SEROTONERGIC NEURONS DIFFERENTIALLY MODULATE THE EFFICACY OF 2 MOTOR-NEURONS INNERVATING THE SAME MUSCLE-FIBERS IN APLYSIA

Feeding behavior in Aplysia shows substantial plasticity. An important site for the generation of this plasticity is the modulation of synap tic transmission between motor neurons and the buccal muscles that gen erate feeding movements. We have been studying this modulation in the anterior portion of intrinsic buccal muscle 3 (I3a), which is innervat ed by two excitatory motor neurons and identified serotonergic modulat ory neurons, the metacerebral cells (MCCs). We have shown previously t hat serotonin (5-HT) applied selectively to the muscle potently modula tes excitatory junction potentials (EJPs) and contractions. All the ef fects of 5-HT were persistent, lasting many hours after wash out. We e xamined whether the release of endogenous 5-HT from the MCC could prod uce effects similar to the application of 5-HT. Stimulation of the MCC s did produce similar short-term effects to the application of 5-HT. M CC stimulation facilitates EJPs, potentiates contractions, and decreas es the latency between the onset of a motor neuron burst and the onset of the evoked contraction. The effects of MCC stimulation reached a m aximum at quite low firing frequencies, which were in the range of tho se previously recorded during feeding behavior. The maximal effects we re similar to those produced by superfusion with similar to 0.1 mu M 5 -HT. Although the effects of MCC stimulation on EJPs were persistent, they were less persistent than the effects of 0.1 mu M 5-HT. Mechanism s that may account for differences in the persistence between released and superfused 5-HT are discussed. Thus activity in the MCCs has dram atic shortterm effects on the behavioral output of motor neurons, incr easing the amplitude and relaxation rate of contractions evoked by bot h B3 and B38 and shifting the temporal relationship between B38 bursts and evoked contractions.