Reinforcement-related regulation of AMPA glutamate receptor subunits in the ventral tegmental area enhances motivation for cocaine.

Chronic cocaine use produces numerous biological changes in brain, but relatively few are functionally associated with cocaine reinforcement. Here we show that daily intravenous cocaine self-administration, but not passive cocaine administration, induces dynamic upregulation of the AMPA glutamate receptor subunits GluR1 and GluR2 in the ventral tegmental area (VTA) of rats. Increases in GluR1 protein and GluR1(S845) phosphorylation are associated with increased GluR1 mRNA in self-administering animals, whereas increased GluR2 protein levels occurred despite substantial decreases in GluR2 mRNA. We investigated the functional significance of GluR1 upregulation in the VTA on cocaine self-administration using localized viral-mediated gene transfer. Overexpression of GluR1(WT) in rat VTA primarily infected dopamine neurons (75%) and increased AMPA receptor-mediated membrane rectification in these neurons with AMPA application. Similar GluR1(WT) overexpression potentiated locomotor responses to intra-VTA AMPA, but not NMDA, infusions. In cocaine self-administering animals, overexpression of GluR1(WT) in the VTA markedly increased the motivation for cocaine injections on a progressive ratio schedule of cocaine reinforcement. In contrast, overexpression of protein kinase A-resistant GluR1(S845A) in the VTA reduced peak rates of cocaine self-administration on a fixed ratio reinforcement schedule. Neither viral vector altered sucrose self-administration, and overexpression of GluR1(WT) or GluR1(S845A) in the adjacent substantia nigra had no effect on cocaine self-administration. Together, these results suggest that dynamic regulation of AMPA receptors in the VTA during cocaine self-administration contributes to cocaine addiction by acting to facilitate subsequent cocaine use.

Emergence of context-associated GluR(1) and ERK phosphorylation in the nucleus accumbens core during withdrawal from cocaine self-administration.

Reexposure to cocaine-associated environments promotes relapse to cocaine seeking and represents a persistent impediment to successful abstinence. Neurobiological adaptations are thought to underlie the preservation of drug-seeking behavior during protracted withdrawal periods, possibly including changes associated specifically with cocaine-paired contexts. We measured GluR(1) (S845) and extracellular signal-regulated kinase (ERK) phosphorylation in rat striatal subregions in an animal model of cocaine relapse. Animals with cocaine self-administration experience and their yoked partners were exposed to extinction conditions for one hour in the drug-paired environmental context after one day or three weeks withdrawal to measure protein phosphorylation induced by the cocaine-paired context in the absence of cocaine reinforcement. GluR(1) (S845) (an index of protein kinase A (PKA) activity) and ERK phosphorylation increased in the nucleus accumbens core of self-administering but not yoked animals after three weeks (but not one day) withdrawal, indicating a time-dependent emergence of context-associated protein phosphorylation in this accumbens subregion. In comparison, animals trained to self-administer sucrose displayed a similar increase in ERK, but not GluR(1) (S845) , phosphorylation following reexposure to a sucrose-paired environment three weeks later, indicating that GluR(1) (S845) phosphorylation did not result solely from lever press behavior per se. In contrast, basal (home cage) GluR(1) (S845) phosphorylation was elevated in the nucleus accumbens shell and caudate-putamen after one day or three weeks cocaine withdrawal regardless of context exposure. These results suggest that time-dependent emergence of context-associated GluR(1) (S845) phosphorylation in the nucleus accumbens core may contribute to the persistence of cocaine-seeking behavior, whereas ERK phosphorylation may be a consequence of this behavior.

Phosphorylation of GluR1, ERK, and CREB during spontaneous withdrawal from chronic heroin self-administration.

