcohol consumption once dependent. The present review will discuss alcohol-induced plasticity in the DYN/KOR system and how these neuroadaptations could contribute to excessive alcohol seeking and consumption.

Dynorphins regulate fear memory: from mice to men

Reexposure to trauma reminders is an integral element of trauma-focused cognitive behavioral therapy (Roberts et al., 2009), but little is known about the physiological processes underlying the therapeutic progress. While it is well established that amygdala, prefrontal cortex and hippocampus are key brain structures in fear memory processing (McGaugh, 2004; Herry et al., 2008; Likhtik et al., 2008), it is not well known which neurotransmitters or neuromodulators are involved. Here with a translational approach we investigated the role of dynorphins in the formation and extinction of fear memories in mice and in humans. Mice lacking dynorphin showed an enhanced cue-dependent fear conditioning, as well as delayed extinction in contextual conditioning/extinction paradigms. The pharmacological blockade of κ-opioid receptors before the extinction trials but not before or after the conditioning produced a similar effect. Analysis of neuronal activity, using the immediate early gene c-fos, demonstrated a reduced neuronal activity in key limbic structures during extinction in the absence of dynorphin. Translating these findings into the human domain, fear conditioning and extinction, coupled with functional MRI was then performed in volunteers preselected for a functionally relevant polymorphism in the dynorphin gene. Human volunteers bearing the (T) allele of PDYN (prodynorphin) at rs1997794 showed reduced fear extinction and a significantly diminished functional connectivity between amygdala and ventromedial prefrontal cortex. Our findings establish a role of dynorphin κ-opioid receptor signaling in fear extinction.

Alcohol dependence as a chronic pain disorder

Dysregulation of pain neurocircuitry and neurochemistry has been increasingly recognized as playing a critical role in a diverse spectrum of diseases including migraine, fibromyalgia, depression, and PTSD. Evidence presented here supports the hypothesis that alcohol dependence is among the pathologies arising from aberrant neurobiological substrates of pain. In this review, we explore the possible influence of alcohol analgesia and hyperalgesia in promoting alcohol misuse and dependence. We examine evidence that neuroanatomical sites involved in the negative emotional states of alcohol dependence also play an important role in pain transmission and may be functionally altered under chronic pain conditions. We also consider possible genetic links between pain transmission and alcohol dependence. We propose an allostatic load model in which episodes of alcohol intoxication and withdrawal, traumatic stressors, and injury are each capable of dysregulating an overlapping set of neural substrates to engender sensory and affective pain states that are integral to alcohol dependence and comorbid conditions such as anxiety, depression, and chronic pain.

Increasing the validity of experimental models for depression

Major depressive disorder (MDD) is a central nervous system disorder characterized by the culmination of profound disturbances in mood and affective regulation. Animal models serve as a powerful tool for investigating the neurobiological mechanisms underlying this disorder; however, little standardization exists across the wide range of available modeling approaches most often employed. This review will illustrate some of the most challenging obstacles faced by investigators attempting to associate depressive-like behaviors in rodents with symptoms expressed in MDD. Furthermore, a novel series of depressive-like criteria based on correlating behavioral endophenotypes, novel in vivo neurophysiological measurements, and molecular/cellular analyses within multiple brain are proposed as a potential solution to overcoming this barrier. Ultimately, linking the neurophysiological and cellular/biochemical actions that contribute to the expression of a defined MDD-like syndrome will dramatically extend the translational value of the most valid animal models of MDD.

Lateralized response of dynorphin a peptide levels after traumatic brain injury

Traumatic brain injury (TBI) induces a cascade of primary and secondary events resulting in impairment of neuronal networks that eventually determines clinical outcome. The dynorphins, endogenous opioid peptides, have been implicated in secondary injury and neurodegeneration in rodent and human brain. To gain insight into the role of dynorphins in the brain's response to trauma, we analyzed short-term (1-day) and long-term (7-day) changes in dynorphin A (Dyn A) levels in the frontal cortex, hippocampus, and striatum, induced by unilateral left-side or right-side cortical TBI in mice. The effects of TBI were significantly different from those of sham surgery (Sham), while the sham surgery also produced noticeable effects. Both sham and TBI induced short-term changes and long-term changes in all three regions. Two types of responses were generally observed. In the hippocampus, Dyn A levels were predominantly altered ipsilateral to the injury. In the striatum and frontal cortex, injury to the right (R) hemisphere affected Dyn A levels to a greater extent than that seen in the left (L) hemisphere. The R-TBI but not L-TBI produced Dyn A changes in the striatum and frontal cortex at 7 days after injury. Effects of the R-side injury were similar in the two hemispheres. In naive animals, Dyn A was symmetrically distributed between the two hemispheres. Thus, trauma may reveal a lateralization in the mechanism mediating the response of Dyn A-expressing neuronal networks in the brain. These networks may differentially mediate effects of left and right brain injury on lateralized brain functions.

Non-opioid nociceptive activity of human dynorphin mutants that cause neurodegenerative disorder spinocerebellar ataxia type 23

We previously identified four missense mutations in the prodynorphin gene that cause human neurodegenerative disorder spinocerebellar ataxia type 23 (SCA23). Three mutations substitute Leu(5), Arg(6), and Arg(9) to Ser (L5S), Trp (R6W) and Cys (R9C) in dynorphin A(1-17) (Dyn A), a peptide with both opioid activities and non-opioid neurodegenerative actions. It has been reported that Dyn A administered intrathecally (i.t.) in femtomolar doses into mice produces nociceptive behaviors consisting of hindlimb scratching along with biting and licking of the hindpaw and tail (SBL responses) through a non-opioid mechanism. We here evaluated the potential of the three mutant peptides to produce similar behaviors. Compared to the wild type (WT)-peptide, the relative potency of Dyn A R6W, L5S and R9C peptides for SBL responses was 50-, 33- and 2-fold higher, and Dyn A R6W and L5S induced the SBL responses at a 10-30-fold lower doses. Dyn A R6W was the most potent peptide. The SBL responses induced by Dyn A R6W were dose dependently inhibited by morphine (i.p.; 0.1-1 mg/kg) or MK-801, an NMDA ion channel blocker (i.t. co-administration; 5-7.5 nmol). CP-99,994, a tachykinin NK1 receptor antagonist (i.t. co-administration; 2 nmol) and naloxone (i.p.; 5 mg/kg) failed to block effects of Dyn A R6W. Thus, similarly to Dyn A WT, the SBL responses induced by Dyn A R6W may involve the NMDA receptor but are not mediated through the opioid and tachykinin NK1 receptors. Enhanced non-opioid excitatory activities of Dyn A mutants may underlie in part development of SCA23.

