Ectopic substance P-immunoreactive boutons are preferentially presynaptic to neurokinin-1 receptor immunoreactive dendrites in the spinal white matter of transgenic mice

A recent immunocytochemical study has shown that substance P (SP) preferent ially innervates targets expressing the neurokinin-1 receptor (NK-1r) in th e superficial spinal dorsal horn of the rat. Based on these findings, we de cided to further investigate the relationship between SP and the NK-1r in a transgenic mouse model in which SP fibres are ectopically located. Double- labelling immunocytochemistry at both the light and electron microscopic le vels was performed to study the association between SP and the NK-1r in the spinal white matter of both control and transgenic mice. Light microscopy revealed NK-1r-immunoreactive (IR) dendrites in the white matter of the dor solateral funiculus in both control and transgenic mice. In transgenic mice , but not in controls, SP-IR fibres were observed in close proximity to the NK-1r-IR dendrites in the white matter. At the ultrastructural level, SP-I R boutons were apposed to NK-1r-IR dendrites in the dorsolateral funiculus of transgenic mice, and a synapse was frequently observed as well. These re sults indicate that, even in conditions in which SP fibres are ectopically located, they still preferentially innervate targets expressing the NK-1r. (C) 1999 Elsevier Science B.V. All rights reserved.