The Reln gene encodes the extracellular glycoprotein Reelin that regulates synaptic plasticity and activity-dependent gene expression with implication in several neuropsychiatric disorders, including substance use disorder. Although reduction of Reln expression has been associated with altered responses to psychostimulants and cannabinoids, its role in opioid-evoked behaviors has not been investigated. Here, we tested whether Reln haploinsufficiency alters behavioral and molecular responses to the synthetic opioid fentanyl. Heterozygous Reeler (Reln+/-) mice and wild-type littermates were assessed using complementary contingent and non-contingent models of fentanyl exposure, including multi-phase fentanyl intravenous self-administration paradigm, conditioned place preference paradigm, locomotor assays, and dorsal striatal immediate early gene expression. Reln haploinsufficiency did not alter acquisition, extinction, or cue-induced fentanyl seeking during self-administration, indicating stable opioid reinforcement and relapse-like behavior. Following acute, passive fentanyl exposure, Reln+/- mice exhibited enhanced fentanyl-induced locomotion and Fos immunoreactivity in the dorsal striatum, while fentanyl conditioned place preference remained unchanged. Together, these findings demonstrate that Reln haploinsufficiency does not modify opioid reinforcement or cue-driven drug seeking but enhances acute drug-evoked neuronal activation in the dorsal striatum. These results identify Reln as a modulatory factor in opioid-responsive neural circuits rather than in the associative learning processes underlying opioid reinforcement.

Traditional kratom (Mitragyna speciosa) products consist of botanical leaf derivatives dominated by mitragynine, with minimal 7-hydroxymitragynine (7-OH) or mitragynine pseudoindoxyl content. Kratom-branded products containing concentrated 7-OH and mitragynine pseudoindoxyl have proliferated since 2024 and are pharmacologically distinct from and more potent than mitragynine at the μ-opioid receptor (MOR). Morphine milligram equivalent (MME) values for mitragynine, 7-OH, and mitragynine pseudoindoxyl were estimated. Functional in vitro and in vivo data from 13 studies informed alkaloid-specific MME estimates, which were applied to alkaloid concentrations in commercial products to derive per-serving and per-day MME totals. Relative to morphine, MOR-referenced potency was lower for mitragynine (geomean MME 0.13) and higher for 7-OH and mitragynine pseudoindoxyl (geomean MME 4.3 and 4.8, respectively). Mean per-serving MME ranged from 2.7 (powders) to 31 (tablets) across product types. Screening of U.S. Food and Drug Administration pharmacovigilance data identified frequent reports of dependence, withdrawal, and psychiatric symptoms among users of 7-OH and mitragynine pseudoindoxyl, but polypharmacy effects could not be ruled out. These findings support pharmacological differentiation between traditional, mitragynine-dominant kratom preparations and concentrated 7-OH/mitragynine pseudoindoxyl products, consistent with the U.S. Drug Enforcement Administration's July 2026 notice of scheduling intent, while traditional preparations retain non-negligible, mitragynine-ceiling-limited opioid activity.

Substance use disorder is a chronic relapsing condition characterized by persistent drug seeking, cognitive impairment, and neurobiological alterations in brain reward and memory circuits. Emerging evidence suggests that dysfunction of the endogenous opioid system and reduced neurotrophic signaling contribute to the pathophysiology of opioid dependence. Cell based therapeutic strategies may provide a novel approach to modulate these neural pathways. In the present experimental study, morphine dependence was induced in adult male rats using an escalating dose regimen. Animals were randomly allocated into control, morphine-dependent vehicle, and cell-treated groups. Proenkephalin-expressing differentiated mesenchymal stem cells (PENK-MSCs) were delivered via the intranasal route. Behavioral assessments were performed using conditioned place preference, novel object recognition, and the elevated plus maze to evaluate addiction related behaviors, cognitive performance, and anxiety like responses. Hippocampal brain derived neurotrophic factor (BDNF) expression was examined using immunohistochemistry. Morphine dependent rats exhibited significant behavioral alterations, including enhanced drug related preference, impaired recognition memory, and increased anxiety like behavior compared with control animals. In addition, hippocampal BDNF expression was significantly reduced following morphine exposure. Intranasal cell delivery significantly attenuated morphine-induced preference, reversed recognition memory deficits, mitigated anxiety-like behaviors, and upregulated hippocampal BDNF expression compared with the vehicle-treated morphine group. These findings demonstrate that intranasal delivery of PENK-MSCs attenuates behavioral deficits and promotes neurotrophic recovery in morphine-dependent rats. This approach may represent a promising experimental strategy for targeting neural mechanisms involved in substance use disorder, although further studies are required to confirm its long-term efficacy and translational potential.

