Modulation of radula opener muscles in Aplysia

We observed fibers immunoreactive (IR) to serotonin (5-HT), the myomodulins (MMs), and FMRFamide on the I7-I10 complex in the marine mollusk Aplysia c alifornica. The I7-I10 muscle complex, which produces radula opening, is in nervated primarily by one motor neuron, B48. B48 is MM-IR and synthesizes a uthentic MMA. When B48 is stimulated in a physiological manner, cAMP levels are increased in opener muscles. cAMP increases also are seen when the MMs are applied to opener muscles but are not seen with application of the B48 primary neurotransmitter acetylcholine (ACh). Possible physiological sourc es of 5-HT and FMRFamide are discussed. When modulators are applied to rest ing opener muscles, changes in membrane potential are observed. Specificall y, 5-HT, MM,, and low concentrations of MMA all depolarize muscle fibers. T his depolarization is generally not sufficient to elicit myogenic activity in the absence of neural activity under "rest" conditions. However, if open er muscles are stretched beyond rest length, stretch- and modulator-induced depolarizations can summate and elicit contractions. This only occurs, how ever, if "depolarizing" modulators are applied alone. Thus other modulators (i.e., FMRFamide and high concentrations of MM,) hyperpolarize opener musc le fibers and can prevent depolarizing modulators from eliciting myogenic a ctivity. All modulators tested affected parameters of motor neuron-elicited contractions of opener muscles. MMA and 5-HT increased contraction size ov er the range of concentrations tested, whereas MMA potentiated contractions when it was applied at lower concentrations but decreased contraction size at higher concentrations. FMRFamide decreased contraction size at all conc entrations and did not affect relaxation rate. Additionally, the MMs and 5- HT increased muscle relaxation rate, decreased contraction latency, and dec reased the rate at which tension was developed during motor neuron-elicited muscle contractions. Thus these modulators dramatically affect the ability of opener muscles to follow activity in the opener motor neuron B48. The p ossible physiological significance of these findings is discussed.