Long-acting κ opioid antagonists nor-BNI, GNTI and JDTic: pharmacokinetics in mice and lipophilicity

Background

Nor-BNI, GNTI and JDTic induce κ opioid antagonism that is delayed by hours and can persist for months. Other effects are transient. It has been proposed that these drugs may be slowly absorbed or distributed, and may dissolve in cell membranes, thus slowing elimination and prolonging their effects. Recent evidence suggests, instead, that they induce prolonged desensitization of the κ opioid receptor.

Methods

To evaluate these hypotheses, we measured relevant physicochemical properties of nor-BNI, GNTI and JDTic, and the timecourse of brain and plasma concentrations in mice after intraperitoneal administration (using LC-MS-MS).

Results

In each case, plasma levels were maximal within 30 min and declined by >80% within four hours, correlating well with previously reported transient effects. A strong negative correlation was observed between plasma levels and the delayed, prolonged timecourse of κ antagonism. Brain levels of nor-BNI and JDTic peaked within 30 min, but while nor-BNI was largely eliminated within hours, JDTic declined gradually over a week. Brain uptake of GNTI was too low to measure accurately, and higher doses proved lethal. None of the drugs were highly lipophilic, showing high water solubility (> 45 mM) and low distribution into octanol (log D_7.4 < 2). Brain homogenate binding was within the range of many shorter-acting drugs (>7% unbound). JDTic showed P-gp-mediated efflux; nor- BNI and GNTI did not, but their low unbound brain uptake suggests efflux by another mechanism.

Conclusions

The negative plasma concentration-effect relationship we observed is difficult to reconcile with simple competitive antagonism, but is consistent with desensitization. The very slow elimination of JDTic from brain is surprising given that it undergoes active efflux, has modest affinity for homogenate, and has a shorter duration of action than nor-BNI under these conditions. We propose that this persistence may result from entrapment in cellular compartments such as lysosomes.

The kappa opioid receptor antagonist JDTic attenuates alcohol seeking and withdrawal anxiety

The role of kappa-opioid receptors (KOR) in the regulation of alcohol-related behaviors is not completely understood. For example, alcohol consumption has been reported to increase following treatment with KOR antagonists in rats, but was decreased in mice with genetic deletion of KOR. Recent studies have further suggested that KOR antagonists may selectively decrease alcohol self-administration in rats following a history of dependence. We assessed the effects of the KOR antagonist JDTic on alcohol self-administration, reinstatement of alcohol seeking induced by alcohol-associated cues or stress, and acute alcohol withdrawal-induced anxiety ('hangover anxiety'). JDTic dose-dependently reversed hangover anxiety when given 48 hours prior to testing, a time interval corresponding to the previously demonstrated anxiolytic efficacy of this drug. In contrast, JDTic decreased alcohol self-administration and cue-induced reinstatement of alcohol seeking when administered 2 hours prior to testing, but not at longer pre-treatment times. For comparison, we determined that the prototypical KOR antagonist nor-binaltorphimine can suppress self-administration of alcohol at 2 hours pre-treatment time, mimicking our observations with JDTic. The effects of JDTic were behaviorally specific, as it had no effect on stress-induced reinstatement of alcohol seeking, self-administration of sucrose, or locomotor activity. Further, we demonstrate that at a 2 hours pre-treatment time JDTic antagonized the antinociceptive effects of the KOR agonist U50,488H but had no effect on morphine-induced behaviors. Our results provide additional evidence for the involvement of KOR in regulation of alcohol-related behaviors and provide support for KOR antagonists, including JDTic, to be evaluated as medications for alcoholism.

The genetics of the opioid system and specific drug addictions

Addiction to drugs is a chronic, relapsing brain disease that has major medical, social, and economic complications. It has been established that genetic factors contribute to the vulnerability to develop drug addiction and to the effectiveness of its treatment. Identification of these factors may increase our understanding of the disorders, help in the development of new treatments and advance personalized medicine. In this review, we will describe the genetics of the major genes of the opioid system (opioid receptors and their endogenous ligands) in connection to addiction to opioids, cocaine, alcohol and methamphetamines. Particular emphasis is given to association and functional studies of specific variants. We will provide information on the sample populations and the size of each study, as well as a list of the variants implicated in association with addiction-related phenotypes, and with the effectiveness of pharmacotherapy for addiction.

Presynaptic inhibition of gamma-aminobutyric acid release in the bed nucleus of the stria terminalis by kappa opioid receptor signaling

BACKGROUND

The kappa opioid receptor (KOR) and its endogenous agonist, the neuropeptide dynorphin, are a critical component of the central stress system. Both dynorphin and KOR are expressed in the bed nucleus of the stria terminalis (BNST), a brain region associated with anxiety and stress. This suggests that KOR activation in this region may play a role in the regulation of emotional behaviors. To date, however, there has been no investigation of the ability of KOR to modulate synaptic transmission in the BNST.

METHODS

We used whole-cell patch-clamp recordings from acutely prepared mouse brain slices to examine the actions of KOR on inhibitory transmission in the BNST. Additionally, we used neurochemical and pathway-specific optogenetic manipulations to selectively stimulate gamma-aminobutyric acid (GABA)ergic fibers from the central nucleus of the amygdala (CeA) to the BNST.

RESULTS

We found that activation of KOR reduced GABAergic transmission through a presynaptic mechanism. Furthermore, we examined the signal transduction pathways that mediate this inhibition and provide the first functional information implicating extracellular signal-regulated kinase in KOR-mediated presynaptic modulation. Moreover, we found that at KOR signaling robustly reduced inhibitory synaptic transmission in the CeA to BNST pathway.

CONCLUSIONS

Together, these results demonstrate that KOR provides important inhibitory control over presynaptic GABAergic signaling within the BNST and provides the first direct functional demonstration of KOR-sensitive long-range GABAergic connections between the CeA and the BNST.

