TRKA EXPRESSION IN THE CNS - EVIDENCE FOR THE EXISTENCE OF SEVERAL NOVEL NGF-RESPONSIVE CNS NEURONS

NGF acts as a neurotrophic factor by binding and activating its recept or on certain neuronal populations in the CNS and PNS. TrkA is a recep tor 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 neostri atum, trkA expression was found in noncholinergic neurons in (1) the p araventricular anterior and reuniens thalamic nuclei, (2) the rostral and intermediate subnuclei of the interpeduncular nucleus (IPN), (3) s cattered neurons in the ventrolateral and paramedian medulla, (4) the prepositus hypoglossal nucleus, and (5) the area postrema. NGF respons iveness was demonstrated for each of these populations. In contrast to trkA, p75(NGFR) was found only in a minority of NGF-responsive popula tions. Our data provide further evidence that expression of trkA marks NGF-responsive CNS neurons and suggests novel roles for NGF in the br ain.