Neural and synaptic mechanisms of the gamma rhythmic activity in local cortical circuitry

Tomoki Fukai,1,3 Takeshi Takekawa,1 Masaki Nomura,3 and Toshio Aoyagi2

1Tamagawa University
2Kyoto University
3CREST, Japan Science and Technology (JST)

The activity of the brain engaging in cognitive process often exhibits rhythmic oscillations. In particular, synchronization of neuronal activity in the gamma band (20-70Hz) is considered to play significant functional roles in sensory perception, motor control and higher cognitive processes. Chattering neuron is a possible neocortical pacemaker for the gamma oscillation (Gray & McCormick, 1996). Based on our recent model of chattering neurons (Aoyagi et al., 2002), we investigate computationally how the gamma-frequency bursting is synchronized in small- and large-scale networks of chattering neurons. In a weak-coupling range, the synchronized bursting of chattering cells entrains activity of the regular spiking pyramidal cells in the gamma band. However, synchronization occurs only slowly with a transient time of about 500 ms. On the other hand, in a strong-coupling range, the chattering cells can quickly synchronize, but they do not entrain regular spiking cells. We find that both transient and steady state properties of synchronization are greatly improved by incorporation of short-term synaptic depression. In addition, we investigate the synchronization phenomena in networks of fast-spiking (FS) interneurons simultaneously interconnected by electric and GABAergic synapses. Our model of FS interneurons is based on the Kv3.1/3.2 type delayed potassium channels characteristic of these neurons. We find that, in a physiologically reasonable range of the intensity ratio between electric and GABAergic synapses, the network of FS interneurons shows bistability between synchronous and asynchronous firing states. How chattering neurons and fast-spiking interneurons together govern the coherence of local cortical activity is now under investigation.