IMMUNOCYTOCHEMICAL LOCALIZATION OF TRKA RECEPTORS IN CHEMICALLY IDENTIFIED SUBGROUPS OF ADULT-RAT SENSORY NEURONS

Immunocytochemistry has been used to examine the location of trkA, the high-affinity receptor for nerve growth factor, in adult rat dorsal r oot ganglia, trigeminal ganglia and spinal cord. TrkA immunoreactivity was observed in small and medium sized ganglion cells and in the dors al horn of the spinal cord. In lumbar L4 and L5 ganglia trkA-immunorea ctive cells constitute 40% of dorsal root ganglion cells and range in size from 15 to 45 mu m in diameter. Double labelling using markers fo r various dorsal root ganglion subpopulations revealed that Virtually all (92%) trkA-immunoreactive cells express calcitonin gene-related pe ptide (CGRP) immunoreactivity. In contrast only 4 and 13% of trkA-immu noreactive cells are labelled by the monoclonal antibody LA4 or the le ctin Griffonia simplicifolia IB4, markers for small non-peptide-contai ning cells. Eighteen percent of trkA-immunoreactive cells belong to th e 'large light' subpopulation, identified by their strong immunostaini ng by the neurofilament antibody RT97. TrkA immunoreactivity in the do rsal horn is heaviest in laminae I and II outer, has a similar distrib ution to CGRP, and is depleted by dorsal rhizotomy. Our results show t hat trkA-expressing cells in dorsal root ganglia correspond almost exa ctly with the CGRP, peptide-producing population. The receptor is pres ent not only on cell bodies but also on central terminals. Non-peptide -containing small cells, which constitute 30% of dorsal root ganglion cells, are not trkA-immunoreactive and therefore most probably are fun ctionally independent of nerve growth factor.