Reelin immunoreactivity in the adult primate brain: intracellular localization in projecting and local circuit neurons of the cerebral cortex, hippocampus and subcortical regions.

Reelin is a large secretable protein which is widely expressed by specific neuronal populations. In the embryonic brain, Reelin plays a signaling role critical for the correct positioning of migrating neuroblasts. Reelin is also expressed in the adult mammalian brain, including humans; however, its function/s there remain poorly understood. To gain insight into which neuronal populations and specific circuits may be influenced by Reelin in the adult, we have conducted a light and electron microscope analysis of Reelin-immunoreactive neuron types in the cerebral cortex and subcortical regions of adult macaque monkeys. Results show that the great majority of brain neurons, including interneurons and projection neurons, are immunoreactive for Reelin although some neuronal populations do not contain Reelin. The immunoreactive protein is located intracellularly, mainly in neuronal somata. Reelin is also present in gray matter neuropil as well as in some long axonal pathways and their terminal arborizations, suggesting that it can be axonally transported over long distances. The staining patterns in the labeled neurons are remarkably diverse. Our observations reveal a wider distribution of Reelin in the adult macaque brain than in any other species investigated to date. The data show that Reelin is in a position to influence most brain circuits in the adult primate brain.