Systemic administration of a delta opioid receptor agonist, KNT-127, facilitates extinction learning of fear memory in rats.

Strategies to facilitate extinction of fear memory have attracted increasing attention for enhancing the effectiveness of exposure therapy for anxiety disorders. Previously, we demonstrated that systemic administration of a delta opioid receptor agonist, KNT-127, has clear anxiolytic-like effects in rats, without impairing memory. These observations led us to hypothesize that KNT-127 might be an appropriate therapeutic agent for anxiety disorders when combined with exposure therapy. In the present study, we demonstrate that KNT-127 (3 mg/kg) facilitates extinction learning of fear memory using the contextual fear conditioning test. As expected, a partial agonist at the glycine-binding site on the glutamatergic N-methyl-d-aspartate receptor, d-cycloserine (15 mg/kg), facilitated extinction learning of contextual fear in rats. In contrast, a benzodiazepine anxiolytic, diazepam (1 mg/kg), impaired the fear extinction learning. Interestingly, the facilitatory effect of KNT-127 on extinction learning was observed not only after a 10-min re-exposure, but also after a much shorter (2-min) re-exposure to the context, while d-cycloserine was ineffective at facilitating extinction when a short-duration exposure was given. Our findings may suggest that administration of a delta opioid receptor agonist might have therapeutic efficacy when combined with exposure therapy for treating a range of anxiety disorders.

Administration of a delta opioid receptor agonist KNT-127 to the basolateral amygdala has robust anxiolytic-like effects in rats.

RATIONALE: We previously reported that systemic administration of a selective delta opioid receptor (DOP) agonist, KNT-127, produced potent anxiolytic-like effects in rats. Interestingly, DOPs are highly distributed in the basolateral region of the amygdala (BLA). OBJECTIVES: In this study, we investigated the effect of intra-BLA administration of KNT-127 on anxiety-like behaviors in rats. METHODS AND RESULTS: In the elevated plus maze test, bilateral injection of KNT-127 into the BLA significantly and dose-dependently increased time spent in the open arms. The magnitude of KNT-127 (0.08 µg/0.2 µl)-induced anxiolytic-like effects was similar to muscimol (0.1 µg/0.2 µl), which is a selective agonist for the gamma amino butyric acid type A receptors. Further, anxiolytic-like effects of KNT-127 were abolished by pretreatment with naltrindole, a selective DOP antagonist, suggesting that KNT-127-induced anxiolytic-like effects are mediated by DOPs. These anxiolytic-like effects were confirmed using another innate anxiety model, the open field test. Interestingly, intra-BLA administration of KNT-127 also induced anxiolytic-like effects in the contextual fear conditioning test. Moreover, these effects were also abolished by naltrindole pretreatment. Finally, we demonstrated that intra-BLA administration of KNT-127 facilitates extinction learning of contextual fear in conditioned rats. CONCLUSIONS: Altogether, our findings clearly demonstrate that intra-BLA administration of KNT-127 in rats has robust anxiolytic-like effects not only in innate anxiety-like behavioral tests but also in the contextual fear conditioning test.

Administration of riluzole to the basolateral amygdala facilitates fear extinction in rats.

A general understanding exists that inhibition of glutamatergic neurotransmission in the basolateral amygdala (BLA) impairs fear extinction in rodents. Surprisingly, we recently found that systemic administration of riluzole, which has been shown to inhibit the glutamatergic system, facilitates extinction learning in rats with a preconditioned contextual fear response. However, the mechanisms underlying this paradoxical effect of riluzole remain unclear. In this study, adult male Wistar rats were bilaterally cannulated in the BLA to examine the effects of intra-BLA administration of riluzole. We also compared the effects of riluzole with those of d-cycloserine, a partial agonist at the glycine-binding region of the N-methyl-d-aspartate (NMDA) receptor. In this study, intra-BLA administration of either riluzole or d-cycloserine facilitated extinction learning of contextual fear in conditioned rats. In addition, both riluzole and d-cycloserine enhanced the acquisition of recognition memory in the same model. However, intra-BLA injections of riluzole, but not d-cycloserine, had a potent anxiolytic-like effect when investigated using an elevated plus-maze test. Our findings suggest that riluzole-induced facilitation of extinction learning in rats with a preconditioned contextual fear reflects an indirect effect, resulting from the intra-BLA administration of the drug, and might not be directly related to inhibition of glutamatergic signaling. Further research is needed to clarify the mechanisms underlying the paradoxical effect of riluzole on fear extinction learning observed in this study.

