Immediate Early Gene Expression in D1-SPNs and D2-SPNs During a Striatum-dependent Reinforcement Learning Task

Abstract

Dopamine signaling is thought to promote movement by differentially altering the excitability of the striatum's principal neurons
(D1- and D2-SPNs). Here, we used immunohistochemistry to quantify the expression of Fos, a marker of neural activity, in mice trained to run in a head-fixed fear conditioning task that requires dopamine signaling in the striatum. Training in the task increased the number of Fos-expressing neurons, and a greater proportion of these neurons were D1-SPNs than D2-SPNs. However, this relative increase in D1-SPN Fos expression was not specific for learning the task, as a similar increase was observed in animals that underwent training, but did not learn to perform the motor response. In those animals, D1-SPN activation may encode something other than the learned response. Although further experiments are necessary to determine what the Fos-active populations encode in learners and non-learners, the training-dependent changes we observed in the levels of Fos expression in D1- and D2-SPNs may correspond to fluctuations in neural plasticity that may contribute to the changes in neural calcium activity previously observed by others in our laboratory. Our findings have implications for understanding disease processes that affect the dopamine system, such as Parkinson's disease and schizophrenia.