Systems Neurobiology Laboratories, The Salk Institute
A comparison of functional layer-specific input to fast-spiking inhibitory and pyramidal neurons in layer 2/3 of rat visual cortex
The precise organization of excitatory and inhibitory connections in the cerebral cortex underlies information processing and shapes cortical response properties. Thus, knowledge of this circuitry is critical to understanding neuronal interactions that mediate cortical processing and can provide the foundation for theoretical models that predict and test patterns of activity in the cortex. Our current understanding of cortical laminar organization is limited mostly to excitatory connections. However, inhibitory neurons have dendritic and axonal structures that can be quite different from neighboring excitatory neurons, suggesting they are likely to follow different rules of connectivity. We investigated this question by mapping layer-specific input patterns to individual fast-spiking inhibitory and pyramidal neurons using laser-scanning photostimulation. Infrared DIC microscopy was used to target layer 2/3 neurons in rat visual cortex slices for whole-cell patch clamp recordings. Excitatory and inhibitory postsynaptic currents were measured in neurons voltage clamped at -65mV or -35mV respectively. Additionally, neurons were iontophoresced with biocytin and their morphologies reconstructed. The number and amplitude of excitatory and inhibitory postsynaptic currents were measured 150 ms after pseudo-random focal uncaging of glutamate in many small regions of each cortical layer. As expected from the anatomy, layer 2/3 pyramidal neurons received the majority of their excitatory input from layer 4. However, some also received strong excitatory input from layer 5A (a very narrow band at the top of layer 5). In contrast, layer 2/3 fast-spiking neurons received the majority of their excitatory input from layer 5B, a layer that provided weak or nonexistent input to layer 2/3 pyramids. Both cell types received the strongest inhibitory input from layer 2/3, although the inhibitory input to pyramidal neurons appeared more widespread. These results suggest that fast-spiking neurons are most strongly driven by a different population of cells than pyramidal neurons. In addition, neurons in layer 5B may be indirectly affecting how information is passed on to higher cortical areas by modulating the firing of layer 2/3 pyramids via inhibitory neurons in layer 2/3.