Members of Edward Kravitz's research team are Charlotte Chui, Kravitz, Christine Couture, Margaret Bradley, Chantal Ly, Henning Schneider and Anja Teschemacher.
A lobster's life is one soggy battle after another. As the
lobster matures, it engages in a series of vicious fights
with other lobsters for status and turf. The outcome of
these highly ritualized battles may determine who is
dominant for hours, days or even weeks to come.
"The losing animal in a fight won't fight with the winner of this fight. It won't fight with the winner of another fight. It won't even fight with the loser of another fight," says Ed Kravitz, George Packer Berry Professor of Neurobiology. "When you're a loser, you're a loser in the lobster world."
However, Kravitz and his colleagues have recently found that injecting serotonin into losing lobsters can produce a stunning reversal. "The losing animal will turn around and start fighting again. Usually they get beaten," says Kravitz. Their findings have not yet been published.
Kravitz believes that serotonin produces such a dramatic turn in behavior by changing how nerves communicate with one another. He suggests that nerve cells that normally release serotonin take up the excess serotonin, thereby increasing their stores. During a fight, they spurt out more serotonin than usual, which in turn affects how other neurons function.
Kravitz, who with others has previously shown that serotonin induces claw-waving and a dominant stance in lobsters, believes that the latest findings strike at something deeper than postural behavior. "I think we're starting to deal with motivation in these animals," says Kravitz. "Now, we don't know what they're thinking. But the point is [serotonin] is changing the animal's behavior so it's now willing to do something it was not willing to do before."
Kravitz spent decades figuring out how lobster neurons communicate with one another before tackling the problem of lobster behavior. When Kravitz first arrived at HMS in 1960, many American researchers believed that neurons communicated by sending electrical impulses, rather than by the release of chemicals from nerve endings.
Kravitz, who was trained in biochemistry, had developed methods for measuring minute amounts of chemicals in single nerve fibers. Working with a band of HMS researchers that included Steve Kuffler and David Potter, he began exploring the role of a chemical, gamma aminobutyric acid (GABA), in the lobster nervous system. He found that GABA was present in the nervous system of lobsters, and that the concentration of GABA was five hundred times greater in inhibitory neurons than in excitatory neurons. "We showed that the GABA content of inhibitory nerves was staggeringly high," says Kravitz.
Kravitz had a hunch that serotonin might be working to change the way neurons fired-that is, to make them either more or less responsive to stimulation. To further explore the effects of serotonin, HMS professor of neurobiology Margaret Livingstone, then a student in Kravitz's lab, tried a rather crude experiment: she injected serotonin from a hand syringe into lobsters. "We said, 'You're not going to see anything, that's not the way this works-it's much more sophisticated,'" Kravitz says.
But Livingstone found that when she injected just the right amount of serotonin, she could get lobsters to stand up tall and put their claws up-in other words, to look like a typically dominant lobster. When she injected another hormone, octopamine, she got animals crouching low to the ground, with their claws straight ahead-a typical subordinate posture. If she got the dosage of octopamine just right, she could even get the lobsters to walk backwards and grovel.
To find out how serotonin exerted its effect, Kravitz and his colleagues tried pouring serotonin directly onto the neurons controlling the claw-muscles of lobsters. They found a striking pattern. Neurons exciting the flexor muscles (the "claws-up" muscles) became more active; the neurons inhibiting the flexor muscles became less active.
On the other hand, neurons exciting the extensor muscles (the "claws-straight-ahead" muscles) became less active, while neurons inhibiting the extensors became more active. Octopamine had the exactly opposite effect.
While poured-on serotonin could exert dramatic changes, it was not clear that serotonin really worked that way in the lobster nervous system. Kravitz and his collaborators scoured the lobster nervous system for serotonin hot spots related to posture. They found the highest concentration of serotonin in clusters of nerve cells in the thoracic region. The serotonin appeared to be produced by four large cells, two on each side of the body.
Next, they tried stimulating the putative serotonin- producing cells. Nothing happened; the lobsters did not stand tall.
"What we found was actually more interesting," says Kravitz. The researchers discovered that by stimulating the neurons that make the lobster stand tall, they also activated the serotonin-producing cells. And the consequences of that indirect serotonin-cell activation was to increase-in a kind of feedforward fashion-the firing of the neurons that make the lobster stand tall.
"So the serotonin-producing cell by itself doesn't do anything. You have to activate the circuitry," says Kravitz. What Kravitz and his colleagues had hit upon was a "gain- setting" mechanism whereby the lobster could, by its own actions (such as standing tall), modulate, or turn up, the functioning of its own circuits.
Typically, lobster fights are highly predictable events: the lobsters display their claws and circle around each other in a kind of do-si-do before locking claws. "In the claw lock, the animals are wrestling and trying to turn each other over. Tearing and ripping-the next phase-is the violent part of the encounter," says Kravitz.
"But after a certain point, one animal backs away. And then they're either ignoring each other or behaving in a hierarchical way."
A lobster fight is a highly ritualized affair. First the lobsters display their claws (1) and take each other's measure (2). After circling around each other (not shown), the lobsters lock claws and try to flip each other over (3). The wrestling escalates into tearing and ripping--the most violent part of the fight (4 & 5). The winner of this brief violent clash assumes the dominant "claws-up" posture while the loser exhibits the "claws-straight-ahead" posture (6).
Normally, the loser will not engage in another fight for hours or even days, not even with other losers. But the researchers found that when they injected lobsters with serotonin, they would usually assume the dominant "claws-up" posture. Then, after the postural component decayed (which took about forty minutes), the lobster would exhibit a surprising turnabout: instead of skulking, it would start fighting once again. Even lobsters who showed no postural change would return to do battle about forty minutes after injection. "So it's a slower response than the postural," says Kravitz.
Kravitz believes that this second slower response is mediated by different serotonin-producing cells and circuits than those controlling the postural component. "There is a giant serotonin-producing cell in the lobster brain that very densely innervates the area of the brain called the accessory lobe. This is a major higher processing center in the lobster brain," Kravitz says. "Although we know little about the physiology [of this area] yet, we think the change may be taking place at this level."
Kravitz suggests that the giant serotonin-producing cell takes up the injected serotonin. "And now every time the animal uses the serotonin nerve terminal in a fight, it releases more serotonin than it did before. And the animal interprets that [excess serotonin in the brain] as saying it's willing to fight."
Kravitz and his colleagues are currently testing this serotonin-uptake hypothesis. They are giving losing lobsters a serotonin-uptake inhibitor, Prozac, which should prevent them from producing the willingness-to-fight response. "So we have lobsters on Prozac now," he says. "Acute Prozac doesn't do much. But acute Prozac doesn't affect people either. It's chronic Prozac you have to have-three or four weeks of Prozac before you see any behavioral consequences."
Whether serotonin-producing cells are producing aggressive behaviors in humans is unclear. Increased serotonin levels in monkeys appear to decrease the frequency of aggressive behaviors-suggesting an opposite response to the one found in lobsters. "With what has been done so far in primates and people, it really does look like amines [such as serotonin] play a role in aggression," says Kravitz.
Meanwhile, the serotonin studies are revealing a nervous system that is less fixed and more flexible than previously believed. "When the original modulating effects of serotonin first came out in the 1950s, modulation was not put in the context of being a normal physiological response. But that's actually part of how synapses work-they're constantly being modified in ways that upregulate and downregulate their physiological role. So they're in constant flux. Synapses are not fixed," says Kravitz. "The nervous system is not a static structure."
--Misia Landau