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All thalamocortical neurones possess a T-type Ca²⁺ 'window' current that enables the expression of bistability-mediated activities

Stuart W. Hughes, David W. Cope, Tibor I. Tóth, Stephen R. Williams and Vincenzo Crunelli

The existence of a non-negligible steady-state ('window') component of the low threshold, T-type Ca²⁺current (I_T) and an appropriately large ratio of I_T to I_Leak conductance (i.e. g_T/g_Leak) have been shown to underlie a novel form of intrinsic bistability that is present in about 15 % of thalamocortical (TC) neurones.

In the present experiments, the dynamic clamp technique was used to introduce into mammalian TC neurones in vitro either an artificial, i.e. computer-generated, I_T in order to enhance endogenous I_T, or an artificial inward I_Leak to decrease endogenous I_Leak. Using this method, we were able to investigate directly whether the majority of TC neurones appear non-bistable because their intrinsic ionic membrane properties are essentially different (i.e. presence of a negligible I_T 'window' component), or simply because they possess a g_T or g_Leak conductance that is insufficiently large or small, respectively.

The validity of the dynamic clamp arrangement and the accuracy of artificial I_T were confirmed by (i) recreating the low threshold calcium potential (LTCP) with artificial I_T following its block by Ni²⁺ (0·5-1 mM), and (ii) blocking endogenous LTCPs with an artificial outward I_T.

Augmentation of endogenous I_T by an artificial analog or introduction of an artificial inward I_Leak transformed all non-bistable TC neurones to bistable cells that expressed the full array of bistability-mediated behaviours, i.e. input signal amplification, slow oscillatory activity and membrane potential bistability.

These results demonstrate the existence of a non-negligible I_T 'window' component in all TC neurones and suggest that rather than being a novel group of neurones, bistable cells are merely representative of an interesting region of dynamical modes in the (g_T, g_Leak) parameter space that may be expressed under certain physiological or pathological conditions by all TC neurones and other types of excitable cells that possess an I_T 'window' component with similar biophysical properties.

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