Rhythmic behaviors such as feeding, locomotion and respiration are critical for sustaining life in multicellular animals. Motor patterns mediating these behaviors are typically generated by neuronal circuits called central pattern generators (CPGs). The motor output of multifunctional CPGs is modulated as a function of environmental stimuli and internal behavioral states so that an appropriate behavior is produced at any given time. 

     Feeding and regurgitation behaviors in gastropod molluscs have rhythmic components mediated by a multifunctional CPG in the buccal ganglia. Many molluscs are robust and resistant to surgical trauma so that they can produce relatively "normal" behaviors while intracellular neuronal recordings are made. Like many invertebrates, gastropod molluscs have a number of relatively large neurons with unique identities. Thus, equivalent neurons can be identified in each specimen, facilitating the analysis of neural circuitry. The modulation of the electrical activities and functional interconnections of identified neurons can be monitored as these neurons alter their functions on a moment to moment basis to contribute to differences in behaviors. 

FIG. 1. Map of buccal neurons 
     The Murphy laboratory has focused pimarily on the neural circuitry underlying feeding and regurgitation in the pond snail, Helisoma trivolvis. The multifunctional CPG in the buccal ganglia has been extensively characterized. Its output can be modulated under experimental control to produce feeding and regurgitation behaviors. A model of the Helisoma CPG has been presented and hypothesized to serve as a "Universal Model" for buccal CPGs in rasping gastropods. 

FIG. 3. Dopaminergic N1a 
     Techniques and experimental approaches range from videomicroscopy of freely behaving intact animals through electrophysiological and pharmacological analyses of neural circuitry, and determination of mechanisms of action of specific neurotransmitters. Immunocytochemistry and a number of neuronal staining techniques are used to determine the morphology of physiologically characterized identifiable neurons. 

3.1. A map of the somata of identified Helisoma buccal neurons. Both the caudal surface and the rostral surface of the animal's left buccal ganglion are depicted. Each identified neuron would have a mirror image homolog in the right ganglion (not depicted). Note that some terminology is altered from previous publications. The rostral and caudal surfaces were previously called ventral and dorsal, respectively; the VBN and LBN were previously designated the HBN and VBN respectively (Kater, 1974). Neurons on the rostral surface were formerly named VB1-VB10 (to indicate ventral buccal neurons) and they have been renamed 101-110. 

The changes were made to conform to the in situ morphology of the structures, and to prior terminology of basommatophoran molluscan morphologists, as well as to facilitate comparisons with other gastropods. CBC, cerebrobuccal connective; ET, esophageal nerve trunk; LBN, laterobuccal nerve; PBN, posterobuccal nerve; VBN, ventrobuccal nerve. 

3.2. Dopamine modulates the multifunctional buccal CPG to generate the feeding motor pattern. A) A schematic diagram of the buccal CPG. Each of the three CPG subunits (S1,S2,S3) is an independent conditional oscillator comprised of one or more pairs of interneurons. The membrane properties of individual CPG interneurons, and the interactions between CPG subunits, can be modulated (diagonal arrows) to permit production of multiple motor neuron activity patterns. Chemosensory afferents stimulated by food in the buccal cavity, and dopamine, either extrinsic to the CPG or released by S1 interneurons, induce the CPG to be active in the feeding mode. Depolarization of S1 interneurons stimulates S1 follower motor neurons (MN) and S2 interneurons. Depolarization of S2 interneurons stimulates S2 follower neurons and simultaneously inhibits S1 and S3 activity. S3 activity is generated by post-inhibitory rebound following the termination of S2 inhibition. Lines ending in horizontal bars represent excitatory pathways and those ending in closed circles represent inhibitory pathways. B) Bath application of dopamine (DA; 10 mM) triggered the production of the triphasic (S1-S2-S3) feeding motor pattern. No buccal CPG activity was observed in physiological saline prior to application of dopamine. Phase 1 motor neuron B6 is excited by S1 and inhibited by S2. Phase 2 motor neuron B27 is excited by S2. Phase 3 motor neuron B19 is inhibited by S2 and excited by S3. 

3.3. Dopaminergic interneuron N1a. A: Morphology of interneuron N1a as revealed by Lucifer yellow injections. A composite tracinc was made from projections of Ektachrome slides photographed at several different focal planes B: The FaGlu histochemical technique indicated the presence of dopamine in interneuron N1a. An electrophysiologically characterized neuron N1a was injected with the dye, reactive red # 4, prior to processing the preparation for FaGlu histochemistry. Left: FaGlu labelling of presumptive dopaminergic neuronal somata in the left buccal ganglion. The prominent bright soma near the left edge is interneuron N1a. The preparation was photographed using a filter combination designed for Lucifer Yellow. Right: The FaGlu-sensitive soma displayed dye that was injected into the electrophysiologically characterized interneuron N1a. The soma of only neuron N1a was visible when the preparation was photographed with a filter combination designed for rhodamine. Calibration bar = 100 mm.