ULTRASTRUCTURAL IMMUNOLABELING SHOWS PROMINENT PRESYNAPTIC VESICULAR LOCALIZATION OF DELTA-OPIOID RECEPTOR WITHIN BOTH ENKEPHALIN-CONTAINING AND NONENKEPHALIN-CONTAINING AXON TERMINALS IN THE SUPERFICIAL LAYERS OF THE RAT CERVICAL SPINAL-CORD

Opioid peptides, Met(5)- and Leu(5)-enkephalin, are known endogenous l igands for the delta-opioid receptor (DOR) associated with opioid anal gesia at the spinal level. To determine the cellular sites for DOR-med iated actions, we examined the ultrastructural localization of DOR and Met(5)-enkephalin (ME) in the spinal cord by combining immunoperoxida se and immunogold-silver labeling for antibodies against DOR and ME, r espectively. Antibodies for DOR localization were raised in guinea pig against peptide 34-47 (p34), an amino acid sequence within the extrac ellular N-terminus of the DOR recently cloned from mouse neuroblastoma glioma (NG-108) cells. Selective immunoperoxidase labeling for DOR wa s detected by light microscopy in NG-108 cells and in the lamina I and II of the dorsal horn of the spinal cord (C2-C4). Electron microscopy of these spinal laminae revealed that the majority of the punctate va ricosities seen by light microscopy were axon terminals. delta-opioid receptor-like immunoreactivity (DOR-LI) in axon terminals was most pro minently associated with large dense core vesicles, and sometimes seen along the membranes of small clear vesicles and segments of the plasm alemma. A semiquantitative analysis of dually labeled sections reveale d that of the terminals showing DOR-LI 23/102 (23%) also contained Met (5)-enkephalin-like immunoreactivity (ME-LI). Conversely, 23/35 (66%) of the terminals showing ME-LI also showed DOR-LI. In addition to the presynaptic localization, selective postsynaptic densities within dend rites were also occasionally (9%) immunolabeled for the opioid recepto r. These results provide the first ultrastructural evidence that DOR m ay serve autoreceptor functions on ME terminals as well as presynaptic modulation of other transmitters in the dorsal horn of the rat spinal cord. Additionally, the vesicular localization of DOR-LI in axon term inals suggests the involvement of these organelles in the transport of the receptors to the plasma membrane.