Behavioral Pharmacology of Novel Kappa Opioid Receptor Antagonists in Rats

Abstract

Background: New treatments for stress-related disorders including depression, anxiety, and substance use disorder are greatly needed. Kappa opioid receptors are expressed in the central nervous system, including areas implicated in analgesia and affective state. Although kappa opioid receptor agonists share the antinociceptive effects of mu opioid receptor agonists, they also tend to produce negative affective states. In contrast, selective kappa opioid receptor antagonists have antidepressant- and anxiolytic-like effects, stimulating interest in their therapeutic potential. The prototypical kappa opioid receptor antagonists (e.g., norBNI, JDTic) have an exceptionally long duration of action that complicates their use in humans, particularly in tests to establish safety. This study was designed to test dose- and time-course effects of novel kappa opioid receptor antagonists with the goal of identifying short-acting lead compounds for future medication development.

Methods: We screened 2 novel, highly selective kappa opioid receptor antagonists (CYM-52220 and CYM-52288) with oral efficacy in the warm water tail flick assay in rats to determine initial dose and time course effects. For comparison, we tested existing kappa opioid receptor antagonists JDTic and LY-2456302 (also known as CERC-501 or JNJ-67953964).

Results: In the tail flick assay, the rank order of duration of action for the antagonists was LY-2456302 < CYM-52288 < CYM-52220 << JDTic. Furthermore, LY-2456302 blocked the depressive (anhedonia-producing) effects of the kappa opioid receptor agonist U50,488 in the intracranial self-stimulation paradigm, albeit at a higher dose than that needed for analgesic blockade in the tail flick assay.

Conclusions: These results suggest that structurally diverse kappa opioid receptor antagonists can have short-acting effects and that LY-2456302 reduces anhedonia as measured in the intracranial self-stimulation test.

Keywords: JDTic; 488; ICSS; LY-2456302; U50; analgesia.

Figure 1.

Effects of systemic (i.p.) vs oral (p.o.) administration of JDTic on U50,488-induced analgesia in the warm water tail flick assay (TFA). Baseline TFA latencies were obtained at least 2 hours prior to any drug treatment. JDTic was administered before the TFA was performed at the times indicated in the legend. U50,488 (15 mg/kg, i.p.) was injected 1 hour before the TFA was performed in each group. n = 12/group for baseline and U50,488 alone, n = 6/group for JDTic groups. **P < .01 compared withU50,488 alone (U50, white bar).

Figure 2.

Dose response and time course effects of LY-2456302 on U50,488-induced analgesia in the warm water tail flick assay (TFA). (a) LY-2456302 was administered orally (p.o.) 1 hour prior to injection of U50,488 (30 mg/kg, i.p.), and TFA latencies were determined 1 hour after U50,488 administration. n = 12 (0.0 mg/kg), 6 (0.1 mg/kg), 7 (0.3 mg/kg), 6 (1.0 mg/kg), 6 (3.0 mg/kg), and 8 (10.0 mg/kg). *P < .05 compared withLY-2456302 (0.0 mg/kg). (b) Based on the dose response, nonlinear regression was used to determine the AD₈₀ dose, which was then used to test the ability of LY-2456302 to attenuate U50,488-induced analgesia when administered 1 hour (n = 18), 2 hours (n = 16), 4 hours (n = 12), 8 hours (n = 7), and 24 hours (n = 12) prior to the TFA test. LY-2456302 (0.9 mg/kg, p.o.) significantly reduced U50,488-induced increases in TFA latency when administered 1 hour before the TFA test. *P < .05 compared withVeh. (c) For each LY-2456302 pretreatment time point, a separate vehicle (3% lactic acid in water) group was tested; total n = 38. There was no effect of Veh pretreatment on TFA latency at any time. As such, Veh groups for each time point were collapsed for the analysis shown in (b).

Figure 3.

