Serotonergic and peptidergic modulation of the buccal mass protractor muscle (I2) in Aplysia

Plasticity of Aplysia feeding has largely been measured by noting changes i n radula protraction. On the basis of previous work, it has been suggested that peripheral modulation may contribute to behavioral plasticity. However , peripheral plasticity has not been demonstrated in the neuromuscular syst ems that participate in radula protraction. Therefore in this study we inve stigated whether contractions of a major radula protraction muscle (I2) are subject to modulation. We demonstrate, first, that an increase in the firi ng frequency of the cholinergic I2 motoneurons will increase the amplitude of the resulting muscle contraction but will not modulate its relaxation ra te. We show, second, that neuronal processes on the I2 muscle are immunorea ctive to myomodulin (MM), RF-amide, and serotonin (5-HT), but not to small cardioactive peptide (SCP) or buccalin. The I2 motoneurons B31, B32, B61, a nd B62 are not immunoreactive to RFamide, 5-HT, SCP, or buccalin. However, all four cells are MM immunoreactive and are capable of synthesizing MMa. T hird, we show that the bioactivity of the different modulators is somewhat different; while the MMs (i.e., MMa and MMb) and 5-HT increase I2 muscle re laxation rate, and potentiate muscle contraction amplitude, MMa, at high co ncentrations, depresses muscle contractions. Fourth, our data suggest that cAMP at least partially mediates effects of modulators on contraction ampli tude and relaxation rate.