Quantitative investigation of calcium signals for locomotor pattern generation in the lamprey spinal cord

Locomotor pattern generation requires the network coordination of spinal ventral horn neurons acting in concert with the oscillatory properties of individual neurons. In the spinal cord, N-methyl-d-aspartate (NMDA) activates neuronal oscillators that are believed to rely on Ca(2+) entry to the cytosol through voltage-operated Ca(2+) channels and synaptically activated NMDA receptors. Ca(2+) signaling in lamprey ventral horn neurons thus plays a determinant role in the regulation of the intrinsic membrane properties and network synaptic interaction generating spinal locomotor neural pattern activity. We have characterized aspects of this signaling quantitatively for the first time. Resting Ca(2+) concentrations were between 87 and 120 nM. Ca(2+) concentration measured during fictive locomotion increased from soma to distal dendrites [from 208 +/- 27 (SE) nM in the soma to 335 +/- 41 nM in the proximal dendrites to 457 +/- 68 nM in the distal dendrites]. We sought to determine the temporal and spatial properties of Ca(2+) oscillations, imaged with Ca(2+)-sensitive dyes and correlated with fluctuations in membrane potential, during lamprey fictive locomotion. The Ca(2+) signals recorded in the dendrites showed a great deal of spatial heterogeneity. Rapid changes in Ca(2+)-induced fluorescence coincided with action potentials, which initiated significant Ca(2+) transients distributed throughout the neurons. Ca(2+) entry to the cytosol coincided with the depolarizing phase of the locomotor rhythm. During fictive locomotion, larger Ca(2+) oscillations were recorded in dendrites compared with somata in motoneurons and premotor interneurons. Ca(2+) fluctuations were barely detected with dyes of lower affinity providing alternative empirical evidence that Ca(2+) responses are limited to hundreds of nanomolars during fictive locomotion.