Chemogenetic Manipulations of Ventral Tegmental Area Dopamine Neurons Reveal Multifaceted Roles in Cocaine Abuse.

Ventral tegmental area (VTA) dopamine (DA) neurons perform diverse functions in motivation and cognition, but their precise roles in addiction-related behaviors are still debated. Here, we targeted VTA DA neurons for bidirectional chemogenetic modulation during specific tests of cocaine reinforcement, demand, and relapse-related behaviors in male rats, querying the roles of DA neuron inhibitory and excitatory G-protein signaling in these processes. Designer receptor stimulation of Gq signaling, but not Gs signaling, in DA neurons enhanced cocaine seeking via functionally distinct projections to forebrain limbic regions. In contrast, engaging inhibitory Gi/o signaling in DA neurons blunted the reinforcing and priming effects of cocaine, reduced stress-potentiated reinstatement, and altered behavioral strategies for cocaine seeking and taking. Results demonstrate that DA neurons play several distinct roles in cocaine seeking, depending on behavioral context, G-protein-signaling cascades, and DA neuron efferent targets, highlighting their multifaceted roles in addiction.SIGNIFICANCE STATEMENT G-protein-coupled receptors are crucial modulators of ventral tegmental area (VTA) dopamine neuron activity, but how this metabotropic signaling impacts the complex roles of dopamine in reward and addiction is poorly understood. Here, we bidirectionally modulate dopamine neuron G-protein signaling with DREADDs (designer receptors exclusively activated by designer drugs) during a variety of cocaine-seeking behaviors, revealing nuanced, pathway-specific roles in cocaine reward, effortful seeking, and relapse-like behaviors. Gq and Gs stimulation activated dopamine neurons, but only Gq stimulation robustly enhanced cocaine seeking. Gi/o inhibitory signaling reduced some, but not all, types of cocaine seeking. Results show that VTA dopamine neurons modulate numerous distinct aspects of cocaine addiction- and relapse-related behaviors, and point to potential new approaches for intervening in these processes to treat addiction.

Lack of increased immediate early gene expression in rats reinstating cocaine-seeking behavior to discrete sensory cues.

Drug-seeking behavior elicited by drug-associated cues contributes to relapse in addiction; however, whether relapse elicited by drug-associated conditioned reinforcers (CR) versus discriminative stimuli (DS) involves distinct or overlapping neuronal populations is unknown. To address this question, we developed a novel cocaine self-administration and cue-induced reinstatement paradigm that exposed the same rats to distinct cocaine-associated CR and DS. Rats were trained to self-administer cocaine in separate sessions. In one, a DS signaled cocaine availability; in the other, cocaine delivery was paired with a different CR. After extinction training and reinstatement testing, where both cues were presented in separate sessions, rats were sacrificed and processed for cellular analysis of temporal activity by fluorescent in situ hybridization (CatFISH) for activity regulated cytoskeleton-associated protein (Arc) mRNA and for radioactive in situ hybridization for Arc and zif268 mRNAs. CatFISH did not reveal significant changes in Arc mRNA expression. Similar results were obtained with radioactive in situ hybridization. We have shown that while rats reinstate drug seeking in response to temporally discrete presentations of distinct drug-associated cues, such reinstatement is not associated with increased transcriptional activation of Arc or zif268 mRNAs, suggesting that expression of these genes may not be necessary for cue-induced reinstatement of drug-seeking behavior.

Effect of methamphetamine neurotoxicity on learning-induced Arc mRNA expression in identified striatal efferent neurons.

Methamphetamine abuse results in lasting, partial depletions of striatal dopamine and cognitive dysfunction. However, the effect of partial dopamine depletions on the expression of an effector immediate early gene, Arc (activity regulated, cytoskeletal-associated protein), known to be involved in synaptic modifications underlying learning and memory, has heretofore not been examined. Male Sprague-Dawley rats were pretreated with a neurotoxic regimen of methamphetamine or saline. Seven weeks later, rats were trained in a motor-response task on a T-maze for five days, and then underwent reversal training on day five. Rats were sacrificed 5 min after reaching criterion on the reversal task, and the brains were removed and processed using double-label fluorescent in situ hybridization for Arc and preproenkephalin (PPE) mRNA expression in the dorsomedial striatum. Rats pretreated with methamphetamine had an average (+/-SEM) 54.4+/-7.9% loss of dopamine in dorsomedial striatum. Interestingly, there was no difference in reversal trials to criterion in methamphetamine- vs. saline-pretreated rats. However, the expression of Arc mRNA in dorsomedial striatum was attenuated in methamphetamine-pretreated animals, particularly in PPE-negative neurons. Furthermore, the correlation between Arc mRNA expression in dorsomedial striatum and learning was abolished in methamphetamine-pretreated animals. These data suggest that methamphetamine-induced partial monoamine loss is associated with disrupted induction of the effector immediate early gene Arc during a behavioral task, particularly in PPE-negative (presumed striatonigral) neurons, as well as with disruption of the relation between Arc mRNA expression in dorsomedial striatum and reversal learning.

Cellular electroporation induces dedifferentiation in intact newt limbs.

Newts have the remarkable ability to regenerate lost appendages including their forelimbs, hindlimbs, and tails. Following amputation of an appendage, the wound is rapidly closed by the migration of epithelial cells from the proximal epidermis. Internal cells just proximal to the amputation plane begin to dedifferentiate to form a pool of proliferating progenitor cells known as the regeneration blastema. We show that dedifferentiation of internal appendage cells can be initiated in the absence of amputation by applying an electric field sufficient to induce cellular electroporation, but not necrosis or apoptosis. The time course for dedifferentiation following electroporation is similar to that observed following amputation with evidence of dedifferentiation beginning at about 5 days postelectroporation and continuing for 2 to 3 weeks. Microarray analyses, real-time RT-PCR, and in situ hybridization show that changes in early gene expression are similar following amputation or electroporation. We conclude that the application of an electric field sufficient to induce transient electroporation of cell membranes induces a dedifferentiation response that is virtually indistinguishable from the response that occurs following amputation of newt appendages. This discovery allows dedifferentiation to be studied in the absence of wound healing and may aid in identifying genes required for cellular plasticity.