Value-based decision making: lessons learnt from a crayfish

We are always at cross roads while making decisions—it can be something trivial as whether to do groceries or to go to the parlor; or, something more important as whether to take a shorter, riskier route or to take a longer but safer route to reach a destination. Like us, other animals also have to assess situations and make decisions constantly. For example, a deer might have to decide whether to feed near a fresh grassy patch where there is a risk of getting eaten by tiger or to move to safer, but distant grounds with drier grass. Thus, animals are often faced with trade off between foraging decisions and risk of predation. A recent study says that crayfish, cousins of lobsters are also faced with such decisions.

Researchers in the Psychology Department of University of Maryland found that when crayfish were simultaneously offered two contrasting choices between finding food and moving away from a potential threat, they responded after weighing in the costs-benefits of the two situations. Furthermore, this study also outlines possible neuronal mechanisms underlying such behavioral choices. Crayfish and other invertebrates are excellent model organisms to examine neurons associated with foraging and antipredaoty behaviors, because of their simplistic neural networks compared to vertebrates.

Juvenile crayfish show two kinds of antipredatory behaviors in response to a moving shadow (which is considered a ‘predator’ in this study): they either freeze or tail-flip, propelling themselves away from the shadow. Tail-flips are mediated by excitation of a pair of neurons called Medial Giant interneuron (MG). Researches found that while slow-moving shadows elicited a tail-flip response, faster shadows caused crayfish to freeze. Tail-flipping facilitated the animals to move away from shadows, but this also took them away from food source. The authors also varied the concentration of the food—high concentration being a more favorable food source than lower concentration. They found that crayfish froze more often in presence of a favorable food source; however a slow-moving shadow (i.e. threat from a predator) evoked a tail-flipping response regardless of an attractive food source. This suggests that crayfish make value-based decisions that are ultimately adaptive, and these behaviors are mediated thru identifiable neural circuits.

A complete version of the article appears in the recent issue of Proceedings of the Royal Society B doi:10.1098/rspb.2010.1000
http://rspb.royalsocietypublishing.org/content/early/2010/06/12/rspb.2010.1000.short?rss=1

This article was contributed by Ms. Divya Bellur Uma, University of Maryland, Dept. of Psychology, Maryland

Prions go the Darwinian way … in cell cultures!

Well… looks like prions also have the propensity to evolve and undergo the natural selection process, just like us living beings. A recent report in the Science journal (Vol. 327. no. 5967, pp. 869 - 872) features new findings that suggests these lifeless-like molecules to adopt a darwinian evolution approach under special conditions in cell cultures. What makes it interesting is that they have shown prions, which are devoid of any nucleic acid material, can selectively amplify and mutate as per the requirement of the environment.

Prions are infectious proteins composed of β-sheet conformers. These β-sheet or PrP Sc conformers are derived from misfolding of the prion protein PrP. These β-sheet rich conformers are responsible for number of diseases like scrapie in Sheep, bovine spongiform encephalopathy (BSE) also known as mad cow disease in cattle and Creutzfeldt–Jakob disease (CJD) and kuru in humans.

Prions are characterized by the incubation time and neuropathology they elicit in a particular host and hence occur in distinct strains. The protein-only hypothesis assumes that each strain is associated with different conformer of PrP Sc. Cell-free conversion experiments and protein misfolding cyclic amplification (PMCA) have shown that conformational templating is possible at protein level.

Prion strain identity is thought to be encoded by PrP Sc conformations. Here the authors determine the cell culture properties of prions using cell panel assays. They show that when prions were transferred from brain to cultured cells, prions strain changed and was more cell adapted. However, when prions were returned from cells to brain, they became brain adapted again. The cell adapted strain outcompeted their brain counterparts in cell culture and brain adapted strain outcompeted when they were returned to brain. Experiments conducted with inhibitor swainsonine showed that when prions were grown in presence of inhibitor the resistant strain outgrew the sensitive strain and viseversa when grown in absence of the inhibitor. The authors suggest a quasispecies concept and Darwainian evolution for prions. They show that pions are subjected to mutations and to selective amplification depending on the different environments. The authors also suggest that drug resisstance would be less in drugs targeting PrP than drugs inhibiting PrP Sc.

This article was contributed by Dr. Shailaja Divekar, Georgetown University Medical Center, Washington DC