NEUROTRANSMITTER INTERACTIONS IN THE STOMATOGASTRIC SYSTEM OF THE SPINY LOBSTER - ONE PEPTIDE ALTERS THE RESPONSE OF A CENTRAL PATTERN GENERATOR TO A 2ND PEPTIDE

Two of the peptides found in the stomatogastric nervous system of the spiny lobster, Panulirus interruptus, interacted to modulate the activ ity of the cardiac sac motor pattern. In the isolated stomatogastric g anglion, red-pigment-concentrating hormone (RPCH), but not proctolin, activated the bursting activity in the inferior ventricular (IV) neuro ns that drives the cardiac sac pattern. The cardiac sac pattern normal ly ceased within 15 min after the end of RPCH superfusion. However, wh en proctolin was applied within a few minutes of that time, it was lik ewise able to induce cardiac sac activity. Similarly, proctolin applie d together with subthreshold RPCH induced cardiac sac bursting. The am plitude of the excitatory postsynaptic potentials from the IV neurons to the cardiac sac dilator neuron CD2 (1 of the 2 major motor neurons in the cardiac sac system) was potentiated in the presence of both pro ctolin and RPCH. The potentiation in RPCH was much greater than in pro ctolin alone. However, the potentiation in proctolin after RPCH was eq uivalent to that recorded in RPCH alone. Although we do not yet unders tand the mechanisms for these interactions of the two modulators, this study provides an example of one factor that can determine the ''stat e'' of the system that is critical in determining the effect of a modu lator that is ''state dependent,'' and it provides evidence for yet an other level of flexibility in the motor output of this system.