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1 The Leslie and Susan Gonda (Goldschmied) Multidisciplinary Brain Research Center, Bar-Ilan University, Ramat Gan, Israel; Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York, NY, USA
2 The Leslie and Susan Gonda (Goldschmied) Multidisciplinary Brain Research Center, Bar-Ilan University, Ramat Gan, Israel; Faculty of Life Sciences, Bar-Ilan University, Ramat Gan, Israel
3 ; Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York, NY, USA
* To whom correspondence should be addressed. E-mail: hurvitz3000{at}hotmail.com.
Tonic stimuli can elicit phasic responses. The neural circuit underlying Aplysia californica consummatory feeding was used to examine how a maintained stimulus elicits repetitive, phasic 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 the wrong phase, 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 the wrong phase. The block was caused by presynaptic inhibition of CBI-2 by a key buccal ganglion retraction phase interneuron, B64, which also causes post-synaptic inhibition of protraction-phase neurons. Presynaptic and post-synaptic inhibition could be separated. First, only presynaptic inhibition affected facilitation of EPSPs from CBI-2 to its followers. Second, a newly identified neuron, B54, produced post-synaptic inhibition similar to that of B64, but did not cause pre-synaptic inhibition. Third, in some target neurons B64 produced only presynaptic, but not post-synaptic inhibition. Blocking CBI-2 transmitter release in the buccal ganglia during retraction functions to prevent CBI-2 from driving retraction phase neurons in the wrong phase, and regulates the facilitation of the CBI-2 induced EPSPs in protraction phase neurons. It may also permit CBI-2 to have different effects in different ganglia.
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