Dynorphin regulates the phagocytic activity of splenic phagocytes in wall lizards: involvement of a κ-opioid receptor-coupled adenylate-cyclase-cAMP-PKA pathway

This in vitro study of the wall lizard Hemidactylus flaviviridis demonstrates the role of the opioid peptide dynorphin A((1-17)) [dyn A((1-17))] in the regulation of the phagocytic activity of splenic phagocytes. Dyn A((1-17)) in a concentration-dependent manner inhibited the phagocytic activity, and the maximum inhibition was recorded at a concentration of 10(-9) mol l(-1). To explore the receptor-mediated effect of dyn A((1-17)), cells were treated simultaneously with the non-selective opioid receptor blocker naltrexone and dyn A((1-17)). Naltrexone completely blocked the inhibitory effect of dyn A((1-17)) on phagocytosis. Moreover, the involvement of selective opioid receptors was investigated using selective opioid receptor antagonists. CTAP and naltrindole, selective μ- and δ-opioid receptor blockers, respectively, failed to block the inhibitory effect of dyn A((1-17)) on phagocytosis. However, the selective κ-opioid receptor blocker NorBNI completely antagonized the inhibitory effect of dyn A((1-17)). Regarding the κ-opioid receptor-coupled downstream signaling cascade, the adenylate cyclase (AC) inhibitor SQ 22536 and protein kinase A (PKA) inhibitor H-89 decreased the inhibitory effect of dyn A((1-17)) on phagocytosis. Furthermore, treatment with dyn A((1-17)) caused an increase in intracellular cAMP content in splenic phagocytes. Thus, it can be concluded that, in H. flaviviridis, dyn A((1-17)) negatively regulates the phagocytic activity of splenic phagocytes by acting through κ-opioid receptors that are coupled with the AC-cAMP-PKA signal transduction mechanism.

The long-lasting effects of JDTic, a kappa opioid receptor antagonist, on the expression of ethanol-seeking behavior and the relapse drinking of female alcohol-preferring (P) rats

The current study assessed the effects of the selective kappa opioid antagonist JDTic on alcohol (EtOH)-seeking behavior, EtOH relapse, and maintenance responding for EtOH. Adult alcohol-preferring (P) rats were trained in 2-lever operant chambers to self-administer 15% EtOH (v/v) on a fixed-ratio 5 (FR-5) and water on a FR-1 schedule of reinforcement during 1-hr sessions. After 10 weeks, rats underwent extinction training for seven sessions. Rats were then maintained in their home cages for 3 weeks without EtOH access. All rats received an injection (s.c.) of 0, 1, 3, or 10 mg/kg JDTic (n=11-14/group) after the first week of the home cage period. Rats were then tested using the Pavlovian Spontaneous Recovery paradigm (PSR; an animal model of alcohol-seeking) for four sessions during which, responses on the EtOH and water levers were recorded but did not produce their respective reinforcer. Following PSR testing rats were returned to their home cages without access to EtOH for one week prior to the start of EtOH relapse testing. To examine EtOH relapse responding, rats were returned to the operant chambers and the EtOH (FR5) and water (FR1) levers were active. Finally, rats were then tested over 17 operant sessions to assess the effects of JDTic on maintenance responding for EtOH. Rats received 0, 1, 3, or 10 mg/kg JDTic (counterbalanced from the initial experiment) 30 minutes prior to the initial maintenance session. JDTic administered 14 and 25 days prior to testing dose-dependently reduced the expression of an EtOH PSR and relapse responding. In contrast, JDTic did not alter EtOH responding under maintenance conditions. Overall, the results of this study indicate that different mechanisms mediate EtOH self-administration under relapse and maintenance conditions and kappa opioid receptors are involved in mediating EtOH-seeking behavior and relapse responding but not on-going EtOH self-administration.

Photoactivatable neuropeptides for spatiotemporally precise delivery of opioids in neural tissue

Neuropeptides activate G protein-coupled receptors to acutely modulate cellular excitability and synaptic transmission. However, due to the lack of reagents for precise delivery of peptides within dense brain tissue, the spatiotemporal scale over which neuropeptides act is unknown. To achieve rapid and spatially delimited delivery of neuropeptides in mammalian brain tissue, we developed photoactivatable analogs of two opioids: [Leu⁵]-enkephalin (LE) and the 8 amino acid form of Dynorphin A (Dyn-8). These peptides are functionally inactive prior to photolysis, and exposure to ultraviolet (UV) light causes clean release of LE and Dyn-8. Recordings from acute slices of rat locus coeruleus (LC) demonstrated that photorelease of LE activates mu opioid receptor-coupled K+ channels with kinetics that approach the limits imposed by G protein-mediated signaling. Temporally precise and spatially delimited photorelease revealed the kinetics and ionic nature of the mu opioid response and the mechanisms that determine the spatial profile of enkephalinergic volume transmission in LC.

Molecular mechanisms of opioid receptor-dependent signaling and behavior

Opioid receptors have been targeted for the treatment of pain and related disorders for thousands of years and remain the most widely used analgesics in the clinic. Mu (μ), kappa (κ), and delta (δ) opioid receptors represent the originally classified receptor subtypes, with opioid receptor like-1 (ORL1) being the least characterized. All four receptors are G-protein coupled and activate inhibitory G proteins. These receptors form homo- and heterodimeric complexes and signal to kinase cascades and scaffold a variety of proteins.The authors discuss classic mechanisms and developments in understanding opioid tolerance and opioid receptor signaling and highlight advances in opioid molecular pharmacology, behavioral pharmacology, and human genetics. The authors put into context how opioid receptor signaling leads to the modulation of behavior with the potential for therapeutic intervention. Finally, the authors conclude there is a continued need for more translational work on opioid receptors in vivo.

ΔFosB enhances the rewarding effects of cocaine while reducing the pro-depressive effects of the kappa-opioid receptor agonist U50488

BACKGROUND

Elevated expression of the transcription factor ΔFosB accompanies repeated exposure to drugs of abuse, particularly in brain areas associated with reward and motivation (e.g., nucleus accumbens). The persistent effects of ΔFosB on target genes might play an important role in the development and expression of behavioral adaptations that characterize addiction. This study examines how ΔFosB influences the responsiveness of the brain reward system to rewarding and aversive drugs.

METHODS

We used the intracranial self-stimulation paradigm to assess the effects of cocaine in transgenic mice with inducible overexpression of ΔFosB in striatal regions (including nucleus accumbens and dorsal striatum). Mice implanted with lateral hypothalamic stimulating electrodes were trained with the "rate-frequency" procedure for intracranial self-stimulation to determine the frequency at which stimulation becomes rewarding (threshold).

RESULTS

A dose-effect analysis of cocaine effects revealed that mice overexpressing ΔFosB show increased sensitivity to the rewarding (threshold-lowering) effects of the drug, compared with littermate control subjects. Interestingly, mice overexpressing ΔFosB were also less sensitive to the pro-depressive (threshold-elevating) effects of U50488, a kappa-opioid agonist known to induce dysphoria and stress-like effects in rodents.

CONCLUSIONS

These data suggest that induction of ΔFosB in striatal regions has two important behavioral consequences-increased sensitivity to drug reward, and reduced sensitivity to aversion-producing a complex phenotype that shows signs of vulnerability to addiction as well as resilience to stress.

