Research has revealed the cluster of neurons that helps the brain's internal GPS remember key landmarks. It is hoped that the findings will provide insight into a range of psychiatric disorders.
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It may be more efficient in some people than in others, but we all have a built-in sense of direction that enables us to navigate the world and imagine a location to arrange a meeting point. In order to achieve this, we develop maps in our head labeled with important locations.
It has been shown that there are specific cells in the hippocampus that give us this ability. Some cells are tuned to location, whereas other record the direction they are facing.
However, although it was clear that there is specialised brain circuitry supporting a sense of direction, the details of these neural circuits were not fully understood
Researchers at Columbia University investigated the circuitry of the brain's learning centre using mice studies to help understand how such flexibility in memory formation was possible.
Memories are fluid, not fixed, but observing precisely how brain cells, or neurons, flexibly lay down or recall memories has long proven challenging to scientists...With today's study we provide, for the first time, visual evidence that a particular type of neurons makes this flexibility possible".
Attila Losonczy, Principal Investigator
It was reported in 2016 that neurons in the CA1 area of the hippocampus encode an animal's location. When a mouse looked for something, like water, neural activity in this area spiked as the animal got close.
The current investigation thus focussed on CA1 to uncover what was directing this spike in activity. In particular, it looked at the activity of inhibitory neurons called vasoactive intestinal polypeptide-expressing (VIP) cells.
The team monitored the VIP-cell activity in mice using a two-photon microscope as they ran on treadmills.
The mice were exposed to a range of sights and sounds as they ran, some of which were familiar and some of which were new. The experiment was repeated, but the mice had the task of finding a water reward that had been placed at a specific, unmarked location along the treadmill's path.
Spikes in VIP-cell activity were observed during both experiments. Further experiments and computational modelling revealed that the VIP cells heavily influenced CA1 neural activity.
CA1 excitatory neurons are usually suppressed by a cluster of neighbouring inhibitory neurons. After learning, the VIP cells, a second cluster of inhibitory neurons, are activated and stop the first cluster of inhibitory neurons from suppressing the activity of the excitatory neurons. This chain reaction activates the entire CA1 memory circuit, whereby allowing the animal to learn.
VIP cells have been classed as disinhibitory neurons as they inhibit inhibitory neurons. Disinhibition provides an ingenious way to achieve an excitatory response.
It illustrates the delicate and fine-tuned nature of learning. This complex feedback system enables the added flexibility needed for the intricate process of memory, providing a subtle way for memory adjustment during learning.
These findings highlight the flexibility of neuronal circuits that is required to navigate an environment.
It is hoped that these latest findings will provide greater insight into psychiatric disorders, such as autism and schizophrenia, which are often characterized by disruptions to this flexibility.
Normal brain function requires a delicate balance of excitation and inhibition, but many neuropsychiatric disorders, such as schizophrenia and autism, are characterized by an imbalance in this type of neural activity...Our work, and the work of others, could very well help to elucidate how these disruptions result in these disorders' devastating symptoms."
Attila Losonczy, Principal Investigator