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My lab:
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I did manage to get a network connection in one of the lecture halls! W00t!

This symposium was all about habituation. The first speaker was Cathy Rankin from University of British Columbia, working on C. elegans. She uses a multi-worm tracker to do high-throughput screening for defects in habituation. These worms will reverse their direction of locomotion to a tap of the petri-dish where they move around. Repeated taps of the dish lead to a habituation of thsi reversal behavior. She compared a pool of 2072 genes with putative nervous system function with the list of known mutants and found 700 available strains. She tested those and found many interesting strains. For instance, two strains habituate more rapidly than wildtype. These mutants were already published so they can serve as controls. She also found strains which are habituating more slowly. In fact, they found almost every deviattion from wildtype in both directions one can think of. They called it the 'strainbow of habituation'. In total, almost 300 strains showed some form of significant deviation from wildtype.
Cathy then continued to describe some of the most interestig mutant strains. For example, she showed lines where variables of habituation (e.g., initial response, response probability, response magnitude, slope, linearity, etc.) which are usually correlated aren't correlated any more. Their lab now continues to characterize some of the strains with the most extreme phenotypes.

Next up was David Glanzman from UCLA, about whom I have blogged here repeatedly. He began by summarizing some older data on the habituation of the gill withdrawal reflex of the marine snail Aplysia. Repeated stimulations of the siphon leads to habituation of the gill-withdrawal reflex. In the 1970s, Carew at al. demonstrated long-term habituation lasting almost one month. Synaptic analysis at the time suggested that a presynaptic mechanism of synaptic depression of the sensory-motor synapse was responsible for habituation. David went on to show that indeed there are other mechanisms involved in habituation in Aplysia, also postsynaptic ones. For instance, he showed that habituation depends on NMDA receptors. Other evidence includes AMPA receptors or the failure of presynaptic depression to induce habituation. In total, there is no agreed upon main mechanism for long term habituation in Aplysia today.

Third speaker in this symposium was Daniel Tomsic from Buenos Aires, working on the crab Chasmagnatus granulatus. In their natural habitat, these animals are predated on by sea-gulls and react to overhead objects with an escape response. Repeated presentations of overhead objects that are not followed by an attack lead to an habituation of the escape response. Using spaced (but not massed or continuous) training, they can get long term habituation lasting more than 5 days. Interestingly, this memory is contect-dependent, i.e., when they test the spaced (but not the massed) trained animals in a different experimental situation with an overhead object, they respond as well as naive animals. Similarly, protein synthesis is engaged by spaced, but not by massed training. Electro-physiologically, they characterized giant motion-sensitive neurons in the third optical neuropil, the lobula, the responses of which to the overhead stimulus matched the behavioral response of the animal quite well. These LG neurons showed less spikes to the stimulus after massed habituation training, which quickly recovered. After spaced training, the reduction in spike frequency in LG neurons was slower and less dramatic, but longer lasting. Interestingly, this 'memory' in the LG neurons is not context-specific, i.e., the associative component of this form of habituation is located elsewhere, downstream of the lobula.

Susanne Schmid from Ontario concluded this session with her talk about habituation in rodents. She works on the acoustic startle response in rats where an auditory stimulus startles the animal. Repeated stimulation leads to habituation, such that the animals eventually cease to startle. The startle pathway comprises a set of three synapses from sensory to motor neurons and short term habituation is intrinsic to this pathway. In slice preparations including the startle pathway they can monitor the crucial synapse where the memory is formed by using electric stimulations to mimic the startling noise. In parallel to the electrophysiological animals, they use pharmacology to manipulazte intact animals during behavioral experiments. The found synaptic depression as a major mechanism of habituation in rats. These synapses are glutamatergic and the depression is based on a presynaptic mechanism and does not rely on postsynaptic gluatame receptors. Building on genetic data from Cathy Rankin's C. elegans, they tested for the involvement of so-called BK channels using BK-channel knockout mice and found that indeed they do not habituate. They also targeted these channels with a BK-channel blocker in rats and found that the treatment blocked habituation. They hypotheize that these channels mediate short-term habituation by shutting down synaptic release by a calcium-dependent potassium current through these channels.
Posted on Wednesday 04 August 2010 - 21:28:38 comment: 0
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