Drug addiction is a chronic disease characterized by an inability to regulate drug-seeking behavior. In order to develop effective behavioral and pharmacological therapies for addiction, it is necessary to have an understanding of the neurochemical and molecular changes that occur following the learned response of drug self-administration. Thus, the goal of my research is to determine biological factors contributing to the long-term behavioral adaptations produced by operant cocaine and alcohol self-administration in rodents. To that end, I employ a multidisciplinary approach to investigate neurochemical and molecular adaptations occurring following chronic drug self-administration. Secondary to this is goal is exploring methods of counteracting these pathological neuroadaptations to reverse the pathologies of learning and memory which drive relapse to drug-seeking.
I use the operant drug self-administration – reinstatement paradigm to model relapse in rats. This paradigm involves training animals to self-administer drug in an operant chamber until behavior becomes stable. The drug-seeking response can then be extinguished and “reinstated” with one of the stimuli known to cause relapse in humans, namely stress, cues associated with drug delivery, or the drug itself. An alternate paradigm does not use extinction training but instead puts animals into “forced abstinence” in the home cage. Both paradigms allow for the assessment of persistent drug-seeking which occurs after a drug-free period of two to three weeks.
I currently have four lines of research which all share the overarching hypothesis that the self-administration of addictive drugs produces persistent alterations in glutamate homeostasis in the nucleus accumbens core.
1. Glutamate Transporters and Cocaine Seeking (NIDA R01 DA033634)
Following cocaine self-administration and extinction training, a reduction in basal extracellular glutamate levels has been observed in the nucleus accumbens core. Furthermore, an increase in synaptically-released glutamate along the prefrontal cortical-nucleus accumbens pathway has been shown to be required for animals to reinstate cocaine-seeking. Together, these observations led to the hypothesis that glutamate homeostasis is disrupted in the nucleus accumbens following cocaine self-administration. Glutamate homeostasis refers to the balance between glutamate release (both synaptic and non-synaptic) and uptake. Work done in my post-doctoral mentor’s lab (Peter Kalivas) has demonstrated that cocaine produces a down-regulation of system xc– which exchanges intracellular glutamate for extracellular cysteine and thereby maintains basal glutamate levels in the nucleus accumbens core. My work has shown that the the expression and function of GLT-1, the glial glutamate transporter that is responsible for 90% of glutamate uptake, is decreased in the nucleus accumbens following cocaine self-administration and 3 weeks of extinction training. It follows that increasing the expression of GLT-1 could be protective against relapse to cocaine-seeking. Concurrent to my finding of decreased GLT-1 expression, a screen of over one thousand FDA-approved substances revealed that the beta-lactam class of antibiotics induces the expression of GLT-1. I was awarded an R21 by NIDA to explore the hypothesis that the beta-lactam ceftriaxone can rescue GLT-1 expression in the nucleus accumbens, thereby restoring glutamate homeostasis following cocaine self-administration and preventing relapse. I found that ceftriaxone increases the expression of both GLT-1 and xCT, the catalytic subunit of system xc– following cocaine self-administration and extinction training. I also showed that ceftriaxone significantly inhibits the relapse of cocaine-seeking, an effect that persists weeks following the last ceftriaxone injection. The positive results of these studies have been featured in articles in both the Journal of the American Medical Association and a National Institute on Drug Abuse publication (NIDA Notes).
While the data gathered in fulfillment of the aims of my R21 indicates that ceftriaxone produces an enduring restoration of glutamate homeostasis, it is unclear at this time whether an up-regulation of both xCT and GLT-1 is needed to attenuate the reinstatement of cocaine-seeking. I am currently employing genetic tools to tease apart the roles of these two proteins in restoring homeostasis. To that end, I have begun to use Vivo-Morpholinos™ (a stable form of antisense) to knock-down the expression of xCT and GLT-1 in the nucleus accumbens core and examine the consequences of this knock-down on glutamate homeostasis and the ability of ceftriaxone to attenuate reinstatement. We are also employing AAV strategies to increase the expression of GLT-1 and xCT to address the same questions.
2. A novel animal model of simultaneous alcohol and cocaine self-administration: effects on glutamate homeostasis.
Cocaine and alcohol may produce opposite effects on glutamate levels in the nucleus accumbens. Thus, I have developed a paradigm to study the consequences of simultaneous self-administration of cocaine and alcohol on accumbens glutamate homeostasis. My hypothesis is that the consumption of alcohol and cocaine together results in neuroadaptations which differ from those arising from self-administration of either drug alone. This research has potential for great translational importance, as the majority (60-90%) of cocaine users consume alcohol with their cocaine. Thus, there exists a great need for animal models of poly-drug use in order to better understand the molecular changes that occur following simultaneous consumption of more than one addictive substance.
3. GLT-1 Enhancers as Drug Candidates for Treating Cocaine Addiction (NIDA R01 DA037270)
While my work and the work of others has consistently demonstrated that ceftriaxone potently inhibits cocaine reinstatement, ceftriaxone is not an ideal candidate for human clinical trials due to its poor brain penetration, antibiotic properties, and inability to survive oral administration. This grant funds medicinal chemistry research to identify ceftriaxone analogs with better oral bioavailability and brain penetration. Our role in the project will be to test these new compounds for their ability to prevent cocaine reinstatement.
4. Development of an animal model and novel treatments for comorbid PTSD and cocaine addiction (Department of Defense, Institute of Molecular Neuroscience Subcontract 804-244)
Substance use disorders (SUDs) and post-traumatic stress disorder (PTSD) are at crisis levels in the United States military. In the general population as well in veterans, there is a high co-occurrence of PTSD and SUD. Patients who display comorbidity for these disorders are more resistant to treatment. An animal model for comorbid PTSD and stimulant addiction is greatly needed in order to screen medications for their ability to reduce symptoms of both disorders. Here we have developed an animal model which can separate stressed rats into those that exhibit long-term symptoms of anxiety and those that are “resilient” to the stressor. Animals that exhibit long-term symptoms of anxiety in our PTSD model also demonstrate enhanced cue-primed reinstatement of cocaine-seeking relative to both unstressed and resilient rats. We plan to investigate the neurobiological substrates of comorbid PTSD and SUD vulnerability with a focus on the hippocampus, PFC and amygdala.