EXPRESSION OF NERVE GROWTH-FACTOR, P75, AND TRK IN THE SOMATOSENSORY AND MOTOR CORTICES OF MATURE RATS - EVIDENCE FOR LOCAL TROPHIC SUPPORTCIRCUITS

Neurotrophins such as nerve growth factor (NGF) are critical for the m aintenance of CNS neurons. We determined the expression of NGF and the neurotrophin receptors p75 and trk in the somatosensory and motor cor tices of mature rats with immunohistochemical techniques. Sections of mature rat cortex were processed immunohistochemically with primary an tibodies directed against NGF, p75, or Irk. The distribution of immuno reactive elements was examined, and stereological techniques were used to determine the density and size of immunoreactive cell bodies. Some sections processed for trk immunoreactivity were examined with an ele ctron microscope. From the size and morphology of the labeled cells, i t appeared that only neurons in the gray matter were NGF-positive. NGF was detected in one-third of the neurons in layers II-m, V, and VI of both somatosensory cortex and motor cortex; however, fewer than 1 in 12 of the layer IV neurons was NGF-positive. With the notable exceptio n of layer V, few cell bodies (2-10% of the total population) were p75 - or trk-immunoreactive. Layer Vb was replete with receptor-positive c ell bodies; more than one-third of the layer Vb neurons were p75- or t rk-positive. All labeled cells appeared to be pyramidal neurons. The d istribution of p75 labeling with the two anti-p75 antibodies was indis tinguishable. In addition, the neuropil in the supragranular laminae w as p75- or trk-positive. Electron microscopy showed that irk immunorea ctivity was also expressed by dendrites. Only rarely were immunoreacti ve axons detected. In summary, NGF is expressed by cortical neurons th roughout cortex, and neurotrophin receptors are widely produced by pos tsynaptic targets. Thus, NGF appears to participate in an intracortica l autoregulatory system. The strong expression of neurotrophin recepto rs by pyramidal neurons in layer Vb (the origin of brainstem and spina l cord projections) suggests that the neurotrophins are especially cri tical for the regulation of corticofugal projection systems.