Immunocytochemical localization of neurokinin B in the rat spinal dorsal horn and its association with substance P and GABA: An electron microscopic study

Substance P and neurokinin B are tachykinins that derive from different pre cursors. Both tachykinins are known to be involved in the processing of pai n-related information. Initial studies suggested an antinociceptive effect for neurokinin B, but more recent data indicate that neurokinin B facilitat es nociception. Unfortunately, morphologic correlates are lacking, as littl e is known about the distribution of neurokinin B, especially at the ultras tructural level. Because of its potentially important role in the processin g of pain-related information, we decided to investigate the synaptic inter actions of neurokinin B-immunoreactive profiles in laminae I-III of the rat cervical spinal dorsal horn and their relation to substance P-immunoreacti ve structures. An antibody raised against a portion of the neurokinin B pre cursor peptide was used for the detection of neurokinin B. Neurokinin B-Lik e immunoreactivity occurred in all superficial laminae, with the highest de nsity in inner lamina II: and the lowest in lamina III. Neurokinin B-like i mmunoreactive axonal boutons were mainly dome-shaped and established symmet ric synaptic contacts with dendrites or cell bodies. Neurokinin B-like immu noreactivity was also detected in dendritic profiles in all superficial lam inae. Some of these dendritic profiles were part of synaptic glomeruli in i nner lamina IE and lamina III. Double-labeling for neurokinin B and substan ce P showed a lack of appositions and synapses between neurokinin B and sub stance P-positive profiles. Furthermore, very few profiles double-labeled f or the two peptides were observed. Double-labeling for gamma-aminobutyric a cid (GABA) and neurokinin B showed a complete absence of neurokinin B/GABA co-localization. Furthermore, neuroldnin B-positive profiles were never pre synaptic to GABA-immunoreactive profiles, but frequently neurokinin B-posit ive dendrites were postsynaptic to GABA-immunoreactive boutons. These resul ts suggest that neurokinin B participates in circuits separate from those i nvolving substance P, as virtually no anatomic correlation was found betwee n the two neuropeptides. J. Comp. Neurol. 420:349-362, 2000. (C) 2000 Wiley -Liss, Inc.