Snails, like all animals, need to remember. They need to remember what is good or bad to eat, what might be trying to eat them, and who they recently mated with. All of these things can prove extremely important in the fight to survive and reproduce; and understanding how a snail forgets such things can teach us about our own memories.
Photo Credit: tillwe via Flickr.
The basic way a snail’s brain works is very similar at the level of individual neurons to that of vertebrates, but when it comes to understanding exactly what is going on in their brains, they have some distinct advantages over model organisms with backbones.
There are far fewer neurons in a snail than in vertebrates, for example – a snail has fewer than 20,000, whereas a mouse has around 75-million and a human around 85-billion. Snail neurons are also much larger. This means we can accurately locate the same neuron in different snails and work out which behaviour a neuron controls.
In a study we published in PLOS ONE, my colleagues and I looked at the effects of stress on a pond snails' memory. We found snails respond to stress and the effects can be detrimental to their memory, much like in mammals – including humans.
There are other similarities too. A snail’s memory responds to drugs in the same way that a mammal’s memory does. For example, hydrogen sulphide and heavy metals, which can have detrimental effects on human memory also do so for a snail. And epicatechin, a flavonol present in among other things dark chocolate, improves snail memory and has a similar effect on mammals.
Similar things are stressful for a snail as those that might stress any animal, including people: not having enough resources such as food or calcium, which is very important for snails to grow their shells and provision for their offspring. Being overcrowded or socially isolated from other snails is also stressful. And feeling threatened, for example, if they smell a predator in their environment. And all of these factors affect the ability of a snail to form memory.
Lymnaea stagnalis, the pond snails we used, can absorb oxygen directly across their skin when oxygen levels are high in the water, but when oxygen levels are low they can also breathe air using a basic lung opened via a hole called the pneumostome. We used a process called operant conditioning to train snails to reduce this aerial breathing behaviour.
Operant conditioning is where a stimulus is applied each time an animal performs a specific behaviour. In our case we applied a negative stimulus, – a gentle poke – to the pneumostome each time a snail tried to breathe air in low oxygen conditions. This process doesn’t harm the snail. In fact they are very well adapted to coping with low-oxygen conditions as they usually live in ponds, ditches and slow-flowing rivers where they frequently encounter less oxygen.
We found that if the snail formed a memory after the training exercise, they reduced the number of times they tried to breathe air during the memory test. Essentially, they learned from the experience.
The neuron in the snail that generates the rhythm for aerial breathing behaviour, inhalation and exhalation, has been identified, and it’s called right pedal dorsal 1 (RPeD1). We can measure activity from this neuron to see how it changes in response to the environment and also following training. In low-oxygen conditions it becomes much more active, which increases the aerial breathing behaviour. But after training there was a big reduction in this activity. From this we were able to measure memory formation both as a change in behaviour, but also as a change in activity in the neuron.
Not all types of stress are equal. Some types of stress block memory formation. For example, chronic stress and elevated cortisol (a hormone present in vertebrates) has been linked with dementia in people. Similarly, some types of stress, such as over-crowding or low-levels of calcium, can be detrimental for memory formation in snails.
However, other forms of stress can actually improve memory formation. An extreme example of this in people would be post traumatic stress disorder (PTSD), where certain memories associated with a traumatic event become so strong that sufferers are unable to forget them. Snails also form strong memories in very stressful times, for example in the presence of predator smell.
We tested the effects of several different types of stress on the activity in the RPeD1 neuron and were able to measure what improved or diminished memory forming from the increases or decreases in the neuron’s activity. And now we know we can directly measure the effects of stress, we can start to look for ways to reduce the effects, and measure again to see whether we have blocked the effects of stress at both a neuronal level and at a behavioural level.
Because we know about the similarities in the response to stress in snails and mammals, we hope that this will lead us towards a better understanding of how to alleviate the effects of stress in mammals – and humans.
Sarah Dalesman does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.