Conditional rhythmicity and synchrony in a bilateral pair of bursting motor neurons in Aplysia.

This investigation examined the activity of a bilateral pair of motor neurons (B67) in the feeding system of Aplysia californica. In isolated ganglia, B67 firing exhibited a highly stereotyped bursting pattern that could be attributed to an underlying TTX-resistant driver potential (DP). Under control conditions, this bursting in the two B67 neurons was infrequent, irregular, and asynchronous. However, bath application of the neuromodulator dopamine (DA) increased the duration, frequency, rhythmicity, and synchrony of B67 bursts. In the absence of DA, depolarization of B67 with injected current produced rhythmic bursting. Such depolarization-induced rhythmic burst activity in one B67, however, did not entrain its contralateral counterpart. Moreover, when both B67s were depolarized to potentials that produced rhythmic bursting, their synchrony was significantly lower than that produced by DA. In TTX, dopamine increased the DP duration, enhanced the amplitude of slow signaling between the two B67s, and increased DP synchrony. A potential source of dopaminergic signaling to B67 was identified as B65, an influential interneuron with bilateral buccal projections. Firing B65 produced bursts in the ipsilateral and contralateral B67s. Under conditions that attenuated polysynaptic activity, firing B65 evoked rapid excitatory postsynaptic potentials in B67 that were blocked by sulpiride, an antagonist of synaptic DA receptors in this system. Finally, firing a single B65 was capable of producing a prolonged period of rhythmic synchronous bursting of the paired B67s. It is proposed that modulatory dopaminergic signaling originating from B65 during consummatory behaviors can promote rhythmicity and bilateral synchrony in the paired B67 motor neurons.