Behavioural and biochemical changes in the feeding system of Lymnaea induced by the dopamine and serotonin neurotoxins 6-hydroxydopamine and 5,6-dihydroxytryptamine

Abstract

The neurotoxins 5,6-dihydroxytryptamine (5,6-DHT) and 6-hydroxydopamine (6-OH-DA) were used to examine the role of monoamines in the feeding system of the snail Lymnaea stagnalis . Biting responses to sucrose were monitored up to 25 days after injection with drugs. Cerebral and buccal ganglia and cerebro-buccal connectives from the same groups of snails were examined for changes in serotonin and dopamine levels by high performance liquid chromatography and the glyoxylic acid histo-fluorescence technique. Twelve to eighteen days after injection with 5,6-DHT only 57% of the snails responded to sucrose with biting movements, compared with 98% of controls. Those that did respond had a longer latency to the first bite and the bites were of shorter duration and occurred at a lower rate when compared with controls. This was accompanied by a 39% drop in 5-HT levels in the cerebro-buccal commissure and nerves and a loss of fluorescence in the axons of the paired cerebral giant cells, the main serotonergic neurons involved in feeding. Earlier behavioural effects of injecting 5,6-DHT at 20 min and 3-4 h after injection could not be explained by specific changes in 5-HT levels. Recovery of both behavioural response and serotonin levels occurred between 22-25 days after injection. Injection of 6-OH-DA also inhibited feeding responses but the effects were quicker (1-3 days) and more dramatic, with only 40% of snails showing any biting response to food compared with a 98% response in controls. Reduction in dopamine (DA) levels of 40%, together with a loss of DA fluorescence in nerve fibres accompanied the reduction of behavioural responsiveness. An early (3-4 h) effect of 6-OH-DA injection could not be correlated with a specific reduction in DA levels. Behavioural responses and DA levels returned to normal by 4-7 days after injection. Both neurotoxins inhibited uptake of their target monoamines and this appeared to be the main mechanism for depleting 5-HT and DA. Early effects of neurotoxin injection probably directly inhibited monoamine uptake, whereas long-term inhibition was a secondary effect because of degeneration of nerve fibres. Neither neurotoxin caused release of monoamines. Results from both neurotoxins suggest that monoamines play an important role in the initiation and maintainance of the feeding response, the consummatory phase of feeding. Neither neurotoxin prevented the occurrence of the initial appetitive phase of feeding when snails orientate towards food in the experimental chamber.