Dopamine Inputs to the Basal Ganglia
Most dopamine (DA) in the brain comes from the Substantia Nigra (located in the midbrain, shown above). It is so named because it appears as dark bands, thus the name “black substance” or substantia nigra. There are two parts of the substantia nigra- substantia nigra pars compacta and substantia nigra pars reticulata. Pars compacta has dopaminergic neurons, while pars reticulata is mostly GABAergic (GABA is a neurotransmitter that is mainly inhibitory).
The substantia nigra pars compacta sends dopaminergic projections to the striatum, therefore effecting the basal ganglia circuitry. In the direct pathway, DA works on D1 receptors and has an excitatory effect. In other words, this means that DA excites the excitatory basal ganglia loops, and therefore increases movements/thoughts/emotions. For neurons in the indirect pathway, DA acts on D2 receptors and has an inhibitory effect. This means that DA inhibits the inhibitory pathway.
For both pathways, the result of DA is to increase actions (movements/thoughts/emotions) by either exciting the direct pathway or inhibiting the indirect pathway.