Anatomy of primary afferents and projection neurones in the rat spinal dorsal horn with particular emphasis on substance P and the neurokinin 1 receptor.
The dorsal horn of the spinal cord plays an important role in transmitting information from nociceptive primary afferent neurones to the brain; however, our knowledge of its neuronal and synaptic organisation is still limited. Nociceptive afferents terminate mainly in laminae I and II and some of these contain substance P. Many projection neurones are located in lamina I and these send axons to various parts of the brain, including the caudal ventrolateral medulla (CVLM), parabrachial area, periaqueductal grey matter and thalamus. The neurokinin 1 (NK1) receptor on which substance P acts is expressed by certain neurones in the dorsal horn, including approximately 80 % of lamina I projection neurones. There is also a population of large NK1 receptor-immunoreactive neurones with cell bodies in laminae III and IV which project to the CVLM and parabrachial area. It has been shown that the lamina III/IV NK1 receptor-immunoreactive projection neurones are densely and selectively innervated by substance P-containing primary afferent neurones, and there is evidence that these afferents also target lamina I projection neurones with the receptor. Both types of neurone are innervated by descending serotoninergic axons from the medullary raphe nuclei. The lamina III/IV neurones also receive numerous synapses from axons of local inhibitory interneurones which contain GABA and neuropeptide Y, and again this input shows some specificity since post-synaptic dorsal column neurones which also have cell bodies in laminae III and IV receive few contacts from neuropeptide Y-containing axons. These observations indicate that there are specific patterns of synaptic connectivity within the spinal dorsal horn.
Todd
A J
AJ
Spinal Cord Group, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK. a.todd@bio.gla.ac.uk
D000818:Animals; D009475:Neurons, Afferent / Q000166:cytology* / Q000502:physiology; D020671:Posterior Horn Cells / Q000166:cytology*; D051381:Rats; D018040:Receptors, Neurokinin-1 / Q000502:physiology*; D013373:Substance P / Q000502:physiology*
D018040:Receptors, Neurokinin-1; D013373:Substance P