Loss of feedback inhibition via D2 autoreceptors enhances acquisition of cocaine taking and reactivity to drug-paired cues

Abstract

A prominent aspect of drug addiction is the ability of drug-associated cues to elicit craving and facilitate relapse. Understanding the factors that regulate cue reactivity will be vital for improving treatment of addictive disorders. Low availability of dopamine (DA) D2 receptors (D2Rs) in the striatum is associated with high cocaine intake and compulsive use. However, the role of D2Rs of nonstriatal origin in cocaine seeking and taking behavior and cue reactivity is less understood and possibly underestimated. D2Rs expressed by midbrain DA neurons function as autoreceptors, exerting inhibitory feedback on DA synthesis and release. Here, we show that selective loss of D2 autoreceptors impairs the feedback inhibition of DA release and amplifies the effect of cocaine on DA transmission in the nucleus accumbens (NAc) in vitro. Mice lacking D2 autoreceptors acquire a cued-operant self-administration task for cocaine faster than littermate control mice but acquire similarly for a natural reward. Furthermore, although mice lacking D2 autoreceptors were able to extinguish self-administration behavior in the absence of cocaine and paired cues, they exhibited perseverative responding when cocaine-paired cues were present. This enhanced cue reactivity was selective for cocaine and was not seen during extinction of sucrose self-administration. We conclude that low levels of D2 autoreceptors enhance the salience of cocaine-paired cues and can contribute to the vulnerability for cocaine use and relapse.

Figure 1

Selective pattern of Cre recombinase expression and loss of D2 autoreceptors in autoDrd2KO mice. (a, b) Cre recombinase expression revealed with the fluorescent reporter tdTomato shown in coronal (a) and sagittal (b) brain sections of autoDrd2KO mice overlaid onto mouse brain atlas outline (Franklin and Paxinos, third edition, 2007). Note fluorescence in the ventral tegmental area (VTA), susbantia nigra compacta (SNC), and projections to the NAc and dorsal striatum (DS). In addition, Cre-expressing neurons were seen in the olfactory bulb (OB). SNR, substantia nigra reticulata; ml, medial lemniscus; ac, anterior commissure. Scale bars=1 mm. (c, d) Confocal images showing fluorescently labeled neuronal cell bodies in VTA (c) and axonal projection in the striatum (d). Scale bars=50 μm. (e) Channelrhodopsin-2 tagged eYFP expression in midbrain DA neurons and their axonal projections to NAc shell (NAcsh), core (NAcc), and dorsal striatum (DS) shown in sagittal brain section of autoDrd2 mice overlaid onto atlas outline. VP, ventral pallidum; LV, lateral ventricle; Tu, olfactory tubercle. (f, top) Representative eDA and oDA transients in NAc shell of autoDrd2KO mice. (f, bottom) Color plots showing voltammetric current (color scale) over time as a function of the applied potential (V). Inset shows current vs voltage plot of DA signals showing characteristic oxidation and reduction peaks. (g) Box-and-whisker plot of peak concentration for eDA and oDA transients recorded in control mice (open) and autoDrd2KO mice (solid): box=25–75 percentiles, whiskers=minimum–maximum, line=median, cross=mean. (h) Traces showing average oDA transients before and after application of the D2-like agonist quinpirole (1 μM, gray) and after subsequent application of the D2R antagonist sulpiride (1 μM, red) in control (top) and autoDRD2KO mice (bottom). (i) Quinpirole (1 μM) inhibit oDA transient in control mice (open), whereas it had no effect on autoDrd2KO (filled) mice. Sulpiride (1 μM) reversed the inhibition in control mice and had no effect on autoDrd2KO mice.

Figure 2

Augmented DA transients after cocaine in mice lacking D2 autoreceptors. (a) Representative oDA transients before (black) and after cocaine (10 μM, red) in control, autoDrd2KO, and control mice in the presence of sulpiride. (b) Decay time constant (τ) of oDA transients before (black) and after cocaine (red) in control (open), autoDrd2KO (solid), and control mice in the presence of sulpiride (shaded). (c) Amplitude of oDA transients after cocaine expressed as percentage of baseline for control (open), autoDrd2KO mice (solid), and control mice in sulpiride (shaded). All data expressed as mean±SEM.

