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The Journal of Neuroscience, February 1, 2001, 21(3):1022-1032

Dynamics of Low-Threshold Spike Activation in Relay Neurons of the Cat Lateral Geniculate Nucleus

Carolina Gutierrez¹, Charles L. Cox¹, John Rinzel², and S. Murray Sherman¹

¹ Department of Neurobiology, State University of New York, Stony Brook, New York 11794-5230, and ² Center for Neural Science and Courant Institute of Mathematical Sciences, New York University, New York, New York 10003

The low-threshold spike (LTS), generated by the transient Ca²⁺ current I_T, plays a pivotal role in thalamic relay cell responsiveness and thus in the nature of the thalamic relay. By injecting depolarizing current ramps at various rates to manipulate the slope of membrane depolarization (dV/dt), we found that an LTS occurred only if dV/dt exceeded a minimum value of ~5-12 mV/sec. We injected current ramps of variable dV/dt into relay cells that were sufficiently hyperpolarized to de-inactivate I_T completely. Higher values of dV/dt activated an LTS. However, lower values of dV/dt eventually led to tonic firing without ever activating an LTS; apparently, the inactivation of I_T proceeded before I_T could be recruited. Because the maximum rate of rise of the LTS decreased with slower activating ramps of injected current, we conclude that slower ramps allow increasing inactivation of I_T before the threshold for its activation gating is reached, and when the injected ramps have a sufficiently low dV/dt, the inactivation is severe enough to prevent activation of an LTS. In the presence of Cs⁺, we found that even the lowest dV/dt that we applied led to LTS activation, apparently because Cs⁺ reduced the K⁺ "leak" conductance and increased neuronal input resistance. Nonetheless, under normal conditions, our data suggest that there is neither significant window current (related to the overlap of the inactivation and activation curves for I_T), rhythmogenic properties, nor bistability properties for these neurons. Our theoretical results using a minimal model of LTS excitability in these neurons are consistent with the experimental observations and support our conclusions. We suggest that inputs activating very slow EPSPs (i.e., via metabotropic receptors) may be able to inactivate I_T without generating sizable I_T and a spurious burst of action potentials to cortex.

Key words: low-threshold spike; thalamus; burst firing; T channel; window current; lateral geniculate nucleus

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Copyright © 2001 Society for Neuroscience  0270-6474/01/2131022-11$05.00/0

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