Spike and burst coding in thalamocortical relay cells

Area of Science:

• Neuroscience
• Computational Neuroscience
• Computational Biology

Background:

• Mammalian thalamocortical relay (TCR) neurons exhibit two primary firing modes: tonic spiking and bursting.
• Understanding how these neurons encode information within their distinct firing patterns is crucial for comprehending neural processing.
• The influence of membrane voltage state on information coding in TCR neurons remains an active area of research.

Purpose of the Study:

• To investigate the specific input signal features represented by single spikes versus bursts in TCR neurons.
• To determine how the membrane voltage state of TCR neurons influences this input-output coding relationship.
• To elucidate the roles of subthreshold voltage-dependent currents (IT and Ih) in modulating firing regimes and information coding.

Main Methods:

• Combined experimental recordings in rat brain slices with a validated three-compartment computational model of TCR neurons.
• Injected identical frozen Gaussian noise current traces into both biological and model neurons.
• Analyzed membrane voltage and spike trains using coherence, impedance, Event-Triggered Average (ETA), and Event-Triggered Covariance (ETC) techniques.
• Calculated information content of different firing events and the overall signal.
• Investigated the impact of subthreshold currents (IT, Ih) and simulated high-conductance states.

Main Results:

• Single spikes selectively represent rapid fluctuations, while bursts encode integrated input signals, with feature selectivity extending up to 300 ms prior to the event.
• Bursts phase-lock and transfer information at lower frequencies compared to single spikes.
• Depolarization shifts TCR neurons from bursting to spiking, enhancing sensitivity to high-frequency fluctuations.
• The low-threshold activated calcium current (IT) and the h current (Ih) dynamics explain the state-dependent firing regimes and their concerted action.
• In high-conductance states, bursts are difficult to initiate due to T-type calcium current inactivation, but can be facilitated by strong or timed inhibition.

Conclusions:

• TCR neurons employ distinct coding strategies for single spikes and bursts, tailored to different aspects of the input signal.
• The interplay between IT and Ih currents is critical for regulating the firing mode and information processing capabilities of TCR neurons.
• Understanding these firing dynamics and the influence of synaptic states is essential for deciphering neural computations in the thalamocortical system.