Dose response and time course effects of CYM-52220 and CYM-52288 on U50,488-induced analgesia in the warm water tail flick assay (TFA). (a) CYM-52220 [n = 12 (0.0 mg/kg), 12 (3.0 mg/kg), 13 (6.0 mg/kg), 16 (8.0 mg/kg), 9 (10 mg/kg), and 6 (30.0 mg/kg)] was administered p.o. 1-hour prior to injection of U50,488 (30 mg/kg, IP), and TFA latencies were determined 1-hour after U50,488 administration. (b) Based on the CYM-52220 dose response, nonlinear regression was used to determine the AD₈₀ dose, which was then used to test the ability of CYM-52220 to attenuate U50,488-induced analgesia when administered 1 hour (n = 14), 2 hours (n = 14), 4 hours (n = 14), 8 hours (n = 14), and 24 hours (n = 8) prior to the TFA test. CYM-52220 vehicle control groups were included for each time point, but similar to data shown in Figure 2C there was no significant difference in tail flick latencies at any vehicle time point, so the values were combined (total n = 60). CYM-52220 (6.0 mg/kg, p.o.) significantly reduced U50,488-induced increases in TFA latency when administered 1, 2, 4 (but not 8), or 24 hours before the TFA test. (c) CYM-52288 [n = 16 (0.0 mg/kg), 17 (1.0 mg/kg), 16 (1.8 mg/kg), 16 (2.4 mg/kg), 19 (3.0 mg/kg), and 16 (10.0 mg/kg)] was administered p.o. 1 hour prior to injection of U50,488 (30 mg/kg, IP), and TFA latencies were determined 1 hour after U50,488 administration. *P < .05, **P < .01 compared with the 0.0-mg/kg dose. (d) Based on the CYM-52288 dose response, nonlinear regression was used to determine the AD₈₀ dose, which was then used to test the ability of CYM-52288 to attenuate U50,488-induced analgesia when administered 1 hour (n = 13), 2 hours (n = 11), 4 hours (n = 9), 8 hours (n = 12), and 24 hours (n = 10) prior to the TFA test. CYM-52288 vehicle control groups were included for each time point, but similar to data shown in Figure 2C, there was no significant difference in tail flick latencies at any vehicle time point, so the values were combined (total n = 20). CYM-52288 (3.2 mg/kg, p.o.) significantly reduced U50,488-induced increases in TFA latency when administered 1, 2, and 4 (but not 8) hours before the TFA test. There was a strong trend (P = .06) for CYM-52288 to reduce U50,488-induced increases in TFA latency when administered 24 hours before testing. *P < .05, **P < .01 compared with vehicle.

Figure 4.

LY-2456302 attenuates U50,488-induced increases in ICSS stimulation thresholds and decreases in maximum rates of responding. LY-2456302 was administered p.o. 30 minutes before injection of U50,488 (10 mg/kg, i.p.). Shown are effects on (a) ICSS stimulation thresholds (Theta 0) and (b) maximum rates of operant responding for: Veh + Veh (n = 9), Veh + U50 (n = 8), and LY-2456302 [1 mg/kg (n = 5), 10 mg/kg (n = 5), and 30 mg/kg (n = 5)] + U50. ICSS thresholds and rates of responding were measured for a total of 90 minutes and data are presented as percent pre-drug baseline. ##P < .01 comparing [Veh + U50] to [Veh + Veh]; **P < .01 comparing [LY (30) + U50] to [Veh + U50] (a), and [LY (10) and LY (30) + U50] to [Veh + U50] (b). LY-2456302 (30 mg/kg, n = 5) on its own had no effect on ICSS thresholds (c) or maximum rates of responding (d) over the 90-minute session. (e) No dose of LY-2456302 significantly altered percent baseline ICSS thresholds in the first 30 minutes post-LY injection, but before Veh was injected at 30 minutes: LY-2456302 [Veh (n = 9), 1.0 mg/kg (n = 5), 10.0 mg/kg (n = 5), 30 mg/kg (n = 8)]. (f) Average of changes in percent baseline ICSS thresholds for each treatment from 30 to 90 minutes post-LY-2456302 treatment, including LY-2456302 (1.0 mg/kg, n = 5). **P < .05 compared with Veh + U50.