ULTRASTRUCTURAL AND IMMUNOCYTOCHEMICAL CHARACTERIZATION OF PRIMARY AFFERENT TERMINALS IN THE RAT CUNEATE NUCLEUS

The cuneate nucleus is a relay center for somatosensory information by receiving tactile and proprioceptive inputs from primary afferent fib ers that ascend in the dorsal funiculus. The morphology, synaptic cont acts, and neurochemical content of primary afferent terminals in the c uneate nucleus of rats were investigated by combining anterograde tran sport of horseradish peroxidase conjugated to wheat-germ agglutinin or to cholera toxin (injected in cervical dorsal root ganglia) with post embedding immunogold labeling for glutamate and GABA. Both tracers gav e similar results. Two types of terminals were labeled: type I termina ls were irregularly shaped, had a mean area of 4.0 mu m(2), synapsed o n several dendrites, and were contacted by other terminals, some of wh ich were GABA positive. Type II terminals were dome-shaped, had a mean area of 2.18 mu m(2), and made synaptic contact on a single dendrite. All the anterogradely labeled terminals (interpreted as endings of pr imary afferents) were enriched in glutamate but not in GABA. The findi ng that identified primary afferent terminals are enriched in glutamat e with respect to other tissue profiles strongly suggests a neurotrans mitter role for glutamate in this afferent pathway to the rat cuneate nucleus. (C) 1994 Wiley-Liss, Inc.