9 MEMBERS OF THE MYOMODULIN FAMILY OF PEPTIDE COTRANSMITTERS AT THE B16-ARC NEUROMUSCULAR-JUNCTION OF APLYSIA

1. Neuromodulation by multiple related peptides with different spectra of physiological effects appears an effective way to integrate comple x physiological functions. A good opportunity to examine this issue oc curs in the accessory radula closer (ARC) neuromuscular circuit of Apl ysia, where, extensive previous work has shown, acetylcholine-induced contractions of the muscle are variously modulated by several families of peptide cotransmitters released under appropriate behavioral circu mstances from the muscle's own two motor neurons. 2. In this work we f ocused on the myomodulins (MMs) released from motor neuron B16. Previo us work has characterized MM(A) (PMSMLRLamide) and MM(B) (GSYRMMRLamid e). We now similarly purified from ARC neuromuscular material and sequ enced MM(C) (GWSMLRLamide), MM(D) (GLSMLRLamide), MM(E) (GLQMLRtamide) , and MM(F) (SLNMLRLamide). Three additional MMs, MM(G) (TLSMLRLamide) , MM(H) (GLHMLRamide), and MM(I) (SLSMLRLamide), are encoded by a know n MM gene. B16 probably synthesizes, and coreleases, all nine MMs. Fur ther MMs have been found in other mollusks. All evidence indicates tha t the MMs are a major, widely distributed family of molluscan neuropep tides important as neuromuscular modulators and probably also central transmitters or modulators. 3. MM effects on motor neuron B 16-elicite d ARC muscle contractions were best analyzed as the sum of three disti nct actions: potentiation, depression of the amplitude of the contract ions, and acceleration of their relaxation rate. We compared the effec tiveness of all nine MMs in these respects. We correlated this with th eir effectiveness in enhancing the L-type Ca current and activating a specific K current in voltage-clamped dissociated ARC muscle fibers, e ffects we previously proposed to underlie, respectively, the potentiat ion and the depression of contractions. 4. All nine MMs were similarly effective in enhancing the Ca current and, as far as it was possible to determine, potentiating the amplitude as well as accelerating the r elaxation rate of the contractions. 5. In contrast, the MMs' ability t o activate the K current and depress the contractions varied greatly. MM(B) and MM(C), in particular, were weak, whereas the other seven MMs were considerably more effective in both respects. 6. Altogether, we were able to explain the potentiating and depressing strengths of the MMs by the magnitude of their modulation of the Ca and K currents, pro viding further support for our hypothesis that the effects on contract ion amplitude are mediated by the effects on the two currents. 7. The net effect on contraction amplitude was determined by the balance betw een the potentiation and depression. Although most MM concentrations h ad both potentiating and depressing actions, potentiated contractions predominated at low and depressed contractions (but with accelerated r elaxation rate) at high concentrations. At 10 nM, all nine MMs increas ed net contraction amplitude by 50-150%; at 1-10 mu M, MM(B) and MM(C) continued to produce net potentiation but the other seven MMs complet ely abolished contractions. 8. Combined application of the MMs in prop ortions likely to be released in vive modulated the contractions no di fferently from the most common individual MM. We discuss the possibili ty that I some of the MM forms may be redundant in this system. 9. Fun ctionally, the potentiation of contractions most likely helps strength en feeding movements to meet behavioral demands, and the acceleration of the relaxation rate, perhaps together with the depression, may serv e to limit the duration of the contractions so as to permit fast, ener getically favorable switching between contractions of antagonistic mus cles.