The allelochemical tannic acid affects the locomotion and feeding behaviour of the pond snail, Lymnaea stagnalis, by inhibiting peripheral pathways

Abstract

(1) The effect of tannic acid (TA), a dominant component of plant allelochemicals, was investigated on the locomotion and feeding of the pond snail, Lymnaea stagnalis. The effect of TA on the neuronal background underlying feeding activity was also analysed. (2) TA affected the spontaneous locomotion and of juvenile snails in a concentration-dependent way. Low (10 μM) TA concentration resulted in an increased (sliding or swimming) activity compared to the control; meanwhile, high (100 μM) TA concentration inhibited the locomotion of the animals. (3) Low (10 μM) TA concentration increased the frequency of sucrose-evoked feeding of intact animals, whereas high (100 μM) TA concentration resulted in significantly longer feeding latency and decreased feeding rate. The feeding changes proved to be partially irreversible, since after 48 h maintained in clear water, the animals tested in 100 μM TA previously still showed lower feeding rate in sucrose. (4) Electrophysiological experiments on semi-intact preparations showed that application of 100 μM TA to the lip area inhibited the fictive feeding pattern of central neurons, the cellular response to sucrose. (5) On isolated CNS preparation, 100 μM TA applied in the bathing solution, however, failed to inhibit the activation of the central feeding (CPG) interneurons following application of extracellular dopamine. Our results suggest that TA affects both afferent and efferent peripheral functions in Lymnaea. TA reduces feeding activity by primarily blocking feeding sensory pathways, and its negative effect on locomotion may imply sensory pathways and/or ciliary activity.

Keywords: Allelochemicals; Feeding; Locomotion; Lymnaea; Peripheral pathways; Tannic acid.

Fig. 1

Time and dose-dependent effect of TA on the locomotion of juvenile snails (Lymnaea stagnalis). X axis shows the different forms of locomotion (Sl sliding, Sw swimming, Fl floating, St sticking) in Balaton water (control) and in 10, 100 μM TA. Y axis: time spent in a particular form of behaviour during the 900 s (15 min) observation period

Fig. 2

Concentration-dependent effect of TA on the locomotion of juvenile snails (Lymnaea stagnalis). In lower concentration (10 μM) TA increased the ratio of active (swimming and sliding) versus passive (floating and sticking) periods of locomotion, seen as higher level of sliding activity and with less sticking periods. Higher, 100 μM TA concentration resulted in the predominance of passive forms with almost equal contribution of sticking and floating. Mean ± SD; n = 10 in each group. Total time of a trial was 60 min (100%). Numbers indicate the percentage values of each form of locomotion

Fig. 3

Feeding tests performed on snails following the application of 100 mM sucrose show altered feeding parameters in the presence of TA. a All three groups (1, 2, 3) tested in Balaton water show similar, not significantly different feeding latency and feeding rate. b An increased feeding rate of group 2 (10 μM TA) and a significantly decreased feeding frequency in group 3 (100 μM TA) are seen compared to the feeding response of control animals tested in water (group 1). c Repeated tests next day (1: sucrose; 2: sucrose + 10 μM TA; 3: sucrose + 100 μM TA) show significantly longer latency and lower feeding rate in both treated groups (2, 3). d After 48 h all the groups (1, 2, 3) were tested in sucrose only, and group 3 still displayed decreased feeding rate. Left axis shows latency in seconds, and the right axis shows the feeding frequencies expressed as bite/min (mean ± SD; n = 12 in each group). *p < 0.05; **p < 0.01; ***p < 0.001

Fig. 4

Preliminary application of TA prevents sucrose response. A Simultaneous recording from two buccal motoneurons (B8, B1) during Balaton water application (W), followed by 100 mM sucrose (Suc) applied to the lip area (a1). Sucrose-evoked rhythmic activity is seen as series of inhibitory inputs on B8 neuron and depolarization with action potentials on B1 neuron, respectively. N1, N2, N3 indicate the phases of fictive feeding (a2). B Application of 100 μM TA did not change the spontaneous activity, and sucrose dissolved in TA solution (Suc +TA) fails to evoke the feeding rhythm (b1), as rhythmic synaptic inputs are not visible in B8 or B1 neurons (b2). C After 20-min washing, the lip with water the sucrose-evoked intracellular response recovered (c1) and the cyclic synaptic inputs re-appear in the intracellular activities of feeding neurons. (c2). Representative example of the experiments carried out on 6 independent preparations

Fig. 5

In isolated central nervous system TA did not prevent fictive feeding evoked by 1 mM DA application. a1 The feeding pattern recorded on the B4 buccal motoneuron in saline; b1 intracellular activity in the presence of 100 μM TA. The feeding cycles marked by the intervals between the N2 phase synaptic inputs on B4 motoneuron (a2) does not show significant changes after DA application (b2). Representative example of independent experiments carried out on 6 preparations

Fig. 6

a In isolated central nervous system TA did not alter the frequencies of DA evoked fictive feeding. a Prior to DA application (control) the spontaneous rhythm of intracellular activity (with frequencies around 5 cycles/min) is switched to a threefold frequency increase after applying 100 μl 1 mM DA in the perfusion chamber (dotted line). b Diagrams showing no significant differences in frequencies in normal saline (NS), in the presence of 100 μM TA, or after washing with saline again (Wash)