The work in our laboratory focuses on cellular and subcellular principles of integration and excitability in dopamine-releasing neurons located in the midbrain. In a recent project, we examined the properties of spine synapses located on the dendrites of dopamine neurons, and compared the relative contribution of spine and shaft synapses to excitability. Prior to this study, evidence for the presence of dendritic spines had been mixed and there had been no functional study examining electrical and Ca2+ signaling in dendritic spines of midbrain dopamine neurons. Using two-photon uncaging of glutamate with imaging in a mouse line that expresses fluorescently tagged-PSD95 to locate glutamatergic synapses, we found that dopamine neurons express functional spines and that the size of synaptic EPSPs correlated positively with the presence of PSD-95. Lastly, we found that the characteristic slow pacemaker firing of dopamine neurons combines with boosting of spine potentials due to the narrow spine neck to produce a novel enhancement of spine Ca2+ that occurs periodically in a window from the middle to the late phase of the spike cycle. The results of this study were published in Elife in 2015. Future work will test the ionic mechanism of this pacemaker-evoked Ca enhancement and explore its implications for synaptic plasticity. Other major interests of the lab include 1) identifying functionally and biochemically unique subpopulations of midbrain dopamine neurons and 2) understanding how these neurons fit into the basal ganglia circuit. To this end, one project compared the responses to evoked and synaptically-generated GABAergic inhibition of dopamine neurons subpopulations projecting to nucleus accumbens and dorsal striatum. Midbrain dopamine neurons pause their firing in response to reward omission or aversive stimuli. Our results show that pauses in dopamine neuron firing, evoked either by stimulation of GABAergic inputs or by hyperpolarizing current injections mimicking inhibition, were enhanced by a subclass of potassium conductances recruited at voltages below spike threshold. Importantly, these A-type potassium currents recorded in mesoaccumbal neurons displayed substantially slower inactivation kinetics which, combined with weaker expression of a second conductance, hyperpolarization-activated conductance or Ih, lengthened hyperpolarization-induced delays in spiking relative to nigrostriatal neurons. Given recent anatomical studies that find that dopamine neuron subpopulations share largely overlapping inputs, these results suggest that integration of these inputs differs among dopamine neurons favoring higher sensitivity to inhibition in mesoaccumbal relative to nigrostriatal neurons, a feature that may be important for aversive signaling. A second project examined functional heterogeneity of dopamine neurons within the substantia nigra. It is known that within the SNc, which is susceptible to cell death in Parkinsons Disease, there are distinct subpopulations of vulnerable and resilient neurons that can be distinguished according to their expression of the calcium (Ca2+) binding protein, calbindin. We found that that vulnerable calbindin-lacking neurons and resilient calbindin-positive dopaminergic neurons differ substantially in their physiology, calcium signaling, dendritic branching, and excitatory synaptic transmission. Interestingly, we found that calbindin-lacking neurons display low-threshold depolarizations that were accompanied by a large increase in dendritic calcium. We found that this Ca2+enters through a subclass of voltage-gated Ca channels called T-type Ca2+ and we are currently exploring the contribution of this channel to physiology of calbindin-lacking cells. Lastly, we participated in a collaborative projects this year which resulted in two publications. One project, a collaboration with Dr. Bruce Bean at Harvard Medical School, examined the potassium channels that contribute to repolarization of action potentials in SNc dopaminergic neurons. This project was published in the Journal of Neuroscience in 2015. A second collaboration with the laboratory of Dr. Ellen Sidransky examined how glucocerebrosidase impacts parkinsonism. A manuscript of this study was recently published as an article in the Journal of Neuroscience in 2016.
