Molecular mechanisms underlying synchronous activity in the central nervous system
A number of compelling studies and computational simulations data provide evidence that networks of inhibitory neurones play a pivotal role in the generation of certain forms of oscillations that arise as a network property. Oscillatory activity in different frequency ranges has been proposed to be critical for a number of cognitive tasks, including object representation, learning and memory. The goal of our studies is to identify ‘key’ molecules in GABAergic interneurones that underlie oscillatory activity and that are involved in controlling synchronous firing of ensembles. Our studies entail analysis from the single cell level to the network activity in vitro (acute slice preparation) and in vivo. Genetically modified mice with altered expression of critical genes (e.g. AMPA receptors, NMDA receptors, connexins) in GABAergic interneurones are a critical tool for subsequent electrophysiological studies to investigate synchronous network activity in cortex and hippocampus in the freely moving mice. These investigations are further complemented by behavioural studies.
Characterization of different interneuronal populations
Given the large diversity of GABAergic interneurones (based on the presence of certain parameters, for instance chemical markers, morphological criteria, connectivity), present projects aim at the identification of GABAergic subpopulations of neurones (e.g. parvalbumin-, somatostatin-, calretininpositive cells). To this end transgenic mice are produced in which these neuronal subpopulations are labelled using the in vivo marker green fluorescent protein. The subsequent electrophysiological studies on fluorescent neurones in these mice should aid in identifying the GABAergic cell types involved in different forms of network oscillations. In transgenic mice that express EGFP in a particular subset of GABAergic interneurones, we have found that unlike the vast majority of GABAergic interneurones that are generated embryonically, certain GABAergic interneurones continue to be generated after birth. Postnatal neurogenesis of GABAergic interneurones that migrate into the cortex and hippocampus is a novel form of plasticity. The analysis is based on anatomical and electrophysiological techniques as well as imaging studies of migrating neurones. Furthermore, these mice are used for gene expression analysis in EGFP-labelled neuronal populations by means of laser dissection microscopy.
A: Example of an in vivo electrophysiological hippocampal recording in transgenic mice.
B: Example of behavioral analysis of transgenic mice.
NMDA receptors neuronal plasticity and vulnerability
The NMDA receptor, a subtype of the glutamate receptor family, is critical for the induction of different forms of plastic changes in the brain. Anatomical and functional characterization of NMDA receptors subtypes has revealed that pyramidal neurones co-express the two NMDA receptor subtypes NR2A and NR2B. The differential developmental regulation of these two NMDA receptor subtype expression with respect to brain areas and cell types exerts an important function in the change of neuronal plasticity during brain maturation.
Projects pertaining to this research programme aim at cell type-specific ablation of the NR2B subunit to study the function of the ‘young’ receptor form in the adult brain. The analysis of the genetically modified mice is performed using electrophysiological and behavioural studies.
AMPA receptor interacting proteins and synaptic plasticity
AMPA receptors, another subtypes of the glutamate receptor family, mediate most of the fast excitatory transmission in the vertebrate central nervous system. They are critical in determining the strength of transmission at glutamatergic synapses, and tight regulation of their function allows for use-dependent and input-specific adaptations of synaptic strength. Their function is influenced by composition, posttranslational modifications and by protein-protein interactions. We have identified a novel brain specific AMPA receptor interacting protein (AIP47) by mass spectrometry of AMPA receptor complexes and we are currently studying the role of this protein in regulating AMPA-receptor mediated synaptic transmission and plasticity of glutamatergic synapses.
A: Schematic drawing of neuroblasts migrating from different migratory stream to the cortex and the olfactory bulb.
B: Picture of fluorescent neuroblasts in the dorsal migratory stream of the 5HT3-EGFP mouse.