Extracellular Sr²⁺ desynchronizes release at thalamocortical synapses in developing barrel cortex. A, Low-power infrared DIC image of the barrel cortex in a thalamocortical slice in the recording chamber. Scale bar, 100 μm. B, High-power infrared DIC image of a neuron in barrel cortex during a patch-clamp recording. Scale bar, 10 μm. C, Biocytin-filled cell in layer IV neonatal barrel cortex. Scale bar, 10 μm. For A-C, pial surface is located toward the top. D, Example EPSCs (single traces) evoked by stimulation in ventrobasal thalamus collected during a whole-cell patch-clamp recording from a layer IV neuron in the barrel cortex in the presence of normal extracellular solution (2.5 mm Ca²⁺, 0 mm Sr²⁺; left) and in the presence of 4 mm Sr²⁺ (2 mm EGTA, 0 mm Ca²⁺; right). Insets show averaged EPSCs from the same cell evoked by paired-pulse stimulation. E, Example experiment (same cell as in D) showing EPSC amplitude (percentage of baseline before application of Sr²⁺) versus time for an experiment in which 4 mm Sr²⁺ (2 mm EGTA, 0 mm Ca²⁺) was applied extracellularly. F, Summary data from 14 cells showing the reduction in EPSC amplitude with Sr²⁺ application. G, Summary data from eight cells showing the change in paired-pulse ratio (EPSC₂/EPSC₁) with Sr²⁺.