Anyone who has made the unfortunate decision to breathe while swallowing a liquid knows that some behaviours are incompatible with each other. The brain therefore has to ensure that, in case conflicting sensory information is received, the right behaviour is chosen: stop the breathing, swallow the liquid. This issue becomes acute when, for instance, an animal has the choice between eating a tasty morsel of food or fleeing a predator. How does the brain ensure that, if conflicting sensory information is received, the right behaviour is chosen? One mechanism, proposed by the pioneering ethologist Niko Tinbergen over 60 years ago, suggests that the brain has encoded a behavioural hierarchy, in which one behaviour can be blocked in favour of another, more important one. There is however little evidence to explain how the brain enacts this hierarchy.
In a study recently published in Current Biology, a team of researchers from the University of Sussex, UK, addressed this issue by looking at how Lymnaea pond snails choose between feeding and fleeing. The snail starts feeding when it detects sugar, but when the front of the animal is touched – mimicking an encounter with a predator – this behaviour is completely blocked. Instead, the snail withdraws, apparently to get away from the perceived predator. The experimenters decided to probe the mechanisms underlying this apparent behavioural hierarchy by applying a tried and tested technique within neuroscience to the snail: recording the electrical activity of identified cells within its simple nervous system.
The team identified a cell, PeD12, which they found to be crucial in the behavioural switch: it is activated when the front of the animal is touched, and, importantly, when the team inactivated the cell, touching the front of the snail no longer elicited an escape response. Moreover, when PeD12 was activated by the researchers the feeding behaviour was blocked, and the withdrawal response initiated, independent of the touch stimulus. These findings suggest that PeD12 is both necessary and sufficient for the implementation of the behavioural hierarchy. How does PeD12 regulate these two behaviours? Another cell type, PIB, has been previously shown to inhibit the neurons responsible for feeding. Could PeD12 simply be switching on PIB?
In order to test this hypothesis, the team performed another set of electrical recordings. They found that, consistent with their hypothesis, there are direct connections between PeD12 and PIB, and that PIB is necessary and sufficient for PeD12 to switch off the feeding behaviour. Moreover, in a separate experiment, the team found that PeD12 is directly connected with the cells responsible for the withdrawal behaviour. This means that the choice between feeding and fleeing is governed by a remarkably simple circuit of neurons: upon the detection of a predator, PeD12 switches off feeding indirectly, through PIB, and switches on fleeing directly, through its connections with the fleeing (withdrawal) circuit.
The study provides direct evidence for the existence of a behavioural hierarchy encoded in the the Lymnaea brain, implemented by simple inhibition of the less dominant behaviour by an identified interneuron. Moreover, it makes a strong case that there is still plenty of mileage in applying age-old experimental techniques to simple model organisms in order to address central questions in neuroscience.