Serotonin keeps crayfish in the dark

Field: Neuroscience (Life Sciences)

There's nothing like a nail-biting FIFA World Cup to remind us what anxiety feels like. Properly defined, anxiety is a behavior brought on by stress that helps us cope with potentially threatening situations. Anxiety is controlled in our brains by the neurotransmitter serotonin, and differs from fear because it persists in the absence of the stressor and in a new context. While fear-like behaviour has been well documented across the animal kingdom, the more complex emotion – anxiety – has so far only been demonstrated in vertebrates. In fact, our understanding of the neurobiology behind anxiety is built almost exclusively on mammalian studies, but a recent Science publication from the joint efforts of Jean-Paul Delbecque and Daniel Cattaert at the Université de Bordeaux, France, offers new insight into the field using a relatively simple invertebrate, the crayfish.The team began their study by comparing how a stressful experience impacted crayfish behavior. First, they exposed some of the animals to 30 min of mild electric pulses and then placed individuals in a plus-shaped aquatic arena with two dark arms and two light arms. Video-recording each animal's movements during a 10 min test period, the team exploited the animals' natural preference for the dark to determine whether the stress had altered their behavior by measuring how long it took stressed and unstressed animals to pluck up the courage to enter a light arm.Compared with unstressed animals, stressed crayfish took longer to enter and spent less time in light arms, suggesting they preferred to stay in their comfort zone – the dark. This behavioral shift meets the defining terms of anxiety in that it exists in the absence of the stressor and in a new context, and is thus the first demonstration of anxiety in a non-vertebrate species. Importantly, the team also measured serotonin levels in the brains of the crayfish and discovered that stressed crayfish had higher levels of serotonin in their brains compared with unstressed controls, suggesting that the neural control of anxiety in crayfish is similar to that of mammals. Then, to confirm this evolutionary conservation, the team injected unstressed crayfish with serotonin and monitored how the crayfish behaved in the test arena. They found that unstressed crayfish given a serotonin injection behaved similarly to the stressed crayfish and stayed mainly in the dark arms, confirming that serotonin controls anxiety-like behavior in crayfish as it does in you and me.The take-home message of this study is that the neural pathways controlling complex behavioral responses to stress are anciently wired. This study paves the way for the use of invertebrate models in studies aimed at understanding the neurobiological mechanisms driving our emotions. It also raises concerns about the broader impact of water-borne pharmaceuticals (e.g. downstream of waste-water treatment plants) on the behavior and physiology of aquatic biota. However, there could be an up-side to this study: perhaps, one day soon, your vet could prescribe Valium for your pet hermit crab who just can't seem to muster-up the courage to seek out a bigger shell.