Investigation of the brain with the technique of encephalography has revealed the existence of six different types of oscillations, which have been named by Greek letters. Electroencephalography has also revealed that some brain regions have their own type of characteristic oscillations. One of these brain regions with their own oscillations is nothing less than the hippocampus. We have two hippocampi, one in each cerebral hemisphere. The hippocampus is essential for memory and spatial orientation and is also one of the first brain regions affected when Alzheimer’s disease is triggered.
The activity of the hippocampus is so important for the normal functioning of the brain that this region has two types of characteristic oscillations that also work in a coordinated manner. These oscillations have been called theta 1 and theta 2.
Theta 1 oscillation has a frequency of 7 to 12 hertz (cycles per second) and appears in behaviors such as exploration and voluntary movements. Theta 2 oscillation has a frequency of 4 to 9 hertz and is associated with states of immobility, anxiety, and behaviors in the face of a predator’s threat, for example, when detecting its scent.
These oscillations generate opposite effects, which is why a group of researchers from the University of Uppsala, in Sweden, in collaboration with researchers from the Federal University of Rio Grande do Norte, in Brazil, address this issue using one of the most sophisticated techniques of the moment, capable of affecting at will in laboratory animals exclusively the functioning of selected neurons. It is the technique of optogenetics.
Optogenetics is simply a trick to be able to activate the neurons that we want without touching them or treating them in any way, but simply illuminating them with a laser light of a certain color. To make the trick work, you need to have a fairly deep understanding of the genetic properties of the neurons you want to activate. In particular, it is necessary to know which gene or genes are working exclusively in them.
Using this knowledge, researchers can generate a transgenic rat or mouse by including an artificial gene in its genome that will produce a protein sensitive to laser light of a certain color. When this protein is activated by a laser light, it acts to exclusively activate the neurons where it is located. This affects the functioning of the brain and also affects the behavior of the animal.
The researchers generate “optotransgenic” animals that possess only one particular type of light-sensitive hippocampal neuron. Previous studies had led scientists to suspect that these neurons could be responsible for one or another of the theta oscillations. These are the so-called interneurons of the region of the hippocampus called oriens-lacunosum moleculare, for those who, knowing Latin, want to know. Interneurons communicate sensory or motor neurons with neurons in the central nervous system.
By illuminating the hippocampus of optotransgenic animals, the researchers discover that only the theta 2 oscillation is generated, allowing them to conclude that optogenetically modified interneurons are involved in the generation of this oscillation. What is really surprising, however, is that illuminating the brains of these animals with laser light modifies their behavior when faced with the smell of a cat, which, as we know at least since Tom and Jerry (1940) was broadcast, is one of the most dangerous predators for rodents. Mice whose interneurons are activated by laser light show riskier or more courageous behavior under these circumstances than normal mice.
These new data indicate that some pathological fear or anxiety states could be treated with specific drugs for these hippocampal interneurons. On a more humane side of the matter, studies may help explain why both animals and people show different degrees of bravery, or recklessness, in the face of obvious, life-threatening risks. That there may be neurons of courage or recklessness is curious, although it is not surprising, since all human abilities and properties, all emotions, all reasoning, depend on the functioning of those small and branched interconnected cells that we call neurons.
Reference: Sanja Mikulovic et al. Ventral hippocampal OLM cells control type 2 theta oscillations and response to predator odor. Nature Communications (2018)
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