Trends in Neurosciences
Genetics moving to neuronal networks
Introduction
To understand the human brain, with its billions of neurons and connections, is a daunting task. Nevertheless, many laboratories around the world are using a multitude of approaches to decipher what is one of the great remaining mysteries. To even begin to understand something as complicated as the brain, it is necessary to simplify and find systems in which meaningful experiments can be done. Central pattern generators (CPGs), which are ensembles of neurons capable of generating a coordinated rhythmic output without external feedback [1], provide a useful system for such work: they are simpler than the brain but more complex than sensory feedback loops.
Charles Sherrington and Graham Brown made important initial discoveries regarding locomotor CPGs. In 1910 and 1913, Sherrington reported that the spinal cord contains enough reflex circuitry to produce alternate extension and flexion during hindlimb movements in cat 2, 3. Brown provided important evidence in 1911 that rhythmic alteration of limb movements can be produced without any central or peripheral incoming rhythmic activity, suggesting that the spinal cord contains an endogenous rhythm generator [4]. Also using the cat spinal cord as a model system, Lundberg, Jankowska and colleagues examined lumbar reflexes and showed that interneurons involved in the flexion reflex can also be part of the locomotor CPG [5]. Since then, studies of CPGs have included neuronal circuits involved in heartbeat, breathing, chewing, vomiting, swimming, crawling, walking and flying in such diverse organisms as worms, grasshoppers, leeches, fruit flies, lampreys, lobsters, crabs, crayfish, sea slugs, pond snails, frogs, zebrafish, mice, rats and humans.
These studies have taken advantage of invertebrate and vertebrate neuronal networks to couple functionality to neuronal populations. This has mainly been done using electrophysiological techniques (Box 1). The introduction of genetic techniques has now begun to reveal identities, development, connections and functions of CPG network neurons. Furthermore, it is now feasible to study groups of neurons, rather than single neurons, as long as they are genetically identifiable. Genetic screens can also be applied to find genes that are crucial for the development and function of neuronal networks (Table 1). This review will focus on recent results from the locomotor CPG in two invertebrate species (the nematode worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster) and two vertebrate species (the zebrafish Danio rerio and the mouse Mus musculus), which currently dominate genetic studies.
Section snippets
Caenorhabditis elegans
Under laboratory conditions, C. elegans mainly move forwards, interrupted by brief sessions of backward movements [6]. A study of the central locomotor network controlling backward and forward movements in C. elegans focused on five interneurons located in each side of the worm (Figure 1a) [7]. The connectivity pattern of all C. elegans neurons has been determined using serial electron microscopy [8], and the connections between these five interneurons can therefore be drawn schematically (
Drosophila melanogaster
That sensory feedback is vital for modulation of locomotor output is well known, but it has been unclear whether sensory feedback is also necessary for development of the CPG. Suster and Bate blocked neurotransmitter release from sensory neurons in Drosophila larvae by expressing the tetanus toxin light chain (TeTxLc) exclusively in the peripheral nervous system [18]. The larval CPGs were still functional, as demonstrated by the ability of the larvae to hatch and move over a substrate. The
Danio rerio
Interneurons in the zebrafish embryonic and larval spinal cord have been classified according to their locations, projections and morphologies 27, 28. In addition, a survey of the neurotransmitter properties of spinal interneurons was recently conducted in embryonic and larval zebrafish, providing a framework for future studies of specific neuronal function [29] (Table 2). In an effort to couple gene expression to interneuron identity, Higashijima and co-workers investigated the nature of
Mus musculus
The location of the locomotor CPG in neonatal rat and mouse has been determined using systematic progressive removal or isolation of spinal cord tissue in fictive locomotion preparations. Several studies demonstrate that the main activity of the CPG resides from the lower thoracic (Th11) to lower lumbar (L6) levels, with a concentration of activity in levels Th11 to L2 (reviewed in Ref. [42]). Removal of the dorsal part of the spinal cord does not affect CPG activity, suggesting that the
Future directions
Fundamental questions regarding the function and development of CPGs remain to be answered. How does the wiring diagram develop? What is the smallest possible unit containing all individual parts required to generate a rhythmic and coordinated locomotor output? One outstanding issue is to identify genetically the participating neuronal populations. To then couple these identities to functionality will be another important step towards a thorough understanding of the CPG network. As discussed in
Acknowledgement
I thank Håkan Aldskogius, Patrik Andersson, Catherina Becker, Henrik Bengtsson, Ted Ebendal, Finn Hallböök, Magnus Lind, Marc Pilon, Helgi Schiöth and Anne Uv for critical reading of the manuscript. The Swedish Research Council, the Knut and Alice Wallenberg Foundation, the Magnus Bergvall Foundation, the Åke Wiberg foundation and Uppsala University support work in my laboratory.
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