Beyond their immune function, microglia have key homeostatic functions in the brain. In particular, it is now well accepted that they control the life of synapses in different contexts: from development to plasticity, and pathologies.
The main objective of this proposal is to start elucidating the neuron-microglia signaling in remodeling nucleus accumbens-(NAc) glutamatergic transmission, through the maturation of newly formed silent synapses, which underlies critical circuit mechanisms promoting cocaine craving (a risk factor for drug relapse), a feature of cue-associated cocaine memories.
Our central hypothesis, based on the current literature and our preliminary results (see below), is as follow: microglia neuron interactions dependent on fractalkine/CX3CR1 signaling contribute to the maturation and plasticity of silent glutamatergic synapses in the NAc, which embed critical memory traces that promote cue-induced cocaine craving.
To verify this hypothesis, we propose to determine the role of fractalkine signaling in modulating incubated cocaine craving after forced abstinence from cocaine-self administration .
We believe that our proposal, combining behavioral, electrophysiological and biochemical approaches, will offer a completely new and promising focus to address a question of great social and clinical relevance.
Expected results:
We are aware that CX3CL1/CX3CR1 pathway is not the only one involved in signaling between neurons and microglia. Indeed, there are multiple bidirectional signaling pathways active in different functional contexts39. However, understanding the role of fractalkine signaling in cocaine plasticity will be an important step in the comprehension of this phenomenon and will enlarge our
understanding on the elusive mechanisms by which the action of microglia on synapses takes place.
Aim 2. After the completion of this study, we will have assessed the efficacy of the CX3CL1 antagonist JMS-17-2 in preventing incubation of cocaine craving. We predict that JMS-17-2, by preventing the maturation of silent synapses, prevents the incubation of cocaine craving. A question of major importance for treating human cocaine users is the extent to which adaptations produced by
chronic drug use recover with a given pharmacological-based or behavioral-based therapy during prolonged abstinence. Preclinical cocaine self-administration studies suggest that incubation of cocaine craving intensifies over time and plateau after ~20 days of forced abstinence5 (and stay high up to 100 days).
Potential pitfalls and contingency plans
Potential age-based differences in rats drug metabolism will not affect seeking tests, occurring in drug-free conditions. In case of systemic effects of JMS-17-2, we will inject the drug intracranially. We do not envision other technical concerns given the expertise on the approaches proposed here, routinely used in the laboratory of Prof. D. Ragozzino and collaborators.
Innovation of the research
This proposal is innovative, in our opinion, on several counts: (i) The incubation of craving model provides a unique approach that specifically focuses on the propensity to relapse, both of which occur in humans. (ii) At present there are several papers available on the incubation of cocaine craving after forced abstinence, focusing primarily on synaptic plasticity, behavioral pharmacology, neural circuitry and epigenetics. Despite the relevance of neuronal signaling, work to date has not considered a role of microglia on cocaineinduced synaptic adaptations, while is known that microglia play a critical role on the formation and elimination of synapses during development. Our focus on the microglial mechanisms modulating synaptic transmission in the adult brain is unique and is one of the aspects of our research that will open novel future directions in the therapeutic setting.