Fast burst firing and short-term synaptic plasticity: a model of neocortical chattering neurons.

We present an ionic conductance model of chattering neurons in the neocortex, which fire fast rhythmic bursts in the gamma frequency range (approximately 40 Hz) in response to stimulation [Gray C. M. and McCormick D. A. (1996) Science 274, 109-113]. The bursting mechanism involves a "ping-pong" interplay between soma-to-dendrite back propagation of action potentials and an afterdepolarization generated by a persistent dendritic Na+ current and a somatic Na+ window current. The oscillation period is primarily determined by a slowly inactivating K+ channel and passive membrane properties. The model behavior is compared quantitatively with the experimental data. It is shown that the cholinergic muscarinic receptor activation can transform the model cell's firing pattern from tonic spiking to rapid bursting, as a possible pathway for acetylcholine to promote 40-Hz oscillations in the visual cortex. To explore possible functions of fast burst firing in the neocortex, a hypothetical neural pair is simulated, where a chattering cell is presynaptic to an inhibitory interneuron via stochastic synapses. For this purpose, we use a synapse model endowed with a low release probability, short-term facilitation and vesicle depletion. This synapse model reproduces the behavior of certain neocortical pyramid-to-interneuron synapses [Thomson A. M. et al. (1993) Neuroscience 54, 347-360]. We showed that the burstiness of cell firing is required for the rhythmicity to be reliably transmitted to the postsynaptic cell via unreliable synapses, and that fast burst firing of chattering neurons can provide an exceptionally powerful drive for recruiting feedback inhibition in cortical circuits. From these results, we propose that the fast rhythmic burst firing of neocortical chattering neurons is generated by a calcium-independent ionic mechanism. Our simulation results on the neural pair highlight the importance of characterizing the short-term plasticity of the synaptic connections made by chattering cells, in order to understand their putative pacemaker role in synchronized gamma oscillations of the visual cortex.