Blockade of kappa opioid receptors attenuates the development of depressive-like behaviors induced by cocaine withdrawal in rats

Drug dependence is characterized by dysregulation of brain reward systems and increased sensitivity to stress. Chronic exposure to drugs of abuse is associated with increased expression of the neuropeptide dynorphin, the endogenous ligand for kappa opioid receptors (KORs). Activation of KORs causes depressive- and aversive-like responses in rodents, raising the possibility that drug-induced upregulation of dynorphin plays a role independence-associated negative states. Here we used "binge" exposure to cocaine (3 daily intraperitoneal injections of 15 mg/kg for 14 days) to examine the development of dependence-like behavior in the intracranial self-stimulation (ICSS) test and the forced swim test (FST). When rats were tested 1 h before their first scheduled injection of each day-a period of drug withdrawal corresponding to 20 h after their last injection on the previous day-there were exposure-dependent increases in ICSS thresholds (a putative indicator of anhedonia) and decreases in latencies to immobility in the FST (a putative indicator of behavioral despair). Administration of the long-lasting KOR antagonist norBNI (20 μg, intracerebroventricular) before the beginning of the binge regimen attenuated the development of cocaine withdrawal-induced anhedonia in the ICSS test. In contrast, administration of norBNI in the midst of the binge regimen had no effect on expression of cocaine withdrawal-induced anhedonia in the ICSS test, although it did attenuate despair-like behavior in the FST. These data suggest that blockade of KORs before exposure to a stressor (in this case, cocaine withdrawal or forced swimming) can attenuate the development of stress-induced behavioral adaptations. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Differential signaling properties at the kappa opioid receptor of 12-epi-salvinorin A and its analogues

The kappa opioid receptor (KOPR) has been identified as a potential drug target to prevent or alter the course of mood, anxiety and addictive disorders or reduce response to stress. In a search for highly potent and selective KOPR partial agonists as pharmacological tools, we have modified 12-epi-salvinorin A, a compound which we have previously observed to be a KOPR partial agonist. Five analogues of 12-epi-salvinorin A were synthesized and their effects on G protein activation as well as β-arrestin2 recruitment were evaluated. Only 12-epi-salvinorin A (1) partially activated signaling through G proteins, yet acted as a full agonist in the β-arrestin 2 DiscoveRx assay. Other salvinorin analogues tested in these functional assays were full agonists in both assays of KOPR activation. By comparison, the non-selective opioid ligand nalbuphine, known to be a partial agonist for G-protein activation, was also a partial agonist for the β-arrestin mediated signaling pathway activated through KOPR.

Naloxone-induced cortisol predicts mu opioid receptor binding potential in specific brain regions of healthy subjects

Investigators have administered the opioid receptor antagonist, naloxone, to interrogate the hypothalamic-pituitary-adrenal (HPA) axis response under the assumption that this technique provides a measure of endogenous opioid activity. However it has never been tested whether provocation of the HPA axis with naloxone provides a surrogate marker for direct measurement of endogenous opioid activity using PET imaging as the gold standard. To test this hypothesis, eighteen healthy subjects underwent a PET scan with the mu-opioid receptor (MOR) selective ligand [(11)C]carfentanil (CFN). The following day ACTH and cortisol responses were assessed using a technique which allows administration of 5 incremental doses of naloxone (0, 25, 50, 100 and 250μg/kg) in a single session. Relationships between ACTH and cortisol responses and [(11)C]CFN binding potential (BP(ND)) were examined in 5 brain regions involved in the regulation of the HPA axis and/or regions with high concentrations of MOR. All subjects mounted graded ACTH and cortisol responses to naloxone administrations. There were significant negative relationships between cortisol response to naloxone and [(11)C]CFN BP(ND) in ventral striatum, putamen and caudate. When sex and smoking were added as covariates to the model, these correlations were strengthened and there was a significant correlation with the hypothalamus. There were no significant correlations between ACTH and any volumes of interest. The opioid receptor antagonist naloxone is not merely a non-specific pharmacologic activator of the HPA axis; it provides information about individual differences in opioid receptor availability.

Might the inability to feel pleasure (anhedonia) explain the symptoms of major depression and schizophrenia, including unmotivated anxiety, delusions and hallucinations?

Inability to enjoy normally pleasurable experiences (anhedonia) is a symptom common both to major depression and schizophrenia. It also regularly accompanies and follows stress, and its presence in the two mental illnesses could depend on the fact that both are facilitated and often preceded by stressful events. Anhedonia might possibly accompany stress because the loss of the pleasure of aiming for a goal and achieving it (including defending oneself and escaping from a danger) could lead to immobility, and immobility (playing dead) offers the extreme chance of safety when an animal is facing the worst possible stressful situation--being seized by a predator--as in this case any movement can further stimulate the predator's aggressiveness. Perceiving and connecting sensory information also gives pleasure, and this appears to enhance the clarity of sensations and is an important factor in learning. We propose that anhedonia, by reducing or eliminating the pleasure, might jeopardize the usual appearance of the environment, which must not only be clearly perceived but also continuously interpreted (for instance a foreshortening, or something far off seen as small, must not be seen as a real deformation; the same holds for words, where the meaning has to be grasped from the single letters, and so on). Consequently, anhedonia could in some cases make the environment's image strange, distorted and frightening, and this could cause anxiety, confusion, and give problems in contacts with people and things. As correct information about images and sounds can inhibit visual and auditory hallucinations (considering them, like delusions, as attempts to reconstruct and make sense again of a world that is becoming confused and alien), we propose that anhedonia, interfering with the correct perceiving and processing of sensations, may facilitate them.

Repeated exposure to the κ-opioid receptor agonist salvinorin A modulates extracellular signal-regulated kinase and reward sensitivity

BACKGROUND

Repeated exposure to drugs of abuse and stress increase dynorphin, a κ opioid receptor (KOR) ligand, in the nucleus accumbens (NAc). Acute KOR activation produces dysphoria that might contribute to addictive behavior. How repeated KOR activation modulates reward circuitry is not understood.

METHODS

We used intracranial self-stimulation (ICSS), a method that provides a behavioral index of reward sensitivity, to measure the effects of repeated administration of the KOR agonist salvinorin A (salvA) (2 mg/kg) on the reward-potentiating effects of cocaine (5.0 mg/kg). In separate rats, we measured the effects of salvA on activation of extracellular signal regulated kinase (ERK), cyclic adenosine monophosphate (cAMP) response element binding protein, and c-Fos within the NAc.

