Dopamine facilitates the response to glutamatergic inputs in astrocyte cell models

by Thiago Ohno Bezerra, Antonio C. Roque

Astrocytes respond to neurotransmitters by increasing their intracellular Ca²⁺ concentration (Ca²⁺ signals). While glutamate released by neurons trigger Ca²⁺ signals through IP₃- and glutamate transporter-dependent mechanisms, dopamine released in distant sites activates astrocytes via dopaminergic receptors. However, little is known about the modulatory effects of dopamine on glutamate-evoked astrocytic activity. To investigate this question, we developed multi-compartment, conductance-based astrocyte models with three distinct morphologies: unipolar; bipolar; and bifurcated-terminal. Glutamate induced localized responses, while dopamine activated all compartments. In the unipolar model, global dopaminergic stimulation reduced the threshold frequency of glutamatergic stimulation required to activate Ca²⁺ signals. Phase-plane analysis of a simplified version of this model revealed that Ca²⁺ signals are influenced by compartment radius and neurotransmitter type. Morphology significantly influenced glutamate-dopamine interactions. In the bipolar model, glutamatergic stimulation in one process minimally affected the other. Conversely, in the bifurcated-terminal model, where a single process bifurcates into two secondary processes, high-frequency glutamatergic stimulation in one secondary process evoked Ca²⁺ signals in the other. Dopamine further facilitated this latter cross-process interaction by lowering the glutamatergic stimulation frequency needed to elicit Ca²⁺ signals in the adjacent secondary process. These findings suggest that dopamine enhances the initiation and propagation of glutamate-evoked Ca²⁺ signals, with the extent of propagation depending on astrocytic morphology and the spatial distribution of glutamatergic inputs.