Chemical and Tactile Inputs to the Lymnaea Feeding System: Effects on Behaviour and Neural Circuitry

Abstract

ABSTRACT Transfer of snails from distilled water to solutions of sucrose or maltose stimulated feeding behaviour. As the concentrations were increased from 10−5mol l−1 to 10−1mol l−1 the proportions of snails showing feeding movements and the median rate of feeding both increased until at 10−3 mol l−1 they reached a plateau. At higher concentrations (10−1 mol l−1), maltose, but not sucrose, reduced the proportion of feeding snails and median feeding rate to values occurring at 10−5 mol l−1. Increases in median feeding rate were due to an increase in the regularity of the feeding rhythm rather than increases in the maximum rates of feeding. Sugars and tactile stimuli were applied to putative sensory structures (interior of the buccal mass, lips and tentacles) in two types of semi-intact preparation whilst intracellular recordings were made from feeding motoneurones (1, 3, 4, 5 and 7 cells) and higher-order interneurones (Cerebral Giant Cells, CGCs, and the Slow Oscillator, SO). Both types of sensory stimuli applied to the lip–tentacle preparation produced strong activation of the CGCs but only long latency weak excitation of motoneurones with no clear initiation of synaptic inputs from central pattern generating (CPG) interneurones. In a more complex lip–tentacle–buccal mass preparation the evoked responses to sugars were comparable to those in the intact snail. Initiation of motoneuronal activity or increased frequency of activity was observed. This was due to increased CPG synaptic inputs as well as activation of a modulatory interneurone, the SO. Multiple sensory pathways from the periphery, present only in the lip–tentacle–buccal mass preparation, were necessary for sensory initiation of the feeding rhythm. The results support the hypothesis that multimodal sensory inputs are likely to be involved in the initiation or modulation of feeding in Lymnaea and act at several levels in the system.