The distribution of zinc selenite and expression of metallothionein-III mRNA in the spinal cord and dorsal root ganglia of the rat suggest a role forzinc in sensory transmission

Zinc appears to play a role in synaptic transmission in the hippocampus. We tested the hypothesis that: zinc is similarly involved in sensory transmis sion by determining whether vesicular zinc and metallothionein-III (MT-III) , a zinc-binding protein, are localized in rat primary afferent neurons. MT -III mRNA, measured using RT-PCR, and MT-III immunoreactivity, were both pr esent in the spinal cord as well as the thoracic and lumbar dorsal root gan glia (DRG). At a time (24 hr) that allows retrograde transport of zinc sele nite to cell bodies, only small-diameter neurons and neurons scattered thro ughout lamina V of the spinal cord were stained by sodium selenite injected intrathecally. This stain disappeared if a ligature was placed on the dors al root to block axonal transport, demonstrating that these cells are, in f act, zinc-containing primary afferent neurons. When assessed 1 hr after sod ium selenite, stain was distributed throughout the neuropil of the spinal c ord, especially in lamina III and the area surrounding the central canal. E ven in rhizotomized animals, large- and small-diameter DRG neuronal cell bo dies were also stained with either selenite (1 hr) or 6-methoxy 8-para-tolu ene sulfonamide quinoline (TSQ). Paradoxically, this unique pool of zinc wa s eliminated in large-diameter DRG neurons after neonatal capsaicin treatme nt, which had no effect on selenite stain or MT-III mRNA content in small-d iameter DRG neurons. In summary, we demonstrate that there is a population of capsaicin-insensitive small-diameter primary afferent neurons that are z inc-containing. In addition, there is a unique pool of capsaicin-sensitive zinc that is associated with large-diameter cell bodies.