Behavioural Physiological and Molecular Mechanisms of Phase Change in Locusts

Many animals undergo profound changes in form and function in response to fluctuating environmental conditions, of which the Desert Locust is a notorious example. It can change reversibly from a cryptic solitary living form to a swarming form that occasionally aggregates in vast numbers to devastating effect. These two forms, the solitarious and gregarious phases, differ considerably in appearance, physiology and behaviour. Since a major goal of Neuroscience is to understand how changes in behaviour are underpinned by modifications of the nervous system, phase change in locusts is a powerful model system in which to pursue this goal. Our aims in this research proposal are three-fold. The first is to understand how signals provided by other locusts act on the central nervous system of solitarious locusts and start to modify the connections between nerve cells so that a previously solitarious locust will behave like a gregarious locust within 4 hours of first contact. We already know that touch signals provided by other locusts drive behavioural gregarization and that these signals trigger the release of two substances in the central nervous system. We wish to know how touch stimuli cause these substances, serotonin and nitric oxide, to be released, which nerve cells they are released from and on which target nerve cells they act. Most importantly, what biochemical reactions do they trigger in these target cells to so profoundly change the locust's behaviour? To find out we will study the changes in brain chemistry that occur as locusts change phase. Can drugs that prevent these changes also prevent a locust from turning gregarious? We also need to ask whether such biochemical processes suffice to switch the behaviour or whether it is necessary to switch genes on or off. Our second aim is to understand how differences in the properties of nerves cells and the connections between them lead to altered behaviours in solitarious and gregarious locusts. To do this we will exploit the fact that insects have many large and identifiable neurons that can be recorded from in many individuals. We have focussed on an identified visual nerve cell that responds to objects on collision course with the locust. We will analyse the connections of this cell to motor nerve cells that control the wings and hind leg and determine whether they are modified in the same way during phase change or if they are independently adjusted to suit particular behaviours. Are the strengths of connections made by another nerve cell that detects wing movements modified in a similar way as the visual nerve cell? Gregarious locusts fly by day and solitarious locusts mostly at night. We wish to know how the responses of the visual system are adjusted to the day- and night-time activities of the two phases and whether an internal clock in the central nervous system changes the sensitivity of the eyes and visual interneurones in anticipation of the onset of daylight or dusk. Our third aim is to analyse the ageing rate of solitarious and gregarious locusts and how this affects the function of nerve cells. Solitarious locusts live 45% longer as adults than gregarious allowing us to manipulate the rate of ageing of locusts by changing their phase. Living cells accumulate the breakdown products of ageing-related damage, called lipofuscin, into granules that can be seen under a microscope allowing us to measure the rate of ageing in individual nerve cells. Furthermore, nitric oxide, one of the substances that are produced in abundance during phase change causes ageing-type damage to cells. We will determine whether the process of phase-change itself causes accelerated ageing over and above that expected from a locust already being in a gregarious phase. We will record from the same identified motor nerve cells detailed above to analyse how ageing changes the way in which a nerve cell responds to incoming signals and communicates with other nerve cells.

Phase change in locusts provides an exceptionally tractable model with which to analyse how neuronal plasticity shapes behaviour to meet changes in the environment. Repeated mechanosensory input to the hind legs causes solitarious locusts to express gregarious behaviour. We use a multi-faceted approach to address 3 interrelated questions. 1) What are the neuronal and molecular mechanisms that underlie the switch? Our data indicate causal roles for 5HT and NO, which mediate plasticity via conserved signalling cascades. We will monitor molecular changes in the CNS during phase transitions, moving from 2nd messengers to protein kinases to transcription factors, confirming causality at each level by drug or dsRNA interference and behavioural assay. Immunostaining will reveal the neurons that generate 5HT and NO during gregarisation, and their target cells. Circuit analysis will uncover how the gregarizing signal is extracted from the patterned input and elucidate the plastic changes in network function that ensue. 2) Are synaptic changes tailored to prepare solitarious and gregarious locusts for their different life styles? Do 2 outputs from one neuron show different phase-related plasticity if they are involved in different behaviours, and likewise are 2 inputs onto one neuron differentially modified? We will address this using 4 identified neurons: 2 sensory neurons of different modality and 2 motor neurons of distinct function (jumping, flight). We will demonstrate how differences in visual processing relate to phase-specific differences in visual ecology. 3) Is there a relationship between NO-mediated oxidative stresses, neural senescence as measured by lipofuscin accumulation, deteriorating performance of identified synaptic connections and lifespan? Solitarious locusts have a 45% longer adult life than gregarious locusts. We will modify longevity experimentally by manipulating phase state or the pathways underlying the transition to address these questions.