A delta opioid receptor agonist, KNT-127, in the prelimbic medial prefrontal cortex attenuates glial glutamate transporter blocker-induced anxiety-like behavior in mice

Astroglial Dysfunctions in Mood Disorders and Rodent Stress Models: Consequences on Behavior and Potential as Treatment Target

Astrocyte dysfunctions have been consistently observed in patients affected with depression and other psychiatric illnesses. Although over the years our understanding of these changes, their origin, and their consequences on behavior and neuronal function has deepened, many aspects of the role of astroglial dysfunction in major depressive disorder (MDD) and post-traumatic stress disorder (PTSD) remain unknown. In this review, we summarize the known astroglial dysfunctions associated with MDD and PTSD, highlight the impact of chronic stress on specific astroglial functions, and how astroglial dysfunctions are implicated in the expression of depressive- and anxiety-like behaviors, focusing on behavioral consequences of astroglial manipulation on emotion-related and fear-learning behaviors. We also offer a glance at potential astroglial functions that can be targeted for potential antidepressant treatment.

Role for μ-opioid receptor in antidepressant effects of δ-opioid receptor agonist KNT-127

Previous pharmacological data have shown the possible existence of functional interactions between μ- (MOP), κ- (KOP), and δ-opioid receptors (DOP) in pain and mood disorders. We previously reported that MOP knockout (KO) mice exhibit a lower stress response compared with wildtype (WT) mice. Moreover, DOP agonists have been shown to exert antidepressant-like effects in numerous animal models. In the present study, the tail suspension test (TST) and forced swim test (FST) were used to examine the roles of MOP and DOP in behavioral despair. MOP-KO mice and WT mice were treated with KNT-127 (10 mg/kg), a selective DOP agonist. The results indicated a significant decrease in immobility time in the KNT-127 group compared with the saline group in all genotypes in both tests. In the saline groups, immobility time significantly decreased in MOP-KO mice compared with WT mice in both tests. In female MOP-KO mice, KNT-127 significantly decreased immobility time in the TST compared with WT mice. In male MOP-KO mice, however, no genotypic differences were found in the TST after either KNT-127 or saline treatment. Thus, at least in the FST and TST, the activation of DOP and absence of MOP had additive effects in reducing measures of behavioral despair, suggesting that effects on this behavior by DOP activation occur independently of MOP.

Under or Absent Reporting of Light Stimuli in Testing of Anxiety-Like Behaviors in Rodents: The Need for Standardization

Behavioral neuroscience tests such as the Light/Dark Test, the Open Field Test, the Elevated Plus Maze Test, and the Three Chamber Social Interaction Test have become both essential and widely used behavioral tests for transgenic and pre-clinical models for drug screening and testing. However, as fast as the field has evolved and the contemporaneous involvement of technology, little assessment of the literature has been done to ensure that these behavioral neuroscience tests that are crucial to pre-clinical testing have well-controlled ethological motivation by the use of lighting (i.e., Lux). In the present review paper, N = 420 manuscripts were examined from 2015 to 2019 as a sample set (i.e., n = ~20–22 publications per year) and it was found that only a meager n = 50 publications (i.e., 11.9% of the publications sampled) met the criteria for proper anxiogenic and anxiolytic Lux reported. These findings illustrate a serious concern that behavioral neuroscience papers are not being vetted properly at the journal review level and are being released into the literature and public domain making it difficult to assess the quality of the science being reported. This creates a real need for standardizing the use of Lux in all publications on behavioral neuroscience techniques within the field to ensure that contributions are meaningful, avoid unnecessary duplication, and ultimately would serve to create a more efficient process within the pre-clinical screening/testing for drugs that serve as anxiolytic compounds that would prove more useful than what prior decades of work have produced. It is suggested that improving the standardization of the use and reporting of Lux in behavioral neuroscience tests and the standardization of peer-review processes overseeing the proper documentation of these methodological approaches in manuscripts could serve to advance pre-clinical testing for effective anxiolytic drugs. This report serves to highlight this concern and proposes strategies to proactively remedy them as the field moves forward for decades to come.

Disulfiram Produces Potent Anxiolytic-Like Effects Without Benzodiazepine Anxiolytics-Related Adverse Effects in Mice

