Jens Herberholz is fascinated by the ways in which crayfish establish their social dominance hierarchies. Put two crayfish together and they will fight it out until suddenly they acknowledge which of the pair is dominant and which subordinate. But what is happening in the crustacean's brain as it makes this decision about its place in the social order? Herberholz, Christopher Mims and Donald Edwards of Georgia State University realised that they needed to look at the crayfish brain to find out(p. 4543).
When Donald Edwards bumped into crustacean expert Barbara Beltz at a conference, she suggested that he could try putting crayfish into magnetic resonance imaging (MRI) scanners to see what was going on inside the crustaceans' heads. So the team got in touch with MRI experts Xiaodong Zhang and Xiaoping Hu at Emory University. With support from the Center for Behavioral Neuroscience in Atlanta, they were ready to scan crayfish brains. But nobody had ever used MRI to investigate crayfish's insides before, and it quickly became clear that conventional MRI wasn't going to work. `We didn't know which imaging parameters to use for crayfish' Herberholz explains, `and there was no contrast between the tissues, so the insides looked like one grey mass.' The team needed to find a way to improve the contrast so that they could see different tissues.
At this point, Herberholz remembered that other researchers had successfully used manganese to light up active brain areas in rats. `Different tissues absorb different amounts of manganese' he explains, `so when an animal is placed in an MRI scanner, some tissues will appear brighter than others.'So manganese was an excellent tissue contrast-enhancing agent in rodent brains, but would it also work for crustaceans?
To see if manganese could improve contrast between crustacean tissues, the team decided to test a crayfish inside an MRI scanner before and after injecting manganese into the animal. They injected manganese chloride into the crustacean's circulatory system using a long tube connected to the animal while it was inside the scanner. `Before injecting the manganese, we couldn't see the animal's brain at all' says Herberholz, `but just a few minutes after injecting the manganese we suddenly saw the brain light up like a Christmas tree.' Clearly, the manganese worked perfectly as a contrast-enhancing agent for crustacean brain tissue. And it wasn't just the brain that became visible;the contrast between other complex anatomical structures like the foregut was also remarkably improved. `We were pleasantly surprised to find that manganese is such an exceptional contrast agent for invertebrates' Herberholz concludes,`and we expect that this simple, inexpensive approach to examine the anatomy of live animals will provide interesting possibilities for future research.'
So will this exciting new imaging tool help Herberholz in his quest to visualise changes in the brain as crayfish determine their place in the social hierarchy? At the moment, the spatial resolution of the brain images simply isn't good enough to make this feasible. But Herberholz is hopeful that, with higher strength MRI, this technique will allow him to see neural activity patterns in the brains of dominant and subordinate animals in the not-too-distant future.