Postnatal development of the T calcium current in cat thalamocortical cells

The burst firing of thalamic cells in the adult cat is mainly controlled by a voltage-dependent membrane current that has recently been characterized as being similar to the low voltage-activated (i.e. low threshold), T-type, Ca2+ current originally described in sensory neurons. In those neurons so far studied, as well as in skeletal muscle, the low threshold Ca2+ current has been shown to decrease in amplitude or even disappear during embryogenesis or the first few weeks of postnatal life. We have now investigated the in vivo postnatal development of the low threshold Ca2+ current present in thalamocortical cells of the dorsal lateral geniculate nucleus of cats aged four to 100 days. The results show that the amplitude of the low threshold Ca2+ current triples from 0.5 nA in the first few days after birth to over 1.5 nA in the adult. However, this increase in amplitude is not accompanied by any change in its inactivation and activation properties, its latency to peak and the time dependence of inactivation removal. Because of these similarities during development and in adulthood it is likely that a major role of the low threshold Ca2+ current during neuronal development is the generation of oscillatory activities similar to those observed in adult thalamocortical cells.