Exploring GABAergic and dopaminergic effects in a minimal model of a medium spiny projection neuron

Striatum is the input stage of the basal ganglia, a collection of nuclei in the midbrain. The basal ganglia are involved in cognitive and motor behaviour, including reward-dependent learning. The reward system in the brain is heavily linked to the dopaminergic system, and many striatal neurons react in a reward-dependent manner. This study explores a minimal model of a striatal medium spiny (MS) projection neuron displaying dopamine-induced bistability. MS neurons mostly fluctuate between two states, a hyperpolarised down-state and a depolarised up-state. MS neurons are only active in the up-state and therefore spiking requires the transition from the down-state. For high dopamine levels in the model, the appearance of a bifurcation results in more distinct state transitions. GABAergic input from local fast-spiking interneurons to MS neurons results in a small depolarisation, but far from causing a transition to the up-state by itself. We investigate if a GABAergic PSP could facilitate the transition to the up-state elicited by glutamatergic input. The model predicts that GABAergic input to MS neurons might facilitate and speed up the transition to the up-state. The prerequisite for this is that the GABAergic enhancement starts slightly before the glutamatergic increase which causes the up-state transition.