Gomes, I. | Search bioRxiv

bioRxiv2026

Ketamine Modulates Endogenous Opioid Peptide Signaling and Reduces Heroin-Seeking in Male Long-Evans Rats

Opioid Use Disorder (OUD) continues to be a pressing public health crisis, marked by high relapse rates and limited treatment options. Ketamine, a non-competitive N-Methyl-D-Aspartate receptor (NMDAR) antagonist, has emerged as a promising therapeutic for numerous psychiatric illnesses due to its robust antidepressant properties and glutamate-mediated synaptic plasticity. However, ketamine's therapeutic mechanisms remain elusive, with emerging evidence implicating the endogenous opioid system. Furthermore, few studies have investigated ketamine's potential as an intervention for OUD. Accordingly, this study assessed ketamine's ability to rewire endogenous opioid and glutamatergic circuitry and reduce heroin-seeking activity in male Long-Evans rats. Using a multi-cohort design, we investigated ketamine's effects on opioid-related transcriptional, protein, and signaling properties in key brain regions associated with reward and addiction, and subsequently on heroin-seeking using a self-administration paradigm. To investigate mechanisms of plasticity, we also quantified transcriptional changes to NMDAR subunits. Molecular analyses demonstrated increased expression of endogenous opioid system-related genes and endogenous opioid peptides at the protein level, elevated mu-opioid receptor protein levels, and enhanced G protein activity, suggesting increased receptor function. Notably, transcriptional changes were particularly prominent in the shell subregion of the nucleus accumbens (NAc). We also revealed that ketamine altered NMDAR subunit expression in the medial prefrontal cortex and NAc, indicating circuit-level synaptic remodeling. Behavioral analyses revealed that ketamine significantly reduced heroin-primed, but not cue-induced, seeking activity. Together, these findings suggest ketamine reduces acute relapse vulnerability to opioids through coordinated modulation of opioid and glutamatergic systems, highlighting its potential as a novel intervention for OUD.

neuroscience↗

bioRxiv2025

Endothelin-converting enzyme 2 differentially regulates kappa opioid receptor trafficking and function

Following activation by endogenous opioid peptides, mu and delta opioid receptors have been shown to undergo differential internalization and recycling; the rate and extent of recycling but not internalization was found to be regulated by the endocytic peptide converting enzyme, ECE2. This study focuses on kappa opioid receptors (KOR) and the ability of endogenous opioid peptides released by post-translational processing of prodynorphin and proenkephalin to induce KOR and ECE2 internalization/recycling, and how ECE2 modulates these processes. Using a proximity-based ligation assay we show that KOR and ECE2 are in close proximity to facilitate co-internalization. In addition, we find that treatment with longer opioid peptides induces fast and robust internalization and recycling of ECE2 at a rate and extent comparable to that of KOR. Next, we directly examined the role of ECE2 in modulating KOR recycling. We find that ECE2 inhibition significantly attenuates KOR recycling. Finally, we examined the role of ECE2 in modulating KOR signaling and find that resensitization of KOR by peptides that are substrates of ECE2 are attenuated by ECE2 inhibition. Taken with the differential expression of ECE2 in the brain (relatively high expression in midbrain & hypothalamus and low expression in the striatum & hippocampus), these results highlight a pivotal role for ECE2 in differentially modulating KOR function. Significance StatementThis study highlights a role for ECE2 in agonist mediated regulation of KOR function by select prodynorphin and proenkephalin derived peptides. Collectively our studies suggest that ECE2 inhibitors could be developed as therapeutics for pathologies involving dysregulations in KOR signaling.

neuroscience↗