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Accueil du site → Doctorat → Allemagne → 2011 → Synaptic plasticity in visual and olfactory brain centers of the desert ant Cataglyphis

Bayerischen Julius-Mawimilians-Universität Würzburg (2011)

Synaptic plasticity in visual and olfactory brain centers of the desert ant Cataglyphis

Stieb Sara Mae

Titre : Synaptic plasticity in visual and olfactory brain centers of the desert ant Cataglyphis

Synaptische Plastizität visueller und olfaktorischer Gehirnzentren der Wüstenameise Cataglyphis

Auteur : Stieb Sara Mae

Université de soutenance : Bayerischen Julius-Mawimilians-Universität Würzburg

Grade : Doctoral Thesis 2011

Résumé partiel
Desert ants of the genus Cataglyphis have become model systems for the study of insect navigation. An age-related polyethism subdivides their colonies into interior workers and short-lived light-exposed foragers. While foraging in featureless and cluttered terrain over distances up to several hundred meters, the ants are able to precisely return back to their often inconspicuous nest entrance. They accomplish this enormous navigational performance by using a path integration system - including a polarization compass and an odometer - as their main navigational means in addition to landmark-dependent orientation and olfactory cues. C. fortis, being the focus of the present thesis, is endemic to the salt flats of western North Africa, which are completely avoided by other Cataglyphis species. The fact that Cataglyphis ants undergo a behavioral transition associated with drastically changing sensory demands makes these ants particularly interesting for studying synaptic plasticity in visual and olfactory brain centers. This thesis focuses on plastic changes in the mushroom bodies (MBs) - sensory integration centers supposed to be involved in learning and memory presumably including landmark learning - and in synaptic complexes belonging to the lateral accessory lobe (LAL) known to be a relay station in the polarization processing pathway. To investigate structural synaptic plasticity in the MBs of C. fortis, synaptic complexes (microglomeruli, MG) in the visual (collar) and olfactory (lip) input regions of the MB calyx were immunolabeled and their pre- and postsynaptic profiles were quantified. The results show that a volume increase of the MB calyx during behavioral transition is associated with a decrease of MG number - an effect called pruning - in the collar and, less pronounced, in the lip that goes along with dendritic expansion in MB intrinsic Kenyon cells. Light-exposure of dark-reared ants of different age classes revealed similar effects and dark-reared ants age-matched to foragers had MG numbers comparable to those of interior workers. The results indicate that the enormous structural synaptic plasticity of the MB calyx collar is primarily driven by visual experience rather than by an internal program. Ants aged artificially for up to one year expressed a similar plasticity indicating that the system remains flexible over the entire life-span. To investigate whether light-induced synaptic reorganization is reversible, experienced foragers were transferred back to darkness with the result that their MBs exhibit only some reverse-type characteristics, in particular differences in presynaptic synapsin expression. To investigate the structure of large synaptic complexes in the LAL of C. fortis and to detect potential structural changes, pre- and postsynaptic profiles in interior workers and foragers were immunolabeled and quantified by using confocal imaging and 3D-reconstruction. The results show that these complexes consist of postsynaptic processes located in a central region that is surrounded by a cup-like presynaptic profile. Tracer injections identified input and output tracts of the LAL : projection neurons from the anterior optic tubercle build connections with neurons projecting to the central complex. The behavioral transition is associated with an increase by 13% of synaptic complexes suggesting that the polarization pathway may undergo some sort of calibration process. The structural features of these synaptic contacts indicate that they may serve a fast and reliable signal transmission in the polarization vision pathway.


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