1. The swimmeret system can be excited by perfusing the
neuropeptide proctolin through the isolated ventral nerve cord of the crayfish. Previously silent preparations begin to generate a characteristic motor pattern, the swimmeret rhythm, in the nerves that innervate the swimmerets. The response to
proctolin is dose dependent and reversible. The threshold concentration of
proctolin perfused through the ventral artery is approximately 10(-8) M. The EC50 is 1.6 X 10(-6) M. 2.
Proctolin-induced motor patterns have periods and phases similar to those of spontaneously generated motor patterns. The durations of the bursts of impulses in power-
stroke motor neurons generated in the presence of
proctolin are, however, significantly longer than those that occur during spontaneous activity. 3. DL-
Octopamine inhibits the swimmeret system, both when the system is spontaneously active and when it has been excited by
proctolin. The inhibition by
octopamine is dose dependent and reversible. The threshold for inhibition is approximately 10(-6) M, and the EC50 is approximately 5 X 10(-5) M. 4.
Octopamine's effect is mimicked by its agonists,
synephrine and
norepinephrine.
Synephrine has a lower threshold concentration than does
octopamine, but
norepinephrine is much less effective than
octopamine. 5.
Octopamine's inhibition is partially blocked by an antagonist,
phentolamine. 6.
Phentolamine also blocks inhibition of the swimmeret system by inhibitory command interneurons. This block is dose dependent and can be partially overcome by stimulating the command interneurons at higher frequencies. 7. Perfusion with 11 other suspected crustacean
neurotransmitters and transmitter analogues did not similarly excite or inhibit the swimmeret system, so we suggest that
proctolin and
octopamine are transmitters used by the neurons that normally control expression of the swimmeret rhythm.