title: Functional differences between neurochemically defined populations of inhibitory interneurons in the rat spinal dorsal horn.
abstract: In order to understand how nociceptive information is processed in the spinal dorsal horn we need to unravel the complex synaptic circuits involving interneurons, which constitute the vast majority of the neurons in laminae I-III. The main limitation has been the difficulty in defining functional populations among these cells. We have recently identified 4 non-overlapping classes of inhibitory interneuron, defined by expression of galanin, neuropeptide Y (NPY), neuronal nitric oxide synthase (nNOS) and parvalbumin, in the rat spinal cord. In this study we demonstrate that these form distinct functional populations that differ in terms of sst(2A) receptor expression and in their responses to painful stimulation. The sst(2A) receptor was expressed by nearly all of the nNOS- and galanin-containing inhibitory interneurons but by few of those with NPY and none of the parvalbumin cells. Many galanin- and NPY-containing cells exhibited phosphorylated extracellular signal-regulated kinases (pERK) after mechanical, thermal or chemical noxious stimuli, but very few nNOS-containing cells expressed pERK after any of these stimuli. However, many nNOS-positive inhibitory interneurons up-regulated Fos after noxious thermal stimulation or injection of formalin, but not after capsaicin injection. Parvalbumin cells did not express either activity-dependent marker following any of these stimuli. These results suggest that interneurons belonging to the NPY, nNOS and galanin populations are involved in attenuating pain, and for NPY and nNOS cells this is likely to result from direct inhibition of nociceptive projection neurons. They also suggest that the nociceptive inputs to the nNOS cells differ from those to the galanin and NPY populations.
authors.affiliation: Spinal Cord Group, Institute of Neuroscience and Psychology, University of Glasgow, Glasgow G12 8QQ, United Kingdom Department of Anatomy, Hokkaido University School of Medicine, Sapporo 060-8638, Japan.
authors.forename: Erika
authors.initials: E
authors.lastname: Polgár
authors.forename: Thomas C P
authors.initials: TCP
authors.lastname: Sardella
authors.forename: Sheena Y X
authors.initials: SYX
authors.lastname: Tiong
authors.forename: Samantha
authors.initials: S
authors.lastname: Locke
authors.forename: Masahiko
authors.initials: M
authors.lastname: Watanabe
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Todd
mesh_terms: D000818:Animals; D019004:Galanin / Q000032:analysis / Q000096:biosynthesis*; D007395:Interneurons / Q000737:chemistry / Q000502:physiology*; D008297:Male; D009433:Neural Inhibition / Q000502:physiology*; D009478:Neuropeptide Y / Q000032:analysis / Q000096:biosynthesis*; D052248:Nitric Oxide Synthase Type I / Q000032:analysis / Q000096:biosynthesis*; D020671:Posterior Horn Cells / Q000737:chemistry / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar
chemical_list: D009478:Neuropeptide Y; D019004:Galanin; D052248:Nitric Oxide Synthase Type I
keywords: Galanin; Neuronal nitric oxide synthase; Neuropeptide Y; Pain; Parvalbumin; Somatostatin receptor 2A