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

1. The existence of a non-negligible steady-state ('window') component of the low threshold, T-type Ca2+ current (I-T) and an appropriately large ration 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) neurons. 2. 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. 3. 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. 4. 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. 5. 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 all 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.