Figure 1: Elevated plus maze,
a test for anxiety in rodents.
Schizophrenia affects 1 in 100 people globally and in the United States. The onset of symptoms is during late adolescence to mid-adulthood (16-30 yrs of age). There are several ideas regarding how brain function changes before, during, and after symptoms begin. The most common hypothesis is that there is an imbalance of the neurotransmitter dopamine in different brain areas. Becoming more popular is the idea that NMDA receptors are not functioning as well as they could be. There have also been some correlative studies showing potential links between maternal stress and schizophrenia in offspring. Dr. Maria Victoria Tejada-Simon’s lab at the University of Houston, is studying the NMDA theory in conjunction with maternal stress.
Alexander Oderhowho, an undergraduate student in Dr. Tejada-Simon’s lab, has been doing preliminary control studies on the maternal stress side of this project. For his experiments, he induced stress in pregnant mice just days after mating and continued the stress throughout the pregnancy. The stressors included confinement to a tube, a test that pairs a tone with a foot shock, tail suspension, and forced swim. He had a control group of mothers, which were not stressed. After the pups were born, Alexander used behavioral testing to assess anxiety behavior and fear-related cognition in the mothers and their offspring. They hypothesized that highly stressful events during pregnancy would result in increased anxiety-like behavior in the mothers and the pups and may impair cognition in the pups.
The tests they used for anxiety included the light dark exploration and the elevated plus maze. Both tests rely on the behaviors mice exhibit as prey animals. Mice prefer small dark spaces and tend to avoid open and bright spaces where they could become the next meal for a predator. In the lab, anxiety can be tested using a light dark exploration box with two sides: small/ dark and large/ open. The elevated plus maze has four arms: two with walls and two without (Figure 1). They also assessed locomotor activity and anxiety behavior in the open field test (a box similar to the light dark box, not a real field of pretty green grass). Similar to the other tests, anxious mice tend to spend less time in the center area of an open field and more time sitting near or walking around the edges. This edge circling behavior is called thigmotaxis.
For cognition, Alexander used a fear-conditioning test that uses a tone paired with a foot shock to assess fear-related learning. Mice freeze when frightened in an attempt to “hide” from predators. The fear-conditioning test assesses freezing behavior, the percentage of time they freeze after a sound is given, as they learn that a mild shock will follow the tone. Alexander’s results were largely similar to what he predicted in his hypothesis.
Mother mice showed increased anxiety following the stressful pregnancy in comparison with unstressed mothers. The previously stressed mothers spent less time in the light side of light dark exploration box and did not venture out into the open arms of the elevated plus maze at all. They even spent less time in the center of the elevated plus maze between the closed and open arms. In the open field test, the stressed mothers had decreased activity overall and decreased thigmotaxis. Normally, decreased thigmotaxis suggests a lower anxiety level. However, because the stressed mothers moved less overall this may account for the reduction in thigmotaxis, despite the increased anxiety response in the other tests.
In the fear-conditioning test, the stressed mothers did not perform well during the test when being trained on the paradigm. Alexander thinks this might have been because they were already familiar with the box used for fear-conditioning test after having experienced it as a stressor during their pregnancy. The offspring, however, had all clearly increased anxiety behaviors but the fear responses were different from mothers.
Pups from stressed mothers, at approximately 40 d of age, also spent a decreased amount of time in the light side of the light dark box and an increase in the amount of time spent in the closed arms of the elevated plus maze. There was no change in the time they spent in the center of the plus maze and they did venture out into the open areas, in contrast to the stressed mothers. There was no change in the basal level of locomotion in the open field when compared with pups from non-stressed mothers. However, thigmotaxis in the pups from stressed mothers was increased, suggesting higher anxiety.
In the fear-conditioning test, pups from stressed mothers had better pairing (more freezing behavior) than pups from non-stressed mothers. This makes sense because the pups tended to be more anxious in the other tests and were likely hyper-aware of scary stimuli.
As hypothesized, mothers stressed during pregnancy showed increased anxiety after birth and pups born from stressed mothers also had higher anxiety. The implication of these results is that there may be epigenetic changes being passed from mother to pup during pregnancy. Epigenetic means the genes themselves are not changing but the way the DNA is wound up together means some genes can be expressed or inhibited that normally are not based on stimuli. Epigenetic changes are not always passed from generation to generation but, in this case, it may be so. In the future, Dr. Tejada-Simon’s lab will combine NMDA receptor hypofunction with stress in mother mice and examine anxiety and cognitive behavior in the mothers and offspring. She would also like to study the biochemical changes occurring in the brain during these periods and morphology of cells in the hippocampus, a brain area important for learning and memory.
|Alexander Oderhowho and his advisor Dr. Maria Victoria |
Tejada-Simon, University of Houston.