Distinct temporal profiles of activity-dependent calcium increase in pyramidal neurons of the rat visual cortex

1. Using fluo-3-based fluorometry, we studied variation in depolarization-induced calcium increases in the proximal dendrites or soma of pyramidal neurons in layer II/III of the rat visual cortex. 2. Depolarization for all durations tested (0.1-2 s; 0.5 nA) evoked a train of action potentials and a small increase in calcium signal (mean 26 %) which peaked within 1 s of the onset of depolarization. With depolarization for longer than 1 s, this small increase was often followed by a larger increase (73 %). This later phase of calcium increase occurred without sudden changes in action potential firing. 3. Application of ryanodine, which suppresses intracellular calcium release, abolished the second phase without affecting the early phase in a use-dependent manner. Meanwhile, no major changes were observed in the pattern of action potential firing. 4. In calcium-free medium, both the early and late phases were almost undetectable, although action potential firing was still evoked by injection of depolarizing currents. Since the late phase depended on intracellular calcium release, this effect of calcium-free medium on the late phase is likely to be indirect through an influence on the early phase. 5. This two-phase profile was observed with somatic depolarization or with antidromic action potentials induced by tetanization. Neocortical pyramidal neurons can thus recruit calcium from different sources, even without chemical sensitization, generating temporally diverse profiles of intracellular calcium signal in response to action potential firing. 6. Such variety in the mechanisms of calcium increase may be relevant to the role of calcium as a versatile second messenger for various types of synaptic plasticity.