Allostatic Changes in the cAMP System Drive Opioid-Induced Adaptation in Striatal Dopamine Signaling.

Opioids are powerful addictive agents that alter dopaminergic influence on reward signaling in medium spiny neurons (MSNs) of the nucleus accumbens. Repeated opioid exposure triggers adaptive changes, shifting reward valuation to the allostatic state underlying tolerance. However, the cellular substrates and molecular logic underlying such allostatic changes are not well understood. Here, we report that the plasticity of dopamine-induced cyclic AMP (cAMP) signaling in MSNs serves as a cellular substrate for drug-induced allostatic adjustments. By recording cAMP responses to optically evoked dopamine in brain slices from mice subjected to various opioid exposure paradigms, we define profound neuronal-type-specific adaptations. We find that opioid exposure pivots the initial hyper-responsiveness of D1-MSNs toward D2-MSN dominance as dependence escalates. Presynaptic dopamine transporters and postsynaptic phosphodiesterases critically enable cell-specific adjustments of cAMP that control the balance between opponent D1-MSN and D2-MSN channels. We propose a quantitative model of opioid-induced allostatic adjustments in cAMP signal strength that balances circuit activity.

Interrogating the Spatiotemporal Landscape of Neuromodulatory GPCR Signaling by Real-Time Imaging of cAMP in Intact Neurons and Circuits.

Modulation of neuronal circuits is key to information processing in the brain. The majority of neuromodulators exert their effects by activating G-protein-coupled receptors (GPCRs) that control the production of second messengers directly impacting cellular physiology. How numerous GPCRs integrate neuromodulatory inputs while accommodating diversity of incoming signals is poorly understood. In this study, we develop an in vivo tool and analytical suite for analyzing GPCR responses by monitoring the dynamics of a key second messenger, cyclic AMP (cAMP), with excellent quantitative and spatiotemporal resolution in various neurons. Using this imaging approach in combination with CRISPR/Cas9 editing and optogenetics, we interrogate neuromodulatory mechanisms of defined populations of neurons in an intact mesolimbic reward circuit and describe how individual inputs generate discrete second-messenger signatures in a cell- and receptor-specific fashion. This offers a resource for studying native neuronal GPCR signaling in real time.

NF1 Is a Direct G Protein Effector Essential for Opioid Signaling to Ras in the Striatum.

It is well recognized that G-protein-coupled receptors (GPCRs) can activate Ras-regulated kinase pathways to produce lasting changes in neuronal function. Mechanisms by which GPCRs transduce these signals and their relevance to brain disorders are not well understood. Here, we identify a major Ras regulator, neurofibromin 1 (NF1), as a direct effector of GPCR signaling via Gßγ subunits in the striatum. We find that binding of Gßγ to NF1 inhibits its ability to inactivate Ras. Deletion of NF1 in striatal neurons prevents the opioid-receptor-induced activation of Ras and eliminates its coupling to Akt-mTOR-signaling pathway. By acting in the striatal medium spiny neurons of the direct pathway, NF1 regulates opioid-induced changes in Ras activity, thereby sensitizing mice to psychomotor and rewarding effects of morphine. These results delineate a novel mechanism of GPCR signaling to Ras pathways and establish a critical role of NF1 in opioid addiction.