Association interneurons of embryonic rat spinal cord transiently express the cell surface glycoprotein SNAP/TAG‐1

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Abstract
SNAP/TAG‐1 is a 135 kDa glycoprotein of the immunoglobulin superfamily that is transiently expressed upon the surfaces of developing axons. In the embryonic rodent spinal cord, this molecule is expressed by motor neurons, dorsal root ganglion cells, and commissural neurons (Yamamoto et al.: J. Neurosci. 6:3576–3594, 1986; Dodd et al.: Neuron 1:105–116, 1988). The commissural cells are a subset of early‐forming dorsal horn interneurons whose axons follow a circumferential course in the embryonic spinal cord. The axons of commissural neurons cross the developing ventral commissure to terminate on contralateral synaptic targets, whereas those of the other subset of circumferential cells, the association interneurons, remain on the same side of the spinal cord to form ipsilateral, terminal synaptic fields. The difference between the axonal trajectories of these two subsets of nerve cells raised the question of whether or not association interneurons would also express the SNAP/TAG‐1 epitope and, if so, how would this expression be related to that of the commissural cells. Immunocytochemistry for SNAP/TAG‐1 and choline acetyltransferase (ChAT) was used to answer these questions. The results indicated that association interneurons expressed SNAP/TAG‐1 epitopes and that this expression began later and lasted longer than that of the commissural neurons. Other new findings of this study included the identification of a lateral subgroup of commissural fibers that expressed SNAP/TAG‐1 later than their more medially located counterparts, and these lateral fibers were more pronounced in the thoracic spinal cord than at cervical levels. Furthermore, interesting developmental relationships were observed between SNAP/TAG‐1‐positive fibers and ChAT‐positive motor neurons in both cervical and thoracic spinal cord. Lastly, SNAP/TAG‐1 immunoreactivity was detected on the terminal collaterals of dorsal root ganglion fibers during the late prenatal period. While the function(s) of SNAP/TAG‐1 remains enigmatic, its expression in developmental time and space, along with the recent suggestion that a released form of this molecule might serve a substrate adhesion function (Furley et al.: Cell 61:157–170, 1990; Karagogeos et al.: Development 112:51–67, 1991), hint that SNAP/TAG‐1 might play a role in the migration of certain cholinergic neurons in the developing spinal cord.

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