Physical exercise recruits endogenous opioid mechanisms in the central nervous system, but whether this physiological opioid tone potentiates the effects of exogenous μ-agonists, and whether such interactions differ between sexes, remains unknown. Here, we used the Straub tail response as a behavioral index of central μ2-opioid receptor activity to investigate whether acute submaximal running modulates the pharmacodynamic effects of meperidine in male and female Swiss mice. In resting animals, meperidine produced a dose- and time-dependent Straub tail response across a 12-fold dose range, and the submaximal doses of 8 and 12 mg/kg were selected for treadmill experiments. Running at approximately 70% of V̇O2 max potentiated the Straub tail response at 12 mg/kg in both sexes, with the effect substantially larger in females, as indicated by a significant Sex × Exercise interaction. Thermal antinociception was equivalent between sexes at 12 mg/kg, indicating that the dimorphism was specific to the exercise-opioid interaction rather than to baseline drug sensitivity. A separate experiment testing whether running alone elicits a naloxone-sensitive response was inconclusive in both sexes; under opioid receptor blockade, however, females maintained higher running-induced scores than males, indicating that the sex difference persists during naloxone treatment. These findings indicate that acute submaximal running potentiates the Straub tail response to meperidine in a dose- and sex-dependent manner. The sexually dimorphic nature of this interaction suggests that biological sex and exercise intensity may jointly modulate opioid pharmacodynamics during physical activity.

A recent driver of opioid-related overdose deaths has been the adulteration of illicitly manufactured fentanyl with alpha-2 (α2)-adrenergic agonists; however, little research has examined their toxicity when combined with fentanyl. The purpose of this study was to examine the effects of fentanyl and the α2-adrenergic agonist, dexmedetomidine, on cardiorespiratory measures that may contribute to the toxicity of these drug combinations (i.e., respiratory rate, heart rate, oxygen (O2) saturation, and pulse distention).

Cognitive impairments during abstinence may complicate recovery from alcohol use disorder, but their persistence across executive domains and accompanying cortical adaptations remain unclear. Neuropeptides are candidate mediators of persistent alcohol (ethanol; EtOH) effects, and emerging evidence implicates endogenous opioid peptides in cortical processing. Here, we examined cognitive function and medial prefrontal cortex (mPFC) enkephalin profiles in rats with a history of chronic EtOH exposure. Using a longitudinal operant-based approach, we assessed cognitive flexibility during early abstinence and delay-dependent working memory during protracted abstinence. We then quantified methionine-enkephalin (Met-ENK) and leucine-enkephalin (Leu-ENK) and profiled the proenkephalin-A (PENK)-derived peptide pool in the mPFC. EtOH history did not alter overall trials or errors to criterion during strategy set-shifting or reversal learning. During reversal, however, EtOH rats showed greater perseverative bias, with response adjustments less aligned with recent feedback and more strongly influenced by prior reinforcement. During initial testing at the longest working memory delay, EtOH rats also showed lower accuracy. Performance across these tasks was correlated in EtOH rats, suggesting individual differences in cognitive susceptibility were maintained across abstinence. Targeted analyses showed comparable levels of Met-ENK and Leu-ENK between groups. Although individual PENK-derived sequences did not differ in abundance, compositional analysis showed a greater summed relative contribution of C-terminal fragments in EtOH samples. These findings indicate that EtOH history produces lasting cognitive vulnerabilities that emerge under greater task demands, with some animals remaining more susceptible than others. C-terminal PENK profiling may reveal novel targets associated with enduring mPFC adaptations during abstinence.

Opioids are widely used for paediatric pain management; however, the long-term impacts of opioid exposure during later stages of neural development on spinal nociceptive circuitry remain unclear. We investigated whether repeated morphine exposure during preadolescence produces alterations in dorsal horn (DH) responses and sensory processing in adulthood. Male and female rats received morphine (3 mg/kg, twice daily) or saline from postnatal days 21-28. Mechanical withdrawal thresholds were assessed longitudinally. In adolescent, in vivo multi-electrode array recordings evaluated lamina-specific DH responses to mechanical and electrical stimulation, including frequency-dependent facilitation (wind-up). Immunohistochemistry quantified parvalbumin (PV), protein kinase C gamma (PKCγ), and somatostatin (SOM) expression. Preadolescent morphine exposure (PME) induced transient mechanical hypersensitivity in males immediately after treatment, whereas both sexes exhibited reduced sensitivity to mechanical stimulation at P40. PME was associated with enhanced mechanical evoked activity in the intermediate DH, alongside an absence of wind-up in this region. In the deep DH, wind-up was reduced in males but enhanced in females. PV expression was increased in both sexes and positively correlated with paw withdrawal thresholds. In contrast, PKCγ expression was reduced in males and SOM expression was reduced in females. These findings demonstrate that PME is associated with alterations in spinal nociceptive processing that is lamina and sex dependent. Changes in inhibitory and excitatory interneuron markers suggest a shift in DH circuit organisation which may contribute to altered sensory processing following early life opioid exposure. PERSPECTIVE: Preadolescent opioid exposure is associated with lasting changes in spinal nociceptive processing. These findings suggest that opioid use during critical developmental periods may influence long-term pain sensitivity, highlighting the importance of understanding how early-life analgesic strategies shape future sensory function.