Behavioural Function of the Putative Glutamatergic Interneuron, N2v, in the Feeding System of the Snail, Lymnaea: Plateauing Properties and Synaptic Connections with Retractor Motor Neurons. MATTHEW J. BRIERLEY, KEVIN STARAS AND PAUL R. BENJAMIN. Sussex Centre for Neuroscience, School of Biological Sciences, University of Sussex, Brighton, East Sussex, BN1 9QG, United Kingdom.
APStracts 4:203N, 1997.
ABSTRACT
Intracellular recording techniques were used to examine the electrical properties and behavioural function of a novel type of retraction phase interneuron, the N2 ventral (N2v) cells in the feeding network of the snail Lymnaea. The N2vs were compared with the previously identified N2 cells that are now renamed the N2 dorsal (N2d) cells. The N2vs are a bilaterally symmetrical pair of electrotonically-coupled plateauing interneurons that are located on the ventral surfaces of the buccal ganglia. Their main axons project to the opposite buccal ganglion but they have an additional fine process in the postbuccal nerve. N2v plateaus which outlast the duration of the stimulus can be triggered by depolarizing current pulses and prematurely terminated by applied hyperpolarizing pulses. Gradually increasing the amplitude of depolarizing pulses reveals a clear threshold for plateau initiation. N2v plateauing persists in a high Mg2+/nominally zero Ca2+ saline that blocks chemical synaptic connections, suggesting an endogenous mechanism for plateau generation. The N2vs fire sustained bursts of action potentials throughout the N2/rasp phase of the fictive feeding cycle and control the retraction phase feeding motor neurons. The N2vs excite the B3 and B9 feeding motor neurons to fire during the rasp phase of the feeding cycle. They also inhibit the B7 and B8 feeding motor neurons. The B8 cells recover from inhibition and fire during the following swallowing phase. These synaptic connections appear to be monosynaptic as they persist in high Mg2+/high Ca2+ (HiDi) saline that blocks polysynaptic pathways. Strong current-induced plateaus in the N2vs generate brief inhibitory postsynaptic responses in the B4CL rasp phase motor neurons but this was due to the indirect N2v N2d B4CL pathway. The N2vs are electrotonically coupled to the N2d cells and triggering plateau in a N2v usually induced one or two spikes in a N2d. Previous experiments showed that the N2ds generate plateau potentials during a fictive feeding cycle. Here we show that the main component of the `plateauing' waveform is due to the electrotonic coupling with the N2v cells. The differential synaptic connections of the N2v and N2d cells with retraction phase motor neurons results in a sequence of motor neuron burst activity B9 B4CL ÿ1A B8 that produces the full retraction (rasp ÿ1A swallow) movements of the feeding apparatus (buccal mass). We conclude that the N2v cells are an essential component of the interneuronal network required to produce feeding motor neuron activity. 

Received 29 October 1996; accepted in final form 8 August 1997.
APS Manuscript Number J862-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997