American Scientist

In this roundup, Katie Burke summarizes notable recent items about scientific research, selected from news reports compiled in the free electronic newsletter Sigma Xi SmartBrief.

The Crocodile’s Sensitive Side

Differences in microscopic pigmented bumps on the jaws and body scales of crocodiles and alligators have been used to distinguish between legitimate and poached species’ hides sold in the fashion industry. Knowledge of these bumps’ functions was speculative. Researchers at Vanderbilt University began exploring the anatomy, innervation and response properties of the bumps, which they call integumentary sensory organs (ISOs). They found that ISOs imparted sensitivity far greater than human fingertips. Within 50 to 100 milliseconds, test subjects snapped at the slightest movements in water.

Leitch, D. B., and K. C. Catania. Structure, innervation and response properties of integumentary sensory organs in crocodilians. Journal of Experimental Biology 215:4217–4230 (November 8)

The Dopamine Effect?

A placebo’s effect has a genetic basis, according to a new study. A gene coding for an enzyme controlling dopamine uptake has two forms, called valine (val) and methionine (met). The val form results in three to four times higher enzyme activity than the met form. People can be homozygotes for the gene and have two copies of the same form, or they can be heterozygotes and have one copy of each. Homozygotes for the met form perform better on cognitive tests and are more sensitive to pain, compared to those with one or two copies of the val form. In this study, 104 patients with irritable bowel syndrome were either told they were waitlisted (no placebo), given machine-administered sham acupuncture (limited placebo), or given the same sham attended by a caregiver (augmented placebo). The placebo response among met/met homozygotes was much stronger than among heterozygotes or val/val homozygotes.

Hall, K. T., et al. Catechol-O-methyltransferase val158met polymorphism predicts placebo effect in irritable bowel syndrome. PLoS One 7:e48135 (October 23)

Slime Memories

An organism that remembers where it has been navigates better than one without this spatial memory. Externalized spatial memory was recently demonstrated in a brainless organism, the slime mold Physarum polycephalum. Its strategy is analogous to another externalized spatial memory system, the use of pheromone trails by many ant species to mark routes to food. The nonreproductive state of P. polycephalum, called a plasmodium, senses and flows toward food, leaving behind a trail of slime. The plasmodium avoids areas with this slime, suggesting it can sense the residue on contact. Thus the slime mold seemed to use its slime to tell where it has already been. To test this hypothesis, researchers borrowed methods from autonomous robot tests that measure a subject’s ability to reach a sensed goal behind a U-shaped barrier. In this case, a U-shaped barrier was placed on an agar surface between the plasmodium and a well containing glucose that diffused through the agar. Some plates had agar coated with fresh slime, whereas others contained blank agar. Significantly more plasmodia successfully reached the food goal on blank agar than slimed agar.

Reid, C. R., et al. Slime mold uses externalized spatial “memory” to navigate in complex environments. Proceedings of the National Academy of Sciences of the U.S.A. (Published online October 8)

Impactful Moon Studies

The leading hypothesis on Moon formation posits that a planetary body collided with proto-Earth, breaking off bits to form the Moon. However, simulations and physical evidence conflicted. Previous simulations suggested that lunar material originated from the impactor. If this were the case, the isotopic signatures of Moon and Earth would differ, but they do not. The simplest explanation is that the Moon formed from Earth’s mantle. Two new simulations rectify these previously conflicting observations. One models an impact between a planetary body and a fast-spinning proto-Earth, producing a debris disk large enough to form the Moon. The second simulates two planets of equal mass colliding. In both cases, the system spun faster than Earth and Moon do today, which earlier simulations suggested would form a smaller moon than Earth’s. But in either study, the spin could have slowed to a 24-hour day through a gravitational interaction between Earth’s orbit around the Sun and the Moon’s orbit around Earth.

Cuk, M., and S. T. Steward. Making the Moon from a fast-spinning Earth: A giant impact followed by resonant despinning. Science (published online October 17) Canup, R. Forming a Moon with an Earth-like composition via a giant impact. Science (published online October 17)

Neural Stem Cell Success

Neural stem cells can differentiate into brain cells called oligodendrocytes, which produce myelin, a material that coats nerves and is essential for efficient electrical signaling between neurons. Neural stem cells were implanted into the frontal lobes of four young boys diagnosed with a rare x-chromosome–linked disorder called Pelizaeus-Merzbacher disease, which results in dysfunctional oligodendrocytes. Over the following year, magnetic resonance imaging showed increased myelin production in each boy. Motor function also increased moderately in three of the four boys. This study was one of the first successful neural stem cell clinical trials in the United States.

Gupta, N., et al. Neural stem cell engraftment and myelination in the human brain. Science Translational Medicine 4:155ra137 (October 10)