The work in our laboratory focuses on 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 and biochemically unique subpopulations of midbrain dopamine neurons and understanding how these neurons fit into the basal ganglia circuit. To this end, one project compared the potassium channels that are active during high-frequency firing in subpopulations of midbrain dopamine neurons. We examined the role of a silent potassium channel subunit, Kv9.3 (Kcns3), that forms functional heterotetramers with Kv2 channels. Because specific channel blockers exist for this subunit are lacking, we generated a novel CRISPR knockout which we used to examine the contribution of Kv9.3/Kv2 channels to firing in dopamine neurons. We found a gradient pattern of Kv9.3 expression in the midbrain, with the highest expression in dopamine neurons of the substantia nigra (SNc) and lateral ventral tegmental area (VTA), but little expression in medial VTA dopaminergic neurons. We then tested the functional contribution of Kv9.3 on firing as well as its modulatory effect on Kv2 channels recorded in voltage clamp. We found that SNc neurons recorded from Kv9.3 knockout mice fire exhibit higher maximal firing rates. In voltage clamp experiments, we found that Kv9.3 subunits boost spike-evoked Kv2 currents to strengthen action potential repolarization in lateral dopaminergic neurons. Kv9.3/Kv2 channels also repolarize slowly which contributes to an afterhyperpolarization. As a consequence of their larger participation in firing, however, we find that the amplitude of Kv9.3/Kv2 currents flowing between spikes is greater which results in slower rates of high-frequency burst firing. We are current preparing a manuscript of this project which will be submitted this year. A second project examined dopamine D2-receptor signaling in midbrain dopamine neurons of the SNc. D2-receptors control the activity of dopamine neurons in an autoinhibitory feedback loop. D2-receptors play a major role in movement and addictive behaviors but the molecular basis of their function is not fully understood. A large literature has focused on two major effectors of D2-receptors voltage-gated calcium channels and Gi/o protein activated inwardly rectifying potassium (GIRK) channels. In this project, we discovered that D2-receptors function through a third major effector, the sodium leak channel called NALCN. First, we established that NALCN channels play a central role in driving pacemaking in dopamine neurons. Furthermore, we found that D2-receptors inhibit NALCN, which then slows spontaneous firing even in absence of GIRK channel signaling. Lastly, we tested whether the modulation of NALCN was specific to D2-receptors. We found that activation of GABA-B receptors also cause inhibition of NALCN. Thus, our data show that NALCN may be general effector for other Gi/o protein coupled receptors. A manuscript of this work has been submitted for publication and is currently under review. In addition to our scientific achievements, we have had some personnel changes in the lab. Manhua Zhu who was a postbaccalaureate researcher in the lab has moved on to neuroscience graduate school at the University of North Carolina Chapel Hill. We have also had a new graduate student, Dana Lewis, join the lab as part of the NIH-George Washington University Graduate Program Partnership. Lastly, we would like to report that the lab has had its second review by the Board of Scientific Counselors in April which was very successful by all standards.
|Philippart, Fabian; Khaliq, Zayd M (2018) Gi/o protein-coupled receptors in dopamine neurons inhibit the sodium leak channel NALCN. Elife 7:|
|Tarfa, Rahilla A; Evans, Rebekah C; Khaliq, Zayd M (2017) Enhanced Sensitivity to Hyperpolarizing Inhibition in Mesoaccumbal Relative to Nigrostriatal Dopamine Neuron Subpopulations. J Neurosci 37:3311-3330|
|Evans, Rebekah C; Zhu, Manhua; Khaliq, Zayd M (2017) Dopamine Inhibition Differentially Controls Excitability of Substantia Nigra Dopamine Neuron Subpopulations through T-Type Calcium Channels. J Neurosci 37:3704-3720|
|Aflaki, Elma; Borger, Daniel K; Moaven, Nima et al. (2016) A New Glucocerebrosidase Chaperone Reduces ?-Synuclein and Glycolipid Levels in iPSC-Derived Dopaminergic Neurons from Patients with Gaucher Disease and Parkinsonism. J Neurosci 36:7441-52|
|Hage, Travis A; Sun, Yujie; Khaliq, Zayd M (2016) Electrical and Ca(2+) signaling in dendritic spines of substantia nigra dopaminergic neurons. Elife 5:|
|Kimm, Tilia; Khaliq, Zayd M; Bean, Bruce P (2015) Differential Regulation of Action Potential Shape and Burst-Frequency Firing by BK and Kv2 Channels in Substantia Nigra Dopaminergic Neurons. J Neurosci 35:16404-17|
|Hage, Travis A; Khaliq, Zayd M (2015) Tonic firing rate controls dendritic Ca2+ signaling and synaptic gain in substantia nigra dopamine neurons. J Neurosci 35:5823-36|