The Substantia Nigra Pars Reticulata

The GABAergic projection neurons are the main neuron type in the substantia nigra pars reticulata (SNr). A striking feature of these SNr GABA neurons is that they fire sustained, spontaneous high frequency spikes and thus tonically inhibit their targets. Compared with their neighboring slow-spiking dopamine neurons, these GABA neurons have a stronger transient voltage-gated sodium current with a faster activation and a faster recovery from inactivation. SNr GABA neurons also express a robust Kv3-like potassium current that activates rapidly and inactivate slowly. Together, these intrinsic ion channels provide SNr GABA neurons with the ability to fire sustain high frequency spikes. Additionally, robust GABA inputs from the striatum and the globus pallidus pause SNr GABA neuron firing, whereas glutamate inputs from the subthalamic nucleus stimulate SNr GABA neuron activity, sculpting the tonic high frequency firing into effective control signals that are further fine-tuned by neuromodulators including dopamine, serotonin, and endocannabinoids.