 Most striatal MSNs contain only D1-type or D2-type dopamine receptors, but a subpopulation of MSNs exhibit both phenotypes.Direct pathway MSNs excite their ultimate basal ganglia output structure (e.g., the thalamus) and promote associated behaviors; these neurons express D1-type dopamine receptors, adenosine A1 receptors, dynorphin peptides, and substance P peptides.Indirect pathway MSNs inhibit their output structure and in turn inhibit associated behaviors; these neurons express D2-type dopamine receptors, adenosine A2A receptors (A2A), heterotetramers, and enkephalin.