Developmental expression of neurotrophin receptor genes in rat geniculate ganglion neurons

Individual neurons dissected from immunohistochemically stained paraffin sections of the developing rat geniculate (VIIth cranial) ganglion were assayed for their content of mRNA of the neurotrophin receptor genes, p75, trkA, trkB and trkC. Fetal and postnatal rats, from the 13th embryonic day (E13) until the 20th postnatal day (P20), were used. Single cells were subjected to RNA amplification, followed by treatment with reverse transcriptase and DNA amplification by the polymerase chain reaction (PCR). The identity of the PCR products was verified by subcloning and sequencing. A total of 227 neurons were examined, of which 212 (93%) gave a PCR signal for at least one neurotrophin receptor. We found: ( 1) Approximately half of the neurons expressed more than one receptor. ( 2) A truncated version of trkB, possessing the ligand-binding region but lacking the tyrosine kinase domain, occurred quite frequently, often in combination with the full-length trkB, with trkA or both. ( 3) The pattern of staining for trkB-like immunoreactivity was usually predictive that either its full length or truncated mRNA would be present. This was not the case for trkC-like immunoreactivity. Western blots on E15 brain tissue showed no band for full-length trkC (similar to 150 kDa), suggesting the antibody may have been immunoreactive with a truncated (similar to 120 kDa) but not a full-length version of the trkC receptor. ( 4) The pattern of neurotrophin receptor gene expression changed during development. ( 5) p75 expression occurred infrequently - in only 7 of the 212 neurons that gave a signal for any receptor.