LONG-DISTANCE VOLUME TRANSMISSION AS A POSSIBLE MODULATORY MECHANISM OF THE CENTRAL PATTERN GENERATOR FOR FEEDING IN MODEL INVERTEBRATE

Volume transmission (VT), a widespread mode of neuron-to-neuron communication in the nervous system, enables broad neuromodulation and plastic changes. Neuromodulatory inputs contribute to behavioural flexibility by modifying networks of central pattern generators (CPGs) in response to internal or external stimuli. The CPG controlling feeding motor programmes in the pond snail Lymnaea stagnalis, located in a symmetrical pair of buccal ganglia, is under neuromodulatory control mostly from projecting neurones in other central ganglia. Earlier studies have suggested a role for volume transmission in the control of feeding behaviour in Lymnaea.
The aim of this study was to determine whether inhibitory input from modulatory FMRFamide-containing pleuro-to-buccal (PlB) neurones to the feeding CPG represents a volume transmission event.
Electrophysiological recordings of PlB neurones were analysed.
Evidence supporting volume transmission includes: (i) the symmetrical pair of electrically coupled PlB neurones does not exhibit 1:1 postsynaptic potentials (PSPs) with any identified neurones belonging to the feeding CPG; (ii) solution that decreases polysynaptic transmission do not abolish the strong inhibition observed at all levels of the CPG; and (iii) application of FMRFamide to the buccal ganglia produces effects similar to those elicited by intracellular stimulation of PlB neurones.
These results suggest that PlB neurones control the feeding CPG via long-distance volume transmission. Previously suggested direct electrical coupling between PlB axons may further enhance volume transmission by modulating the membrane potential of contralateral PlB axon terminals in buccal ganglia without requiring action potential initiation at the PlB soma in the contralateral pleural ganglion.