REEVALUATION OF SYNAPTIC RELATIONSHIPS OF CEREBELLAR TERMINALS IN THEVENTRAL LATERAL NUCLEUS OF THE RHESUS-MONKEY THALAMUS BASED ON SERIALSECTION ANALYSIS AND 3-DIMENSIONAL RECONSTRUCTION

Terminals of cerebellar afferents (CB) to different regions of the ven tral lateral nucleus (VL) of the rhesus monkey thalamus were labeled w ith wheat germ agglutinin-horseradish peroxidase following injections into the dentate nucleus. Synaptic relationships of 17 CB with project ion neuron dendrites (PNd) and local circuit neuron dendrites (LCNd) w ere analyzed in serial ultra thin sections from dorsal and ventral VL regions, which are known to differ cytoarchitecturally and functionall y. Three terminals were reconstructed using three-dimensional (3D) com puter image analysis techniques to obtain volumetric and planar measur ements. C-B in the ventral VL were often flat and elongated with synap tic vesicles arranged in clusters. Each CB was engaged with one PNd an d one to four LCNd. A single bouton formed 8-50 synaptic contacts, wit h those on PNd outnumbering the ones on LCNd 4.1:1. Only some CB in th e ventral VL were engaged in complex synaptic arrangements such as tri ads and serial synapses. Most CB in the dorsal VL displayed a roundish shape and numerous uniformly distributed synaptic vesicles. They form ed 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 we re also involved in serial synapses with two LCNd or LCNd and another PNd, and serial sequential synapses with two LCNd and a PNd. Three glo merulus-like structures were encountered in the dorsal VL. 3D reconstr uction and volumetric measurements revealed that synaptic contacts for med by CB on PNd had varying shapes and sizes (0.022-0.274 mu m(2)). S ynapses formed on LCNd were larger (0.09-0.407 mu m(2)). The total are a of all active zones of a single CB on LCNd was either equal to or ab out 40% smaller than that of synapses on PNd. The entire active zone a rea 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 mu m(2 )). The results suggest that: synapses formed by CB on PNd and LCNd, a nd synapses formed by LCNd on PNd may differ in strength; a variety of different circuits participate in the processing of cerebellar affere nt information in the primate VL; and these circuits differ in functio nally different VL subdivisions.