Administration of riluzole into the basolateral amygdala has an anxiolytic-like effect and enhances recognition memory in the rat.

It is widely thought that inactivation of the glutamatergic system impairs recognition memory in rodents. However, we previously demonstrated that systemic administration of riluzole, which blocks the glutamatergic system, enhances recognition memory in the rat novel object recognition (NOR) test. The mechanisms underlying this paradoxical effect of riluzole on recognition memory remain unclear. In the present study, adult male Wistar rats were bilaterally cannulated in the basolateral amygdala (BLA) to examine the effects of intra-BLA administration of riluzole. We also compared the effects of riluzole with those of d-cycloserine, a partial agonist at the glycine binding site on the N-methyl-d-aspartate (NMDA) receptor. The BLA plays a critical role not only in recognition memory, but also in the regulation of anxiety. In the present study, intra-BLA administration of riluzole or d-cycloserine enhanced recognition memory in the NOR test. It was previously suggested that recognition memory can be strongly affected by the state of anxiety in rodents. Interestingly, intra-BLA administration of riluzole, but not d-cycloserine, produced a potent anxiolytic-like effect in the elevated plus-maze test. Thus, the enhancement of recognition memory by riluzole might be an indirect effect resulting from the anxiolytic-like action of the intra-BLA administration of the drug, and may not be directly related to inhibition of the glutamatergic system. Further studies are needed to clarify the mechanisms underlying the memory enhancing effect of riluzole.

The voltage-gated sodium channel activator veratrine induces anxiogenic-like behaviors in rats.

In this study, we investigated the anxiogenic-like effects of systemically administered veratrine in rat models of anxiety. In the light/dark test, veratrine (0.6 mg/kg, s.c.) significantly and dose-dependently decreased the time rats spent in and the number of entries into a light box 30 min after administration, suggesting that veratrine increases anxiety-like behaviors. These findings were also supported by results from the elevated-plus maze test and the tail-swing behavior test. In addition, veratrine (0.6 mg/kg, s.c.) significantly increased the plasma concentration of corticosterone, an endogenous biomarker for anxiety, compared to vehicle. On the basis of these results, we conclude that veratrine induces anxiogenic-like behaviors in rats. The anxiogenic-like behaviors induced by veratrine (0.6 mg/kg, s.c.) were completely abolished by co-treatment with the typical benzodiazepine anxiolytic diazepam (1 mg/kg, s.c.), when assessed in the elevated-plus maze test. Similar results were obtained with co-treatment with riluzole (10 mg/kg, p.o.), which directly affects the glutamatergic system and has recently been suggested to have anxiolytic-like effects. In conclusion, this study provides evidence that systemically administered veratrine induces anxiogenic-like behaviors in rats. We propose the veratrine model as a novel pathological animal model to explore possible candidate drugs for anxiolytics.

Systemic administration of riluzole enhances recognition memory and facilitates extinction of fear memory in rats.

Strategies to enhance recognition memory and facilitate extinction of fear memory have attracted increasing attention for enhancing the effectiveness of exposure therapy for anxiety disorders. Previously, we demonstrated that systemic administration of riluzole has clear anxiolytic-like effects, without impairing memory, in rats. In the present study, we examined whether riluzole could have therapeutic potential for anxiety disorders when combined with exposure therapy. Both riluzole and D-cycloserine enhanced recognition memory in the novel object recognition test and facilitated extinction learning in the contextual fear conditioning in rats. Interestingly, the facilitatory effect of riluzole on extinction learning was clearly observed even after a short re-exposure to the context, while D-cycloserine was ineffective at facilitating extinction when a short duration exposure session was given. In contrast, diazepam impaired both recognition memory and the extinction of fear memory. Our findings strongly suggest that systemic administration of riluzole may have therapeutic efficacy when combined with exposure therapy for treating a range of anxiety disorders. Clinical trials to examine the efficacy of riluzole in combination with exposure therapy in these patients are warranted.

Glucagon-like peptide-2-induced memory improvement and anxiolytic effects in mice.

