How does the human brain store information? Neuronal circuits in a premotor area of zebrafish brains can store information for a number of seconds by maintaining a persistent level of activity. This is how such cells memorise eye positions. Dr. Aristides Arrenberg from the University of Freiburg did his doctorate in the USA where he further developed a method that can be used to silence or activate specific zebrafish nerve cells at will. The method is based on light stimuli that trigger cellular switches. Working with colleagues, Arrenberg has also developed a potential light-controlled alternative to electrical cardiac pacemakers. In addition, the researchers have recently published an article on research into how eye movements are stored in the hindbrain. Standard network models used for so-called integrator circuits now need to be reconsidered.
The method used by Arrenberg and his colleagues to address molecular switches in cells with light pulses is known as optogenetics. The method is based on light-gated ion channels of microorganisms which the scientists can genetically transfer into excitable vertebrate cells such as neurons. Triggered by specific light pulses, the ion channels open or close, ions then flow across the membrane, the membrane potential changes and a nerve cell produces a short-lived succession of action potentials that reaches its neighbours. A neuronal circuit can thus alter its activity, and what’s more, it can do so “at the push of a button”. Dr. Artistides Arrenberg originally studied biochemistry, did his doctorate at the University of San Francisco where he established special optogenetic tools for investigating neuronal circuits of zebrafish and he now works in the Department of Development Biology at the Institute of Biology I at the University of Freiburg. “I am interested in the function of neuronal circuits,” said Arrenberg. “The optogenetic tools enable us to specifically interfere with and investigate such circuits.”
The light-controlled molecular switches can be switched on and off within 10 milliseconds. This is quite revolutionary compared to the time it takes for pharmaceuticals to do the same thing. In addition, the genetic manipulation can be restricted to specific cell types, thereby enabling the targeted manipulation of specific cells of a neural network. Sophisticated microscopic methods can be used to shape a beam of light so that only certain cells are activated. Optical fibres can be used to pinpoint and activate distinct areas with a diameter of 50 micrometers. Zebrafish embryos are transparent and therefore suitable for microscopic investigation and they also display behaviour that can be investigated at an early stage of development. Arrenberg and his colleagues from San Francisco therefore used zebrafish to investigate the development of cardiac pacemaker cells. These cell types are characterized by rhythmic activity and are responsible for regular heartbeat; they tend to lose their function as people get older or after they have suffered a heart attack and therefore they need to be supported by electrical pacemakers. “We used light to specifically control the heart rate in zebrafish,” said Arrenberg explaining that one of the many advantages of using optogenetic pacemakers rather than electrical ones is the ability to limit the development of toxic gases associated with longer pulses. However, it is unlikely that optical pacemakers will become a clinical reality due to the widespread use of electrical pacemakers. However, the researchers’ results show that the method is a promising tool for answering as yet unanswered questions in the biological sciences. In their most recent study, published in the renowned journal Nature Neuroscience, Arrenberg and his US collaboration partners used the method to investigate the mechanisms that store information in neuronal circuits in the hindbrain of zebrafish. These circuits act as models of short-term memory in humans.
In zebrafish and humans, the eyes can make quick jerky movements when changing focus from one point to another (referred to as “saccade”). A saccade is triggered by a population of nerve cells in the hindbrain of zebrafish which sends impulses to motor neurons, thereby indirectly activating the eye muscles. After every saccade, the eyes remain in their new position for several seconds. A specific eye position is maintained by a second circuit, which is referred to as a neural integrator for eye movements. This integrator receives its input from the same cells that are connected to the motor neurons. It is not yet known why the cells of the integrator circuit are able to keep up their electrical activity for several seconds. The electrical activity leads to the creation of a position signal that is then transmitted to the motor neurons, and the eyes remain in the new position until a new movement is triggered. The question is, how can the cells of the integrator store activity?Previous models worked on the assumption that all integrator cells store their activity for the same amount of time. However, the optogenetic manipulation of individual cell groups in the region provided Arrenberg and his collaborators with information that proved exactly the opposite, namely that the cells are an inhomogeneous cell population. “The cells in the circuit have different temporal constants and we could map a gradient in persistence time,” said the neurobiologist. “In contrast to current thinking, the circuits in the integrator generate a wide diversity of time constants.” In addition, the researchers found out that cells with similar time constants are located in the same area. The integrator therefore appears to have a highly ordered spatial structure. Inside the hindbrain, the cells located towards the spinal marrow are arranged behind each other in relation to the increasing length of storage capacity. “We hypothesise that the cells transfer their activity within the hindbrain in the direction of the spinal marrow from one cell to another and that the cells in the vicinity of the spinal marrow are able to store the activity for the longest period of time and send the signal to the motor neurons,” said Arrenberg. It is not yet clear whether this so-called feed-forward pattern actually exists. However, it is clear that previous assumptions need to be reconsidered.
Arrenberg initially carried out his investigations using optical fibres, which, as mentioned above, are able to silence and activate brain areas with a diameter of around 50 micrometres. In order to be able to manipulate smaller regions comprising around ten cells, the neurobiologist is planning to use a special microscope in Prof. Dr. Wolfgang Driever’s laboratory in the Department of Developmental Biology at the Institute of Biology I. “We hope that future experiments will help us to shed light on how the spatial order of eye movements develops in the integrator,” said Arrenberg going on to add “we are aiming to obtain in-depth insights by investigating step by step how the circuit pattern between the individual cells is created.” In cooperation with theoretical neuroscientists, the researchers from Freiburg also plan to develop new models that are better suited to representing the reality in this zebrafish brain region. The researchers also hope that the models will help them obtain a better understanding of the function of similar circuits in the human prefrontal cortex, the region where our working memory is located.
Further information:Dr. Aristides ArrenbergUniversity of FreiburgInstitute of Biology IFreiburgTel.: +49 (0)761/203-2581E-mail: Aristides.Arrenberg(at)biologie.uni-freiburg.de