Ultrastructural analyses of afferent terminals in the subthalamic nucleus of the cat with a combined degeneration and horseradish peroxidase tracing method.

The synaptic organization of the feline subthalamic nucleus (STN) was studied electron microscopically. Following horseradish peroxidase (HRP) injections into the globus pallidus (GP) and electrolytic lesions of the nucleus tegmenti pedunculopontinus pars compacta (TCP) in the same cat, both degenerating and HRP-labeled terminals were found in the STN with abundant retrogradely HRP-labeled neurons. Degenerating terminals of TPC origin were medium-sized and characterized by asymmetric synaptic contacts. They synapsed widely on the STN neuronal surface, including the somata, dendrites of varying dimensions, dendritic spines and vesicle-containing processes. They formed 25.1%, 65.1%, 4.7%, and 4.7%, respectively, of all TPC efferent terminals. Some of the postsynaptic components were labeled with HRP. Occasionally both degenerating terminals and HRP-labeled terminals were in synaptic contact with the same HRP-labeled neuron: therefore, afferents of TPC and GP converge on the same STN projection neuron. In order to discover the origin of these HRP-labeled terminals, a mixed solution containing HRP and kainic acid was injected into the GP. Numerous degenerating terminals were observed to synapse with HRP-labeled STN neurons, but no HRP-labeled terminal was observed. These degenerating terminals were similar in appearance to the above-mentioned HRP-labeled terminals. They were characterized by their relatively large size, predominantly symmetric synapses, and preferential distribution on the somata and large or medium-sized dendrites. They formed 39.6%, 20.1%, and 31.1%, respectively, of all GP efferent terminals. Therefore, it became clear that both the HRP-labeled terminals of the first experiment and the degenerating terminals of the second experiment originated from the GP. Following surgical ablations of the primary sensorimotor cortex (Cx), some axon terminals in the STN showed degeneration. These degenerating terminals were small and formed asymmetric synapses mainly with dendritic spines, small dendrites and vesicle-containing processes. They formed 48.0%, 28.0%, and 12.0%, respectively, of all Cx efferent terminals. These electron microscopic investigations reveal the convergence of TPC and GP afferents and that STN projection neurons relay the TPC and pallidal inputs directly to the GP.