Intra-axonal neurobiotin™ injection rapidly stains the long-range projections of identified trigeminal primary afferents in vivo: comparisons with HRP and PHA-L

Currently available methods for studying the morphology of physiologically characterized primary afferents are limited by difficulties inherent in impaling thin fibers and by the limited distances over which conventional tracers move during the course of a recording session. We have encountered an alternative method that overcomes these limitations. Neurobiotin™ (NB; Vector) injections into rat trigeminal (V) primary afferents in the brain stem or V ganglion provided rapid, long-range staining with recording and electrophoretic parameters that are commonly used to eject horseradish peroxidase (HRP) or Phaseolus vulgaris leucoagglutinin (PHA-L). When NB was injected into brain stem fibers responsive to vibrissal deflection with A-beta conduction velocities, collaterals were darkly stained in each of the 4 V subnuclei, as well as the cervical dorsal horn. Labeled fibers were also seen in the V root and peripherally in the infra-orbital nerve for a distance up to 15 mm from the injection site (30 mm total). Cell bodies in the ganglion were never labeled. When NB was injected into V ganglion cells with low- or high-threshold receptive fields and A-beta or A-delta conduction velocities, parent axons were stained in the V spinal tract to the level of the obex, and collaterals were visible in each of the 4 V subnuclei. Such long-range staining occurred within 4 h of tracer injection. HRP never stained brain stem fibers following ganglion cell injections and, when injected centrally with the same survival intervals used with NB, dark staining was limited to within 4 mm of the injection site. Unlike NB or HRP, PHA-L injections rarely produced useful data, either because of the high mortality accompanying attempts to achieve a 1-2 week survival period or because injected neurons were not recovered. Due to its rapid and robust transport, NB is a more convenient and reliable tracer than PHA-L for producing long-range staining of the projections of identified ganglion cells. Intracellular injection of NB also produces rapid Golgi-like staining of fibers over much greater distances than HRP under equivalent staining parameters.