The specificity of connections in visual cortex explored with two-photon imaging and Patchstar whole-cell recordings

Ho Ko, Sonja B. Hofer, Bruno Pichler, Katherine A. Buchanan, P. Jesper Sjostrom & Thomas D. Mrsic-Flogel Functional specificity of local synaptic connections in neocortical networks. Nature, 5 May 2011/ Vol 473

Researchers at the University College London have combined in-vivo two-photon imaging with in-vitro whole cell recordings to expound our understanding of the intricate synaptic connections between neighbouring neurons in the mouse visual cortex. These connections, and many like them, are crucial to information processing throughout the animal brain, and therefore pose important questions for understanding fine-scale networks within the brain.

The team concentrated on the L2/3 layer of the mouse visual cortex where neurons with diverse preferences for sensory stimuli locally intermixed. They first carried out in-vivo two-photon imaging to sample somatic calcium signals in response to presentations of different visual stimuli, including 8 different orientations of drifting gratings and natural movie sequences. They imaged a complete population of L2/3 neurons by imaging layers at consecutive depths from the cortical surface.hey followed this with in-vitro whole-cell patch clamp recordings using the Patchstar micromanipulator, recording from up to four neighbouring pyramidal cells at once, at this stage they were blind to the functional identity of the neurons they were recording from. They determined connectivity between neurons by evoking action potentials in each neuron whilst recording membrane potentials in other neurons.

They were then able to accurately match the in-vivo image stack with the in-vitro stack using affine transformation.

They identified that connection probability is linked to shared responses to specific visual stimuli, put another way neurons which responded similarly to distinct orientations of visual stimuli (as identified in-vivo by two-photon imaging) were more likely to be connected to form neocortical networks. The same applied for neurons responding to the naturalistic stimuli. This is demonstrated statistically with the findings that "neurons with the same preference for orientated stimuli were connected at twice the rate of neurons with orthogonal orientation preferences".

These findings importantly highlight the great level of detail and reliability two-photon microscopy and patch-clamp recordings can provide us with, and the benefits of combining these techniques to map connection patterns between neighbouring neurons.