Conduction Velocities of Excitatory Axons Innervating Parvalbumin Interneurons in Primary Somatosensory Cortex

Abstract

Inhibitory interneurons that express parvalbumin (PV) play critical roles throughout the brain. Their rapid-spiking characteristics enable them to control the dynamics of neural circuits across a range of time scales, but the timing of their activation by different cortical pathways remains unclear. Here, we use a genetically encoded hybrid voltage sensor, hVOS, to image PV interneuron voltage changes with sub-millisecond precision in primary somatosensory barrel cortex (BC) of adult male and female mice. Electrical stimulation evoked depolarizing responses with a latency that increased with distance from the stimulating electrode, allowing us to determine conduction velocity. By focusing on conduction between cortical layers or between barrel columns we were able to measure interlaminar or intralaminar conduction velocity, respectively. Velocities ranged from 74 to 473 m/msec depending on trajectory, and we found that interlaminar conduction velocity was about 71% faster than intralaminar conduction velocity. This suggests that computations within columns can be processed more rapidly than between columns. BC circuitry integrates thalamic and intracortical input for a variety of functions including texture discrimination and sensory tuning. The difference in timing between intra- and interlaminar activation of PV interneurons could impact these functions. This study illustrates how hVOS imaging of PV interneuron electrical activity can reveal differences in the dynamics of signaling within different elements of cortical circuitry, and this approach offers a unique opportunity to investigate conduction in populations of axons based on their targeting specificity.