RESULTS

SalvA had biphasic effects on reward: an immediate effect was to decrease the rewarding impact of ICSS, whereas a delayed effect was to increase the rewarding impact of ICSS. Repeated salvA produced a net decrease in the reward-potentiating effects of cocaine. In the NAc, both acute and repeated salvA administration increased phosphorylated ERK, whereas only acute salvA increased c-Fos and repeated salvA increased phosphorylated cAMP response element binding protein. The KOR antagonist nor-binaltorphimine (20 mg/kg) blocked the immediate and delayed effects of salvA and prolonged the duration of cocaine effects in ICSS.

CONCLUSIONS

Repeated salvA might trigger opponent processes such that "withdrawal" from the dysphoric effects of KOR activation is rewarding and decreases the net rewarding valence of cocaine. The temporal effects of salvA on ERK signaling suggest KOR-mediated engagement of distinct signaling pathways within the NAc that might contribute to biphasic effects on reward sensitivity.

The delta opioid receptor: an evolving target for the treatment of brain disorders

Compared to the better-known mu opioid receptor, delta opioid receptors have been relatively understudied. However, the development of highly selective delta opioid agonists and the availability of genetic mouse models have extended our knowledge of delta opioid receptors in vivo. Here we review recent developments in the characterization of delta opioid receptor biology and aspects of delta opioid receptor function that have potential for therapeutic targeting. Preclinical data have confirmed that delta opioid receptor activation reduces persistent pain and improves negative emotional states; clinical trials have been initiated to assess the effectiveness of delta opioid agonists in chronic pain and depression. Furthermore, a possible role for these receptors in neuroprotection is being investigated. The usefulness of targeting delta opioid receptors in drug abuse remains open and a role for these receptors in impulse control disorders is emerging. Finally, the recent demonstration of biased agonism at the delta opioid receptor in vivo opens novel perspectives towards targeting specific therapeutic effects through drug design.

Kappa opioid receptor signaling in the basolateral amygdala regulates conditioned fear and anxiety in rats

BACKGROUND

The kappa opioid receptor (KOR) system contributes to the prodepressive and aversive consequences of stress and is implicated in the facilitation of conditioned fear and anxiety in rodents. Here, we sought to identify neural circuits that mediate KOR system effects on fear and anxiety in rats.

METHODS

We assessed whether fear conditioning induces plasticity in KOR or dynorphin (the endogenous KOR ligand) messenger RNA (mRNA) expression in the basolateral (BLA) and central (CeA) nuclei of the amygdala, hippocampus, or striatum. We then assessed whether microinfusions of the KOR antagonist JDTic (0-10 μg/side) into the BLA or CeA affect the expression of conditioned fear or anxiety. Finally, we examined whether fear extinction induces plasticity in KOR mRNA expression that relates to the quality of fear extinction.

RESULTS

Fear conditioning upregulated KOR mRNA in the BLA by 65% and downregulated it in the striatum by 22%, without affecting KOR levels in the CeA or hippocampus, or dynorphin levels in any region. KOR antagonism in either the BLA or CeA decreased conditioned fear in the fear-potentiated startle paradigm, whereas KOR antagonism in the BLA, but not the CeA, produced anxiolytic-like effects in the elevated plus maze. Effective fear extinction was associated with a 67% reduction in KOR mRNA in the BLA.

CONCLUSIONS

These findings suggest that fear conditioning and extinction dynamically regulate KOR expression in the BLA and provide evidence that the BLA and CeA are important neural substrates mediating the anxiolytic-like effects of KOR antagonists in models of fear and anxiety.

A primary care focus on the diagnosis and treatment of major depressive disorder in adults

Major depressive disorder (MDD), a highly prevalent psychiatric condition, is encountered in 1 of every 10 to 20 patients seen in the primary care setting. While awareness of MDD has increased, timely and accurate diagnosis and adequate treatment remain formidable challenges. Treatment options for patients with MDD should be individualized according to each patient's clinical and medication history, pharmacologic tolerability profile, and personal preferences, to maximize long-term adherence. The most robust and consistently maintained positive outcomes occur in patients who are administered effective treatment with recommended antidepressant pharmacotherapy, psychotherapy, or a combination of these treatment modalities. This manuscript reviews the epidemiology, biologic and clinical features, diagnosis, and treatment of patients with MDD. A focus is placed on guidelines and strategies that target the achievement of MDD symptom remission. The article also includes details on individualizing treatment selection and novel and emerging therapies. Primary care physicians must be prepared to adjust, substitute, or augment antidepressant treatments to optimize patient response and enhance the chances of achieving remission. Considerations that strongly influence long-term patient adherence, including tolerability and cost, are also reviewed.

Acute oxycodone induces the pro-emetic pica response in rats

Oxycodone, a semisynthetic opioid analgesic, is frequently prescribed for the management of pain. Side effects of nausea and emesis affect patient compliance and limit its therapeutic use. The present study established that an antinociceptive dose of oxycodone (15 mg/kg; oral) induces the pica response. We found sex differences in the temporal course of pica, with females having a longer duration. Opioid receptors mediated the pica response, as 1.0 mg/kg naloxone transiently attenuated and 2.0 mg/kg naloxone blocked pica. A κ-selective antagonist failed to block the response, suggesting mediation by μ opioid receptor. For further validation, we used the well established kaolin intake model to assess pica with the chemotherapeutic drug cisplatin as a positive control. Oxycodone and cisplatin significantly increased kaolin intake 4- to 7-fold, and the wet weight of stomach was elevated 2- to 3-fold. To examine the underlying neural circuitry, we investigated c-fos activation in the area postrema and nucleus of solitary tract (NTS). Oxycodone treatment significantly increased the number of c-fos-positive neurons in the area postrema and NTS compared with water controls. As expected, cisplatin also increased the number of c-fos-positive cells in these regions. In the area postrema, the oxycodone effect was greater than cisplatin, especially at 2 h. These results indicate that an antinociceptive dose of oxycodone is associated with the expression of pica, a pro-emetic response.

Tissue-specific DNA methylation of the human prodynorphin gene in post-mortem brain tissues and PBMCs

OBJECTIVE

Dynorphins, the endogenous ligands for the κ opioid receptor, are implicated in neuropsychiatric disorders through modulation of basal and stimuli-induced dopaminergic, glutamatergic, and serotonergic tones. Expression of the prodynorphin gene (PDYN) is critical for rewarding properties of drugs of abuse and stress-induced responses. Epigenetic factors, such as DNA methylation, play an important role in modulation of gene expression.

METHODS

We analyzed DNA methylation patterns of three CpG-rich regions of PDYN, a CpG island, and cluster A in the proximal promoter, and cluster B in coding exon 4, by bisulfite sequencing of DNA from the caudate and anterior cingulate cortex from post-mortem brain of 35 individuals (22 HIV seropositive), and in peripheral blood mononuclear cells from 21 of these individuals.

