We investigated the development of a low (T-type) and two high voltage-activated (N- and L-type) calcium channel currents in large diameter dorsal root ganglion neurones acutely isolated from embryonic mice using the whole-cell patch-clamp technique. The low and high voltage-activated barium currents (LVA and HVA) were identified by their distinct threshold of activation and their sensitivity to pharmacological agents, dihydropyridines and ω-conotoxin-GVIA, at embryonic day 13 (E13), E15 and E17–18, respectively, before, during and after synaptogenesis. The amplitude and density of LVA currents, measured during a –40 mV pulse from a holding potential of –100 mV, increased significantly between E13 and E15, and remained constant between E15 and E17–18. The density of global HVA current, elicited by 0 mV pulse, increased between E13 and E15/E17–18.
The density of the N-type current studied by the application of ω-conotoxin-GVIA (1 μm) increased significantly between E13 and E15/E17–18. The use of the dihydropyridine nitrendipine (1 μm) revealed that the density of L-type current remained constant at each stage of development. Nevertheless, application of dihydropyridine Bay K 8644 (3 μm) demonstrated a significant slowing of the deactivation tail current between embryonic days 13 and 15, which may reflect a qualitative maturation of this class of calcium channel current.
The temporal relationship between the changes in calcium channel pattern and the period of target innervation suggests possible roles of T-, N- and L-type currents during developmental key events such as natural neurone death and onset of synapse formation.