Neuron labeling by extracellular delivery of horseradish peroxidase in vivo: a method for studying the local circuitry of projection and interneurons at physiologically characterized sites.

An anatomical method is described that yields individual neurons with continuously labeled dendrites and axons following the extracellular deposition of horseradish peroxidase (HRP) at neurophysiological recording sites in vivo. The method is a logical evolution of previous methods for iontophoretic delivery of HRP: Parameters critical to the ultimate concentration of HRP at the labeling site are reduced by an order of magnitude relative to standard practice. In successful cases one neuron or two in the immediate vicinity (50 microns) of recording sites is/are labeled. Labeling of other processes traversing the injection site, if any, is subliminal at highest light microscopic magnification. Due to the labeling of so few cells and the absence of other labeled processes, dendritic trees and local axonal arbors can be reconstructed without ambiguity. In addition to recovering neurons at sites characterized with physiological (e.g., sensory) stimuli, the method offers the further advantage of being fully compatible with subsequent electron microscopy. Both large (> 20 microns) and small (approximately 8 microns) neuron types and glia have been labeled.