ULTRASTRUCTURAL-STUDY OF DELTA-OPIOID RECEPTORS IN THE DORSAL HORN OFTHE RAT SPINAL-CORD USING MONOCLONAL ANTIIDIOTYPIC ANTIBODIES

The ultrastructural localization of delta-opioid receptors was studied using monoclonal anti-idiotypic antibody prepared with an anti-D-Ala( 2)-D-leu(5)-enkephalin. Immunocytochemical techniques were used on vib ratome sections from rats perfused with paraformaldehyde. A high densi ty of immunoreactivity was observed in the dorsal horn of the spinal c ord, particularly the two superficial layers, the dorsolateral funicul us and the area surrounding the central canal. The labelling was absen t when the antibody was preincubated with the immunogen. Competition b etween the anti-idiotypic antibody and different ligands, sigma or mu, was controlled by preincubation of tissue sections with the ligand in the presence of peptidase inhibitors for 3-4 h before addition of the antiidiotypic antibody. Enkephalin, dermenkephalin and naltrindole in duced disappearance of the labelling at 10(-9) M while dermorphin or d ermorphin Lys(7) were ineffective at the same concentration. Lamina II of the dorsal horn was studied by electron microscopy. The immunolabe lling was mainly localized on cell membranes at appositions between tw o neurons. About one third were localized between an axon terminal and a dendrite, the same proportion of labellings were between two axon t erminals. Labelling was occasionally observed at appositions between a glomerular terminal and a dendrite or a terminal or at axoglial appos itions. Axosomatic localizations were rare. The presynaptic localizati on of the labelling is in favor of a presynaptic mechanism of action f or delta-opioids in the spinal cord, providing that these receptors ar e functional. delta-Opioid peptides probably act non-synaptically sinc e receptors were never localized on synaptic differentiations.