In situ hybridization detection of trkA mRNA in brain: distribution, colocalization with p75NGFR and up-regulation by nerve growth factor.

In situ hybridization techniques were used to examine the distribution and the nerve growth factor (NGF) regulation of trkA mRNA in the adult rat brain in order to identify neurons in discrete regions of the brain that may be NGF responsive. In agreement with previous studies, trkA mRNA was detected within cells located in the medial septum (MS), diagonal band of Broca (DBB), and caudate. trkA mRNA was also detected in many other regions of the brain, including the nucleus basalis of Meynert, substantia innominata, paraventricular nucleus of the thalamus, interpeduncular nucleus, prepositus hypoglossal nucleus, vestibular nuclei, raphe obscuris, cochlear nucleus, sensory trigeminal nuclei, and gigantocellular as well as perigigantocellular neurons in the medullary reticular formation. By combining in situ hybridization detection of trkA mRNA with immunocytochemical detection of p75NGFR, it was determined that the vast majority (> 90%) of the trkA mRNA-containing cells detected in the MS and DBB also express p75NGFR. Likewise, the vast majority of p75NGFR-IR cells detected in the MS and DBB expressed trkA mRNA. Intracerebroventricular infusions of NGF into the third ventricle adjacent to the preoptic area resulted in a 58% increase in relative cellular levels of trkA mRNA in the horizontal limb of the DBB. These data provide evidence that both p75NGFR and trkA are expressed by NGF-responsive neurons in the MS and DBB. In addition, we note that areas that contained trkA mRNA and that also have been reported to contain p75NGFR are areas where high-affinity NGF binding sites have been observed autoradiographically, whereas areas that contain either trkA or p75NGFR alone are areas where no high-affinity NGF binding has been reported. Together, these findings suggest that both trkA and p75NGFR play an important role in the formation of high-affinity NGF receptors in brain and, furthermore, suggest that NGF may have physiological effects within many regions of the brain outside of the basal forebrain.