We investigated the effectiveness of glucagon-like peptide-2 (GLP-2) on memory impairment in lipopolysaccharide (LPS)-treated mice, and anxiety-like behavior in adrenocorticotropic hormone (ACTH)-treated mice. In the Y-maze test, LPS (10 µg/mouse, i.c.v.) significantly decreased spontaneous alternation, which was prevented by pretreatment with GLP-2 (0.01-0.3 µg/mouse, i.c.v.). The GLP-2 treatment just before the Y-maze test also improved LPS-induced memory impairment. Continuous treatment with GLP-2 (3 µg/mouse, i.c.v.) had no effect on the open-field test in saline-treated or ACTH-treated mice. Chronic ACTH treatment did not cause anxiogenic effects in the elevated plus-maze test. GLP-2 showed weak anxiolytic-like effects in the elevated plus-maze test in ACTH-treated, but not saline-treated mice. Moreover, GLP-2 increased 5-HT, but not 5-HIAA and tryptophan hydroxylase 2 levels in the amygdala of ACTH-treated mice. Pharmacological depletion of 5-HT prevented the anxiolytic effects of GLP-2. These results suggest that GLP-2 protected and improved memory function in LPS-treated mice, and also had anxiolytic effects due to changes in the 5-HT system.

Activation of the prelimbic medial prefrontal cortex induces anxiety-like behaviors via N-Methyl-D-aspartate receptor-mediated glutamatergic neurotransmission in mice.

We investigated the possible roles of the prelimbic medial prefrontal cortex (PL) in the regulation of anxiety-like behaviors by pharmacologically activating the terminals of neuronal inputs or postsynaptic efferent neurons with a sodium channel activator veratrine. The extracellular glutamate levels were measured by in vivo microdialysis, and the behaviors were assessed with the open field (OF) test in mice simultaneously. The samples were collected every 10 min for 60 min, as basal levels of glutamate. The medium containing drugs were perfused for 30 min. The OF test was performed in the last 10 min of drug perfusion. After the drug treatments, the perfusion medium containing drugs was switched back to perfusion medium without drugs, and then samples were collected for another 90 min. The extracellular glutamate levels were significantly elevated after local perfusion of veratrine in the PL. At the same time, perfusion of veratrine in the PL produced anxiety-like behaviors in mice. Local coperfusion of a sodium channel blocker, lamotrigine, completely diminished the veratrine-induced elevated extracellular glutamate levels and the behavioral changes. Local coperfusion of an NMDA receptor antagonist, MK-801, but not a non-NMDA (AMPA/kainate) receptor antagonist, CNQX, completely diminished the behavioral changes without any effects on the veratrine-induced elevated extracellular glutamate levels. This study demonstrates that the activation of the PL with veratrine induces anxiety-like behaviors via NMDA receptor-mediated glutamatergic neurotransmission in mice.

The impairment in spatial learning and hippocampal LTD induced through the PKA pathway in juvenile-onset diabetes rats are rescued by modulating NMDA receptor function.

Childhood- or early adulthood-onset type 1 diabetes is associated with modest impairments in cognition, and has an elevated risk of cognitive decline. Although an earlier onset age of diabetes has been identified as one of the strongest risk factors associated with cognitive dysfunction, little is known about the effects of cognitive performance associated with hippocampal function. Our previous study showed impaired working memory and hippocampal long-term depression (LTD) deficits in juvenile-onset diabetes mellitus (JDM) rats. Here, we demonstrated that treatment with the NMDA open-channel blocker, memantine, rescued hippocampal LTD and hippocampal-dependent memory in JDM rats. In addition, the impairment in LTD was attributed to a malfunction in NR2B-containing NMDA receptors. JDM rats exhibited excessive PKA activity, which may play a role in altered NMDA receptor function and impaired LTD. The changes in NR2B-containing NMDA receptors and PKA activity may be involved in learning impairments in JDM rats. Our findings suggest that NMDA open-channel blockers offer a potential strategy to treat cognitive deficits in childhood-onset diabetes.

DOR(2)-selective but not DOR(1)-selective antagonist abolishes anxiolytic-like effects of the δ opioid receptor agonist KNT-127.

