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 t he low threshold, T-type Ca2+ current (I-T) and an appropriately large rati on of I-T to I-Leak conductance (i.e. g(T)/g(Leak)) have been shown to unde rlie 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 intr oduce into mammalian TC neurones in vitro either an artificial, i.e. comput er-generated, I-T in order to enhance endogenous I-T, or an artificial inwa rd I-Leak to decrease endogenous I-Leak. Using this method, we were able to investigate directly whether the majority of TC neurones appear non-bistab le because their intrinsic ionic membrane properties are essentially differ ent (i.e. presence of a negligible I-T 'window' component), or simply becau se 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 artifi cial I-T were confirmed by (i) recreating the low threshold calcium potenti al (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 o f an artificial inward I-Leak transformed all non-bistable TC neurones to b istable cells that expressed the full array of bistability-mediated behavio urs, i.e. input signal amplification, slow oscillatory activity and membran e 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 gr oup of neurones, bistable cells are merely representative of all interestin g region of dynamical modes in the (g(T), g(Leak)) parameter space that may be expressed under certain physiological or pathological conditions by: al l TC neurones and other types of excitable cells that possess an I-T 'windo w' component with similar biophysical properties.