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Morphology and neurokinin 1 receptor expression of spinothalamic lamina I neurons in the rat spinal cord

Distinct morphological types of spinothalamic tract (STT) lamina I (LI) neurons have been identified in the cat and monkey spinal dorsal horn. Because these morphological types appear to differ in functional properties and receptor expression, we examined their distribution in the rat to test how their identification relates to earlier classification schemes. LI STT cells were retrogradely labeled with cholera toxin subunit b (CTb). Three types were recognized on the basis of cell body shape and proximal dendrites in the horizontal plane: fusiform, multipolar, and pyramidal. The relative distribution of these types was: 43, 26, and 28%, respectively, similar to that observed in the cat and monkey. 3D reconstructions were used to view each cell in all three major projection planes: horizontal, parasagittal, and transverse. Most LI STT neurons appeared fusiform in the parasagittal plane even though they belonged to different types based on their appearance in the horizontal plane, except in the most lateral portion of the dorsal horn, where LI curves ventrally. The proportion of STT neurons within LI was quantified by using the optical dissector method. To label all LI neurons, we used an anti‐neuron‐specific nuclear protein (NeuN) antibody. We found that approximately 9% of LI neurons projected to the thalamus. We also investigated neurokinin 1 receptor (NK‐1r) expression in LI STT neurons. As in the monkey, most pyramidal STT neurons did not express NK‐1r. These results provide further evidence that distinct morphological types of neurons differ in phenotype but not in their projection pattern. J. Comp. Neurol. 491:56–68, 2005. © 2005 Wiley‐Liss, Inc.