Abstract
NGF acts as a neurotrophic factor by binding and activating its receptor on certain neuronal populations in the CNS and PNS. TrkA is a receptor for NGF. Recent findings
in vitro
indicate that this NGF-activated receptor tyrosine kinase transduces the NGF signal. To further define NGF actions in the CNS, we examined
trkA
expression in the adult rat brain. We found that
trkA
mRNA and immunoreactivity (IR) coincided in specific, defined neuronal populations in the forebrain and brainstem. In addition to cholinergic neurons in the basal forebrain and neostriatum,
trkA
expression was found in noncholinergic neurons in (1) the paraventricular anterior and reunions thalamic nuclei, (2) the rostral and intermediate subnuclei of the interpeduncular nucleus (IPN), (3) scattered neurons in the ventrolateral and paramedian medulla, (4) the prepositus hypoglossal nucleus, and (5) the area postrema. NGF responsiveness was demonstrated for each of these populations. In contrast to trkA, p75
NGFR
was found only in a minority of NGF-responsive populations. Our data provide further evidence that expression of
trkA
marks NGF-responsive CNS neurons and suggests novel roles for NGF in the brain.
