Expression of brain-derived neurotrophic factor, neurotrophin-3 and their receptor messenger RNAs in monkey rhinal cortex.

The primate rhinal cortex, consisting of areas 36 and 35 of the perirhinal cortex and the entorhinal cortex (area 28), plays a crucial role in perception and memory. We investigated the expression of messenger RNAs for brain-derived neurotrophic factor and neurotrophin-3, as well as those for their respective tyrosine kinase receptors, TrkB and TrkC, in the monkey rhinal cortex. Results from in situ hybridization revealed that each of these messenger RNAs was expressed in neurons with distinct laminar and areal patterns of distribution. Brain-derived neurotrophic factor messenger RNA was principally detected in layers V/ VI of area 36, and layers II/III and V of the entorhinal cortex. Some of the messenger RNA-positive cells in the deep layers of the rhinal cortex were confirmed to exhibit a pyramidal cell-like morphology. Neurotrophin-3 messenger RNA expression was confined to layers II/III of the entorhinal cortex. In contrast, trkB and trkC messenger RNAs were expressed rather homogeneously and abundantly throughout the rhinal cortex. The laminar and cellular distributions of brain-derived neurotrophic factor and neurotrophin-3 messenger RNAs indicate the predominant expression of these neurotrophins in projection neurons. These results suggest that brain-derived neurotrophic factor and neurotrophin-3 regulate neuronal connectivities of forward and backward projections from the rhinal cortex and contribute to functional reorganization underlying the formation and maintenance of long-term memory in primates.