Messenger RNAs for trk and the low-affinity NGF receptor in rat basal forebrain

Abstract

NERVE growth factor (NGF) acts by binding to specific sets of neurons. Low-affinity binding (K_d of 10⁻⁹M) is mediated via a 75 kDa glycoprotein (LNGFR). Recently, a 140 kDa tyrosine protein kinase encoded by the protooncogene trk has been found to bind NGF with high affinity (K_d of 10⁻¹¹M) and to evoke neurotrophic responses. In addition, the related trkB protein and the LNGFR have been shown to bind brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) with an affinity of K_d 10⁻⁹M. We demonstrate the distribution of mRNA encoding the neurotrophin-binding proteins in the forebrain of the 21-day-old Sprague-Dawley rat by in-situ hybridization. Expression of trk and LNGFR mRNA showed co-localization and was restricted to the medial septal nucleus and the nucleus of Broca's diagonal band, suggesting a receptor function in these cells for both proteins encoded. In contrast, expression of trkB was widely spread in many areas. Thus trkB protein might serve general functions in the forebrain.