Muscular hyperalgesia and hypoalgesia after stimulation of the ureter in rats.

Vocalization electric thresholds were measured bilaterally in muscles of the lower back and in the tail after electrical stimulation of the left ureter in rats implanted with electrodes. 'Painful' stimulation immediately produced a hyperalgesia lasting about half an hour in the left lower back muscles and tail, followed by a 10 min hypoalgesia limited to the back muscles. No modification or only a delayed hypoalgesia appeared in the right muscle. Strong but 'non-painful' stimulation produced no effect or gave rise to an immediate hypoalgesia in the left muscle. Weak non-painful stimulation produced a delayed hypoalgesia in the tail.