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  • Addiction, Drugs
  • Information from Lay-Language Summaries is Embargoed Until the Conclusion of the Scientific Presentation

    733—Cocaine Reinforcement, Seeking, and Reinstatement I

    Wednesday, November 13, 2013, 8:00 am - 12:00 noon

    733.10: Effect of DREADD-mediated modulation of G-protein coupled signaling in the lateral habenula on cocaine reinforced operant responding

    Location: Halls B-H

    *S. G. NAIR, D. SMIRNOV, J. F. NEUMAIER;
    Psychiatry and Behavioral Sci., Univ. of Washington, Seattle, WA

    Abstract Body: The lateral habenula (LHb), part of the habenular complex in the dorsal diencephalon, is an important regulator of midbrain dopaminergic systems that are known to be involved in the reinforcing properties of cocaine. However, very little is known about the precise role of the LHb in cocaine reinforced behaviors. Here, we examined the role of the LHb in cocaine reinforced operant responding using a chemical-genetic approach utilizing viral mediated gene transfer of engineered G-protein coupled DREADD’s (Designer Receptors Exclusively Activated by Designer Drugs). Male, Long-Evans rats were trained to self-administer cocaine (0.75 mg/kg/infusion) on a fixed ratio 1, 20 second timeout reinforcement schedule. Following self-administration training, hemagglutinin-tagged, Gi/o coupled human M4 (hM4Di) receptors were expressed in the LHb. Activation of hM4Di by the pharmacologically inert synthetic ligand clozapine-N-oxide CNO (1 and 3 mg/kg, i.p), increased cocaine reinforced operant responding. The LHb projects to the dopaminergic ventral tegmental area (VTA) and the GABAergic rostromedial tegmental nucleus (RMTg), among other brain regions. Since the LHb, VTA and RMTg are all known to be involved in the processing of reward-related information, we examined the effect of modulation of G-protein coupled signaling in neurons projecting from the LHb to the VTA in cocaine reinforced operant responding. A Cre-recombinase dependent viral vector based flip-excision method was employed that involved injecting a combination of floxed, inverted hM4Di into LHb neurons and a canine adenovirus 2 (CAV-2) engineered to express Cre recombinase into the VTA. CAV-2 efficiently infected VTA axon terminals and was retrogradely transported to the neuronal cell bodies in the LHb, where it flipped the inverted DREADD into the proper orientation to allow transcription. CNO-mediated activation of hM4Di in LHb neurons projecting to the VTA had no effect on cocaine reinforced operant responding. Studies are currently underway to determine the effect of modulating G-protein coupled signaling in LHb neurons projecting to the RMTg on cocaine reinforced operant responding.

    Lay Language Summary: Addiction to psychostimulant drugs such as cocaine is a critical health problem, with serious economic and social consequences. Despite decades of pre-clinical and clinical research, no pharmacological treatment currently exists to specifically treat cocaine addiction. We believe that a better understanding of the connections between neurons in the brain that are involved in cocaine’s addictive properties is critical to further the development of effective interventions to treat this addiction.
    Our current research indicates that silencing the lateral habenula, a brain region involved in the processing of aversive stimuli, causes rats to consume higher amounts of cocaine. The lateral habenula is connected anatomically and functionally to several brain regions that are involved in the rewarding properties of drugs of abuse. This suggests that the lateral habenula normally blunts the reinforcing effects of cocaine that drives individuals into becoming addicted to this drug of abuse.
    To control the activity of lateral habenula neurons, we used the DREADD (Designer Receptors Exclusively Activated by Designer Drugs) strategy. DREADD’s are a family of receptors that are engineered to respond specifically to a chemical that has no other effects on the brain. By introducing DREADDs into neurons using gene therapy methods, we can activate or inhibit signaling in these cells with high specificity. This strategy may hold promise for human therapeutics for a variety of neuropsychiatric disorders where it would be beneficial to activate or inhibit a specific group of cells with negligible “off-target” effects that often occur with conventional therapeutics.
    We used the well-established rat self-administration procedure to examine the behavioral effects of modulating lateral habenula signaling on cocaine reinforced behaviors. Specifically, rats were implanted with intravenous catheters into their jugular veins. Following approximately 10 days of post-operative recovery, these rats were trained to lever press for intravenous injections of cocaine. We then expressed inhibitory DREADDs in the lateral habenulae of these rats and measured the number of lever presses for cocaine upon administration of the drug that exclusively activates DREADD receptors.
    We found that when lateral habenula activity was silenced with DREADDs, the rats increased their lever pressing and self-administration of cocaine. In an attempt to reveal which connections from the lateral habenula are responsible for this effect, we have been systematically testing specific pathways from the lateral habenula to other brain regions by expressing DREADDs in just one pathway at a time. We do this by using a combinatorial strategy where an inactive form of the DREADD gene is expressed in lateral habenula neurons that must be activated by an enzyme, Cre recombinase. This enzyme is encoded by a second viral vector that is injected into a brain region to which the lateral habenula projects. The second viral vector introduces the gene for Cre recombinase into the projections from the lateral habenula in the targeted brain region. This viral vector is retrogradely transported back to cell bodies in the lateral habenula, Cre recombinase is then synthesized and activates expression of DREADDs only in those neurons that contain both viral transgenes. Using this strategy we are testing which lateral habenula projections are responsible for the ability to control the consumption of cocaine.
    The results of our study suggest that activity of neurons in the lateral habenula play a significant role in cocaine-induced behavioral responses. Further, our study points to the significance of establishing the role of not only brain regions that are involved in the processing of reward-related information, but also brain regions involved in processing of aversive information to fully parse out the neuronal circuitry that is perturbed by this drug of abuse.