The highly selective dopamine D3R antagonist, R-VK4-40 attenuates oxycodone reward and augments analgesia in rodents

Abstract

Prescription opioid abuse is a global crisis. New treatment strategies for pain and opioid use disorders are urgently required. We evaluated the effects of R-VK4-40, a highly selective dopamine (DA) D₃ receptor (D₃R) antagonist, on the rewarding and analgesic effects of oxycodone, the most commonly abused prescription opioid, in rats and mice. Systemic administration of R-VK4-40 dose-dependently inhibited oxycodone self-administration and shifted oxycodone dose-response curves downward in rats. Pretreatment with R-VK4-40 also dose-dependently lowered break-points for oxycodone under a progressive-ratio schedule. To determine whether a DA-dependent mechanism underlies the impact of D₃ antagonism in reducing opioid reward, we used optogenetic approaches to examine intracranial self-stimulation (ICSS) maintained by optical activation of ventral tegmental area (VTA) DA neurons in DAT-Cre mice. Photoactivation of VTA DA in non-drug treated mice produced robust ICSS behavior. Lower doses of oxycodone enhanced, while higher doses inhibited, optical ICSS. Pretreatment with R-VK4-40 blocked oxycodone-enhanced brain-stimulation reward. By itself, R-VK4-40 produced a modest dose-dependent reduction in optical ICSS. Pretreatment with R-VK4-40 did not compromise the antinociceptive effects of oxycodone in rats, and R-VK4-40 alone produced mild antinociceptive effects without altering open-field locomotion or rotarod locomotor performance. Together, these findings suggest R-VK4-40 may permit a lower dose of prescription opioids for pain management, potentially mitigating tolerance and dependence, while diminishing reward potency. Hence, development of R-VK4-40 as a therapy for the treatment of opioid use disorders and/or pain is currently underway. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Keywords: Brain-stimulation reward; D(3) receptor antagonist; Opioid analgesia; Oxycodone; R-VK4-40; Self-administration.

Figure 1.

The chemical structure and receptor binding affinities of R-VK4-40 and comparisons to previously reported D₃R antagonists/partial agonists CAB2-015, BAK4-54 and (±)VK4-116.

Figure 2.

Phase I metabolic stability of R-VK4-40 in rat liver microsomes fortified with NADPH. R-VK4-40 was stable in rat liver microsomes with 86 ± 2 % intact remaining following 60 min incubation. Control experiments without cofactors were conducted in parallel with >95% compound remained at 60 min.

Figure 3.

Pharmacokinetic evaluation of R-VK4-40 following peri-oral administration in rats at 10 mg/kg. (A) Plasma and brain concentration vs time profile of R-VK4-40; data is expressed as mean ± SD, n=3, per time-point. (B) Mean ± SD pharmacokinetic parameters in plasma and brain. Brain to plasma ratio is calculated from mean total AUC_brain to AUC_plasma.

Figure 4.

Effects of R-VK4-40 on oxycodone self-administration under FR1 reinforcement in rats. A: Oxycodone self-administration dose-response curves in the presence or absence of R-VK4-40, illustrating that R-VK4-40 dose-dependently inhibited oxycodone self-administration. B: Pretreatments with R-VK4-40 also dose-dependently reduced oxycodone intake (mg/kg) during self-administration. C: Pretreatment with R-VK4-40 had no effect on inactive lever responses during oxycodone self-administration. på<0.01 (10 mg/kg R-VK4-40), ***p<0.001 (20 mg/kg R-VK4-40) compared to vehicle group.

Figure 5.

Oxycodone self-administration under progressive-ratio (PR) reinforcement. A: Break-points for oxycodone self-administration in the presence or absence of R-VK4-40. B: Normalized break-points over baseline immediately before the testing day (% change in break-point). *p < 0.05 compared to vehicle group.

Figure 6.

Effects of R-VK4-40 and/or oxycodone on optical brain-stimulation reward in DAT-Cre mice. A: Schematic diagrams illustrating the target brain region (VTA) of the AAV-DIO-ChR2-EGFP microinjections and intracranial optical fiber implantation (left). B, C, D: Representative images of TH-immunostaining (red) and fluorescent ChR2-EGFP expression (green) in the VTA. E: Representative rate-frequency curve under baseline conditions. F: Systemic administration of oxycodone produced dose-dependent biphasic effects – lower doses upward-shifted, while higher doses downward-shifted stimulation-response curves. G: Systemic administration of R-VK4-40 alone dose-dependently shifted the stimulation-response curve downward. H: Pretreatment with R-VK4-40 dose-dependently blocked 1 mg/kg oxycodone-enhanced optical ICSS. *p < 0.05; **p < 0.01, ***p <0.001 compared to the vehicle control group; ^##p < 0.01 (10 mg/kg R-VK4-40), compared to control groups.

Figure 7:

Effects of R-VK4-40 and/or oxycodone on thermal nociceptive responses in rats. A: The time course of the analgesic effects of oxycodone or R-VK4-40 alone as assessed in the hotplate test. B: Thirty-minute pretreatment with R-VK4-40 produced an upward shift in the dose-response curve of oxycodone-induced analgesia at 15 minutes post-injection, suggesting enhanced antinociceptive effects. C: Thirty-minute pretreatment with R-VK4-40 dose-dependently suppressed sucrose self-administration in mice. D: Open-field locomotor behavior indicating that R-VK4-40 had no effect on locomotion in rats. E: Rotarod mobility is similarly unaffected by R-VK4-40 treatment in mice. F: Combination of R-VK4-40 with 1 mg/kg oxycodone produced a non-significant trend towards locomotor sedation in mice. *p<0.05, ** p<0.01, ***p<0.001 compared to control groups.