Substantia nigra dopamine neurons fire tonically at low rates resulting in action potential backpropagation. Therefore, we have begun to look at how the rate of background tonic firing influences dendritically-located conductances and synaptically-driven firing responses. In our first study, we were surprised to find that increasing the tonic rate from 1 to 6 Hz generated Ca signals up to three-fold greater than predicted by linear summation of single spike-evoked Ca-transients (or what we call a Ca supralinearity). In addition, two-photon glutamate-uncaging and synaptic stimulation revealed that fast tonic firing enhances NMDA receptor-dependent dendritic Ca signaling and intensifies burst firing output. These results show that modulation of background firing rate precisely tunes dendritic Ca signaling and provides a simple yet powerful mechanism to dynamically regulate the gain of synaptic inputs. A manuscript of this study has been submitted for review (Hage and Khaliq, submitted). During reward behaviors, dopamine neurons fire bursts of action potentials driven by excitatory synaptic input. Behavioral experiments using knockout mice as well as single-cell recordings performed in vivo have established that NMDA receptors are key mediators of burst firing. The subthalamic nucleus, which strongly innervates the compacta, contains neurons that fire at high basal rates of 10-30 Hz and fire bursts as high as 100 Hz for brief periods, raising the question of how release of glutamate and subsequent activation of synaptic receptors occurs under conditions of repetitive synaptic input. Therefore, we are examining synaptic transmission from the subthalamic nucleus to SNc dopamine neurons during high-frequency stimulation patterns. We are finding a likely role of glutamate spillover and activation of extrasynaptic NMDA receptors. Surprisingly, however, rather than simply promoting excitation of SNc neurons as might be expected, our data suggest that spillover also triggers a negative feedback mechanism that functions to reduce spiking output through presynaptic group II metabotropic receptor (mGluRII) dependent inhibition of synaptic input. We expect to submit a manuscript of these results this Fall 2014 (Girasole, Hage, and Khaliq, in preparation). We were approved to perform intracranial injections in mice last year and spent a significant amount of time working out the coordinates for injection sites. Currently, we are using this procedure to retrogradely label particular dopamine subpopulations based on the axonal projection firing pattern (eg. prefrontal cortex, nucleus accumbens, etc). So far, we find significant differences in the excitability of dopamine neurons (mesocortical vs mesoaccumbal), and we are currently studying the ionic mechanisms underlying those differences. We expect to have the experiments for this study complete this Fall 2014 and look to submit this manuscript early in 2015 (Tarfa and Khaliq). We continue to make good progress in staffing the laboratory. There are currently five (5) members of the lab - Travis Hage, postdoctoral fellow;Rebekah Evans, postdoctoral fellow;Rahilla Tarfa, graduate fellow;Yujie Richie Sun, postbaccalaureate fellow;and Renshu Sherry Zhang, research technician. Dr. Evans started working the lab earlier this year in January. We are also participating in collaborative projects with Dr. Ellen Sidransky and Dr. Andres Buonanno (NIH).
