Figure 1.

Rhythmic activity and heterogeneity among neighboring V1 interneurons. (A) Fluorescence image of upper lumbar, ventral horn En1:GCaMP3 neurons imaged through the ventral white matter in a P1 mouse. Thirteen neurons were circled as ROI in this 300 × 300 mm field of view. Lateral is up. Inset illustrates En1:GCaMP3 expression in upper lumbar spinal cord. Box indicates approximate location of imaged neurons within the ventral horn ∼50 μm deep within the ventral grey matter. (B) Time series traces of fluorescence intensity fluctuations from ROIs defined in A. Cells 5, 6, 7, and 10 are highlighted in panels C and D. During neurochemically-induced fictive locomotion, 12/13 interneurons in this field of view showed rhythmic activity significantly phase locked to electrically-recorded ventral root output (bottom trace). Oscillation amplitudes varied between 10–60% ΔF/F. Grey shading highlights two arbitrary cycles of activity to clarify phase relationships between the imaged neurons. (C) The phase of V1 interneuron bursts identified in 1B cluster into two groups. Phase relationships relative to the ventral root were calculated from smoothed imaging traces. Each group contains similar numbers of cells and are ∼180 degrees out of phase. In this example, 7/13 neurons are approximately in phase with the cyan neuron (blue shading), whereas the remaining six neurons are out of phase with the cyan neuron (orange shading). (D) Pair-wise cell comparisons of relative burst amplitudes. Trough to peak amplitudes were calculated for each locomotor cycle in each cell and plotted against one another; linear regressions determined whether correlated amplitude fluctuations were present between neurons. A high amplitude oscillation in neuron 10 (cyan) predicts a high amplitude oscillation in neuron five (blue, R = 0.87). In contrast, high amplitude oscillations in neuron seven (orange) predict a low amplitude oscillation in neuron six (red, R = -0.72). (E) Neighboring V1 interneurons are not spatially segregated by phase. ROIs were color coded according to the phase groups identified in C. Neighboring V1 interneurons were often active in opposite phases (e.g., cells five and six) such that no clear spatial segregation was noticed. Superimposed, expanded traces from all 13 neurons in the field of view are shown below the image. Traces are color coded according to their phase group from C, highlighting the two alternating cell groups in the field of view. Imaging trace amplitudes scaled arbitrarily, scale bar 10 s.