title: A possible involvement of the central endorphin system in the autoanalgesia induced by chronic administration of Freund's adjuvant solution in rats.
abstract: Chronic pain was induced in rats by daily injections of complete Freund's adjuvant into hind paws. Daily changes of pain threshold and endorphin (ED) content and their receptors in 4 divided parts: cortex, diencephalon-mesencephalon containing striatum (D-M), pons-medulla (P-M) and spinal cord, were measured. Decrease in pain responsiveness was observed in the adjuvant-injected group with concomitant increase of ED content in P-M and spinal cord. This decrease in pain responsiveness in the adjuvant-injected group was significantly different from that in the non-treated control group being partially reversed by naloxone. Furthermore, [3H]met-enkephalin binding sites increase in number in P-M of the adjuvant-injected group when maximal decrease of pain responsiveness was observed, returning to control level thereafter. Scatchard analysis revealed the increase of the low affinity binding site in P-M of the adjuvant-injected group. In cortex and D-M, on the other hand, ED content tended to decrease and no change was observed in number of [3H]met-enkephalin binding sites. These results indicate that the ED system in P-M and spinal cord may be more substantially involved in autoanalgesia than in cortex and D-M.
mesh_terms: D000818:Animals; D001921:Brain / Q000187:drug effects / Q000502:physiology*; D004723:Endorphins / Q000502:physiology*; D004744:Enkephalin, Methionine / Q000378:metabolism; D008297:Male; D009270:Naloxone / Q000494:pharmacology; D009619:Nociceptors / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D011930:Reaction Time / Q000502:physiology; D011957:Receptors, Opioid / Q000502:physiology*; D012684:Sensory Thresholds; D013116:Spinal Cord / Q000187:drug effects / Q000502:physiology*
chemical_list: D004723:Endorphins; D011957:Receptors, Opioid; D009270:Naloxone; D004744:Enkephalin, Methionine