Recently, we reported that the δ opioid receptor (DOR) agonist KNT-127 produces anxiolytic-like effects in behaving rats. Here, we report on the roles of DOR subtypes ( DOR(1) and DOR(2)) play in mediating KNT-127-induced anxiolytic-like effects. Pretreatment with the DOR(2)-selective antagonist naltriben (NTB; 0.05mg/kg, s.c.) completely abolished KNT-127 (3.0mg/kg, s.c.)-induced anxiolytic-like effects in rats performing the elevated plus-maze task. By contrast, the DOR(1)-selective antagonist 7-benzylidenenaltrexone (BNTX; 0.5mg/kg, s.c.) produced no effect at a dose that completely blocked the antinociceptive effects of KNT-127. These findings were also supported by results from a light/dark test and open-field test. We clearly demonstrated that the DOR(2)-selective antagonist, but not the DOR(1)-selective antagonist, abolishes the anxiolytic-like effects of the DOR agonist KNT-127, suggesting different roles of these DOR subtypes in anxiety. We propose that DOR(2)-selective agonists would be good candidates for future development of anxiolytic drugs.

Riluzole does not affect hippocampal synaptic plasticity and spatial memory, which are impaired by diazepam in rats.

We have previously demonstrated that riluzole has anxiolytic-like effects in rats, without affecting spontaneous alternation performance in the Y-maze test. However, the effects of riluzole on hippocampal synaptic plasticity were still unclear. In this study, we showed that bath application of riluzole did not impair long-term potentiation and long-term depression, whereas a benzodiazepine anxiolytic, diazepam, significantly impaired them. Furthermore, the acquisition of spatial memory in the Morris water maze test was impaired in diazepam-treated but not riluzole-treated rats. We thus provide further evidence for the potential usefulness of riluzole as an anxiolytic that does not cause amnesia.

Induction of galanin after chronic sertraline treatment in mouse ventral dentate gyrus.

A number of studies implicate neuroplasticity in the therapeutic mechanisms of antidepressants, specifically neuroplasticity in the dentate gyrus of the hippocampal formation. The dorsal hippocampal region in rodents is preferentially involved in spatial learning and memory, while the ventral hippocampal region plays a more important role in stress, emotion, and affective behaviors. These findings led us to investigate behavioral changes and gene expression changes in the ventral and dorsal dentate gyrus differentially after chronic treatment in mice with the antidepressant sertraline. Four-week treatment with sertraline significantly decreased immobility in the modified forced swim test, a behavioral test for assessing antidepressant-like effects in rodents. In the novelty-suppressed feeding test, performance of which is affected by functional changes in the dentate gyrus, sertraline treatment significantly decreased latency to feed. Next, we examined the expression of several neuroplasticity-related genes (those for Notch receptors, basic helix-loop-helix transcription factors and related factors, SoxC transcription factors, and glial-related genes) by real-time RT-PCR in the ventral and dorsal dentate gyrus of mice after the sertraline treatment. The gene encoding the neuropeptide galanin was significantly induced in only ventral dentate gyrus, not in dorsal dentate gyrus. These results suggest that sertraline-related galanin induction in ventral dentate gyrus may play an important role in therapeutic mechanisms for depression.

Glucagon-like peptide-2 but not imipramine exhibits antidepressant-like effects in ACTH-treated mice.

We investigated the effectiveness of glucagon-like peptide-2 (GLP-2) against refractory depression in adrenocorticotropic hormone (ACTH)-treated mice as a model of tricyclic antidepressant (TCA)-resistant depression. Chronic ACTH treatment (0.45 mg/kg, s.c., 14 days) weakened the antidepressant-like effects of imipramine (20 mg/kg, i.p., 6 days) in the forced-swim test (FST). Conversely, GLP-2 (3 µg/mice, i.c.v., 6 days) induced antidepressant-like effects in the ACTH-treated mice in the FST. ACTH-treatment increased basal serum corticosterone levels, with an additional increase induced by the FST. Imipramine or GLP-2 had no effect on the basal corticosterone level, but GLP-2 attenuated the additional increase caused by the FST. Moreover, GLP-2 increased 5-HT levels, but not 5-HIAA. These results suggest that GLP-2 induced antidepressant-like effects under imipramine-resistant conditions through increase in 5-HT levels.

The novel δ opioid receptor agonist KNT-127 produces distinct anxiolytic-like effects in rats without producing the adverse effects associated with benzodiazepines.