Figure 3

Enhanced acquisition of cue-paired intravenous cocaine self-administration in autoDrd2KO mice. (a, b) Representative responding of a Drd2^loxP/loxP (a) and an autoDrd2KO (b) mouse across 15 training sessions. (c) Percent of mice from each genotype that met cocaine self-administration acquisition criteria on each day of training. (d) Number of sessions required to meet acquisition criteria for cocaine self-administration for each individual animal (symbol) and the mean for each genotype (bars). Symbols above the gray dotted line represent mice that failed to reach acquisition criteria by session 15 (p=0.006, n=24–20). (e) Percentage of control (open blue) and autoDrd2KO (solid blue) mice that met criteria for acquisition of sucrose self-administration throughout the sessions (n=10–13). (f) Number of sessions required to meet acquisition criteria for sucrose self-administration for each individual animal (symbol) and the mean for each genotype (bars). Symbols above the gray dotted line represent mice that failed to reach acquisition criteria by session 15 (n.s., p=0.79, n=10–13). (g, h) Comparison of the acquisition curves for cocaine (red) and sucrose (blue) displayed by Drd2^loxP/loxP (g) and autoDrd2KO (h) mice. Reported error is SEM.

Figure 4

Once mice acquire, autoDrd2KO mice show similar daily intake, sensitivity, and breakpoint for cocaine. (a) Rate of earned cocaine infusions for autoDrd2KO (solid) and Drd2^loxP/loxP (open) mice that met acquisition criteria. (b) Daily cocaine intake during the last 3 training sessions (sessions 13–15) before extinction sessions for all mice that met acquisition criteria; n.s., not significant difference p=0.44. (c) Mean breakpoint value achieved during progressive-ratio session for autoDrd2KO (solid) and Drd2^loxP/loxP littermates (open); n.s., not significant difference p=0.59, n=17–17. (d) Distribution of breakpoint values for autoDrd2KO (top, solid) and Drd2^loxP/loxP littermate (bottom, open) mice achieved during the progressive-ratio session. (e) Mean number of active pokes performed during progressive-ratio session for autoDrd2KO (solid) and Drd2^loxP/loxP littermates (open); n.s., difference p=0.64, n=17–17. (f) Rate of cocaine infusions earned by autoDrd2KO (solid) and Drd2^loxP/loxP littermate (open) mice when varying the unitary dose of cocaine infusion; n=6–6. All data, except for (d), are expressed as mean±SEM.

Figure 5

Resistance to extinction in the presence of drug-paired cues and enhanced cue-induced reinstatement. (a, b) Representative raster plot showing responding during 15 sessions of cues-ON extinction after cocaine self-administration for a Drd2^loxP/loxP (a) and an autoDrd2KO (b) mouse. (c, d) Rate of earned saline infusions during cues-ON (c) or cues-OFF (d) extinction sessions after cocaine self-administration by autoDrd2KO (solid red) and Drd2^loxP/loxP littermates (open red) (n=10–10 for ON and n=10–12 for OFF). *Genotype: F(1, 18)=4.65, p=0.045. (e) Rate of earned saline infusions during cues-OFF extinction (5 last sessions) (black) and during cue-induced reinstatement session (red) for autoDrd2KO mice (solid) and littermates (open) (n=13–10). *2W-ANOVA F(1, 42)=8.09, p=0.007, Tukey's p<0.05. (f) Rate of undelivered sucrose rewards during cues-ON extinction sessions after sucrose self-administration by autoDrd2KO (solid blue) and Drd2^loxP/loxP littermates (open blue) (n=9–12). *Genotype: F(1, 13)=0.08, p=0.78. (g, h) Normalized rate of earned saline infusion or undelivered rewards (to last 3 training sessions) for Drd2^loxP/loxP (g) and autoDrd2KO (h) for cues-ON (squared symbol, cocaine in red, sucrose in blue) and cues-OFF (cocaine in red, round symbol). Data expressed as mean±SEM.