Developmental plasticity of immature glutamatergic synapses in the mouse matosensory (barrel) cortex
Final Report Abstract
The postnatal development of the mammalian brain is characterized by the formation of complex, synaptically connected neuronal networks. In addition to massive synaptogenesis, the functional maturation of immature synapses is of major importance for brain function. In this project, we studied the functional properties of immature glutamatergic synapses in the early postnatal period by patchclamp recording in brain slices of the mouse somatosensory cortex. Although so-called silent synapses lacking postsynaptic AMPA-type glutamate receptors have been described previously, our knowledge about immature synapses is rather limited. In particular, the coupling of pre- and postsynaptic maturation processes has hardly been addressed. Using extracellular stimulation and patch-clamp recording from immature layer Vb pyramidal neurons at postnatal day 7, we found two types of AMPA receptor-expressing synapses: exhibiting either low or relatively high efficiency of transmitter release (release probability). During further postnatal development the synapses with very low release probability disappeared suggesting functional maturation. We also studied activity-induced long-term potentiation of these presynaptically immature synapses, which might contribute to activity-dependent developmental maturation. In addition to presynaptically immature synapses, we confirmed the presence of postsynaptically silent synapses lacking AMPA receptors. We also attempted to obtain paired recordings from both the pre- and the postsynaptic cells (immature layer Vb pyramidal neurons), because this would allow for a more controlled stimulation of presynaptic transmitter release. Unfortunately, most likely because paired recordings involve an extended search procedure with stimulation, it turned out to be extremely difficult to get stable recordings. This was mainly caused by a rapid run-down of postsynaptic responses upon repeated stimulation. We further characterized the subunit composition of postsynaptic NMDA-type glutamate receptors at immature synapses, because this is well known to drastically change with developmental synapse maturation. We found that both pre- and postsynaptically immature synapses lacked postsynaptic NR2A subunits, which were only found in more mature, high release probability synapses. These results indicate that there is a strong coupling between pre- and postsynaptic maturation processes leading ultimately to the formation of fully functional glutamatergic synapses. We also started to address the molecular mechanisms of pre- and postsynaptic coupling of developmental maturation by analysing the role of trans-synaptic signaling. Therefore, we transplanted genetically modified, immature pyramidal neurons (expressing green fluorescent protein) into wildtype cortical slice cultures, and studied their synapses by patch-clamp recording and presynaptic stimulation using photolysis of caged glutamate. Interestingly, in transplanted brainderived neurotrophic factor (BDNF) knockout neurons, we found no change in the formation of excitatory glutamatergic synaptic inputs, but unexpectedly a strong reduction of inhibitory GABAergic inputs. This finding will be further investigated in follow up studies.
Publications
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(2009). BDNF signaling in the formation, maturation and plasticity of glutamatergic and GABAergic synapses. Exp Brain Res 199: 203-234
Gottmann K, Mittmann T, Lessmann V
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(2009). Developmental alterations in the functional properties of excitatory neocortical synapses. J Physiol 587: 1889-1896
Feldmeyer D, Radnikow G
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(2010). Properties of glutamatergic synapses in immature layer Vb pyramidal neurons: coupling of pre- and postsynaptic maturational stages. Exp Brain Res 200: 169-182
Walz C, Elßner-Beyer B, Schubert D, Gottmann K
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(2012). Improved biocytin labeling and neuronal 3D reconstruction. Nat Protoc 7: 394-407
Marx M, Günter RH, Hucko W, Radnikow G, Feldmeyer D
