Changes in 3H-leucine enkephalin binding in spinal cord of frog after dorsal rhizotomy, cordotomy, transcutaneous stimulation and temperature variations: correlation with nociceptive sensitivity.

In the frog spinal cord about 50% of the 3H-leucine enkephalin (3H-LE) binding sites (b.s.) were blocked by an endogenous ligand. Three days after deafferentation and cordotomy the number of free b.s. increased by 44 and 56%, respectively. In spinal frogs the threshold of the flexor reflex responses evoked by nociceptive stimuli decreased. More than 7 days after deafferentation and cordotomy the number of both total and free 3H-LE b.s. decreased, while the threshold of the flexor reflex responses returned to that before spinalization. Transcutaneous electrical stimulation (TES) of the hind limbs (30 Hz, 5 minutes) in frogs spinalized 3 hours earlier increased 3H-LE binding at low intensities of stimulation (0.2 mA) and decreased the threshold of the flexor reflex responses. TES at higher intensities (1.0 mA) decreased 3H-LE binding and increased the threshold. Three days after spinalization TES even at low intensity diminished 3H-LE binding and raised flexor reflex threshold. A decrease in the number of free 3H-LE b.s. was found when the frog body temperature was elevated (from 15 to 24 degrees C) or lowered (from 15 to 1 degrees C) for 14 days and was accompanied by an increase in flexor reflex threshold. The data suggest the existence of an endogenous opioidergic system in the frog spinal cord which has a high degree of tonic activity.