We previously reported that a δ opioid receptor agonist SNC80 produced potent anxiolytic-like effects in rodents. Recently, we succeeded in synthesizing a novel δ opioid receptor agonist KNT-127. In this study, we investigated the anxiolytic-like effects of KNT-127 using three different rat models of innate anxiety. In an elevated plus-maze test, KNT-127 (0.3, 1, and 3.0 mg/kg, s.c.) significantly and dose-dependently increased the time rats spent in the open arms 30 min after administration. The magnitude of the KNT-127 (3.0 mg/kg, s.c.)-induced anxiolytic-like effects was similar to that produced by diazepam (1.0 mg/kg, s.c.), a benzodiazepine anxiolytic. The anxiolytic-like effects of KNT-127 (3.0 mg/kg, s.c.) were abolished by pretreatment with naltrindole (0.1 mg/kg, s.c.), a selective δ opioid receptor antagonist, suggesting that KNT-127-induced anxiolytic-like effects are mediated by δ opioid receptors. These findings were supported by results obtained from light/dark and open-field tests. Interestingly, in contrast to diazepam (1.0 mg/kg, s.c.), KNT-127 (3.0 mg/kg, s.c.) caused no significant performance changes in the Y-maze test, the ethanol-induced sleeping test, and footprint test. This is the first study to demonstrate that the novel δ opioid receptor agonist KNT-127 produces distinct anxiolytic-like effects in rats, without producing the adverse effects associated with benzodiazepines.

Riluzole produces distinct anxiolytic-like effects in rats without the adverse effects associated with benzodiazepines.

In this study, we investigated the anxiolytic-like effect of riluzole using three different innate anxiety models in rats. In the elevated plus-maze test, riluzole significantly increased the time spent in, and entries into, the open arm after 60 min administration. This finding was supported by results obtained from light/dark and open-field tests. The magnitude of the anxiolytic-like effects of riluzole in each of the behavioral models was similar to those produced by a benzodiazepine, diazepam, suggesting that riluzole has a robust anxiolytic-like activity in rats. To clarify the involvement of sodium channels in this anxiolytic activity, we examined the effect of a co-administered sodium channel activator, veratrine. The anxiolytic-like action of riluzole was diminished by veratrine in the elevated plus-maze, light/dark and open-field tests. Based on these results, it is suggested that the anxiolytic mechanism of riluzole is clearly distinct from that of diazepam. In addition, to examine whether riluzole directly and non-selectively affected the GABA(A)-benzodiazepine receptor complex, we performed three behavioral tests (footprint analysis, Y-maze test and the ethanol-induced sleeping time test) that are closely related to the GABA(A)-benzodiazepine pathways. In contrast to diazepam, riluzole produced no significant effects in these tests. Here, we provide the first report demonstrating that riluzole produces distinct anxiolytic-like effects in rats without the adverse effects associated with benzodiazepines.

The novel δ opioid receptor agonist KNT-127 produces antidepressant-like and antinociceptive effects in mice without producing convulsions.

We previously reported that the δ opioid receptor (DOP) agonists SNC80 and TAN-67 produce potent antidepressant-like and antinociceptive effects in rodents. However, SNC80 produced convulsive effects. Recently, we succeeded in synthesizing a novel DOP agonist called KNT-127. The present study examined the convulsive, antidepressant-like, and antinociceptive effects of KNT-127 in mice. In contrast to SNC80, KNT-127 produced no convulsions at doses of up to 100mg/kg. In mice subjected to the forced swim test, a screening model for antidepressants, KNT-127 (1mg/kg, s.c.) significantly decreased the duration of immobility and increased the duration of swimming without influencing spontaneous locomotor activity. These behavioral changes were similar to that observed for the tricyclic antidepressant imipramine (6mg/kg). The antidepressant-like effect of KNT-127 in mice was antagonized by pretreatment with naltrindole (NTI), a selective DOP antagonist, or naltriben, a putative DOP(2) subtype antagonist. In addition, KNT-127 (3mg/kg, s.c.) significantly reduced the number of acetic acid-induced abdominal constrictions and the duration of licking time, respectively, in mice subjected to a writhing test and a formalin test. These antinociceptive effects were antagonized by pretreatment with either NTI or 7-benzylidenenaltrexone, a putative DOP(1) subtype antagonist. We propose that KNT-127 should be considered as a candidate compound for the development of DOP-based antidepressants and/or analgesics that lack convulsive effects.