%0 Generic
%A Pelzer, Patric
%D 2016
%F heidok:20515
%K Piriform cortex, mediodorsal thalamus, giant driver synapse, class I synapse, cortico-thalamo-cortical loops, cortico-thalamic synapse, giant synapse, synaptic transmission
%R 10.11588/heidok.00020515
%T A Paleocortico-Thalamo-Cortical Circuit Operating Giant Synapses
%U https://archiv.ub.uni-heidelberg.de/volltextserver/20515/
%X The thalamus is a critical relay station in the pathway for sensory information to the cortex and additionally important for the intercortical information transfer. An exception is the olfactory system, as it does not require a thalamic relay step before the information reaches the cortex. Olfactory receptor neurons send axonal projections to the olfactory bulb, from where the information proceeds to the primary olfactory cortex. It is only from the piriform cortex (PIR) that the information is passed to the prefrontal cortex via direct projections and via the mediodorsal thalamus (MD). Aside the for a sensory system exceptional connectivity, does this circuit also stand out against other cortico-thalamo-cortical loops. The PIR belongs to the paleocortex instead of neocortex, the contributor to most other cortico-thalamo-cortical loops.  This situation raises the question if paleocorticothalamic projections have the same function as their subcorticothalamic or neocorticothalamic equivalents?  In this thesis, I utilize a highly precise spatiotemporal gene transfer system via adeno-associated viruses to label PIR synapses. In acute slice preparations these synapses may be excited individually by juxtapositioned near field simulation electrodes. Based on the kinetics of the evoked postsynaptic currents and immunohistological stainings, I propose glutamate as the principle neurotransmitter. The PIR-MD synapse displays short-term depression, as it has been shown for other thalamic afferences, classified as “drivers”.  In an electronmicroscopic preparation the complex dendritic interface of PIR-MD synapses becomes apparent. Often multiple dendritic excrescences invade the presynaptic profile. The presynaptic lumen is filled with vesicles and mitochondria. Altogether the morphology is that of a typical driver synapse in the thalamus.  Surprisingly, I found chemical synapses onto intermediate stretches of labeled axons, a constellation that has not been described in MD or elsewhere previously.  In summary, the results show that the olfactory brain circuit may have an additional level of complexity, imposed by axo-axonal contacts, and that PIR-MD synapses function like driver synapses in other transthalamic projections. However, as the term “driver” suggests that it always evokes postsynaptic action potentials, which is not true for the PIR-MD synapse, the recently proposed term “class I” synapse is adopted.