Figure 1.

Embryonic amphibian spinal neurons exhibit spontaneous Ca²⁺ spike activity during growth in vitro and in vivo. A, Ca²⁺ spike activity of a X. tropicalis neuron at 3 h in culture. Spikes are often generated in bursts. B, C, Distinct periods of increased spike activity occur during neuronal development in two different frog species. In X. tropicalis neurons in vitro, Ca²⁺ spike frequency is highest between 3 and 5 h of development. Ca²⁺ spike frequency of X. laevis neurons in vitro is highest between 5 and 9 h of development (*p < 0.05, **p < 0.01; n = 9–23 neurons/group from >5 cultures). D, Ca²⁺ spike activity of a X. tropicalis neuron on the ventral surface of the spinal cord at stage 22. E, Ca²⁺ spike activity is elevated in X. tropicalis neurons in vivo at the corresponding time of development in vitro. Neurons on the ventral surface of the spinal cord exhibit high frequencies of Ca²⁺ spiking between stages 21 and 24 (*p < 0.05, **p < 0.01; n = 3–10 embryos/stage). F, The incidence of spiking on the ventral spinal cord is also high at these stages, although the variance prevents it from achieving significance. All values are means ± SEM.