U50,488 Enantiomers Differentially Contribute to its Analgesic, Diuretic, and Discriminative Stimulus Effects

Abstract

Kappa (κ) opioid receptor agonists can produce prominent analgesic and diuretic effects. However, these agonists also produce dysphoria, which markedly reduces their clinical utility. The purpose of the present experiment was to determine if the optical isomers of the κ agonist U50,488 ((±)‐trans‐3,4‐dichloro‐N‐methyl‐N‐[2‐(1‐pyrrolidinyl)‐cyclohexyl]benzeneacetamide) might be differentially effective in producing these κ‐mediated effects. In addition, the effects of specific cannabinoids were tested alone and in combination with the U50,488 racemate on several measures of κ activity. To establish some of U50,488's subjective effects as a discriminate stimulus, a group (n=12) of rats were trained to discriminate 5.6 mg/kg from saline under a fixed‐ratio 20 (FR‐20) schedule of food reinforcement. Then, following sessions in which the enantiomers or the cannabinoids were administered, antinociception was assessed in these rats using both a warm‐water tail‐withdrawal procedure and a paw‐withdrawal (Von Frey) procedure. In separate groups of rats (n=6/group), U50,488 and its enantiomers, (−)‐(1S,2S)‐U50,488 and (+)‐(1R,2R)‐U50,488, were administered intracerebroventricularly (ICV) to determine their capacity for producing an aquaresis. With respect to the discrimination procedure, both (−)‐(1S,2S)‐U50,488 and delta‐9‐tetrahydrocannabinol (Δ9‐THC) produced dose‐dependent increases in U50,488‐lever responding, but only (−)‐(1S,2S)‐U50,488 produced full (>80%) substitution. In contrast, both (+)‐(1R,2R)‐U50,488 and the mu opioid receptor agonist morphine (negative control) engendered less than 20% U50,488‐lever responding up to doses that significantly decreased overall response rate. Doses of U50,488 and (−)‐(1S,2S)‐U50,488 that produced U50,488‐appropriate responding also produced antinociception by dose‐dependently increasing both tail‐ and paw‐withdrawal latencies, whereas (+)‐(1R,2R)‐U50,488 was inactive in both assays. Finally, a synergistic interaction between the cannabinoids and κ opioids occurred when a dose of 0.056 mg/kg of CP‐55,940, a non‐selective CB1/CB2 receptor agonist, was administered prior to ineffective doses of U50,448. This dose of CP‐55,940 produced a small effect alone and potentiated the effects of low doses of U50,488 in the paw‐withdrawal assay (i.e., there was an upward shift in the dose‐effect curve). While all of the κ agonists tested produced an aquaresis, as measured by an increase in urine output, an equivalent dose of each produced a rank order of (−)‐(1S,2S)‐U50,488 > U50,488 > (+)‐(1R,2R)‐U50,488. These increases in urine output were also blocked by a pretreatment with the κ antagonist norbinaltorphimine (nor‐BNI), indicating these effects were mediated by κ opioid receptors. Taken together, these data indicate that the (−)‐(1S,2S)‐U50,488 enantiomer produces discriminative, antinociceptive, and diuretic effects that are most similar to those of the racemate, and provide evidence for a synergistic interaction between κ opioids and cannabinoid receptors. Further studies are needed in order to reevaluate the clinical potential of κ opioid receptor agonists as therapeutic targets as either analgesics or diuretics.Support or Funding InformationNIH P30GM106392 (DRK)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.