Functional characterization of a novel opioid, PZM21, and its effects on the behavioural responses to morphine

Abstract

Background and purpose: The concept of opioid ligands biased towards the G protein pathway with minimal recruitment of β-arrestin-2 is a promising approach for the development of novel, efficient, and potentially nonaddictive opioid therapeutics. A recently discovered biased μ-opioid receptor agonist, PZM21, showed analgesic effects with reduced side effects. Here, we aimed to further investigate the behavioural and biochemical properties of PZM21.

Experiment approach: We evaluated antinociceptive effects of systemic and intrathecal PZM21 administration. Its addiction-like properties were determined using several behavioural approaches: conditioned place preference, locomotor sensitization, precipitated withdrawal, and self-administration. Also, effects of PZM21 on morphine-induced antinociception, tolerance, and reward were assessed. Effects of PZM21 on striatal release of monoamines were evaluated using brain microdialysis.

Key results: PZM21 caused long-lasting dose-dependent antinociception. It did not induce reward- and reinforcement-related behaviour; however, its repeated administration led to antinociceptive tolerance and naloxone-precipitated withdrawal symptoms. Pretreatment with PZM21 enhanced morphine-induced antinociception and attenuated the expression of morphine reward. In comparison to morphine, PZM21 administration induced a moderate release of dopamine and a robust release of 5-HT in the striatum.

Conclusions and implications: PZM21 exhibited antinociceptive efficacy, without rewarding or reinforcing properties. However, its clinical application may be restricted, as it induces tolerance and withdrawal symptoms. Notably, its ability to diminish morphine reward implies that PZM21 may be useful in treatment of opioid use disorders.

Figure 1

Effects of PZM21 on acute thermal antinociception. (a) Administration of morphine (10 mg·kg⁻¹, i.p.), used as a positive control, resulted in an attenuated sensitivity to painful stimulus in the tail flick test (b) Treatment with PZM21 (20, 40, and 80 mg·kg⁻¹, i.p.) caused dose‐dependent antinociceptive effect measured in the tail flick test. When compared to saline, the antinociceptive effect of 20 mg·kg⁻¹ of PZM21 was statistically significant 2 and 4 hr after the drug administration, while treatment with doses of 40 and 80 mg·kg⁻¹ of the compound induced antinociception that lasted from 1 to 4 hr after the treatment. Morphine and PZM21 groups are compared to the same saline controls. (c) A selective μ‐receptor antagonist, cyprodime (10 mg·kg⁻¹, i.p.), administered 15 min prior to PZM21 (40 mg·kg⁻¹), prevented antinociception in the tail flick test. (d) PZM21 had no effect on the paw flinching reaction in the hot plate test. However, at a dose of 80 mg·kg⁻¹, it increased the latency to paw licking/jumping behaviour. Treatment with morphine significantly attenuated both types of reactions. Both responses were measured 90 min after drug administration. (e) Pretreatment with cyprodime attenuated the effects of 40 mg·kg⁻¹ of PZM21 on both types of reaction in the hot plate test. * P < .05, significant effect of cyprodime. (f) Intrathecal administration of PZM21 (at doses of 5 and 7.5 μg) caused antinociceptive effects in the tail flick test in rats. Data are presented as the mean ± SEM. * P < .05, significant effect of treatment compared with appropriate controls. Numbers of animals used in experiments presented in Table S5. Cyp, cyprodime; MPE, maximum possible effect; Sal, saline