Disulfiram is an FDA approved drug for the treatment of alcoholism. The drug acts by inhibiting aldehyde dehydrogenase, an enzyme essential to alcohol metabolism. However, a recent study has demonstrated that disulfiram also potently inhibits the cytoplasmic protein FROUNT, a common regulator of chemokine receptor CCR2 and CCR5 signaling. Several studies have reported that chemokine receptors are associated with the regulation of emotional behaviors in rodents, such as anxiety. Therefore, this study was performed to clarify the effect of disulfiram on emotional behavior in rodents. The anxiolytic-like effects of disulfiram were investigated using an elevated plus-maze (EPM) test, a typical screening model for anxiolytics. Disulfiram (40 or 80 mg/kg) significantly increased the amount of time spent in the open arms of the maze and the number of open arm entries without affecting the total open arms entries. Similar results were obtained in mice treated with a selective FROUNT inhibitor, disulfiram-41 (10 mg/kg). These disulfiram-associated behavioral changes were similar to those observed following treatment with the benzodiazepine anxiolytic diazepam (1.5 mg/kg). Moreover, disulfiram (40 mg/kg) significantly and completely attenuated increased extracellular glutamate levels in the prelimbic-prefrontal cortex (PL-PFC) during stress exposure on the elevated open-platform. However, no effect in the EPM test was seen following administration of the selective aldehyde dehydrogenase inhibitor cyanamide (40 mg/kg). In contrast to diazepam, disulfiram caused no sedation effects in the open-field, coordination disorder on a rotarod, or amnesia in a Y-maze. This is the first report suggesting that disulfiram produces anxiolytic-like effects in rodents. We found that the presynaptic inhibitory effects on glutaminergic neurons in the PL-PFC may be involved in its underlying mechanism. Disulfiram could therefore be an effective and novel anxiolytic drug that does not produce benzodiazepine-related adverse effects, such as amnesia, coordination disorder, or sedation, as found with diazepam. We propose that the inhibitory activity of disulfiram against FROUNT function provides an effective therapeutic option in anxiety.

[Neural mechanism underlying the regulation of emotional behavior via δ-opioid receptors]

The delta opioid receptor (DOP) belongs to the G protein-coupled receptor family and is abundant in the limbic system. In recent years, consistent with their distribution, they have been suggested to be involved in the regulation of emotional behavior. In particular, DOP agonists have been shown to exhibit antidepressant and anxiolytic-like effects, and clinical trials are underway as targets for the development of new psychotropic drugs with mechanisms of action different from those of existing monoamine drugs. In this article, we review the roles and mechanisms of DOP in emotion regulation that are being elucidated in basic studies using rodents, and also introduce the current status of its clinical application.

Modulation of glutamatergic synaptic transmission and neuronal excitability in the prelimbic medial prefrontal cortex via delta-opioid receptors in mice

The medial prefrontal cortex (mPFC) plays a vital role in the processing of emotional events. It has been shown that activation of the glutamatergic transmission in prelimbic subregion of the mPFC (PL-PFC) evoked anxiety-like behavior in rodents. We previously reported that local perfusion of a selective agonist to delta-opioid receptor (DOP), KNT-127, attenuated the veratrine-induced elevation of extracellular glutamate in the PL-PFC and anxiety-like behavior in mice. These results suggested the possibility that KNT-127 suppresses glutamate release from the presynaptic site in the PL-PFC. To examine this possibility directly, we performed whole-cell patch-clamp recording from principal neurons in the PL-PFC and examined the spontaneous and electrically-evoked excitatory postsynaptic currents (EPSC)s. We found that bath application of KNT-127 significantly decreased the frequency of spontaneous and miniature EPSCs. Conversely, amplitude, rise time, and decay time of spontaneous and miniature EPSCs were not affected by bath application of KNT-127. Also, KNT-127 increased paired-pulse ratios of electrically-evoked EPSCs in the PL-PFC principal neurons tested. Further, we analyzed the firing properties of pyramidal neurons in the PL-PFC and found that KNT-127 treatment significantly reduced the number of action potentials and firing threshold. These results suggested that KNT-127 suppresses glutamatergic synaptic transmission by inhibiting glutamate release from the presynaptic site and reduces neuronal excitability in the mouse PL-PFC. We propose the possibility that these suppressing effects of KNT-127 on PL-PFC activity are part of the underlying mechanisms of its anxiolytic-like effects.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

Research and development of κ opioid receptor agonists and δ opioid receptor agonists

Delta opioid delta receptor (DOP) agonists were expected to be analgesics and many researchers tried to develop the SNC80 derivatives. However, the derivatives were dropped at the stage of early clinical trials because of undesirable side effects and weak analgesia. On the other hand, DOP agonists have been proposed as attractive candidates for the novel psychotropic drugs. We recently succeeded in synthesizing a novel selective DOP agonist KNT-127. KNT-127 produced neither catalepsy nor convulsive effects. We have demonstrated that KNT-127 has potent anxiolytic-like effect in rat models of innate anxiety. This anxiolytic-like effect was independent from known adverse effect of benzodiazepine, such as memory impairment, motor coordination deficits, and ethanol interactions. We have also demonstrated that KNT-127 showed potent and rapid antidepressant-like effects in rat models of depression. This antidepressant-like effect was independent from known adverse effect of selective serotonin reuptake inhibitor (SSRI), such as digestive symptoms. Therefore, we propose that DOP should be considered as an attractive target for the development of novel psychotropic drugs, without producing the adverse effects associated with benzodiazepine anxiolytics and SSRI antidepressants. Very recently, we developed another delta agonist NC-2800 with a different structure. NC-2800 is now in the preclinical stage using the CiCLE fund supported by AMED (Japanese Agency for Medical Research and Development).