Substantia nigra pars reticulata:

the region of the substantia nigra that contains high numbers of GABA neurons and is less populated with dopamine neurons than the substantia nigra pars compacta (the other part of the substantia nigra).

Further reading: Know your brain- Substantia nigra

Know your brain - Basal ganglia

Watch this 2-Minute Neuroscience video to learn more about the substantia nigra.