The cellular and synaptic architecture of local circuits in the spinal dorsal horn

Lamina II of the spinal dorsal horn is a major target of nociceptive primary afferents, especially C–fibers and plays a role in modulating and transmitting incoming sensory information. However, its structural and functional organization, and its role in the neuronal circuitry for processing pain information, remain poorly understood due to the difficulty in identifying functional populations among interneurons. Virtually all lamina II neurons are interneurons, and are allocated to inhibitory or excitatory types. Furthermore, these interneurons show morphological, neurochemical and electrophysiological diversity. Thus, it is very important to dissect the local neuronal circuits involving different types of lamina II neurons, in order to understand the output from spinal cord. Recently, we investigated these interneurons by using a combined electrophysiological and anatomical approach. We revealed mechanisms that might con-tribute abnormal pain states through modulation of local circuitry consisting of excitatory and inhibitory interneurons in lamina II. We found a possible local neuronal circuit that might convey signals from low–threshold mechanoreceptors (LTMRs) to lamina I projection neurons through vertical cells (excitatory interneurons). Vertical cells have dendrites spreading ventrally into laminae III ⁄ IV and axons terminating on lamina I projection neurons. We found contacts between dendritic spines of vertical cells and terminals of LTMRs in laminae III ⁄ IV. This circuit is therefore a potential route through which tactile inputs can activate lamina I projection neurons and thus could play a role in tactile allodynia. We also investigated mechanisms to gate these inputs, because this circuit could be inhibited by axo–axonic synapses on the central terminals of LTMRs in normal conditions. It is likely that particular types of lamina II neurons have specific roles in modulating local circuitry, so that the input–output relation could be changed through interactions among different types of interneurons.