A second type of striatonigral neuron: a comparison between retrogradely labelled and Golgi-stained neurons at the light and electron microscopic levels

In a light and electron microscopic examination of the neostriata of rats that had received injections of horseradish peroxidase into the ipsilateral substantia nigra, two morphologically distinct types of horseradish peroxidase-labelled neurons were observed. In confirmation of previous findings, one type was of medium-size and was characterized by Golgi-staining and gold-toning as the densely spinous type. The second type of neuron was in contrast, larger, had an indented nucleus and numerous cytoplasmic organelles. The synaptic input to the perikarya of the latter neurons consisted of numerous boutons containing large round and oval vesicles. The boutons formed symmetrical synaptic contacts and were similar to those of the local axon collaterals of medium-size densely spiny striatonigral neurons.In an attempt to establish what type of Golgi-impregnated neuron the second type of horseradish peroxidase-labelled neuron was, seventeen Golgi-stained or gold-toned neurons were examined in the electron microscope. Three of them were very similar in their ultrastructural features and synaptic input to the horseradish peroxidase-labelled neurons. All three were of a similar morphological appearance in the light-microscope and characteristically had long (up to 700 μm), essentially smooth dendrites. Both the large horseradish peroxidase-labelled neurons and the Golgi-impregnated neurons with long dendrites have so far only been found in the most ventral regions of the neostriatum.It is concluded that there are at least two morphologically distinct types of striatonigral neurons.