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 re ceptors, To determine the potential cellular sites for the delta-opioi d receptor mediated opioid actions in nociception, we examined the ult rastructural immunoperoxidase localization of guinea pig antisera agai nst 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 localizat ion of delta-opioid receptor-like immunoreactivity (delta-ORLI) in lay ers I and II of the rat cervical spinal cord. This labeling was remove d by preadsorption with excess peptide. Ultrastructural localization o f the antibody revealed the most prominent localization along the plas ma 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 res ults provide the first ultrastructural evidence for a prominent presyn aptic, but also postsynaptic, action of Met(5)-enkephalin acting at de lta-opioid receptors in nociceptive regions of the rat spinal cord.