When animals and humans learn, the neural circuits in the brain change, thus establishing a memory trace. This allows the individual to better control its behavior in the future. The difficulty in locating memory within the gridwork of many thousands of neurons is due to the fact that the neurons involved in learning cannot be directly observed while they create the memory trace. It is therefore advantageous to study a relatively simple nerve system, which is however still able to learn quickly and form multiple memory traces including that of a stable long-term memory. We investigate this topic using the honeybee. Bees learn landmarks in order to navigate within their territory. They associate the odors, colors, shapes and locations of flowers which offer nectar and pollen. They learn from each other when they perform the waggle dance to indicate the direction and the distance of rich feeding spots or a new nest. In doing so, their learning behavior is remarkably elaborate. They generalize on the basis of the common visual characteristics of visual patterns (e.g. symmetry or asymmetry), and they adjust their decision according to perceived signals or the situation in which they find themselves. Especially important for us as neuroscientists is the fact that bees can learn when optical and electrical recordings are being made in their brains. In these situations, an odor is used as a stimulus for a sugar reward, and the animals learns, just as Pavlov’s dog did, to expect the reward after experiencing the stimulus. This makes it possible to search for the locations where memory is formed and to measure the changes in the neuronal circuits. We can, for example, find that an odor that was learned as an important one has a stronger and more precise neuronal representation in the brain than other odors. These memory traces can be tracked all the way to the individual, identified neurons and their circuitry. This opens up the possibility of detecting the crucial elements which establish the memory trace in the bee’s brain.
A unique feature of memory is its dynamic aspect, a characteristic which the bee’s memory shares with the memories of humans and many animals. This means that a sensitive short-term memory phase follows the learning experience, in which memory can be easily changed or impaired. In the next phase, a mid-term memory can direct behavior for several hours after learning; the following phase is long-term memory, which, in bees, is divided into an early (1 - 2 days) and a late (more than 2 days) long-term memory. We found that the memory phases are directly related to specific reaction chains of signal molecules in the neurons involved in establishing that memory. Certain enyzmes (protein kinases) play a key role; their activation leads to a functional change in existing molecules and later to synthesis of new proteins, thereby creating new structures. The cellular reaction paths that become active here are nothing special; they are present in almost all the cells in the body. They are also not specific to the honeybee; they are found in the cellular mechanisms of the memory trace in other animals, from snails to humans. The memory content is not stored in certain special molecules, rather, it is retained in the pattern of the changes in the circuitry created by the common molecules. This principle of memory retention is also present in humans, therefore the bee brain can be used as a model system for studying the general mechanisms of memory formation. Observing the temporal dynamics of memory traces, we find vast differences among different animals. These dynamics seem to be coupled with the behavioral conditions in which memory is used. We can show that in bees the food-gathering cycle is closely aligned to the temporal dynamics of the memories used in foraging.
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