A Possible involvement of the central endorphin system in the autoanalgesia induced by chronic administration of freundʼs adjuvant solution in rats

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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 nontreated control group being partially reversed by naloxone. [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, ED content tended to decrease and no change was observed in number of [3H]met-enkephalin binding sites. The ED system in P-M and spinal cord may be more substantially involved in autoanalgesia than in cortex and D-M.