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 precursors. Both tachykinins are known to be involved in the processing of pain-related information. Initial studies suggested an antinociceptive effect for neurokinin B, but more recent data indicate that neurokinin B facilitates nociception. Unfortunately, morphologic correlates are lacking, as little is known about the distribution of neurokinin B, especially at the ultrastructural level. Because of its potentially important role in the processing of pain-related information, we decided to investigate the synaptic interactions of neurokinin B-immunoreactive profiles in laminae I-III of the rat cervical spinal dorsal horn and their relation to substance P-immunoreactive structures. An antibody raised against a portion of the neurokinin B precursor peptide was used for the detection of neurokinin B. Neurokinin B-like immunoreactivity occurred in all superficial laminae, with the highest density in inner lamina II and the lowest in lamina III. Neurokinin B-like immunoreactive axonal boutons were mainly dome-shaped and established symmetric synaptic contacts with dendrites or cell bodies. Neurokinin B-like immunoreactivity was also detected in dendritic profiles in all superficial laminae. Some of these dendritic profiles were part of synaptic glomeruli in inner lamina II and lamina III. Double-labeling for neurokinin B and substance P showed a lack of appositions and synapses between neurokinin B and substance P-positive profiles. Furthermore, very few profiles double-labeled for the two peptides were observed. Double-labeling for gamma-aminobutyric acid (GABA) and neurokinin B showed a complete absence of neurokinin B/GABA co-localization. Furthermore, neurokinin B-positive profiles were never presynaptic to GABA-immunoreactive profiles, but frequently neurokinin B-positive dendrites were postsynaptic to GABA-immunoreactive boutons. These results suggest that neurokinin B participates in circuits separate from those involving substance P, as virtually no anatomic correlation was found between the two neuropeptides.