Separate neuronal populations of the rat substantia nigra pars lateralis with distinct projection sites and transmitter phenotypes.

The topographic organization of the nigral cells sending axons to the striatum, amygdala and inferior colliculus was studied in the rat substantia nigra pars lateralis by using retrograde fluorescent tracers. Nigral perikarya projecting to the inferior colliculus were located dorsolaterally within the substantia nigra pars lateralis, whereas nigral perikarya projecting to the striatum or to the amygdala were mostly situated ventromedially within the substantia nigra pars lateralis. The transmitter substances of the nigrotectal cells were examined by combining a retrograde tracing method with immunohistochemistry for tyrosine hydroxylase or glutamate decarboxylase. Nigral neurons projecting to the inferior colliculus lacked tyrosine hydroxylase immunoreactivity, but exhibited immunoreactivity for glutamate decarboxylase. The substantia nigra pars lateralis is made up of different neuronal populations: one projecting to the inferior colliculus and another directed to the striatum and amygdala. The pars lateralis pathway to the inferior colliculus utilized GABA as a neurotransmitter, whereas the previously characterized nigral cells projecting to the striatum and superior colliculus use GABA and dopamine as neurotransmitters.