NOVEL INTERNEURON HAVING HYBRID MODULATORY-CENTRAL PATTERN GENERATOR PROPERTIES IN THE FEEDING SYSTEM OF THE SNAIL, LYMNAEA-STAGNALIS

1. We used intracellular recording techniques to examine the role of a novel type of protraction phase interneuron, the lateral N1 (N1L) in the feeding system of the snail Lymnaea stagnalis. 2. The N1Ls are a b ilaterally symmetrical pair of electrotonically coupled interneurons l ocated in the buccal anglia. Each N1L sends a single axon to the contr alateral buccal ganglia. Their neurite processes are confined to the b uccal neuropile. 3. In the isolated CNS. depolarization of an N1L is c apable of driving a full (N1 --> N2 --> N3), fast (1 cycle every 5 s) fictive feeding rhythm. This was unlike the previously described N1 me dial(N1M) central pattern generator (CPG) interneurons that were only capable of driving a slow, irregular rhythm. Attempts to control the f requency of the fictive feeding rhythm by injecting varying amounts of steady current into the N1Ls were unsuccessful. This contrasts with a modulatory neuron. the slow oscillator(SO), that has very similar fir ing patterns to the N1Ls. but where the frequency of the rhythm depend s on the level of injected current. 4. The N1Ls' ability to drive a fi ctive feeding rhythm in the isolated preparation was due to their stro ng. monosynaptic excitatory chemical connection with the N1M CPG inter neurons. Bursts of spikes in the N1Ls generated summating excitatory p ostsynaptic potentials (EPSPs) in the N1Ms to drive them to firing. Th e SO excited the N1M cells in a similar way, but the EPSPs are strongl y facilitatory, unlike the the N1L --> NIM connection. 5. Fast (1 cycl e every 5 s) fictive feeding rhythms driven by the NIL occurred in the absence of spike activity in the SO modulatory neuron. In contrast. t he N1L was usually active in SO-driven rhythms. 6. The ability of the SO to drive the N1L was due to strong electrotonic coupling. SO --> N1 L. The weaker coupling in the opposite direction, N1L --> SO, did not allow the N1L to drive the SO. 7. Experiments on semiintact lip-brain preparations allowed fictive feeding to be evoked by application of 0. 1 M sucrose to the lips (mimicking the normal sensory input) rather th an by injection of depolarizing current. Rhythmic bursting, characteri stic of fictive feeding, began in both the SO and NIL at exactly the s ame time. indicating that these two cell types art activated in ''para llel'' to drive the feeding rhythm. 8. The N1L is also part of the CPG network. It excited the N2s and inhibited the N3 phasic (N3p) and N3 tonic (N3t) CPG interneurons like the N1MS. The N1L in turn was inhibi ted strongly by the N2. N3p. and N3t interneurons. These synaptic conn ections with other CPG interneurons were necessary for fictive feeding to occur. For instance, suppressing N1L activity in a food-driven rhy thm in a semiintact preparation stopped activity in the whole CPG netw ork. Similar experiments where activity in the modulatory SO was suppr essed merely slowed the rhythm. 9. The N1L has a limited set of connec tions with the buccal motor neurons. It only excited the B1 salivary g land motor neuron and inhibited the B4 retractor motor neuron. making it similar re the SO modulatory interneuron, but different from the N1 M cells that have widespread connections with all 10 previously descri bed types of motor neurons. 10. The data shows that the N1Ls have some properties that resemble a modulatory neuron such as the SO, but thei r cells also form parr of the CPG network. They are thus a hybrid cell type that is normally activated in parallel with the SO and CPG neuro ns (N1M, N2, N3p and N3t) to generate the Lymnaea feeding rhythm.