Transforming tonic firing into a rhythmic output in the Aplysia feeding system: presynaptic inhibition of a command-like neuron by a CpG element.

Tonic stimuli can elicit rhythmic responses. The neural circuit underlying Aplysia californica consummatory feeding was used to examine how a maintained stimulus elicits repetitive, rhythmic movements. The command-like cerebral-buccal interneuron 2 (CBI-2) is excited by tonic food stimuli but initiates rhythmic consummatory responses by exciting only protraction-phase neurons, which then excite retraction-phase neurons after a delay. CBI-2 is inhibited during retraction, generally preventing it from exciting protraction-phase neurons during retraction. We have found that depolarizing CBI-2 during retraction overcomes the inhibition and causes CBI-2 to fire, potentially leading CBI-2 to excite protraction-phase neurons during retraction. However, CBI-2 synaptic outputs to protraction-phase neurons were blocked during retraction, thereby preventing excitation during retraction. The block was caused by presynaptic inhibition of CBI-2 by a key buccal ganglion retraction-phase interneuron, B64, which also causes postsynaptic inhibition of protraction-phase neurons. Pre- and postsynaptic inhibition could be separated. First, only presynaptic inhibition affected facilitation of excitatory postsynaptic potentials (EPSPs) from CBI-2 to its followers. Second, a newly identified neuron, B54, produced postsynaptic inhibition similar to that of B64 but did not cause presynaptic inhibition. Third, in some target neurons B64 produced only presynaptic but not postsynaptic inhibition. Blocking CBI-2 transmitter release in the buccal ganglia during retraction functions to prevent CBI-2 from driving protraction-phase neurons during retraction and regulates the facilitation of the CBI-2 induced EPSPs in protraction-phase neurons.