RESULTS

We found remarkably similar patterns of methylation across CpG sites in these tissues. However, there were tissue-specific differences in methylation levels (P=0.000001) of the CpG island: higher levels in peripheral blood mononuclear cells (82%) than in the brain tissues, the caudate (62%), and the anterior cingulate cortex (44%). But there was higher PDYN expression in the caudate than in the anterior cingulate cortex. In contrast, cluster A near the transcription start site is hypomethylated.

CONCLUSION

This DNA methylation profile of the PDYN gene is typical for primary responsive genes with regulatory elements for both basal and tissue-specific transcription. Our findings provide a rationale for further studies of the role of other epigenetic factors in the regulation of PDYN expression in individuals with psychiatric and neurological disorders.

Effects of recovery from immobilization stress on striatal preprodynorphin- and kappa opioid receptor-mRNA levels of the male rat

Previously, we have reported that brain regions that are thought to be involved in motivated behavior are altered in animals undergoing repeated exposures to immobilization stress. The goal of the present study was to determine the effects of recovery from this type of stress on these same mesolimbic brain regions. For this purpose, adult male Sprague-Dawley rats were initially exposed to immobilization stress either once (2 h) or repeatedly (2 h×10 days). Rats were then either allowed to recover from the stressor for a shorter (2 days) or longer period of time (9 days) in their home cages. At the end of this recovery period, rats were euthanized and trunk blood and brains were processed for serum corticosterone (CORT) and neurochemistry, respectively. Brain mRNA levels were determined via in situ hybridization for the opioid preprodynorphin (DYN) and its cognate receptor (kappa, KOR), in striatal and accumbal subregions. A pattern of selective transcriptional activation emerged in the four resultant treatment conditions where a short recovery from either a single or repeated exposure to immobilization produced increases in KOR-mRNA levels in striatal and nucleus accumbens (Acb) subregions. Relative to controls, these differences were diminished after a longer recovery period. Interestingly, DYN-mRNA levels were unchanged after the shorter recovery period and after single or repeated immobilizations but appeared to be induced after a longer recovery period after repeated immobilizations. A relative amount of weight loss occurred after immobilization following repeated but not single exposure to stress. In addition, only those rats recovering from repeated stress exposures had higher CORT levels compared with non-immobilized controls. These results suggest that recovery from immobilization stress may alter the motivational system after as little as a single immobilization and that a possible dysphoric effect on appetitive behavior may be reflected by an altered striatal dynorphin system.

Extending pharmacological spectrum of opioids beyond analgesia: multifunctional aspects in different pathophysiological states

Opioids are well known to exert potent central analgesic actions. In recent years, the numerous studies have unfolded the critical role of opioids in the pathophysiology of various diseases as well as in biological phenomenon of therapeutic interest. The endogenous ligands of opioid receptors are derived from three independent genes and their appropriate processing yields the major representative opioid peptides beta-endorphin, met-enkephalin, leu-enkephalin and dynorphin, respectively. These peptides and their derivatives exhibit different affinity and selectivity for the mu-, delta- and kappa-receptors located on the central and the peripheral neurons, neuroendocrine, immune, and mucosal cells and on many other organ systems. The present review article highlights the role of these peptides in central nervous system disorders such as depression, anxiety, epilepsy, and stress; gastrointestinal disorders such as diarrhea, postoperative ileus, ulceration, and irritable bowel syndrome; immune system and related inflammatory disorders such as osteoarthritis and rheumatoid arthritis; and others including respiratory, alcoholism and obesity/binge eating. Furthermore, the key role of opioids in different forms of pre- and post-conditioning including ischemic and pharmacological along with in remote preconditioning has also been described.

Neurobiology of aversive states

Hoebel and colleagues are often known as students of reward and how it is coded in the CNS. This article, however, attempts to focus on the significant advances by Hoebel and others in dissecting out behavioral components of distinct aversive states and in understanding the neurobiology of aversion and the link between aversive states and addictive behaviors. Reward and aversion are not necessarily dichotomous and may reflect an affective continuum contingent upon environmental conditions. Descriptive and mechanistic studies pioneered by Bart Hoebel have demonstrated that the shift in the reward-aversion spectrum may be, in part, a result of changes in central dopamine/acetylcholine ratio, particularly in the nucleus accumbens. The path to aversion appears to include a specific neurochemical signature: reduced dopamine release and increased acetylcholine release in "reward centers" of the brain. Opioid receptors may have a neuromodulatory role on both of these neurotransmitters.

Identification of short-acting κ-opioid receptor antagonists with anxiolytic-like activity

The κ-opioid receptor plays a central role in mediating the response to stressful life events. Inhibiting κ-opioid receptor signaling is proposed as a mechanism for treating stress-related conditions such as depression and anxiety. Preclinical testing consistently confirms that disruption of κ-opioid signaling is efficacious in animal models of mood disorders. However, concerns about the feasibility of developing antagonists into drugs stem from an unusual pharmacodynamic property of prototypic κ-opioid receptor-selective antagonists; they inhibit receptor signaling for weeks to months after a single dose. Several fundamental questions include - is it possible to identify short-acting antagonists; is long-lasting inhibition necessary for efficacy; and is it safe to develop long-acting antagonists in the clinic. Here, we test representative compounds (AZ-ECPC, AZ-MTAB, and LY-DMPF) from three new chemical series of κ-opioid receptor ligands for long-lasting inhibition. Each compound dose-dependently reversed κ-opioid agonist-induced diuresis. However, unlike the prototypic antagonist, nBNI, which fully inhibited evoked diuresis for at least four weeks, the new compounds showed no inhibition after one week. The two compounds with greater potency and selectivity were tested in prenatally-stressed rats on the elevated plus maze, an exploration-based model of anxiety. Spontaneous exploration of open arms in the elevated plus maze was suppressed by prenatal stress and restored with both compounds. These findings indicate that persistent inhibition is not an inherent property of κ-opioid-selective antagonists and that post-stress dosing with transient inhibitors can be effective in a mood disorder model. This further supports κ-opioid receptor as a promising target for developing novel psychiatric medications.

