Golgi-like labeling of a single neuron recorded extracellularly

We describe a novel and powerful extracellular method for the staining of a single neuron identified by electrophysiological criteria. This single-unit technique involves the use of glass micro-electrodes (tip diameter: 1.5-2.5 microns) filled with a saline solution (NaCl; 0.5 M) containing 1.5% of biocytin or Neurobiotin. Once a neuron is recorded, isolated and identified, the tracer is delivered by anodal current pulses of a few nA. The cell must remain well isolated and alive during the ejection procedure to ensure optimal staining. Evidence is provided that the labeled neuron is actually the one that was recorded. Our simple method is reliable with a success rate exceeding 85%. The advantages and pitfalls are discussed. This single-unit labeling technique could further be combined with any other procedures ranging from biological to behavioral studies.