INFLAMMATION OF THE RAT PAW ENHANCES AXONAL-TRANSPORT OF OPIOID RECEPTORS IN THE SCIATIC-NERVE AND INCREASES THEIR DENSITY IN THE INFLAMED TISSUE

The effect of inflammation, induced by unilateral intraplantar injecti on of Freund's adjuvant, on opioid receptors transported in the sciati c nerve and on opioid receptors present in the paw of the rat was stud ied by means of in vitro receptor autoradiography using [I-125]beta-en dorphin (human) as ligand. In the absence of inflammation, human beta- endorphin binding sites accumulated proximally and distally to a ligat ure placed on the sciatic nerve in a time-dependent manner, indicating bidirectional axonal transport. Some human beta-endorphin binding was also visible in non-inflamed paw tissue. Inflammation of the paw tiss ue massively increased human beta-endorphin binding on both sides of t he sciatic nerve ligature and in the ipsilateral paw tissue. In inflam ed paw tissue, beta-endorphin binding accumulated in the cutaneous ner ve fibers as well as in the immune cells infiltrating the surrounding tissue. In the sciatic nerve and paw tissue, beta-endorphin binding wa s displaced by (D-Ala2, N-methyl-Phe4, Gly-ol5)enkephalin and (D-Pen2, D-Pen5)enkephalin, selective mu- and delta-opioid receptor agonists, respectively, and by the universal opioid antagonist naloxone, but not by U-50,488H, a k-selective receptor agonist. Taken together, these d ata provide neuroanatomical evidence for local inflammation-induced en hanced axonal transport of opioid receptors in rat sciatic nerve and a ccumulation in paw tissue.