Presynaptic kainate receptors in primary afferents to the superficial laminae of the rat spinal cord

Subunits of glutamate receptors participate in the regulation of sensory tr ansmission at primary afferent synapses in the superficial laminae of dorsa l horn (DH). We report here on the distribution of kainate receptors (GluR5 /6/7) in these laminae by using light microscope (LM) and electron microsco pe (EM) immunocytochemistry. Standard (4%) paraformaldehyde fixation result ed in immunostaining for GluR5/6/7 in perikarya and fine processes in lamin a TI, especially its inner part (IG). Preembedding EM revealed immunostaini ng of dendrites, perikarya, and occasional terminals, presumed to be from p rimary afferent fibers, at the center of glomerular arrangements. In rats p erfused with 0.5% paraformaldehyde, LM showed a more punctate staining, mai nly in the ventral part of lamina IG and lamina III, than in material fixed with 4% paraformaldehyde. Approximately two-thirds of GluR5/6/7 puncta wer e also immunostained with synaptophysin, suggesting that in material fixed with 0.5% paraformaldehyde, a large fraction of these are synaptic terminal s. Double immunostained puncta disappear 4 days after dorsal rhizotomy, sug gesting that most of GluR5/6/7-immunopositive terminals are from primary af ferent fibers. EM material fixed with 0.5% paraformaldehyde confirmed the e xpression of GluR5/6/7 in numerous synaptic endings with morphology of prim ary afferents. To determine the type of primary afferent terminals that exp ress GluR5/6/7, two neuroanatomic tracers were injected in the sciatic nerv es. The lectin from Bandeiraea simplicifolia (IB4) is selectively taken up by unmyelinated primary afferent fibers that terminate in the outer part of lamina II (IIo) and dorsal part of lamina IIi, whereas the B subunit of th e cholera toxin (CTB) is selectively taken up by a broader class of primary afferents which, in superficial DH, terminate mainly in laminae I, ventral part of IG, and III. Approximately 20% of GluR5/6/7-immunoreactive puncta colocalized with IB4, whereas approximately 40% of GluR5/6/7-immunoreactive puncta colocalized with CTB. The present study shows that (Ij GluR5/6/7 do es not have a clear and consistent spatial relation with postsynaptic sites , (2) a large number of primary afferents express GluR5/6/7, and (3) these are not limited to one functional class. Thus, modulation by glutamate of p rimary afferent terminals by means of kainate receptors in the superficial laminae of DH predominantly involve presynaptic mechanisms.(C) 2001 Wiley-L iss, Inc.