Synaptic modulation contributes to firing pattern generation in jaw motor neurons during rejection of seaweed in Aplysia kurodai

Synaptic modulation contributes to firing pattern generation in jaw motor n eurons during rejection of seaweed in Aplysia kurodai. J. Neurophysiol. 82: 2579-2589, 1999. Japanese species, Aplysia kurodai, feeds well on Ulva but rejects Gelidium (Geli.) or Pachydictyon (Pach.) with different rhythmic p atterned movements of the jaws and radula. During ingestion the jaws open a t the radula-protraction phase and remain half open at the initial phase of the radula-retraction, whereas during rejection the jaws open similarly bu t start to close immediately after the onset of the radula-retraction. Thes e can be induced not only by the natural seaweed but by the extract solutio ns. We previously showed that the change of the patterned jaw movements fro m the ingestion to the rejection may result from the decrease in the delay of the firing onset of the jaw-closing (JC) motor neurons during their depo larization. This diminished delay produces a phase advance relative to the radula-retraction phase. In that study, we showed that during ingestion the buccal multiaction (MA) neurons may generate the delay of firing onset of the JC motor neurons by producing monosynaptic inhibitory postsynaptic pote ntials (IPSPs) during the initial portion of their depolarization In the pr esent experiments, the firing patterns in the MA neurons induced by applica tion of the Geli or Pach. extract to the Lips were explored in the semiinta ct preparations. During the Pach. response the duration and the firing freq uency of the MA firing at each depolarizing phase were decreased in compari son with the Ulva response. No decreases in the MA firing were observed dur ing the Geli. response, suggesting that the similar patterned jaw movements during rejection of Geli. and Pach. may be generated by different neural m echanisms. Moreover, the size of the MA-induced IPSP in the JC motor neuron s was largely decreased by application of the Geli. or Pach. extract to the lips in the reduced preparations consisting of the tentacle-lips and the c erebral-buccal ganglia. Voltage-clamp experiments on the JC motor neurons s howed that the size of synaptic current induced by the MA spikes was decrea sed by application of these solutions to the lips. The decrease was induced when the buccal ganglia were bathed in a solution to block polysynaptic pa thways. These results suggest that the advance of the onset of the JC firin g at each depolarizing phase during the Geli. or Pach. response may be main ly or partly caused by the decrease in the size of the MA-induced IPSP in t he motor neurons. Modulatory action of cerebral neurons or peripheral affer ent neurons in the lip region may contribute to this synaptic plasticity.