Figure 2

Influence of PZM21 on addiction‐like behaviour in mice. (a) In contrast to morphine (10 mg·kg⁻¹, i.p.), PZM21 (20, 40, and 80 mg·kg⁻¹, i.p.) did not induce a preference towards drug‐associated compartment in a CPP test at any of the tested doses. (b) Repeated treatment with morphine induced locomotor sensitization and expression, whereas that effect was not observed after PZM21 administration. Mice treated with 80 mg·kg⁻¹ PZM21 presented a slight expression of sensitization after an 8‐day incubation period. (c) Chronic administration of PZM21 (80 mg·kg⁻¹, but not 20 or 40 mg·kg⁻¹) as well as morphine induced naloxone‐precipitated jumps, considered as a physical sign of withdrawal. (d) Repeated treatment with 40 and 80 mg·kg⁻¹ PZM21 resulted in a decrease of antinociceptive efficacy of the compound. Tolerance was assessed using tail flick test performed on each experimental day, 1 hr after the drug administration. Data are presented as the mean ± SEM. In (a, c), * P<.05, PZM21‐ and morphine‐treated groups significantly different from saline controls; in (b, d) within group effects significantly different from the first day of experiment. # P < .05, expression of locomotor sensitization within groups significantly different from the last day of sensitization development. Numbers of animals used in experiments presented in Table S5. CPP, conditioned place preference; Morph, morphine; MPE, maximum possible effect; Sal, saline

Figure 3

Evaluation of PZM21 effects on intravenous self‐administration in rats. (a) Rats that self‐administered oxycodone (0.06 mg·kg⁻¹ per infusion, i.v.), but not PZM21 (0.05 and 0.5 mg·kg⁻¹ per infusion, i.v.), presented an increasing number of infusions over time. (b) Only rats from the oxycodone group presented an increasing number of active lever responses. (c) No differences between groups were observed in inactive lever presses during self‐administration training. (d) Unlike the oxycodone group, rats in the saline and PZM21 groups did not present drug‐seeking behaviour after abstinence period, as they did not discriminate between active and inactive levers and made a similar number of responses on both levers. Data are presented as the mean ± SEM. In (a), * P < .05, within group effects significantly different from the first day of experiment; in (b–d), active and inactive lever responses significantly different within experimental groups. Numbers of animals used in experiments presented in Table S5. Oxy, oxycodone; Sal, saline

Figure 4

Effects of PZM21 on striatal dopamine and 5‐HT levels. (a) Administration of 40 and 80 mg·kg⁻¹ PZM21 (i.p.) as well as 10 and 20 mg·kg⁻¹ of morphine (i.p.) increased extracellular level of dopamine in the striatum. Basal extracellular levels were 3.71 ± 0.51 pg in a volume of 10 μl (n = 30). (b) All doses of PZM21 and morphine potentiated striatal 5‐HT release when compared to saline. Basal extracellular levels were 0.40 ± 0.06 pg in a volume of 10 μl (n = 30). Data are presented as the mean ± SEM. Bar graphs presenting cumulative data are expressed as AUC. * P < .05, significantly different from saline. Numbers of animals used in experiments presented in Table S5. DA, dopamine; Morph, morphine; Sal, saline

Figure 5

Influence of PZM21 on behavioural effects of morphine. (a) PZM21 (at dose of 40 mg·kg⁻¹, i.p.), administered 30 min prior to morphine, enhanced antinociception evoked by 5 mg·kg⁻¹ of morphine (i.p.) in the tail flick test. (b) Pretreatment with PZM21 had no effect on the development of tolerance to antinociception induced by 10 mg·kg⁻¹ of morphine. Tolerance was assessed using tail flick test performed on each experimental day, 1 hr after the drug administration. (c) Pre‐administration of PZM21 at a dose of 40 mg·kg⁻¹, but not 20 mg·kg⁻¹, prevented the formation of conditioned response to morphine (10 mg·kg⁻¹). (d) Pretreatment with PZM21 resulted in a tendency towards reduced development, but not expression, of locomotor sensitization induced by repeated administration of morphine (10 mg·kg⁻¹). Data are presented as the mean ± SEM. * P < .05, significantly different from morphine controls. Numbers of animals used in experiments presented in Table S5. CPP, conditioned place preference; Morph, morphine; MPE, maximum possible effect; Sal, saline