A Modeling Study Suggests Complementary Roles for GABAA and NMDA Receptors and the SK channel in Regulating the Firing Pattern

Midbrain dopaminergic (DA) neurons in vivo exhibit two major firing patterns: singlespike firing and burst firing. The firing pattern expressed is dependent upon both the intrinsic properties of the neurons and their excitatory and inhibitory synaptic inputs. Experimental data suggest that the activation of NMDA and GABAA receptors is a crucial contributor to the initiation and suppression of burst firing, respectively, and that blocking Ca-activated potassium SK channels can facilitate burst firing. A multicompartmental model of a DA neuron with a branching structure was developed and calibrated based on in vitro experimental data, in order to explore the effects of different levels of activation of NMDA and GABAA receptors, as well as the modulation of the SK current on the firing activity. The simulated tonic activation of GABAA receptors was calibrated by taking into account the difference in the electrotonic properties in vivo versus in vitro. Although NMDA evoked currents are required for burst generation in the model, currents evoked by GABAA-receptor activation can also regulate the firing pattern. For example, the model predicts that increasing the level of NMDA receptor activation can produce excessive depolarization that prevents burst firing, but a concurrent increase in the activation of GABAA receptors can restore burst firing. Another prediction of the model is that blocking the SK channel current in vivo will facilitate bursting, but not as robustly as blocking the GABAA receptors.