PLASTICITY IN THE MULTIFUNCTIONAL BUCCAL CENTRAL PATTERN GENERATOR OFHELISOMA ILLUMINATED BY THE IDENTIFICATION OF PHASE-3 INTERNEURONS

1. The mechanism for generating diverse patterns of buccal motor neuro n activity was explored in the multifunctional central pattern generat or (CPG) of Helisoma. The standard pattern of motor neuron activity, w hich results in typical feeding behavior, consists of three distinct p hases of buccal motor neuron activity. We have previously identified C PG interneurons that control the motor neuron activity during phases 1 and 2 of the standard pattern. Here we identify a pair of interneuron s responsible for buccal motor neuron activity during phase 3, and exa mine the variability in the interactions between this third subunit an d other subunits of the CPG. 2. During the production of the standard pattern, phase 3 excitation in many buccal motor neurons follows a pro minent phase 2 inhibitory postsynaptic potential. Therefore phase 3 ex citation was previously attributed to postinhibitory rebound (PIR) in these motor neurons. Two classes of observations indicated that PIR wa s insufficient to account for phase 3 activity, necessitating phase 3 interneurons. 1) A subset of identified buccal neurons is inhibited du ring phase 3 by discrete synaptic input. 2) Other identified buccal ne urons display discrete excitation during both phases 2 and 3. 3. A bil aterally symmetrical pair of CPG interneurons, named N3a, was identifi ed and characterized as the source of phase 3 postsynaptic potentials in motor neurons. During phase 3 of the standard motor pattern, intern euron N3a generated bursts of action potentials. Stimulation of N3a, i n quiescent preparations, evoked a depolarization in motor neurons tha t are excited during phase 3 and a hyperpolarization in motor neurons that are inhibited during phase 3. Hyperpolarization of N3a during pat terned motor activity eliminated both phase 3 excitation and inhibitio n. Physiological and morphological characterization of interneuron N3a is provided to invite comparisons with possible homologues in other g astropod feeding CPGs. 4. These data support a model proposed for the organization of the tripartite buccal CPG. According to the model, eac h of the three phases of buccal motor neuron activity is controlled by discrete subsets of pattern-generating interneurons called subunit 1 (S1), subunit 2 (S2), and subunit 3 (S3). The standard pattern of bucc al motor neuron activity underlying feeding is mediated by an S1-S2-S3 sequence of CPG subunit activity. However, a number of ''nonstandard' ' patterns of buccal motor activity were observed. In particular, S2 a nd S3 activity can occur independently or be linked sequentially in rh ythmic patterns other than the standard feeding pattern. Simultaneous recordings of S3 interneuron N3a with effector neurons indicated that N3a can account for phase-3-like postsynaptic potentials (PSPs) in non standard patterns. The variety of patterns of buccal motor neuron acti vity indicates that each CPG subunit can be active in the absence of, or in concert with, activity in any other subunit. 5. To explore how C PG activity may be regulated to generate a particular motor pattern fr om the CPG's full repertoire, we applied the neuromodulator serotonin. Serotonin initiated and sustained the production of an S2-S3 pattern of activity, in part by enhancing PIR in S3 interneuron N3a after the termination of phase 2 inhibition.