Medium spiny neuron projection from the rat striatum: an intracellular horseradish peroxidase study.

The morphological features of striatal projection neurons and the responses of these neurons to electrical stimulation of the substantia nigra were studied in rats through the methods of intracellular recording and intracellular labeling with the enzyme horseradish peroxidase. Under urethane anesthesia, single striatal neurons were first analyzed for responsiveness to nigral stimuli and then filled electrophoretically with the enzyme in order to permit subsequent serial reconstruction. The axon of the medium spiny neuron was found to form an extensive collateral plexus within the striatum before entering the globus pallidus or the internal capsule. These medium spiny projection neurons responded to nigral stimuli with monosynaptic excitation.