Negative motivational symptoms are observed soon after withdrawal from chronic opiate administration, and are thought to mediate dependence. Examination of brain region-specific signaling changes that accompany early withdrawal may shed light on neural mechanisms underlying negative reinforcement and dependence. Thus, we measured alterations in protein phosphorylation in multiple limbic brain regions in rats undergoing 24 h spontaneous or naltrexone-precipitated withdrawal from chronic (6 h/day) i.v. heroin self-administration. Region-specific increases in cyclic AMP-dependent GluR(1) (S845) phosphorylation were found in the nucleus accumbens shell, basolateral amygdala, hippocampal CA1 and CA3 subregions, and premotor cortex from 12 to 24 h of spontaneous withdrawal, and there were no changes in prefrontal cortex, nucleus accumbens core or caudate-putamen. Increased GluR(1) (S845) phosphorylation was detected earlier (12 h withdrawal) in the central amygdala and ventral tegmental area. In contrast, prominent increases in extracellular signal-regulated kinase phosphorylation were found in both prefrontal and premotor cortex, and CA1 and CA3 between 12 and 24 h withdrawal. Phosphorylation of striatal cyclic AMP response element binding protein increased in the caudate-putamen but not in the nucleus accumbens. Naltrexone administration after 24 h withdrawal increased extracellular signal-regulated kinase phosphorylation in the central amygdala, and nucleus accumbens core and shell. Thus, spontaneous withdrawal from heroin self-administration produces region- and time-dependent changes in cyclic AMP and extracellular signal-regulated kinase activity that could contribute to the behavioral manifestation of opiate dependence.

Addiction-related alterations in D1 and D2 dopamine receptor behavioral responses following chronic cocaine self-administration.

The cocaine-addicted phenotype can be modeled in rats based on individual differences in preferred levels of cocaine intake and a propensity for relapse in withdrawal. These cocaine-taking and -seeking behaviors are strongly but differentially regulated by postsynaptic D1 and D2 receptors in the mesolimbic dopamine system. Thus, we determined whether addiction-related differences in cocaine self-administration would be related to differential sensitivity in functional D1 and D2 receptor responses. Using a population of 40 outbred Sprague-Dawley rats trained to self-administer cocaine for 3 weeks, we found that animals with higher preferred levels of cocaine intake exhibited a vertical and rightward shift in the self-administration dose-response function, and were more resistant to extinction from cocaine self-administration, similar to phenotypic changes reported in other models of cocaine addiction. After 3 weeks of withdrawal from cocaine self-administration, high intake rats were subsensitive to the ability of the D1 agonist SKF 81297 to inhibit cocaine-seeking behavior elicited by cocaine priming, but supersensitive to cocaine seeking triggered by the D2 agonist quinpirole, when compared to low intake rats. Additionally, high intake rats developed profound increases in locomotor responses to D2 receptor challenge from early to late withdrawal times, whereas low intake rats developed increased responsiveness to D1 receptor challenge. In a second experiment, responses to the mixed D1/D2 agonist apomorphine and the NMDA glutamate receptor antagonist MK-801 failed to differ between low and high intake rats. These findings suggest that cocaine addiction is related specifically to differential alterations in functional D1 and D2 receptors and their ability to modulate cocaine-seeking behavior.

Patterns of responding differentiate intravenous nicotine self-administration from responding for a visual stimulus in C57BL/6J mice.

RATIONALE: Testing genetically engineered mice in a reliable nicotine self-administration procedure could provide important insights into the molecular mechanisms underlying nicotine reinforcement. OBJECTIVES: We assessed operant responding for intravenous nicotine infusions in C57BL/6J male mice under a fixed-ratio 3 schedule of reinforcement in which a visual cue was contingently associated with drug delivery. METHODS/RESULTS: Acquisition, dose-response function, extinction, and cue-induced reinstatement of operant behavior were characterized. Low nicotine doses (0.001-0.06 mg/kg/infusion) elicited response rates similar to those supported by saline, whereas a higher dose (0.1 mg/kg/infusion) decreased responding. Using an identical procedure to assess cocaine self-administration in an independent group of mice yielded an inverted U-shaped dose-response curve. Other mice trained to respond exclusively for the visual stimulus earned a similar number of reinforcers as mice self-administering saline or low nicotine doses, although with a lower selectivity for the active lever and their response rates were sensitive to the discontinuation and resumption of cue light presentation. Finally, patterns of responding for nicotine, cocaine, or the visual stimulus alone were analyzed using frequency distributions of inter-response intervals and extended return maps. These analyses revealed unique properties of nicotine, which dose-dependently delayed the first response post-timeout and increased the regularity of lever pressing activity. CONCLUSIONS: Nicotine did not enhance the reinforcing properties of the visual cue paired with drug delivery. Interestingly, however, patterns of responding could differentiate nicotine self-administration from responding for a visual stimulus or saline and indicated that nicotine functioned as a salient stimulus driving highly regular operant behavior.