Retrograde tracing of zinc-enriched (ZEN) neuronal somata in rat spinal cord

The zinc selenide autometallographic (ZnSeAMG) technique for tracing the re trograde axonal transport of zinc ions in zinc-enriched (ZEN) neurons was u sed to map thy distribution of ZEN neuronal somata in rat spinal cord. Afte r a local injection of sodium selenide into the dorsal or ventral horn. ZnS eAMG-labeled ZEN neurons appeared in Rexed's laminae V.VII and X while lami nae I and II were void. A few scattered ZEN somata were observed in the rem aining laminae. The labeled neurons differed in shape and size, and the rel atively high level of labeled somata around the injection site suggests tha t many ZEN neurons have relatively short axons or boutons en passage close to the neuronal origin. Ultrastructurally, the retrogradely transported zin c selenide clusters were found in the lysosomes of ZEN somata and proximal dendrites. Electron microscopic studies also revealed two different kinds o f ZEN terminals: (1) terminals with flat synaptic vesicles making symmetric synaptic contacts: and (2) terminals with round vesicles making asymmetric synaptic contacts. The present study suggests the existence of propriospin al systems of ZEN neurons comprising both segmental and intersegmental ZEN connections and having either inhibitory or excitatory ZEN terminals. The Z EN neurons seem to form a vast network of terminals located primarily in th e gray matter, but also contacting dendrites radiating into the white matte r. Important functions of this rather massive system of ZEN terminals can n ot be deduced from our present knowledge, but the systems appear to be invo lved in both motor and sensory functions. (C) 2001 Elsevier Science B.V. Al l rights reserved.