Innate immune signaling in the ventral tegmental area contributes to drug-primed reinstatement of cocaine seeking

Abstract

Cocaine addiction is a chronic relapsing disorder characterized by persistent perturbations to an organism's homeostatic processes that result in maladaptive drug seeking. Although considerable attention has been directed at the consequences of neuronal changes following chronic cocaine taking, few studies have examined the role of microglia, the brain's resident immune cells, following chronic cocaine administration. Toll-Like Receptor 4 (TLR4) is a molecular pattern receptor that recognizes pathogens, danger signals, and xenobiotics and induces proinflammatory signaling in the central nervous system. TLR4 is generally considered to be expressed primarily by microglia. Here, we used a rodent model of cocaine addiction to investigate the role of TLR4 in the ventral tegmental area (VTA) in cocaine seeking. Male Sprague-Dawley rats were trained to self-administer cocaine in daily 2-h sessions for 15days. Following self-administration, rats underwent extinction training and were tested in a drug-primed reinstatement paradigm. Pharmacological antagonism of TLR4 in the VTA using lipopolysaccharide from the bacterium Rhodobacter sphaeroides (LPS-RS) significantly reduced cocaine-primed reinstatement of drug seeking but had no effect on sucrose seeking. TLR4 activation within the VTA using the TLR4 activator, lipopolysaccharide, was sufficient to moderately reinstate cocaine seeking. We also assessed changes in proinflammatory cytokine expression in the VTA following cocaine self-administration. Cocaine self-administration increased the expression of mRNA for the proinflammatory cytokine interleukin-1ß, but not tumor necrosis factor alpha, in the VTA. Pharmacological antagonism of the interleukin-1 receptor in the VTA reduced cocaine-primed drug seeking. These results are consistent with the hypothesis that chronic cocaine produces inflammatory signaling that contributes to cocaine seeking.

Keywords: Addiction; Cocaine; Cytokines; Interleukin-1ß; Neuroinflammation.

Fig. 1.

Intra-VTA TLR4 antagonism reduces drug seeking in the cocaine primed reinstatement model of relapse. (A) Rats self-administered cocaine (0.5 μg/kg/infusion) during 2-h daily sessions over 15 days. (B) Rats extinguished formerly drug-paired lever pressing over 7 days of 2-h daily sessions. (C) The TLR4 antagonist LPS-RS (5 μg/side) or vehicle was administered bilaterally into the VTA 5 min prior to cocaine administration (15 μg/kg, i.p.) or saline (0.9% NaCl wt./vol., i.p.). N = 11–13 rats per group. Drug seeking at the previously drug-paired lever increased in the cocaine/vehicle group compared to the vehicle/vehicle group (*Bonferroni’s test, p < 0.001). Intra-VTA LPS-RS decreased drug seeking compared to the cocaine/vehicle group (^#Bonferroni’s test, p < 0.001). (D) Cannula placements were verified by post-mortem visual inspection.

Fig. 2.

Intra-VTA TLR4 antagonism does not alter reinstatement of sucrose seeking. (A) Rats were food restricted and maintained at 340–360 g throughout the experiment. (B) Rats self-administered sucrose pellets during 2-h daily sessions over 15 days as measured by active lever responses and sucrose consumption. (C) Rats extinguished lever pressing on the formerly sucrose-paired lever during 2-h daily sessions over 7 days. (D) LPS-RS (5 μg/side) was administered bilaterally into the VTA 5 min prior to a sucrose reinstatement session. N = 11–12 rats per group. Sucrose seeking increased during the reinstatement session compared to the last extinction session in both the vehicle (*Bonferroni’s test, p < 0.001) and LPS-RS (*Bonferroni’s test, p < 0.05) treated groups. A significant effect of treatment was not found between the vehicle and LPS-RS treated groups (^ns2-way ANOVA, p = 0.267).

Fig. 3.

Intra-VTA TL4 activation moderately induces drug-seeking following extinction of cocaine self-administration. (A) Rats self-administered cocaine (0.5 μg/kg/infusion) during 2-h daily sessions over 15 days. (B) Rats extinguished formerly drug-paired lever pressing over 7 days of 2-h daily sessions. (C) Bilateral intra-VTA LPS (20 ng/side) increased cocaine seeking at the active lever compared to the vehicle treated group. N = 8 rats per group. (*Bonferroni’s test, p < 0.001). (D) Cannula placements were verified by post-mortem visual inspection.

Fig. 4.

Cocaine self-administration increases the expression of IL-1ß and GFAP mRNA in the VTA. (A) Rats self-administered cocaine (0.5 μg/kg/infusion) during 2-h daily sessions over 15 days. (B) Rats extinguished formerly drug-paired lever pressing over 7 days of 2-h daily sessions. (C) Cocaine self-administration history increased IL-1ß, NFκB, and GFAP mRNA expression, but had no effect on TNFa mRNA expression in the VTA. NFκB mRNA expression was increased by a cocaine challenge only in rats having a history of cocaine self-administration. N = 3–6 rats per group. CD11 b mRNA expression was greatest in the cocaine self-administering group that received an acute cocaine challenge. (*p < 0.05; ^#Bonferroni’s test, p < 0.05 compared with saline control group.)

Fig. 5.

Intra-VTA interleukin-1 receptor antagonist reduces drug-seeking behavior in the cocaine-primed model of relapse. (A) Rats self-administered cocaine (0.5 μg/kg/ infusion) during 2-h daily sessions over 15 days. (B) Rats extinguished formerly drug-paired lever pressing over 7 days. (C) IL-1ra (10 ng/side and 20 ng/side) or vehicle was administered bilaterally into the VTA 5 min prior to non-contingent injection of cocaine (15 μg/kg, i.p.) or vehicle (NaCl 0.9%, wt./vol.) N = 4–12 rats per group. Vehicle/cocaine treated rats increased cocaine seeking compared to the vehicle/vehicle treated group (*Bonferroni’s test, p < 0.001). Intra-VTA IL-1ra reduced drug-seeking behavior at both the 10 ng/side (^#Bonferroni’s test, p < 0.05) and 20 ng/side (^#Bonferroni’s test, p < 0.01) doses, although responding following 10 ng/side remained elevated compared with the vehicle/vehicle control group (*Bonferroni’s test, p < 0.05). (D) Placements were verified by post-mortem visual inspection.