Kv7 channel opener retigabine reduces self-administration of cocaine but not sucrose in rats

Abstract

The increasing rates of drug misuse highlight the urgency of identifying improved therapeutics for treatment. Most drug-seeking behaviours that can be modelled in rodents utilize the repeated intravenous self-administration (SA) of drugs. Recent studies examining the mesolimbic pathway suggest that K_v7/KCNQ channels may contribute to the transition from recreational to chronic drug use. However, to date, all such studies used noncontingent, experimenter-delivered drug model systems, and the extent to which this effect generalizes to rats trained to self-administer drugs is not known. Here, we tested the ability of retigabine (ezogabine), a K_v7 channel opener, to regulate instrumental behaviour in male Sprague Dawley rats. We first validated the ability of retigabine to target experimenter-delivered cocaine in a conditioned place preference (CPP) assay and found that retigabine reduced the acquisition of place preference. Next, we trained rats for cocaine-SA under a fixed-ratio or progressive-ratio reinforcement schedule and found that retigabine pretreatment attenuated the SA of low to moderate doses of cocaine. This was not observed in parallel experiments, with rats self-administering sucrose, a natural reward. Compared with sucrose-SA, cocaine-SA was associated with reductions in the expression of the K_v7.5 subunit in the nucleus accumbens, without alterations in K_v7.2 and K_v7.3. Therefore, these studies reveal a reward-specific reduction in SA behaviour and support the notion that K_v7 is a potential therapeutic target for human psychiatric diseases with dysfunctional reward circuitry.

Keywords: Kv7 channel; cocaine self‐administration; retigabine.

FIGURE 1

Retigabine reduces the acquisition of cocaine conditioned place preference (CPP). (A) CPP testing included a pretest and 5 days of twice daily intraperitoneal injections of cocaine (black box; 10 mg/kg) and saline (white box; 1 mL/kg) in alternating counterbalanced fashion. At 15 min before cocaine, rats received intraperitoneal pretreatments of either vehicle (1 mL/kg; n = 11) or retigabine (5 mg/kg; n = 11). On Day 7, rats were tested for side preference. (B) CPP score was calculated as the difference in the time spent in individual chambers during pre‐test and post‐test. A 2W ANOVA indicated an interaction of conditioning with cocaine × retigabine (F_{(1, 40)} = 4.551, p = 0.0391). Rats pretreated with vehicle showed a preference for the cocaine‐paired chamber (t ₍₄₀₎ = 3.530; **p = 0.0011) compared with pre‐test. Rats pretreated with retigabine showed reduced preference for the cocaine‐paired chamber compared with vehicle‐treated controls (t ₍₄₀₎ = 2.567 *p = 0.0141). For these and all other figures, the error bars indicate the mean ± SEM.

FIGURE 2

Training for self‐administration (SA) of cocaine or sucrose and expression of the K_v7 channel prior to retigabine testing. (A) Cohort design for rats learning to self‐administer cocaine (cocaine‐SA, Group 1; 0.5 mg/kg unit dose) or sucrose‐SA (Group 2; 45 mg pellet). After Days 10–12 daily sessions, cocaine‐SA rats remained in the same paradigm (Group 1A, 0.5 mg/kg/infusion; fixed‐ratio 1 [FR1]) or transitioned to a different dose (Group 1B, 0.1 mg/kg/infusion; FR1) or dose and reinforcement schedule (Group 1C, 0.1 mg/kg/infusion; FR3). (B, C) Summary of behavioural responding when rats were presented with levers that resulted in the activation of a light and tone cue paired with (B) a cocaine infusion or (C) a sucrose pellet. (D, E) Western blots of nucleus accumbens (NAc) and ventral tegmental area (VTA) punch lysates from rats that learned to self‐administer cocaine (Group 1, n = 4) or sucrose (Group 2, n = 4) showing K_v7 subunit expression (7.2, 7.3, 7.5) normalized to Glyceraldehyde 3‐Phosphate Dehydrogenase (GAPDH) and expressed as a percentage of sucrose controls. (E) In the NAc, Kv7.5 expression decreased, whereas in the VTA, K_v7 subunit expression did not differ between treatment groups. **p = 0.0052 compared with sucrose‐SA using an unpaired t‐test.

FIGURE 3

Retigabine reduces cocaine‐ but not sucrose‐self‐administration (SA). (A) Schematic depicting retigabine testing on a fixed‐ratio 1 (FR1). Each group underwent 3 cycles (4 sessions per cycle) of testing with pretreatments with saline (1 mL/kg; Sessions 1, 2 and 4) or increasing doses of retigabine (2, 5, 7 mg/kg, i.p.; Session 3) in a repeated‐measure design administered 15 min before SA. (B, C, D) Summary data showing active lever responding in consecutive sessions (expressed as a percent of baseline) during an FR1 reinforcement schedule for cocaine at unit doses of (B) 0.5 mg (C) 0.1 mg or (D) sucrose (45 mg). Retigabine altered the active‐lever responding for (B, C) cocaine at both unit doses, but not for (D) sucrose. Grey bars indicate retigabine pretreatment (R) at doses denoted by the numbers in parentheses (2, 5, 7 mg/kg, i.p.). (E) Summary of the total reward consumed after pretreatment with saline or different doses of retigabine (▼2, ●5 or 7 mg/kg). Significance compared with saline at the cocaine unit doses of 0.5 mg (7 mg/kg, **p = 0.0133) and 0.1 mg (2 mg/kg, **p = 0.0027; 5 and 7 mg/kg, ****p < 0.0001) using a Šídák's multiple comparison test.

FIGURE 4

Retigabine reduces the motivation for cocaine in the progressive‐ratio task. (A) Schematic depicting retigabine testing in a progressive‐ratio (PR) schedule of reinforcement. Group 1C rats received pretreatments with saline (1 mL/kg; Sessions 1, 2, 4–6) 15 min prior to session of cocaine‐self‐administration (SA) (0.1 mg unit dose) on the fixed‐ratio 3 (FR3) reinforcement schedule. In Sessions 3 and 7, rats received vehicle (1 mL/kg) or retigabine (5 mg/kg, i.p.) pretreatments 15 min before cocaine (0.06, 0.1 or 0.25 mg/kg) in a PR reinforcement schedule. Separate groups of rats were used for the different doses of cocaine. (B, C) Effects of retigabine on (B) infusions and (C) breakpoints under a PR schedule of cocaine reinforcement at various doses. Numbers in parentheses denote the number of subjects. ***p = 0.0002 (0.06 mg unit dose, infusions), **p = 0.0021 (0.1 mg unit dose, infusions) compared with vehicle using a Šídák's multiple comparison test; in (B), main effect, F_2,29 = 12.71, p < 0.0001; retigabine F_1,24 = 10.63, p < 0.0033; dose × retigabine F_2,24 = 0.7867, p = 0.4667.