Multiple neuropeptides in cholinergic motor neurons of Aplysia: Evidence for modulation intrinsic to the motor circuit

Changes in Aplysia biting responses during food arousal are partially mediated by the serotonergic metacerebral cells (MCCs). The MCCs potentiate contractions of a muscle utilized in biting, the accessory radula closer (ARCM), when contractions are elicited by stimulation of either of the two cholinergic motor neurons B15 or B16 that innervate the muscle. We have now shown that ARCM contractions may also be potentiated by peptide cotransmitters in the ARCM motor neurons. We found that motor neuron B15 contains small cardioactive peptides A and B (SCP(A) and SCP(B)) - i.e., whole B15 neurons were bioactive on the SCP-sensitive Helix heart, as were reverse-phase HPLC fractions of B15 neurons that eluted like synthetic SCP(A) and SCP(B). Furthermore, [³⁵S]methionine-labeled B15 peptides precisely coeluted with synthetic SCP(A) and SCP(B). SCP(B)-like immunoreactivity was associated with dense-core vesicles in the soma of B15 and in neuritic varicosities and terminals in the ARCM. B16 motor neurons did not contain SCP(A) or SCP(B) but contained an unidentified bioactive peptide. RP-HPLC of [³⁵S]methionine-labeled B16s resulted in one major peak of radioactivity that did not coelute with either SCP and which, when subject to Edman degradation, yielded [³⁵S]methionine in positions where there is no methionine in the SCPs. Exogenously applied B16 peptide potentiated ARCM contractions elicited by stimulation of B15 or B16 neurons. Thus, in this system there appear to be two types of modulation; one type arises from the MCCs and is extrinsic to the motor system, whereas the second type arises from the motor neurons themselves and hence is intrinsic.