Cell-specific effects of variants of the 68-base pair tandem repeat on prodynorphin gene promoter activity

A polymorphic 68-bp tandem repeat has been identified within the promoter of the human prodynorphin (PDYN) gene. We found that this 68-bp repeat in the PDYN promoter occurs naturally up to five times. We studied the effect of the number of 68-bp repeats, and of a SNP (rs61761346) found within the repeat on PDYN gene promoter activity. Thirteen promoter forms, different naturally occurring combinations of repeats and the internal SNP, were cloned upstream of the luciferase reporter gene, transfected into human SK-N-SH, H69, or HEK293 cells. Cells were then stimulated with TPA or caffeine. We found cell-specific effects of the number of 68-bp repeats on the transcriptional activity of the PDYN promoter. In SK-N-SH and H69 cells, three or four repeats led to lower expression of luciferase than did one or two repeats. The opposite effect was found in HEK293 cells. The SNP also had an effect on PDYN gene expression in both SK-N-SH and H69 cells; promoter forms with the A allele had significantly higher expression than promoter forms with the G allele. These results further our understanding of the complex transcriptional regulation of the PDYN gene promoter.

Annual Research Review: New frontiers in developmental neuropharmacology: can long-term therapeutic effects of drugs be optimized through carefully timed early intervention?

Our aim is to present a working model that may serve as a valuable heuristic to predict enduring effects of drugs when administered during development. Our primary tenet is that a greater understanding of neurodevelopment can lead to improved treatment that intervenes early in the progression of a given disorder and prevents symptoms from manifesting. The immature brain undergoes significant changes during the transitions between childhood, adolescence, and adulthood. Such changes in innervation, neurotransmitter levels, and their respective signaling mechanisms have profound and observable changes on typical behavior, but also increase vulnerability to psychiatric disorders when the maturational process goes awry. Given the remarkable plasticity of the immature brain to adapt to its external milieu, preventive interventions may be possible. We intend for this review to initiate a discussion of how currently used psychotropic agents can influence brain development. Drug exposure during sensitive periods may have beneficial long-term effects, but harmful delayed consequences may be possible as well. Regardless of the outcome, this information needs to be used to improve or develop alternative approaches for the treatment of childhood disorders. With this framework in mind, we present what is known about the effects of stimulants, antidepressants, and antipsychotics on brain maturation (including animal studies that use more clinically-relevant dosing paradigms or relevant animal models). We endeavor to provocatively set the stage for altering treatment approaches for improving mental health in non-adult populations.

Neuropharmacology of the naturally occurring kappa-opioid hallucinogen salvinorin A

Salvia divinorum is a perennial sage native to Oaxaca, Mexico, that has been used traditionally in divination rituals and as a treatment for the "semimagical" disease panzón de borrego. Because of the intense "out-of-body" experiences reported after inhalation of the pyrolized smoke, S. divinorum has been gaining popularity as a recreational hallucinogen, and the United States and several other countries have regulated its use. Early studies isolated the neoclerodane diterpene salvinorin A as the principal psychoactive constituent responsible for these hallucinogenic effects. Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of KOP receptors, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable, because 1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist, and 2) its effects are not mediated by the 5-HT(2A) receptor, the classic target of hallucinogens such as lysergic acid diethylamide and mescaline. Rigorous investigation into the structural features of salvinorin A responsible for opioid receptor affinity and selectivity has produced numerous receptor probes, affinity labels, and tools for evaluating the biological processes responsible for its observed psychological effects. Salvinorin A has therapeutic potential as a treatment for pain, mood and personality disorders, substance abuse, and gastrointestinal disturbances, and suggests that nonalkaloids are potential scaffolds for drug development for aminergic G-protein coupled receptors.

Activation of CREB in the nucleus accumbens shell produces anhedonia and resistance to extinction of fear in rats

Stress triggers psychiatric conditions including depressive and anxiety disorders. The mechanisms by which stress produces persistent changes in behavior are not fully understood. Here we show in rats that stress (footshock) activates the transcription factor cAMP response element binding protein (CREB) within the nucleus accumbens shell (NAS), a brain area involved in encoding reward and aversion. To examine the behavioral significance of altered CREB function in the NAS, we used viral vectors to elevate or disrupt CREB in this region. Elevated CREB produced increases in intracranial self-stimulation thresholds, a depressive-like sign reflecting anhedonia (decreased sensitivity to reward), whereas disruption of CREB function by expression of a dominant-negative CREB had the opposite effect. To determine whether neuroadaptations that produce anhedonia subsequently affect vulnerability to stress-induced behavioral adaptations, we subjected rats with altered CREB function in the NAS to fear conditioning. Although neither elevation nor disruption of CREB function altered the development of conditioned fear, elevation of CREB impaired extinction of conditioned fear. To mimic downstream effects of CREB activation on expression of the opioid peptide dynorphin, we microinjected the κ-opioid receptor (KOR) agonist U50,488 directly into the NAS. KOR stimulation produced anhedonia but had no effect on expression or extinction of conditioned fear. These findings demonstrate that activation of CREB in the NAS produces multiple behavioral signs (anhedonia, impaired extinction) characteristic of experience-dependent psychiatric conditions such as posttraumatic stress disorder. Although CREB activation is a common trigger, expression of these individual signs appears to involve divergent downstream mechanisms.

Cross-species affective functions of the medial forebrain bundle-implications for the treatment of affective pain and depression in humans

Major depression (MD) might be conceptualized as pathological under-arousal of positive affective systems as parts of a network of brain regions assessing, reconciling and storing emotional stimuli versus an over-arousal of parts of the same network promoting separation-distress/GRIEF. In this context depression can be explained as an emotional pain state that is the result of a disregulation of several sub-systems that under physiological conditions are concerned with bodily or emotional homeostasis of the human organism in a social context. Physiologically, homeostasis is maintained by influences of the SEEKING system represented - amongst others - by the medial forebrain bundle (MFB). Neuroimaging studies show that the MFB has a proven access to the GRIEF/Sadness system. A functional decoupling of these systems with a dysfunctional GRIEF pathway might result in MD. Therewith GRIEF and SEEKING/PLEASURE systems play important roles as opponents in maintenance of emotional homeostasis. Chronic electrical modulation of the reward SEEKING pathways with deep brain stimulation might show anti-depressive effects in humans suffering from MD by re-initiating an emotional equilibrium (of higher or lower activity) between these opposing systems.

The role of kappa-opioid receptor activation in mediating antinociception and addiction

The kappa-opioid receptor (KOR), a member of the opioid receptor family, is widely expressed in the central nervous system and peripheral tissues. Substantial evidence has shown that activation of KOR by agonists and endogenous opioid peptides in vivo may produce a strong analgesic effect that is free from the abuse potential and the adverse side effects of mu-opioid receptor (MOR) agonists, such as morphine. In addition, activation of the KOR has also been shown to exert an inverse effect on morphine-induced adverse actions, such as tolerance, reward, and impairment of learning and memory. Therefore, the KOR has received much attention in the effort to develop alternative analgesics to MOR agonists and agents for the treatment of drug addiction. However, KOR agonists also produce several severe undesirable side effects such as dysphoria, water diuresis, salivation, emesis, and sedation in nonhuman primates, which may limit the clinical utility of KOR agonists for pain and drug abuse treatment. This article will review the role of KOR activation in mediating antinociception and addiction. The possible therapeutic application of kappa-agonists in the treatment of pain and drug addiction is also discussed.

