(a–c) Parasagittal 50-μm-thick sections of lumbar enlargement from CalcaCreER/tdTomato mice that received intrathecal capsaicin treatment to reduce tdTomato-immunoreactivity from primary afferents. In most cases, the axons (arrows) of the dorsal horn CGRP-tdTomato interneurons travel ventrally. (a) An axon arises from a ventrally-projecting primary dendrite, rather than the cell body. (b) The axon (arrows) of this tdTomato-positive neuron arises from the caudal ventral surface of the cell body and travels ventrally and rostrally. The cell body also emits a very fine dendritic process, defined by the presence of spines (double-headed arrow). (c) This heavily spine-laden, multipolar CGRP-tdTomato interneuron emits a ventrally directed axon from one of its dendrites. (d) Drawings of 23 dorsal horn CGRP-tdTomato interneurons whose axons could be identified and traced. Each drawing shows the neuronal cell body and its axon as well as initial portions of its major primary dendrites. Nineteen of the axons originate from the ventral region of the cell body; 3 (d14, d18, d22), from a ventrally projecting primary dendrite and one (d17) from a secondary dendrite close to its branch point off a primary dendrite. Most of the axons travel ventrally and caudally; some travel rostrally (e.g. d1, d5) and an occasional axon courses directly ventral (e.g., d15). After initially travelling ventro-caudally, 2 of the axons (d11, d19) looped dorsally and then began to travel rostrally. Eight of the axons bifurcated (d5, d6, d12, d13, d14, d15, d16, d19), all within 120 μm of their origin from a cell body. Scale bars: 20 μm.