Major Depressive Disorder (MDD) is a common and debilitating mental illness primarily characterized by depressed mood and anhedonia, as well as a heterogenous mixture of other somatic symptoms. Existing therapies for MDD act primarily on monoamine transporters and receptors, with only partial success. A more recent advancement in depression treatment is the identification of the N-methyl-d-aspartate receptor (NMDAR) antagonist (R,S)-ketamine, which has shown efficacy in individuals with treatment resistant depression. Studies have raised the possibility of a mu opioid receptor (MOR)-dependent component to the actions of (R,S)- ketamine, through direct activation of MOR or indirect effects of NMDAR antagonism on endogenous opioids.
Considering the ongoing global opioid epidemic, in which over prescribing of opioid painkillers and greater availability of illicit opioids has caused significant morbidity and mortality, studies suggesting an opioid component to ketamine’s actions have caused concern. We undertook the current experiments to better understand 1) the contribution of MORs to the behavioral effects of ketamine, and 2) how very low efficacy agonism of MOR may lead to a better separation between the desirable and undesirable effects of opioid drugs. Previous work from collaborators had established that a single dose of (R,S)-ketamine, specific hydroxynorketamine (HNK) metabolites, and the memantine derivative fluoroethylnormemantine (FENM), given one week before a contextual fear conditioning stressor could prevent the development of learned fear and behavioral despair.
Moreover, some of these drugs also prevented the development of learned fear and behavioral despair when given shortly after the contextual fear conditioning stressor. We were interested in how (R,S)- ketamine’s reported MOR activity may contribute to these behavioral effects. We began by testing these compounds in cell-based signaling assays to determine their ability to directly activate MORs. We found that the parent (R)- and (S)-ketamine enantiomers could activate MORs with low potency in a high amplification G protein activation assay. In contrast, in low amplification miniG-based assays, the compounds tested showed little to no activation of MOR, suggesting that (R,S)-ketamine activates MOR with low potency and low efficacy.
We moved to experiments in mice with the pseudo-irreversible MOR antagonist methocinnamox (MCAM) and attempted to block various behavioral effects of (R,S)-ketamine and the more selective NMDAR antagonist (FENM). We found that MCAM pretreatment blocked effects of (R,S)-ketamine on antinociception, behavioral despair, and learned fear, while other effects such as anesthesia and changes in hippocampal electrophysiology were not prevented by MCAM. With FENM the antinociceptive effects were less potent and less impacted by MCAM; however, the effects on behavioral despair and learned fear were still MORdependent.
These results suggest that the protective effects of (R,S)-ketamine and FENM against stress may be an indirect effect of NMDAR antagonism on endogenous opioids. In a separate line of experiments, we used a range of doses of MCAM to antagonize the behavioral effects of morphine, the MOR agonist antidepressant tianeptine, and the MOR partial agonist 7-OH mitragynine. 7-OH mitragynine is a metabolite of mitragynine, the major alkaloid in kratom, which some reports suggest may have antidepressant and anxiolytic properties. Based on fundamentals of pharmacology, we hypothesized that inherent differences in the presence of “spare receptors” or receptor reserve between brain circuits could be revealed by differential inhibition by MCAM across behaviors and agonists. We assessed the inhibitory potency of MCAM against these drugs in tests of antinociception, hyperlocomotion, behavioral despair, respiration, and gastrointestinal motility.
We found that MCAM pretreatment more potently inhibited the low efficacy agonist 7-OH mitragynine in the tests of antinociception, behavioral despair, and constipation. These data suggest that in circuits modulating antinociception, behavioral despair, and constipation, differences in receptor reserve likely facilitate the response to low efficacy agonists. However, our data also argue that the wider therapeutic window of G protein biased, low intrinsic efficacy MOR agonists is not solely a result of differences in the number of “spare” MORs regulating the effects of opioids in different circuits.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/vyvt-2460 |
| Date | January 2024 |
| Creators | Langreck, Cory |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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