PRESYNAPTIC LOCATION OF DELTA-OPIOID RECEPTOR IMMUNOREACTIVITY IN DORSAL HORN OF THE RAT SPINAL-CORD

Endogenous opioid peptides, Met(5)- and Leu(5)-enkephalin elicit many physiological effects in the CNS through activation of delta-opioid receptors, To determine the potential cellular sites for the delta-opioid receptor mediated opioid actions in nociception, we examined the ultrastructural immunoperoxidase localization of guinea pig antisera against a peptide sequence located within the extracellular region of the cloned delta-opioid receptor, corresponding to amino acids 34-47 (p34). Additionally, we combined the labeling of this peptide with a rabbit anti Met(5)-enkephalin. Light microscopy showed a selective localization of delta-opioid receptor-like immunoreactivity (delta-ORLI) in layers I and II of the rat cervical spinal cord. This labeling was removed by preadsorption with excess peptide. Ultrastructural localization of the antibody revealed the most prominent localization along the plasma membrane and dense core vesicles along unmyelinated axons and axon terminals. However, selective postsynaptic junctions were also labeled. In sections dually labeled for the delta-opioid receptor (peroxidase) and Met(5)-enkephalin(immunogold/silver), delta-ORLI was present in axon terminals both with and without detectable Met(5)-enkephalin (ME). In addition, delta-ORLI was observed at the synaptic specializations on dendrites, postsynaptic to axon terminals containing ME. These results provide the first ultrastructural evidence for a prominent presynaptic, but also postsynaptic, action of Met(5)-enkephalin acting at delta-opioid receptors in nociceptive regions of the rat spinal cord.