Glutamic acid decarboxylase and enkephalin immunoreactive axon terminals in the rat neostriatum synapse with striatonigral neurons.

Synaptic interactions between striatal projection neurons and axon terminals containing immunoreactive glutamic acid decarboxylase (GAD) or Leu-enkephalin were examined in the rat neostriatum using a combined method of horseradish peroxidase retrograde transport from the substantia nigra and immunohistochemistry at the electron microscopic level. Results showed that numerous immunoreactive GAD and enkephalin boutons formed synapses with the cell bodies and dendrites of medium-sized striatonigral neurons. These findings demonstrate that within the neostriatum GABA and enkephalin directly influence caudate output pathways.