Contrary effects of octopamine receptor ligands on behavioral and neuronal changes in locomotion of lymnaea

Monoamines, Insulin and the Roles They Play in Associative Learning in Pond Snails

Molluscan gastropods have long been used for studying the cellular and molecular mechanisms underlying learning and memory. One such gastropod, the pond snail Lymnaea stagnalis, exhibits long-term memory (LTM) following both classical and operant conditioning. Using Lymnaea, we have successfully elucidated cellular mechanisms of learning and memory utilizing an aversive classical conditioning procedure, conditioned taste aversion (CTA). Here, we present the behavioral changes following CTA training and show that the memory score depends on the duration of food deprivation. Then, we describe the relationship between the memory scores and the monoamine contents of the central nervous system (CNS). A comparison of learning capability in two different strains of Lymnaea, as well as the filial 1 (F1) cross from the two strains, presents how the memory scores are correlated in these populations with monoamine contents. Overall, when the memory scores are better, the monoamine contents of the CNS are lower. We also found that as the insulin content of the CNS decreases so does the monoamine contents which are correlated with higher memory scores. The present review deepens the relationship between monoamine and insulin contents with the memory score.

The antidepressants venlafaxine ("Effexor") and fluoxetine ("Prozac") produce different effects on locomotion in two species of marine snail, the oyster drill (Urosalpinx cinerea) and the starsnail (Lithopoma americanum)

Human antidepressants have been previously shown to induce foot detachment from the substrate in aquatic snails. Prior to foot detachment, antidepressants also affect snail crawling speed. We tested two commonly prescribed antidepressants, venlafaxine ("Effexor") and fluoxetine ("Prozac") on crawling speed and time to reach the air-water interface in two species of marine snail, the oyster drill Urosalpinx cinerea and the American starsnail Lithopoma americanum. Exposure to venlafaxine increased crawling speed in both species, while fluoxetine slowed them down. Our lowest LOEC (lowest observed effect concentration) was 31.3 μg/L venlafaxine in Urosalpinx. Similarly, snails (L. americanum) exposed to venlafaxine tended to move faster and more often to the air-water interface, but exposure to fluoxetine slowed them down. Our lowest LOEC was 345 μg/L fluoxetine in Lithopoma. These results indicate that venlafaxine boosts locomotion, while fluoxetine reduces it, and both behaviors are preludes to foot detachment. The different effects of these two antidepressants on snail locomotion suggest differing physiological mechanisms of action in marine snails as well as possible ecological consequences.

Effect of acute exposure to low environmental calcium on respiration and locomotion in Lymnaea stagnalis (L.)

Environmental calcium is a major factor affecting the distribution of freshwater gastropods. Whilst the effects on growth and morphology are fairly well understood, little is known about how calcium availability affects other aspects of gastropod biology. Lymnaea stagnalis (L.) is considered a calciphile and exhibits reduced growth and survival in environments containing less than 20 mg l(-1) Ca(2+). Many freshwater systems exhibit fluctuations in calcium concentration over time: where calcium levels are normally high there may be periods of low [Ca(2+)], for example following periods of flooding. Here we examined the effects of acute periods of low (20 mg l(-1)) environmental calcium on the physiology and behaviour of L. stagnalis, specifically measuring how locomotion and respiration differ between high calcium (80 mg l(-1)) and low calcium (20 mg l(-1)) environments. We found that in a low calcium environment crawling speed is reduced, and that this coincides with an increase in cutaneous respiration, indicating that the increased metabolic demands of calcium acquisition at low [Ca(2+)] reduce the energy available for locomotion. Conversely we found a decrease in aerial respiration in hypoxic conditions in the low calcium relative to the high calcium environment. In conclusion, we found that acute exposure to low environmental calcium has a highly significant effect on locomotion and respiration, which may have consequences for snail fitness when no morphological effects are apparent.