Synaptic organization of the cat entopeduncular nucleus with special reference to the relationship between the afferents to entopedunculothalamic projection neurons: an electron microscope study by a combined degeneration and horseradish peroxidase tracing technique.

The synaptic organization of the feline entopeduncular nucleus was studied electron microscopically. After horseradish peroxidase injections into the ventral anterior and ventral lateral nuclear complex of the thalamus, normal axon terminals synapsing with entopedunculothalamic projection neurons were classified into four types on the basis of the size and shape of synaptic vesicles in them, and types of the postsynaptic membrane differentiation. Type I and type II axon terminals were characterized by symmetrical synaptic contacts, and large ovoid or small ovoid synaptic vesicles, respectively. Type II axon terminals were further classified into two subtypes as to their sizes: one was small (IIa), the other large (IIb). Type III and type IV axon terminals were characterized by asymmetrical synaptic contacts, and large ovoid or small ovoid synaptic vesicles, respectively. To determine the origin of each type of terminal, electrolytic lesions of the caudate nucleus or the subthalamic nuclear region were combined with horseradish peroxidase injections into the thalamus or the subthalamic nuclear region. After electrolytic lesions of the caudate nucleus, degeneration was seen in type I axon terminals contacting entopedunculothalamic projection neurons. Following electrolysis or horseradish peroxidase injection into the subthalamic nuclear region, type IIa and type IV axon terminals showed degenerations or horseradish peroxidase labelling. Such terminals also synapsed with entopedunculothalamic projection neurons. It was demonstrated that these projection neurons relay the striatal or subthalamic inputs directly to the thalamus. After horseradish peroxidase injection into the thalamus, many labelled type II axon terminals were observed to synapse with entopedunculothalamic projection neurons. Type III axon terminals were left unchanged throughout these experiments. In addition, the entopeduncular neuron was observed to receive convergent inputs from both the caudate nucleus and probably the subthalamic nucleus. Axoaxonal synapses were also found to be involved in the synaptic triad. These results indicate that type I axon terminals originate from the caudate nucleus, part of type IIa and type IV axon terminals originate from the subthalamic nucleus or caudal to the subthalamic nuclear region, and part of type IIa and type IIb terminals come from intrinsic axon collaterals.