PLASTICITY IN SPINAL OPIOID CONTROL OF LOWER URINARY-TRACT FUNCTION IN PARAPLEGIC CATS

SPINAL cord injury disrupts micturition reflexes, which produces morbi dity. The contribution of endogenous opioid systems to urinary retenti on were assessed in chronic spinal cats by administering the opioid re ceptor antagonist, naloxone (5-500 mu g kg(-1), i.p.), to unanesthetiz ed paraplegic cats while monitoring lower urinary tract function and o bserving hind limb reflexes. While naloxone had no overt effect in acu te spinal cats, in chronic spinal cats naloxone induced the release of large volumes of urine and produced marked hind limb hyper-reflexia. Prominent tachyphylaxis and tolerance to the effects of naloxone were evident. Immunohistochemical studies indicated a marked increase in le ucine enkephalin and dynorphin in sacral spinal neurons. Together, the se data indicate hyperactivity of the endogenous spinal opioid system following recovery from spinal cord injury and, furthermore, suggest t hat the spinal neural circuitry may become 'dependent' upon elevated l evels of endogenous opioid peptides.