Pivotal role of orexin signaling in the posterior paraventricular nucleus of the thalamus during the stress-induced reinstatement of oxycodone-seeking behavior

Abstract

Background: The orexin (OX) system has received increasing interest as a potential target for treating substance use disorder. OX transmission in the posterior paraventricular nucleus of the thalamus (pPVT), an area activated by highly salient stimuli that are both reinforcing and aversive, mediates cue- and stress-induced reinstatement of reward-seeking behavior. Oral administration of suvorexant (SUV), a dual OX receptor (OXR) antagonist (DORA), selectively reduced conditioned reinstatement of oxycodone-seeking behavior and stress-induced reinstatement of alcohol-seeking behavior in dependent rats.

Aims: This study tested whether OXR blockade in the pPVT with SUV reduces oxycodone or sweetened condensed milk (SCM) seeking elicited by conditioned cues or stress.

Methods: Male Wistar rats were trained to self-administer oxycodone (0.15 mg/kg, i.v., 8 h/day) or SCM (0.1 ml, 2:1 dilution [v/v], 30 min/day). After extinction, we tested the ability of intra-pPVT SUV (15 µg/0.5 µl) to prevent reinstatement of oxycodone or SCM seeking elicited by conditioned cues or footshock stress.

Results: The rats acquired oxycodone and SCM self-administration, and oxycodone intake correlated with signs of physical opioid withdrawal, confirming dependence. Following extinction, the presentation of conditioned cues or footshock elicited reinstatement of oxycodone- and SCM-seeking behavior. Intra-pPVT SUV blocked stress-induced reinstatement of oxycodone seeking but not conditioned reinstatement of oxycodone or SCM seeking or stress-induced reinstatement of SCM seeking.

Conclusions: The results indicate that OXR signaling in the pPVT is critical for stress-induced reinstatement of oxycodone seeking, further corroborating OXRs as treatment targets for opioid use disorder.

Keywords: Orexin; conditioned reinstatement; paraventricular nucleus of the thalamus; prescription opioid; stress-induced reinstatement.

Figure 1.

Experimental design for animals that were trained to self-administer (A) oxycodone and (B) SCM. Self-admin, self-administration; pPVT, posterior paraventricular nucleus of the thalamus; S^D, discriminative stimulus; EXT, extinction; S^N, neutral stimulus.

Figure 2.

Representation of injection sites aimed at the pPVT in (A) oxycodone-trained (n = 32) and (B) SCM-trained (n = 12) rats. ○, rats with correct injection sites; ×, rats with missed injection sites.

Figure 3.

(A) Oxycodone self-administration (n = 26). **p ≤ 0.01, vs. inactive lever (Bonferroni’s post hoc test). (B) Somatic withdrawal in oxycodone-dependent rats (n = 26). *p ≤ 0.05, **p ≤ 0.01, ****p ≤ 0.0001, vs. session 1 (Bonferroni’s post hoc test). Inset: Correlation plot between somatic withdrawal score at 15 h of abstinence and the number of oxycodone infusions that were earned during the prior self-administration session. pPVT, posterior paraventricular nucleus of the thalamus.

Figure 4.

Extinction training in the (A) absence (n = 13) and (B) presence (n = 13) of the S^D in oxycodone-dependent rats. ^nsp > 0.05, vs. inactive lever (Bonferroni’s post hoc test).

Figure 5.

Effect of intra-pPVT injection of SUV on the reinstatement of oxycodone-seeking behavior that was elicited by (A) the S^D (n = 13) or (B) footshock stress (n = 13). *p ≤ 0.05, **p ≤ 0.01, vs. EXT, ^#p ≤ 0.05, vs. VEH (Bonferroni’s post hoc test). EXT, extinction; S^N, neutral stimulus; VEH, vehicle; SUV, suvorexant; S^D, discriminative stimulus.

Figure 6.

Sweetened condensed milk self-administration (n = 10). *p ≤ 0.05, vs. inactive lever (Bonferroni’s post hoc test). pPVT, posterior paraventricular nucleus of the thalamus.

Figure 7.

Extinction training in the (A) absence (n = 10) and (B) presence (n = 10) of the S^D in rats that were trained to self-administer SCM. ^nsp > 0.05, vs. inactive lever (Bonferroni’s post hoc test).

Figure 8.

Effect of intra-pPVT injection of SUV on the reinstatement of SCM-seeking behavior that was elicited by (A) the S^D (n = 10) or (B) footshock stress (n = 10). *p ≤ 0.05, vs. EXT (Bonferroni’s post hoc test). EXT, extinction; S^N, neutral stimulus; VEH, vehicle; SUV, suvorexant; S^D, discriminative stimulus.