Basic brain mechanisms underlying drug addiction, craving, and relapse

National Institute On Drug Abuse

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Abstract

During the present reporting period, we continued our prior work on cannabinoids and endocannabinoid signaling in the brain. We previously reported that cocaine self-administration upregulates cannabinoid CB2R expression in midbrain dopamine (DA) neurons. We now investigated whether cocaine or heroin also alters CB2R expression in striatal medium-spiny neurons that express dopamine D1 or D2 receptors (D1-MSNs, D2-MSNs) and microglia. We found that a single injection of cocaine failed to alter, while repeated cocaine injections or self-administration dose-dependently upregulated CB2R gene expression in neocortex, striatum, and periphery (spleen). In contrast, repeated heroin administration produced dose-dependent reduction in striatal CB2 mRNA expression. We found that CB2R upregulation occurred mainly in D1-MSNs, not in D2-MSNs or microglia, in the nucleus accumbens rather than the dorsal striatum. These findings indicate that repeated cocaine exposure may upregulate CB2R expression in both brain and spleen, with regional and cell type-specific profiles. In striatum, CB2R upregulation occurs mainly in D1-MSNs in the nucleus accumbens. Given the important role of D1-MSNs in brain reward function, these findings provide new insight into mechanisms by which brain CB2Rs modulate cocaine action. Glutamate plays an important role in drug-induced and cue-induced reinstatement of drug seeking. But the role of glutamate in drug reward itself is unclear. We therefore studied the effects of multiple glutamate transporter (GLT) inhibitors on extracellular glutamate and dopamine (DA) in the nucleus accumbens (NAc), i.v. cocaine self-administration, intracranial brain-stimulation reward (BSR), and reinstatement of cocaine seeking in male and female rats. Among the five GLT inhibitors tested, TFB-TBOA was the most potent. Microinjections of TFB-TBOA into NAc, but not ventral tegmental area (VTA) or dorsal striatum (DS), dose-dependently inhibited cocaine self-administration under fixed-ratio and progressive-ratio (PR) reinforcement, shifted the cocaine dose-response curve downward, and inhibited intracranial BSR. Selective downregulation of astrocytic GLT-1 expression in the NAc by GLT-1 antisense oligonucleotides also inhibited cocaine self-administration. The reduction in cocaine self-administration following TFB-TBOA administration was NMDA GluN2B receptor dependent, and rats self-administering cocaine showed upregulation of GluN2B expression in NAc DA- and cAMP-regulated phosphoprotein 32 (DARPP-32)-positive medium-spiny neurons (MSNs). In contrast, TFB-TBOA, when locally administered into the NAc, VTA, or ventral pallidum (VP) dose-dependently reinstated cocaine-seeking behavior. Intra-NAc TFB-TBOA-evoked drug-seeking was long-lasting and NMDA/AMPA receptor dependent. These findings, for the first time, indicate that glutamate in the NAc negatively regulates cocaine's rewarding effects, while an excess of glutamate in multiple brain regions triggers reinstatement of drug-seeking behavior. We also explored the role(s) of ghrelin in cocaine addiction. We showed that: (1) elevation of ghrelin by cocaine plays a critical role in maintenance of cocaine self-administration and cocaine-seeking motivated by cocaine-conditioned stimuli; (2) acquisition of cocaine-taking behavior is associated with the acquisition of stimulatory effects of cocaine by cocaine-conditioned stimuli on ghrelin secretion, and with an upregulation of VTA ghrelin receptor mRNA levels; (3) blockade of ghrelin signaling by pretreatment with JMV2959, a selective ghrelin receptor antagonist, dose-dependently inhibits reinstatement of cocaine-seeking triggered by either cocaine or yohimbine in behaviorally extinguished animals with a history of cocaine self-administration; (4) JMV2959 pretreatment also inhibits BSR and cocaine-potentiated BSR maintained by optogenetic stimulation of VTA dopamine neurons in DAT-Cre mice; (5) blockade of peripheral adrenergic 1 receptors by atenolol potently attenuates the elevation in circulating ghrelin induced by cocaine and inhibits cocaine self-administration and cocaine reinstatement triggered by cocaine. However, the response of the ghrelin system to opioid-motivated behaviors and the role of ghrelin in oxycodone self-administration was unstudied. We investigated the reciprocal interactions between the endogenous ghrelin system and oxycodone self-administration behaviors in rats and the role of the ghrelin system in BSR driven by optogenetic stimulation of midbrain reward circuits in mice. Oxycodone self-administration significantly elevated plasma ghrelin, des-acyl ghrelin and growth hormone and showed no effect on plasma LEAP2, a newly identified endogenous ghrelin receptor (GHS-R1a) antagonist. Oxycodone self-administration produced significant decreases in plasma gastric inhibitory polypeptide and insulin. Acquisition of oxycodone self-administration significantly upregulated GHS-R1a mRNA levels in VTA DA neurons. Pretreatment the selective GHS-R1a antagonist JMV29959 dose-dependently reduced oxycodone self-administration and decreased the breakpoint for oxycodone under progressive ratio reinforcement. The inhibitory effects of JMV2959 on oxycodone self-administration is selectively mediated by GHS-R1a as JMV2959 showed a similar effect in Wistar wildtype but not in GHS-R knockout rats. JMV2959 pretreatment significantly inhibited BSR driven by selective stimulation of VTA DA neurons, but not by stimulation of striatal GABA neurons projecting to the VTA in mice. These findings suggest that elevation of ghrelin signaling by oxycodone or oxycodone-associated stimuli is a causal process by which oxycodone motivates oxycodone drug-taking and targeting the ghrelin system may be a viable treatment approach for opioid use disorders in addition to cocaine use disorder. Furthering our cannabinoid work, we explored the therapeutic potential of PIMSR, a neutral CB1R antagonist lacking an inverse agonist profile, against cocaine's behavioral effects. We found that systemic administration of PIMSR dose-dependently inhibited cocaine self-administration under fixed-ratio 5 (FR5), but not FR1, reinforcement, shifted the cocaine self-administration dose-response curve downward, decreased incentive motivation to seek cocaine under progressive-ratio reinforcement, and reduced cue-induced reinstatement of cocaine seeking. PIMSR also inhibited oral sucrose self-administration. Importantly, PIMSR alone is neither rewarding nor aversive as assessed by place conditioning. We then used BSR to explore the possible involvement of the mesolimbic DA system in PIMSR's action. We found that PIMSR dose-dependently attenuated cocaine-enhanced electrical BSR, but by itself failed to alter electrical BSR, while dose-dependently inhibiting BSR maintained by optical stimulation of midbrain DA neurons in transgenic DAT-Cre mice, suggesting the involvement of DA-dependent mechanisms. We also found that PIMSR pretreatment attenuated THC- or ACEA (selective CB1R agonist)-induced reduction in optical BSR. The neutral CB1R antagonist PIMSR deserves further research as a promising pharmacotherapeutic for cocaine use disorder. Finally, we found that long-term physical exercise produced a robust increase in red nucleus c-fos expression, and activates a subset of medial red nucleus (RNm) glutamate neurons projecting to VTA DA neurons. Optogenetic stimulation of this pathway was rewarding, as assessed by BSR and conditioned place preference. Running wheel access decreased cocaine self-administration and cocaine-seeking. Optogenetic stimulation of the RNm-to-VTA glutamate pathway inhibited responding to cocaine. These findings indicate that physical exercise activates a specific RNm-VTA glutamatergic pathway, producing exercise reward and reducing cocaine's appeal.