Localization of brain-derived neurotrophic factor and TrkB receptors to postsynaptic densities of adult rat cerebral cortex

Although neurotrophins are critical for neuronal survival and differentiati on, recent studies suggest that they also regulate synaptic plasticity. Bra in-derived neurotrophic factor (BDNF) rapidly increases synaptic transmissi on in hippocampal neurons, and enhances long-term potentiation (LTP), a cel lular and molecular model of learning and memory. Loci and precise mechanis ms of BDNF action remain to be defined: evidence supports both pre-and post synaptic sites of action. To help elucidate the synaptic mechanisms of BDNF action, we used antisera directed against the extracellular and intracellu lar domains of trkB receptors, anti-trkBout and anti-trkBin, respectively, to localize the receptors in relation to synapses. Synaptic localization of BDNF was examined in parallel using anti-BDNF antisera. By light microscop y, trkBin and trkBout immunoreactivities were localized to hippocampal neur ons and all layers of the overlying visual cortex. Immunoelectron microscop ic analysis of the cerebral cortex revealed that trkB and BDNF localize dis cretely to postsynaptic densities (PSD) of axo-spinous asymmetric synaptic junctions, that are the morphological correlates of excitatory, glutamaterg ic synapses. TrkB immunoreactivity was also detected in the nucleoplasm by light and electron microscopy. Western blot analysis indicated that both an ti-trkBout and anti-trkBin antisera react with a protein band in the PSD co rresponding to the molecular weight: expected for trkB; however, molecular species distinct from that for trkB were recognized in the nuclear fraction by both anti-trkBin and anti-trkBout antisera,indicating that the nuclear immunoreactivity, seen by immunocytochemistry, reflects cross-reactivity wi th proteins closely related to, but distinct from, trkB. The PSD localizati on of both BDNF and trkB supports the contention that this receptor/ligand pair participates in postsynaptic plasticity. (C) 2000 Wiley-Liss, Inc.