MODELS ALLOW SCIENTISTS TO STUDY MECHANISMS OF FORAGING, SEX AND HEARING
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MODELS ALLOW SCIENTISTS TO STUDY MECHANISMS OF FORAGING, SEX AND HEARING.
ORLANDO, Monday, Nov. 4 - Several animal models now enable scientists to study specific types of behaviors and how they are controlled by genes. These include how honey bees become transformed from a 'nurse' to a 'forager;' how mice communicate with each other by using pheromones; and how birds are closely related to crocodilians as illustrated by very similar designs in auditory coding.
The new studies were reported today during the 32nd annual meeting of the Society for Neuroscience.
Neuroscience research has greatly benefited from the use of animal models because different species are particularly suited to answering different questions. The prominence of a particular behavior and the feasibility and ease of performing experiments combine to make insects, mice, reptiles and birds attractive for investigation of the basis of particular behaviors.
Moreover, the conduct of experiments with animal models is generally simpler than comparable experiments with people. On the other hand, the contemporary finding that nervous systems across species are organized according to similar principles encourages the belief that the results are relevant to man, and convergent results obtained from different models lead to the conclusion that the findings can be generalized and are likely to have relevance to the human situation.
Gene Robinson, PhD, and his colleagues at the University of Illinois study how the honey bee can be used as a model for the neural mechanisms that underlie sociality. The researchers reported last spring that a gene they studied has played a key evolutionary role in the changes of food-gathering behaviors in many creatures.
Honey bees live in a social world known for its distinct age-related division of labor. They begin their adult life working inside the hive as sanitation workers and nursemaids, among other roles. Foraging begins at two to three weeks of age, or whenever the needs of a colony require it; the age at which a bee leaves the hive to forage is socially regulated.
When honey bees grow up and begin foraging, their transition is helped along by an increase in the activity of the foraging gene (for). It stimulates PKG, an activity-boosting enzyme in some of the brain's visual and multi-modal processing centers. Precocious foragers - bees stimulated to forage as early as one week of age - also show the increased gene activity. This assures that the brain changes were not simply a natural result of age.
Additionally, the researchers used a pharmacological approach, feeding an experimental group of young bees with a compound that stimulated PKG activity. The treated bees started to forage precociously, while untreated bees did not.
Before becoming foragers, honey bees make the transition from their role as nurses by going through a series of gradually widening orientation flights. During this time there are changes in brain chemistry and structure, endocrine activity and gene expression. Robinson and colleagues theorized that increased gene activity was necessary to drive behavioral change. Other genes had been implicated, but for is the first one shown to actually affect division of labor in honey bee colonies.
Two forms of for previously had been found to influence naturally occurring variation in foraging behavior in Drosophila. "Rover" flies that cover large areas have high levels of PKG, while "sitter" flies that gather food nearby have low levels. Nurse bees "loosely resemble sitter flies because they obtain food only in the more restricted confines of the beehive, while forager bees display rover-like behavior by ranging widely throughout the environment," the researchers say. PKG also has been linked to feeding arousal in some other invertebrates and vertebrates. The researchers believe that for and other genes that affect similar behaviors in different species might represent a class of genes that are particularly important to understanding the ways that genes influence behavior.
To survey more broadly for genes involved in the highly flexible behavior of the honey bee, Robinson and his colleagues recently studied the expression profiles of thousands of genes using a 'gene chip' that they developed. They found clear differences in the expression profiles from the brains of nurses and foragers, with about one third of the genes differentially expressed. The researchers also used social manipulations that result in bees that perform nursing and foraging at atypical times in their lives, and discovered that the activity of most genes was more closely associated with the occupation of the bee rather than its age. These results demonstrate that changes in the activity of many genes play an important role in orchestrating neural processes that underlie naturally occurring changes in behavior.
In other work with animal models, Catherine Dulac, PhD, of Harvard University studies how animals communicate with each other within a given species by using specific chemical cues called pheromones. In mammals, pheromones are thought to be mostly detected by a dedicated olfactory sensory organ called the vomeronasal organ (VNO), and they have been implicated in triggering male-male aggressive behavior and male-female mating.
Using very sensitive cloning techniques, Dulac and her colleagues have identified key signaling molecules and receptors of the pheromone response in mammals. "Remarkably these molecules appear unique to the pheromone response and are exclusively found in the VNO and not in the rest of the body or in any other sensory organ," she says.
Dulac has generated a mutant mouse line which is deficient in all pheromone detection. These animals appear unable to display male-male aggression. Moreover males deficient in pheromone detection attempt to mate with mice of both sexes with equal frequency, demonstrating that they cannot distinguish the gender of their conspecifics. In other words, mutant males appear unable to discriminate males from females and when put in presence of both, they attempt to mate with members of each sex with equal frequency.
These finding provide a fundamental understanding of the role of pheromones in regulating mouse behavior, says Dulac. "It identifies the role of pheromones, not as a direct trigger of mating behavior as previously thought, but rather as an indicator of the gender of potential mating partners. In other words, it clearly dissociates two aspects of the mating behavior: the behavior itself that does not rely on a functional VNO, and the choice of the mating partner that requires VNO activity."
This work provides an essential framework to investigate the neural basis of animal behavior and of gender identification. The next step will be to identify the circuits in the brain that are responsible for these events.
In other research, scientists have found that the auditory systems of birds and their close relatives, the crocodilians, share very similar designs to the auditory systems of mammals.
"We have studied how crocodiles and birds such as barn owls localize sound," says Catherine Carr, PhD, of the University of Maryland. "When sound comes from one side of the body, it reaches one ear before the other. That time difference is translated into location in space by similar circuits in the brains of birds, crocodilians and mammals. They are not identical, but they employ the same rules."
Carr and her colleagues have recorded with metal electrodes the responses to sound from the brains of chickens, owls and alligators and have also recorded from brain slices from chicken and alligator embryos.
Examinations of brain slices show that chicken and alligator auditory neurons that encode the timing of sounds are biophysically similar to mammalian cells with the same function. "Specializations, such as the shedding of dendrites in favor of a large terminal that envelops the cell body of the postsynaptic cell, have evolved repeatedly to perform the same function in different animals," says Carr.
Recordings from barn owl brains have shown us that birds and mammals use similar circuits to detect sound location. Information from the barn owl has been used to construct robots that localize sound, using barn owl algorithms in the robot.
"When different creatures use the same neural codes, the assumption is that these codes are good solutions to the hard problem of telling the brain about the outside world," says Carr. "We can argue that birds and crocodilians converged on the same solution as mammals because there is strong evidence from paleontology that the ears of birds and crocodilians evolved in parallel with mammalian ears. Therefore, when the brains of these animals all code sounds the same way, we can argue that biology has found common solutions to the problem of knowing where and what sounds are."
Scientists know very little about how the brain codes information. So the appearance of similar codes in different animals suggests that these codes are useful. Furthermore, the comparisons of how different animals process sound can be used to derive some general principles of how the auditory system works.