Region-specific regulation of voltage-gated intrinsic currents in the developing optic tectum of the Xenopus tadpole.

Across the rostrocaudal (RC) axis of the Xenopus tadpole optic tectum exists a developmental gradient. This gradient has served as a useful model to study many aspects of synapse and dendrite maturation. To compliment these studies, we characterized how the intrinsic excitability, the ease in which a neuron can fire action potentials, might also be changing across the same axis. Whole-cell recordings from tectal neurons at different points along the RC axis revealed a graded increase in intrinsic excitability: compared with neurons at the caudal end of the tectum, neurons at the rostral end fired more action potentials in response to current injection and expressed greater peak Na⁺ and K⁺ currents, the major intrinsic currents in these neurons that underlie the action potential. We also observed, along the same axis and in the same direction, a previously described increase in the amount of synaptic drive received by individual neurons (Wu GY, Malinow R, Cline HT. Science 274: 972-976, 1996). Thus as synaptic activity ramps up across the RC axis, so does intrinsic excitability. The reduction of overall circuit activity induced a compensatory scaling up of peak Na⁺ and K⁺ currents only in the caudal portion of the tectum, suggesting a region-specific, compensatory form of plasticity.