Role of projections from ventral subiculum to nucleus accumbens shell in context-induced reinstatement of heroin seeking in rats

Abstract

Rationale and objective: In humans, exposure to contexts previously associated with heroin use can provoke relapse. In rats, exposure to heroin-paired contexts after extinction of drug-reinforced responding in different contexts reinstates heroin seeking. We previously demonstrated that the projections from ventral medial prefrontal cortex (vmPFC) to nucleus accumbens (NAc) shell play a role in this reinstatement. The ventral subiculum (vSub) sends glutamate projections to NAc shell and vmPFC. Here, we determined whether these projections contribute to context-induced reinstatement.

Methods: We trained rats to self-administer heroin (0.05-0.1 mg/kg/infusion) for 3 h per day for 12 days; drug infusions were paired with a discrete tone-light cue. Lever pressing in the presence of the discrete cue was subsequently extinguished in a different context. We then tested the rats for reinstatement in the heroin- and extinction-associated contexts under extinction conditions. We combined Fos with the retrograde tracer Fluoro-Gold (FG) to determine projection-specific activation during the context-induced reinstatement tests. We also used anatomical disconnection procedures to determine whether the vSub → NAc shell and vSub → vmPFC projections are functionally involved in this reinstatement.

Results: Exposure to the heroin but not the extinction context reinstated lever pressing. Context-induced reinstatement of heroin seeking was associated with increased Fos expression in vSub neurons, including those projecting to NAc shell and vmPFC. Anatomical disconnection of the vSub → NAc shell projection, but not the vSub → vmPFC projection, decreased this reinstatement.

Conclusions: Our data indicate that the vSub → NAc shell glutamatergic projection, but not the vSub → vmPFC projection, contributes to context-induced reinstatement of heroin seeking.

Keywords: Baclofen; Conditioned cues; Dopamine; Drug environment; Extinction; Fluoro-Gold; Fos; GABA; Glutamate; Heroin self-administration; Hippocampus; Muscimol; Opiates; Relapse; SCH 23390.

Figure 1. Heroin self-administration training and extinction of drug-reinforced responding

(A) Training: Mean±SEM number of infusions and active and inactive lever responses during the 12 d of heroin self-administration training for Exp. 1–4. (B) Extinction: Mean±SEM number of presses on the active lever and inactive lever during the first 12 extinction sessions conducted in the absence of heroin in a different (extinction) context for Exp. 1–4 (total n=132).

Figure 2. Activation of vSub→NAc shell projection during context-induced reinstatement

(A) Reinstatement test: Total number of active lever presses in rats tested in the Extinction (Control A-B-B) or the Heroin (Renewal A-B-A) context. (B) FG-IR cells: Number of FG-IR cells per mm² in ipsilateral vSub of rats tested in the Extinction or Heroin context. (C) Fos-IR cells: Number of total Fos-IR nuclei per mm² (ipsilateral and contralateral hemispheres were averaged) in vSub of rats tested in the Extinction or Heroin context. (D) Fos+FG double-labeled cells: Percentage of Fos+FG-IR cells in the ipsilateral vSub of rats tested in the Extinction context or Heroin context. (E) Representative photomicrographs of FG injection into NAc, FG labeling, and Fos+FG cells in ipsilateral vSub. *Different from the Extinction context, p<0.05, n=8–9 per group.

Figure 3. Disconnection of vSub→NAc shell projection decreases context-induced reinstatement

(A) Reinstatement Test: Total number of active lever presses in rats tested in the Extinction and Heroin context after unilateral injections of vehicle or M+B into vSub and contralateral or ipsilateral injection of vehicle or SCH23390 into NAc shell before exposure to Heroin context or Extinction context. (B). Cannula placements and representative pictures. Approximate placements (mm from Bregma) of the injector tips for vSub and contralateral (left) or ipsilateral (right) NAc shell (Paxinos and Watson 2008) and representative photomicrographs of cannula placements are shown. Because implantation of cannula into the left or right hemisphere was counterbalanced, injector tip placements are depicted in both hemispheres (light gray circles = vehicle, black circles = drug). *Different from the Vehicle condition, p<0.01 (n=10–19 per group)

Figure 4. Activation of vSub→vmPFC projection during context-induced reinstatement

(A) Reinstatement Test: Total number of active lever presses in rats tested in the Extinction (Control A-B-B) or the Heroin (Renewal A-B-A) context. (B) FG-IR cells: Number of FG-IR cells per mm² in ipsilateral vSub of rats tested in the Extinction or Heroin context. (C) Fos-IR cells: Number of total Fos-IR nuclei per mm² (ipsilateral and contralateral hemispheres were averaged) in vSub of rats tested in the Extinction or Heroin context. (D) Fos+FG double-labeled cells: Percentage of Fos+FG-IR cells in the ipsilateral vSub of rats tested in the Extinction context or Heroin context. (E) Representative photomicrographs of FG injection into vmPFC, respectively, FG labeling, and Fos+FG cells in ipsilateral vSub. *Different from the Extinction context, p<0.05 (n=11–12 for A and C, and n=10 per group for B and D)

Figure 5. Disconnection of the vSub→vmPFC projection had no effect on context-induced reinstatement

(A) Reinstatement test: Total number of active lever presses in rats tested in the Extinction and Heroin context after unilateral injections of M+B into vSub and contralateral or ipsilateral injection of M+B into vmPFC before exposure to Heroin context or Extinction context. (B) Cannula placements and representative pictures. Approximate placements (mm from Bregma) of the injector tips for vSub and contralateral (left) or ipsilateral (right) vmPFC (Paxinos and Watson 2008) and representative photomicrographs of cannula placements are shown. (n=11–14 per group).