Nociceptive primary afferents generally have small myelinated (Aδ) or unmyelinated (C) axons and terminate mainly in the superficial laminae (I and II) of the dorsal horn of the spinal cord. By using a combination of immunocytochemistry, transganglionic tracing and lectin binding it is possible to identify various ‘neurochemical’ types of fine diameter primary afferent and examine their distribution and connections within the spinal cord.

Many C and Aδ afferents synthesise one or more neuropeptides: in the rat all of these peptidergic afferents are thought to contain calcitonin gene-related peptide (CGRP), while many also have the tachykinin peptide substance P. There is physiological evidence that all substance P-containing primary afferents function as nociceptors (Lawson et al. 1997). Substance P-containing afferents (which can be revealed by the presence of both substance P and CGRP) terminate densely in lamina I and the outer part of lamina II, but are also found in deeper laminae of the dorsal horn. Many C fibres do not appear to contain neuropeptides, and these can be revealed with the lectin Bandeiraea simplicifolia isolectin B4 (BSI-B4). Since some peptidergic afferents also bind this lectin, the non-peptidergic C fibres are best demonstrated by combining fluorescent detection of BSI-B4 (applied to tissue sections) with immunofluorescence for CGRP (Sakamoto et al. 1999). Non-peptidergic C afferents terminate in a narrow band within lamina II, and the majority form the central terminals of synaptic glomeruli. Aδ nociceptors arborise mainly in lamina I, and some of these can be identified by transganglionic transport of the B subunit of cholera toxin (CTb) injected into an intact somatic nerve. We have found that following injection of CTb into the rat sciatic nerve most labelled axons in lamina I were not immunoreactive with antibodies against substance P or CGRP, which suggests that CTb may specifically label those Aδ nociceptors that lack neuropeptides.

The spinal dorsal horn contains neurons that project to various supraspinal targets, including the caudal ventrolateral medulla (CVLM), the lateral parabrachial area (LPb), the periaqueductal grey matter (PAG) and the diencephalon. Many of these projection neurons are located in lamina I, and we have demonstrated that approximately 80 % of lamina I cells projecting to each of these areas express the neurokinin 1 (NK1) receptor, on which substance P acts (Todd et al. 2000). Most lamina I projection neurons have dendrites that remain within the lamina. We have found that following injection of two different tracers into the CVLM and LPb the majority of retrogradely labelled neurons in lamina I are double-labelled, which suggests that many lamina I cells project to both of these targets. Another prominent group of dorsal horn projection neurons are those which have cell bodies located in lamina III or IV and long dorsally directed dendrites that enter the superficial laminae. These cells also express the NK1 receptor. Virtually all of these cells project to the CVLM, while most can be labelled from LPb and some from PAG or thalamus (Todd et al. 2000).

We have used immunocytochemistry with confocal and electron microscopy to investigate possible connections between fine diameter primary afferents and projection neurons in the rat spinal dorsal horn. Substance P-containing primary afferents form numerous contacts with NK1 receptor-immunoreactive projection neurons in lamina I and with those in laminae III/IV. In the latter case, the primary afferents frequently run along dendrites which pass through the superficial dorsal horn (Naim et al. 1997). By combining confocal and electron microscopic immunocytochemistry, we have been able to show that synapses are present at the majority of contacts which the cells receive from substance P-containing primary afferents. This suggests that in addition to the action of substance P on the NK1 receptors expressed by these cells, glutamate will act as a fast transmitter at these contacts. Although the dendrites of lamina III/IV NK1 receptor-immunoreactive neurons pass through lamina II, they received very few contacts from non-peptidergic C afferents (Sakamoto et al. 1999).

These results suggest that fine diameter primary afferents belonging to different neurochemical populations have specific patterns of synaptic connection within the superficial dorsal horn.We are grateful to The Wellcome Trust for support.

Lawson, S.N., Crepps, B.A. & Perl, E.R. (1997). J. Physiol. 505, 177-191. abstract

Naim, M., Spike, R.C., Watt, C., Shehab, S.A.S. & Todd, A.J. (1997). J. Neurosci. 17, 5536-5548.

Sakamoto, H., Spike, R.C. & Todd, A.J. (1999). Neuroscience 94, 903-908.

Todd, A.J., McGill, M.M. & Shehab, S.A.S. (2000). Eur. J. Neurosci. 12, 689-700.