Opioid peptides as possible neuromodulators in the frog peripheral nerve system.

Sciatic nerves of the frog Rana ridibunda were examined for the effects of applied opioid peptide, methionine-enkephalin, synthetic enkephalin analogue, leucine-enkephalin-NH(2) and opiate antagonist, naloxone. The effect of both peptides in concentrations of 1x10(-6) and 1x10(-5)M or naloxone in 1x10(-6)M was investigated on the action potential parameters using electrophysiological techniques. The isolated nerves were stimulated by single square pulses each of which lasted for 0.5ms at supramaximal strength. Effect of each single dose of peptides at 0min was compared with the remaining time segments. Both peptides produced changes in action potential of nerve when compared with untreated nerves. Methionine-enkephalin in both concentrations reduced the amplitude between 7% and 41% and conduction velocity at about 26-61%. This peptide in the same concentrations prolonged the duration around 12-53% and increased the stimulating voltage at about 9-50%. In contrast, leucine-enkephalin-NH(2) in both concentrations caused a decrease in amplitude between 13% and 48% and in conduction velocity around 20-50%. The same concentrations of this peptide prolonged the duration at about 3-33% and increased the stimulating voltage at about 10-56%, but naloxone in 1x10(-6)M antagonized the responses of both peptides over 75%. The results indicate that both opioid peptides produce changes in action potential parameters in frog peripheral nerve system and these changes are partially reversed by naloxone.

Evidence for the involvement of an opioid system in sciatic nerve of Rana ridibunda.

The effect of opioid peptide, D-alanine2-leucine-enkephalin and opioid homolog peptide, des-tyrosine-methionine-enkephalin in concentrations of 1 x 10(-6) and 1 x 10(-5) M was investigated on the action potential parameters of frog sciatic nerve. Des-tyrosine-methionine-enkephalin was used as the control to prove the opioid action of the peptide. The effects of both peptides were examined by means of the extracellular electrophysiological technique. The isolated sciatic nerves were stimulated by single square pulses each of which lasted for 0.5 ms at supramaximal strength. Effect of each single dose of peptides at 0 min was compared with the remaining time segments. Both peptides produced changes on action potential of Rana ridibunda sciatic nerve when compared with untreated nerves. D-alanine2-leucine-enkephalin decreased significantly the amplitude at about 34-83%, the area at about 34-92%. The same concentrations of this peptide decreased significantly the conduction velocity around 35-78%. In contrast, des-tyrosine-methionine-enkephalin reduced the action potential amplitude between 8% and 80%. The same concentrations of this peptide decreased significantly the area at about 12-76% and the conduction velocity around 42-70%. The depression of both peptides in action potential parameters was partially blocked by 1 x 10(-6) M naloxone.