Behavioral stress may increase the rewarding valence of cocaine-associated cues through a dynorphin/kappa-opioid receptor-mediated mechanism without affecting associative learning or memory retrieval mechanisms

Stress exposure increases the risk of addictive drug use in human and animal models of drug addiction by mechanisms that are not completely understood. Mice subjected to repeated forced swim stress (FSS) before cocaine develop significantly greater conditioned place preference (CPP) for the drug-paired chamber than unstressed mice. Analysis of the dose dependency showed that FSS increased both the maximal CPP response and sensitivity to cocaine. To determine whether FSS potentiated CPP by enhancing associative learning mechanisms, mice were conditioned with cocaine in the absence of stress, then challenged after association was complete with the kappa-opioid receptor (KOR) agonist U50,488 or repeated FSS, before preference testing. Mice challenged with U50,488 60 min before CPP preference testing expressed significantly greater cocaine-CPP than saline-challenged mice. Potentiation by U50,488 was dose and time dependent and blocked by the KOR antagonist norbinaltorphimine (norBNI). Similarly, mice subjected to repeated FSS before the final preference test expressed significantly greater cocaine-CPP than unstressed controls, and FSS-induced potentiation was blocked by norBNI. Novel object recognition (NOR) performance was not affected by U50,488 given 60 min before assay, but was impaired when given 15 min before NOR assay, suggesting that KOR activation did not potentiate CPP by facilitating memory retrieval or expression. The results from this study show that the potentiation of cocaine-CPP by KOR activation does not result from an enhancement of associative learning mechanisms and that stress may instead enhance the rewarding valence of cocaine-associated cues by a dynorphin-dependent mechanism.

The role of the dynorphin-kappa opioid system in the reinforcing effects of drugs of abuse

BACKGROUND

Initial hypotheses regarding the role of the kappa opioid system in drug addiction suggested that kappa receptor stimulation had anti-addictive effects. However, recent research suggests that kappa receptor antagonists may reverse motivational aspects of dependence. In the present review, we revisit the studies that measured the effects of kappa receptor ligands on the reinforcing and rewarding effects of drugs and postulate underlying neurobiological mechanisms for these effects to elaborate a more complex view of the role of kappa receptor ligands in drug addiction.

RESULTS

The review of studies indicates that kappa receptor stimulation generally antagonizes the acute reinforcing/rewarding effects of drugs whereas kappa receptor blockade has no consistent effect. However, in a drug dependent-like state, kappa receptor blockade was effective in reducing increased drug intake. In animal models of reinstatement, kappa receptor stimulation can induce reinstatement via a stress-like mechanism. Results in conditioned place preference/aversion and intracranial self-stimulation indicate that kappa receptor agonists produce, respectively, aversive-like and dysphoric-like effects. Additionally, preclinical and postmortem studies show that administration or self-administration of cocaine, ethanol, and heroin activate the kappa opioid system.

CONCLUSION

kappa receptor agonists antagonize the reinforcing/rewarding effects of drugs possibly through punishing/aversive-like effects and reinstate drug seeking through stress-like effects. Evidence suggests that abused drugs activate the kappa opioid system, which may play a key role in motivational aspects of dependence. Kappa opioid systems may have an important role in driving compulsive drug intake.

Comparison of the kappa-opioid receptor antagonist DIPPA in tests of anxiety-like behavior between Wistar Kyoto and Sprague Dawley rats

RATIONALE

Recent evidence suggests a role for the dynorphin/kappa-opioid receptor (KOR) system in the expression of stress-induced behaviors. Wistar Kyoto (WKY) rats exhibit increased depression-like and anxiety-like responses in behavioral tests compared to other strains and may be a model of comorbid depression and anxiety characterized by increased activity within the dynorphin/KOR system. Though KOR antagonists produce antidepressant-like effects in WKY rats, their effects in tests of anxiety-like behavior have not been examined in the WKY strain.

OBJECTIVE

The aim of the current study was to investigate the effects of the KOR antagonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide hydrochloride (DIPPA) on the behavior of WKY rats and Sprague Dawley (SD) rats in tests of anxiety-like behavior.

METHODS

The novelty-induced hypophagia and defensive burying tests were used to measure anxiety-like behavior in WKY and SD rats and determine the effects of DIPPA on anxiety-like behavior in both strains.

RESULTS

WKY rats displayed greater amounts of anxiety-like behavior compared to SD rats. DIPPA produced anxiolytic-like effects in both tests in both strains.

CONCLUSIONS

WKY rats display more anxiety-like behavior at baseline compared to SD rats, and DIPPA produced anxiolytic-like effects in both WKY and SD rats. These findings support previous research suggesting that KOR antagonists possess anxiolytic-like properties and may potentially represent a novel class of treatments for mood disorders.

Role of kappa-opioid receptors in the effects of salvinorin A and ketamine on attention in rats

BACKGROUND

Disruptions in perception and cognition are characteristic of psychiatric conditions such as schizophrenia. Studies of pharmacological agents that alter perception and cognition in humans might provide a better understanding of the brain substrates of these complex processes. One way to study these states in rodents is with tests that require attention and visual perception for correct performance.

METHODS

We examined the effects of two drugs that cause disruptions in perception and cognition in humans-the kappa-opioid receptor (KOR) agonist salvinorin A (salvA; 0.125-4.0 mg/kg) and the non-competitive NMDA receptor antagonist ketamine (0.63-20 mg/kg)-on behavior in rats using the 5-choice serial reaction time task (5CSRTT), a food-motivated test that quantifies attention. We also compared the binding profiles of salvA and ketamine at KORs and NMDA receptors.

RESULTS

SalvA and ketamine produced the same pattern of disruptive effects in the 5CSRTT, characterized by increases in signs often associated with reduced motivation (omission errors) and deficits in processing (elevated latencies to respond correctly). Sessions in which rats were fed before testing suggest that reduced motivation produces a subtly different pattern of behavior. Pretreatment with the KOR antagonist JDTic (10 mg/kg) blocked all salvA effects and some ketamine effects. Binding and function studies revealed that ketamine is a full agonist at KORs, although not as potent or selective as salvA.

CONCLUSIONS

SalvA and ketamine have previously under-appreciated similarities in their behavioral effects and pharmacological profiles. By implication, KORs might be involved in some of the cognitive abnormalities observed in psychiatric disorders such as schizophrenia.