Thursday, May 29, 2008

Neuroscientist studies bizarre mammals

Ken Catania studies the brains of some of the strangest-looking mammals alive: the star-nosed mole and the naked mole rat.

"I used to be a little defensive about studying such weird-looking animals," the assistant professor of biological sciences acknowledges. "But then I realized that what makes these animals so strange is their extreme specialization and, for that very reason, there is a great deal that we can learn from studying them.

"This research strategy appears to be paying off. Catania has just been awarded one of only 15 fellowships given annually by the Searle Foundation, a highly competitive honor that will provide him with $240,000 to use on his research for the next three years. In addition, his latest paper appeared as the cover article in this month's issue of the journal Nature Neuroscience.

It was Catania's interest in the sense of touch that led him initially to the community of moles. In their underground world there is little light, so vision is not very important. Also, sounds are attenuated and hearing is not that valuable either. That leaves the senses of touch and smell pre-eminent. His first research subject was the star-nosed mole-an animal that looks very much like an ordinary mole except that has a peculiar star of fleshy appendages ringing its nose. More recently he has begun studying the naked mole rat, an animal famous in animal behavior circles because it lives in insect-like colonies organized around single breeding females, or queens.

Catania got acquainted with the star-nosed mole when he was an undergraduate at the University of Maryland. He worked at the National Zoo, where his job was to collect small mammals, including star-nosed moles. "I learned where to find star-nosed moles and how to collect them, which is a skill not many people have," he explains. Although they range from Canada, down through the Eastern United States as far as Georgia, people rarely see these unusual-looking creatures because they are the only mole that lives in marshes and wetlands.

As its name implies, the star-nosed mole has a very unusual snout, ringed with 11 pairs of pink appendages that form a fleshy star. There have been a lot of different ideas about the function of this distinctive feature. Some have proposed that it is a super-sensitive olfactory organ that helps the nearly blind moles to sniff there way around underground. Others have suggested that serves as an extra "hand" for grasping prey or other objects. Still others have argued that it serves as an antenna that detects electric fields as the moles swim through muddy marsh water.

It was not until Catania studied this structure in detail as part of his doctoral thesis at the University of California, San Diego that the star's true function came to light. Working with noted neuroscientist Glenn Northcutt, he showed that these appendages serve as an extraordinary touch organ, covered with more than 25,000 microscopic sensory receptors, called Eimer's organs, that allow the hamster-sized mole to literally feel its way around its subterranean environment.

"I just showed some pictures of the nose around the lab and everybody got very excited," he recalls.

Catania was able to generate equally high levels of interest in this unusual creature when he came to Vanderbilt as a post-doctoral fellow in the lab of Centennial Professor of Psychology Jon Kaas. He and Kaas determined that more than 100,000 nerve fibers run from the moles star nose to its brain, more than six times the number that connect the human hand and brain.

"Given this structure, it is possible that the mole can feel distinctive differences in the textures of different materials at a microscopic level," Catania speculates.

He and Kaas also discovered that the mole's fleshy nose rays develop in a way unlike any other animal appendage. The basic strategy employed to make everything from human fingers, insect legs, fish fins, and porcupine quills is to start as an outgrowth of the body wall and grow directly outward. By contrast, the rays develop first as fleshy ridges along the side of the nose. Then they become little cylinders that separate from the body beginning at the ends farthest from the nostrils. These ends move forward to become the tips of the freed appendages.

Next, Catania took a closer look at the how the nerve connections between the star-nosed mole's brain and appendages develop. The region connected to each of the appendages is clearly marked on the mole's cortex, which makes it very easy to study, Catania says. The mole's star is made up of two different kinds of appendages. The central, lowermost pair is much shorter than the other ten and contains a much higher number of nerve fibers per touch receptor than do the longer pairs. The area of the cortex that is connected to this pair is also much bigger than that devoted to the others, the neuroscientist has found.

"This organization has surprising parallels to that of the human visual system," Catania observes.

Humans, like most animals that rely primarily on sight, continually shift their eyes. When an interesting or important image enters their peripheral vision, a person instinctually shifts his or her eyes to move the image into the central part of the retina, called the fovea. The fovea has a much higher concentration of nerve fibers than the periphery of the retina, which allows it to provide the brain with a more highly defined image.

Similarly, active star-nosed moles continually wave their nose appendages around. When one detects something of potential interest, such as an unfortunate earthworm, then the mole moves its nose quickly to bring one of the central rays into contact, giving it a superior tactile image of the object so it can determine whether it is something good to eat. For small prey the entire process from first touch to complete ingestion takes about a fifth of a second.

In the April issue of Nature Neuroscience, Catania set out to determine whether the central appendages gain control of larger regions of the cortex because they have been genetically programmed to do so or because they are used more intensively. By looking at the development of the mole's nose and brain, he was able to determine that the central appendages get a head start in the development process. Largely because they get their major growth earlier in development than do the peripheral appendages, the central rays establish a larger number of nerve connections and interconnect with a larger area of the cortex than its fellow appendages, he reports.

In his application to the Searle Foundation, Catania emphasized his desire to study the organization of the brains of highly specialized animals like the star-nosed mole. "When you stop and think about it, most scientific research is concentrated on just a handful of species, so I got the idea of looking at other, non-traditional species," he says.

In pursuit of this goal, he recently arranged to obtain a colony of naked mole rats. The mole rats have a much different tactile system than do the star-nosed moles. Instead of putting the vast majority of their touch-sensors in one organ, the mole rat has sensory hairs spread all over its body, including its tail.

"The mole rats are known for their ability to travel rapidly backward through tunnels. The sensory hair on their tails probably allow them to do this," Catania says.

Mole rats also have an exceptional set of front teeth. They have four incisors that are located entirely outside of their mouth so they can close their mouth tightly when they are using their teeth for digging. The moles also have the ability to move each front tooth independently. For example, they can spread them apart or move them together. An indication of just how important these teeth are to the animal is the fact that fully 30 percent of its cortex is devoted to processing information from the region where the teeth are located, Catania says.

"Are these just super-sensitive chop sticks, or do they have other functions as well?" Catania asks. That is just one of a number of questions that he hopes further study of these unusual animals will help answer.


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