GLUTAMATERGIC N2V CELLS ARE CENTRAL PATTERN GENERATOR INTERNEURONS OFTHE LYMNAEA FEEDING SYSTEM - NEW MODEL FOR RHYTHM GENERATION

We aimed to show that the paired N2v (N2 ventral) plateauing cells of the buccal ganglia are important central pattern generator (CPG) inter neurons of the Lymnaea feeding system. N2v plateauing is phase-locked to the rest of the CPG network in a slow oscillator (SO)-driven fictiv e feeding rhythm. The phase of the rhythm is reset by artificially evo ked N2v bursts, a characteristic of CPG neurons. N2v cells have extens ive input and output synaptic connections with the rest of the CPG net work and the modulatory SO cell and cerebral giant cells (CGCs). Synap tic input from the protraction phase interneurons N1M (excitatory), NI L(inhibitory), and SO (inhibitory-excitatory) are likely to contribute to a ramp-shaped prepotential that triggers the N2v plateau. The prep otential has a highly complex waveform due to progressive changes in t he amplitude of the component synaptic potentials. Most significant is the facilitation of the excitatory component of the SO --> N2v monosy naptic connection. None of the other CPG interneurons has the appropri ate input synaptic connections to terminate the N2v plateaus. The modu latory function of acetylcholine (ACh), the transmitter of the SO and N1M/N1Ls, was examined. Focal application of ACh (50-ms pulses) onto t he N2v cells reproduced the SO --> N2v biphasic synaptic response but also induced long-term plateauing (20-60 s). N2d cells show no endogen ous ability to plateau, but this can be induced by focal applications of ACh. The N2v cells inhibit the N3 tonic (N3t) but not the N3 phasic (N3p) CPG interneurons. The N2v -->t N3t inhibitory synaptic connecti on is important in timing N3t activity. The N3t cells recover from thi s inhibition and fire during the swallow phase of the feeding pattern. Feedback N2v inhibition to the SO, N1L protraction phase interneurons prevents them firing during the retraction phase of the feeding cycle . The N2v --> N1M synaptic connection was weak and only found in 50% o f preparations. A weak N2v --> CGC inhibitory connection prevents the CGCs firing during the rasp (N2) phase of the feeding cycle. These dat a allowed a new model for the Lymnaea feeding CPG to be proposed. This emphasizes that each of the six types of CPG interneuron has a unique set of synaptic connections, all of which contribute to the generatio n of a full CPG pattern.