Anterograde transport of opioid receptors in rat vagus nerves and dorsal roots of spinal nerves: pharmacology and sensitivity to sodium and guanine nucleotides

Summary

We have utilized the technique of in vitro autoradiography to ascertain that opioid receptors are transported in the rat vagus nerve and in the rat dorsal spinal root fibers. In the dorsal roots, opioid receptors accumulated on both sides of the ligatures. In the vagus nerve, a distal accumulation of binding sites was difficult to detect, however, proximal to the ligatures, vagal receptors accumulated in a linear fashion during the first 12 h of ligation. At longer periods after ligation, accumulation was less than expected and the receptors appeared to migrate retrogradely. The receptor transport could be blocked by intravagal colchicine injection and the receptor translocation could be elicited in isolated vagal nerve segments suggesting that the receptors move by fast transport. Sodium chloride, present in the incubation medium, inhibited [³H]dihydromorphine ([³H]DHM) binding to receptors adjacent to and far from the proximal aspect of the ligature with IC₅₀'s of 42 mM and 51 mM, respectively. The addition of GTP in the incubation medium also inhibited [³H]DHM binding to “proximal’ and “far proximal” receptors with IC₅₀'s of 0.27 μM and 1.0 μM, respectively. The presence of GTP also inhibited [³H]naloxone ([³H]Nal) binding to “proximal” and “far proximal” receptors with IC⁵⁰'s of 0.34 μM and 0.66 μM, respectively. The transported vagal opioid receptors bound the ligands in a stereospecific manner. Using [³H]DHM, [³H]D-ala²-D-leu⁵-enkephalin ([³H]DADL), and [³H]ethylketocyclazocine ([³H]EKC), we found that most of the transported vagal receptors have mupharmacology although kappa and delta receptors are present.