Effects of (2R,6R)-hydroxynorketamine in assays of acute pain-stimulated and pain-depressed behaviors in mice.

Ketamine has been shown to produce analgesia in various acute and chronic pain states; however, abuse liability concerns have limited its utility. The ketamine metabolite (2R,6R)-hydroxynorketamine (HNK) has been shown to produce antidepressant-like effects similar to ketamine without abuse liability concerns. (2R,6R)-HNK produces sustained analgesia in models of chronic pain, but has yet to be evaluated in models of acute pain. The present study evaluated the efficacy of acute (2R,6R)-HNK administration (one injection) in assays of pain-stimulated (52- and 56-degree hot plate test and acetic acid writhing) and pain-depressed behavior (locomotor activity and rearing) in male and female C57BL/6 mice. In assays of pain-stimulated behaviors, (2R,6R)-HNK (1-32 mg/kg) failed to produce antinociception in the 52- and 56-degree hot plate and acetic acid writhing assays. In assays of pain-depressed behaviors, 0.56% acetic acid produced a robust depression of locomotor activity and rearing that was not blocked by pretreatment of (2R,6R)-HNK (3.2-32 mg/kg). The positive controls morphine (hot plate test) and ketoprofen (acetic acid writhing, locomotor activity, and rearing) blocked pain-stimulated and pain-depressed behaviors. Finally, the effects of intermittent (2R,6R)-HNK administration were evaluated in 52-degree hot plate and pain-depressed locomotor activity and rearing. Intermittent administration of (2R,6R)-HNK also did not produce antinociceptive effects in the hot plate or pain-depressed locomotor activity assays. These results suggest that (2R,6R)-HNK is unlikely to have efficacy in treating acute pain; however, the efficacy of (2R,6R)-HNK in chronic pain states should continue to be evaluated.

Differential effects of repeated fluoxetine and ketamine administration on behavioral and pharmacological stressor-induced depression of digging behavior in mice.

Major depressive disorder is a multifactorial disorder that originates from a complex web of variables and overlaps with similar disorders (e.g., depression and anxiety). As such, animal models should account for the considerable symptom overlap between psychiatric disorders. We sought to extend the findings of behavioral assays that encompass both anxiety and stress/depression components. To do so, we have focused on digging behavior, a compulsive-like behavior displayed in mice, in which we employed behavioral and pharmacological stressors to reduce digging behaviors, producing a depression-like state. Locomotor activity was assessed during each test session. We found that digging behavior remains consistent, but locomotor activity decreased when exposed to multiple test sessions over 4 weeks and no sex differences were observed. A time-course study showed a single swim stress significantly reduced digging behavior for at least 3 days but rebounded to baseline levels by Day 7. Repeated treatment of 10 mg/kg/day fluoxetine, but not ketamine, partially reversed swim stress-induced depression of digging behavior on Days 3 and 7. The pharmacological stressor yohimbine (1.0-5.0 mg/kg) dose-dependently decreased digging behavior. Repeated treatment of 10 mg/kg/day ketamine, but not fluoxetine, reversed yohimbine-induced depression of digging behavior on Days 3 and 7. These data suggest that digging behavior is a stable and consistent behavior displayed by all mice. We were able to depress digging behavior with both behavioral and pharmacological stress. However, the reversal of stress-induced depression of digging behavior was stimulus- (e.g., behavioral vs. pharmacological) and drug-dependent and will require further investigation. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Methodological approach: using a within-subjects design in the marble burying assay.

In 2016, the National Institutes of Health mandated the use of both male and female mice in funded research. The use of both sexes is an important variable to consider; however, it comes with negative consequences such as increased animal expenses. One way to combat these negatives is to explore the option of using a within-subjects design (repeated measures) in behavioral assays that historically use a between-subjects design. Our study aimed to determine if a within-subjects design can be utilized in the marble burying assay. The marble burying assay is used as a tool for screening putative anxiolytic compounds as the assay is thought to measure obsessive-compulsive disorder- or anxiety-like behaviors. First, we compared the effects of sex and digging medium (corn cob or Sani Chip) on the number of marbles buried using CD-1 mice. Second, we determined if mice would continue to bury marbles after repeated exposures to the test arena. Lastly, we tested three positive controls (buspirone, ketamine, and fluoxetine). We found that mice buried significantly more marbles within Sani Chip digging medium, and no sex differences were observed. Next, the number of marbles buried and locomotor activity remained consistent across four test sessions. The positive controls buspirone (3.2-10 mg/kg) ketamine (32 mg/kg), and fluoxetine (10 mg/kg) decreased the number of marbles buried using the within-subjects design. These data suggest that a within-subjects design is optimal for the marble burying assay as it will reduce the number of animals and increase statistical power.