Two-photon mapping of synaptic connectivity in the neocortex

• Applicant: Professor Dr. Jan Jasper Hirtz
• Subject Area: Cognitive, Systems and Behavioural Neurobiology ; Molecular Biology and Physiology of Neurons and Glial Cells
• Term: from 2012 to 2013
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 228457509

The organization of the synaptic microcircuits in the neocortex still remains poorly understood. While some studies suggest specific connections, others favor the idea that cortical connections are formed in a random, stochastic manner. Two-photon photostimulation has recently demonstrated that the inhibitory connectivity of interneurons on pyramidal neurons is promiscuous, with inhibitory neurons often contacting every neighboring cell. At the same time, it is unclear if excitatory connections are specific or not. In fact, the specific computations of the neocortex could be generated from specficic mechanisms at any one of three levels: i) specific structural connectivity between pyramidal neurons, ii) specific weight of synaptic strengths, or iii) specific postsynaptic integration. I propose to examine these possibilities, employing cutting-edge optical methods, including two-photon uncaging, optogenetics and spatial light modulator microscopy in combination with patch-clamp recordings in brain slices from genetically engineered mice. In a first part of the project, the network of potentially connected pyramidal neurons will be mapped, providing data about the spatial patterns of total connectivity and synaptic strength of connected neurons. To do so, 500-4000 pyramidal neurons will be subsequently activated while recording synaptic responses of a single pyramidal neuron. In a second part, I will address the question whether excitatory inputs to pyramidal neurons are integrated in a linear (unspecific) or nonlinear (specific) manner. Pyramidal neurons connected to a recorded neuron will first be separately activated, then simultaneously to test the linearity of their integration. The possible role of inhibition mediated by interneurons for the integration of excitatory inputs will also be examined by activating connected interneurons in addition to pyramidal neurons. These data will determine the degree of precision among excitatory connections in the cortex and contribute fundamentally to the understanding of how neuronal activity is generated by the neocortical microcircuits.

• DFG Programme: Research Fellowships
• International Connection: USA
• Host: Professor Rafael Yuste, Ph.D.