Many neurons with cell bodies in laminae III or IV of the spinal dorsal horn possess the neurokinin 1 receptor and have dorsal dendrites that arborize in the superficial dorsal horn. We have performed a confocal microscopic study to determine whether these cells receive inputs from substance P-containing primary afferents. All neurons of this type received contacts from substance P-immunoreactive axons, and in most cases the contacts onto dorsal dendrites were very numerous. A great majority (90-100%) of substance P-immunoreactive varicosities in contact with these cells were also immunoreactive with antibody to calcitonin gene-related peptide, indicating that they were of primary afferent origin. The density of contacts from substance P-immunoreactive varicosities onto these cells was significantly higher than that seen on cholinergic neurons in lamina III (which do not possess the receptor). Electron microscopy revealed that synapses were present at points of contact between substance P-immunoreactive boutons and dorsal dendrites of cells with the neurokinin 1 receptor. Some cells of this type belong to the spinothalamic tract, and we therefore examined neurons with cell bodies in laminae III or IV that possessed the neurokinin 1 receptor and were labeled retrogradely after thalamic injection of cholera toxin B subunit. These cells also received contacts from substance P-immunoreactive axons on their dorsal dendrites. The results of this study indicate that neurons of this type are a major target for substance P-containing primary afferents.

We have previously demonstrated that neurons which have cell bodies in laminae III or IV of the rat spinal cord, dendrites that enter the superficial laminae and which possess the neurokinin-1 receptor receive a major synaptic input from substance P-containing primary afferent axons. In this study we set out to determine whether these cells also receive monosynaptic input from myelinated primary afferents by using transganglionic transport of the B subunit of cholera toxin to identify the central terminals of myelinated afferents from the sciatic nerve. Dual-immunofluorescence and confocal microscopy revealed apparent contacts between labelled primary afferent terminals and all of the neurokinin-1 receptor-immunoreactive cells examined, although these contacts were much less numerous than those which the cells receive from substance P-containing primary afferents. By using a combined confocal and electron microscopic technique we were able to confirm that synapses were present at some of the contacts between primary afferents and neurokinin-1 receptor-immunoreactive neurons. These results suggest that cells of this type will have wide-dynamic range receptive fields, but with a relatively strong input from nociceptors.

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