Our laboratory studies the cellular and subcellular principles of integration and excitability in dopamine-releasing neurons located in the midbrain. A major interest of the lab is identifying functionally unique subpopulations of midbrain dopamine neurons and understanding how these neurons contribute to the basal ganglia circuit. To this end, one project in the lab focuses on understanding how inhibitory inputs may control subpopulations of dopaminergic neurons within the substantia nigra pars compacta (SNc). SNc dopaminergic neurons pause their activity in response to during aversive events. In vivo experiments show that a subset of these neurons fire rebound bursts of action potentials or show rebound calcium activity following the aversive pause in activity. However, the local neural circuits that underly this behavior are currently unknown. We have been using two-photon imaging and local optogenetic activation to functionally map the inhibitory inputs from basal ganglia nuclei onto dopamine neurons. We compare the strength and location of five (5) separate genetically-defined inhibitory subpopulations in the striatum (striosome and matrix), globus pallidus (Pvalb and Lhx6), and substantia nigra pars reticulata (SNr). We find that the striosomal inputs selectively inhibit the ventrally-projecting SNr dendrite of the dopamine neurons. Although isolated to the SNr dendrite, this connection exerts strong control over the entire cell, pausing action potentials and facilitating rebound firing. Furthermore, we find that striosomal input facilitates rebound firing through activation of GABA-B receptors, which strongly hyperpolarize the SNr dendrite. Therefore, inhibition from striosomes onto SNc dopamine neurons is optimally placed to produce rebound firing. In a second project, we set out to directly test for axonal receptors and their influence over axonal excitability and ultimately dopamine release. Specifically, GABA-A receptors modulate transmitter release in some neurons, including potentially dopamine neurons, but the mechanisms of this modulation are debated. To address this knowledge gap, we performed direct recordings from the cut ends of dopaminergic neuron axons, including from branching axons within the dorsal striatum. Our results provide definitive evidence for the existence of GABA-A receptor-mediated conductances. In contrast to their function at the soma, we found axonal GABA-A receptors were depolarizing with a chloride reversal potential of -56 mV relative to resting membrane potential of -68 mV. In addition, we found that activation of GABA-A receptors decreased the amplitude of a propagating action potential through shunting inhibition. Finally, we found that diazepam, a broad-spectrum benzodiazepine, decreased the input resistance of striatal dopamine neuron axons, suggesting an underappreciated mechanism of action for these drugs. In conclusion, direct recordings from dopamine neuron axons demonstrate that GABA-A receptors are important modulators of axonal action potential propagation and dopamine release. In addition, these receptors are targets of benzodiazepines, as well as potentially other drugs that target GABA-A receptors like ethanol and barbiturates. Aside from these two major projects, we have had one study published that identified a sodium leak channel, NALCN, as the main driver of spontaneous firing in SNc dopaminergic neurons (Philippart and Khaliq, eLife 2018). Importantly, we found that both dopamine D2 receptors as well as GABA-B receptor negatively modulate the activity of dopaminergic neurons through inhibition of NALCN. Therefore, this study identifies NALCN as a novel effector Gi/o protein coupled receptors in dopaminergic neurons. Lastly, the personnel in the lab are finding success in their own professional careers. This May, for example, Rebekah Evans was awarded the Brain Initiative K99 grant. She was also asked to deliver talks at the Organization for Computational Neuroscience in Barcelona Spain and at the GRS Catecholamines where she was one of two GRS speakers chosen to deliver a talk at the main GRC meeting. Paul Kramer also presented his work at the GRC Catecholamines meeting with great success. Lastly, Emily Twedell has recently completed her postbaccalaureate fellowship this summer and will enrolling this Fall as a student at UCSF in their Neuroscience Graduate Program. To fill this open position, a new postbaccalaureate fellow, Alexander Sukharev, has joined the lab.
