The electron microscopic localization of methionine-enkephalin within the superficial layers (I and II) of the spinal cord

The synaptic relationships of methionine-enkephalin containing axon terminals within layers I and II of the rat spinal cord have been investigated using immunocytochemical techniques. Labelled terminals contained large numbers of spherical synaptic vesicles and formed synaptic contacts with dendritic shafts and spines and to a lesser extent with cell bodies within the superficial layers of the dorsal horn. A large number of labelled terminals were seen in apposition to profiles containing pleomorphic vesicles, particularly within layer I and outer layer II. Following rhizotomy, degenerating primary afferent axon terminals were found throughout layers I and II but only in one case was a synaptic relationship with a labelled terminal observed.Thus we were unable to find a morphological correlate of the reported effects of opiates on sensory axons and terminals.