Reevaluation of synaptic relationships of cerebellar terminals in the ventral lateral nucleus of the rhesus monkey thalamus based on serial section analysis and three-dimensional reconstruction

Terminals of cerebellar afferents (CB) to different regions of the ventral lateral nucleus (VL) of the rhesus monkey thalamus were labeled with wheat germ agglutinin-horseradish peroxidase following injections into the dentate nucleus. Synaptic relationships of 17 CB with projection neuron dendrites (PNd) and local circuit neuron dendrites (LCNd) were analyzed in serial ultrathin sections from dorsal and ventral VL regions, which are known to differ cytoarchitecturally and functionally. Three terminals were reconstructed using three-dimensional (3D) computer image analysis techniques to obtain volumetric and planar measurements. CB in the ventral VL were often flat and elongated with synaptic vesicles arranged in clusters. Each CB was engaged with one PNd and one to four LCNd. A single bouton formed 8–50 synaptic contacts, with those on PNd outnumbering the ones on LCNd 4.1∶1. Only some CB in the ventral VL were engaged in complex synaptic arrangements such as triads and serial synapses. Most CB in the dorsal VL displayed a roundish shape and numerous uniformly distributed synaptic vesicles. They formed 5–25 synaptic contacts with a 3∶1 ratio of contacts on PNd compared with those on LCNd. CB in the dorsal VL participated in a variety of complex synaptic arrangements. Two types of triads were found: classic with CB, PNd and LCNd, and unconventional with CB and two LCNd. CB were also involved in serial synapses with two LCNd or LCNd and another PNd, and serial sequential synapses with two LCNd and a PNd. Three glomerulus-like structures were encountered in the dorsal VL. 3D reconstruction and volumetric measurements revealed that synaptic contacts formed by CB on PNd had varying shapes and sizes (0.022–0.274 μm2). Synapses formed on LCNd were larger (0.09–0.407 μm2). The total area of all active zones of a single CB on LCNd was either equal to or about 40% smaller than that of synapses on PNd. The entire active zone area comprised 1–1.6% of the total CB surface area and did not seem to correlate with the volume. Synaptic contacts formed by associated LCNd on PNd in complex arrangements were usually small (0.021–0.044 μm2). The results suggest that: synapses formed by CB on PNd and LCNd, and synapses formed by LCNd on PNd may differ in strength; a variety of different circuits participate in the processing of cerebellar afferent information in the primate VL; and these circuits differ in functionally different VL subdivisions.

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