Background and Purpose —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 sst2A receptor expression and in their responses to painful stimulation.
Methods—Male Wistar rats were deeply anaesthetized and perfused withiut any stimulation or after noxious stimulation. Four different types of noxious stimulus (heat, pinch, or injection of capsaicin or formalin) were studied. After perfusion ﬁxation, midlumbar (L4–5) segments were removed from all animals and cut into 60-micro m-thick sections with a Vibratome and stained for Galanin, pERK, FOS, NPY, nNOS, Parvalbumin, sst2A, GABA and NeuN.
Results—The sst2A 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.
Conclusions— 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.