BDNF is found in neurons of the central nervous system. It is expressed predominantly in hippocampus, cortex, and synapses of the basal forebrain. The biological activity of BDNF is mediated by a receptor that belongs to the trk family of receptors encoding a tyrosine-specific protein kinase. BDNF only binds weakly to the gp140trk receptor (to which NGF binds with high affinity), and it binds to the NGF receptor known as LNGFR. BDNF selectively supports the survival of primary sensory neurons and retinal ganglia. The factor supports survival and differentiation of certain cholinergic neurons and also some dopaminergic neurons in vitro. BDNF does not appear to act on sympathetic ganglia. In specific neurons of the central nervous system located in the hippocampus and the cortex the synthesis of BDNF is influenced by neuronal activity either positively (glutamate transmitter system) or negatively (GABA transmitter system).