Computational Analysis of Modulatory Function in Small Motor Networks in Insects
The biogenic amine octopamine is a neuromodulatory transmitter in insects that plays an important role in a variety of behavioral contexts (e.g. Mentel et al, 2003). We address the functional role of octopaminergic neurons (ONs) in the locust motor system combining experimental and modeling approaches. A well described network of motor neurons reliably generates rhythmic locomotory patterns and a parallel small-sized network of ONs connects to this motor network, the connectivity of which has been described in detail (Field et al., 2008). Amongst the latter, octopaminergic unpaired median neurons, with either dorsal or ventral cell bodies (DUM or VUM neurons, respectively) are clustered along the midline of the thoracic and abdominal ganglia. Both networks receive 'top-down' input from central areas of the suboesophageal ganglion and the brain, and are activated in parallel with high synchrony (Burrows & Pflüger, 1995; Duch & Pflüger, 1999). Previous results (Bräunig & Pflüger, 2001) suggest that the ON network modulates synaptic efficacy of neuromuscular synapses and of excitatory synapses onto inter- and motor neurons, providing a mechanism for gating and/or gain control that specifically acts during each cycle. In our project we study the functional integration of both these networks and the specific role of neuromodulation in in vivo experiments and in realistic network simulations.
A unique feature of octopaminergic DUM neurons is the generation of action potentials in the soma (active soma spikes). Na+-, Ca2+-, and K+-currents as well as a hyperpolarization-activated current were characterized with respect to their activation/inactivation properties. The overshooting soma-action potentials are carried by sodium and calcium, whereas repolarisation is caused by K+-currents. An opposing hyperpolarization-activated current contributes to maintaining the resting potential and induces “rebound-behavior” after phases of inhibition. (Heidel & Pflüger, 2006; Ryglewski et al., 2007).
To extend these studies we are now tracking DUM neuron soma spiking using calcium-sensitive fluorescent dyes enabling us to record Ca2+ levels optophysiologically from the whole population of metathoracic DUM-neurons during motor activity. Calcium probes will be transferred into the cell bodies by bath application or neuronal back fillings via nerve N3, N4 and/or N5. Calcium imaging techniques allow the translation of changing calcium levels into a shift of fluorescence emission of the fluorescent dye.
In particular we want to compare the default pattern of the system under reduced sensory input with activation pattern during induced flight and walking behavior. Such data would allow an inside in the functionality and connectivity within the ensemble of DUM neurons and their connectivity to other parts of the neuronal network. Additionally, we will use computational network models to study (1) the effect of common vs. separate control of ONs and motor networks, (2) the effect of phasic or gradually phasic-tonic neuro-modulatory release on network output variability (Nawrot et al., 2008) and motor precision, and (3) the role of sensory feedback control acting onto motor and ON networks.
This research project is funded by the Federal Ministry of Education and Research (BMBF) through grant 01GQ1001D to the Bernstein Center for Computational Neuroscience Berlin.
Dr. Dorothea Eisenhardt (Free University of Berlin, Germany)
Dr. Björn Brembs (Dr. Julien Colomb; Christine Damrau - Free University of Berlin, Germany)
Behavioral experiments in bees and fruit flies suggest that the response to sugars is under aminergic control. On the other hand, the reward signal was suggested to go via octopaminergic neurons in the bee brain. Using genetically encoded calcium sensors, we will investigate the activity of octopaminergic neurons during sugar application to the fly proboscis, and whether this putative activity is modified by starvation state. Preliminary work using a pan-neuronal expression of the sensor allowed us to measure neuronal activity in the antennal lobe and the mushroom bodies.
Prof. Dr. Martin Giurfa (Université Paul Sabatier, Toulouse, France ; Dr. Jean-Christophe Sandoz, CNRS, Gif sur Yvette, France)
“Stimulus dominance in compound processing: uncovering the mechanisms of olfactory overshadowing in honeybees”
The overshadowing effect has long attracted attention of researchers interested in the learning and processing of sensory compounds. It occurs when a subject trained with a binary compound of two stimuli responds significantly more to one component at the expense of the other. Within-mixture interactions between components as well as differential, independent processing of compound components have been proposed as underlying mechanisms. We study overshadowing in honeybees trained to associate odor mixtures with sucrose solution.
Dr. Silke Sachse (MPI for Chemical Ecology, Jena, Germany; Dr. Jürgen Krieger - University of Hohenheim, Germany)
“Reception and coding of pheromone signals in insects”
Olfactory coding in insects is based on widely unknown aspects of molecular signal recognition in the antenna and primary processing events in the antennal lobe. Focusing on the pheromone signaling system of Heliothis virescens, the project aims on elucidating early events in pheromone detection and coding. Since certain odorants enhance the pheromone response of the sensory cells in moth, we will assess if a contribution of binding proteins or receptors may underlie this peripheral synergism. In order to approach the question if there may also be a central interplay between the pheromone and odor responses, in vivo imaging experiments will be performed monitoring the activity in the pheromone-processing macroglomerular complex (MGC) and odorant-processing glomeruli.